MALE REPRODUCTIVE CELLS 



449 



prophase stages of fine, coarse, and segmented spireme (Fig. 412, 

 26-27). The stages of the regular homeotypic mitosis follow (Fig. 

 413, 28-30). A pair of chromosomes, larger than their fellows, lags 

 behind in its entrance into the spindle and its passage to the poles in 

 some of the cells. This pair, seen in Figures 28-29, are the products of 

 an equation division of the accessory chromosome. Obviously again, 

 equatorial plates of different spindles ought to show a chromosome 

 count varying between 17 and 18, the latter count including the accessory 

 chromosome. Figure 31 shows the metaphase groups of chromosomes 

 of four contiguous, secondary spermatocytes. These are pairs of 

 daughter cells of a pair of primary spermatocyte mother cells. The 

 chromosome count alternates between 18 and 17 among the groups. 

 The first and third are seen to contain a large U-shaped chromosome 

 at the periphery of the complex. This is the accessory chromosome. 

 Telophase stages of this 

 mitosis (Figs. 32-33) do 

 not reveal the accessory 

 as a distinct body within 

 the general chromatin 

 mass; but as soon as the 

 nuclear wall of the result- 

 ing spermatid is recon- 

 structed, the accessory, of 

 typical form and location, 

 again presents itself in the 

 pale staining reticulum 

 (Fig. 414, 34). In Figure 



35 is shown a Spermatid FIG. 414. Aplopus Mayeri. 33, telophase of second re- 



,i r- e duction; no accessory chromosome ; 34, spermatid, and 



in the first Stages Of meta- 3S> spermatozoon) with accessory chromosome; 36 and 



morphosis tO become a 37, a younger and an older pair of spermatozoa, one of 



spermatozoon. It contains each pair with and th ; ther u ith ut F an T n CC 7v7 *"* 



mosome. X 1400. (Drawn by H. E. JORDAN.) 



the accessory as a chro- 

 matic spherical eccentric body. A middle piece has grown out from 

 a centrosome-like granule applied to the nuclear wall, and terminates 

 in a long slender filament or tail about which later develops a cyto- 

 plasmic spiral fin. Later stages in the metamorphosis are shown in 

 Figures 36 and 37, each figure representing a pair of spermatids with 

 and without the accessory, respectively. Finally the accessory disap- 

 pears in the head of the ripening spermatozoon. 



The observed facts, then, that may be employed in forming a theory 

 concerning the role of the accessory chromosome in the determination 

 of sex as it has been developed by McClung, Castle, and most fully 

 elaborated and most widely applied by Wilson are these: (i) an odd 



