REPRODUCTION 119 



short and blurred together. The lining up of the chromosomes on the 

 spindle constitutes the metaphase of the second division. There is no 

 further doubUng of the chromosomes; they simply separate and pass to 

 the poles so that in the anaphase of the second meiotic division a hap- 

 loid set of single chromosomes passes to each pole. In the telophase, 

 the cytoplasm divides, the chromosomes become longer, thinner and less 

 easily seen, and a nuclear membrane forms around them. The net re- 

 sult of the two meiotic divisions is a group of four cells, each of which 

 contains the haploid number of chromosomes, that is, one and only one 

 of each kind of chromosome. These cells are mature gametes and do 

 not undergo any further mitotic or meiotic divisions. 



The term gonad refers to the glands which produce gametes, the 

 testis of the male and the ovary of the female. The meiotic process is 

 fundamentally the same in ovary and testis but there are a few differ- 

 ences in detail. 



Spermatogenesis. A typical testis consists of thousands of cylindri- 

 cal sperm tubules, in each of which develop billions of sperm. The walls 

 of the sperm tubules are lined with unspecialized germ cells called 

 spermatogonia. Throughout development, the spermatogonia divide by 

 mitosis and give rise to additional spermatogonia to provide for the 

 growth of the testis. After sexual maturity, some spermatogonia begin 

 to undergo spermatogenesis, which includes the two meiotic divisions 

 followed by the cellular changes which result in mature sperm. Other 

 spermatogonia continue to divide mitotically and produce additional 

 spermatogonia for spermatogenesis at a later time. In most wild animals, 

 there is a breeding season, either in spring or fall, during which the 

 testis increases in size and spermatogenesis occurs. Between breeding 

 seasons the testis is usually smaller and contains only spermatogonia. 

 In other animals, including man and most domestic animals, spermato- 

 genesis continues throughout the year once sexual maturity has been 

 attained. 



The first step in spermatogenesis is the growth of the spermato- 

 gonia into larger cells, the primary spermatocytes (Fig. 6.3). Each 

 primary spermatocyte divides, by the first meiotic division, into two cells 

 of equal size, the secondary spermatocytes. These in turn divide by 

 the second meiotic division to yield four spermatids. The spermatid 

 is a spherical cell with quite a bit of cytoplasm. Although it is a mature 

 gamete (it has the haploid number of chromosomes), further changes 

 (but no cell division) are required to convert it into a functional 

 spermatozoan. The nucleus shrinks in size, becomes more dense, and 

 forms the head of the sperm (Fig. 6.4). Most of the cytoplasm is shed, 

 but some of the Golgi bodies aggregate at the anterior end of the 

 sperm and form a point which may be of some value in puncturing the 

 cell membrane of the egg. A bit of the cytoplasm is converted into a 

 long flexible tail, the beating of which drives the sperm forward. The 

 mitochondria aggregate at the junction of the head and tail to form 

 the middle piece which is believed to supply the energy for the beating 

 of the tail. 



The mature spermatozoa of different species exhibit a wide range 



