428 FRANK R. LILLIE 
which is necessary if cleavage of the ovum is to take place. The 
present study is a contribution to the analysis of the latter phase 
based on observations concerning the fertilizing power of por- 
tions of the spermatozoon. 
It is questionable whether any direct and universal method for 
such an experiment could be devised, for one would have to over- 
come the difficulties of isolating a spermatozo6n, of operating on 
it, and of ensuring the entrance of a part into the ovum, under 
precautions that would preclude the possibility of fertilization by 
an intact spermatozoon. These difficulties might be overcome 
by an instrument sufficiently delicate to enable one to amputate 
parts of the attached spermatozo6n before its entrance into the 
egg. The same result has been obtained in Nereis by a method 
that enables one to operate in bulk, to remove fractions of the 
attached sperm head of varying size, to observe the entrance of 
the part remaining attached to the egg, and to study its fertiliz- 
ing effect, at least to a certain extent. 
In brief, the method consists in centrifuging the inseminated 
eggs of Nereis at five minute intervals before penetration and pre- 
serving the centrifuged eggs at appropriate times. The effective- 
ness of the method depends on conditions already described (see 
part 1 of the present paper, study 3 of the series) which may be 
summarized briefly as follows: The spermatozo6n remains exter- 
nal to the vitelline membrane with its perforatorium embedded 
in the entrance cone for about fifty minutes, more or less, depend- 
ing on the temperature, after insemination. It is embedded inthe 
thick viscous jelly secreted by the egg (text figs.-1 and 2). If, 
now, a quantity of eggs be centrifuged with sufficient force, they 
first accumulate at the distal ends of the tubes in a mass which 
becomes closely packed together. The jelly, which is of less 
specific gravity than the eggs, then separates from the latter and 
forms a layer above the eggs. In squeezing through the narrow 
interstices between the closely packed eggs the jelly rubs over 
the surface of each egg and in many cases carries the attached 
spermatozo6n away with it, leaving, however, the perforatorium 
and attachment granules in the cone as evidence of its former 
presence. In other cases, especially if the eggs be centrifuged 
shortly before the time of penetration of the spermatozoon, it 
