149 



as central, the under surface of the placenta becomes naturally 

 concave; and this results in the next period in a curious trans- 

 formation of the inner angle of the yolk-sac sinus. This is the 

 point at which the low epithelium of the distal wall passes into 

 the columnar cells of the proximal; and here portions of this 

 inner angle are, so to speak, gathered up into the rniddle of the 

 placenta, and appear in the sections as isolated portions of the 

 yolk-sac lined on one side by a low, on the other by a columnar 

 epithelium (Fig. 11). 



With respect to the maternal circulation (Fig. 11) it has been 

 demonstrated by injection that the arteries pass through the centre 

 of the subepithelial tissue of the sinuses in the glycogenic tro- 

 phoblast, and thence by wide trophoblastic channels to the foetal 

 side of the placenta; here the blood passes in all directions from 

 them into the lacunae in which it gradually ascends to the outer 

 and upper region of the trophoblast. 



By the same method it raay be equally welf shown that the 

 large veuous sinuses, which in earlier stages radiated from the 

 trophoblastic lacunae outwards, upwards, and then inwards to the 

 mesometrium but which now, by the great increase in diameter 

 of the placenta, pass merely upwards and inwards, receive their 

 blood from a broad annular tract of trophoblastic lacunae sur- 

 rounding a central area traversed by the arteries. It will be 

 remembered that it is in the peripheral region that megalokaryo- 

 cytes are formeel '). 



c.) The third and last period of gestation is marked in the allan- 

 toic portion of the placenta merely by a further multiplication 

 and elongation of the allantoic villi; on the maternal side of the 

 placenta on the contrary, the changes that take place are more 

 complicated, and their interpretation is attended with much greater 

 difficulty. The conclusion to which 1 have come, and which I will 

 here, for the sake of clearness, state somewhat dogmatically, is 

 that the whole of the subepithelial glycogenic tissue of the mother 



1) A similar arrangement of maternal arteries and veins is found in other Rodents, 

 and in Carnivora; see Part II. 



