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EVOLUTION, GENETICS, AND EUGENICS 



Maturation of the sperm-cell. — In the maturation of the male 

 gamete the germ-cell, now known as a spermatogonium, increases 

 greatly in size to become a primary spermatocyte. In each primary 

 spermatocyte the pairing of the chromosomes already alluded to 

 occurs as indicated in Fig. 47, where six is taken arbitrarily to indicate 

 the ordinary or diploid number of chromosomes, and three the reduced 

 or haploid number. The division of the primary spermatocyte gives 

 rise to two secondary spermatocytes (c), the paired chromosomes 

 separating in such a way that a member of each pair goes to each 



Fig. 48. — Diagram to illustrate oogenesis, a, showing the diploid number of 

 chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and in 

 oogonia; b, the pairing of corresponding mates preparatory to reduction; c, d, 

 the reduction division, giving off the first polar body; e, egg preparing to give off the 

 second polar body, first polar body ready for division; /, second polar body ready 

 for division; g, second polar body given off, division of first polar body completed. 

 The egg nucleus, now known as the female pronucleus, and each polar body contain 

 the reduced or haploid number of chromosomes. (From Guyer.) 



secondary spermatocyte. Each secondary spermatocyte (d) soon 

 divides again into two spermatids (e), but in this second division the 

 chromosomes each split lengthwise as in an ordinary division so that 

 there is no further reduction. In some forms the reduction division 

 occurs in the secondary spermatocytes instead of the primary. Each 

 spermatid transforms into a mature spermatozoon (/). The sper- 

 matozoa of most animals are of Hnear form, each with a head, a 

 middle-piece and a long vibratile tail which is used for locomotion. 

 The head consists for the most part of the transformed nucleus and is 

 consequently the part which bears the chromosomes. 



