EMBRYONIC DEVELOPMENT 



195 



While the chromosomes have been coming together, they may also 

 have become duplicated; that is, each chromosome is in some way con- 

 verted into two. Each pair thus comes to consist of four half chromo- 

 somes, and the quadruple body formed is called a tetrad. Owing to 

 its origin, two of the parts of each tetrad are maternal, the other two 

 paternal. 



The Divisions in Spermatogenesis. — In the two divisions that follow, 

 the tetrads are divided in two planes, first into double bodies called 

 dyads, next into their single components. A spindle is formed on which 

 the tetrads take their place. How the tetrads are divided depends 

 on the way they are placed on the spindle. In some animals the tetrad 

 may be turned so that its maternal half faces one end of the spindle, 

 the paternal half the other end. In other animals the maternal and 

 paternal halves of the tetrad may be turned toward the sides of the 

 spindle (Fig. 165). In either posi- 

 tion the tetrad is cut in two in such 

 a way that the two parts facing an 

 end of the spindle go to that end in 

 the cell division. In Fig. 164 it is 

 assumed that the tetrads were so 

 placed that the maternal half was 

 separated from the paternal half. 

 It is a matter of chance, however, 

 whether the paternal half is turned 

 toward one end of the spindle or 

 toward the other. It may happen, 

 therefore, that all the paternal dyads go into one cell and all the maternal 

 dyads into the other or, as in the figure, part into one cell and part 

 into the other. The cells produced by this division are called secondary 

 spermatocytes. 



It 'is important to note that in the division just described no chromo- 

 some has divided. The tetrads have divided, but merely by the sepa- 

 ration of the two chromosomes which had previously come together. 

 Such a division is called a reduction division, or meiosis;^ it never occurs 

 in divisions of somatic cells. 



The secondary spermatocytes now divide by a mitosis in which the 

 dyads are divided into two components. The resulting cells are called 

 spermatids. A given spermatid may contain only paternal chromosomes, 

 or only maternal, or both paternal and maternal in any proportion. 

 The number of these chromosomes is only half that of the original 

 spermatogonium. 



^ The term meiosis is sometimes applied to the whole process of spermatogenesis 

 and oogenesis, including both divisions. 



Fig. 165. — The t'^^-o possible positions 

 of a tetrad on the spindle of the first 

 division in spermatogenesis, and the kinds 

 of cells resulting from them. 



