284 NORMAt HISTOLOGY AND ORGANOGRAPHY. 



ing two kinds of secondary spermatocytes. When 

 the latter divide the accessory chromosome also di- 

 vides, thus giving rise to two kinds of spermatozoa in 

 equal numbers, one-half of them having eight and 

 one-half nine chromosomes. The mature ova have 

 no such complications, and each one has nine chro- 

 mosomes. When fertilization takes place two com- 

 binations are possible. Should a spermatozoon with 

 eight chromosomes fertilize the egg, then a male de- 

 velops with somatic cells that have seventeen chro- 

 mosomes, and if one with nine is used, then a female 

 is produced with somatic cells having eighteen 

 chromosomes. 



In some cases the accessory chromosome has a 

 small mate in synapsis. Two classes of spermatozoa 

 develop, one-half with a large and the other half 

 with a small chromosome. The class with the large 

 chromosome produces females, and that with the 

 small produces males. 



Two kinds of spermatozoa, differing in quality 

 and quantity of chromatin material, have been des- 

 cribed in the whole animal phylum, even in man, and 

 it seems to be proved that the quality of the sper- 

 matozoon in these forms determines the sex. But 

 there are many forms of life, particularly in plants, 

 where parthenogenetic and other asexual develop- 

 ment prevail and where sex cycles arise from fac- 

 tors other than spermatozoa. In some of these the 

 quantity and quality of food are sex factors, while 

 in others we do not know the determining agents. 



Structure of Spermatozoa. A spermatozoon is a 

 minute cell, about 0.055 mm. long and consisting of 

 a nucleus or head, a middle piece or body, and a vibra- 



