NATURE OF BIOLOGICAL RESPONSES 121 



types of agent acting upon the skin of the frog. In the one 

 case, the skin has split between the cornified layer and the 

 prickle cells; in the second case, the skin has split in the 

 middle of the prickle cell layer; and in the third case, 

 the prickle cell layer and the cornified layer have become 

 detached from the dermis. If the action of these three 

 agents had been assessed as either "vesication" or "not 

 vesication", all three would have been assessed as vesi- 

 cants and presumed to act through the same mechanism. 

 But the details of their biological action are in fact so 

 diflterent that it must be concluded that only part of 

 their action at most can be exerted upon a common 

 mechanism. 



From these examples, as from many others which 

 could be adduced, it must be concluded that a cell or 

 tissue is commonly designed to fulfil a particular pur- 

 pose, and that it responds to stimuli of very diverse types 

 in a manner which is characteristic of the cells involved, 

 and not necessarily of the natureof the stimulus. Differ- 

 ent stimulating agents may act upon quite different 

 cellular systems, but the design of a cell is commonly 

 such that these mechanisms are funnelled to result in the 

 elicitation of a response characteristic of the particular 

 design of cell. With this point in mind we can proceed to 

 examine a number of types of biological response in 

 rather more detail. These will include artificial partheno- 

 genesis and mitotic abnormalities, and the responses of 

 genetic systems to drugs. 



