LINVILLE : PULMONATE GASTEROPODS. 239 



the sperm-head at the end nearest the egg-aster. The fact that it lies 

 on the side of the sperm-head which is directed toward the egg-aster is, 

 I believe, a strong argument that this particular structure is the sperm- 

 aster. The discussion of examples of undoubted sperm-asters I shall 

 leave for the next sub-division of this paper. 



II. The Sperm-Centrosome. 



Kostanecki und Wierzejski ('96) have shown that after the spermato- 

 zoon has entered the egg and the sperm-head has taken on a spherical 

 form, the centre of activity is within the sperm-aster. These authors 

 have also shown that at the earliest appearance of the sperm-aster in the 

 egg it is very small and near the sperm-head. As the aster increases 

 in size, it is found sometimes at a considerable distance from the sperm- 

 head. Later in the progress of development the sperm-aster and sperm- 

 head (the latter now considerably vacuolated) are found near together 

 moving toward the egg-aster, the sperm-aster leading the way. Fre- 

 quently, even at a very early stage, the sperm-centrosome divides into 

 two parts, and a gradually developing spindle forms between them. 



It has been supposed generally that the sperm-aster in its course 

 toward the egg-aster takes a position somewhere between the egg-aster 

 and the sperm-head. Some of the figures by Kostanecki und Wierzejski 

 show exceptions to this. For example, in their Figure 3, showing tiie 

 first maturation spindle at the top of the figure, the sperm-head lies to 

 the left near the periphery of the egg, but the sperm-aster is below the 

 centre. Likewise in their Figure 14, the sperm-aster, instead of lying 

 between the sperm-nucleus and the deep end of the second maturation 

 spindle, is situated so that the line joining it to the sperm-nucleus 

 makes a right angle with the line uniting sperm-nucleus and the inner 

 end of the second maturation spindle. At first I was inclined to think 

 that a second sperm-head in the egg had escaped their observation ; but 

 since that time I have found that in my own material it is far easier 

 to make out sperm-nuclei than sperm-asters. Furthermore, I am able 

 to corroborate for Limncea (Figure 7) some of the conditions which 

 they have shown for Physa. I have no important additions to make to 

 the accounts of these authors on the early changes of the sperm-centro- 

 some in the egg. The facts I have collected are all corroborative of the 

 results obtained by Kostanecki und Wierzejski. I shall therefore de- 

 scribe my observations only briefly. 



The sperm-nuclei shown in Figures 5 and 7 are pointed on the side 



