VERTEBRATE DEVELOPMENT 227 



At the posterior end of the body the neural tube also bends ven- 

 trally, and in that region its cavity connects for a time through the 

 neurenteric canal with the cavity of the enteron. (W. f. 174, 

 K.) 



Lying ventral to the neural tube is the solid rod-like notochord, 

 and lying below this the enteron, the walls of which are composed 

 of endodermal cells. The enteron runs the entire length of the 

 body. In the head region the cavity of the enteron is very much 

 larger than it is posteriorly and this enlarged portion constitutes 

 the primitive pharynx. Various portions of the alimentary canal 

 differentiate from it later. A projection of the pharynx runs pos- 

 teriorly under the yolk mass, close to the ventral body wall. The 

 liver is formed from this diverticulum, and it is consequently 

 known as the liver diverticulum. Anterior to this, between 

 the ventral wall of the pharynx and the body wall, is a space which 

 contains a few scattered embryonic mesoderm cells. These cells 

 represent the rudiment of the future heart. 



Back of the pharynx, the enteron continues as an undifferen- 

 tiated tube to the posterior end of the body, where it communicates, 

 as noted above, with the cavity of the neural tube. At this time 

 the permanent anal opening has not been formed, but a little later 

 it breaks through the body wall, just below the embryonic blasto- 

 pore, and the temporary connection with the neural tube is also 

 lost. A considerable mass of yolk material is present in the embryo 

 at this stage. The cells containing this material are largely 

 grouped in the posterior half of the embryo, between the ventral 

 wall of the enteron and the ventral wall of the embryo. By the 

 time this food has been utilized, the mouth and anal openings have 

 been formed and the embryo can begin to feed. 



Lying on either side of the neural tube and notochord, in a some- 

 what triangular space bounded externally by the ectoderm, is the 

 mesodermal tissue. The mesoderm is first formed as two sheets of 

 tissue running anteriorly from the blastopore, one on either side of 

 the neural tube and notochord. They grow laterally and ventrally, 

 and soon become separated by a longitudinal division into a dorsal 

 portion (verterral plate) and a ventral portion (lateral plate). 

 The vertebral plates soon show evidence of segmentation, and 

 divide transversely into a number of muscle segments, or myo- 

 tomes. The lateral plates of mesoderm grow ventrally and 

 extend around the ventral half of the embryo until they meet and 



