EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 



59 



underlying tissues, remaining attaclied to the wall of the uterus by the 

 stalklike umbilical cord (Fig. 4.10; and Fig. 9.12, p. 191). The umbilical 

 cord ends in the placenta, the beginning of which was noted above. The 

 cord contains blood vessels carrying the embryo's blood to and from the 

 placenta, where it comes in close contact with the mother's blood, though 



branchioil grooves. 



somites 



^olk sac 



FIG. 4.9. Human embryo of the fourth week (3.9 mm. long). (Mainly after Patten, Hu- 

 man Embryology, The Blakiston Company, 1946.) 



the two bloods are separated by thin membranes. "Buds" representing the 

 beginnings of arms and legs have appeared, as have, also, the eyes 

 (Fig. 4.10). 



"Gill Slits" in Human Embryos 



Figs. 4.9 and 4.10 give evidence of another new development, some- 

 times called "gill shts," although branchial grooves is a better term for 

 them, since most of them do not form actual slits in the human embryo. 

 They constitute a series of grooves in the lower head and neck region. On 



