122 OUTLINES OF CHORDATE DEVELOPMENT 



3. The Enteron 



At the close of gastrulation and notogenesis the archenteron 

 is a nearly hemispherical cavity on the dorsal side of the embryo, 

 open posteriorly through the blastoporal opening. Its roof 

 and sides are left as a thin layer of endoderm the enteroderm 

 after the chorda and mesoderm have been split off; its floor is 

 formed of the thick mass of yolk cells. By the time the neural 

 tube is completed and the embryo has elongated slightly, the 

 enteric cavity or mesenteron has enlarged considerably, chiefly 

 in front of the yolk mass, which retains a postero- ventral posi- 

 tion in the wall of the gut (Fig. 37). This anterior enlarged 

 region of the mesenteron is known as the fore-gut, the entire 

 wall of which is but one cell thick; this is the region of the 

 embryonic pharynx and later of the oesophagus and stomach 

 as well. At the posterior end of the mesenteron there is a slight 

 enlargement, just in front of the neurenteric canal, which is the 

 hind-gut or rectal portion of the intestine. Connecting these 

 two regions the narrow mid-gut or intestinal region proper, is 

 that containing the yolk cells, which are also to be regarded as 

 enteroderm. 



In the fore-gut of this stage there is an antero-ventral out- 

 pocketing toward the ectoderm just below the fore-brain, indi- 

 cating the region where the mouth will form later. A postero- 

 ventral outgrowth beneath the anterior end of the yolk mass is 

 the rudiment of the liver. Sections passing through the sides 

 of the fore-gut show that even in this early stage the rudiments 

 of the first two or three visceral pouches are present in the form 

 of vertical outgrowths from the side walls of the pharynx to 

 the ectoderm, with which they fuse. Along the region of the 

 fusion the ectoderm is depressed so that these are externally 

 visible as vertical depressions just back of the head (Fig. 22, E, 

 F). Externally two of these are marked at this time as the 

 external branchial grooves, and the ridges left between and in 

 front of them are the rudiments of the second or hyoid, and first 

 or mandibular arches respectively. The hyoid arch is sometimes 

 known here as the "gill plate"; it extends dorsally nearly to 



