138 



The lower surface of the proximal wall of the yolk-sac is 

 covered by a cubical epithelium which is not thrown into folds; 

 the cells contain fat granules. 



The ragged edge of the capsularis may be seen even in the 

 latest stages still attached to the edge of the allantoidean tro- 

 phoblast, that is to say of the placenta (Fig. 11). In this region 

 where the distal wall turns over to pass into the proximal, the 

 upper portion, or sinus, as Duval has termed it, of the cavity of 

 the yolk-sac undergoes further changes; the outer or distal rim 

 of the sinus is carried up round the edge of the placenta, while 

 the inner is carried towards the umbilical cord where it under- 

 goes a curious invagination into the substance of the latter, which 

 will be better understood when the development of the placenta 

 has been described. To this description we are now at liberty 

 to proceed. 



Formation of the Placenta proper. 



a.) The development of the allantoidean trophoblast and the 

 changes that take place in the adjacent subepithelial tissues during 

 the first period, that is to say from the time of the fixation of 

 the embryo up till the attachment of the allantois. 



The development of the allantoidean trophoblast commences as 

 a proliferation of the trophoblastic cells at the embryonic pole of 

 the blastocyst, at this time already situated in the depression on 

 the anti-mesometric side of the uterine lumen; in this pit the 

 embryo appears to be subject to a considerable lateral compress- 

 ion. The trophoblastic cells, which contain fat granules, divide 

 rapidly. The nuclear spindles being parallel to the long axis of 

 the blastocyst and the cells themselves elongated in the same 

 direction ; and as a result the embroyonic knob is pushed into 

 the interior of the blastocyst in such a manner that it simulta- 

 neously invaginates the proximal wall of the yolk-sac (Fig. 13). 



A continuatiou of the process leads to the formation of a mass 

 of tissue (Selenka's 'Trager') which below, in the interior of the 

 blastocyst, is coutinuous with the embryonic knob; in the latter 



