296 



AMPHIBIA 



the lower " vegetal pole " has no pigment but is rich in yolk and 

 much heavier. Frog eggs are deposited in a mass of jelly which 

 encloses green algae aiding in aeration, and which unquestionably 

 aids in the absorption and radiation of heat. The jelly also pre- 

 serves the eggs from friction and the attacks of enemies. (Figure 

 160.) 



Fig. 160. The metamorphosis of the frog. (After Brehm, from Galloway- Welch, 

 Textbook of Zoology. Courtesy of P. Blakiston's Son & Co.) 



Cleavage. — The first vertical cleavage occurs about 3 hours after 

 fertilization, and divides the ovum into a right and a left half. The 

 second vertical cleavage about three-fourths of an hour after the 

 first is at right angles to the first while the third cleavage, an equa- 

 torial one, divides the dorsal pole from the ventral. Segmentation 

 which is total but unequal results in the development of a ball of cells 

 of which the dorsal ones are smaller and more numerous than the 

 yolk cells beneath. At this stage the egg is called the blastula. 



At the close of segmentation the egg has developed into a hollow 

 sphere with the cavity or blastocoel nearer the dark upper pole. 

 The upper hemisphere has two quite distinct layers. A crescentic 

 groove appears at one side of the egg between the layer of white and 

 dark cells. The horns of the crescent extend until they form a cir- 

 cle, the blastopore, which is filled with a mass of white cells called the 

 yolk plug. Rapid division of the marginal black cells in the dorsal 

 region reduces the diameter of the blastopore. At this stage the 



