476 HENEY F. OSBORN. 



brane at any point. Still, no opinion could be formed as 

 to its subsequent relations, for its development is evidently 

 very rapid, and the embryos were in an early stage of 

 growth. 



The greatest interest naturally was directed to the villous area 

 of the subzonal membrane. This could be separated with ease 

 from the subjacent portion of the yolk-sac, revealing the rich 

 capillary network of the latter. At this point a careful draw- 

 ing of the foetus was made (Plate XXXIII, fig. 1), magnified 

 about five diameters, and representing the proportions as nearly 

 as could be done by the eye. The membrane was composed of 

 a single layer of polygonal epithelial cells. When seen from 

 above the villi appeared as rings of thickened epithelium of all 

 sizes in profile (fig. 3) ; they were seen to be composed of a 

 single layer of columnar cells, some of which were produced 

 into minute processes. The villi varied considerably in height; 

 they were hollow, and beneath them was a layer of flattened 

 epithelium ; whether the latter was derived from the subzonal 

 membrane or had been torn off from the yolk-sac could not be 

 ascertained. A portion of the villous area near the sinus ter- 

 minalis, containing one of the vitelline arteries and a section 

 of the vena terminalis (Plate XXXIII, fig. 2), shows thatattliis 

 period there was no especial enlargement of the capillary 

 vessels near the villi ; in fact, none of the latter showed any 

 trace of vascularity. Two facts, however, indicated that this 

 would appear in a subsequent stage: — 1. The villi were appa- 

 rently beginning a similar line of development to that which 

 they pursue over the attached allantoic area in the Placentalia. 

 2. The villous area in each foetus was in close contact 

 with the uterine furrow, whereas the remainder of the sub- 

 zonal membrane floated free in the uterine cavity. The word 

 '' attachment" would be incorrect in this connection, but tlie 

 union with the uterine wall was sufficiently close to prevent 

 separation, even when there was considerable motion in the 

 water in which the uterus was placed. 



By an unfortunate blunder in the laboratory one horn of the 

 uterus containing the embyos in situ Avas thrown away, so 



