236 bulletin: museum of comparative zoology. 



the comparison of the sperm-heads in Figures 14 and 21 would seem to 

 lead to the conclusion that the centrosome in the egg represented in 

 Figure 21 had moved from the side to the end of the head (if the 

 change in form shown in Figure 14 is typical) ; but such a change in po- 

 sition it seems to me would be meaningless. The difference between the 

 two may be explained by the fact that the sperm-head would naturally 

 present the smallest surface while being drawn through the yolk gran- 

 ules and the protoplasm. 



Besides the modification of the sperm-head, there is another phe- 

 nomenon which, like the one just described, is incidental rather than 

 essential. I refer to the early appearance of a clear area about the sperm- 

 head. The conditions are represented in Figures 14, 15, 21, etc. The 

 outer limit of this area is usually marked by a fine sharp line, so that the 

 whole has the appearance of a clearly defined vacuole, with the sperm- 

 head at the centre. One can make out fine threads radiating from the 

 sperm-head to the margin of the vacuole. In the eggs that I have ex- 

 amined there is no visible modification of the form of the sperm-head 

 by these radiating threads in the vacuole, except in two cases. One 

 of these is represented in Figure 15, where I find that the radiating 

 threads have caused a modification of the outline of the sperm-head. 

 Wherever a thread comes in contact with the sperm-head, a projection, 

 apparently from the head, is formed. The origin and meaning of these 

 radiating threads seem to me of considerable interest in connection with 

 the subsequent changes in the sperm-nucleus, and hence I shall discuss 

 that subject in the next division of this paper. 



3. The Sperm-Head and the Beginning of the Sperm-Centrosome. 



The change of most importance in the early history of the spermato- 

 zoon in the egg is the breaking away of the sperm-head from the tail. 

 The interval of time between the penetration of the sperm-head and its 

 separation from the tail may be short or long according as we interpret 

 the conditions shown in the figures I have made. In some cases it ap- 

 pears probable that the spermatozoon has travelled over a considerable 

 part of the substance of the egg before the head is separated ; in others 

 (Figures 1, 7, 8, 11, 16, 17, etc.) either the sperm-head on entering has 

 left the tail outside, or the tail, though entering, has been resorbed. 

 However, the time of separation of the head from the tail is not so im- 

 portant as the manner of separation. It is to the manner, and still 

 more to the cause, that I desire now to direct attention. 



