378 THE EVOLUTION OF MAN. 



become more branched. Though these tufts at first covered 

 the whole surface, they afterwards degenerate over a great 

 part of this ; they develop in consequence all the more 

 vigorously at a particular point, at the place where the 

 allantois forms the placenta. 



On opening the chorion of a human embryo of three 



Fig. 134. — Human embryo with its membranes, six weeks old. The outer 

 covering of the embryo forms the chorion, which is covered with numerous 

 branching tufts, and is lined internally by the serous membrane. The embryo 

 is surrounded by the delicate membrane of the amnion-sac. The yelk-sac is 

 reduced to a little pear-shaped navel-vesicle ; its thin stalk, the long yelk- 

 duct, is enclosed in the navel-cord. In this cord, behind the yelk-duct, lies 

 the much shorter stalk of the allantois, the inner layer of which (intestinal- 

 glandular layer) in Figs. 132 and 133 presented a bladder of considerable 

 size ; while the outer layer attaches itself to the inner wall of the outer 

 egg-membrane, and at this point forms the placenta. 



