STUDIES OF FERTILIZATION 381 
We are not, of course, free to infer that fertilization is complete 
as the stimulus to development immediately after penetration 
of the spermatozoon. The experiments prove directly that in 
the egg of Nereis the stimulus of the spermatozoon as the impulse 
to development consists of two distinct parts: (1) an external 
stimulus that causes membrane formation and whichissufhicient of 
itself to induce the maturation; (2) an internal stimulus dependent 
on penetration of the spermatozoon. How long after penetration 
fertilization is still incomplete cannot be decided on the basis of 
these experiments. 
In concluding this section, we may note that the cause of 
failure to segment following centrifuging during the anaphase of 
the first cleavage is an entirely different one. The cause in 
this case is the breaking up of the karyokinetic figure and func- 
tion, and dispersing the chromosomes. ‘This is readily demon- 
strated in sections. In the case of eggs centrifuged at the ‘critical 
stage’, the sections show that the maturation spindle receives 
no injury from centrifuging, but appears coherent and normal in 
the sections. The sections of eggs centrifuged during the ana- 
phases of the first cleavage show the chromosomes dispersed 
through the cytoplasm and the cleavage spindle no longer 
coherent, but broken up. In the first case the cause of failure 
to segment is elimination of the sperm-nucleus, as shown by 
study of series 27. This cannot be the cause in the second case, 
and a sufficient explanation of the failure to segment is found in 
the destruction of the karyokinetic figure. 
C. Comparison of completely and partially fertilized eggs vn later 
stages 
We have noted so far that definite proportions of eggs centri- 
fuged at definite periods in the process of fertilization fail to 
develop a sperm-nucleus, and that similar proportions of the 
same lots of eggs when left to develop fail to undergo segmenta- 
tion. The facts (1) that all the control eggs of the same lot seg- 
ment, and (2) that the centrifuged eggs that fail to segment, 
nevertheless had formed the fertilization membrane and under- 
