318 THE POPULAR SCIENCE MONTHLY 



the so-called " germinal epithelium/' but are the direct descendants of 

 cells that have never lost their germ-cell characteristics. 10 



Artificial parthenogenesis is also easily produced among the low 

 inetazoa, and this has been known for a long time. 0. and E. Hertwig 

 published in 1887 their experiments, which showed that various poi- 

 sons might artificially induce segmentation in the eggs of echinoderms. 

 Loeb carried the process further in 1899 and produced plutei from the 

 unfertilized eggs of the sea urchins by raising the concentration of the 

 sea water. " It was immaterial which substance was used to raise the 

 concentration of the sea water, except for the fact that no substance 

 could be used that injured the eggs too much." 11 Parthenogenesis 

 may be initiated by such a variety of substances as chloroform, urea, 

 sugar, salts and certain acids; but only in low forms of life. At least 

 it is the legitimate inference that it is much easier to produce this 

 effect among the simpler invertebrates. Experiments of this sort on 

 the eggs of fishes are noticeable by their absence. Experiments on the 

 eggs of frogs and Petromyzon have not, as far as I know, resulted in 

 carrying the process beyond the segmentation stage. The experiments 

 on insects which will be discussed under that heading have only a 

 doubtful bearing on the present point, since parthenogenesis is among 

 them more or less of a normal process. 



Mollusks 



Modification experiments on mollusks have been relatively rare in 

 comparison with the lower invertebrates. This must be, to some ex- 

 tent, due to the practical difficulties which the experimenter would 

 meet on attempting to modify an animal enclosed in a shell. 



There is, however, much evidence to show that mollusks may be 

 greatly influenced by their surroundings, at least as regards size. This 

 especially concerns the question of the sizes of snails in relation to 

 the extent of the media in which they are forced to live. The manner 

 in which the volume of water affects the snail's growth is a matter of 

 dispute and does not interest us here. The question for us is, how 

 much are they affected? The answer is, that after about two months 

 the snails grown under optimum environment were more than three 

 times the length, or even five times the length, of those grown under the 

 least favorable conditions. 12 Compare this with similar experiments 

 on plants where the extreme variations artificially induced may be ten 

 or twenty fold, and also with the experiments on the higher vertebrates 

 where extreme variations resulting from any experimentally pre- 



10 For literature, see B. M. Allen, Anatomischer Anzeiger, Band XXIX., 

 1906, pp. 217-236. 



11 Loeb, "Dynamics of Living Matter," p. 167. 



12 Vernon, " Variations," p. 302. C. B. Davenport, " Exp. Morph.," p. 474. 



