STUDIES OF FERTILIZATION 367 
I have been able to perform this experiment on the eggs of 
Nereis and have found that eggs in which the spermatozoon is 
removed after the cortical changes have occurred proceed but 
little farther in their development than eggs mechanically or 
chemically stimulated, and they do not undergo segmentation. 
Fertilization is therefore still incomplete after the formation of 
the fertilization membrane. 
It will be seen thatif the results above indicated be demon- 
strated, the process of fertilization is obviously something more 
than a beginning of cytolysis or a mere alteration of permeability 
of the peripheral cell membrane. It would appear to be a pro- 
gressive change, starting at the periphery and gradually involving 
the more central portions of the cell. We would, at least, have 
to distinguish two stages in the fertilizing action of the sperma- 
tozoon, one before and the other after penetration. 
I shall consider first the evidence for the statement that in 
the egg of Nereis elimination of the spermatozoon after membrane- 
formation leaves the process of fertilization incomplete. In 
the second place I shall note the respects in which the com- 
pletely fertilized egg differs from the partially fertilized egg, and 
finally, shall consider the bearing of the facts on the theory of 
fertilization. Inasmuch as it will be necessary to make frequent 
comparisons with the normal fertilization, a brief account of 
the salient features of this process will be given first. 
A. Salient features of the normal fertilization 
The egg of Nereis is difficult to fix in a thoroughly satisfactory 
fashion; owing, no doubt, to the presence of the large oil-drops 
and yolk-granules, uneven fixation with shrinkage is hard to 
avoid. The eggs appear likewise difficult of penetration, owing 
probably to the rather viscid jelly from which they cannot be 
separated; this also makes any considerable number of eggs very 
bulky and the killing fluid is apt to be much diluted if used in 
ordinary: amounts. After considerable experimenting with 
picric acid, corrosive sublimate and osmic acid fixing fluids, I 
finally found one which gives practically perfect results in all 
