46 THE FOUNDATION Part I 



the two chromosomes of each similar or homologous pair, one derived from 

 the male and one from the female parent, come together and lie parallel to 

 one another. This is called synapsis. Soon each chromosome duplicates itself 

 as in mitosis, so that there is a cluster of chromatids (potential chromosomes), 

 a quartet or tetrad in which two chromatids are of male and two of female 

 parental origin (Fig. 3.10). A spindle forms and in the metaphase the tetrads 

 become arranged on its equator. In the anaphase, the two chromatids of 

 female parental origin in the tetrad go to one pole of the spindle and the two 

 chromatids of male parental origin go to the other. Each of the resulting cells 

 is a secondary spermatocyte with three chromosomes, each of which contains 

 two chromatids. In these secondary spermatocytes a spindle soon forms for 

 the second meiotic division, and in the metaphase the two chromatids of 

 each chromosome separate and one goes to each pole. Each of the cells 

 (spermatids) that result contains three chromosomes. Some of the cells may 

 hold chromosomes entirely of male or entirely of female parental origin; some 

 may hold chromosomes of both origins. Meiosis is now completed, the 

 chromosome number being reduced by half, i.e., to the haploid number. The 

 rest of the process is a change in form. The nucleus becomes more compact 

 and the cell body relatively minute with a slender cytoplasmic tail or flagellum 

 that acts as a swimming organ. At its base is the bead-shaped middle piece 

 that holds the centrioles (Fig. 3.10). Thus, from each primary spermatocyte 

 four sperm cells (gametes) are formed. The foregoing process is usually com- 

 pleted before the sperm cells leave the testis. 



Oogenesis. Fewer and larger sex cells (gametes) are produced in oogenesis. 

 Great numbers of oogonia result from divisions in the period of multiplication 

 (Fig. 3.10). Following this period certain of the oogonia become primary 

 oocytes which grow to be larger than the spermatocytes, the comparable stage 

 of the male germ cells. But they are similar to them in the behavior of the 

 chromosomes, in synapsis, tetrad formation, and the reduction of the number 

 of chromosomes in the first meiotic division. In this division, however, one 

 secondary oocyte receives practically all of the cytoplasm along with its three 

 chromosomes, while the other one, called the first polar body, has very little 

 cytoplasm with the same number of chromosomes. Likewise in the second 

 meiotic division, the large secondary oocyte divides unevenly. The bulk of the 

 cytoplasm surrounds the nucleus of the incipient egg (ootid or ovum) with its 

 three chromosomes. The little remaining cytoplasm and the nucleus contjain- 

 ing three chromosomes compose the second polar body, actually a rudimentary 

 egg. The first polar body goes through a division that parallels the second 

 meiotic one. Thus there are three polar bodies and the egg, each with three 

 chromosomes assorted as in spermatogenesis (Fig. 3.10). The polar bodies 

 with their loads of precious hereditary substance eventually degenerate and 

 come to nothing. The egg keeps its form and is enlarged by its supply of yolk. 



