Development of the Mammal 313 



bilicus is reduced to a solid cord of fibrous 

 tissue, the urachus. 



FIG. 103. HUMAN EMBRYO AT THE END OF THE SEVENTH WEEK. 

 (From Kollmann,) 



;//, amnion. <:<?, external coelom. cf, chorion frondosum. cl, chorion laeve. 

 j, serosa. ys, yolk-sac. 



THE YOLK-SAC 



The yolk-sac as seen in the frog and the 

 chick has already been described. In the 

 mammal it varies in size but is a very incon- 

 spicuous structure, and contains a liquid instead 

 of the food yolk, as has already been said. 



The early structure of the yolk-sac in the 

 mammals is shown in Figures 93 and 94. 



