SPERMATOGENESIS 23 



presumably indefinite. As seen in sections, the spermatogonia, sper- 

 matocytes, and spermatids may be described as follows, using for illus- 

 trations Davis's figures of a common grasshopper Dissosteira Carolina. 



Spermatogonia. The nucleus of each spermatogonium contains the full 

 number of chromosomes, which in most of the grasshoppers (Acrididae) 

 is 23. With every spermatogonial division, each chromosome is split 

 lengthwise. In this and other respects the mitotic figures are quite like 

 those occurring elsewhere in the body. They are shown in Fig. 20, A, 

 B, and C. When the twenty-three chromosomes have formed the equa- 

 torial plate, it is sometimes possible to see all of them in a single trans- 

 verse section of the cell (Fig. 19, A). It then appears, as found by Mont- 

 gomery (1901) in certain Hemiptera, and a year later by Sutton in grass- 

 hoppers, that the chromosomes vary in size, but the "gradations in volume 

 are not between individual chromosomes but between pairs, the two 

 members of which are of approximately equal size." In Fig. 19, A, twelve 

 forms of chromosomes have been identified by Davis; and all of these are 

 paired except the one numbered 4. The members of a pair are often, but 

 by no means invariably, side by side. In some cases, owing to foreshort- 

 ening, their resemblance in size is not apparent in the drawing. The be- 

 havior of the odd or accessory chromosome is of special interest, since accord- 

 ing to McClung's hypothesis, now well established, this accessory chromo- 

 some is the bearer of those qualities which determine sex. 



Primary spermatocytes. After the last spermatogonial division, the 

 cells begin their "growth period." At this time the chromatin tends to 

 collect on one side of the nucleus, in a condition known as synapsis (or 

 more recently as synizesis). This distribution of the chromatin has been 

 frequently observed, but it has not been shown to be of special signifi- 

 cance. In the primary spermatocytes drawn in Fig. 20, D, E, and F, the 

 chromatin is evenly distributed. All of the chromosomes, except the 

 accessory chromosome, have become filamentous, but the accessory chro- 

 mosome remains as a compact, darkly staining body close to the nuclear 

 membrane. It resembles a nucleolus, for which in fact it has been mis- 

 taken. True nucleoli may occur in these cells, together with the acces- 

 sory chromosome, but they stain differently. 



As the primary spermatocytes prepare for the next division, the spi- 

 reme becomes resolved into eleven loops, each of which represents the two 

 members of a pair of chromosomes joined end to end. The granules im- 

 bedded in the linin thread divide as usual, so that each loop contains a 

 double row of granules (Fig. 20, F). These loops contract to form eleven 

 chromosomes, which, because of their four parts, are known as tetrads. 

 The structure of the tetrads is shown in Fig. 19, B-G. The filaments seen 

 in the upper row of drawings contract into corresponding solid forms of 



