VERTEBRATA. 



423 



the unpaired postcaval. We have already noticed in the 

 rabbit, as in all mammals, that the right cardinal persists as 

 an azygos vein (page 387). 



Fig. 304. — The Arterial Arches of Vertebrates. 

 Ventral view. 



A, Frog and reptiles. B, Birds. C, Mammals. 



Ccelom. — The coelom or primary cavity of the meso- 

 derm arises throughout the Vertebrata as a schizocoele, the 

 mesoblast splitting into somatic and splanchnic layers. 



Fig. 305. — Diagrammatic Transverse Section of a Vertebrate 

 Embryo. 



(Mainly after Van Wijhe.) 



Myotome. 



Pronephros. 



Perivisceral '\\^ 

 Ccelom. 



Nevertheless it has the same relationships as in the entero- 

 coelic ccelom of Amphioxus. As in the latter, the myocoele (or 



