THE MUTATED GENE 



217 



1927). Such cases, e.g., fruit size, therefore permit a similar 

 treatment, and they are pertinent to our problem when hereditary 

 differential (heterogonic) growth is involved in the form of 

 hereditary control of shape. The problem is then obviously the 

 same as in the Drosophila wing (see p. 212) though somewhat 

 complicated by the tridimensional aspect. Nearer to the 

 Drosophila wing are the shapes of leaves which, as is well known, 



Fig. 39. — Diagrammatic representation of one half of a spread feather germ 

 (collar) of a regenerating feather. The germ has been split along the ventral 

 superficial axis (left) and also in the center of the shaft (right). Barbs and shaft 

 are drawn at right angles to the base of the germ. C v Cd, locus of primary 

 growth by cell division which is extended to the level h ; P v Pj, a locus of simul- 

 taneous pigmentation. Ridges join the shaft at u. (From Fraps and Juhn, 

 1936, Phys. Zool. 9, Fig. 2.) 



are controlled by Mendelian genes in innumerable instances. 

 We have pointed out that size and shape of fruit are controlled by 

 Mendelian genes, sometimes by a single one (tomato, Lindstrom, 

 1928). In Cucurbita pepo (Sinnott, 1927; Sinnott and Ham- 

 mond, 1930), two independent allelomorphs with cumulative 

 effect (cf. the Drosophila case, page 212) control the disklike or 

 spherical shape of the summer-squash fruit. In this case, 

 Sinnott (1935a) made it clear that shape is not a physiological 

 consequence of size but controlled by independent genes, as is 



