LINVILLE : rULMONATE GASTEROPODS. 221 



In addition to the fact that in Figure 21 the ceutrosomes have moved 

 further apart than in Figure 17, the more completely fused condition of 

 the elements composing the " Zwischenkorper " in Figure 21 gives con- 

 clusive evidence that it has progressed in development further than 

 Figure 17. I have not been able to follow closely the fate of this cen- 

 tral reticulated area after the stage represented in Figure 21. 



It will be noticed that in Figure 1 7 the long diameter of the centro- 

 sphere is nearly coincident with the chief axis of the egg, and likewise 

 with a line connecting the two minute centrosomes. In Figure 21, on 

 the contrary, the long diameter of the centrosphere is nearly perpen- 

 dicular to the chief axis of the egg, and forms a very sharp angle 

 with the line connecting the two small centrosomes. This relation of 

 centrosphere-axis with centrosomes suggests a causal connection, but 

 when compared with the position of the centrosomes in Plate 3, Figure 

 22, it seems probable that the reason for the variation in the direction 

 of the long axis of the centrosphere, as compared with the chief axis of 

 the egg, in these three cases must be sought in something else than the 

 position of the centrosomes. 



Before turning to the consideration of the centrosome, it will be inter- 

 esting to call attention to the astral rays of Figure 17. Here one set of 

 the astral rays begins at the centrosphere and extends two thirds of the 

 way to the vegetative pole, while another set is composed of very short 

 rays scattered among the long rays. Beginning near the distal end of 

 the long rays and continuing in the same general direction to the 

 periphery of the egg are rather broad, indistinct band-like radiations 

 (unfortunately not well reproduced from the drawing). On examination 

 with a one-twelfth inch immersion lens, the bands were found to be 

 composed of exceedingly fine granules, different from the ordinary yolk 

 granules, and distinct also from the " microsomes " which are visible 

 among the long astral rays. The same faint bands appeared in other 

 eggs of the same stages as those shown in Figures 17 and 21. 



There is a point concerning the astral rays in Figure 21, to which I 

 wish to call particular attention, since it may serve to throw new light 

 on the question of the centrosphere. Beginning at the periphery of the 

 central finely reticulated area, the rays extend outward a short dis- 

 tance as a set of extremely fine fibres, then suddenly become thicker, 

 and retain this condition to their peripheral ends. I cannot demonstrate 

 any ring of microsomes at the region of transition from the finer to the 

 coarser fibres, but nevertheless the zone embracing the finer rays is a 

 distinct modification of the aster, and is of interest because of its bear- 



