248 THE POPULAR SCIENCE MONTHLY 



specific structures in the abnormal location. These experiments 

 strongly suggest, even if they do not rigorously prove, that such sub- 

 stances are essential ingredients in definite developmental processes. 



I am indebted to Dr. Eiddle for the following illustration: The 

 various colors of mammals, such as black, brown, red, yellow, are due 

 to chemical substances known as melanins. The chemistry of these 

 substances starts out from a simple colorless base or chromogen, from 

 which the series of colors, yellow, red, brown, black, is derived as 

 successive stages of oxidation. The chromogen base is found in all 

 mammals; the color then would appear to be due to the varying powers 

 of the cells of different individuals to oxidize the given base. Tornier 

 has shown in his experiments on the coloration of Amphibia that the 

 particular color developed is a function of nutrition, varying in the 

 order of oxidation value (as was later ascertained) according to the 

 degree of nutrition. The development or inheritance of color, there- 

 fore, can certainly not be due to the presence of black or brown or red 

 or yellow determinants in the germ, assumed for theoretical purposes 

 by some students of heredity, but to a specific power of oxidation of the 

 protoplasm. This faculty in its turn is no doubt capable of resolution 

 into other physiological terms. 



We are only at the beginning of the study of correlations of em- 

 bryonic metabolism. The role that the internal secretions of the 

 embryo may play in the processes of development is practically unknown ; 

 but we may expect to find here biological reactions of fundamental 

 significance, especially when we consider such phenomena of the adult 

 as the influence of pregnancy on the organism, the possibility of in- 

 ducing lactation, with all that this implies, by injection of fcetal tissues; 

 the relations between the sex organs and secondary sexual characteristics 

 and indeed the entire habitus of the organism; the influence of a small 

 gland like the thyroid, or the pituitary body, etc. Biochemical reaction 

 runs through every phase of development and is unquestionably the 

 decisive factor in the appearance of many characters of the organism. 



Self-differentiation. — The conception of self-differentiation in mor- 

 phogenesis is a vague and unsatisfactory one. In a sense it is a contra- 

 diction in biological terms, for assuredly environment enters into every 

 biological process. On the one hand, the term covers the fact of the 

 specificity of primordia, which means only a certain stability of metab- 

 olism and reaction capacity; on the other hand, it may have specific 

 meaning in one large class of developmental phenomena, viz., polariza- 

 tion and localization. If, for instance, the term self-differentiation 

 might be applied to the appearance of definite axes, angles, points and 

 faces of a crystal, it would with equal propriety be applicable to the 

 appearance of polarity, bilaterality, etc., the axes of embryonic develop- 

 ment. But if the term should come to hold simply this restricted 

 meaning, then all reason for its maintenance would be gone. 



