LINVILLE : PULMOXATE GASTEROPODS. 243 



cells and the egg-nucleus. Frequent observation of the stages in the 

 development of the sperm-nucleus enables one to know, even by a hasty 

 examination of the series of sections, which nucleus is the sperm-nucleus. 

 When, therefore, we find, as in Figure 27, that the sperm-nucleus, 

 although partly overlying the egg-nucleus, is nearer to the two cen- 

 trosomes, and indeed, between the egg-nucleus and the centrosomes, we 

 know that both centrosomes more definitely belong to the sperm-nucleus 

 than to the egg-nucleus. More important than the position of the cen- 

 trosomes with reference to the sperm-nucleus, and still dependent on 

 that position, is the fact that the astral rays from either centrosome 

 have extended toward the sperm-nucleus ; and, having touched the 

 nuclear membrane, have caused indentations in it, and have even pene- 

 trated the nucleus itself and united to form a spindle there. The most 

 careful scrutiny of the sections through the egg-nucleus fails to reveal 

 the slightest trace of its being affected yet by the presence of the 

 asters. 



Although many changes take place in the nucleus between the stages 

 represented in Figures 27 and 26, the formation of the cleavage spindle 

 has proceeded just far enough to show that the process of first involving 

 the sperm-nucleus in the spindle continues without interruption until that 

 nucleus is wholly within the spindle, while the egg-nucleus is still out- 

 side of it. The series of changes leading up to the formation of a perfect 

 first cleavage spindle with the chromosomes in their prophase position is 

 nearly completed in the stage represented in Figure 28. 



In conclusion I desire to answer an objection that may possibly be 

 raised and then to call attention to an important additional fact. It 

 may be urged that at least one of the centrosomes represented in each 

 of the Figures 12, 18, and 19 may still be the egg-centrosome, since it is 

 as near to the egg-nucleus as it is to the sperm-nucleus. In reply to 

 this possible contention, I should say, that, while the various positions 

 which the two nuclei may occupy with reference to each other, would 

 easily bring one centrosome into a situation where it would be as near 

 to the egg-nucleus as to the sperm-nucleus, nevertheless such a position 

 in the light of the subsequent history of the centrosome can be shown 

 to be temporary and incidental. It must be remembered also that the 

 sperm-centrosome, sometimes single and sometimes divided, precedes the 

 sperm-nucleus as the two move toward the egg-nucleus in the early 

 sta^res of fertilization. The fact that one and sometimes both centre- 

 somes later migrate from a point between the two nuclei and away from 

 the egg-nucleus toward the sperm-nucleus tends to emphasize the iuter- 



VOL. XXXV, — NO. 8 3 



