]\rorvPTTOLOc;Y of cyclostoimata. HOV 



peculiar cells are, I think, the equivalent of the primitive 

 genital cells found in the corresponding part of the Amphibian 

 and Selachian body. 



Up to Stage Tir, these cells can not be distinguished from 

 other mesoblastic cells which are equally rich iu yolk-granules. 

 In Stage iv, they become conspicuous ; and in Stage v, again 

 indistinguishable from other constituent cells of this part. 



C. — The Vascular System in the Fro?iephros. 



In early stages, no trace of the vascular system is perceived 

 in the pronephros. What is recognisable as a fore-runner of the 

 vessel is represented by mesenchymatous cells scattered in the 

 space between the primary germinal layers (figs. 77 and 82, 

 7?ich.). These free cells are detected, during Stage iv,^' in three 

 tracts, viz., beneath the chorda, beneath the ventral wall of the 

 enteric canal and outside the jn-onephric tubules on either side (figs. 

 77, 79, 80, 81, and 82, mch.). In Stage y, or at the end of Stage iy, 

 the cells below the hind section of the fore-gut are converted into 

 the endothelium of the heart and of the vessels which are its 

 direct continuations. The cells beneath the chorda are destined 

 to be transformed into the dorsal aorta, and the cells on either 

 side of the pronephros constitute the first indication of the cardinal 

 veins. It is these three vessels — the aorta and the two cardinal 

 veins — which come iu relation wüth the pronephros. 



In the embryos in which the degeneration of the tubules is 

 still going on, there is no special vessel supplying the pronephros; 

 but when the process is over, a pair of long blood-spaces (figs. 

 100-104, bs.) is found in communication with the aorta-tract. 



1) A few of them are observed here and there already in Stage III. 



