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Occasionally one finds two such cells, directly opposite each other, one in each myotome of the two sides. Such a find is interesting as being an indication of a former greater symmetry of the apparatus. The peripheral or vagrant ganglion-cells are usually bipolar with two processes, but some give no evidence of the existence of any processes whatever. Often the cells form chains reaching from the centre to, and even over, the tip of the myotome.

Transient nerves of the system. Young embryos also reveal an early formation of nerve-processes spun by the ganglioncells themselves. Especially is this true of the central ganglion-cells. The process of the spinning of fibres is illustrated and described in detail. All the processes tend either in the direction of the epiblast, or in that of the apex of a myotome. In the latter case they make their way along its outer side lying closely under the epiblast. They often, but by no means invariably, occur in pairs one on each side of the body, or so nearly opposite, that a conclusion as to their frequently paired character seems to find a warrant. No more than any other parts of the system do these nerves manifest traces of segmental arrangement. Usually the nerves arch over the myotome, sometimes they pierce it in their course. It is a little doubtful whether the final termination of any of these nerves has been determined, although many of them have been followed downwards far below the segmental duct. Probably some of them make their way on to the yolk-sac, and this is the more probable seeing that they cease where the embryo leaves the yolk-sac.

For a long time the nerves are merely naked axis-cylinders, and they are very seldom made up of more than one fibre. In later stages "nuclei" occur upon them.

Whilst most of them are undoubtedly only processes of single cells, others have a more complicated build, being composed of a plexiform arrangement of ganglion-cells and nerve-processes. Often there occur ganglion-cells, applied end to end, in the course of the nerve, still more frequently there is a coating of such cells upon the fibre.

In the build of these nerves all sorts of bewildering variations are met with in different embryos.

As to the functions of these nerves the conclusion now drawn is that it is a sensory one, and this is held to be true of the whole apparatus.

The system as described is often well-developed in embryos of

9 mm, in which other organs of the animal are only in their very beginnings; it persists intact, until the embryo is upwards of 43 mm in length, and about then reaches its culmination. Degeneration does not as a rule set in, however, until the embryo attains a length of nearly 70 mm.

The period of functional activity appears to be a prolonged one in Raja batis, far longer than in any other form yet investigated. In other forms its life-period may be represented by weeks, or even by days, instead of by months.

In Raja batis it remains quite normal for two months or longer of the 17 months of the embryonic development within the egg-capsule. Then it enters upon the long slow path leading to complete degeneration, the end of which is reached in 12 months or more.

The degeneration of the various elements is described in detail in the paper. Apart from odd members of the vagrant ganglion-cells the nerves appear to be the first to die. Their degeneration is followed by a slow atrophy of the ganglion-cells, and in fact, as indicated in the Lepidosteus paper 1) and now extended to include the nerves, the whole series of changes undergone by these cells corresponds exactly to that degeneration and death of nerve-cells and nerve-fibres which the pathologists term "simple atrophy". This occurs normally in every embryo, and in the paper the histological changes are illustrated and described in some detail.

After a section devoted to a discussion of the mode of nervedevelopment in the transient apparatus, a few lines are assigned to the description of a process termed "degradation of ganglion-cells". "By degradation of ganglion - cells is meant the losing of specific ganglionic characters and functions, when a cell, whose phylogenetic history is ganglionic, sinks to the position of a mere nucleus in the conducting fibre to which it has given origin."

Another section is given over to "preliminary remarks on permanent giant ganglion-cells" in other forms, including those of Amphioxus. Against the views of others it is urged that in older Raja and Scyllium embryos ganglion-cells occur which are perhaps the homologues of those of Amphioxus, and several decisive objections are stated to VON KUPFFER'S attempts to seek equivalents to the transient nerves in the posterior nerve-roots of Amphioxus.

Still further on it is pointed out that, even at its prime, the

1) op. cit. p. 117.

transient apparatus exhibits signs of degeneration, i. e. of being in a somewhat vestigeal form. And in another section it is briefly shown, that there exists no morphological resemblance whatever between the transient and permanent nervous systems, and that the one cannot be regarded as an earlier developed portion of the apparatus, which succeeds it.

The concluding section of the memoir treats, in a preliminary fashion, of development by substitution of organisms, or antithetic alternation of generations.

Among other things an attempt is made to show how in the development of the skate a sexual form (the skate embryo) arises by substitution on an asexual foundation or larva, the former gradually replacing the latter.

There would appear to be, in disguised form, two generations contained in what is usually designated "a young skate embryo". The one generation (here the sexual one) begins to arise on the other (the asexual one) at a very early period after the fertilisation of the egg. At first the asexual form, or reduced larva, obtains the start, and gets into a position to develop its nervous system. Meanwhile the future sexual form has been making slow but sure progress in its development, and, anon, a period arrives when the two cannot co-exist as of equal functional value. As shown by commencing degeneration of its nervous system, the asexual form, or larva, begins to get suppressed, and in the skate-development this appears to be brought to pass by a sudden spurt on the part of the sexual generation.

Previously the organism apparent to our vision lacked many of the most essential characters of the skate, and its sex could not be determined. Now, and now for the first time, it becomes markedly flattened, its pectoral fins commence their forward growth, its organs are getting into something like the adult form and histological condition, the cells of the cord and of the spinal ganglia are becoming distinctly ganglionic, the permanent canal of the central nervous system is in course of formation, and, of some import, the sex of the organism becomes obvious even from the external characters.

These points indicate the great changes in operation, changes all tending towards the attainment of the adult form. The thesis is stated that all Metazoan development from the egg appears to be fundamentally an antithetic alternation of generations, and that directly or indirectly there is invariably a substitution of organisms.

It is also suggested that, while in the higher plants the sexual generation becomes more and more reduced with the higher evolution of the asexual generation, in the animal kingdom the conditions would appear to be reversed, as we ascend the scale; for here the sexual generation would be the one to attain higher and higher morphological differentiation with a progressive reduction and degeneration of the asexual generation or larva.

Appendix a.

Table of sizes, number of somites and state of gill-clefts or pouches

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Appendix b.

The comparison with RABL's finds in Pristiurus in the following summary may be useful:

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It is difficult to bring the two tables into exact correspondence, for there is some variation on Raja in the period of the actual rupture of the various pouches. Thus in No. 564 the spiracle has lagged behind and is still closed, although the third branchial is open on one side. RABL correct ly states that the spiracle usually only opens after the first and second branchials.

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