16 ARTHUR DENDY. 



cavity, doubtless belong also to tbe lower layer, as already 

 observed, but they take no part in the formation of the embryo, 

 and may in future be disregarded.) 



In the embryo itself the epiblast and a lower layer can still 

 be distinguished, but with a totally different histological struc- 

 ture. Immediately behind the blastopore both layers merge 

 into a thick, dense mass of cells of very characteristic appear- 

 ance. Each cell contains a very large but faintly staining 

 oval nucleus, containing a few small, darkly staining granules. 

 Under a low power the nuclei look as if they were the actual 

 cells. This mass of cells doubtless represents the primitive 

 streak (figs. 3, 4, 5, Pr. S.). The blastopore {B. P.) appears 

 as a pitting in of its upper surface, as yet extending down- 

 wards and forwards only for a short distance, but destined later 

 on to form a very distinct neurenteric passage opening below. 

 The cells immediately surrounding the blastopore show a slight 

 trace of radial arrangement. 



In front of the blastopore the general cellular mass of the 

 primitive streak passes into two perfectly distinct layers — an 

 outer epiblast composed of several tiers of radially elongated 

 cells, and an inner layer of cells not diiFering in any way from 

 those of the primitive streak itself. This inner layer, which 

 I propose to consider as at any rate chiefly mesoblastic, appears 

 to be simply a forward prolongation of the primitive streak 

 beneath the epiblast. 



The epiblast of the embryo itself differs much in appearance 

 from that of the surrounding blastoderm ; the outlines of the 

 cells are much less distinct, while their nuclei stain with much 

 less intensity, and are coarsely granular instead of compara- 

 tively homogeneous ; moreover they are arranged in several 

 layers. Just in front of the blastopore the epiblast, though 

 thick, is not nearly so thick as the layer beneath it; but as we 

 trace it forwards its thickness greatly increases, attaining its 

 maximum about the anterior margin of the head-fold, and then 

 somewhat suddenly diminishing again as it turns downwards 

 and backwards to join the epiblast of the area pellucid;i. In 

 the region of the head-fold the epiblast forms a darkly stain- 



