History of a transient nervous apparatus in certain Icbthyopsida. 335 



epiblast some little way behind the point where the embryo passes off 

 the yolk-sac. 



The centrally lying ganglion-cells are in great part already re- 

 cognisable as ganglionic, but the nerves of the transient system are 

 as yet only represented by a few spun fibrils in the region of the pro- 

 nephros. 



Thus, the system as a whole is only in the first stages of for- 

 mation and differentiation. There are no figures from this embryo. 



Of an embryo of R. clavata, 8 mm in length and with 86 somites, 

 it may be incidentally recorded that, although the pronephros is fully 

 formed, and much of the segmental duct has arisen, the cells of the 

 future transient system are not yet ganglionic, and there are no tran- 

 sient nerve-fibres. 



The next two embryos (Nos. 141 and 142) are almost exactly of 

 identical age. Both measured proximately 9 mm, 83 somites were 

 counted in No. 141, 85 in No. 142. The neurenteric canal is, of course, 

 persistent. The 3rd gill-pouch is complete, but the 4»^ is still merely 

 a slight evagination of the gut. The first branchial cleft is open, the 

 spiracle still closed, and the second branchial cleft on the point of 

 opening. The optic vesicles are constricted but not invaginated, and 

 the lens is represented by a very slight thickening of epiblast. The 

 auditory ganglion is closely attached to the auditory epithelium. The 

 segmental duct ceases some distance in front of the anus. 



A very remarkable appearance for Raja is the great number of 

 "primitive ova " in both somatopleure and splanchnopleure of No. 141. 

 Mater's segmental transverse blood-vessels are also prominent features 

 of this embryo. On plate 27, Fig. 95 a small portion of a horizontal 

 section of the summit of the cord is shown. Some few central 

 ganglion-cells (gl.c.c) are seen, as well as two transient nerves {tn) 

 ending just under the epiblast. In this series of horizontal sections 

 the top of the cord in the region of the pronephros, and for some 

 distance further caudalwards, is crowded with ganglion-cells. 



Capsule cells are as yet undeveloped. 



In the series of transverse sections of embryo No. 141 a great 

 many interesting transient nerves, with and without intercalated nerve- 

 cell nuclei, are to be seen. It also offers very pretty examples of 

 what may be termed subepiblastic nerves, i. e. transient nerves, either 

 simple or of several fibres, running along from cells in the cord as 

 their centre closely underneath the epiblast between it and the 

 myotome. 



