192 BIOLOGY: GENERAL AND MEDICAL 



that of the ovum with its reduced number of chromo- 

 somes as the female pronucleus. 



Fertilization is effected by the entrance of the male 

 pronucleus into the female cell whose nucleus appears to 

 advance to meet it. Coming together near one pole 

 of the cell, the two pronuclei conjugate, mingle their 

 chromosomes, and so form a new nucleus for the zygote 

 or fertilized cell which thus comes into possession of the 

 full somatic number of chromosomes. 



The process of chromosome reduction generally per- 

 vades the world of multicellular living beings. Related 

 phenomena also make their appearance among unicellu- 

 lar organisms. 



Many interesting examples of the special means by 

 which reduction of chromosomes is effected among 

 plants might be given, but at an expense of space that 

 would scarcely be worth while in a writing not particu- 

 larly devoted to plant physiology. In dismissing the 

 subject, however, one fact should be mentioned, that is, 

 that among the ferns, mosses, and algae, where alterna- 

 tion of generations exists, the asexual generations 

 (sphorophytes) have spores possessing the somatic 

 number of chromosomes, while the sexual generations 

 (gametophytes) produce sexual cells, or gametes, having 

 the reduced number. 



This fertilized cell or zygote is immediately ready for 

 development into the new individual, which through 

 the receipt of an equal number of chromosomes from 

 each parent inherits characteristics from each. The 

 development of the zygote into the new individual forms 

 a new phase for study known as ontogenesis. 



REFERENCES. 

 Same as for Chapter VIL 



