280 



ELEMENTS OF BIOLOGY 



long (Fig. 191). It is traversed by the blood vessels that connect the 

 placenta with the embryonic circulation and by the stalk of the yolk 

 sac and the stalk of the endodermal structure that in early embry- 

 onic life is the origin of the embryonic portion of the placenta. The 



PLACENTA 



UMBILICAL CORD 



WALL or UTERUS 



■FOETAL MEMBRANES 



MOUTH OF UTERUS 



Fig. 191. — Diagram to show the position of the human foetus in the uterus, the 

 umbilical cord, and the position of the placenta. (After Ahlfeld.) 



large amnionic cavity and the relatively long umbilical cord permit 

 considerable movement of the developing offspring. The amnionic 

 fluid serves to equalize pressure on the embryo and as a protection 

 against contact between the rapidly growing embryo and extra-em- 

 bryonic structures. 



Embryonic Vestigial Structures. Recapitulation. The adult 

 of higher animals has a considerable number of structures that are 



