212 PHYSIOLOGICAL GENETICS 



determination of growth. Before analyzing differential growth, 

 we may review shortly the pertinent genetic and other facts 

 concerning growth in general. In animals, as well as in plants, 

 the size of an organ (or the whole body) depends upon two proc- 

 esses, cell division and increase in cell size. Usually these follow 

 each other in individual development ; i.e., at a certain time, cell 

 division ceases, and growth of cells begins. There are many 

 examples, one of them found in the butterfly wing where epidermal 

 cell division ceases when the formation of scales with corre- 

 sponding cell growth begins. In the Drosophila wing, we have 

 seen that cell growth begins after the structure of the wing is 

 completed. The same two processes are found in the develop- 

 ment of a bird's feather or in the growth of a fruit. The most 

 extreme case is found in such cell-constant animals as the 

 nematodes (Goldschmidt, Martini), the Rotatoria (Martini), and 

 the Acanthocephala (Van Cleave) in which a limited number of 

 cells composes all organs, cells that might grow to an immense 

 size without further division. Genetically, the two consecutive 

 types of growth may be controlled by different genes. It is 

 known that in Drosophila the wing-mutant Miniature prevents 

 only the cell growth following a normal development during the 

 phase of cell division and differentiation (Dobzhansky, 1929; 

 Goldschmidt, 1935c, 1937). The wing mutant Expanded (large 

 broadened wing), however, is produced by a different amount of 

 growth in the period of cell division (Goldschmidt, 1937). It 

 is possible to combine both genes in one individual, with the 

 result that a broad wing is formed in the first phase which, how- 

 ever, does not grow in the second (Csik, 1934a). These facts 

 show that the size produced by growth may be controlled 

 by genes affecting at least these two processes of growth, maybe 

 differentially. But differences in size may also be influenced by 

 other genes acting on growth in general, irrespective of the two 

 phases. It is therefore to be expected that in different cases 

 different relations might be found. To mention some: Hough- 

 taling (1935) has studied fruit size in tomatoes which, according 

 to Lindstrom (1928), is controlled by a number of genes with 

 different effect. He found that in the ovary of the large forms 

 more cells are present and that, in addition, larger cells are found 

 in the larger fruits; i.e., both processes of growth are changed by 

 mutant genes. The first, cell division, is ended about flowering 



