52 



ATTACHMENT OF THE OVUM TO THE DECIDUA. 



arborescent tuft of villi continuous with the foetal chorion, and traversed by blood- 

 vessels which are supplied from the branches of the umbilical arteries, These 

 blood-vessels form a capillary loop in each villas, and these capillary loops are 

 separated from the maternal blood in the placental sinuses not only by the capillary 

 walls and the connective tissue of the villas, but also by a double layer of flattened 

 epithelium-like cells derived either from the chorionic epithelium (Minot), 1 or from 

 the decidual tissue, and, perhaps, in part representing an endothelial membrane 

 belonging to the placental sinuses, which, according to Waldeyer, are lined by endo- 

 thelium prolonged from that of the uterine vessels. 



Some of the chorionic villi are attached (1) by comparatively stout bands of 

 fibrous tissue to the basal stratum of the decidua, (2) by finer bands of similar 

 substance both to one another and to the 

 septal prolongations of the decidua ; others 

 hang freely into the placental sinuses. 

 These sinuses are supplied directly with 

 arterial blood from tortuous branches of 

 the uterine arteries which pass through the 



a.?nni.cn. 



chcricTL 



Fig. 55. DIAGRAM SHOWING THE TISSUES COVERING 



THE VILLI IN THE HUMAN PLACENTA, AND 

 THEIR RELATION TO THE DECIDUA ACCORDING 



TO TURNER. 



F, foetal tissue ; M, maternal tissue ; d, blood- 

 vessels in villus ; d', blood in placental sinus ; 

 e', layer of cells covering villus : x. basement mem- 

 brane covering villus (? endothelium) continued from 

 uterine vessels ; ds, decidua serotina ; t, tissue 

 uniting villus to decidua ; ca, up, uterine vessels 

 opening into sinus. 



a, 



Fig. .". DIAGRAM OP THE PLACENTA (E. A. S. ). 



am, amnion ; ck, chorion ; s, placental sinus ; 

 ds, decidua serotina ; sp, spongy layer ; m, 

 muscularis ; a, v, uterine artery and vein open- 

 ing into placental sinus. 



spongy stratum of the decidua serotina and through the basal stratum of the 

 placenta to open into the sinuses without the intervention of capillaries. From 

 the sinuses veins, which run very obliquely through the decidua, carry off the blood, 

 and eventually pass into the veins of the muscular wall. The foetal villi are thus 

 bathed by slowly flowing maternal arterial blood, and respiratory and nutritive 

 exchanges may occur between the two kinds of blood, but there is no actual mixing 



1 Minot describes botli the villi and the sinuses of the placenta as bounded by the chorionic epithelium. 

 This would agree very well with recent observations in the bat, hedgehog, and other mammals, which 

 show that these spaces are originally developed in hypertrophied epiblast (see next page). 



