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 
3 =} zt 
2 2 ois 
cI a AR 
1S) 1S) Od 
€ 
B : 3 
| 4 £ 
= ¢ 3 
ic - —Au 
& 5 
ban = 
a a : 
a a iS 
: 
2 g a 
& ‘3 ES 
3 o n 
a 3 
a ES 
a 7) 
A, Frog and reptiles. B, Birds. C, Mammals. 
Ccelom.—The ccelom or primary cavity of the meso- 
derm arises throughout the Vertebrata as a schizocele, the 
mesoblast splitting into somatic and splanchnic layers. 
Fig. 305.—DIAGRAMMATIC TRANSVERSE SECTION OF A VERTEBRATE 
EMBRYO. 
(Mainly after VAN WIJHE.) 
Nerve Cord. 
Myotome. 
Notochord... 
Aorta. 
Pronephros.. 
Perivisceral 
‘celom. 
Intestine. 
Nevertheless it has the same relationships as in the etero- 
celic celom of Amphioxus. As in the latter, the myoccele (or 
