All of these factors — like all other character-forming factors — can be brought under 

 the two headings of: 



(1). Oenotypical (internal) growth factors . determined by hereditary "dispoaition" , 

 These may be characteristic forj 



(a). The species of fish. 



(b). The race of fish. 



(c). The individual fish. 



(2). Paratypical (external) growth factors, i.e. environmental factors. These 

 Trill influence grovrth within the field of heredity and constitutional 

 possibilities. 



With regard to the part played by nutrition, one is entitled to ccaisider separately 

 the influence of external factors during the following periods of development: 



(a). Existence in ovum. 



(b). Nutrition in the vitelline sac (exclusively). 



(c). Nutrition in the vitelline sac (in part). 



(d). Period of free feeding. 



In a general sense, we can therefore formulate the dictum that every external phenomenon, 

 every characteristic — growth in this case — is the functional result of two variables, to witi 

 Internals and Externals. 



In contrast to warm-blooded animals, some fundemental differences in the effects ol 

 these variable factors upon growth are of special importance to the fishbreeder. 



(1). Fish have no "normal size" which maybe spoken of as full size. Their 

 growth is "unlimited", so to speak, and in this respect fish resemble trees and plants more 

 than warm-blooded animals (in regard to growth, be it understood). 



(2). Fish can be greatly retarded and even entirely stopped in their growth — 

 through persistent undernourishment — ^without injury to their existence as well as to 

 their faculty for growth. 



(3). Through no amount of over-feeding can the growth of fish be advanced. 

 The reason for this lies in the already mentioned relations between reproductive anabolism 

 and environmental factors (even here the law of "Needs regulate rate" operates). The 

 increase in weight through the deposit of unorganized reserve materials, such as fat and 

 glycogen, is also negligible, and is not even desirable when production of meaty fish is 

 aimed at. 



Physiologists' (Zentz and Knauthe) have stated, nevertheless, that growing fish may 

 be masted. This observation is based upon the fact that growing fish, usually, do not 

 find sufficient sustenance necessary for the fullest function of reproductive anabolism. 



M, A still further step above and beyond mere standstill in growth is 

 possible in fish: "Negative" growth, viz. excessive loss in weight. The explanation for 

 this lies in the fact — already mentioned — that the necessary calories for proper metabolic 

 functions will be provided, in case of xindemourishmentj at the expense of bodily sub- 

 stance. This, of course, must lead to loss in weight. 



The resistance to starvation, in fish, is great. They can especially withstand it 

 in winter, when their v;ants and needs are few, on account of low v.-ater temperatures. 



