7i6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



in volume or weight, to which comparatively little attention has been 

 given in studies of growth : that is, negative growth or reduction. Growth 

 as increase in size or weight is not irreversible. In the absence or inade- 

 quacy of nutritive material or other substances contributing to increase, 

 or under physiological or pathological conditions that increase catabolism, 

 reduction in size and in weight of the organism may occur, and this 

 may also be differential as regards parts of the body. In the warm- 

 blooded animals reduction soon results in death, but cold-blooded verte- 

 brates can undergo much greater reduction; and some of the lower 

 invertebrates — for example, planarians — can undergo reduction by 

 starvation to a minute fraction of the original size and weight and still 

 remain active and in good condition, and even capable of reconstitution. 

 When feeding is resumed, positive growth begins, and the original size 

 may be again attained. In planarian starvation the digestive tract under- 

 goes reduction more rapidly than other organs, with progressive disappear- 

 ance of its branches from their tips basipetally. In absence of digestive 

 function this organ serves, to a greater extent than any other, as a source 

 of nutrition. Even in vertebrates absence of function in a differentiated 

 organ — muscle, for example — results in its reduction, often in its complete 

 disappearance. 



Perhaps, then, growth in living organisms should be regarded as in- 

 cluding not merely increment but both increment and decrement of sub- 

 stance, as transfer of substance either to or from an organismic system 

 as a consequence of protoplasmic constitution and activity. 



GROWTH AND DEVELOPMENT 



The relation of growth to development is by no means uniform, either 

 spatially or chronologically. An organism may undergo extensive develop- 

 ment without growth increment of the whole, or even during reduction, 

 as in planarian reconstitution during starvation and in early stages of 

 embryonic development of most animals. Certain regions of the reconsti- 

 tuting individual grow at the expense of others, and in the embryo proto- 

 plasm grows at the expense of nutritive material; but the total result in 

 both cases is reduction. The histological differentiation of cells and 

 growth, associated with cell division and synthesis of protoplasm, appear 

 to be more or less mutually exclusive. When cells undergo visible histo- 

 logical differentiation, they usually cease to divide and very commonly 

 cease to grow or grow but little or in an entirely different way from the 

 dividing cell, perhaps by swelling or by increase in size of vacuole, as in 



