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, ). 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- 

 gut is increased by the forward 

 extension of the splanchno- 

 pleural tail- fold. The region 

 beneath the hind-gut ; between 

 the splanchnopleural and 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, B). 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). 





al 



FIG. 120. Model of part of the 

 caudal end of a four-day chick, show- 

 ing the relation of the amnion to the 

 allantoic stalk. After Ravn ( modi- 

 fied). The broken line marks the 

 cavity of the hind-gut and allantoic 

 stalk, a, Cut edge of amnion; al, allan- 

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

 face of hind-limb bud; o, vitelline 

 artery; t, base of tail (the tip has 

 been removed). 



