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TEXTBOOK OF ZOOLOGY 



membrane changes and prevents entrance of others. The head of the 

 sperm carries the nucleus and soon takes the form of a rounded male 

 pronucleus inside the cytoplasm of the egg. The egg nucleus is known 

 as the female pronucleus. The male and female pronuclei finally fuse 

 to form the fusion nucleus, and the fertilization is complete. The 

 significance of fertilization is largely centered around two important 

 functions. First, it is the impetus for the development of an embryo 

 from the egg under most normal circumstances; however, partheno- 

 genesis replaces this function in some cases. Secondly, it brings about 



Fig. 48. — Diagrams showing cleavage in the young embryo of Asterias. 1, 

 Fertilized ess (zygote) ; 2, two-celled embryo following first cleavage division; S, 

 the four-cell stage ; //, the eight-cell stage ; 5, the sixteen-cell stage ; 6, morula 

 stage (solid) ; 7, blastula stage (hollow) ; 8, early gastrula stage (infolding of cell 

 layer at one side) ; 9, later stage of gastrulation. The infolded layer is the en- 

 doderm. (Drawn by T. C. Evans.) 



the means for inheritance of characteristics from two different lines 

 of ancestry. This union also restores the diploid number of 

 chromosomes. 



Cleavage is a series of mitotic cell divisions beginning in the 

 zygote immediately following its formation. These divisions occur 

 in rapid order with but very little intervening growth, and the 

 resulting cells adhere to each other in a body. In eggs where the 

 yolk material is scant and evenly distributed, the ensuing cleavage 



