THE F(ETAL MEMBRANES OF MAN. 273 



(c) The yolk-.sac (umbilical vesicle) is reduced to an exceedingly 



small vesicle, and is connected with the embryonic 

 intestine by means of a long thread-like stalk, the 

 vitelline duct. 



(d) By the enlargement of the amnion, which at length fills the 



entire blastodermic 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 this 

 way is formed the umbilical cord (funiculus umbilicalis), 

 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 iinited with it by means of 

 villi which lag behind in growth becomes the chorion 

 laeve. 



(/3) The region which abuts upon the decidua serotina, 

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

 branched tufts, is converted into the chorion 

 frondosum. 



5. By the penetration of the villous tufts of the chorion frondosum 

 into the decidua serotina and their firm union with it, there is formed 

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

 placenta. 



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

 (1) the placenta foetalis or the chorion frondosum, and (2) the pla- 

 centa uterina or the original decidua serotina. 



(a) The placenta fcetalis consists 



First, of the membrana chorii, in which the chief 

 branches of the umbilical blood-vessels spread them- 

 selves out, and to which the umbilical cord is attached, 

 ordinarily in the middle (insertio centralis), rarely at the 

 margin (insertio marginalis), still more rarely at a 

 distance from the margin (insertio velamentosa) ; 



Secondly, of bundles of chorionic villi, the " attachment- 



18 



