140 GROWTH PRINCIPLES AND THEORY 2 



The more important aspects of the problem of growth can be enumerated as follows : 



1. Growth as synthesis of species-specific substances. Growth signifies, first, the 

 synthesis of high-molecular organic compounds, especially of proteins. This does 

 not merely mean that proteins are synthesized, but proteins are produced in such 

 way that their species, tissue, cell, and even individual specificity is maintained. 



2. Growth as identical reproduction. Among the components of the cell, there are 

 certain systems that show a characteristic not found in inanimate ones, namely, the 

 capability of self-reproduction. These may be termed as "elementary biological 

 units"; genes and viruses are the most important representatives. Besides self- 

 reproduction, these systems show the further characteristic that they direct cell 

 processes and, so far as genes are concerned, particularly direct protein synthesis 

 in the way of the specificities mentioned. According to modern research, self- 

 duplication is essentially connected with nucleoproteins. 



3. Growth as a cellular process. The growth of an organism takes place as a cellular 

 process by cell multiplication (mitosis), increase in cell size, and formation of 

 intercellular substances. The surplus of anabolism over catabolism leads first to 

 an increase of cell size. However, in spite of continuous potency for synthesis, 

 a cell cannot surpass a certain size so that cell division takes place after this size has 

 been reached. 



4. Growth of the organism as a whole. Surveying the development of a multi- 

 cellular organism, a considerable number of isolable component processes can be 

 distinguished (J. Needham, 1942). For the present consideration the following 

 are the more important: (a) Formative movements, i.e. migrations and displacement 

 of cells (gastrulation, invaginations in the formation of germ layers, etc.); — 

 (b) Segregation, i.e. progressive determination and separation of embryonic areas 

 and organ anlagen; — (c) Differentiation, i.e. specialization of the structure and 

 function of cells; — (d) Growth, i.e. enlargement and multiplication of cells, and 

 consequent increase of body size. To a certain extent these processes are independ- 

 ent or can be isolated in experiment. Early embryonic processes (gastrulation, 

 neurulation) take place without increase in size as the embryo does not have an 

 intake of nutrition during this period. Segregation may take place during or late 

 in segmentation (regulation ova) or may have taken place before segmentation 

 (mosaic ova). Cell multiplication may or may not go along with simultaneous 

 differentiation, etc. 



5. Growth as a morphogenetic process. Animal development leads to the establish- 

 ment of certain shapes or forms. In early developmental stages, morphogenetic 

 changes take place essentially by way of formative movements. In later stages 

 they are mainly caused by relative growth, i.e. different growth rates exhibited 

 in various directions or by various components of the body. In this sense the 

 problem of organic form is to a wide extent a problem of growth. 



This viewpoint applies not only to ontogenetic, but also to phylogenetic changes. 

 Evolutionary changes to a wide extent are changes in proportion, i.e. morpho- 

 genetic changes accomplished by change of the rates of relative growth. Hence, 

 many evolutionary changes can be considered as problems of growth. 



6. Factors of growth. Animal growth is determined or influenced by a great 

 number of factors which are either in the organism itself, or in its environment. 



