52 - The Cell 



UNFERTILIZED EGG (haploid) 



PRE-FERTILIZATION 



EGG NUCLEUS 

 A SPERM NUCLEUS 



FERTILIZATION 



FERTILIZED EGG (diploid) 



JN.&UES OflDAUtft 



Fig. 3-9. Fertilization: origin of diploid cells. 



so forth, take part in both kinds of cell divi- 

 sion. 



The meiotic divisions take place in the 

 ovaries or the testes, in the case of animals, 

 and in the spore capsules, in the case of 

 plants. At present it will suffice to describe 

 meiosis as it occurs in the testis of a male 

 animal. Later, in connection with the study 

 of heredity, the similar development of eggs 

 in the ovary, and of spores in the sporan- 

 gium, will also be described. 



The testes of man and other multicellular 

 animals contain a group of numerious special 

 cells, called the spermatogonia (Fig. 3-10). 

 Sooner or later these cells eventually give 

 rise to the sperm cells. The spermatogonia 

 are diploid cells. They have arisen, along 

 with all other cells of the organism, from 

 the repeated mitoses of the original diploid 

 fertilized egg. Furthermore, the spermato- 



gonia in the testes may continue to multiply 

 by mitosis, giving rise to a steady supply of 

 more spermatogonia, all diploid. 



Finally, a few at a time, the germ cells in 

 the testes stop multiplying by mitosis. Now 

 each begins to grow and can be recognized as 

 a primary spermatocyte (Fig. 3-10). Each 

 of the primal) spermatocytes is destined 

 to undergo only two further divisions and 

 these will be the meiotic divisions. Conse- 

 quently each primary spermatocyte will form 

 only lour sperm cells (Fig. 3-10). 



The prophase ol the first meiotic division 

 commences almost as soon as the primary 

 spermatocyte begins to enlarge. This pro- 

 phase differs in two ways horn that of an 

 ordinary mitosis. First it endures much 

 longer, usually for days, rather than for 

 hours or minutes, and during all this period 

 the elongate chromosomes are definitely vis- 



