LIXVILLE : PULMONATE GASTEROPODS. 235 



A very suggestive phenomenon in the early history of the sperma- 

 tozoon in the egg is shown in Plate 3, Figure 15. It seems as if the 

 attraction which caused the spermatozoon to penetrate the egg were 

 only a general attraction, aud not located in a definite region of the egg ; 

 for after penetration the head made almost a complete circle before it 

 came within the more definite influence of the egg-centrosome, or before 

 the egg-centrosome and the sperm-centrosome had entered upon the 

 proper phases for attracting each other. At other times, as in Figure 

 14, the spermatozoon on entering moved straight ahead, not stopping 

 until it had come in contact with the membrane on the other side of 

 the esrs:. Here it would have remained until the head aud tail had 

 separated. 



In a preceding paragraph I have referred to the preliminary processes 

 through which a spermatozoon in the egg may be said to go. These 

 processes are : first, the change in form of the sperm-head ; and, 

 secondly, the separation of the head from the tail. I shall describe 

 these processes in turn. 



2. Changes in the Sperm-Head. 



The sperm-head in its fully developed condition, and before it has 

 entered the egg, has the form of a skewer with one, more or less com- 

 plete, spiral turn. The sperm-head in Figure 15, although broken away 

 from the tail, still retains the general form it had before it entered the 

 egg. As a rule, however, immediately after penetration it undergoes a 

 modification in form. The nature of this modification is well shown in 

 Figure 14. The fact that the head is still attached to the tail, enables 

 one to see that, in this case at least, the long axis of the head is now at 

 right angles to its original long axis. Kostauecki und Wierzejski ('96, 

 Figure 1) show the same condition for Physa. Apparently the change 

 begins very soon after the spermatozoon enters the egg, for in only two 

 instances (one of them shown in Figure 15) have I seen the normal 

 form preserved. It also seems probable that the sperm-head, after be- 

 coming elliptical, does not increase in size for a considerable time, for 

 many sperm-heads are found having the same elliptical form, and 

 apparently the same size, both when they are at the periphery of the 

 egg (Plate 1, Figure 1, Plate 2, Figure 8, and Plate 3, Figure 17) and 

 when advanced in their course toward the egg-aster (Plate 3, Figure 21). 

 This fact is brought out better by the figures of Kostanecki und Wier- 

 zejski than by mine. I do not believe any especial significance can be 

 attached to this change in the form of the sperm-head. At first thought, 



