OUE EMBRYONIC DEVELOPMENT. 69 



the villi on one part of the chorion are withdrawn ; 

 while on the other part they grow proportionately 

 stronger, and unite intimately with the mucous 

 membrane of the womb. It is in consequence of this 

 intimate blending that a portion of the uterus is 

 detached at birth, and carried away with loss of 

 blood. This detachable membrane — the decidua — 

 is a characteristic of the higher placentalia, which 

 have, consequently, been grouped under the title of 

 deciduata ; to that category belong the carnassia, 

 rodentia, simile, and man. In the carnassia and 

 some of the ungulata (the elephant, for instance) the 

 placenta takes the form of a girdle, hence they are 

 known as the zonoplacentalia ; in the rodentia, the 

 insectivora (the mole and the hedge-hog), the apes, 

 and man, it takes the form of a disc. 



Even ten years ago the majority of embryologists 

 thought that man was distinguished by certain pecu- 

 liarities in the form of the placenta — namely, by the 

 possession of what is called the decidua rejiexa, and by 

 a special formation of the umbilical cord which unites 

 the decidua to the fcetus. It was supposed that the 

 rest of the placentals, including the apes, were without 

 these special embryonic structures. The funiculus 

 umbilicalis is a smooth, cylindrical cord, from sixteen 

 to twenty-three inches long, and as thick as the little 

 finger. It forms the connecting link between the 

 fcetus and the maternal placenta, since it conducts 

 the nutritive vessels from the body of the fcetus 

 to the placenta ; it comprises, besides, the pedicle 

 of the allantois and the yelk-sac. The yelk-sac 

 in the human case forms the greater portion of the 

 germinal vesicle during the third week of gesta- 

 tion ; but it shrivels up afterwards, so that it was 



