312 JORDAN. [Vol. VIII. 



the spermatozoon has penetrated some distance into the o^gg 

 substance. 



I have not succeeded in discovering the earliest stages in the 

 metamorphosis of the sperm-head into the male pronucleus. 

 When the spermatozoon first enters the t.%^ (Figs. 23B, 23C) 

 the head does not appear to be sharply defined, and the earliest 

 stage at which I have been able to recognize a well-differentiated 

 portion is shown in Fig. 2 3D. Here the head is already far on 

 the way towards becoming a true pronucleus. The most obvious 

 point of difference between this stage and the mature pronucleus 

 shown in Fig. 23 A is one of size. This is somewhat exaggerated 

 by the difference in magnification of the two figures, but even 

 making the necessary deduction for the difference in magnifica- 

 tion the increase in size will be seen to be considerable, and 

 amounts to at least a doubling in diameter. Sometimes during 

 the maturation of the male element the yolk granules in the 

 neighborhood assume a radial arrangement, but this astral 

 phenomenon is not of invariable or indeed frequent occurrence. 

 The membrane of the pronucleus in the early stages is less 

 distinct than it afterward becomes {cf. Figs. 23 A and 2 3D). 



My friend Dr. Watase ('92) has determined by Auerbach's 

 method of differential staining that the sperm-nucleus of the 

 newt reacts differently at different stages of its maturation 

 period. When it first enters the Qg^ it is, like the original 

 sperm-head, strongly cyanophilous, but the reaction gradually 

 alters until, just before union with the female pronucleus, both 

 9nale and female proniLclei stain identically. 



The rapidity of movement of the male element when once 

 inside the ^gg does not seem to be great. The spermatozoa 

 enter the egg from one to two hours after deposition, and 

 although they sometimes enter in close proximity to the female 

 element (Fig. 6), union does not usually occur until from four 

 to six hours later. It is obvious that the fallacy incident to all 

 studies upon preserved eggs lurks in this estimate of rapidity 

 of movement. Since, however. Fig. 23 C shows a spermatozoon 

 in an q%% two hours old, and since Fig. 2 3D is from an Qgg 

 four hours old there would seem to be a fair basis for the 

 assumption of slow penetration. It is of course likely that the 



