488 EMBRYOLOGY OF THE CHICK 



into the extra-embryonic coelom, the attached end lying caudal to, and 

 parallel to, the yolk-stalk. The proximal portion is called the allantoic 

 stalk, and the extended bladder-like distal portion the allantoic vesicle. 



It grows very rapidly from the fourth to the tenth day, and extends 

 into the sero-amniotic cavity in a flattened manner. Ultimately it en- 

 compasses the entire embryo and yolk-sac, and in so doing the mesoder- 

 mic layer of the allantois fuses with the layer of mesoderm of the serosa 

 which comes to lie in direct contact with it. This means that there is 

 thus formed a double layer of mesoderm, the serosal portion being de- 

 rived from the somatic mesoderm, and the allantoic portion derived 

 from the splanchnic mesoderm. A very rich vascular network now de- 

 velops between these two layers connected with the vascular circulation 

 by the allantoic arteries and veins. 



The allantois thus becomes an organ of respiration, as well as cir- 

 culation, to the developing embryo. As the allantois lies just beneath 

 the porous shell, there is a wide area presented for an exchange of the 

 carbon dioxide developed within the embryo and the oxygen from the 

 outer world. 



In addition to this function, however, the allantois also serves as a 

 reservoir for the secretions from the excretory organs of the embryo, 

 and likewise takes part in absorbing the albumen. 



THE CHORION 



The serosa will become a part of the chorion in the higher forms, 

 and consequently, should be clearly understood at this point. The 

 allantoic vessels mentioned above and the mesoderm which lies between 

 the serosa and amnion, later fuse with the inner layer of the serosa to 

 form the chorion, which is the embryo's organ of attachment to the 

 uterine wall of the mother. How very important the allantoic circu- 

 lation becomes in mammals may be surmised by realizing that there 

 is little yolk in mammalian eggs. This forces the embryo to receive all 

 of its nourishment from the blood of the mother through the uterine 

 walls. The allantoic circulation thus performs the function of the vitel- 

 line circulation also. 



