PROBLEM OF DIFFERENTIATION 7 



mentally produced developmental monsters out of chick embryos, 

 and rightly concluded that since there cannot have been any 

 preformation of these experimentally induced monstrosities, normal 

 embryos need not be preformed either. A better known death-knell 

 for the preformationist hypothesis is Driesch's demonstration that 

 in many forms, the parts (blastomeres or groups of blastomeres) of 

 the dividing egg could, if separated, develop into complete little 

 embryos. It is impossible to imagine any theory of preformation, 

 however elastic, which will explain the fact that an egg normally 

 develops into a single embryo, and yet can be made to give rise to 

 two or four whole embryos. 



§ 3 

 The inevitable conclusion is that development involves a true in- 

 crease of diversity, a creation of differentiation where previously 

 none existed, and that the interpretation of embryonic development 

 must be sought along the lines of some epigenetic theory. The 

 problem is narrowed down to a search for a principle on which it is 

 possible to understand how the determinations of the future em- 

 bryo can arise out of a non-diversified egg. It is the great merit of 

 C. M. Child to have shown in theory how this is possible. Briefly, 

 his view (which will be considered in detail later) is that certain 

 external factors set up quantitative differentials in the egg and 

 embryo, as a result of which qualitative differences of structure 

 ultimately ensue. The egg contains a complex of inherent factors, 

 notably the genes of Mendelian theory, which have been trans- 

 mitted from its parents and ensure that it shall develop in a specific 

 fashion, and that if the environment is normal it shall develop so 

 as to resemble other members of its kind. However, these internal 

 inherent and transmitted factors of the egg, though genetically pre- 

 formed, cannot be regarded as a preformation in a spatial or em- 

 bryological sense. What they do is to confer upon the developing 

 organism the capacity to respond in a specific way to certain stimuli 

 which in the first instance are external to the organism. It is, as 

 Ray Lankester and Herbst first suggested, these responses of a 

 specific hereditary outfit to stimuli outside themselves, which 

 constitute development. 



Differentiation is evoked out of the egg afresh in each and every 



