DEVELOPMENT. 



189 



the foetus and give it equal mechanical support on all sides. The 

 outer layer of the amnion becomes very thin and adheres to the 

 chorion. 



FIG. 60. Diagram of the Fecundated Egg, farther advanced: a, umbilical vesicle; 6, am- 

 niotic cavity ; c, allantois. 



FIG. 61. Diagram of the Fecundated Egg, with allantois nearly complete: a, inner lam- 

 ina of amniotic fold ; b, outer lamina of amuiotic fold ; c, point where the amniotic 

 folds come in contact. The allantois is seen penetrating between the inner and outer 

 laminae of the amniotic tolds. 



What is the composition of the amniotic fluid ? 



It is water containing small quantities of albumin and urea. 



What is the allantois? 



During the development of the amnion from the somatoblast 

 a change has occurred in the splanchno- 

 blast. From the inferior extremity of 

 the included portion of the yelk-sac, 

 which is to become intestine, there has 

 budded a small mass which develops 

 rapidly, following the outline of the 

 amnion (see Figs. 60, 61, 62), and grows 

 fast to its chorion layer. This structure, 

 the allantois, soon becomes very vascular 

 and carries its blood-vessels to the chorion. 



What is the function of the allantois ? 



It nourishes the growing embryo. The 

 chorion has already become tufted with 

 capillary loops, and has established a 

 connection with the decidua of maternal 



growth. As the vessels of the allantois communicate more and 

 more with the chorion, the embryo derives more of its sustenance 



Diagram of the Fecundated Egg, 

 with the allantois fully form- 

 ed : a, umbilical vesicle ; b, am- 

 nion ; e, allautois. 



