58 INTRODUCTION. 



ficial parthenogenesis, brought about by the use of chemicals and other 

 stimuli, have thrown some light upon the subject, but in the opinion 

 of the author they are, as yet, far from furnishing an adequate solution 

 of the problem. 



In Marsilia vestita Nathansohn (1900) found that it was possible 

 to stimulate the egg-cell to a parthenogenetic development by exposing 

 the germinating macrospores to a temperature of 35 C. for 24 hours, 

 and allowing them to continue their development at a temperature of 

 27 C. As a result about 7 per cent, of the spores gave rise to par- 

 thenogenetic embryos. So far as we know, this is the only case among 

 plants above the Thallophyta in which parthenogenesis has been 

 brought about artificially, and it may be that Marsilia lends itself to 

 this sort of experiment more readily because of the fact that in certain 

 species the tendency toward normal parthenogenesis is strongly mani- 

 fested. In Marsilia drummondii Shaw ('97) found normal parthe- 

 nogenesis to be of frequent occurrence. In these cases of Marsilia 

 the morphological side of the question, especially the behavior of the 

 nucleus, is not known, nor have the number of chromosomes been 

 determined in the cells of the parthenogenetic embryo. 



On the animal side of the question the experimenter finds, fortu- 

 nately, an abundance of most favorable material in the eggs of sea- 

 urchins and of certain marine worms. The results of several investi- 

 gators (Wilson, Morgan, Loeb, and others) have shown that the eggs 

 of Arbacia and Toxopenustes may be made to develop parthenoge- 

 netically through certain earlier stages by subjecting them for a certain 

 time to a solution of sea-water, whose osmotic power is increased by 

 the addition of a solution of magnesium chloride. The action of the 

 Mg-solution seems to be similar to the growth stimulus imparted to 

 the egg by a spermatozoon in normal fecundation. 



Equally instructive are the experiments of Winkler (1901) on nucle- 

 ated and enucleated fragments of the egg of Cystosira barbata, one 

 of the Fucacece, which were fecundated by the spermatozoids. Both 

 the enucleated fragments and those containing the nuclei developed 

 into small embryo plantlets which were exactly alike and attained 

 about the same stage of development. 



The development of normally fecundated fragments of egg-cells and 

 that of the entire eggs induced to develop parthenogenetically by 

 chemical or physical stimuli are phenomena which seem to fall into 

 the same category. They show that in all probability the growth 

 stimulus, or the restoration of the power of division and the blending 

 of hereditary characters are phenomena which in a measure are inde- 

 pendent of each other. Experiments similar to the foregoing have 



