V THE DEVELOPMENT OF THE FROG 95 



thins out toward the edges, which gradually extend around 

 the lower pole of the egg. In this way the crescent becomes 

 converted into a circle, and the circle gradually becomes 

 smaller and smaller until only a small part of the light-colored 

 yolk, known as the yolk plug, appears in the midst of the dark 

 area. The white pole is thus overgrown by the dark, but 

 not with equal rapidity from all sides, the closing-in taking 

 place much more rapidly on the side where the crescentic 

 fold originally appeared, and which subsequent events prove 

 to be the anterior end of the embryo. 



If we make a vertical section through the embryo at right 

 angles to the crescentic blastopore, we shall find the latter 

 is the mouth of a cavity which ex tends soiji g distance into 

 the egg. Above this cavity, which is called the archenieron, 

 is a comparatively thin roof, closely applied to the upper 

 wall of the embryo, and at the floor of the cavity is a large 

 mass of yolk cells. The archenteron represents the cavity 

 produced by the process of gastrulatipn. It is due, in great 

 measure at least, to the overgrowth of the dorsal lips of the 

 blastopore, the cells forming the floor being formerly at the 

 surface" of the egg. According to Marshall, the cavity 

 arises in great part through the splitting apart of the yolk 

 cells, but while this may be a factor in the case, it certainly 

 cannot be the predominant one. (See Robinson and Asshe- 

 ton '91,^ Assheton '94,- Morgan '97.^) As the archenteron 

 increases in size, the blastoccel or segmentation cavity neces- 

 sarily becomes smaller. According to Marshall the former 

 breaks through into the latter, and the two form one cavity. 



The Germ Layers. — The formation of the gastrula pro- 

 duces a two-layered embryo, each layer being several cells 



1 Robinson and Assheton, Quart. Jour. Mic. ScL, Vol. 32, 1891. 



2 Assheton, Ibid., Vol. 37, 1894, 



8 Morgan, " The Development of the Frog's Egg," 1897. 



