DECIDUAL MEMBRANES 



373 



layers respectively. These ancient terms are of obscure derivation. Chorion (Gr., 

 x6pu>v) is the same as the Latin corium, which is applied to the vascular layers of the 

 skin. In its Greek form it is used to designate the vascular chorioid coat of the eye, 

 and the chorioid plexuses of the brain, but it refers particularly to the vascular embryonic 

 membrane. Amnion is derived indirectly from d/wos (a sheep) and Hyrtl reason- 

 ably asks "How came the sheep to have his name enrolled in anatomy?" Whether 

 the amnion was first observed in the sheep, or was so named because of its softness, 

 or for some very different reason, is discussed by the early commentators. The 

 allantois was first observed in the lower mammals in which it attains great size. For 

 example, in the sheep and pig it forms an elongated sac filled with fluid and attached 

 like the arms of a "T" to the distal end of the allantoic duct. This duct, which 

 corresponds with the entire human allantois, issues from the ventral abdominal wall 

 and divides into its two branches, as seen indistinctly through the chorion in Fig. 

 372 (over the body of the embryo). The allantoic sac extends almost the entire length 



FIG. 372. A PIG EMBRYO REMOVED FROM THE UTERUS, "SURROUNDED BY ITS THREE MEMBRANES. 



(Fabricius ab Aquapendente. 1687.) 



of the chorion, and its ends break through the chorionic membrane, projecting freely 

 as the allantoic appendages. In Fig. 372, the one at the right extends upward, and 

 the one at the left, downward. Such an allantois was sought for in man, between the 

 amnion and chorion, where a corresponding structure should be located. Hale (1701) 

 was among those who thought that he found one, but he declared that "most of the 

 ancients allow a human allantois not from their experience of it, but because they 

 took it for granted that men and other animals were alike in their viscera." It was 

 not until 1885 that it was clearly and finally stated that the human allantois was 

 merely a blind tube in the body stalk, never being free or vesicular (His, Anatomic 

 menschlicher Embryonen). 



As to the appropriateness of the term allantois (sausage-like, from the Gr. dAAas 

 there is difference of opinion. Fabricius (De formato fcetu, 1600) one of whose 

 drawings is reproduced in Fig. 372, considers that the word really means "intestinal," 

 or like a sausage skin. 



DECIDUA VERA, AMNION, AND CHORION L^EVE. 



The three structures named above may readily be included in a single 

 vertical section of the wall of the uterus, in the latter part of pregnancy. 

 Care must be taken, however, not to detach the amnion. In Fig. 373 the 



