426 THE PHYSIOLOGY OF REPRODUCTION 



of Eternod (Bryce 1 ). This communication appears to exist before 

 anv vessels appear in the embryo itself. From the third week 

 onwards, saccnlar dilatations of the entodermal lining of the yolk-sac 

 are produced, and from their walls solid masses of cells are budded 

 ofi, resembling liver-tissue in its simplest form and perhaps function- 

 ing as such (von Sp'ee 2 ). The sac grows up to the end of the fourth 



Fia. 111. Hypothetical section of the human ovum embedded in the decidua, 

 somewhat younger than Peters' ovum. The trophoblast is greatly 

 thickened, and lined with mesoderm, which surrounds also the embryonic 

 rudiment, with its yolk-sac and amnio-embryonic cavity (T. H. Bryce in 

 Quain'g Anatomy). The embryonic rudiment is proportionally on too 

 large a scale. 



week. It is then pear-shaped, and is united to the intestine by ;i 

 long neck in which the cavity is obliterated. The vesicle persists 

 throughout pregnancy. Little is kiiM\\n of its contents; at the end 

 of pregnancy it contains variable quantities of fatty substances and 

 carbonates (Schultze 3 ). 



1 See (jii'iin'x . I //"/</,//, vol. i., Part I., 1908. 

 - //<//. 



3 Schultze, "Ueber die Em bryonalhullen und die Placenta der Saugethiere 

 und des Menschen," Sitzunggh. <1. ll'/v/-:A//r//er /<////.<//-.-/;/-</. /;,..//., 1896. 



