THE HUMAN CHORION. 



41 





Fig. 48. 



.>7 ^.rii -■■'•^«, 



Extraplacental area 

 {Chorion lizve) 



// 





marked B. Z. , which are merely portions of the uterine mucosa lying against the 

 trophoblast, the tissue is oedematous in character. This tissue is described by Peters 

 as the bordering zone. In other portions of the mucous membrane there are seen 

 parts of some of the uterine glands (,^). In the region marked Cap., is seen the 

 beginning of the decidua capsularis, growing in over the area through which the 

 embryonic vesicle broke into the surface of the uterus. This layer, decidua capsu- 

 laris, is at this stage scarcely developed, only the beginning of it is apparent. 



This embryo, described by Peters, is one of the youngest which has been accu- 

 rately studied. The inner dimensions of the vesicle, as given by Peters, are as follows: 

 1.6 by 0.8 by 0.9 mm. The youngest human embryo is that described by Bryce and 

 Teacher, and is probably several days earlier than the one recorded by Peters. In a gen- 

 eral way, it presents the relations of the amniotic and vitelline sacs already described. 



The Human Chorion. — The vascular chorionic vilH, although becoming 

 more complex by the addition of secondary branches, are for a time equally well 

 developed over the external surface of the entire embryonic vesicle ; subsequently, 

 from the end of the second month, a noticeable differentiation takes place, the villi 

 included within the field that later corre- 

 sponds to the placental area undergoing 

 unusual growth and far outstripping those 

 covering the remaining parts of the chorion. 

 This inequality in the development of the 

 villi led to the recognition of the chorion 

 fro7idosum and the chorio7i lave, as the 

 placental and non-placental portions of the 

 chorion respectively are termed (Fig. 48). 

 The vascular supply of the villi also shares 

 in this differentiation, the vessels to those 

 of the placental area becoming progres- 

 sively more numerous, while, on the con- 

 trary, those distributed to the remaining 

 villi gradually atrophy as the chorion 

 comes into intimate apposition with the 

 uterine tissue. When well developed, the 

 chorionic villi possess a distinctive appear- 

 ance, the terminal twigs of the richly 

 branched projections being clubbed and 

 slightly flattened in form. Their recogni- 

 tion in discharges from the vagina often 

 afTords valuable information as to the ex- 

 istence of pregnancy. 



The Amniotic Fluid. — The amnion 

 at first lies closely applied to the embryo, 



but soon becomes separated by the space which rapidly widens to accommodate the 

 increasing volume of the contained liquor amnii. The accumulation of fluid within 

 the amniotic sac, which in man takes place with greater rapidity than in other mam- 

 mals, results in the obliteration of the cleft between the chorion and amnion until 

 the latter envelope lies tightly pressed against the inner surface of the chorion. 

 The union between the two envelopes, however, is never very intimate, as even after 

 ihe expulsion of the membranes at birth the attenuated amnion may be stripped off 

 from the chorion, although the latter is then inseparably fused with the remaining 

 portions cf the deciduae. 



The amniotic fluid, slightly alkaline in reaction, is composed almost entirely 

 of water ; of the one per cent, of solids found, albumin, urea, and grape-sugar are 

 constituents. The quantity of liquor amnii is greatest during the sixth month of 

 gestation, at which time it often reaches two litres. With the rapid increase in the 

 general bulk of the foetus during the later months of pregnancy, the available space 

 for the amniotic fluid lessens, resulting in a necessary and marked decrease in the 

 quantity of the liquid ; at birth, less than one litre of amniotic fluid is usually 

 present. Sometimes, however, the amount of the liquor amnii may reach ten 



Placental area 

 ( Chorion frojido sum) 



External surface of part of the human chorion of 

 the third month ; the lower portion is covered with the 

 highly developed villi of the placental area. 



