466 THE AMERICAN NATURALIST. [Vou 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 ( : oo) found that a small proportion (about 

 7 Jfe ) 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. arid 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," i. e., the growth stimulus, 

 can be introduced experimentally as in artificial parthenogenesis 

 then they cease to be fundamental qualities of the sexual act. 



