24 HISTOLOGY 



chromosomes seen in the lower row, in which the place of attachment to 

 the spindle fibers has been indicated. 



Each tetrad represents two chromosomes joined end to end and split 

 lengthwise. The simplest forms are shown in Fig. 19, B and C, which 

 illustrate respectively two ways in which the tetrad may later divide. 

 The two component chromosomes may simply be pulled apart, as indi- 

 cated in Fig, 19, B, in which the spindle fibers are attached to the ends of 

 the rod. If this takes place, each secondary spermatocyte will receive 

 one member of every pair of chromosomes which occurred in the sper- 

 matogonium, but no part of the other member. Such a division, which 

 eliminates one-half of the chromosomes from the daughter cell, is known 

 as a reductional division. The other form of chromosome division is 

 known as equational. When it takes place, every chromosome divides 

 lengthwise, and the daughter cells receive one-half of every chromosome in 

 the parent cell. This occurs in ordinary cell division, and also in the di- 

 vision of the tetrads provided that the spindle fibers are attached to the 

 place where the two component chromosomes come together (Fig. 19, C). 



FIG. 19. A, POLAR VIEW OF THE METAPHASE OF A SPERMATOGENIAL DIVISION IN Dissosteira Carolina. 

 X 1450 (After Davis.) The pairs of chromosomes have been numbered. B-G, various forms of tetrads, 

 rom primary spermatocytes. (After Davis and Robertson.) 



As a stage in the separation of the two halves of a rod-shaped tetrad, cross- 

 shaped forms are produced (Fig. 19, D). If the separation is almost 

 complete, such shapes are seen as in Fig. 19, E. The arms of the tetrad 

 which are not attached to the spindle fibers may bend toward one another 

 and unite, so as to form rings (F), or they may twist about like a figure 

 8, as shown in G. If the spindle fibers are attached to the points xx in the 

 upper figure in G, the division will be equational; if as shown in the lower 

 figure it will be reductional. 



Usually it is considered that the division of the tetrads into double 

 bodies or dyads, is equational, and that the division of the dyads, which 

 takes place when the secondary spermatocytes divide, is reductional. 

 According to Davis, however, the first division of the tetrads is reductional 

 and the second division is equational. In either case the end-result is the 

 same. Each spermatid will contain one of the four parts of each tetrad, 

 and thus one member of every pair of chromosomes will be eliminated from 

 any given spermatid. 



