BLACKMAN: SPERMATOGENESIS OF THE MYRIAPODS. 65 



tetrads, without having passed through any of the intervening stages. 

 Several of these tetrads may often be seen in a nucleus, and among 

 them are flaky masses of chromatin from which the rest are to be 

 formed. In these nuclei the accessory chromosome is always irregular 

 in form and granular toward the outside, showing that the substance 

 from which the tetrads are formed is derived from its substance. These 

 tetrads assume all the typical forms described for them in A7iasa and 

 Jlippiscus (fig. 6), and, as in the cells of those animals, may all be 

 referred to a single tyjje, their apparent diversity of form being caused 

 either by the point from which they are seen or by a slight natural 

 modification. 



The tetrads, as they first appear, are rather elongated, the longi- 

 tudinal si^lit being much more pronounced than the transverse one. 

 At first they are composed of rather coarse granules or flakes of 

 chromatin loosely arranged, but, as the cell approaches division, the 

 granules become flner and arrange themselves in masses, which stain 

 much more densely with chromatin stains. ( Figs. 6, 7. ) 



It is at about this time that the centrosome appears. (Fig. 7.) It 

 is first to be seen in the yolk material, which at this stage is collected 

 in a large, irregular mass at one side of the nucleus. The centrosome 

 is a rather prominent dumb-bell-shaped body contained in a clear 

 sj^ace, fairly well differentiated from the surrounding deutoplasm. 

 At this stage it is not nearly so large as it becomes later, in the pro- 

 phase and in the metaphase. 



From this time the movements of the centrosome can be traced 

 clearly, up to the telophase of the succeeding division. The dumb- 

 bell-shaped figure elongates and finally constricts (fig. 8), forming 

 two spherical bodies. Astral radiations then appear and the centro- 

 somes move toward the nucleus and finally come to rest upon the nu- 

 clear membrane. (Fig. 9.) The astral radiations become more marked 

 and the centrosomes move slowly apart upon the membrane. (Figs. 

 9, 10, 11, 12). 



When they have reached points about 100 degrees apart, the nu- 

 clear membrane begins to disappear. That part over which the cen- 

 trosomes have not passed in their migration is the first to be dissolved, 

 while the portion between the centrosomes persists for some time, as 

 is shown by the numerous cells in which it is still to be found. 

 (Figs. 11, 12.) At this stage the astral rays are more marked than 

 at any preceding time, but are not so prominent as they become later, 

 in the metaphase and anaphase. 



While these phenomena have been occurring, the tetrads have also 

 changed. They have lost their granular character and ragged out- 

 line, have become much more homogeneous, and have assumed an ap- 

 proximately spherical form. They can no longer be distinguished 



