54 formation of the layers. 



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 

 liypoblast. 



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. 



Shortly after the floor of cells has appeared, the whole seg- 

 mentation cavity becomes obliterated. 



When the embryonic rim has attained to some importance, 

 the position of the embryo becomes marked out by the appear- ^ 

 ance of the medullary groove at its most projecting part. The 

 embryo extends from the edge of the blastoderm inwards to- 

 Avards the centre. 



At about the time of the formation of the medullary groove, 

 the mesoblast becomes definitely constituted. It arises as two 

 independent plates, one on each side of the medullary groove, 

 and is entirely derived from lower layer cells. 



