168 OUTLINES OF CHORDATE DEVELOPMENT 



Later the dorsal rudiment unites with the fused ventral 

 parts, and the entire pancreas is then connected by the pan- 

 creatic duct with the ventral wall of the gut. The opening of 

 the pancreatic duct marks the boundary between the fore- and 

 mid-gut regions during these early stages; later the opening of 

 the duct shifts just within the margin of the bile-duct. 



The chief steps in the differentiation of the oesophagus, 

 stomach, and intestines occur just after hatching. The region 

 between the lung rudiments and the openings of the hepatic; 

 and pancreatic ducts, elongates as the region from which the 

 oesophagus and stomach are formed (Fig. 59). Shortly after 

 hatching (8 mm.) the anterior end of the oesophagus becomes 

 completely occluded by a proliferation of its wall just anterior 

 to the laryngeal opening (Fig. 58, A). The oesophagus remains 

 closed until shortly after the opening of the mouth (10-11 mm.) 

 when it reacquires cummunication with the pharynx. The 

 stomach appears as a dilation of the posterior portion of the 

 fore-gut. Its axis is at first longitudinal, but soon it becomes 

 bent so as to lie transversely. Throughout the larval period 

 the stomach remains comparatively small and not clearly 

 marked off from oesophagus and intestine. 



2. The Derivatives of the Mid-gut 



Up to the time of hatching the mid-gut remains as a narrow 

 opening, dorsal to the yolk-mass which forms the floor of this, 

 the intestinal region; its roof and sides are but one cell thick 

 (Fig. 37). After hatching, the yolk is rapidly absorbed and 

 the intestine begins" to elongate. The process of yolk absorp- 

 tion is most rapid during the first week after hatching; in part 

 the yolk cells degenerate, and in part they become modified 

 into the glandular epithelium of the intestinal wall. Some of 

 the cells of the endodermal lining of the intestine seem to 

 wander outside the wall of the gut, into the mesentery (see 

 below) and contribute to the formation of lymphatic tissue. 



As a result of the elongation of the intestine it becomes 



_ 



thrown into a transverse or duodenal J,oop, extending across 



