THE PHYSICAL BASIS OF HEREDITY. 173 
spermatozoon in some cases containing less than zgq4599 
(Wilson), and in extreme cases less than zo5pho005 
(Hertwig) of the volume of the egg cell. 
A discussion of the method by which the reduction 
of the chromosomes in the germ nuclei is brought about 
may profitably be deferred until the es- 
sential features of fertilization have been 
examined. The phenomena of the fusion of egg and 
sperm can best be studied in some such form as the sea 
urchin, in which the egg is very small and, in some spe- 
cies, quite transparent. As fertilization takes place free 
in the sea water, the germinal cells being cast out from 
the parents, it is possible to collect the eggs and sperm 
separately from mature individuals and bring them to- 
gether in small dishes of sea water, and at such times 
as may suit one’s convenience. Then in the living egg 
much of the process may be followed under the micro- 
scope, and properly prepared sections of the eggs killed 
by reagents at the various stages enable conclusions to 
be drawn as to matters of minute detail. Fig. 10, 4 to 
fF, presents a series of diagrams, taken from Boveri, illus- 
trating the principal facts in the process of fertiliza- 
tion. In Fig. 10, A, the egg is represented with its 
clear nucleus in the centre, surrounded by the egg mem- 
brane. Clustered around the periphery are a number of 
spermatozoa endeavouring to find their way into the 
substance of the egg. On the right-hand side in the 
figure one has penetrated the membrane and is shown 
passing into the egg cytoplasm, which puts forth a 
small conical prominence to meet it. As soon as the 
head of one sperm enters the egg cytoplasm a new 
membrane is formed around the egg which effectually 
prevents the entrance of any others. The head and 
middle piece penetrate into the egg, the tail usually re- 
maining imbedded in the membrane where it soon de- 
Fertilization. 
