History of a transient nervous apparatus in certain Ichthyopsida. 393 



the remains of their withered processes (Fig. 110) are readily identified. 

 Afterwards it is largely a matter of chance if such can be distinguished 

 at all. Occasional remains may be found, even in late stages, i. e. 

 in embryos of 19 cm, as in Fig. 129 b. In the final stages no traces 

 of the former processes appear to exist (Fig. 131a— d). 



The changes in the nucleus are briefly the following. At first 

 there seems to be a breaking-up and running-together of its contents, 

 the nucleoli presently become few in number, and finally, in late 

 stages, have entirely vanished, leaving the nucleus as a very faintly 

 defined empty membrane (Figs. 126, 129, etc.). 



Even this seems to vanish, though traces of it often persist until 

 the final disappearance of the cell. It ought also to be stated that 

 throughout the course of the atrophy the nucleus is always becoming 

 more indistinct, and is always losing by degrees the power of "fixing" 

 colouring matters such as haematoxylin or carmine, until it becomes 

 absolutely indifferent to all dyes. As to the cell-protoplasm, this from 

 the start takes on a glassy appearance , often it becomes fissured 

 (Figs. 128 and 130), and at times cells with large vacuoles in their 

 protoplasm may be encountered (Figs. 115 and 128). 



The remains of capsule-cells , loosely hanging on to the dying 

 ganglion-cells, may for a time be observed (Figs. 120 — 124), but they 

 degenerate much more rapidly than do the latter. 



In late stages (embryos of 19 cm) the cells become very few in 

 number, and this scarcity is hardly accounted for by increase in size 

 of the embryo; probably it is due to the much greater rapidity of 

 degeneration in some of the cells. 



An examination of the figures on plate 29 beginning with Fig. 114, 

 taken from an embryo in which the degeneration had barely begun, 

 and leaving -off with Fig. 131, from a newly hatched jR. batis 

 some 17 months old and more than 19 cm in length, reveals, among 

 other things, the remarkable dwindling in size which the cells undergo. 



Figs. 106 a, b and Fig. 131 are drawings under the same magni- 

 fication , in the former the cells are only in the earlier stages of 

 atrophy, in the latter it is almost completed. 



In this final stage of their history, the limit to which they have 

 been followed, they stain intensely with both haematoxylin and eosin. 



Even now the transient ganglion-cells of Raja have not quite 

 quitted the scene. But, surely, they have been traced far enough! 

 How seldom does it not fall to the lot of the embryologist to follow 

 out developmental processes whose history extends over a period of 



