488 A. A. W. HUBREOHT. 



considerable proliferation of maternal epithelium, which, how- 

 ever, does not enter into the constitution of the ripe placenta, 

 but affords facilities of fixation and nutrition for the embryonic 

 neo-forraation in its earliest stages. 



The discoid placenta is, in the later stages of pregnancy, the 

 only connection between foetus and mother. The zonary con- 

 nection in the omphaloidean region is only temporary. Below 

 this another riug-shaped modification of trophoblast appears 

 at an early stage, and persists till the end as a thickened circular 

 membrane, forming an annular constriction round the allan- 

 toidean vessels that connect the foetus with the placenta. 

 There is reason to suppose that both this subdivision of the 

 trophoblast and that which is applied against the mesometrical 

 surface of the uterus-lumen play a part in the transport of 

 nutritive material to the inner cavity of the yolk-sac, where it 

 is being absorbed by specially modified hypoblast-cells. 



C. Details of the Process of Placentation. 

 1. The Changes in the Uterine Tissues. 



The various and successive changes by which the uterine 

 tissues are affected during pregnancy visibly influence the 

 outward aspect of the uterus. The position of the embryo and 

 placenta is not only marked by a swelling which increases in 

 size as pregnancy advances, but this swelling undergoes certain 

 unexpected changes of shape which we shall have to notice in 

 some detail. Several figures of the uterus in early stages of 

 pregnancy were already given in my paper on the development 

 of the germinal layers of Sorex (this Journal, vol. xxxi, 1890) ; 

 of these I will here refer to figs. 13, 14, 15, 43, 65, and 83. 



The swellings are knob-like projections separated by the more 

 tubular portions (fig. 47). With advancing pregnancy the 

 rounded knob becomes more ovoid, the tubular connecting 

 portions being, if possible, yet narrower (fig. 39). Of the 

 figures here alluded to, the following correspond to the figures 

 of transverse sections given in this paper : 



