364 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



Fig. 174, B, the cavity was perfectly continuous towards the head 

 end, while posteriorly it was still in the form of isolated vacuoles. 



The cells which form the rudiment of the dorsal aorta are from 

 their coarsely-yolked character clearly derived ultimately from the 

 primitive endoderm, but the question remains whether they are 

 derived from the definitive endoderm directly or through the inter- 



FlG. 174. — Portions of horizontal sections through Polypterus larvae of stages 21+ (A) and 

 25 (B) showing the rudiment of the aorta in longitudinal section. 



A, aortic rudiment ; vae, vacuoles. 



mediary of the mesoderm. Such a section as that shown in Fig. 175 

 indicates that the latter is the case. The definitive endoderm shows 

 a perfectly sharply defined surface, clearly marked off from the 

 aortic rudiment, while the mesoderm of the sclerotome on the other 

 hand is continuous with the aortic rudiment. We may say therefore 

 with high probability that the aortic cells are derived from the 

 sclerotome. 



A remarkable feature has been noticed in the development of the 



