BLACKMAN. — SPERMATOGENESIS OF THE MYllIAPODS. 507 



relatively larger amount of achromatic material contained in the fully 

 formed chromosomes. In Figures 26 and 27 are seen various late 

 stages in the evolution of the tetrads. The younger stages here shown 

 are practically identical with those of the other species. The later 

 stages, however, are here much clearer. The larger quantity of achro- 

 matic material causes the component parts to be more loosely bound 

 together, and the individual chromatids may be readily distinguished. 

 It is possible even to recognize the planes of the cleavage, although 

 it cannot be said with certainty which is longitudinal and which is 

 transverse. 



When all of the chromosomes have left the karyosphere, the body 

 remaining is purely nucleolar in nature and stains lightly (Plate 1, 

 Figures 24, 25). It is considerably smaller than the karyosphere, and 

 in succeeding stages of the prophase gradually disintegrates. Usually 

 it breaks up into a number of irregular granular bodies, which may 

 persist even to the stage of the metaphase or anaphase (Figures 30, 32). 

 In such cells these particles are usually grouped irregularly in the 

 cytoplasm near the poles of the nuclear spindle, as has been observed 

 in both Lithobius by P. Bouin (:0l), by Meves und von Korff (:0l), 

 and in Scutigera by Medes (:05). In other cells, however, the nucle- 

 olus seems to disappear entirely, there being no residue at the time of 

 the formation of the spindle (Figure 29). 



Often the nucleus, instead of breaking up into several granular 

 masses, becomes dissolved in the nuclear sap while retaining its 

 spherical shape. In such cases its circumference often remains 

 about the same, and sometimes even increases. An example of 

 this is shown in Plate l. Figure 23, the increase in circumference 

 being due to the presence of a large vacuole, which represents nucle- 

 olar material converted into a liquid. The contents of the vacuole 

 remain entirely unstained. In later stages more and more of the 

 nucleolus undergoes this change, until eventually none remains. 



The changes in the achromatic structures in chilopods, and the 

 peculiarly modified spindle occurring in these forms, have been the sub- 

 ject of several papers. Meves und von KorfF (:0l) confined their ob- 

 servations to the first spermatocyte division. The centrosomes during 

 the early prophase are not upon the nuclear membrane, but lie at a 

 considerable distance Irom it. In later stages of the prophase they 

 diverge and pass through the cytoplasm to the cell membrane, where 

 they take up a position at opposite sides of the nucleus. Later, the 

 nuclear spindle arises, but the mantle fibres have no connection with the 

 centrosomes, each fibre ending free in the ctyoplasm. The astral fibres 

 radiating from the centrosomes have no connection with the spindle. 



