CLASS AMPHIBIA 



533 



The eggs are fertilized externally by tlie male, who is clasping 

 the female as the eggs are laid and discharges spermatozoa into the 

 water. The first spermatozoon to swim to the Q^g and enter it by 

 piercing the vitelline membrane initiates fertilization. After the 

 sperm has entered, a fertilization membrane is formed which prevents 

 the entrance of additional spermatozoa. Only the head of the sper- 

 matozoon enters, the remainder being discarded. This head, which 

 is composed primarily of the male spermatozoon nucleus, fuses with 

 the nucleus of the e^g to complete fertilization and start development. 



B/aifomtrts- 



2 Ce// 



VCe// 



6 Cell 



/e Cell 



8laslul<i 



Gasl-rula 



{Yolk P/ua) 



Fig. 287. — Diagrams of early cleavage stages, blastula and gastrula of the frog. 

 This is holoblastic, unequal type of cleavage. The upper, shaded portion of each 

 of the first five diagrams represents the animal hemisphere, and the lower portion 

 of each, the vegetal hemisphere. The circular plug seen in the gastrula stage is 

 the yolk plug. 



Development begins with cleavage which is a series of mitotic divi- 

 sions. Cleavage results in the rearrangement of nuclear material in 

 relation to the cytoplasm. The furrow made by cleavage cuts through 

 the entire egg, and such cleavage is known as total, or holoblastic. 



Cleavage and Blastula Formation. — The first and second divisions 

 run from pole to pole at right angles to each other and divide the 

 egg into four equal hlastomeres. The third cleavage is parallel to 



