PROCESS OF FERTILIZATION IN THE CRAB bi 
8. FERTILIZATION 
In eggs taken from the oviduct or just after leaving it, many 
sperm-vesicles may be found lying on the edge of the cytoplasm 
as shown in figures 102 and 103, while one is found down in the 
cytoplasm (figs. 104 to 108). Here it is evident that the move- 
ment of the cytoplasm has carried the vesicle below the surface. 
Before the sperm-vesicle enters, there is a layer of cytoplasm 
just inside of the egg-shell. The rest of the egg is filled with 
spherules of food material, in the interspaces of which the cyto- 
plasm extends from the peripheral layer, by fine strands, all 
through the egg. The fact that the spherules of food move 
apart and a small mass of cytoplasm accompanies the sperm- 
vesicle into the egg, is best explained by supposing that the 
first vesicle which comes in contact with the cytoplasm, initiates 
a flowing movement of the latter along the inner surface of the 
shell, from all sides towards the newly entered vesicle. The cyto- 
plasm, moving thus along the inner surface of the shell towards 
one point, would be deflected in towards the center of the egg, 
and would tend to carry the vesicle in with it. We may suppose 
further that, when the cytoplasm has once responded to such a 
stimulus, its physiological state is so changed that it will not 
respond to another. As a result only one vesicle becomes im- 
bedded in the cytoplasm of the egg where it is to be transformed 
into the male pronucleus. 
The vitelline membrane (fig. 104, v.) is formed just after the 
entrance of the sperm-vesicle into the cytoplasm and the vesicles 
which failed to enter, lie between it and the shell of the egg. 
The first polar body (fig. 114), which is cast off while the eggs 
are passing through the oviduct, is also found between the vitel- 
line membrane and the shell. 
The first step in the transformation of the sperm-vesicle into 
the male pronucleus is a thickening of its lateral walls. This 
may be observed in figures 105 to 111. Accompanying this there 
is an increase of affinity for the stains used (thionin and Dela- 
field’s hematoxylin). Next, there seems to be an extrusion of 
the old capsular wall which, if we recall the method of the ever- 
