FORMATION OF THE GERMINAL LAYERS 115 



lateral, and ventral lips of the blastopore appear successively 

 during its downward movement. From this point its progress 

 is again shifted. The egg rotates about its horizontal axis 

 through about 100 owing to the development within it of 

 an eccentric cavity which causes it to topple over. This 

 brings the original vertical axis of the egg downwards and 

 the blastopore up to the equator again. 



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 

 circular 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 blasto- 

 pore. 



The epiblast covers the whole 'Surface except at the blasto- 

 pore. 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 differentiation of the subequatorial intermediate 

 cells during overgrowth of the lip 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. 



Between this ingrowing layer of hypoblast and the yolk- 

 cells there is a space. This is a very narrow chink near the 

 blastopore, but farther 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 ex- 

 terior through the blastopore, though the aperture is 



