240 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



nearest the sperm-aster. This condition was first noticed by Mark for 

 Limax campestris in 1881. In Fignres 13, 20, 29, of Kostanecki und 

 Wierzejski are shown striking modifications in the form of the egg-aster 

 and sperm-aster. Apparently each aster is repelled or possibly, as Kos- 

 tanecki und Wierzejski suggest, is being assimilated by the other. The 

 interesting facts shown in these figures and in my own Figures 2 and 

 7 are : first, in each of the five cases the process of maturation is not 

 completed, and, secondly, there has been an attraction existing, else the 

 sperm-aster and the egg-aster never would have come as near together 

 as they have. The fact that the functions of the egg-aster are not yet 

 completed, may be sufficient to account for the temporary repulsion. 



Considerable attention was given in Part A of this paper to the vari- 

 able form of the centrosome. In the study of the egg-centrosome, the 

 centrosphere afforded a sort of standard for comparison — a very unsat- 

 isfactory one, however. The sperra-centrosome has no centrosphere. 

 The condition of the sperm-centrosome in the specimens I have observed 

 which is most nearly typical of the conditions of centrosome structures 

 in general is that shown in Figure 5. The centrosome in that figure is 

 a small dense granule, which is at the point of origin of the astral rays. 



The sperm-centrosome shown in Plate 2, Figure 10 is the largest I 

 have seen. In fact, I imagined when I first saw it that it was a sperm- 

 head with possibly a centrosome beneath it, but I now believe that the 

 dark body seen in close proximity to the radiations from the large cen- 

 trosome is the sperm-nucleus belonging to it. Another sperm-head is 

 visible in the egg, but it is near the periphery and is still homogeneous, 

 whereas the more central sperm-head gives evidence of being vacuolated 

 (a condition not shown in the figure). 



Another condition of the sperm-aster is shown in Figure 2. I have not 

 been able to make out anything in the central region of the aster that 

 has the appearance of a centrosome, there being instead q. large, faintly 

 stained, thickly reticulated area, from which astral rays extend. This 

 central region is very much flattened, apparently owing to a repulsion 

 between this aster and the deep aster of the maturation spindle. The 

 repulsion apparent in the astral rays of both these asters is manifest 

 also in the flattened condition of their central reticulated areas. The 

 peculiar distortion and partial displacement of the reticulated area of 

 the deep maturation spindle affords perhaps the strongest evidence 

 I have presented of a repulsion of one aster by another. The centro- 

 somes could be found only in the peripheral aster of the maturation 

 spindle. 



