linyille: pulmonate gasteropods. 227 



8, for Figure 13 shows that the small centrosphere is very faintly out- 

 lined and that the centrosome is very small. There can be no doubt 

 that rays begin at the small centrosphere and continue through the 

 outer centrosphere and beyond ; in fact the outer centrosphere is limited 

 by a very faint outline, which does not interrupt the course of the fibres. 

 There appear to be vacuolations within the space enclosed by the irreg- 

 ular outline of the outer centrosphere, a condition which is not well 

 shown in the figure. The outline itself is not a distinct membrane, but 

 on the contrary marks the extreme limit of what seems to be a progres- 

 sive vacuolation, which advances outward in all directions except toward 

 the animal pole. Neither in the first nor in the second polar cell of the 

 specimen from which Figure 13 was drawn could any trace of a centro- 

 some or centrosphere be found. 



The question of the fate of the deep centrosome of the second matu- 

 ration spindle involves the question of the origin of the first cleavage 

 spindle. The principal part of the discussion of the latter question I 

 shall leave for another division of this paper. There are, however, cer- 

 tain points which may be considered here. 



Unlike the egg-nucleus in sea-urchins and tunicates, 1 the egg-nucleus 

 in gasteropods moves but slightly from the region where the polar cells 

 ai'e formed. Hill ('95) has shown for Sphaerechinus that while the egg- 

 nucleus is in the resting stage at the centre of the egg, it has no rays 

 indicating the presence of a centrosome. The sperm-nucleus, with its 

 centrosome, may be seen at this time a sufficient distance away from 

 the egg-nucleus to enable one to determine readily the relationship of 

 the single aster present in the egg. The case with gasteropods is quite 

 different. Not only is the egg-nucleus eccentric in position, but the 

 astral rays belonging to it persist till a very late stage in the develop- 

 ment of the two nuclei. Kostanecki und Wierzejski find for Physa that 

 as the sperm-nucleus, with the aster in advance, moves toward the egg- 

 nucleus, the astral rays of the egg-nucleus begin to disappear, as if they 

 were being assimilated by the sperm-aster, while the two are still a 

 considerable distance apart. In both Limax and Limnpea I have seen 

 quite the opposite conditions and have never seen the phenomena these 

 authors describe. Instead of a diminution of the area affected by the 

 deep aster of the second maturation spindle, I find that, as the egg- 

 nucleus develops, the centrosphere enlarges and the extent of the rays 



1 The statement with regard to the movement of the egg-nucleus in tunicates 

 is based on unpublished evidence, which Dr. H. E. Crampton of Columbia Univer- 

 sity has kindly permitted me to refer to. 

 vol. xxxt. — no. 8. 2 



