462 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



glutamine. In birds, and presumably also in reptiles, it combines with 

 diaminovaleric acid (ornithin), the principal endproduct of protein metabo- 

 lism in these classes of animals. Or, to mention another example : while in 

 man and mammals in general, in amphibia and fish the principal endproduct 

 of amino acid metabolism is urea, in birds and reptiles the principal end- 

 product of protein breakdown is uric acid. 



Individual differences in the electric potential of the grey matter of the 

 brain, originating presumably in the structure and function of the ganglia 

 cells, are found when electric currents are obtained with electrodes placed on 

 different parts of the skull or brain surface (Hallowell Davis). A similar 

 individualization in electric potentials also exists in different parts of the 

 eye, and the totality of such potential gradients in the adult and in the 

 embryo seems to be characteristic of different species. Similar findings may 

 be obtained presumably in every organ and every tissue, and we may assume 

 that at least in the higher organisms it might be possible not only to discover 

 species and strain differences in all the organs and their functions, but also 

 individual differences, in accordance with inherited constitutional character- 

 istics, if only we had fine enough methods to recognize them. 



In a general way, such organ and tissue differences parallel, in their de- 

 velopment, the phylogenetic evolution of these species, but there are many 

 exceptions to this rule. Two examples in which a strict parallelism does not 

 exist may be mentioned, namely ( 1 ) the substances which control the expan- 

 sion and contraction of the melanophores of the skin, become effective in 

 some classes of animals mainly through the nervous system, when they 

 function as neurohormones ; in other classes, through the blood, when they 

 function as ordinary hormones. Sex determination depends, in part, on the 

 distribution of two chromosomes in male and female in two possible ways ; in 

 vertebrates as well as in insects, these two modes of distribution are found 

 irregularly present, without reference to phylogenetic relationship ; likewise 

 the means of control of the state of contraction of the chromatophores are 

 irregularly distributed. 



The organism consists therefore of a mosaic of organs and tissues; but 

 the units in this mosaic are subdivided again into smaller units and thus the 

 mosaic is really much finer than it might appear if only gross methods of dif- 

 ferentiation are used. It is by means of a more detailed microscopic examina- 

 tion and a study of the mode of reactions, of different tissues to various 

 hormones that very fine subdivisions are revealed, as for instance, in vagina, 

 cervix and uterus of the guinea pig ; and this is true of connective tissue as well 

 of epithelial structures. As mentioned already, in tissue cultures of various 

 embryonal structures R. C. Parker has shown that fibroblasts derived from 

 different organs behave differently in regard to rapidity of growth, produc- 

 tion of acid, as well as solution of fibrin, and these characteristics remain 

 constant in vitro, although, on the whole, they may change with advancing 

 development. The existence of definite units constituting the organism is also 

 indicated by the study of inheritance of organ characteristics in accordance 

 with Mendelian principles. Furthermore, those factors whose development is 

 controlled by gene-hormones in various insects represent mosaic character- 



