34 PROGRESS OF MICROSCOPICAL SCIENCE. 
much more granular appearance in the sections. Both epiblast and 
hypoblast throughout almost the whole extent are composed of a 
simple layer of flattened cells joined edge to edge. Most of the 
epiblastic cells exhibit in a high degree a condition of the nucleus 
which is frequently met with in epithelial cells elsewhere—a ten- 
dency, namely, to become separated into a clear colourless part, and 
a highly refracting and usually somewhat irregular body, which 
readily becomes stained by the usual colouring reagents. This change 
is no doubt a post-mortem effect, probably produced by the action of 
the reagents employed. The hypoblastic cells do not for the most 
part present this appearance; their nucleus remains large, round, and 
clear; and the cell-substance does not become stained as much as that 
of the epiblastic cells. 
“Tt was mentioned above that the epiblastic and hypoblastie 
vesicles come into closer proximity at one part of the circumference 
than elsewhere; even here, however, they do not come into actual 
contact. At this place they are no longer formed, as elsewhere, of a 
single layer of cells; but their component elements, besides being 
rounder in shape and smaller, are two or three deep, although not 
arranged in as many definite strata. Both layers are in consequence 
somewhat thickened just here, the thickening (which is mies marked 
in the epiblastic vesicle) extending over an area of about 1, inch in 
diameter ; not sharply defined, however, but gradually shading off 
into the thin part. Both epiblast and hypoblast are, it may be 
repeated, perfectly well defined and distinct from one another here as 
elsewhere ; and there are no cells to be seen which do not clearly 
belong to one or the other. Moreover, they are not only separated 
by a small but obvious amount of the granular material (coagulated 
fluid) previously mentioned, but there is in addition an exquisitely 
fine pellicle, which in the sections appears as a mere line passing over 
and forming a definite boundary to the upper (outer) surface of the 
hypoblast at the thickened area. This membranous pellicle, for 
which I would venture to propose the name of membrana limitans 
hypoblastica, is, as made out in teased preparations, perfectly homo- 
geneous, and continues so throughout its extent. It becomes stained 
slightly by carmine, but apparently not at all by logwood, and is 
probably to be looked upon as a cuticular formation produced by the 
hypoblastic cells. Whether the delicate pellicle may extend around 
the whole hypoblast in the natural condition, I am unable to say; in 
the sections, at any rate, it appears to terminate towards the periphery 
of the thickened area, and to have become curled somewhat away from 
the hypoblast. 
“T am not aware that a similar structure has yet been noticed in 
the early blastoderm of any animal; but its importance in this case 
in bounding the hypoblast is evident. Indeed, if it should be found 
that the membrane in question is of general occurrence in the mam- 
malian germ, and that the first appearance of the mesoblastic cells 
occurs external to it, as from its proximity to the hypoblast there is 
little reason to doubt, the fact of the existence of such a film between 
the commencing mesoblast and hypoblast is strongly in favour of the 
