GENERAL ZOOLOGY 



During gametogenesis the cytosome undergoes modifications that differ in 

 male and female germ cells and that result in the formation of a male gamete 

 smaller than the female gamete. Gametes of different sizes are known as 

 anisogametes (Fig. 5.3) in contrast to the isogametes found in some of the uni- 

 cellular animals (p. 267). The nuclear changes in gametogenesis are closely 

 comparable in both types and constitute the process of meiosis. In other 

 words, meiosis is the term applied to the distinctive nuclear phenomena that 

 occur during the maturation of the germ cells of vertebrates and other 

 animals. The essential characteristic of meiosis is that it results in the 

 distribution to each mature germ cell of only one chromosome from each of 

 the pairs of homologous chromosomes present in the primordial germ cells of 

 an organism. Consequently, each mature germ cell has only half as many 

 chromosomes as the somatic cells and the primordial germ cells had; these 

 chromosomes are unpaired, and the number present constitutes the haploid 

 number of the species (see Fig. 2.12C, p. 42). This fact is very significant 

 in the light of the activities of the germ cells and the relation of the chro- 

 mosomes to the hereditary determiners. 



If we consider the process of spermatogenesis, we find numerous undiflTeren- 

 tiated germ cells, known as spermatogonia, in the testes (Fig. 5.7). Sperma- 

 togonia give rise to other spermatogonia by the process of mitosis during the 

 period of division. When any spermatogonium is ready to begin its diflferen- 

 tiation, it enters the growth period, during which there is some increase in the 

 size of the cytosome and synapsis occurs in the nucleus. The chromosomes of 

 a spermatogonium occur in pairs of similar size and shape with the exception 

 of the X-chromosome; this either occurs alone or has a companion Y-chromo- 

 some, which may not be the same size or shape (see Fig. 2A2E, p. 42). 

 During the growth period the homologous chromosomes come to lie side by 

 side in pairs, a phenomenon known as synapsis. The chromosomes at this 

 stage are comparable to the prophase threads of mitosis. As they shorten and 

 thicken, it is seen that each member of a pair is double in a way that is com- 

 parable to a metaphase chromosome during mitosis. Thus, each "pair" of 

 chromosomes is really a group of four half-chromosomes, or chromatids, and for 

 that reason is called a tetrad (Fig. 5.9B). There are half as many tetrads as 

 there were chromosomes, one tetrad for each pair of homologous chromosomes. 

 The cell is now known as a primary spermatocyte. A spindle appears in this 

 cell, and the tetrads become arranged at the equatorial plate. It can be 

 observed that the two chromatids derived by duplication from a single 

 chromosome are connected by its kinetochore. At anaphase, the two kine- 

 tochores of a tetrad separate toward opposite poles of the spindle; that is, the 

 two chromatids derived by duplication from a single chromosome and known 

 as a dyad pass to one pole, and the two chromatids derived by duplication 

 from its synaptic mate pass to the opposite pole. This is known as the first 

 meiotic or first maturation division; it is the disjunctional division because the 

 two homologous chromosomes which had been paired are now separated, or 

 disjoined, in such a way that they pass to separate cells. The cells arising 



136 



