28 HISTOLOGY 



contain twelve. The mature ova all contain twelve chromosomes. If a 

 spermatozoon with eleven chromosomes unites with an ovum with twelve, 

 a male animal will be produced, in every cell of which there will be twenty- 

 three chromosomes. But if the spermatozoon contains twelve chromo- 

 somes, a female animal is formed, containing twenty-four chromosomes 

 in every cell. Thus sex appears to be determined by the presence or 

 absence of a chromosome within the spermatozoon. 



In some cases, as in several Hemiptera described by Wilson, the acces- 

 sory chromosome is paired, but its mate is of small size. Thus the sperma- 

 tozoa all have the same number of chromosomes; but half of them contain 

 the large member of the pair and will produce females, and the other half 

 contain the small member and will produce males. The mature ova all 

 contain the large member. In tfyese insects, therefore, both sexes con- 

 tain the same number of chromosomes, but the cells of the male contain 

 a small chromosome, whereas the corresponding one in the female is 

 large. From these observations it is reasonable to conclude that sex may 

 be determined by a difference in the nature of certain chromosomes in 

 those animals in which there are no appreciable differences in size or 

 number. 



In man, a difference in the number of chromosomes in the sexes has 

 been reported, but the observations have not been confirmed. It is sup- 

 posed that the spermatogonia contain twenty-four chromosomes, but it 

 has not been shown that they exist as pairs. The spermatocytes, sperma- 

 tids and spermatozoa apparently contain twelve. As the principal con- 

 stituents of the spermatozoon, the chromosomes are believed to be the 

 essential agents in the transmission of all qualities inherited from the male 

 parent, and certain of them may determine sex. 



OtfGENESIS. 



Mature ova result from a succession of cell divisions closely comparable 

 with those which produce spermatozoa. The primitive female sexual 

 cells correspond with the spermatogonia, and are called oogonia. They 

 are provided with the full number of chromosomes, and divide an indefi- 

 nite number of times. After a period of growth they become primary 

 oocytes, in which the number of chromosomes is reduced one-half. The 

 primary oocytes divide to form secondary oocytes; and these again divide 

 to produce the mature ova, which are incapable of further division 

 unless fertilization takes place. (The term ovum is ordinarily loosely 

 applied, so that it includes not only the mature cells, but also oocytes, 

 and the clusters of cells resulting from the division of the fertilized ovum.) 



Although the mature ovum and the spermatozoon are closely similar 



