600 



CIRCULATORY ORGANS 



CHAP. 



yolk by the embryo. From this area vasculosa 

 (Fig. 159) the blood is returned by splanchnopleuric 

 (p. 580) vitelline veins, which join with a subintestinal 

 vein (compare Amphioxus, Fig. no) and open into 

 vessels which eventually give rise to the hepatic portal 

 veins. The chief somatopleuric veins in all embryonic 

 Craniates are, as in the dogfish, the jugulars and the 

 cardinals (Figs. 160 and 164). In all Vertebrates above 



n.cr 



-mes" 



end" 



FIG. 161. Transverse section across the head of a Chick embryo at about the fortieth 



hour of incubation, in the region oi the hind-brain and heart. ( X about go.) 

 ao. aorta ; au. beginning of auditory pit ; end', endoderm of pharynx ; end". 

 endoderm of yolk-sac ; h'. muscular wall of heart ; h". epithelial lining of heart 

 (the septum, which subsequently disappears, indicates the paired origin of the 

 heart) ; h.b. hind-brain ; mes'. parietal, and mes". visceral layer of mesoderm; 

 nc. notochord ; n. cr. neural crest ; pc. pericardial coelome ; ph. pharynx. 



the fishes, the cardinals become subsequently more or 

 less entirely replaced functionally by the postcaval 

 (compare p. 456) : the anterior part of one or both 

 cardinals may, however, persist as the azygos vein or 

 veins (e.g. rabbit, p. 525). 



Urinogenital Organs. The excretory organs, speak- 

 ing of craniate Vertebrates as a whole, arise as a series 

 of tubules having the general character of nephridia 



