i in: HKTAI. MI.M "F MAN. 273 



(c) The yolk-sac ( umbilical v Hrln ix n-duced to an exceedingly 



small vesicle, and is COMM-CUM! with the embi \ 

 intestine by means of a long tin-rail like >talk, the 

 vitelline duct. 



(d) By the enlargement of tin- ainnion, which at length lills the 



-nt in- Mastodermic vesicle (increase of amniotic fluid), 

 the canal of the allantois and the vitelline duct, together 

 with the umbilical and vitelline blood-vessels, become 

 completely enveloped by the amniotic sheath; in thi- 

 way is formed the umbilical cord (funiculus umhilicali> . 

 a cord-like connection between the inner surface of the 

 egg-membrane and the navel of the embryo. 



(e) The serosa at a remarkably early period (second week) 



develops villi over its whole surface, and by the ingrowth 

 of the connective tissue of the allantois into the latter it 

 becomes the villous membrane (chorion). 



(f) The villous membrane is differentiated into a chorion laeve 



and a chorion frondosum : 



(a) The part which lies in contact with the decidua 

 reflexa and is firmly united with it by means of 

 villi which lag behind in growth becomes the choi 

 Iseve. 



(/3) The region which abuts upon the decidu; 



and in which the villi giow out into large, much- 

 bra nched tufts, is converted into the chorion 

 frondosum. 



5. By the penetration of the villous tufts of the chori>n frondosum 

 into the decidua serotina and tin -ir firm union with it, th'iv is formed 

 an especial organ of nutrition for the embryo, the after-birth, >i 

 placenta. 



6. One distinguishes a foetal and a maternal part of the placenta : 

 lie placenta fo-talis or the chorion frondosun,. the pla- 

 centa uterina or the original decidua serotina. 



(a) The placenta fretalis consists 



First, of the membrana chorii. in which th- 

 branches of th umbilical blood pread them- 



selves out, and to which the iimbilic;,; stlached, 



ordinarily in the middle i - ntralis). rarely at the 



margin (insertio marginalia, still more 

 distance from the mai-trin (in- i-tio \<-lamentosa); 



Secondhj+Qi bundles of <hoi ionic villi, the ^attachment- 



18 



