132 



THE HUMAN EMBRYO. 



mm. The number of villi was about one hundred, of which some seventy are 

 clustered about the region where the embryo was found. The others are scattered 

 over the surface of the membrane. They are considerably branched. Each one is 

 .covered by ectoderm which consists of two layers, an inner distinctly cellular, and 

 an outer one in which the cell-boundaries are indistinct and which is known, there- 

 fore, as a syncytium and represents the remains of the original trophoderm. Each 



Am. 



Yk. 



Cho 







FIG. 75. EMBRYO OF A GIBBON (HYLOBATES CONCOLOR) IN THE THIRD STAGE. 

 Am, Amnion. Yk, Yolk-sac. Cho, Chorion. Vi, Villi. (After E. Selenka.) 



villus contains a core of mesodermic tissue. The chorionic membrane is repre- 

 sented as open in order to show the size and position of the yolk-sac, Yk, and of 

 the amnion, Am, which encloses the embryo as it rests upon the yolk-sac. The 

 embryo itself is not shown in the illustration. Both the yolk-sac and the amnion 

 are, of course, covered by a layer of mesoderm. The entire space between these 

 two inner structures and the chorion corresponds to the extra-embryonic coelom, the 

 very precocious and enormous development of which is a special characteristic of 



