JIAMMALIA. 



203 



it was at an earlier stage. The allantoic stalk is shewn at al. Both it 

 and the stalk of the yolk-sack are enveloped by the amnion (am). The 

 chorion with its vascular processes surrounds the whole embryo. 



It may be noted that the condition of the chorion at this stage is very 

 similar to that of the normal diffused type of placenta, described in the 

 sequel. 



While the above changes ai-e taking place in the embryonic membranes, 

 the blastodermic vesicle greatly increases in size, and forms a considerable 

 projection from the upper wall of the uterus. Three regions of the uterine 

 wall, in relation to the blastodermic vesicle, are usually distinguished ; 

 and since the superficial parts of all of these are thi'own off with the after- 

 birth, each of them is called a decidua. They are represented at a some- 

 what later stage in tig. 151. There is (1) the part of the wall reflected 

 over the blastodermic 

 vesicle, called the de- 

 cidua reflexa (ch) ; 

 (2) the part of the 

 wall forming the area 

 round which the re- 

 flexa is inserted, called 

 the decidua scro- 

 ti na (ds) ; (3) the 

 general wall of the 

 uterus, not related to 

 the embryo, called the 

 decidua vera (du). 



The decidua reflexa 

 and serotina together 

 envelop the chorion, 

 the processes of which 

 tit into crypts in them. 

 At this period both of 

 them are highly and 

 nearly uniformly vas- 

 cular. The general 

 cavity of the uterus is 

 to a large extent ob- 

 literated by the ovum, 

 but still pei'sists as a 

 space tilled with mu- 

 cus, between the decidua reflexa and the deciJua vera. 



The changes which ensue from this period onwards are fully known. 

 The amnion continues to dilate (its cavity being tensely tilled with amniotic 

 fluid) till it comes very close to the chorion (tig. 151, am); from which, 

 however, it remains separated by a layer of gelatinous tissue. The villi of 

 the chorion in the region covered by the decidua reflexa, gradually cease 

 to be vascular, and partially atrophy, but in the region in contact with the 

 decidua serotina increase and become more vascular and more arborescent 

 (tig. 151, z). The former region becomes known as the chorion Iseve, 

 and the latter as the chorion frondosum. The chorion /rondosum, 

 toyetlcer with the decidua serotina, gives rise to the 'placenta. 



Fig, 151. Diagrammatic section of pregnant human 

 UTERUS WITH CONTAINED FCETUs. (From Huxlcj after 

 Longet.) 



al. allantoic stalk; nh. umbilical vesicle; am. amnion; 

 ch. chorion ; ds. decidua serotina ; dw. decidua vera ; dr. 

 decidua reflexa ; I. Fallopian tube; c. cervix uteri; u. uterus; 

 z. foetal villi of true placenta ; z'. villi of non-placental 

 part of chorion. 



