CHAPTER XII 



INTERPRETATIONS OF THE EXPERIMENTS; AND CON- 

 CLUSIONS 



THE results of the experiments of Pfliiger, of Roux, and of 

 others have given rise to much discussion in respect to the 

 relation existing between the unsegmented egg and the embryo. 

 The old questions of evolution and epigenesis have been once 

 more brought into the foreground, but divested of their historic 

 meaning. 



The results of the experiments on the frog's egg are, how- 

 ever, in the first place, too insufficient in themselves, and in the 

 second place are as yet too uncertain on many points, to warrant 

 general conclusions based on these results alone. The experi- 

 ments can only be understood if considered in connection with 

 similar experiments on other groups of animals. 



Roux's MOSAIC THEORY OF DEVELOPMENT 



Roux's discussion of the problems of development is deserv- 

 ing of most careful examination, for even in his earliest papers 

 we see foreshadowed many of the possible interpretations that 

 have later been accepted in one or another form. Roux pointed 

 out that the known facts of development showed that a certain 

 formal self -differentiation of many parts of the egg takes place 

 during development. This self-differentiation may result from 

 an unequal growth of different substances in the egg which 

 come into activity at different times ; and if so, it should be our 

 aim to discover the stimuli that bring these different substances 

 into action, and thus cause the consecutive series of events. 

 The stimuli must come either from without at each stage of 

 development, or the egg may contain within itself the power of 

 progressive development as soon as it is once set into activity. 

 That the egg needs during its development certain things from 



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