PROCESSES RELATED TO SEXUAL REPRODUCTION 227 



number of chromosomes ; but sooner or later some of these descend- 

 ant cells can be recognized as the early germ cells of the male, or 

 the spermatogonia. Hence, the body cells, or somatoplasm, and 

 the germ cells, or germplasm, are represented in the figure as two 

 distinct lines of descent among the cells of the individual, although 

 both originally came from the same zygote. The spermatozoa 

 are differentiated from the spermatogonia by the nuclear changes 

 of maturation and by the changes in size and shape of the cell 

 body as described in the following section. The separation of 

 germplasm from somatoplasm in a female is similarly shown in 

 Fig. 114. 



Maturation of the Male Germ Cells: Spermatogenesis. — The 

 final changes in the maturation by which the spermatogonia and 

 oogonia become transformed into the spermatozoa and ova are 

 known respectively as spermatogenesis and oogenesis. In sperma- 

 togenesis (Fig, 113), there first occurs a union of the chromosomes 

 in pairs, which is known as synapsis. 



Following this union, there is a cell divison known as the first 

 maturation division, producing two cells as shown by the figure. 

 These resemble the cell shown in synapsis, because each chromo- 

 some splits lengthwise, as is regularly the case in mitosis, and thus 

 no change in number is produced. In the second maturation divi- 

 sion, however, the mitotic cell division is unique; for the chromo- 

 somes do not split lengthwise as in all other mitotic divisions in both 

 body and germ cells. Instead of this, the members of the pairs of 

 chromosomes separate, producing cells in which there are only 

 one-half the number of chromosomes that occur in all the other 

 cells of the organism. In some instances reduction occurs in the 

 first instead of the second maturation division. The final result in 



on the right. Like the body cells, the earlj germ cells or sperniatogonia undergo mitotic 

 cell divisions in which the number of chromosomes remains unchanged. At the time of 

 synapsis the paired chromosomes, or autosomes, unite in the manner shown, but the sex 

 chromosome remains unpaired, since it has no synaptic mate. In the first maturation 

 division there is no change in the number and relationships of the chromosomes, because 

 each one splits lengthwise as in typical cases of mitosis. In the second maturation division, 

 the pairs of autosomes, which united in synapsis, separate and pass without further division 

 to the two cells that become the spermatozoa, while the sex chromosome passes undivided 

 to one or the other of these cells. In this manner each spermatozoon receives one member 

 of each pair of autosomes, and one-half of all the spermatozoa receive an "X" chromosome, 

 while the remaining spermatozoa are without an "X" chromoso-tie. Hence, at the time 

 of fertilization {c.f. Fig. 114), one-half the oosperms or zygotes receive but one ' 'X " chromo- 

 some and become males, while the other half receive two "X" chromosomes and become 

 females {c.f. p. 448). 



