426 



THE PHYSIOLOGY OF REPRODUCTION 



of Eternod (Bi'jce ^). This communication appears to exist before 

 any vessels appear in the emhryo itself. From tlie third week 

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

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

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

 ing as sncli (von Spee^). Tlie sac grows up to tlie end of tlie fourth 



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

 somewhat j'ounger than Peters' ovum. The trophoblast is greatly 

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

 rudiment, with its volk-sae and amnio-enibi'yonio cavitj' (T. H. Bryce in 

 Quain's Ariatojiii/). The embryonic rudiment is proportionally on too 

 large a scale. 



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

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

 throughout pregnancy. Little is known of its contents ; at the end 

 of pregnancy it contains ^-arialile quantities of fatty substances and 

 carbonates (Schultze ^). 



' See (Jmiiii^s Anatdinii, vol. i.. Part I., 1908. 

 -' Hid. 



3 Schultze, "Leber die Ernbryonalhullen und die Placenta der Siiugethiei'e 

 und des Mensehen," Sitzungsh. <1. Wiirdrurger phi/sil-.-med. Gi'xeU., 1896. 



