148 



cular layer stain inuch more intensely with acid stains; nor are 

 the stretched cells elastic fibres, as raay be demonstrated by the 

 use of orcein. Their surfaces are inclented as though distorted by 

 the pressure of wavy bundies of fibres; and they lie in a matrix 

 which stains faintly with orcein and deep blue with nigrosin; in 

 fact we have here to do with rnerely a inodified fibrous tissue 

 (Fig. 26). With the accomplishment of the histological changes 

 which have now been detailed the placenta, as a structure in 

 which the raaternal and foetal circulations are brought into in- 

 timate relation, may be said to be properly established; and the 

 processes that subsequently take place duriug the later phases of 

 pregnancy are merely an elaboration and perfection of an organ 

 which is already in a state of functioual activity. 



Before however we proceed to the consideration of these pro- 

 cesses we must briefly refer to the grosser anatomical changes 

 which have by now been brought about in the topographical 

 relations of the embryonic and maternal tissues, and to the ar- 

 rangement of the maternal arteries and veins. 



Whereas formerly tbe inner surface of the whole trophoblast 

 fornied together a continuous spherical investment for the embryo 

 with its yolk-sac, and extra-embryonic coelom, the allautoidean 

 portion bas now so completely outstripped the omphaloidean, as 

 to project inwards into the centre of the uterus. Since, further, 

 there is a continuous increase not merely in the depth but also 

 in the diameter of the placenta as well, it comes about that the 

 upper portion or sinus of the yolk-sac is invaginated by the 

 margin of the placenta, up the outer side of which the annular 

 line of attachment of the capsularis appears to be carried (Fig. 11) 



It is perfectly clear that when once the allantoic villi have 

 firmly rooted themselves in the trophoblast, their subsequent 

 elongation, accompanied by the stretching and flattening of the 

 trophoblast which covers thern, is centripetal; that is to say the 

 inner surface of the layer of trophoblastic glycogen cells forms a 

 relatively fixed point starting from which the villi grow inwards. 

 By the continued elongation of all the villi, peripheral as well 



