368 AN INTRODUCTION TO ZOOLOGY 



chromosomes that is characteristic of the body cells, and this is 

 termed the reduced or haploid number. The exceptions are found 

 in those animals that produce germ cells that can undergo develop- 

 ment without fertilisation ; a form of reproduction termed partho- 

 genesis. Thus the number of chromosomes in a germ cell, i.e. the 

 haploid number, can be represented in a general way by the constant 

 n, and the number in the body cells, or diploid number, will be 

 therefore 2n* The processes undergone in the production of the 

 germ cells differ slightly in the two sexes, and need to be treated 

 separately, although, as will be seen, they are fundamentally the 

 same. Cells destined to give rise only to the germ cells appear at a 

 very early stage in the development of animals, including the 

 higher forms ; these are known as the primitive germ cells. They 

 take up a position in the tissue that, with their descendants, will give 

 rise to the gonads, and, after a certain period of multiplication, are 

 termed the spermatogonia in the male, and the oogonia in the 

 female. 



The process in the male resulting in the production of 

 the male gametes is termed spermatogenesis. The spermatogonia, 

 together with certain other nutritive or nurse cells, form the lining 

 of the seminiferous tubules of the testes. They have been repro- 

 duced from the primitive germ cells by a series of ordinary mitotic 

 divisions, and while in the tubule they grow until they attain a 

 certain size, when they again divide into two by mitosis. One 

 daughter cell, the outer, persists as a parent spermatogonium, which 

 grows to regain its original size and then divides again, and so on. 

 The other, and inner cell, forms a sperm-mother cell or primary 

 spermatocyte. This cell also grows slightly and then divides to 

 form the secondary spermatocytes ; but this division is not an 

 ordinary mitotic one, and its main points need to be considered. 



The number of chromosomes entering this cell is, of course, the 

 diploid or somatic number. As the spireme appears it passes into 

 an unusual condition, for its fibres all mass together at one pole of 

 the nucleus near the centrosomes to form a dense clump, such as is 

 not encountered in ordinary division, and this condition is termed 

 the contraction phase or synizesis. After a time this chromatin 

 knot begins to unravel, and almost as it does so it segments into 

 chromosomes ; but the number of chromosomes produced in this 

 way is only half the- ordinary, i.e. it is the haploid number. Closer 

 examination shows that each apparent chromosome is really a pair 

 of chromosomes in close apposition, but they are not completely 



* There are certain exceptions to this also in the case of animals that 

 possess an extra chromosome in one sex, but this is a point into which it is not 

 necessary to enter. 



