FERTILIZATION REACTION IN ECHINARACHNIUS PARMA. 7 



the egg immunizing it to other sperm; the direct opposite pole of 

 the site of sperm entry is the last point affected. 



The site of sperm entry becomes a "point of injury" and 1 's 

 "negative" for any other sperm arriving at this point, all other 

 'portions around the egg being positive. This "wave of nega- 

 tivity" moves over the egg, its rate varying with the variations 

 in the time that marks the disappearance of the sperm within 

 the cytoplasm. When only the tip of the sperm head has en- 

 tered the cytoplasm, the immediate vicinity of the site of pene- 

 tration alone can not engulf sperm. As more of the sperm head 

 disappears within the cortex of the egg, the "negativity" of the 

 cortex for sperm entry progresses still farther around the egg 

 until at -the moment the head has disappeared the egg can engulf 

 sperm only at one point the pole opposite that at which the 

 sperm entered the egg. A "wave of negativity" thus progres- 

 sively sweeps over the egg from the point of sperm entry pre- 

 ceding the actual beginning of membrane lifting. Before the 

 membrane begins lifting at the site of sperm entry sperm can no 

 longer enter at any point on the egg. 1 From the point of sperm 

 entry a definite gradient of membrane elevation is established, 

 the last point of membrane elevation being at the pole opposite 

 that of successful sperm entry. This gradient, therefore, follows 

 that of diminished susceptibility to sperm penetration initiated 

 at the entrance-place of the sperm. 



If several spermatozoa become attached to the vitellus at the 

 same instant membrane elevation starts at each point of attach- 

 ment no matter what the distance separating them. Thus, 

 two, three or more sperm may be attached at very nearly the 

 same point or at varying distances from each other; in these 

 cases other sperm later in arrival at the periphery of the egg, do 

 not become engulfed, though they may be affixed by the egg. 

 The membrane lifts at these points as blisters; each point behaves 

 with reference to the space between it and an adjacent point of 

 attachment as does the point in monospermic eggs, except that 

 the "waves of negativity" spreading from each move to meet 



1 In 1878 O. Hertwig expressed the view that it is the egg plasma itself, if its 

 vitality be unimpaired, which alone can prevent the entrance of more than one 

 sperm. 



