390 JOHN BEARD, 



of ganglion-cells from the present paper and its continuation, but his 

 material, from its very nature, can never afford him the completeness 

 of a progressive series of stages of the degeneration such as that 

 furnished by an embryological collection Uke mine. This normal 

 degeneration of ganglion-cells and of nerves is now for the first time 

 described and figured for Vertebrate animals, in which hitherto such 

 an occurence is without precedent. The process takes place in every 

 embryo, and in its details it corresponds to what the pathologists term 

 "simple atrophy". 



The degeneration of the various portions of the transient system 

 may be conveniently described in the following order ; viz. a) the 

 nerves, b) the peripheral ganglion-cells, and c) the central ganglion- 

 cells. 



a) The nerves. These demand priority; because, with exceptions 

 to be mentioned anon , they are apparently the first parts of the 

 system to leave the scene. 



Observation on the degeneration of the nerves of the apparatus 

 is beset with difficulties both on account of their small size as also 

 because of the density of the connective tissue in which they lie. 

 When atrophy commences, in embryos of 55 — 60 mm, as recorded in 

 the preceding pages, the nerves are no longer so numerous, they are 

 difficult to find, and, when found, are seen to be shrivelling up. Thus 

 in the nerves the degeneration is apparently of a very rapid nature. 

 In embryos 10 mm longer than the above the degenerating nerves of 

 the apparatus can still be detected , and even their connection with 

 ganglion-cells in the mesoderm may be noted, but none of them can 

 be traced to ganglion-cells at the top of the cord. 



Sometimes the transient nerves in such embryos can be followed 

 for long stretches, even for the whole length of the myotome, but in 

 slightly older embryos the nerves have shrivelled to nothingness, and 

 from now onwards the sole evidence of their former existence is mani- 

 fested by short withered processes of the once multipolar ganglion-cells. 

 In younger stages than this observations have also been made of a 

 gradual breaking up of nerves in the mesoderm. 



b) Peripheral ganglion-cells. In general terms it may 

 be stated that the degeneration of some of these sets in earlier than 

 that of the central cells. 



Examples of such dying cells may be seen on plate 23, Fig. 27, 

 plate 25, Figs. 59, 04, 67 and 68, plate 28, Figs. 107 and HI, and 

 on plate 29, Fig. 125. 



