196 BIOLOGY: GENERAL AND MEDICAL 



abortive and disappear from view without subserving 

 any known function. 



The purpose of reducing the chromosomes in this 

 manner seems to be two-fold: first, to prevent the cells 

 from becoming burdened with an overwhelming number 

 of chromosomes, as must occur if they were doubled with 

 each generation of organisms, and second, to permit the 

 admission to the zygocyte, or fertilized egg, of an equal 

 quantity of essential substance (chromosomes) from each 

 parent, amphimixis, a matter the importance of which 

 will be better understood when the subject of conformity 

 to type has been discussed. 



The nucleus of the spermatozoon with its reduced 

 number of chromosomes is known as the male pronudeus; 

 that of the ovum with its reduced number of chromo- 

 somes as the female pronudeus. 



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. Wherever 

 definite gametes are produced, reduction of chromosomes 

 occurs. Related phenomena also make their appearance 

 among unicellular organisms. 



Though insufficient data are at hand to enable accurate 

 generalization to be made, it seems safe to assume that 

 in such plants and animals as are subject to parthenoge- 

 netic development, or development from unfertilized eggs 

 or germinal cells, reduction of chromosomes does not occur. 



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 



