Studies on the Nervous System of Crustacea.

Transcription

1 STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 33 Studies on the Nervous System of Crustacea. By Edgar.1. Allen, B.Sc, Director of the Plymouth Laboratory of the Marine Biological Association. With Plate 4. IV. Further Observations on the Nerve Elements of the Embryonic Lobster. IN the first part of these studies 1 an account was given of certain nerve elements which had been demonstrated by the action of dilute solutions of methylene blue upon the ganglia of the embryonic lobster (Homarus vulgaris). The elements then described arose, in most cases, from cells which lay in the anterior ganglia of the thorax. In the present paper a number of additional elements occurring in these ganglia will be noticed, together with those found in the posterior thoracic and in the abdominal ganglia. The method of investigation has been the same as that described in my former communication, the results in this case, however, having been obtained principally from embryos which were near the point of hatching. Several of the new elements differ in essential points from any of those previously described, and are of interest in throwing additional light upon the manner in which different portions of the central nervous system, or different movements of the body, are co-ordinated one with another. 1 This Journal, vol. 36, pt. 4, 1894; a preliminary notice occurs iu 'Proceed. Roy. Soc.,' vol. Iv. VOL. 39, PAET 1. NEW SER. C

2 34 EDGAR J. The observations will be dealt with in the following order: I. Elements arising from cells situated in the THORACIC GANGLIA. (a) Elements of types the same as or similar to those previously described. (b) Elements belonging to types not previously described. II. Elements arising from cells situated in the ABDOMINAL GANGLIA. III. Elements arising from cells outside the central nervous system. I. THORACIC GANGLIA. (a) ELEMENTS SIMILAR TO THOSE PREVIOUSLY DESCRIBED. B, THORAX XI 1 (fig. 1). The cell lies in the anterior portion of the lateral mass of ganglion-cells of Thorax xi, and gives off a fine fibre, which passes into the neuropile (" punkt-substance"), and there breaks up into three main branches. Of these branches one passes forwards into the next ganglion in front, another backwards to the ganglion behind, whilst the third curves round the central ganglioncells, and, keeping close to these, runs forward towards the brain on the opposite side of the cord to that on which its cell lies. By actual observation this fibre has been traced as far forwards as Th. VTII, but there can be no doubt that it behaves in the same way as B, Th. v, and B, Th. vm (Pt. I, pi. 35, fig. 3), s and runs forward to the brain. A comparison of the three elements makes it evident that they belong to one and the same system, B, Th. vm, and B, Th. xi, especially resem- 1 The denomination of the elements to be adopted here will be a continuation of the system used in the first paper. On comparingfig.1 of this paper with fig. 1 on PI. 35 of Part I (this Journal, vol. 36), it will be seen that in the present case the lateral mass of ganglion-cells in each ganglion (shaded blue) is more completely divided into an anterior and a posterior portion by the neuropile (left white in the figure) and thefibresof the anterior nerve-root. The present diagram represents more nearly the appearance presented by very late embryos, whilst the former figure represents that found in earlier ones. a This Journal, vol. 36.

3 STUDIES ON THE NERVOUS SYSTEM OP CRUSTACEA. 35 bling one another in details of arrangement. Also, in a preparation of an earlier embryo, in which the fibres B, Th. v, and B, Th. vm, were completely stained, a third fibre was observed ending with them in the brain, running close to them along the cord, but proceeding backwards beyond Th. vm. Its destination at that time remained undetermined, but there can be little doubt that it was the fibre of this element, B, Th. xi. It will be observed, too, that Th. xi is exactly the ganglion in which we should expect to find another element of this class. Such elements have already been described (see Pt. I, pi. 36, fig. 3) in Th. n (with branches to Th. i and Th. m), in Th. v (with branches to Th. iv and Th. vi), in Th. vm (with branches to Th. vn and Th. ix), and now we find a similar one in Th. xi (with branches to Th. x and Abd. i). In this way each ganglion of the thorax must be influenced by these elements, which end in a particular region of the brain. The series of elements of. which the element B, Th. xi, is a type resembles in many respects the series of elements described by Retzius, 1 in Amphioxus lanceolatus, the cells of which give rise to the giant fibres in the nerve-cord of that animal. These fibres, after leaving the cell and crossing to the opposite side of the cord, are described as running forwards for some distance, but their ultimate fate was not determined. A knowledge of the nature and position of their endings would be of great interest. PROBABLE MOTOR ELEMENTS OF ANTERIOR ROOTS. The elements E, G, and H (fig. 1. Compare also Pt. I, pi. 35, fig. 1), the fibres of which leave the ganglia by the anterior nerve-roots, and which have already been described for the anterior ganglia, occur also in Th. ix, x, and xi. The elements E (b) and E (c) (fig. 1) resemble in general features the element E, but differ from it in the number and mode of branching of the fibres which they give off to the neuropile. Their characteristic appearance and the situation of the cells may be seen from the figure. They occur upon both sides in the ganglia from Th. vm to Th. xi. 1 Retzius, f Biol. Untersuch. Neue Folge,' ii, 1891.

4 36 EDGAR J. ALLEN. The element F (b) (fig. 1), which occurs in Th. x and xi, resembles the element F (Pt. I, pi. 35, fig. 1), and may be the representative of that element in these two ganglia. The element P,Th.vi (fig. 1), presents a striking appearance when stained, and differs in important respects from the elements previously described. The cell lies in the anterior portion of the lateral mass of ganglion-cells. The fibre curves backwards to the neuropile, and after giving off what, from its relatively small diameter, may be regarded as a subsidiary branch, divides into two main branches, one of which passes immediately out of the ganglion through the anterior nerveroot, whilst the other runs across as a stout transverse fibre to the opposite side, where it turns back again forming a loop with itself, and was traced as far as the centre of the ganglion. When the element has stained upon both sides the two transverse branches lie close together and appear as one stout fibre. The subsidiary branch, which leaves the fibre before it bifurcates, takes the somewhat complicated curved course represented in the figure, and was traced to the neuropile of the opposite side. As, however, neither this nor the main branch was observed to break up into finer branches, it seems probable that the staining was in all cases somewhat incomplete. Similar elements have not been found in any other ganglion. PROBABLE MOTOR ELEMENTS OF POSTERIOR ROOTS. The cells of the elements just described all send out fibres through the anterior nerve-root of the ganglion in which the cell is situated. In the two following cases, however, the fibre leaves by the posterior root. In Th. VIII, an element (fig. 1, K, Th. vm) similar to K, Th. in (Pt. I, pi. 35, fig. 1), has stained. The cell is situated in the central mass of ganglion cells, the fibre decussates with its fellow of the opposite side and passes out at the posterior root of the ganglion. This element has been found only in Th. in and Th. VIII. In Th. xi is an element which most nearly resembles the element J of the anterior ganglia. This is denominated J (b) (fig. I). The cell is situated at the posterior end of the central

5 STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 37 mass of ganglion cells of the ganglion, and is relatively large. From the cell the fibre passes forwards and slightly outwards to the centre of the neuropile, where after turning downwards it bifurcates, sending one branch out of the ganglion through the posterior root, whilst the other runs as a transverse fibre to the neuropile of the opposite side. The elemeut has not stained in any other ganglion of the thorax, but as will be seen later, what is probably the same element occurs in the abdominal ganglia. The element 0 in Th. vn (fig. 1) also sends its fibre through the posterior nerve-root. The cell lies in the anterior portion of the central mass of ganglion cells and pursues the course indicated in the figure, giving off branches to the neuropile on the side only on which the cell lies. The element occurs on both sides of the ganglion Th. vn, but corresponding elements have never stained in any other ganglion. ELEMENTS BELONGING TO NEW TYPES. The motor elements referred to in Part I, and those already noticed in the present paper, are all characterised by the fact that the fibre leaves the central nervous system through one of the roots of that ganglion in which the cell attached to it is situated. The portion of the element which lies within the central nervous system is therefore entirely confined to one ganglion. In the cases now to be described, whilst the cell lies in one ganglion the fibre passes out of the cord by the nerve-root of some other ganglion. The element Q, Th. vn, is an instance of this class. The cell lies in the anterior portion of the central mass of ganglion cells of Th. VII, gives off a fibre which runs outwards and then upwards to Th. vi, where it passes out by the posterior root of the ganglion. The fibre gives off a stout arborescent branch in Th. VII, and a straight transverse branch in Th. vi, which passes across to the opposite side of that ganglion. This pair of elements has only stained in Th. VII. Three pairs of elements having many of the characteristics of the above, but differing in detail, are found in Th. iv, v,

6 38 EDGAR J. ALLEN. and vi. The element R, Th. vi, shows their principal characters. The cell in this case lies in the posterior portion of the central mass of ganglion cells of Th. vi. 1 It is somewhat smaller than the cell of the element Q, and its fibre runs forwards for some distance before turning outwards. After proceeding in the outward direction, the fibre again turns forwards and runs into Th. v, leaving the central nervous system by the posterior root of that ganglion. Two principal branches are given off during its course, one arborescing in the neuropile of Th. vi, the other in that of Th. v. Another pair of elements, whose cells lie in one ganglion whilst the fibres pass out through the posterior roots of the ganglion in front, is that lettered S in Th. xi (fig. 1). In this case, however, the elements of the opposite sides decussate, giving rise to the characteristic figure shown (fig. 1). The cell of this element is small, and lies on the central surface of the ganglion. The element occurs also in the abdominal ganglia, and will be subsequently described in more detail. It has not stained in any other ganglion of the thorax, except Th. xi. ELEMENTS HAVING TWO OK MORE BEANCHES WHICH PASS OUT OF THE CENTRAL NERVOUS SYSTEM BY THE NERVE-ROOTS OF DIFFERENT GANGLIA. Two pairs of elements of this kind have stained in the thorax, the cells of one pair being found in Th. vn, those of the other in Th. VIII. In the latter case (Fig. 1, T, Th vm) the cell lies in the anterior portion of the lateral mass of ganglion cells, near the point at which the anterior nerve-root leaves the ganglion. The cell gives off a moderately fine fibre, which very soon bifurcates, one branch passing immediately out of the ganglion through the anterior nerve-root, whilst the other runs forwards along the ganglionic cord. The forward branch, keeping close to the lateral masses 1 As was mentioned in Part I (see this Journal, vol. 36, p. 465) the central masses of ganglion-cells of the embryo, shaded blue infig.1, divide at a later stage into an anterior and a posterior portion, the two portions belonging to two adjacent ganglia. The Hue of demarcation is seen in Th. IX (fig. 1).

7 STUDIES ON THE NKRVOUS SYSTEM OF CRUSTACEA. 39 of ganglion cells, pursues a perfectly straight course until it reaches Th. HI, where it gives off a branch, which passes out through the posterior root of that ganglion. After giving off this branch the fibre continues to Th. n, where it turns and leaves the ganglion through the posterior root. In one or two preparations, another branch appeared to be given off from the fibre in Th. iv, and to pass through the posterior root of that ganglion, but I was never able to make myself quite sure of this point. From the preceding description and the figure (fig. 1), it will be seen therefore that the element T, Th. VIII supplies fibres to three (possibly four) nerve-roots of different ganglia, namely, the anterior nerve-root of Th. vm, the posterior root of Th. iv (probable), the posterior root of Th. in, and the posterior root of Th. II, and that all these fibres have their origin in a single cell. A corresponding element occurs also in Th. vn (fig. 1, T, Th. vn). The cell is similarly situated to that of Th. vm, and a fibre passes almost immediately to the anterior root of Th, vn. A second fibre pursues a straight course forwards, keeping close to the corresponding fibre of Th. vm, until it reaches Th. in, where it gives off a branch to the posterior nerve-root of that ganglion. The fibre then continues to run forwards, but its ultimate destination has not been satisfactorily determined. It appears to turn inwards in Th. n. Mention may here be made of a fibre which stains in almost every preparation from the earliest stages onwards. It is inserted in fig. 1, T (a). The fibre appears to run longitudinally through the ganglionic cord from the level of Th. n to the posterior region of the brain, and to give off branches to two nerve-roots. One branch passes out through the posterior root of Th. II, whilst the second leaves the brain by a small nerve which arises immediately anterior to the nerve which supplies Antenna II. A few small fibres are given off to the neuropile of the brain at the angle which the latter branch makes with the longitudinal fibre. No cell has ever been seen to stain in connection with the

8 40 EDGAR J. ALLEN. fibre just described, and whether the originating cell lies inside or outside the central nervous system remains unknown. It is possible that we are here dealing with an element similar to the elements T in Th. vn and vin, the cell of which however has never taken up the methylene blue. Elements in which one ganglion cell gives rise to two or more fibres passing out of the ganglion by different nerve-roots have been described by Retzius in Aulastomum gulo. 1 In that case the two fibres traced passed out by the two roots of the ganglion in which the cell lay, whilst a third, whose ultimate fate was not determined, passed into the general mass of longitudinal fibres of the ganglionic chain. MISCELLANEOUS ELEMENTS. In Th. iv a system of elements occurs, which offers certain difficulties in resolving it into its component parts. It is found to stain either completely or incompletely in embryos at almost all stages of development. Its position and relations to the neighbouring ganglia are shown in fig. 1 (U, Th. iv), whilst the details will be best seen in fig. 4. As may be gathered from the latter figure only two cells, belonging to corresponding elements of the opposite sides, have stained, but it appears to me to be most probable that the system contains several sets of elements, the cells of some of which have never taken up the methylene blue. The only clue which it has been possible to obtain as to the course pursued by the individual elements is the fact that in a number of preparations of late embryos (near hatching) the portion of the system represented in fig. 5 has been alone stained, and may therefore represent a single element. If this be the case, the course of the element may be described as follows (cf. fig. 1, U., figs. 4 and 5) : Starting from a cell in the anterior portion of the lateral mass of ganglion cells of Th. iv, the fibre runs near the dorsal surface of the ganglion with an almost straight course inwards towards its centre, where it gives off a pair of branches (figs. 4 and 5, c.) which run down- 1 Retzius, ' Biol. Untersuch. Neue Folge,' ii. ' Zur Kenntuiss des centralen nervensystems der Wiirmer,'

9 STUDIES ON THE NERVOUS SYSTEM OP CRUSTACEA. 41 wards and break up into tufts of fibres near the ventral surface. The original fibre continues its course to a point a little beyond the centre of the dorsal surface of the ganglion, where it bifurcates, one branch turning upwards and curving outwards to the posterior root of Th. in (figs. 4 and 5, a), whilst the second branch continues in a transverse direction for some distance, and then turns suddenly forwards and runs into the neuropile of Th. in (figs. 4 and 5, b). Its fate here is a little uncertain, but it often presents an appearance which suggests that it ends in a tuft of fine branches, as in fig. 4, b. (on the right hand side of the figure). It should be mentioned that there is one difficulty in regarding the portion represented in fig. 5 as a single element, namely, that two central branches (e) are stained, whereas there appear to be only two when the complete system (fig. 4) is stained. Returning to fig. 4, it will be seen that the whole system there represented contains, in the first place, two elements similar to that in fig. 5, and lying upon opposite sides of the ganglion. In addition to this pair of elements, there is a fibre upon each side (fig. 4, d.), which appears to start from the point where the transverse fibre turns forwards to form the branch b. This fibre (d) runs backwards for a short distance and then turns outwards to the posterior root of Th. iv. It is, however, probably not simply a branch of the element already described, but has a transverse portion of its own running parallel and close to the transverse portion of that element. In some preparations it is clear that the main transverse fibre of the whole system is of a composite nature. This is indicated on the right-hand side of fig. 4. A third fibre belonging to the system (fig. 4, e) appears to spring from near the centre of the transverse fibre, to curve forwards, and finally to pass out at the posterior nerve-root of Th. in. It seems to be impossible to determine from embryos whether the fibres d and e are independent elements whose cells have never stained, or merely branches of the other elements of the system. It is to be hoped that the study of young adults may throw light upon this point.

10 42 EDGAR J. ALLEN. Two other fibres in the same region often stain, and are shown in fig. 4, / and g. The fibre / appears to enter by the posterior nerve-root of Th. in, to pass transversely across the ganglion, and leave by the posterior root of the opposite side, the straight course being broken by a slight indentation at the middle line. It is not unlikely that there are in reality two elements which decussate in the middle line, and are there connected with cells which have remained unstained. The fibre g has a similar relation to the posterior roots of Th. iv. Traces of a system of elements similar to that just described occur in Th.in. A pair of elements exists in each ganglion from Th. vi to Th. x, which unfortunately seldom stains, but when stained presents several points of interest. The appearance is generally that shown in fig. 1, Th. ix, W., but in one preparation the element stained on one side only of the ganglion, and fortunately showed the position of the cell. This element, which actually occurred in Th. vi is inserted in fig. 1 for the sake of clearness in Th. x. (W.). Since in another preparation in which the element was stained, a cell just commenced to take up a similar position, I feel little doubt that this figure (fig. 1, Th. x.j W.) represents the true condition of the element. Its course may then be described as follows: Starting from a cell, which lies in the anterior portion of the central mass of ganglion cells, the fibre passes first backwards, decussates with its fellow of the opposite side, and then turns outwards to the neuropile, upon entering which it gives off a small tuft of fine branches. After giving off this tuft, the fibre continues to run transversely through the neuropile to about the level of the lateral mass of ganglion cells, where it bends suddenly forwards, giving off a short richly arborescent branch at the angle, and then runs to the next ganglion in front, in the neuropile of which it ends in another tuft of fine branches. No branch has stained which passes out through either of the nerve-roots. If the above description represents in reality the entire course of the element, it must serve to put different parts of two adjacent ganglia into communication with each other.

11 STUDIES ON THE NERVOUS SYSTEM OF CRUSTACEA. 43 II. ABDOMINAL GANGLIA. METHOD OF PREPARATION. Staining of elements in the abdominal ganglia can be observed in two ways. In the case of embryos at an early or medium stage of development which have been prepared, as described in Part I, for the staining of elements in the thorax and in which the abdomen is allowed to remain undisturbed, fibres which have taken up the methylene blue in the thorax often continue to absorb the colouring matter in the abdomen, and the cells with which they are connected are thus brought to light. The best results for the abdominal ganglia can, however, be obtained by special preparation of embryos which are very near the hatching point. In such embryos the abdominal ganglia may be dissected out from the surrounding tissue by careful manipulation with needles. Special care must be taken not to injure or stretch the ganglia and their continuity with the ganglia of the thorax should be maintained. If the embryos, thus prepared, be placed with the dorsal surface uppermost in very dilute methylene blue (1 :100,000 may be used to commence with, and the strength gradually increased), satisfactory staining of many of the elements of the abdomen will soon take place. A variation of this method, which often gives good results, consists in removing, or even simply tearing, with needles the cuticle on the ventral surface of the abdomen (after having first turned the abdomen backwards, and caused it to lie in a line with the thorax), and allowing the embryo to lie in the methylene blue with the ventral surface uppermost. In this case, as soon as the staining is thought to be satisfactory, the ganglia must be dissected out before they are examined. A cover-glass may be placed upon them, and the elements rapidly drawn, or the preparation may be fixed with ammonium picrate, and mounted in glycerine diluted with an equal volume of saturated solution of the picrate. Such preparations, however, only retain their full colour for a comparatively short time. For practical purposes a large number of fresh preparations, examined with a cover-glass, have been found more instructive

12 44 EDGAE J. ALLKN. and satisfactory. The same elements are seen again and again with such frequency, that little doubt can remain as to the accuracy of an observation. The elements of the abdomen are similar in kind to those described for the thorax, and will be considered under the same general headings. ELEMENTS OF WHICH BOTH CELL AND FIBRE LIE ENTIRELY IN THE GANGLIONIC CHAIN. In each of the abdominal ganglia, from Abd. n to Abd. vi, a pair of elements similar to that represented in fig. 6, B (b) has been found to exist. Each element has its origin in a large cell situated in the anterior lateral portion of the ganglion near its ventral surface. From this cell a fibre arises which runs inwards and backwards, decussating with its fellow of the opposite side at the middle line and subsequently giving rise to two fibres, one of which runs backwards, whilst the other runs forwards towards the brain. The backward branch breaks up into fine fibres in the neuropile of the ganglion in which the cell lies. The forward branch runs as a longitudinal fibre along the ganglionic cord and in all probability enters the brain. Unfortunately, owing to difficulties of technique, it has never been possible to trace a single longitudinal fibre of one of these elements through its entire course, but a consideration of the whole evidence leaves little doubt as to their entrance into the brain. One of the most satisfactory direct observations was made upon an embryo near the point of hatching, in which the abdominal ganglia had been dissected out, as described above. In such preparations the pair of elements under consideration most frequently stains in Ab. n, and the fibres can generally be traced forwards through Th. x and ix. In one preparation, however, where there was a wound at the level of Th. in, the fibres of the elements upon both sides were distinctly visible as far forwards as this point. But it is in embryos at an earlier stage, in which the abdomen has been left uninjured, that these elements most frequently appear. In order to make the evidence clear it is

13 STUDIES ON THE NERVOUS SYSTEM OP CRUSTACEA. 45 necessary to again draw attention to the method of preparation adopted for such embryos, when it is desired to obtain staining of the longitudinal fibres. The yolk is removed with needles, and the embryo placed with the exposed thoracic ganglia uppermost in the dilute methylene blue, the abdomen being allowed to remain turned in underneath the thorax. The ganglionic cord is then cut across, generally at the level of the oesophagus, and the methylene blue enters the fibres at the wound. In preparations so made the elements under consideration (B (b), figs. 6 and 7) have frequently taken up the blue, and can be well seen on turning the embryo over and lifting back the abdomen. The longitudinal fibres of the posterior ganglia can be traced through the anterior ones and show no sign of terminating. They are clearly continued forward into the thorax, although they cannot be individually followed on account of the flexure of the abdomen. On turniug the embryo back again so that the thoracic ganglia can be examined, all the longitudinal fibres coming from the abdomen are seen to commence at the wound which has been made, and when this wound lies at the level of the oesophagus it is obvious that they must be fibres which enter the brain. No endings of longitudinal fibres coming from the abdomen have ever been observed in the thorax, and preparations of the kind just described have been so frequently made that there can be practically no doubt that the fibres in question enter the brain. These elements (B (5), figs. 6 and 7) are evidently the same as those seen by Retzius in the abdominal ganglia of the adult Astacus. (See ' Biol. Untersuch.' Neue Folge I, pi. xi, fig. 1; pi. ix, fig. 4.) A pair of elements of this kind, as has been already stated, occurs in Abd. vi, and is represented in fig. 7. In this ganglion a second pair of elements, whose cells are situated in the posterior portion of the ganglion, also frequently stains. The fibres from these cells (fig. 7, B (c)) have been traced as far forwards as the abdominal flexure, in preparations in which the only wound in the thorax has been at the level of the

14 46 EDGAR J. ALLEN. oesophagus, and they probably therefore enter the brain. The fact that two pairs of elements in Abd. vi send fibres to the brain, whilst only one pair has been found in the other abdominal ganglia is in accordance with the known composite nature of this ganglion. PROBABLE MOTOR ELEMENTS. Of elements consisting of a cell in a ganglion and a fibre passing out of the cord by one of the nerve-roots, two principal kinds have been found in. the abdomen, as in the thorax, namely (1) those in which the fibre passes out through one of the roots of the ganglion in which the cell lies, the whole of the element within the central nervous system being confined to one ganglion; and (2) those in which the fibre passes out through a nerve-root of some ganglion other than that in which the cell lies. These elements are represented in figs. 2 and 3; fig. 2 giving the appearance presented when the elements upon both sides of a ganglion are stained ; fig. 3, that when they are stained upon one side only, and indicating therefore the course of the individual elements. Each of the elements to be described has been found in all the ganglia from Abd. i to Abd. v. ELEMENTS CONFINED TO ONE GANGLION. The element o, shown in figs. 2 and 3, takes its origin in a cell, which lies near the centre of the ganglion and at its ventral surface. The fibre passes first upwards and outwards, turns inwards, and after a short course divides into two branches, one of which passes out of the ganglion by the posterior nerve-roots, whilst the other runs across to the opposite side, keeping close to the corresponding branch from the element of that side (fig. 2) and subsequently breaks up on the opposite side of the ganglion. The fibre gives off numerous fine branches during its course through the ganglion, which have not been represented in the somewhat diagrammatic figs. 2 and 3, but which may be seen in fig. 8, drawn from a preparation fixed with ammonium picrate (fig. 8, a; Abd. n and in). Element b (figs. 2, 3 and 8) is similar in most respects to

15 STUDIES ON THJi NERVOUS SYSTEM OF CRUSTACEA. 47 the element just described. It differs chiefly in the position of the cell, which lies in the posterior lateral portion of the ganglion. The fibre curves first upwards and forwards, and then downwards, when it divides into two branches similar to those of element a. The element J (b), which has already been described as occurring in the last ganglion of the thorax (fig. 1, Th. xi) is found also in the ganglion of the abdomen (Abd. i-v), and is shown in figs. 2 and 3. The cell lies near the middle of the posterior end of the ganglion. The fibre runs at first forwards and outwards, and then turns inwards and backwards, giving off a small branch to the neuropile at the angle. Subsequently it divides into two branches, one of which passes out at the posterior root, whilst the other runs over to the opposite side of the ganglion, where it breaks up in the neuropile. The element D (figs. 2 and 3) arises from a comparatively small cell, which lies in the anterior half of the ganglion near the middle line. The fibre runs backwards and slightly outwards, decussates with its fellow of the opposite side forming the characteristic figure represented in fig. 2, gives off a small arborescent branch to the neuropile, and then runs backwards to the posterior root of the ganglion through which it passes. ELEMENTS NOT CONFINED TO ONE GANGLION. Two pairs of elements have taken up the stain in each of the abdominal ganglia, whose fibres pass out through one of the roots of the ganglion immediately anterior to that in which the cell lies. The element e (figs. 2 and 3) has the cell situated near the middle line at the anterior end of the ganglion. Prom the cell, which is small, the fibre runs outwards and backwards, and then, turning sharply inwards, runs as a transverse fibre close to its fellow of the opposite side. On reaching the other side it turns forwards, giving off an arborescent branch at the angle, runs into the next ganglion in front, and there passes out at the posterior root. When the elements upon opposite sides are stained it is generally impossible to distinguish the two transverse fibres, which lie so close together in this and

16 48 EDGAB J. ALLEN. in the other similar cases in the abdominal ganglia, that they appear as one. The element S (figs. 2 and 3) is similar to the element S already described in the thorax (Th. xi, fig. 1). The cell, which is very small, lies near the centre of the ganglion and at the ventral surface. The fibre runs first upwards, backwards, and outwards, and then curves forwards and inwards. After running for some distance in this direction it decussates with its fellow of the opposite side, the two fibres lying for a short distance close together. The fibre then turns forwards and outwards, gives off an arborescent branch to the neuropile, and then proceeds forwards in a straight line to the ganglion in front, where it passes out by the middle root of the three which spring from that ganglion. From the foregoing description of the motor elements found in the abdomen, it will be noticed that they, in nearly every case, supply fibres to the posterior nerve-roots of the ganglia, whilst the greater number of those described for the thorax in this and the previous paper send their fibres to the anterior nerve-roots, a few only supplying the posterior roots. The probable reason for this difference is not difficult to find. The anterior roots chiefly supply the appendages, which are well developed in the thorax of the embryo, whilst in the abdomen they are wanting. The posterior roots, on the other hand, supply the muscles of the body itself, which are well developed in the abdomen, but less so in the thorax owing to the considerable space occupied by the still unabsorbed yolk. III. ELEMENTS ARISING FROM CELLS OUTSIDE THE CENTRAL NERVOUS SVSTEM. In Part I a number of elements were described, arising from cells which lay in the ectoderm of the dorsal surface of the abdomen. These cells give off fibres which enter the abdominal ganglia and there bifurcate in the Y-shaped figure, which is characteristic of sensory nerve-fibres in all divisions of the animal kingdom, sending one branch forwards and the

17 STUDIES ON THE KEKVOUS SYSTEM OF OBUSTAOEA. 49 other backwards along the ganglionic cord. Numerous elements of this kind have stained in embryos near the point of hatching, and it has been possible to trace the fibres inside the central nervous system for considerably greater distances than was done before. In the thorax, elements having the characteristic Y-shaped bifurcation enter the various ganglia by the posterior roots. One such fibre is represented in fig. 1 at M, Th. x. Prom this ganglion (Th". x) it has been traced forwards by direct observations as far as Th. i, but there was no indication of a definite ending there, the blue colour becoming gradually less distinct and suggesting that the true termination had not been reached. 1 From Th. xi the element has also been traced as far as Th. i. In the abdomen, elements of this kind enter by the middle root of the three which belong to each ganglion. From Abd. i the forward branch has been actually traced as far as Th. 11, but gave no indication of a definite ending there. From the three ganglia already mentioned the forward fibre has been directly observed to pass through at least nine or ten ganglia. Cases in which the fibre could be traced from these and neighbouring ganglia through five, six, and seven ganglia occurred in a large proportion of the numerous preparations of late embryos that were made. The fibres from spindle-shaped cells lying in the telson can, in embryos of moderate age, be seen to enter the last abdominal ganglion and then to run forward through the anterior ganglia. Such fibres have been traced as far as Abd. i, but could not be followed further on account of the flexure of the abdomen. The fact mentioned in Part I, that elements of the kind 1 If an element stains at all frequently it is generally possible to form an opinion as to whether the actual termination has been reached, or whether the staining is incomplete. In the former case the terminal portion is, generally, at least as deeply stained as any other part of the element, and it may continue to take on a stronger colour after there has been a considerable fading of the other parts. In the case of incomplete staining, on the other hand, the colour becomes more and more faint towards the end of the fibre until Itfinallydisappears. VOL. 39, PART 1. NEW SBE. D

18 SO EDGAR J. ALLEN. under consideration frequently stain in the abdomen, when the longitudinal fibres commence to take up the methylene blue at a wound made at the level of the oesophagus, from considerations of a similar nature to those already adduced in the case of the abdominal elements B (b), leaves little doubt that the fibres actually go to the brain. The observations now recorded, although they do not directly demonstrate the point, render it still more probable. With regard to the second branch given off by these elements after entering a ganglion, which is directed backwards along the ganglionic cord, no definite termination has been found. It has never been seen to pass through more than three ganglia, and can generally only be followed through two. This, however, is probably due to incomplete staining, and the entire course of this branch remains yet to be determined. EXPLANATION OF PLATE 4. Illustrating Mr. Edgar J. Allen's " Studies on the Nervous System of Crustacea." PIG. 1. Brain and thoracic ganglia of Homarus embryo, ma. (Esophagus. tr.br. Transverse bridge behind oesophagus, com. (Esophageal commissure, ant. II. Ganglion of Autenna n. Th. I ix. Thoracic ganglia. For individual elements see text. Somewhat diagrammatic. FIQ. 2. Three abdominal ganglia of Homarus embryo. Motor elements inserted upon both sides. For individual elements see text. Somewhat diagrammatic. FIG. 3. Ditto. Motor elements inserted upon one side only. FIGS. 4 and 5. System of nerve elements in Th. in and Th. rv of Homarus embryo. FIG. 6. Abdominal ganglion of Homarus embryo. B (fi). Element sending fibre to brain. FIG. 7. Sixth abdominal ganglion of Homarus embryo. B (b) and B (c). Two elements sending fibres to brain. FIG. 8. Second and third abdominal ganglia of Homarus embryo. Camera drawing from preparation preserved in ammonium picrate. a. b. motor elements.

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