ONTOGENY 25 



its environment. Either set of conditions alone can lead to 

 no action; for organismal activity is reaction. 



Just so the developing organism, at whatever stage it be 

 considered, reacts to the stimuli of its environment in a manner 

 determined for the moment, on the one hand by its own state 

 or "organization," both morphological and physiological, and 

 on the other by the character of the stimuli acting. The ovum 

 is not to be regarded as a mechanism wound up, ready upon 

 receipt of a single stimulus, to go through its development 

 into an adult organism. It is rather to be regarded as an 

 organism which reacts to its surroundings by undergoing certain 

 changes. This changed organism then reacts further by under- 

 going certain other changes. One reaction of the fertilized 

 ovum is to cleave, of the blastula to gastrulate, and so on. 

 Step by step, one condition succeeding another and leading to 

 still another, the organism gradually alters its morphological 

 and physiological characteristics. Throughout its whole exist- 

 ence the organism shows transformations of substance, energy, 

 and form; we agree to set apart certain of these transformations 

 occurring at a very early period, and to refer to them as pro- 

 cesses of " development." The normal " behavior" of the egg 

 or of the embryo is to develop. The processes of development 

 are neither easier nor more difficult to explain than the phe- 

 nomena of adult behavior, and they have just the same basis 

 in the relation between the internal conditions, within the 

 organism, and the external conditions, without the organism, 

 at the time. 



From this point of view the question why the egg develops 

 is a problem not different, in its essentials, from why the 

 organism grows, or why it seeks or avoids the light. None of 

 these is to be solved by consideration of the organism alone, 

 whether egg or adult, apart from the conditions acting upon 

 the organism; both must be studied together. 



With this conception of development in mind, we should here 

 mention briefly one of the great generalizations that has come 

 from the study of organic development, namely, the Biogenetic 

 Law or the Theory of Recapitulation. Briefly stated this 



