FORMATION OF THE GERMINAL LAYERS 113 



yolk this groove becomes horseshoe-shaped, and a little later 

 circular. 



The egg has now the appearance shown in Fig. 24, the 

 epiblast covering the whole surface except a circular patch, 

 where alone the yolk-cells are visible from the surface. This 

 circular aperture in the epiblast is called the blastopore ; it is 

 situated at what will become the posterior end of the embryo ; 

 and it is bordered by a distinct rim or lip, round which the 

 epiblast turns inwards into the interior of the egg. The cir- 

 cular plug of yolk-cells filling up the blastopore is spoken of as 

 the yolk plug. 



The structure of the egg, or rather of the embryo at a 

 slightly later stage, is shown in Fig. 25, which represents a 

 vertical section passing through the middle of the blastopore. 



The epiblast covers the whole surface except at the blastopore. 

 From the lip of the blastopore a layer of cells appears to grow 

 into the egg concentrically with its surface. 



This layer is called the hypoblast : it appears to be formed 

 partly by an ingrowth of cells round the whole margin of the 

 blastopore, partly by differentiation of yolk-cells ; it extends 

 much more rapidly dorsally than ventrally, so that while on the 

 upper or dorsal surface it extends to the anterior end of the 

 embryo, at the sides and below it only extends a very short 

 way as yet. From the mode of its formation it is necessarily 

 continuous with the epiblast all round the lip of the blasto- 

 pore. 



Between this ingrowing layer of hypoblast and the yolk-cells 

 there is a space. This is a very narrow chink near the blasto- 

 pore, but further forwards it dilates on the dorsal surface 

 to form a cavity of some size (Fig. 25, MN), wider from side to 

 side than it is dorso-ventrally. (Cf. Fig. 26.) 



This cavity, which is named the mesenteron, is the future 

 alimentary canal : it communicates with the exterior through 

 the blastopore, though the apertiu-e is reduced to a narrow 

 chink and is almost stopped up by the yolk plug. The per- 

 manent mouth and anus are not yet formed. 



During the process of formation of the mesenteron the 

 segmentation cavity gets pushed out of place. It has been 

 asserted to ultimately open into and form part of the mesen- 

 teron. 



The Notochord. Along the roof of the mesenteron in the 



