AMNIOTES. 



289 



ventral side, through which, as will be seen from Fig. 288, D, 

 pass the stalks of the yolk sac and of the allantois next to be 

 mentioned. 



FIG. 288. Diagram of the foetal envelopes of an amniote. A, before the union 

 of the amniotic folds ; B, transverse section of A ; C, union of amniotic folds ; 

 D, outgrowth of allantois and reduction of yolk sac. a, amnion ; a/, allantois ; am, 

 amniotic folds; ac, cavity of amnion ; t, ccelom; i, alimentary tract; s, serosa; so, 

 somatopleure ; sp, splanchnopleure ; y, yolk sac ; ys, yolk stalk. The somatic meso- 

 derm by dashes, the splanchnic layer dotted ; ectoderm and entoderm a continuous 

 line. 



The allantois is represented in the amphibia by a ventral 

 outgrowth from the hinder portion of the alimentary canal, 

 which never extends beyond the body walls, but develops in 

 situ into the urinary bladder. In the amniotes, on the other 

 hand, this outgrowth is more extensive, and extends outward be- 



