446 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



in the reactions of epidermal tissues. Another instance of differences in the 

 constitution of an apparently homogeneous tissue has already been noted; 

 it was shown that different areas of skin of amphibian embryos exhibited dif- 

 ferent degrees of responsiveness to the contact action of the optic disc, some 

 areas possessing, others lacking the ability to form a lens. 



In agreement with these conclusions is the observation that fibroblasts ob- 

 tained from the connective tissue of different areas of the embryo may behave 

 differently when cultivated in vitro (R. C. Parker). They differed in their ra- 

 pidity of growth, in the amount of acid produced, and in their power of resist- 

 ance to injurious conditions, and these differences were permanent in certain 

 strains of fibroblasts and seemed to be inherent in the cells. Not only were 

 variations found in these respects between periosteal, perichondral and ordi- 

 nary connective tissue cells, but even between connective tissue cells taken 

 from the stroma of various organs. Although in these cases we have to deal 

 with lower organisms and with not yet fully differentiated embryonal or 

 larval instead of with adult tissues, it is evident from our findings in the 

 uterus that in principle the same condition holds good also in the case of 

 adult mammalian organisms. 



We may then conclude that the differentiation of tissues is in reality much 

 furthergoing than has been assumed on purely morphological grounds. Fur- 

 thermore, the possibility must be considered that the contact substances, and 

 in certain cases perhaps also the hormones, given off by tissues which are 

 morphologically indistinguishable from one another, may correspondingly 

 differ. 



More recent studies of various authors prove the still wider applicability 

 of this mode of experimental analysis of the specific character of certain 

 tissues in embryonal development, as well as in adult organisms. By these 

 means Ritter and Blacher have studied the cause of the differences in pig- 

 mentation which are observed in two races of urodele amphibia, the black 

 and white Axolotl, and in different areas of the skin of the same individual 

 Axolotl. 



The white and black races of Axolotl differ in the proportion of the pig- 

 mented and unpigmented parts of their skin; in the former the white, and 

 in the latter the black color predominates. Now, it is known that in the 

 hypophysis there is produced a hormone which causes a black coloration of 

 amphibian skin by inducing the expansion of the chromatophore pigment and 

 also by increasing the number of these pigment cells. The question arose, 

 therefore, as to whether the inherited difference in the behavior of the skin 

 of the white and black Axolotls might be due to inherited differences in the 

 amount of hormone produced by the pituitary glands of these two races, or 

 whether it was due to differences in the recipient skin. Experiments by E. 

 Ritter have shown that the second interpretation is correct, no difference being 

 noticeable between the hypophysis and its pigment-regulating hormone of 

 the black and the white Axolotls. The difference between these two races con- 

 sists not only in the greater number of pigmented cells in the black as com- 

 pared with the white race, but also in the reactivity of these two kinds of 



