448 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



depend upon different threshold reactions of the pigment cells to the hor- 

 mone, the amount of hormone needed in order to obtain an effect being differ- 

 ent in the pigment cells in different races, as well as in different areas within 

 the same individual. There remains, however, the possibility that different 

 cells may vary primarily in their ability to attract and to bind a certain amount 

 of the hormone, rather than in the amounts necessary to call forth a reaction. 

 As to the causes of the differences in the behavior of these different types of 

 cells, nothing definite is known, but it may be suggested that a substance is 

 produced within the cell which increases the sensitiveness of the latter to the 

 hormone, a condition analogous to the sensitization to mechanical stimuli 

 which is produced in the uterine mucosa by the lutein hormone. 



We see, then, that the same mechanism applies to a condition of pure 

 organ- or tissue-specificity, and to a condition of combined organismal- and 

 tissue-specificity. 



Analogous are certain findings in adult mammals. Here in women past the 

 menopause the ovary no longer reacts to the stimulating action of pituitary 

 gonadotropic hormones with maturation of follicles and corpus luteum for- 

 mation, although the human anterior hypophysis is still potent (Saxton and 

 Loeb) ; the lack of ovarian responsiveness must, in such instances, be due 

 to changes which have taken place in the recipient organ, the ovary. 



In mammals differences in the reaction of analogous organs to the same 

 kind of hormones have been observed in different species. Thus, for instance, 

 the ovary of the guinea pig, rat and rabbit react quite differently to the same 

 gonadotropic hormones of the pituitary gland and to changes in the constitu- 

 tion of hormones which follow hysterectomy. These differences depend on 

 the structure of the ovaries in these species and in particular on the power 

 of resistance of follicles and corpora lutea to injurious conditions and on the 

 ability of the theca interna to undergo luteinization. In these instances we 

 have to deal with organismal specificities in the reaction of tissues to the 

 same kind of hormones. 



A similar problem as to the relative significance of substratum and stimulus 

 in determining the specificity of the reaction arises in the field of regeneration. 

 Triton is able to regenerate tail as well as anterior and posterior extremities ; 

 anuran amphibia, such as toads, do not possess this regenerative power. In 

 the lizard the condition is intermediate; the tail is able to regenerate, while 

 the posterior extremities regenerate only in a rudimentary way, and the 

 anterior extremities not at all. Weiss transplanted in Triton the regenerative 

 bud from a tail to a cut surface in the anterior extremity, a piece of which 

 had previously been excised. It seemed that the grafted tail material became 

 transformed into a leg under the influence of the leg stump, which thus acted 

 as an organizer and caused the transformation of potential tail material into 

 a limb. In this case evidently the stimulating tissue and not the recipient tissue 

 determined the fate of the tail bud. However, when a similar experiment was 

 carried out in the lizard, where the tail still has the power to regenerate but 

 the anterior extremity lacks it, the transplanted tail bud was not transformed 

 into a leg, because the wound surface of the limb to which it was attached 



