24 GENERAL EMBRYOLOGY 



both material and dynamic (i.e., energetic). And further this 

 organization is definitely related to the structure of the future 

 embryo and adult, having reference, but not resemblance, to the 

 adult. The organization of the cytoplasmic part of the germ 

 is itself a condition which develops (epigenesis) under the 

 influence of the primary structure, or organization, of the 

 nucleus. At present this inherited organization or predelinea- 

 tion of the nucleus seems primary and fixed, and to represent 

 the only strictly predelineated portion of the germ, controlling 

 and directing the later and epigenetic developmental processes, 

 which may be said often to have commenced in the germ cells 

 even before syngamy has occurred. But history warns us 

 against believing that this organization of the nucleus will 

 prove the ultimate organization. As knowledge becomes more 

 complete this will be thrown farther back to restricted elements 

 of the nucleus; indeed it seems probable now that the primary 

 organization concerns, not the entire nucleus nor perhaps its 

 chromosomal elements alone, but some, as yet invisible, 

 problematic, chemical and physical configurations of its 

 structure. 



But we must not search for an explanation of the whole 

 process of development, alone in the structure of the germ 

 cells. We must look upon development as upon other forms 

 of activity in living things, as a succession of reactions on the 

 part of the organism to the normal stimuli of its surroundings. 

 The things that an adult organism does are obviously reactions; 

 it reacts to the conditions of its environment by making certain 

 movements, forming certain substances, undergoing certain 

 structural modifications; in short, by doing certain things 

 collectively termed its behavior. The precise character of an 

 animal's behavior is determined not alone by its structure, 

 by the organs it has to react with, nor alone by its physiological 

 condition at the time, nor alone by the nature of the external 

 conditions acting; but by all of these combined. What the 

 adult organism does at any particular moment is therefore 

 determined by two interacting sets of conditions, one within 

 the organism its organization, the other without the organism 



