THE DEVELOPMENT OF VERTBBRATA. 



229 



Amphioxus, but is far slower, owing to the fact that the 

 great amount of yolk present impedes the activity of the 

 protoplasm. For the same reason differences soon show 

 themselves between the behaviour of the animal and vege- 

 tative hemispheres, the latter of which contains so much 

 more yolk than the former. The first two divisions occur 

 along meridional planes, as in Amphioxus, and the third in 

 a plane at right angles to both these ; but instead of being 

 equatorial, it is decidedly nearer the animal pole (fig. 117, 

 A,B). From this point onwards two kinds of cells can be dis- 

 tinguished smaller black cells in the animal hemisphere, 

 and larger yolk-laden cells forming the vegetative hemi- 

 sphere. These correspond approximately to the epiblast 



ABO 



Fig. 117. SEGMENTATION OF OVUM OF FROO. 



(After Howes and Marshall.) A, ei^ht-celled stage, side view ; B, the 

 same in vertical section ; C, blastosphere, vertical section. 



and primitive hypoblast cells of Amphioxus, respectively : 

 we shall call them the primitive epiblast and the yolk-cells, 

 respectively. At the blastosphere stage (fig. 117, c) we find 

 that, owing to the larger size of the yolk-cells, the seg- 

 mentation cavity lies nearer the animal pole. 



3. The Gastrula of the Frog. The next stage early 

 gastrula in Amphioxus (fig. 1 13, B) showed us the primitive- 

 hypoblast cells partially invaginated. The yolk-cells of the 

 frog are too bulky for this to take place, and so, as they 

 continue to divide, they become a somewhat irregular mass 

 (fig. 118, A). This is much the same as what, we should get in 

 Amphioxus if the primitive hypoblast cells were to greatly 

 enlarge in size and fill up the archenteron cavity. And we 



