1876.] in an early condition of Development. 401 



of an intermediate layer of cells or mesoblast. The epiblast closely lines 

 the zona throughout ; but the hypoblastic vesicle is considerably smaller, 

 so that except at one part, where it comes into closer proximity than else- 

 where with the epiblast, it is separated from the latter by a considerable 

 interval, filled in the fresh condition by a clear fluid. ■ This fluid would 

 seem to be of a different nature from that which occupied the cavity of 

 the hypoblastic vesicle, for the coagulum produced in it by the action of 

 the hardening liquid has a 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. Those 

 of the former are represented in fig. 2, as seen flat in a separated portion, 

 those of the latter in fig. 3. 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 tendency, namely, to become separated into a 

 clear colourless part, and a highly refracting and usually somewhat irre- 

 gular 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. 



It was mentioned above that the epiblastic and hypoblastic 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 siugle layer 

 of cells, but their component elements, besides being rounder iu 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 most marked in the epiblastic vesicle) 

 extending over an area of about -g 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 (coagu- 

 lated 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 Tiypoblastica, is, 

 as made out in teased preparations, perfectly homogeneous, 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 



