BLACKMAN: SPERMATOGENESIS OF THE MYRIAPODS. 63 



III. — OBSERVATIONS. 



1. The Spermatocytes. The spermatocytes, as they arise from the 

 telophase of the last spermatogonial division, are very small in com- 

 parison with the truly enormous size attained by them later. Even 

 in this stage their size varies somewhat, but the average diameter is 

 not more than ten micra. At this time the nucleus fills nearly the en- 

 tire body of the cell, and its chromatin is in the form of a number of 

 short, granular segments. (Fig. 1.) 



These segments, by their disintegration and subsequent union, 

 form the spireme seen in a later stage. (Fig. 2.) This spireme is 

 similar in all respects to that found in the spermatocytes of insects at 

 the same stage of the prophase. 



The resemblance of the spermatocytes of myriapods to those of in- 

 sects is still further emphasized by the presence, within the nuclear 

 cavity, of that peculiar chromatic element known as the accessory 

 chromosome. 



As is the case in insects, this element lies immediately against the 

 nuclear membrane. It stains intensely with all chromatin reagents 

 employed, and in other ways behaves as does the accessory chromo- 

 some of insects. It is peculiar, however, in that it is approximately 

 spherical in form. 



At this stage, the cytoplasm, although much more voluminous 

 than in the spermatogonia, is relatively small in amount as com- 

 pared with what it becomes later. It is finely reticular in structure and, 

 as far as observed, contains no centrosome or idiozome. Yolk ma- 

 terial is sometimes present in small quantities. 



When the cell has reached the stage represented in fig. 2, the 

 chromatin spireme gradually breaks down, and the nucleus is filled 

 with a looser, less densely staining reticulum, as represented in fig. 3. 

 This reticulum continues to become finer and less dense, until the cell 

 finally reaches the stage represented in fig. 4, where the nuclear 

 cavity is filled with a beautiful regular network composed of faintly 

 staining fibers. Most of the cells remain in this condition until the 

 active prophase begins with the formation of the tetrads. Some, 

 however, go still farther, and in these the reticulum of the nucleus 

 could not be distinguished from that of the cytoplasm were it not for 

 the nuclear membrane enclosing it. These cells are, moreover, larger 

 than the ordinary ones at this stage. 



Meanwhile the accessory chromosome has undergone very marked 

 changes. It has increased in size much out of proportion to the rest 

 of the nucleus, as is shown in fig. 4. It is still approximately spher- 

 ical in form and shows the same affinity for chromatin stains which 

 is characteristic of it in preceding stages. 



5— K.U.Qr. A-x2 



