REPRODUCTION 191 



three abortive. A minor difference, not to be neglected, 

 is that in spermatogenesis the whole transformation is 

 completed in the spermatogonium (testis in animals), 

 whereas in the oocyte much of it takes place after it 

 leaves the oogonium (ovary), and sometimes after the 

 conjugation process has begun. 



Like the human spermatocyte, the human oocyte 

 contains thirty-two chromosomes which are at first fila- 

 mentous, but which become globular and collected into 

 tetrads as the time of maturation arrives. Like the 

 spermatocyte, the oocyte also undergoes two successive 

 divisions for the purpose of reducing its chromosomes. 

 During these divisions, which are equal in size and im- 

 portance as regards the nuclear substance, but very 

 unequal as regards the cytoplasm, the nucleus moves 

 to the extreme periphery of the cell, so that the comple- 

 tion of each nuclear division results in the formation of 

 what appears to be a bud upon its surface. 



In the first reduction division the tetrad chromosome 

 formations become divided transversely so that an 

 equal number of dyads pass out into the tiny cell and 

 remain in the mother cell. Each has now sixteen chromo- 

 somes of dyad shape. After a brief interval the process 

 of division is repeated both by mother and daughter 

 cells, so that eventually there are four cells, each with 

 eight chromosomes of monad or globular form. 



Among the higher animals the ovum alone persists for 

 conjugation and development, the smaller cells resulting 

 from the reduction division, and known as "polar bodies/' 

 disappear as far as present knowledge goes. 



The purpose of reducing the chromosomes in this 

 manner seems to be the admission to the zygocyte of an 

 equal quantity of essential substance (chromosomes) 

 from each parent, 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; 



