REPRODUCTION 



93 



region, carrying with them the coelom, and meet mid-ventrally to form a 

 double vertical layer, a ventral mesentery, extending from the enteron 



prosencephalon 



proamnion 



anterior neuropore 



optic vesicle 



■ mesencephalon 

 border of fore-gut 



omphalomesenteric vein 



rbombencephalon 



lateral mesodean 



notochord 



sinus rhomboidalis primitive streak 



Pig. 65. — Dorsal view of an entire chick embryo having 12 pairs of mesodermal 

 somites; after about :is hours incubation. X14. (From Patten, Embryology of the 

 Chick.) 



to the outer body wall and separating right and left coelomic cavities 

 (Fig. 63). 



The splitting of the original sheet of mesoderm extends so far dorsally 

 as to involve the somite which accordingly contains a more or less definite 

 cavity, the myocoele — "myo" because the somite is mainlv muscle- 



