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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 perves 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. The spiracular pouch is still closed, as is also the second branchial. 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 1200 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 17th 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 19 th 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 11 th 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,
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 1/135 mm, i. e. the nerves do not extend over quite 2 mm of the 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, afte 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) Derve.