THE MUTATED GENE 213 



time and has then already produced size differences in the 

 different stocks. Then growth of the individual cells begins and 

 also lasts relatively longer in the larger races, thus producing 

 larger cells. In this case, then, both phases of growth are deter- 

 mined together at an early stage (as opposed to the quoted case 

 in Drosophila). A similar result was also derived by Sinnott, 

 Houghtaling, and Blakeslee (1934) for polyploids and trisomies 

 of Datura, where it is concluded that some genes control increase 

 in cell size, others increase the rate of cell division. Somewhat 

 different is the case of inherited size differences in Lymantria 

 dispar (Goldschmidt, 1932d, 1933a). Here the growth by cell 

 division starts with eggs of the same size in races of different 

 genetic size, the differences of which are controlled by a number 

 of genes. As is indicated by the growth curves of the larval 

 stages, cell division is faster from the beginning in the larger 

 races (with the addition of sexual differences between the large 

 females and small males). This agrees with the findings of 

 Castle and Gregory (1929) in rabbits of different hereditary 

 size. But in Lymantria there is also an additional growth by 

 increase in cell size, as it was proved that size of spermatocytes 

 and of wing scales — two types of cells with considerable post- 

 division growth— increases in proportion to inherited body size. 

 But the same increase of size of these cells is also found in larger 

 plus variants of a small race. The nonhereditary factors, then, 

 act in the same direction upon cell size. This might mean — 

 though not necessarily — that the second phase of growth is 

 dependent upon actual size reached at time of pupation which in 

 all cases will be followed by a definite percentage cell growth. 

 These few examples show that growth of a body or organ may be 

 determined by genie control of one primary process of rate of cell 

 division, possibly by the production of growth hormones at 

 definite times and in definite quantities; further, by control of 

 the secondary process of increase in cell size (in plants known to 

 be under auxin control); further, by control of the relative times 

 of beginning and ending of both processes and also of their 

 relative intensity. The integrated effect of such of these ele- 

 ments as may be present in the individual case is the measured 

 linear growth. 



Now to the problem of this chapter — the pattern of form by 

 differential growth and its eventual explanation by a single or a 



