CH. LIX.] THE NOTOCHORD 835 



that in the tail fold is the hind-gut, and the remainder is the 

 mid-gut (fig. 631). 



The constriction where the body of the embryo becomes con- 

 tinuous with the remainder of the ovum, is known ultimately as the 

 umbilicus. It remains pervious till birth, when the embryo is 

 separated from the rest of tbte ovum, and through it the mid-gut is 

 connected with the remainder of the archenteron (which is henceforth 

 called the yolk sac) by a narrow hypoblastic tube, the vitello-intestinal 

 duct (tig. 630, 10). 



The portion of the mesoblastic cavity enclosed in the embryo is 

 called the body cavity. It gradually differentiates into the pericardial 

 pleural and peritoneal cavities, which are eventually entirely separated 

 from each other. 



In the early stages the gut is close to the posterior wall of the 

 body, but after a time it advances into the body cavity ; it remains 

 connected, however, with the dorsal wall by a fold of the splanchnic 

 portion of the mesoblast, which is called the dorsal mesentery. A 

 similar mesentery is found connecting the ventral wall of that portion, 

 fore-gut, which becomes stomach and duodenum, with the ventral 

 wall of the body. 



Before the neural groove is closed and becomes the neural canal, 

 the hypoblast beneath the middle of the groove becomes thickened to 

 form a longitudinal ridge (fig. 628). This ridge is the notochord or 

 primitive skeletal axis. It soon separates from the remainder of the 

 hypoblast, and forms a round cord, which lies at first immediately 

 beneath the neural groove, and afterwards beneath the neural tube, 

 extending from the anterior end of the primitive gut, which lies 

 beneath that region of the neural tube which afterwards becomes 

 the mid-brain, to the caudal end of the embryo (figs. 630, 631). 



It follows from what has already been stated, that the embryo 

 attains its distinct form by a process of folding ; but for some time 

 after it is separated off from the remainder of the ovum (except at the 

 margins of the umbilical orifice), it has no limbs. After a time a ridge 

 appears on each side of the body, along the line of the intermediate 

 cell mass in the interior ; this is the Wolffian ridge, and from its 

 anterior and posterior parts, the limbs grow out as small horizontal 

 ledges. 



The differentiated embryo contains parts of all the layers of the 

 blastoderm, and from each of these certain organs are formed as 

 indicated in the following list. 



1. Prom Epiblast. a. The epidermis and its appendages. 

 &. The nervous system, both central and peripheral. 



c. The epithelial structures of the sense-organs. 



d. The epithelium of the mouth, the enamel of the teeth. 



