FORMATION OF THE LAYERS. 267 



in the yolk are frequently larger than those in the blastoderm. 

 (2) That there are present a very large number of nuclei in the 

 yolk which precisely resemble the nuclei of the cells under 

 discussion. (3) That in some cases (PL 7, fig. 13) cells are seen 

 indistinctly circumscribed as if in the act of being formed. 



Between the blastoderm and the yolk may frequently be 

 seen a membrane-like structure, which becomes stained with 

 hsematoxylin, osmic acid etc. It appears to be a layer of 

 coagulated albumen and not a distinct membrane. 



SUMMARY. 



At the close of segmentation, the blastoderm forms a some- 

 what lens-shaped disc, thicker at one end than at the other ; the 

 thicker end being termed the embryonic end. 



It is divided into two layers an upper one, the epiblast, 

 formed by a single row of columnar cells ; and a lower one, con- 

 sisting of the remaining cells of the blastoderm. 



A cavity next appears in the lower layer cells, near the 'non- 

 embryonic end of the blastoderm, but the cells soon disappear 

 from the floor of this cavity which then comes to be constituted 

 by yolk alone. 



The epiblast in the next stage is reflected for a small arc at 

 the embryonic end of the blastoderm, and becomes continuous 

 with the lower layer cells ; at the same time some of the lower 

 layer cells of the embryonic end of the blastoderm assume a 

 columnar form, and constitute the commencing hypoblast. The 

 portion of the blastoderm, where epiblast and hypoblast are 

 continuous, forms a projecting structure which I have called the 

 embryonic rim. This rim increases rapidly by growing inwards 

 more and more towards the centre of the blastoderm, through 

 the continuous conversion of lower layer cells into columnar 

 hypoblast. 



While the embryonic rim is being formed, the segmentation 

 cavity undergoes important changes. In the first place, it receives 

 a floor of lower layer cells, partly from an ingrowth from the 

 two sides, and partly from the formation of cells around the 

 nuclei of the yolk. 



1 8 2 



