144 The Mechanistic Conception of Life 



VII 



The experiments on the artificial parthenogenesis of other 

 forms of animals show that the eggs of different animals possess 

 a varying tendency for parthenogenetic development. There 

 are eggs which can easily be induced to develop, so easily in 

 fact, that the experimenter cannot always be sure whether he 

 has caused the development by a substance applied by him or 

 whether some accidental condition of the experiment was 

 responsible. The eggs of the silkworm, of the star-fish, and 

 of certain worms belong to this class. In working with star- 

 fish eggs we can observe that occasionally a few of them develop 

 in normal sea-water, apparently without any demonstrable 

 cause, into swimming larvae. The eggs of the Calif ornian 

 sea-urchin Strongylocentrotus purpuratus, on the other hand, 

 show not the slightest tendency to segment parthenogenetically ; 

 only the above-mentioned very specific and quantitative method 

 causes them to develop. For this reason I selected these eggs 

 for the investigation of the nature of the experimental stimulus, 

 since I could always be sure that the same stimulus gave the 

 same results; while, e.g., in the star-fish eggs we can never be 

 perfectly certain that some internal condition in the egg or some 

 overlooked unimportant secondary condition in the experiment 

 may not have caused the development. Although eggs with 

 such a strong tendency for spontaneous development as the 

 star-fish eggs are not the best material for the study of the 

 nature of the developmental stimulus yet we have to answer 

 the question how it happens that some eggs have a greater 

 tendency for parthenogenetic development than others. 



Mathews has observed that by gently shaking the star-fish 

 eggs the number of eggs which develop '^ spontaneously" can 

 be increased. I made a similar observation in the eggs of 

 Amphitrite, an annelid. In the eggs of the sea-urchin nobody 

 has ever been able to obtain such a result. I am inclined to 

 believe that if a sea-urchin should be found, the eggs of which 



