i02 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



rated from the surface of the ovum, forming a perivitelline space. 



The egg is therefore encased 

 by the jelly, but free to 

 rotate within a central cav- 

 ity ; and since the vegetative 

 pole is the heavier, the 

 developing egg normally 

 remains with the darker- 

 colored animal pole upper- 

 most. Although the func- 

 tion of the jelly is obviously 

 protective, there is evidence 

 that it also tends to raise 

 the temperature of the egg 

 by allowing free entrance of 

 the sun's rays and checking 

 the heat radiation, after the 



Fig. 211. 



-The frog's egg with its layers of 

 jelly. 



1, 2, and 3, the three layers of jelly; m, vitelline 



membrane; p.h., polar bodies; s.t., pigmented manner of the glaSS COVCF- 



track left by nucleus of spermatozoon as it moved • ^^ ^^ ^ hot-bed A similar 



toward the nucleus of ovum (c/. Fig. 213 A to C). ^ ' -i i 



function has been ascribed 

 to the layer of pigment that lies just beneath the egg membrane 

 of the animal hemi- 

 sphere (Fig. 212 B). The 

 sun's rays may also be 

 somewhat focused by 

 passing through the 

 curved surfaces of the 

 jelly layers. 



The testes of the male 

 reproductive system, as 

 described in Chapter 3 

 (Fig. 30, p. 52), are a 

 pair of small bodies at- 

 tached by a mesentery 

 near the anterior end of ^ig. 212.-Gametes of the frog. A two 

 , ... ™, spermatozoa. B, an ovum m section showmg 



^ ■ , ^ pigment of animal hemisphere, nucleus, and yolk 



matozoa, which are pro- granules in cytoplasm. The magnification of A 

 duced within the testes is much greater than that of B. 



(Fig. 212 A), arise from 

 primordial germ cells by a process similar to that described 



