388 JAS. P. HILL. 



of the syncytium, and are firmly adherent thereto. In corre- 

 lation with this close adherence of the chorionic ectoderm, this 

 area of the uterine syncytium is markedly thicker than the 

 remainder, and forms the allantoic placental area. In the 

 allanto-chorionic mesenchyme, and in close relation to the 

 inner surface of the chorionic ectoderm, run the allantoic 

 capillaries. (2) Outside the discoidal allanto-chorionic area 

 a somewhat annular zone of the yolk-sac wall is also brought 

 into intimate relation with the maternal syncytium around the 

 above-mentioned allantoic placental area, by means of a close 

 approximation of its exceedingly thin ectodermal cells. This 

 annular zone of yolk-sac wall corresponds to the embryonic 

 vascular area, and at this stage the portion of the syncytium 

 in relation to it is more highly vascular than the allantoic 

 placental syncytium itself. This structural arrangement can 

 hardly be considered other than a yolk-sac placental forma- 

 tion, functional at a time when the allantoic placenta is yet 

 only being formed. 



(b) Formation of the Functional Allantoic Placenta. 

 — This is brought to pass through the gradual degeneration 

 and resorption of the enlarged chorionic ectoderm cells over 

 the placental area proper. These cells thus take no further 

 part in placental formation. The allantoic capillaries can now 

 directly reach the vascular surface of the allantoic placental 

 syncytium, to which they become intimately attached, dipping 

 down into the depressions in its surface, and forming in places 

 a regular interlocking system. The foetal and maternal blood- 

 streams are now only separated by their thin endothelial walls, 

 and perhaps a thin layer of syncytial protoplasm. 



III. Parturition. 



At birth not only is there no loss of maternal tissue (i.e. no 

 decidua is formed), but the vesicular portion of the allantois 

 remains persistently attached to the placental syncytium, and 

 is gradually absorbed in situ along with the latter through 

 the agency of maternal leucocytes. 



The foetus, whilst still connected with the placental area by 



