LINVILLE: PULMOXATE GASTEROPODS. 241 



III. The Origin of the First Cleavage Spindle. 



The fact that a fully formed sperm-spindle exists in the egg while the 

 processes of maturation are going on, does not of itself prove that the 

 sperm-spindle will become the tirst cleavage spindle; but nevertlieless a 

 strong presumption is created that such is the case. If a spindle 

 arising from the division of the egg-centrosome should also be found 

 after the formation of the second polar cell, the reason for the existence 

 of a sperm-spindle could not be easily explained. As far as I know, 

 however, the egg-centrosome has never been found to divide and produce 

 a spindle after the formation of the second polar cell ; but that does not 

 end the matter. In the first place, it is impossible to know whether 

 every sperm-centrosome divides to form a spindle ; and in the second 

 place, the sperm-spindle, whether formed, or only potentially existing in 

 the sperm-centrosome, nearly disappears from view during the resting 

 stage of the germ-nuclei. 



Kostanecki und Wierzejski maintain that they have followed the his- 

 tory of the sperm-centrosome through even this almost quiescent stage 

 to the actual formation of a cleavage spindle. Their Figure 33a repre- 

 sents the centrosome as having nearly disappeared. Even on this 

 evidence, how is one to prove that the egg-centrosome has totally 

 disappeared " because of inability to carry on the division of the cell," 

 while the sperm-centrosome, a new organ, becomes predominant and de- 

 velops into the first cleavage spindle 1 If at any time a link is missing 

 in the chain of the history of the sperm-spindle, it seems to me we can- 

 not exclude the possibility that the egg-centrosome may come again to 

 view, and actually take part in forming at least a part of the first 

 cleavage spindle. 



In my discussion of the changes of the egg-centrosome, I have said 

 that, although the disappearance of the egg-centrosome and the astral 

 rays is long delayed, they nevertheless come to a condition in wliich it 

 is impossible to distinguish them from the yolk granules and the proto- 

 plasm. I have also said that I have not seen a stage such as Kosta- 

 necki und Wierzejski represent in their figures, where the rays of the 

 egg-aster are apparently being assimilated by the rays of the sperm- 

 aster ; but I have not seen exactly the conditions of germ-nuclei which 

 these authors find at the time assimilation is going on. However, their 

 figures as a whole do not bear out the " assimilation " theory completely, 

 as, for example, in Figure 27, where the egg-aster (even though the 

 germ-nuclei are nearer together than in Figure 25 or 28) is still appar- 



