ZOOLOGY: E. G. CONKLIN 
495 
the second polar body the influence of the spermatozoon on the egg pro- 
toplasm during this time is not suflicient to start development in the 
second polar body even though it may contain the greater part of the 
egg substance. 
This indicates that the second factor concerned in the process of nor- 
mal fertilization is not to be found in the diffusion through the egg of 
some chemical substance carried in by the spermatozoon but is some 
non-diffusable substance, probably an organic structure. 
Long ago Boveri (1887) showed that under certain circumstances 
the egg of Ascaris may divide at the first cleavage so that half of the 
egg nucleus passes into each daughter cell while the sperm nucleus does 
not divide but goes entire into one of the first two cells. Such a condi- 
tion he called ^partial fertilization/ and in such cases he found that 
both halves of the egg develop, thus showing that the activating in- 
fluence of the spermatozoon has affected both halves. Since in this 
case the centrosome is the only structure derived from the spermatozoon 
which is known to go into both cleavage cells he reached his well known 
conclusion that the essential thing in fertilization is the addition of a 
centrosome to the egg cell. 
It is possible of course that other unrecognized structures are intro- 
duced by the spermatozoon and serve to activate the egg. Meves 
(1911) found that the spermatozoon of Ascaris introduces into the 
egg a number of coarse granules, -the ^plastochondria,' which he thinks 
unite with similar granules in the egg and are then distributed to the 
cleavage cells. However, in one of the Echinids he finds that the large 
granule or 'plastosome' which is derived from the middle-piece of the 
spermatozoon goes into one only of the first two cleavage cells and yet 
both develop. I have found that the ^plastosomes' in the eggs of gas- 
tropods and ascidians may be distributed very unequally to the first 
two cleavage cells without interfering with the further division of both 
cells, and there is no evidence whatever that the activating influence 
of the spermatozoon is due to these granules. 
On the other hand many investigators have held that fertilization 
is essentially a chemical process and that the activation of the egg de- 
pends upon the introduction by the spermatozoon of certain chemical 
substances which diffuse through the egg. 
The observations recorded in this paper indicate that the second or 
internal factor in normal fertilization is a non-diffusable substance 
which is introduced by the spermatozoon, and they strongly suggest 
that this factor is the sperm centrosome, a position which Boveri has 
long maintained and which I have hitherto contested. 
