276 EVOLUTION AND ANIMAL LIFE 



Whether the new individual to be exists in the germ cell 

 as a more or less nearly completely preformed embryo needing 

 only to expand, unfold, and grow to be the fully developed 

 new creature, or whether the fertilized egg cell is a bit of prac- 

 tically undifferentiated protoplasm, endowed with a limited 

 and specific potentiality, but depending for its marvelous out- 

 come chiefly on extrinsic imposed influences this question 

 has been a matter of contention since the beginning of the 

 study of generation and development. 



From our scrutiny of the phenomena of mitosis, it is ap- 

 parent that, while the germ cell is certainly considerably 

 differentiated as regards its fine structure, on the other hand it 

 as certainly contains no preformed embryo of the individual 

 into which it is to develop, as the old school of preformationists 

 held. But the testimony from mitosis by no means settles the 

 controversy between the modern preformationists and the 

 modern epigenesists. This rages hotly, and furnishes a great 

 incentive to the pushing on of the study of development. 



What is most interesting, perhaps, about this present-day 

 embryological study is, perhaps, its method. Where hereto- 

 fore the study of development has been almost purely descrip- 

 tive and comparative, as, indeed, all biological study has, the 

 modern embryologist is an experimenter. Experiment, the 

 method of the study of inorganic nature, is being resorted to 

 and relied on for the determination of biological problems, and 

 in particular that one that has for its subject the seeking of 

 the factors and actual causes of individual development. 

 This has been aptly named preformation versus epigenesis. 

 It might also pertinently be called intrinsic versus extrinsic 

 factors or, more broadly, vitalism versus mechanism. 



The new phase or mode of the study of development has 

 been variously called developmental mechanics, experimental 

 development, or, more broadly, experimental morphology, 

 because the experimental method has been extended to the 

 study of phenomena not strictly, or at least not usually in- 

 cluded in the immature, or developing stage of the animal's 

 life; the study of regeneration, of reactions to stimuli, and of 

 reflexes and movements in general, has all been illuminated 

 by the decisive results of the substitution of experiment for 

 haphazard observation in nature. And the further extension 

 of experimental and statistical modes of investigation to the 



