10 JAS. P. HILL. 



D. It may be meutioned that small numbers of maternal 

 leucocytes occur in the allantoic cavity. 



A fairly typical section of the allantoic placental area is 

 shown in PL 2, fig. 7, from which it will be seen that, except 

 perhaps for an increase in size of both foetal and maternal 

 capillaries, the description already given of the allantoic 

 placenta in Stage D holds good certainly up to this stage of 

 gestation, and without doubt also for the whole period of 

 intra-uterine life, a conclusion already arrived at from the 

 conditions presented by the post-partum uterus described in 

 my previous paper. 



(c) Vascular Omphalopleure and Yolk-sac Placenta. 

 — The entodernial cells of the vascular omphalopleure, some of 

 which were found in Stage D to be undergoing enlargement, 

 are now found to have all increased in size, and to form a 

 uniformly much thicker layer than in that stage. The cells 

 are mostly somewhat cubical in form, with rounded projecting 

 ends (PI. 2, fig. 8, ent.). Their protoplasm and nuclei stain 

 deeply. Over the sinus terminalis (fig. 8, s. t.) the entoderm 

 cells are also enlarged and, as in previous stages, club-shaped 

 in form, with the nucleus situated in the outer projecting 

 part of the cell. 



It seems highly probable that this hypertrophy of the 

 entodermal cells of the vascular omphalopleure, commencing 

 as it does about the time the allantoic placenta begins its 

 functional activity, is to be associated with the functional 

 retrogression of the yolk-sac placenta, consequent upon the 

 establishment of the allantoic one. For, whether the ecto- 

 derm of the vascular omphalopleure be adherent to the 

 surface of the syncytium or not, the condition of the yolk-sac 

 vessels shows us that the yolk-sac placenta, if we can any 

 longer speak of such, is now functionally the mere shadow of 

 its former self. The ectoderm of the vascular omphalopleure, 

 though still a comparatively thin layer, is slightly thicker 

 than in preceding stages, the cell bodies being richer in 

 protoplasm (PI. 2, fig. 8.). 



{d) Bilaminar Omphalopleure. — The ectoderm of the 



