278 



INTRODUCTION TO CYTOLOGY 



the time they meet the male pronucleus has usually, but not always, 

 become equal in size and appearance to the female pronucleus. The 

 union of the two pronuclei to form a fusion nucleus, or sijnkaryon, usually 



Fig. 108. — Diagram of fertilization and cleavage in an animal. It is assumed that 

 in this case the egg has undergone maturation before the penetration of the 

 spermatozoon. 



mmm 











?mm 



C M^ r^-^tiM^^^S 



Fig. 109. — Independence of parental chromatin contributions in the cleavage 



of the egg of Cryptobranchus. 

 A first cleavage mitosis. B, C, prophase and metaphase of fourth cleavage mitosis 

 (After Smith, 1919.) 



occurs at once after they meet. In a great many cases there may be no 

 actual fusion of the pronuclei at all: as they come close to one another 

 each passes through the prophase stages and gives rise independently to 



