SEXUAL REPRODUCTION. 



43 



After resting and growth of the segmentation-nucleus 

 a series of cell-divisions takes place called segmejitation. 

 These cell-divisions continue throughout the life of the 

 individual, but the earlier and more evident divisions are 

 called segmentation. 



We may illustrate the process of sexual reproduction 

 thus — 



Fig- 15. — Diagram Illustrating Nuclear Changes during 

 Sexual Reproduction. 



Primitive germ cell grows 

 to form sperm-mother 

 cell /. .. 



Sperm-mother cell divides 

 into two 



And into four to form four 

 spermatozoa 



Male. 



Female. 



Primitive germ cell grows 

 to form egg-mother cell. 



Egg-mother cell divides 



into two. 

 And four to form ovum 



and two polar bodies. 



Spermatozoon and ovum 

 fuse. 



J.Cell-diA 



After hundreds of these cell-divisions, one N. becomes 

 the nucleus of a primitive germ-cell, increases to 2N. in the 

 mother cell, leaves the organism as § in the pronucleus, 

 and the cycle recommences. 



If this diagram be carefully studied it will be clear 



that the process of maturation has for its object the forma- 



tlon of cells which have only half the usual nuclear element^ 



whereas fertilisation consists of the fusiofi of these halves to 



form nuclei of normal proportions. Further, the dimorphism 



