THE DEVELOPMENT OF THE FROG 



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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, feo that the egg 

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



Fig, i8. — Vertical section through the blastula of a frog in different stages. 

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



A cavity makes its appearance near the center of the egg 

 and gradually increases in size as cleavage proceeds. This 

 is the b lastoccel or seg mentation ca vity, and the egg at this 

 time is called the b lastul a. 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 gastrula. 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 



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