FCETAL NUTRITION: THE PLACENTA 393 



the yolk-sac are produced, and from their walls sohd masses of 

 cells are budded off, resembhng liver-tissue in its simplest form 

 and perhaps functioning as such (von Spee ^). The sac grows 

 up to the end of the fourth week. It is then pear-shaped, and 







Fig. 86.— 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-embryonlc cavity (T. H. Bryce 

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

 large a scale. 



is united to the intestine by a long neck in which the cavity is 

 obliterated. The vesicle persists throughout pregnancy. Little 

 is known of its contents ; at the end of pregnancy it contains 

 variable quantities of fatty substances and carbonates 

 (Schultze % 



' See Quain's Anatomy, vol. i., Part I., 1908. 



^ Schultze, " Ueber 9ie Embryonalhiillen und die Placenta der Saugethiere 

 und des Menschen," Sitzungsb. d. Wurzburger physik.-med. Geaell., 1896. 



