SUBSTRATUM AND MORPHOGENIC SUBSTANCES 447 



skin. If through extirpation of the pituitary gland the number of chromato- 

 phores has been diminished, and if subsequently the pituitary hormone is 

 experimentally again introduced into such an animal, either through trans- 

 plantation of hypophyseal gland tissue or through injection of the active sub- 

 stance, the skin of the black race responds more readily with the new forma- 

 tion of pigment cells than the skin of the white race. But under these conditions 

 injection of hormone or transplantation of hypophyseal tissues does not en- 

 tirely restore the normal characteristics of the skin, the number of new pig- 

 mented spots remaining smaller in the hypophysectomized than in the normal 

 individuals of the black race of Axolotls. The essential point, however, is 

 that there is no noticeable difference between the action of hypophysis of the 

 white and of the black race, both being about equally effective. The dis- 

 tinguishing features in the pigmentation of these two races depend upon 

 conditions inherent in the structure of the skin; after transplantation of 

 skin from the white to the black Axolotls, and vice versa, the transplants 

 retain their race characteristics. Therefore, factors inherent in the substratum 

 on which the hormone acts primarily determine the pigmentation of the skin. 

 On the other hand, if through transplantation of an excess of hypophyseal 

 tissue into an Axolotl belonging to the white race the quantity of hormone 

 action on the substratum is much increased, then also the skin of the white 

 race can be converted into black skin. The conclusion may then be drawn that 

 the threshold of hormone action necessary to call forth production of pigment 

 cells is greater in the white race than in the black race, and that correspond- 

 ingly more hormone is needed in the skin of the former to obtain the same 

 amount of pigmentation as in the black race. In this case we have to deal 

 with an example of the second type of specificity, the organismal specificity. 



In various classes of animals the skin of the same individual may be white 

 in certain areas, while in others it is black ; here, also, the coloration depends 

 not upon differences in the activity of the hypophysis but upon differences 

 inherent in the skin ; and again, the threshold in the reaction to pituitary hor- 

 mone differs in the pigment cells in different areas of the skin. Thus Blacher 

 has shown that after extirpation of the hypophysis the pigment contracts first 

 in the chromatophores of the abdominal skin, next in the chromatophores of 

 the tail and dorsal skin, and lastly, in the corresponding cells in the skin of 

 the head. As a result of the contraction a whitening of the skin takes place. 

 Corresponding to this difference in the reactivity of the chromatophores is the 

 greater tendency of the skin of the head to be black, than of the skin else- 

 where ; the same difference between the different areas of the skin is found in 

 the black as well as in the white races ; also in the latter the skin of the head 

 has the greatest tendency to assume a black color under the influence of 

 the hypophyseal hormone. While thus the differences in the behavior of 

 pigment cells in different areas of the body are of the same type in the 

 white and black races, the threshold of hormone action necessary to cause 

 expansion of the pigment and call forth a new formation of chromatophores 

 differs in the two races. 



Blacher and Ritter assume that the differences in the reaction of these cells 



