466 THE AMERICAN NATURALIST. (VoL. XXXIX. 
terizing the first (vegetative fertilization) should really be con- 
sidered a part of the sexual act. Strasburger regards the proc- 
esses of “ generative fertilization ” as essential to the sexual act. 
The growth stimulus “vegetative fertilization ” is always to be 
expected as an accompaniment of fertilization. It may be given 
to cells in other ways than by the sexual act and is found in cell 
and nuclear fusions which for phylogenetic reasons are plainly 
not sexual. 
The experimental work of recent years on the conditions 
determining artificial parthenogenesis have done much to define 
the sorts of factors which stimulate growth and division of sexual 
cells when the process of fertilization is suppressed. Klebs for 
plants and Loeb for animals have been foremost in these studies 
and they have shown that what seem to be very minor changes 
in the environment of the sexual cell may suffice to give a gamete 
the power of immediate development without fertilization. Thus 
the egg of the sea urchin will develop parthenogenetically to an 
advanced stage when placed for a short time in sea water contain- 
ing magnesium chloride and then brought back to normal sea 
water. Nathansohn (:00) found that a small proportion (about 
7 %) of the eggs of Marsilia vestita would germinate partheno- 
genetically when the megaspores were cultivated for 24 hours at 
the rather high temperature of 35? C. and then left to continue 
their development at 27° C. There are then a number of fac- 
tors such as varying osmotic pressure, temperature, and in some 
cases chemical reagents which may induce gametes to further 
development without the usual sexual processes. These reac- 
tions seem to be of a similar character to the processes in that 
phase of sexual reproduction termed * vegetative fertilization " 
by Strasburger. They give the stimulus to growth but without 
that essential feature of sexuality, the mingling of germ plasm of 
different parentage which distinguishes the processes of “ gener- 
ative fertilization.” 
It seems to the author, for the sake of clearness, that we are 
trying to include too much under the term fertilization. If the 
features of “vegetative fertilization," 7. e., the growth stimulus, 
can be introduced experimentally as in artificial parthenogenesis 
then they cease to be fundamental qualities of the sexual act. 
