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of these three are on the left side and one only on the right. Of these only the second and third appear to form a pair, of which the one on the right is much bigger than its fellow.

In the next 26 sections there are again four transient nerves.

The first two of these lie on the left side and one of them is a fine one reaching down the length of the myotome. The last two, situated near the end of the row, appear to form a pair.

In the 23 sections of the succeeding row there are only two transient nerves, both on the left side, and the second is much the larger; it has ganglion-cells near its base, as well as one in its course under the epiblast.

Four transient nerves are seen in the 25 sections of the ensuing row; of these the two anterior may perhaps form a pair, the remaining ones lie both on the left side.

The next fact to be noticed is a decussation of fibres between centrally lying ganglion cells, as in Fig. 76. Moreover, it should be mentioned that in this row there are two cases of mitotic figures of cells of the transient system. There is in this series of 21 sections only one transient nerve, and that a fine one, on the left side.

Altogether there are five transient nerves in 20 sections of the next row. The first of these lies to the right side, as does also the second. The third and fourth form a pair, of which the third is much larger than the fourth, although both have a prolonged course. The fifth nerve lies to the left.

After 20 sections more only one additional nerve has been encountered, it appears to be the last of the lot and lies to the left side. In the succeeding sections there are no nerves.

A summary of the nerves of this series is as follows:

There are 21 nerves, paired or unpaired, in all, as well as many small fibres, there are 14 unpaired nerves on the left side and only 4 on the right. These are all that were seen in 211 sections of about the nerves do not extend over quite 2 mm of the

1/133 mm, i. e. embryo 1).

Embryo No. 564 (size about 10,5 mm) showed externally traces of five gill pouches or clefts. In the sections it is seen that of these the first and second branchials possess openings to the exterior, on one side the third branchial has opened, but it is still a pouch on the other. The fourth branchials are in the form of gut-pouches, and the spiracle is still without opening on either side. There are about 1) A very significant fact.

99 somites posterior to the last gill-pouch. In its other general characters the present embryo resembles No. 562.

In the sections there are indications of the transient system for some fifty sections in front of the pronephros, i. e. there are some ganglion-cells scattered here and there on the cord, and at times nerve-processes are encountered. These are not numerous.

With the commencement of the pronephros elongated transient nerves make their appearance; thus, 17 sections from the anterior border of the first pronephric funnel there is a pair of such. In the following 20 sections there are indications of three fibres, all to the right.

In the ensuing row of 19 sections there appear an elongated nerve to the right side, 4 sections further on a pair of spun fibrils, after yet other 3 sections an exquisite nerve (Fig. 88, Plate 5) passing from a basal group of ganglion cells along the outer side of the myotome; after this a pair of fine fibres, and, finally, a single nerve fibre on the left side.

In the first row of the next slide the first four sections contain a pair of elongated nerves proceeding from ganglion - cells on each side of and outside (lateral of) the cord.

Five sections further back there is a second pair of such nerves, and then, in the remaining 17 sections, there are four nerves, two to the right alternating with two to the left.

One of these nerves is depicted in Fig. 87, Plate 5. Its composition reminds one of Fig. 24, but here the fibre is only in course of development. There are 27 sections in the row.

In the second row there is a single nerve (to the right) with ganglion-cells at its base. A little further on the crossing of fibres at the top of the cord, as in Fig. 76, is very obvious. One of the spinning ganglion-cells of Fig. 78 a-g, is followed by two "ganglionated" nerves, one to the right alternating with one to the left, which is 7 sections caudalwards; after other 6 sections there comes a nerve with ganglion-cells in the myotome and others under the epiblast. There are 28 sections in this row.

The following row of 23 sections only contains two nerves, both to the right, and there is an interval of 13 sections between them.

Near the top of the next row of 24 sections lie one of the spinning ganglion-cells of Fig. 78 a-g, then in 3 sections a pair of transient nerves, and some 10 sections beyond this a simple (left)

nerve.

The fifth row (20 sections) only reveals spinning ganglion-cells. In the first row of the next slide (23 sections) there are two large transient nerves 22 sections apart.

In the following three rows of sections (70 sections in all) there are no large "ganglionated" transient nerves, but there are still ganglion-cells on the cord, and here and there short nerve-processes. In the third of these rows the presence of one or two longish fibres, stretching along the outer side of the myotome, should be mentioned. The embryo is now just off the yolk-sac, and in the following row of 28 sections there is a total cessation of fibres and transient nerves to be recorded.

This proves that, when the embryonic body leaves the yolk-sac, the nerves of the system disappear.

Of this series it may be stated in general terms that it is a good one for demonstrating the transient nerves, which are as well represented as in No. 563. Sometimes, perhaps, the extension is not so great, but both embryos (Nos. 563 and 564) differ remarkably from No. 562.

It may also be remarked of No. 564 that there is a marked tendency among the transient nerves of this series to the formation of plexuses, and numbers of small short fibres passing to the epiblast, as in Fig. 76, are very apparent.

As already mentioned, embryo No. 419 exhibits very similar features to those recorded of embryo No. 4101). Like the latter it possesses many subepiblastic nerves of the transient system, and here also the nerves appear to pass in the direction of the yolk-sac. It is quite an easy matter to trace many of them beyond the segmental duct, often they are seen to lie closely under even applied to the epiblast in their course. In this embryo all such nerves met with were in the form of single fibrils without applied nuclei and with no intervening ganglion-cells.

One instance, out of many which might have been depicted, is that shown in Fig. 72, Plate 4.

The figure, like some already described, is a combination or reconstruction. Again in the sections the dorsal part of the cord possesses a crest or crown of transient ganglion-cells (gl. c. c). From each side there project other ganglion-cells, which on the one hand

1) page 20.

remain in contact with those actually on the cord, and on the other pass into long nerve-processes (s. n), the products of their own spinning activities. The two fibres could be followed just as far as, and no further than, represented in the figure. The one on the left comes to an abrupt end nearly opposite the segmental duct, whereas the other could be traced for some little distance further ventralwards.

2) Embryos of 12-13 mm.

In the first embryo (No. 180) to be described under this heading the characters of the transient nervous apparatus resemble in many respects those of No. 141, but the system, so far as its ganglioncells are concerned, has made great progress in its development. Some of its conditions are so striking and remarkable that, were it not for the constant recurrence of similar appearances in a large series of embryos, one might be inclined to suspect some abnormality. R. batis No. 180 measured 12 mm and possessed about 111 somites behind the last gill-cleft. Of the gill-pouches all six are present, all but the last have openings to the exterior, and there are external gill-buds on two arches.

The neurenteric canal is still persistent until the last somite has been formed.

it does not disappear

In the eye the front wall of the retina is thickened and invaginated into the posterior wall. The lens has the form of an epiblastic thickening projecting into the optic cup.

The paired fins are appearing as bud-like outgrowths of the body wall. The segmental duct reaches as far as the cloaca, but does not open into it, being fused with the epiblast in front of the cloaca. Like No. 141 the series contains abundant "primitive ova" in the region where the sexual organs will arise, but here there are none in the somatopleure, which is actively proliferating formative tissue into the limb-outgrowths and for the cutis.

Several sections from the series are figured on Plate 3. (Figs. 51, 52, 53, 54 and 55).

Fig. 51, one of the most anterior, is also one of the most noteworthy of these figures. As the drawing shows, there are within or upon the cord (in which the "Randschleier" of His has begun to be formed), five large ganglion-cells in the usual position. On the left side, and out from the cord, there project five other similar ganglioncells (w.gl. c) forming a chain, the one end of which is applied to a short stumpy process of the outermost of the five central cells, while

the other end, by means of a process of the external ganglion-cell, extends over the tip of the myotome. This chain of applied ganglioncells is, as the sequel will prove, hardly to be looked upon as merely a number of cells caught in the act of migrating outwards from the spinal cord. Similar chains exist in much older stages, and of such many only differ from the present one in the respect that in them the bridge is not solely composed of ganglion-cells, but that there are one or more axis-cylinders forming part of it. In Fig. 53 there is another somewhat one-sided group of six centrally-lying cells, and, in addition, a seventh one (sp. c) excentric in position, which is just beginning to spin a process in the direction of the epiblast.

In the next figure of the series (Fig. 52) a number of central ganglion-cells are again encountered, of these one has a process, which can be followed to the myotome, running behind and closely applied to two ganglion-cells (w.gl. c) placed bluntly end to end. Of these two cells the one is in touch, by means of its nerve-process, with its central fellows, the other possesses a process pointing in the opposite direction over the myotome.

Of another section no drawing is given, because it exactly resembles Fig. 46 from a different series.

Fig. 54 next calls for attention. In it one sees the long nerveprocess of a central cell a process which differs from that in the next figure in that near the myotome it becomes applied to a ganglion-cell (gl. c1). The last figure (Fig. 55) from the series is a combination of the appearances met with in two consecutive sections. It is intended to show among other things that the transient system has in this embryo not yet reached the maximum of its development. The summit of the cord is still in a state of unrest, as demonstrated by the half-developed wandering ganglion-cell (w.gl. c) just beneath the epiblast, and also by the cells projecting from the cord to the right of the figure. On this side also there is a large wandering ganglion-cell (w. gl. c) in close association with the cells of the spinal ganglion. The figure was chosen out of many which revealed this unrest chiefly on account of the short axis-cylinder process, with embedded nucleus, which passes from among the ganglion-cells in the cord upwards, until it touches the epiblast. The figure was made into a combination so as to put in it the long naked axis-cylinder process (n. p) of a central ganglion-cell which was present in the section following that originally drawn. This process becomes lost to sight among the cells forming the tip of the myotome.

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