98 



ESSENTIALS OF ZOOLOGY 



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

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

 water. The first spermatozoon to swim to the egg and enter it by 

 piercing the vitelline membrane initiates fertilization. After the 

 sperm has entered, a fertilization memhrane 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 spermatozoan nuclens, fuses with 

 the nucleus of the Qgg to complete fertilization and start development. 



micromcrcs 



m&cromerc5 



TROaEGGL, CK065 6ECT10M Or BLA'STULA 



(Courtesy of General 



Fig. 33. — Section through blastula stage of developing frog 



Biological Supply House.) 



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 blast omeres. The third cleavage is parallel to 

 the equator of the egg and somewhat above it. This produces four 

 cells at the animal pole that are smaller than the four cells at the 

 vegetal pole. Subsequently the cells at the animal pole (micr omeres) 

 divide more rapidly than the cells at the vegetal pole, for they con- 

 tain less yolk. Such cleavage is known as unequal, and as divisions 



