THE BEARERS OF THE HERITAGE 299 
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 
Fic. 48.—Diagram to illustrate odgenesis. a, showing the diploid number of 
chromosomes (six is arbitrarily chosen) as they occur in ordinary cells and in 
odgonia; 6, 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; f, 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 spermatozoén (f). The sper- 
matozoa of most animals are of linear 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. 
