REPRODUCTION 249 



The sexual cells differ widely in appearance. The spermato- 

 zoon consists of an elliptical head, a short middle piece, and a 

 tapering tail. It is undoubtedly a cell which arises from a 

 testicular cell known as the spermatocyte. The latter divides 

 into four spermatids which grow directly into spermatozoa. 

 It is important as well as interesting to know that the number 

 of chromosomes in the head of the spermatozoon is one-half 

 the number normally present in the body cells of the individual. 

 The spermatozoon is adapted to vigorous activity. It seeks 

 the ovum by means of the movements of its tail, which is lashed 

 from side to side, causing it to progress and at the same time to 

 rotate. The rapidity with which it moves is from 1.2 to 3.6 

 mm. per second. Spermatozoa will live in the male genital 

 passages for months, and they probably will live in the female 

 for a long while, but the exact time is not known. They are 

 produced in large numbers. One estimate puts the production 

 at 226,257,000 per week. The spermatozoa are contained in a 

 fluid which comes from the testes partly, but chiefly from acces- 

 sory sexual glands the seminal vesicles, the prostate gland, 

 and Cowper's glands. Together these constituents form the 

 semen, which may be described as a whitish viscid fluid with a 

 characteristic odor. The amount passed at a time is from 0.5 

 to 6 c.c. In some animals it contains fibrinogen, which enables 

 it to clot within the female passages, thus preventing escape 

 of the spermatozoa. 



The ovum in its perfected state as it leaves the Graafian 

 follicle is found to be a minute globular cell containing a nucleus 

 and nucleolus as well as a cell membrane. It undergoes a 

 process analogous to what takes place in the formation of a 

 spermatozoon, which is known as maturation. It begins as 

 the ovum is leaving the ovary, and consists of a karyokinetic 

 division of the nucleus twice in succession. With each division 

 half of the nucleus is extruded together with a small amount of 

 protoplasm as the polar bodies. The first polar body usually 

 divides into two parts, making three polar bodies, all of which 

 degenerate. As the result of these divisions the ovum has left 

 one-half of the number of chromosomes of a body cell. The 

 union of the nuclei of ovum and spermatozoon restores to 

 their original number the chromosomes of the species. Ova 

 are developed within specialized cavities of the ovary lined by 



