230 LILLIE. [Vol. XVII. 



more than two or three cases of polyspermy in the thousands 

 of eggs examined. The head of the spermatozoon is shaped 

 like a blunt cone, and so should furnish evidence of rotation, 

 if this really occurs. But as it rounds off almost immediately 

 on entering, and as the sperm-aster does not form at once, the 

 evidence is inconclusive. 



The usual path of the spermatozoon is shown in PI. XXIV, 

 Figs. I and 2. It is always towards one side of the &%^ and 

 in the direction of the animal pole. The path is frequently 

 quite direct, as in PL XXIV, Figs, i and 2, but is often 

 curved more or less. When the sperm-nucleus has passed 

 above the equator a little way, its movement stops and it 

 remains quiescent until after the formation of the second 

 polar body. 



During its penetration the sperm-nucleus swells up some- 

 what by the accumulation of caryolymph within it, forming 

 a nucleus with a thick chromatic wall and a reticulum of a 

 few strands only. At this time it possesses almost invariably 

 a sharp apex directed forwards and a broad base, thus corre- 

 sponding very nearly in form to the head of the spermatozoon 

 before its entrance. But as it has power, after entering, to 

 change its form very considerably, it does not of necessity 

 follow that the apices correspond in the two cases. 



Arrived at a point of equilibrium in the egg-substance, its 

 place of rest, the sperm-nucleus rounds off and contracts in 

 volume. As it does so, the chromatic reticulum becomes very 

 regular (PI. XXIV, Figs. 7, 8, 9 « and 9 b). By careful study 

 it is possible to determine that there are about sixteen regular 

 hexagonal interchromatic areas on the surface ; and as this 

 agrees with the number of the chromosomes that ultimately 

 develop from it, it seems probable that each of these areas 

 corresponds to a chromatin-vesicle or chromosome. The con- 

 traction proceeds during the anaphase of the first maturation 

 division, until the cavity of each vesicle becomes indistinguish- 

 able, and the sperm-nucleus is then a small mulberry-like body, 

 staining intensely black. Sobotta ('97, p. 46) has described in 

 Amphioxus a similar enlargement and subsequent contraction 

 of the sperm-nucleus. 



