PAR THENOGENESIS 



genetic in origin, is a normal organism though he lacks the 

 sperm-borne chromosomes of his sisters. 



With the aid of chemical solutions, changes in tempera- 

 ture, mechanical shock or radiations, eggs which normally 

 never develop without fertilization can be induced to 

 develop parthenogenetically. Up to the present we have 

 succeeded in inducing this artificial or experimental par- 

 thenogenesis in eggs of echinoderms, worms, molluscs, a 

 spider, fishes, and frogs. As we shall later learn, the only 

 period in which experimental parthenogenesis is possible 

 coincides with the stage in which the egg is fertilizable. 

 Since, for the eggs of the animals named, this fertilizable 

 period falls, for some of them, in the germinal vesicle stage, 

 for others during first, for others during second, and for the 

 remainder after complete maturation, we can not regard 

 only one of these stages as the prerequisite for induced 

 parthenogenesis. 



Unfortunately, of the eggs of the various species of ani- 

 mals that have been treated with means for inducing par- 

 thenogenesis only those of a sea-urchin, of a starfish and 

 of frogs have been reared to sexual maturity. The 

 composition of the nucleus can generally be given only for 

 early stages of development. With respect to the end- 

 result of development our knowledge of induced partheno- 

 genesis falls far short of that of the natural. Of the few 

 cases known the sea-urchins' and the starfish eggs show 

 throughout development a single set of chromosomes. The 

 adult frog derived from a parthenogenetic ^gg is different; 

 such an individual has either cells with single or such with 

 double sets of chromosomes or cells of both types. Noth- 

 ing indicates that the normality of these adults varies with 

 chromosome-garniture.^ The same can be said for the 



^ We must not overlook the fact that there are variations in the 

 chromosome number in cells of adults from fertilized eggs. 



2og 



