208 BIOLOGY: GENERAL AND MEDICAL 



These cells, unlike the secondary spermatocytes, are not 

 of uniform size. One is very large, the other very small 

 The reduction division is immediately followed by division 

 of both of the newly formed cells, but this time by homo- 

 type mitosis, so that all of the four resulting cells contain 

 the reduced number of chromosomes. Again the division 

 lacks uniformity, the large cell again dividing into a large 

 and a small cell, and the small cell into two small cells of 

 uniform size. The final outcome is four cells, of which 

 one, the ovum, is large, and three, the so-called polar 

 bodies, very small in comparison. The ovum seems to be 

 the only functionally active cell (gamete), the others are 

 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 pronucleus; 

 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. Wherever 

 definite gametes are produced, reduction of chromosomes 



