464 On the Structural Constituents of the Nucleus, etc, 



manifested during ontogeny. Of course, in calling them primordia, I only 

 mean to imply that they are the agents that determine the particular 

 sequence of chemical changes which shall occur in the unstable cytoplasm. I 

 do not mean that they are themselves the characters in parvo. The very 

 fact that, as agents, they may persist in a dormant state, and thus fail to 

 excite the appearance of the characters appropriate to them, as may happen 

 in the concurrent presence of dominant and recessive allelomorphs, suffi- 

 ciently emphasises this. For as long as the course of metabolism can be 

 completely impelled by one of the two allelomorphs, the influence of the 

 other will remain latent. We may, perhaps, find an analogy in the behaviour 

 of Penicillium, which, although it is provided with a powerful battery of 

 ferments, does not indiscriminately attack all the nutrient materials of the 

 substratum when plentifully provided with suitable carbohydrates. 



Such an example serves also to illustrate another aspect of the problem, 

 and one that perhaps often meets with less attention than it deserves. For 

 admitting that there are good grounds for assigning the production of 

 hereditary characters to the action of discrete bodies, we are still without an 

 explanation of the obvious facts of functional correlation and adaptation. The 

 environment, in the widest sense of the term, often determines which of the 

 many potential characters of a cell shall actually develop. 



We know something as to the manner in which this may be achieved in 

 certain instances. For example, nutritional, as well as other and more 

 obscure, stimuli were found by Klebs to determine the particular one out of 

 several alternative courses of development which was followed in the case of 

 certain algae. And again, the Hormones of Starling may be quoted as 

 types of substances, apparently of relatively simple chemical structure, 

 which serve as excitors of specific cellular activities in the complex animal 

 body. 



The conceptions of primordia which are responsible for the appearance of 

 hereditary characters on the one hand, and also of specific exciting substances 

 on the other, seem to be both necessary, and the one to supplement the 

 other. 



Every organism and every cell has larger potentialities than are ever 

 realised in any single ontogeny. The hereditary mechanism imposes the 

 limits within which development can take place ; but within those limits other 

 conditions may determine the path actually followed. 



