History of a transient nervous apparatus In certain Ichthyopsida. 369 



Another embryo ^ (No. 456) of about the same age measured 

 53,5 mm, and from it a series of horizontal sections were made. 



In the figure from it (Fig. 90, pi. 27) a small portion of the summit of 

 the cord, in the region where the ganglion-cells of the transient system 

 are most abundant, is depicted. The ganglion-cells are still large and 

 healthy, and their nerves as as marked features as in earlier embryos. 



Of another embryo (No. 239, 56—57 mm) it is recorded in my 

 notes that degeneration appears to be commencing in the nerves ; for 

 they are no longer so numerous, difficult to find, and, when found, 

 seem to be shrivelling up. 



Embryo No. 100 measured 60 mm. In it the external gills are 

 long, but the characteristic skate-flattening of the body cannot be said 

 to have commenced. 



Only one section from this embryo has been figured (Fig. 104, 

 plate 28). In this figure three central ganglion- cells, with their capsule- 

 cells (c. c), are seen. The cells appear to be still healthy, but, in the 

 present embryo at any rate, it is now no longer possible to find long 

 axis-cylinder processes passing off" from them. With picro-carmine 

 the cell-protoplasm is hardly stained: it is of a grayish-yellow tint with 

 a slight tendency to pink; the nucleus and contents are of a delicate 

 pink colour, and the capsule- cells also. 



Degenerating ganglion-cells, like those already figured from other 

 embryos, are encountered here and there embedded in the upper part 

 of the myotome, but "mesodermal" ganglion-cells are rare in this 

 particular embryo, — an individual peculiarity. Traces of the transient 

 nerves are few and far between, and hence their degeneration in this 

 embryo may be surmised. 



Embryo No. 256 measured 61 mm. Of this embryo horizontal 

 sections were prepared. In these sections the nerve-processes of the 

 transient ganglion-cells are as clearly defined and as numerous as 

 ever. Degeneration does not appear to have as yet attacked the 

 ganglion-cells. 



8. Embryo of 71 mm. — Critical stage. 

 Embryo No. 255 measured 71 mm. The external gills are over 

 13 mm in length. Of the total length 21 mm is covered by the body, 

 the rest, some 50 mm, by the tail. It is of some significance 



1) This embryo was preserved in Flemming's chrom-osmic-acetic 

 mixture. 



