334 OUTLINES OF CHORD ATE DEVELOPMENT 



2. Organs of the Hind-gut 



Embryologically the most important appendage of the hind- 

 gut is the allantois. In describing the formation of this organ 

 we mentioned the essential steps in the establishment of the 

 hind-gut itself, and described the formation of the postanal gut 

 and the beginning of the cloaca (Figs. 120, 121). On the ventral 

 side of the hind-gut, between the outgrowing allantois and the 

 base of the tail the ectoderm becomes pitted in toward the 

 cloaca forming the proctodceum. The ectodermal epithelium 

 of the proctodaBum fuses with the endodermal lining of the 

 cloaca forming the anal plate. This was originally toward the 

 dorsal surface of the embryo, but was pushed into a ventral 

 position by the formation and growth of the tail-bud (Fig. 121). 



The cloaca becomes a deep but narrow cavity; it receives, 

 antero-dorsally, the opening of the terminal portion of the 

 intestine, and the allantoic stalk connects antero-ventrally. 

 Laterally, either side of the rectal opening, the urinogenital 

 ducts discharge into the cloaca. During the fifth and sixth 

 days the postero-ventral portion of the cloaca is temporarily 

 closed by the fusion of its walls. The cavity which re-forms 

 here is the rudiment of the bursa Fabricii, which acquires an 

 opening directly into the proctoda3um just outside the anal 

 plate. Thus the embryonic cloaca gives rise chiefly to that 

 part of the adult cloaca into which the urinary and genital 

 ducts open. The anal plate finally becomes perforated early in 

 the third week of incubation. 



3. The Mid-gut 



The embryonic mid-gut gives rise to the intestinal tract ex- 

 tending from the hepatic and pancreatic diverticula to the cloaca. 

 Its establishment through the approach and fusion of the 

 lateral splanchnopleural folds has been described in the pre- 

 ceding chapter. It remains open by the splanchnic stalk to 

 the yolk-sac until toward the close of the embryonic period 

 (see above). 



