296 OUTLINES OF CHORDATE DEVELOPMENT 



will be better understood if we recall certain facts regarding the 

 development of the tail. The tail-fold of the blastoderm differs 

 essentially from the head-fold in that the blastoderm where it 

 appears includes mesoderm, and is already differentiated into 

 somatopleure and splanchnopleure (Fig. 109, A}. Preceding 

 the formation of the tail-fold of the somatopleure, the splanch- 

 nopleure is folded forward beneath the posterior end of the 

 embryo, establishing a short hind-gut. Soon after, the true, or 



somatopleural, tail-fold pushes 

 inward, a short distance be- 

 hind the splanchnic fold. The 

 embryonic region immediately 

 anterior (morphologically) to 

 the somatopleural tail-fold is 

 the rudiment of the tail, the 

 tail bud (Fig. 109, B). As this 

 elongates, a posterior extension 

 of the hind-gut continues into 

 it, as the postanal gut, for the 

 anus is formed on the ventral 

 side of the base of the tail. 

 The anterior extent of the hind- 



a 



ap 



FIG. 120. Model of part of the 

 caudal end of a four-day chick, show- 

 ing the relation of the aninion to the 



aiiantoic stalk. After Ravn ( modi- gut is increased by the forward 



fied). The broken line marks the 



cavity of the hind-gut and aiiantoic 



stalk. a, Cutedgeofamnion;aZ, allan- 



toic stalk; ap, anal plate; hi, cut sur- , 



face of hind-limb bud; o, vitelline beneath the hind-gut, between 



. f .-, 



extension of the 



pleura! tail-fold. The 

 l 



regon 



thesplanclmopleuraland somat- 

 opleural tail-folds, is mesoder- 

 mal and is in reality a deep ventral mesentery. The cavity of 

 the hind-gut now pushes downward into this ventral mesentery, 

 forming an elongated depression, just in front of the anal 

 region (Fig. 109, 5). This groove-like outgrowth is the rudi- 

 ment of the allantois, which thus makes its appearance about 

 the commencement of the third day of incubation (about 28 

 pairs of somites). In front of this the hind-gut narrows again; 

 the dorsal portion of this expansion, and the region just posterior 

 to it, form the rudiment of the cloaca (Fig. 120). 



