THE REALISATION OF THE NUCLEAR FACTORS 87 



have been formed, and may influence the chemical, physical, 

 morphological, or physiological properties of these cells only. 

 Probably the pigment cells of newts (see p. 81) and the 

 skeleton-building cells of sea urchins (see p. 84) are examples 

 of this kind. In other cases, the substances produced under the 

 influence of certain genes may diffuse from the cells where 

 they originated into the blood. They will then be transported 

 throughout the body, and will be able to influence other parts 

 of the embryo as well. Such substances have been called ''gene 

 hormones''. Their existence has been proved, and their activity 

 studied, in particular in insects (Plagge, 1939; Ephrussi, 1942). 

 Research on this problem has been carried out mainly on the 

 moth Ephestia kuhniella by German investigators (Caspari, 

 Plagge, Becker, and others) in Kiihn's laboratory, and on the 

 fruit fly Drosophila melanog aster by a great number of French 

 and American workers (Ephrussi, Beadle, Clancy, Tatum, and 

 others). Similar results, however, were also obtained in the 

 parasitic wasp Habrobracon juglandis (Whiting, 1932-34) and 

 in the moth Bombyx mori (Morohosi, Kikkawa, Kawaguchi, 

 1937-38). We shall take Drosophila as an example because the 

 greatest progress has been made in this species. 



In Drosophila two mutations occur, known as "vermilion" 

 (v), and "cinnabar" (en), respectively. The eye colour of these 

 mutants is lighter than that of normal individuals. The eyes of 

 the latter are coloured dark red by the combination of a red 

 and a brown pigment. The brown pigment is absent both in v 

 and in en mutants. Both mutants are due to single gene muta- 

 tions. If grafted early into a normal animal, the eye of a 'U'-mutant 

 developed the same dark red colour as is found in the wild form. 

 The same was seen on transplantation of v-eyes into a cn- 

 animal. Eyes of a cn-larva also became dark red when trans- 

 planted into a normal animal. However, they remained light if 

 grafted into a v-animal. The result did not depend upon which 

 particular part of the body the eyes were grafted into. It can 

 be concluded from these observations that the whole body, 

 both of the normal animal, and of the cn-mutant contains a 

 factor, probably a substance, which gives rise to brown pigment 

 in the eyes of the -u-mutant, and which is evidently absent in 



