BULLFROG AS TYPICAL VERTEBRATE ANIMAL 



97 



1% to 2 inches. It usually takes about three to four years for this 

 young frog to attain maturity and begin egg laying. 



The embryology of the bullfrog does not differ materially from 

 that of the leopard frog, and the following account is based, except 

 where otherwise noted, upon the development of the latter. 



The egg when laid is a single cell. The upper portion of the 

 egg has considerable pigmentation, making it black. This part of 

 the egg is known as the animal hemisphere, and it is thought that 



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 (Yolk P/u^) 



Thi'^'f; hni^^itni^^'^^.^^ ^f^P cleavage stages, blastula and gastrula of the frog. 

 ;;f ili « * «^^^^i?' unequal type of cleavage. The upper, shaded portion of earl 

 ot ^^o^^t.^l%^^f^^^^:^^ y^r^^^"^^^ the animal /tem^/ie.'erand the low^r porttSn 

 toe yolk pmg ^^ ^^ hemisphere. The circular plug seen in the gastrula stage i? 



the pigmentation serves to absorb and retain heat necessary for 

 development. The lower portion is white and is known as the 

 vegetal hemisphere. The bullfrog egg is surrounded by a layer of 

 transparent jelly, but does not have an inner envelope of jelly, as 

 does the leopard frog egg. This jelly protects the egg and helps 

 It to retain heat. The nucleus of the egg, or germinal vesicle, lies 

 near the animal pole. The boundary of the egg is known as the 

 vitelline membrane. 



