CHANGE IN VOLUME OF EGGS AT FERTILIZATION. 89 



This result, further discussion of which is unnecessary, en- 

 couraged me to undertake a more careful study of a few eggs. 

 Although involving more labor, this alternative was chosen in 

 preference to the "wholesale" method, because the behavior of 

 individual eggs could be definitely recorded. The probable 

 advantage of this was indicated by the discovery by Glaser ('13) 

 that the number of sperm used in insemination plays a role 

 in the result. Accordingly "wholesale" measurements, which 

 are satisfactory enough to decide whether or not a determinable 

 change in volume occurs, have the disadvantage of covering up 

 the fact that the loss is by no means a constant quantity. 



In making these particular measurements, special precautions 

 were taken. The eggs were traced one at a time, no other eggs 

 were on the slide, and the focus before and after fertilization 

 remained unchanged. This was not easy to accomplish, for 

 the sperm are apt to set the egg in motion and change its position. 

 Whenever this occurred, the attempt to secure the second meas- 

 urement was abandoned. Naturally only a small number of 

 cases could be collected under these conditions. 1 



In order to fulfill the aforesaid requirements, and keep the 

 magnification of the tracings absolutely constant, it was necessary 

 to use small quantities of sperm. This however introduced 

 other difficulties. The appearance of the fertilization membrane 

 is the only convenient index of impregnation, but the membrane 

 does not appear unless the spermatozoa are present in sufficient 

 numbers. As a consequence, I often failed to observe the only 

 available indication of fertilization. 



While the results which I have seem to me to point in but one 

 direction, the changes in volumes registered are in most cases 

 probably minimal, for they depend on the amount of surface 

 alteration, and this in turn is, within limits, a function of the 

 number of sperm involved. Indeed with great excesses of sperm 

 I have not infrequently observed a destructive alteration of the 

 surface of the egg. This is more easily observable in Asterias 

 than in Arbacia. It appears likely therefore that normally, or 

 perhaps better, frequently, greater changes in volume occur 

 than I have recorded. 



The results on individual eggs dealt with singly in the manner 

 described are given in the subjoined table. 



