REPRODUCTIVE ORGANS IN THE MALE. 283 



and therefore equal to one-half the original number 

 of chromosomes. A new spindle quickly forms in 

 the secondary spermatocytes, the diads take an 

 equatorial position and then separate to form 

 monads, each daughter cell or spermatid receiving 

 an equal number. The number of monads is equal 

 to the number of diad groups and therefore equal to 

 one-half the original number of chromosomes. The 

 division of tetrad groups to form diads is usually 

 considered an equational or somatic process, while 

 the division of the diad groups to form monads is 

 looked upon as a reduction process. The splitting 

 of the tetrads is then interpreted as a longitudinal 

 division of the chromosomes, and that of the diads 

 as an end-to-end division. Each spermatid ulti- 

 mately moulds to form ripe spermatozoa and thus 

 every primary spermatocyte produces four sperma- 

 tozoa. 



Sex Determination. Cytological and experimental 

 work in recent years have revealed facts which show 

 that certain chromosomes play an important part in 

 the determination of sex. In the grasshopper (Steno- 

 bothrus viridulus) the cells of the male have seven- 

 teen and the cells of the female eighteen chromo- 

 somes. In case of the primary spermatocytes, each 

 with seventeen chromosomes, when synapsis occurs, 

 one chromosome is left without a mate. This odd 

 one is called the accessory chromosome, and can be 

 recognized by its condensed form, heavy staining 

 qualities, and its position near the nuclear mem- 

 brane. When the primary spermatocyte divides, 

 this accessory or univalent chromosome remains un- 

 divided within one of the daughter cells, thus mak- 



