;i2 



BULLETIN OF THE 



any of the cavities of the embryo; with a cavity which is part of the 

 original "cavity of invagination," and is surrounded by a wall of cells 

 derived from the endoderm : and an.outer chamber bounded on the in- 

 side by the cells of the endoderm, and on the outside by the ectoderm, 



Fig. 19. 



-73 



Fig. 21. 



Successive stamps in the formation of the embryo ; b, outer tunic of male ; c, wall of branchial 

 sac of male ; li, branchial cavity of male : 1. sinus system of male; 10, opening of inner cham- 

 ber of placenta ; 11, club-shapi il organ ; 12, outer chamber <>f placenta ; 13, branchial cavity 

 of embryo; 11, nucleus of embryo. 



having no communication with the sinus svstem or other cavities of 

 the nurse, but being directly continued into the body cavity of the 

 embryo. 



The Placenta. — For the sake of clearness we will neglect for the 

 present the changes which are now taking place in the body of the 



