370 JOHN BEARD, 



to note that the embryo is now making for the adult 

 form: it is flattened from above downwards, and the expanding 

 pectoral fins are also growing forwards to the snout, on each side of 

 which there is a deep notch between it and the fin. The sex can 

 now be determined from the condition of the pelvic fins. The present 

 embryo is a male. The thymus is now free from the epithelium of 

 the gill-cleft. 



Though a longer period than one cares to think about was 

 demanded for the making of sections of this embryo , it has been 

 drawn upon for only two figures (Fig. 105, plate 28, and Fig. 27, 

 plate 23). 



Of these Fig. 105 furnishes a picture of five central cells. Nerve- 

 processes from these were not to be found. Capsule-cells are no 

 longer very obvious. The cells, with picrocarmine, after the platinum- 

 chloride mixture of Rabl, are of a darkish yellow-brown , and the 

 nucleoli are not so distinct. The whole cell has taken on that "glassy" 

 appearance which characterises the degeneration. 



The nerves of the transient system appear to be in process of 

 degeneration, here and there one can be detected in the mesoderm, 

 often connected with ganglion- cells there , but none of them can be 

 followed to the centre. 



Even for long stretches such nerves can still be traced; one in 

 this series could even be followed the whole length of the myotome. 

 It is, however, anything but an easy task to make out the nerves at 

 all, owing as much to the great development of connective tissue as 

 to the degeneration. Degenerating ganglion-cells occur at intervals 

 in the myotome, and they are very numerously represented in the 

 mesoderm. A group of such ganglion-cells, as drawn under high 

 magnification, is shown in Fig. 27, plate 23. 



The cells in question were found near the epiblast on the level 

 of the notochord. Their degenerate condition is obvious. In the 

 section they are glassy in appearance and with irregular but sharp 

 outlines. Nuclei are indistinct and nucleoli hardly represented. We 

 have now reached a stage where there is no longer any 

 question of a general degeneration of the apparatus. 

 There remains to trace this process in a series of embryos. To 

 demonstrate it most of the figures on ])lates 28 and 29 have been 

 drawn. All the following embryos then illustrate : 



