THE EMBRYO. 



1157 



entire length of the trunk. These bodies, as will be explained hereafter, are not 

 the representatives of the permanent vertebrae, but are differentiated, partly into 

 the vertebrae and partly into the muscles and true skin. On either side of the 

 proto vertebrae the lateral mesoblast splits into two layers; the upper becomes 

 applied to the epiblast, forming with it the somatopleure or body wall, while the 

 lower becomes attached to the hypoblast and with it forms the splanchnopleure or 

 wall of the alimentary tube (Fig. 699). The space between them is the ccelum or 



Hindbrain. 



Auditory vesicle. 



1 1 ullary ridge. 



Remains of primitive 

 streak. 



FIG. 701. Chick embryo of thirty- 

 Duval's Atlas d'Embryologie.) 



.-Head fold of amnion. 



, Forebmin. 



*-- Optic vesicle. 



3 Heart. 



QlH Omphalo-mesenteric vein. 



^^^^B 1 Protovertebrse or 



9... ) mesoblastic somites. 



'-9~. -Sinus rhomboidalis. 



mrs' incubation, viewed from the dorsal aspect ; X 30. (From 



pleuro-peritoneal cavity (Fig. 700). While the parietes of the body are still 

 unclosed, this cavity is continuous with the space between the amnion and chorion, 

 as seen in Fig. 705. The embryo, which at first seems to be a mere streak, ex- 

 tends longitudinally and laterally. As its grows forward the cephalic end becomes 



