substance of SCHWANN" it must be exceedingly slight in amount, for they are only "browned" and not blackened with osmic acid.

Figs. 22 (Pl. 1) and 24 (Pl. 2) furnish in detail rather different appearances than those above described. Of the two Fig. 22 shows only a part of what is to be seen in the row of sections. In it the spinal cord, with a number of transient ganglion-cells, is figured. On each side is a portion of a nerve whose full course, as far as determined, is given again in Fig. 24, Pl. 2. This latter, and the mode of drawing it, may now be described. The section in which the ganglion-cells at the root of the nerve appeared was first drawn, and then from the sections in front and behind (in this instance it happened that all were posterior to that drawn) the details of the two nerves were filled in section by section. In this way long stretches of the two nerves could be completely formed anew. These particular nerves have not quite the actual extension given to them in the figure, or rather, to speak more precisely, they could not be followed so far ventralwards. But in the same row of sections there are other similar nerves, the details of whose extension, when filled into the figure, carried them as far as shown. It is unlikely that they terminated even there. This course, by the way, was adopted, not because it made any difference in the actual composition of each nerve, for each of the original nerves could be followed far beyond the point where the "ganglionic nodes" occur, but to avoid unnecessary multiplication of figures. The figure (Fig. 24) shows in section spinal cord, notochord, myotomes, somatopleure etc. etc. It depicts in more diagrammatic fashion the central cells and the two ganglionated nerve-roots seen in Fig. 22. To the right it shows the ganglionated root of the nerve of that side, passing into a nerve formed by a plexiform arrangement of the root ganglion-cells with one near the tip of the myotome (see Fig. 22, gl). Here the nerve ends with, hardly in, a ganglion-cell (gl. c1). To this ganglion-cell a similar one (gl. c2) is applied, and the latter gives off the nerve-fibre (s. n), which passes down under the epiblast in the way shown in the figure. And ends where? Probably on the yolk-sac. On the left side the nerve starts in the same way from two ganglion-cells - or perhaps it may be more correct to describe it as starting at the ganglioncell gl.c3 and passes upwards to end by being applied to the two "root ganglion-cells". The cell gl. c3 is itself very closely applied to another cell gl. c, this could only be made out with the

best and highest lenses, for with ordinary powers there appeared to be only one cell here.

A sketch of this by my pupil, Mr. J. A. MURRAY, will be found on Pl. 5, Fig. 24 a.

The cell gl. c gives off a fibre (s. n) which passes down in the same way as that of the other side. The remarkable nature and the close similarity of these two nerves are very striking.

Another combination from the same series is given in Fig. 25. Here on the left side the nerve (s. n), when followed through three sections, was seen to have exactly the extension given to it in the drawing. How much further it proceeded was an insolvable problem.

The pieces of the nerves were, as just stated, completely present in one or other of the three sections. Thus, the 6th section of the row furnished the "root" piece to nearly the tip of the myotome, as also a small bit opposite the segmental duct, the 7th section revealed a piece near the tip of the myotome, the continuation of that in the 6th, and the 8th section contained a long intact stretch of fibre reaching from just above the segmental duct to nearly the top of the myotome.

might one not add significant?

It is very curious, to note that these nerves are most apparent in the region where the embryonic layers pass on to the yolk-sac, and that where the body is shut off from the yolk, as in the next slide of the series, they cease, or, at any rate, have not so great an extension 1).

There are many nerves like those of Figs. 23 and 25 in this series, but others were noted which had the same composition as those in Fig. 24.

In embryo No. 562 (size 10 mm) there were four pouches or clefts externally visible. In the sections it is seen that of the gillclefts the first and the second branchials are open to the exterior, the spiracle is still closed. The primary optic vesicles are constricted, but as yet not invaginated. The auditory depression is widely open. The segmental duct extends some way back, but as yet it does not reach the cloaca.

In the region of the pronephros the cells of the future transient

1) This point is more fully discussed in a foot-note in a subsequent section. The above was written before a suspicion of the cessation of the nerves, where the embryonic body quitted the yolk-sac, became abundantly verified by observation in other embryos.

system are taking on ganglionic characters aud a few short nerveprocesses of some of the cells are encountered. A little posterior to the pronephric region there are a few transient nerves with a ventral extension as low down as the segmental duct, the total number of these is, however, very limited; and only some half a dozen were noted.

But the transient system as a whole is only in course of development, and there is as yet a marked absence of fully developed transient ganglion-cells on the cord. In the present embryo the migration of the future spinal ganglion-cells from the lips of the cord is not completed in that region in which the transient system first develops.

In embryo No. 551 (10,25 mm) five gill-pouches or clefts were externally visible. Of these the spiracular pouch appears to be about to rupture, and the first and second branchials are actually open. There are about 97 somites behind the gill-region. The auditory invagination is still widely open. The segmental duct terminates in the epiblast some distance in front of the cloacal region.

The most anterior transient nerve (on one side only) lies some 20 sections anterior to the commencement of the pronephros. Only 14 nerves were counted as yet developed in this embryo and of these only four seemed capable of resolution into pairs.

The ganglionic system and the nerves are only in process of development, and the number of cases of mitosis in which future members of the transient system are concerned affords striking evidence of this.

All the same it can be noted that there is a practical cessation of transient ganglion-cells at about the point where the embryo is leaving the yolk-sac.

Two figures from the present embryo are intended to show more especially the way in which the transient ganglion-cells strive towards the epiblast. In Fig. 76 a the single ganglion-cell has spun its process (n.p) into direct touch with the epiblast. In the other figure (Fig. 76) several ganglion-cells have processes reaching to the skin. The latter figure also illustrates the intertwining or anastomosis of processes of centrally lying cells. It proves, as is in fact frequently enough manifested, that the centrally lying cells are in contact by means of a meshwork of nerve-processes.

Embryo No. 563 measures about 10,5 mm. is still closed, as is also the second branchial.

The spiracular pouch Behind this there are

indications of two other pouches. Only one gill-cleft is actually open, i. e.: the first branchial. There are about 106 somites. The auditory invagination is still widely open. The optic vesicles are somewhat constricted, and there is a very slight epiblastic thickening on the site of the lens.

The first traces of the transient system are encountered some twenty sections (of about 1/200 mm thickness) in front of the pronephros. Here they take the form of occasional ganglion-cells spinning out short processes, but, as the pronephros is being reached, ganglionic groups, forming extended nerves, are seen on the right side. A few sections further on a pair of such nerves is very prominent. In this row there are in all twenty-nine sections.

At the very commencement, i. e. in the first three sections of the following row, a pair of transient nerves, as fine as any of those depicted in the plates, comes to view, and in the remaining twenty sections of the row there are three other nerves on the right and a curious chain of ganglion-cells. The pronephros now terminates. The following row of twenty-two sections reveals a large transient nerve to the right side, and, 10 sections further on, a pair of such nerves. Both proceed from ganglion-cells projecting from the cord.

The fifth row of this slide reveals a spinning ganglion-cell like those of Fig. 78. In some of its sections the crossing or decussation of fibres on the top of the cord is quite obvious, as in Fig. 76. Then in the 8th section there is a long fine fibril extending down the outer side of the myotome on the right. And in the 17 th and 18 th sections the beautiful pair of nerves of Fig. 75 is contained. In all there are twenty-three sections in the row.

In the twenty-four sections of the first row of the next slide are the following: A large transient nerve on the left with ganglioncells at its basis, a spinning ganglion-cell, and, finally, in the next section a transient nerve, with ganglion-cells at its root, which possibly completes the pair.

In the second row there are twenty-two sections and three pairs of transient nerves. Of these the first has a coating of ganglioncells at the root-end, whilst the other two are free from such cells and proceed from ganglion-cells on the top of the cord.

In the third row there is already an apparent diminution in the number of nerves; for in 33 sections there are only one pair of nerves with ganglion-cells at the root, a similar single one on the right, and, 18 sections from this, a fibril on the left.

In the 4th row a lengthy nerve occurs on the left side of the second section, and in the 19th section there is a plexiform nerve on the right. These are the only nerves in twenty-nine sections.

In the twenty-five sections of the fifth row only one fibril, on the right side of the 11th and 12th sections, was detected.

Finally, in the twenty-three sections of the last row there is practically a cessation of the nerves, only two short fibrils, both to the right, being noticed. The embryo at this point is leaving the yolk-sac, but is not yet quite off it.

In this slide there are in various sections plenty of short fibrils reaching to the skin, as in Fig. 76.

Transient cells, or rather cells developing into such, were also met with in mitotic division in this series.

The development of the system is perhaps not completed.

In embryo No. 552 (size about 10 mm) the number of somites could not be counted there were probably about 100 present. The auditory organ is an open vesicle, and cells are being proliferated from its epithelium into the applied ganglionic mass.

The spiracle is still closed, the first and second branchials are open, the third is a pouch reaching to the epiblast, and the fourth a mere pouch not yet in contact with the skin.

The oesophagus behind the gill pouches is closed. The optic vesicles are constricted, and the invagination is just commencing. The epiblast is slightly thickened on the site of the future lens. The segmental duct does not yet reach the cloaca.

There are indications of the transient system a little in front of the pronephric region. These are met with some 25-28 sections before the first pronephric funnel, but anterior to this latter there appear to be vestiges of pronephric tubes.

The first well-marked transient nerve lies some 28 sections anterior to the first pronephric funnel, it is in the form of a fine fibril extending along the outer side of the myotome almost as far down as the level of the segmental duct. Eleven sections further back the top of the cord is seen to possess a cap of transient ganglion-cells.

These are the sole very obvious elements of the transient system anterior to the pronephros. In the following 21 sections in the beginning of the pronephric region there are two pairs of transient nerves, followed by a larger one, on the left side only, with ganglioncells near its basal portion and others in its course under the epiblast.

In the next row of 28 sections there are four transient nerves,