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 



r f i(^i ^^M ~ r of the hmd ~ gut continues into 



it, as the postanal gut, for the 

 anus is formed on the ventral 

 FIG. 120. Model of part of the side of the base of the tail. 



The anterior extent of the hind - 



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



fied). The broken line marks the , ,. ,-, -, , 



cavity of the hind-gut and aiiantoic extension ot the splancnno- 



stalk. a .Cut edge of amnion; Dalian- pleura! tail-fold. The region 



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



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



*"* <the ** *~ the spknchnopleural 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). 



