LIXVILLE: PULMONATE GASTEROPODS. 237 



Kostanecki und Wierzejski in their Figure 1 give a stage of an egg 

 of Physa which is slightly more advanced than my Figure 14. The 

 head in their figure, as in mine, is represented as being still attached to 

 the tail. Between the head and the stainable tail there is the so-called 

 middle piece, containing the densely stained body supposed to be the 

 centrosome. I have tried to demonstrate the existence of a middle 

 piece and a centrosome in the specimen from which my figure was 

 drawn. Although the sperm-head and a portion of the tail lie in a sin- 

 gle section, I am unable to make out any differentiation in the tail near 

 the point of union with the head, the only modification of the tail at that 

 point being a slight swelling. Unfortunately I have but this single 

 specimen showing the head still counected with the tail. 



In the work of Wilson and others on the changes of the spermatozoon 

 in the egg, it has been established that the centrosome, in many animals 

 at least, lies in the middle piece of the spermatozoon, and the discov- 

 ery of any granule within the middle piece has come to be considered 

 as the discovery of a centrosome. Waiving the question of the existence 

 of a differentiated middle piece in the spermatozoon of gasteropods, 

 there are proofs, beside those brought forward by Kostanecki und 

 Wierzejski, that a centrosome exists near the base of the sperm-head. 

 Wilson and ^Mathews ('95) among others have shown that the sperm- 

 head soon after penetration turns so that its basal end is nearest the 

 egg-aster. At the time of turning a small aster is visible at the base of 

 the sperm-head. An important point in the turning of the head is, 

 that the centrosome is located at the base of the sperm-head. Now, if 

 the tail is present in the egg, it serves as a landmark to show when the 

 turning of the sperm-head takes place and how great is the angle 

 through which the axis of the sperm-head passes. In sea-urchins the 

 tail does not enter the egg with the sperm-head and centrosome ; hence 

 the degree of turning cannot be noted as accurately as is possible in 

 the eggs of gasteropods. The difficulty with gasteropod spermatozoa, 

 however, is that because of the early change in the form of the sperm- 

 head and its separation from the tail, the observer is unable to distin- 

 guish apex from base. Fortunately this difficulty does not exist in two 

 of the series of preparations that I have. Figure 14 illustrates one of 

 them. In this case the head and tail are still attached. In the other 

 case (Figure 15) the form of the detached sperm-head is very nearly 

 the same as that of a mature sperm-head before penetration. Whatever 

 movements the tail may have gone through while still attached to the 

 head, it is fair to assume that the position of the tail shown in the 



