286 EDUARD UHLENHUTH 



spot and (4) the number of spots. As a rule in one individual 

 all four characteristics are developed in the same direction; thus, 

 almost uniformly black animals are produced at one end of the 

 scale, while very bright, almost orange yellow spotted animals are 

 found at the other end. 



If it were true that this second factor belonged to the specific 

 properties of the skin and was specific for the individual, we 

 should expect that a piece of skin grafted from an individual with 

 the 'almost orange yellow' type of spots, to an 'almost black' 

 animal would nevertheless develop on the 'almost black' host, 

 'almost orange yellow' spots, for the 'almost orange yellow' 

 type of the factor should have been carried within the graft to 

 the new host. That such is actually the case is shown by some 

 of the experiments of series XXV. It is particularly well ex- 

 pressed in experiments 11-12, and 21-22. Since in each pair 

 both grafts were taken from the same individual we should ex- 

 pect that if the spot developed by one graft is 'almost orange 

 yellow,' the spot of the other graft — if this graft developed a 

 spot at all — should be 'almost orange yellow' too; and this is 

 what actually happened in these two pairs. A similar result 

 was obtained in experiment 33 and experiment 45. 



In a similar way must we look upon experiments 13, 28, 45, 

 11-12 in regard to the behavior of the hosts. In these experi- 

 ments the hosts were of the black variety. This variety not 

 only is almost bare of yellow pigment, but develops the few faint 

 spots much later — if at all — than the majority of the individuals 

 of this species, and at a time when metamorphosis is almost 

 complete. In these individuals the appearance of the yellow 

 spots cannot be said to indicate that the metamorphosis of the 

 skin has started since the spots appear when it is almost finished, 

 in regard to the rest of the adult characteristics of the skin. If 

 to such an individual a normal piece of skin is grafted it will 

 react in a normal way to the action of the metamorphosis factor 

 and the spots will appear on it ; while the host will not react in a 

 normal way and no spots will appear, or only much later if at 

 all, since the second factor necessary for the development of the 

 yellow spots, being a specific factor contained in the skin itself, 



