CHAPTER XVII. 

 DEVELOPMENT. 



Maturation of Male Germ Cells, the Spermatozoon. The general 

 effect of maturation of germinal cells is to reduce the number of chromo- 

 somes, in order that there may not be more than the normal supply in the 

 new cell that will be formed by the union of the ovum and the sperm 

 in the fertilization of the ovum. We shall describe the process of 

 maturation in terms that will apply to the production of spermatozoa 

 in nearly all animals. 



In each species of animals the number of chromosomes in every cell 

 nucleus is constant. Although they are of various sizes and shapes, each 

 nucleus in the body appears to have the same assortment as every other. This 

 is explained by the fact that whenever a cell divides, each chromosome divides 

 individually, and the two resulting daughter chromosomes from each original 

 chromosome are distributed to the two new cells. Thus the chromosomes 

 can be traced back individually through innumerable cell divisions to the 

 assortment that existed in the egg cell at the very beginning of its development. 

 The source of this supply of chromosomes found in the developing egg is 

 double, one set having been derived from the original nucleus of the egg, and 

 the other provided by the sperm. Now the developed organism, which has 

 obtained in this way a double set of chromosomes in every cell, must like its 

 ancestors be able to produce germ cells that each contain a single set. 



This maturation reduction of chromosomes, from a double to a single set, 

 is always carried out by the joint effect of a process of synapsis and of two 

 peculiar cell divisions known as the maturation divisions. In the nucleus of 

 the primary spermatocyte the chromatin materials become crowded together, 

 and pass through the stage known as synapsis, during which the chromosomes 

 become closely united with each other in pairs. The double set of simple, i.e., 

 uncompounded chromosomes occurring in the spermatogonia is thus 

 transformed in the primary spermatocyte into an apparently single set of 

 double chromosomes. Each of these double structures is believed to repre- 

 sent the union of corresponding chromosomes out of the two sets previously 

 existing. They grow immediately into a four-parted condition, which has 

 given them the name tetrads. In the cell division of the primary spermato- 

 cytes, the first maturation division, the tetrads are divided equally, and the 

 result is two secondary spermatocytes, each containing a single set, called 

 diads. The second maturation division follows promptly, producing two 



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