THE CONTINUITY OF CELLS 



555 



Fig. 22-3. A single egg and several different kinds of sperms are schematically shown here. 



cytoplasm must be achieved before division 

 can take place. When there is too much 

 cytoplasm in relation to the surface area 

 of the nucleus, exchange of vital materials 

 cannot keep pace with the requirements 

 essential to further growth. This may be 

 an important factor in initiating cell divi- 

 sion, for by nuclear division the nuclear sur- 

 face area is doubled with no increase in 

 cytoplasm, thus allowing growth to con- 

 tinue. 



The significance of mitosis should be ap- 

 parent when it is recalled that all the cells 

 of our body have arisen by mitosis from 

 the original fertilized egg and that every 

 one of the quadrillions of cells contains the 

 same numbers and kinds of chromosomes 

 and genes that were present in that original 

 cell! 



A special kind of mitosis— meiosis 



It has been implied that all cells divide in 

 the manner outlined in the preceding sec- 

 tion. We must hasten to discuss a very spe- 

 cial kind of mitosis which does not follow 

 this process in every detail. This occurs in 

 the process by which germ cells ( gametes or 

 sex cells ), namely, eggs and sperms, are pro- 

 duced (Fig. 22-3). The process is called ga- 

 metogenesis — spermatogenesis for sperms 

 and oogenesis for eggs. Obviously if, upon 

 fertilization, the chromosome number is 



doubled, at some previous time the number 

 must have been halved. If this were not 

 true the number of chromosomes would 

 double with each generation, which is 

 not the case. Therefore, during gameto- 

 genesis the number of chromosomes is re- 

 duced from the diploid number (2N), 

 which is the number found in the body 

 cells, to the haploid number (N), just one- 

 half the diploid number which is the num- 

 ber found in sex cells. As a matter of fact, 

 all body or soma cells contain duplicate sets 

 of chromosomes, one from the paternal 

 parent and one from the maternal parent. 

 Man, for example, has 24 pairs of chromo- 

 somes, 48 in all. His germ cells contain one 

 full set or 24 chromosomes, in other words, 

 only one complete complement of genes or 

 chromosomes, not two as in the body cells. 

 At fertilization the two sets are restored. 

 How are these haploid cells produced? 



Spermatogenesis 



The formation of sperm cells takes place 

 in the testis, of course, and varies only in 

 detail among various animals. Let us con- 

 sider the case of mammalian spermatogene- 

 sis. 



Sperms have their beginning in the pe- 

 riphery of the seminiferous tubules (Fig 

 22-4), where the cells are very similar to 

 those of any other body cells, namely, 



