V THE DEVELOPMENT OF THE FROG 93 



segmentation the cells are much larger at the vegetative pole 

 and gradually become smaller toward the opposite side of 

 the egg. In the first few cleavages the planes of division lie 

 at right angles to the surface of the egg, but subsequently 

 planes of division occur parallel to the surface, so that the egg 

 comes to consist of more than one layer of cells in thickness. 



Fig. 18. — Vertical section through the blastula of a frog in different stages. 

 B, segmentation cavity or blastocoel. (After Marshall.) 



A cavity makes its appearance near the center of the egg 

 and gradually increases in size as cleavage proceeds. This 

 is the blastoccel or segmentation cavity, and the egg at this 

 time is called the blastula. It is essentially a hollow sphere 

 of which the wall on the vegetative side is very much thicker 

 than it is above and composed of large yolk-laden cells. 



Gastrulation. — In the embryonic development of most of 

 the many-celled animals a stage is passed through which is 

 known as the gastriila. In its typical form a gastrula is a sort 

 of double-walled sac such as may be produced, according to 

 a well-worn illustration, by pushing in one side of a hollow 

 rubber ball with the finger. The mouth of the gastrula is 

 called the blastopore, and this opening naturally becomes 



