PATTERNS OF DEVELOPMENTAL PROGRESS 715 



tion of any other parts in the intact animal than those completely domi- 

 nant from the beginning, such as the hydranth region of a hydroid, may 

 at least be questioned. 



GROWTH : THE QUESTION OF DEFINITION 



In any consideration of the many problems involved in the changes that 

 biologists have called "growth" in living organisms, the question of defini- 

 tion must be raised. Many definitions of "growth" have been given, and 

 the term has often been used without definition for various features of 

 development. There is still no general agreement as to what constitutes 

 growth. For some biologists growth is increase in size or weight. Such 

 increase may occur in many ways — by increase of living protoplasm ; by 

 increase of fat, or in plants of starch; by swelling of protoplasm or other 

 cell constituents in consequence of absorption of water; by increase in 

 size of cell vacuoles; and by deposition of products of metabolism — chitin, 

 keratin, cellulose, mineral salts in various structural forms, skeletons, 

 shells, etc. Some of the plant physiologists have maintained that growth 

 involves change of form. For example, Sachs (1887) defined growth as 

 increase in volume intimately associated with change of form. According 

 to Pfeffer (1901), growth is permanent change of form in the protoplasmic 

 body, and increases in volume or mass are not correct criteria of growth. 

 Thompson (1917, p. 52) says: "The transference of portions of matter 

 into the system from without and from one widely distant part of the 

 organism to another" is what is usually regarded as growth. He points 

 out, further, that on the basis of this concept of growth the modifications 

 of form in organisms depend essentially on difference in rates of growth, 

 in different directions, except in so far as purely molecular forces are con- 

 cerned. In his study of relative growth Huxley (1932, p. 6) says: "One 

 essential fact about growth is that it is a process of self-multiplication of 

 living substance." Elsewhere (p. 149), however, he distinguishes the "mul- 

 tiplicative, intussusceptive or compound interest type of growth," in 

 which the increment of material is alive and contributes to further growth 

 and "the additive, accretionary or simple interest type," in which the in- 

 crement consists of nonliving material that makes no further contribution 

 to growth. It is obvious that the additions of different kinds of materials 

 take place under different conditions and involve different activities. If 

 growth includes all additions, the physiology of growth presents not one 

 but many problems. 



There is still another aspect of protoplasmic activity involving change 



