22 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



parts of the body assume definite forms and sizes? 

 Why does the skin of the human thumb assume mark- 

 ings so sharply defined as to be in a singular degree 

 characteristic of the individual sufficiently so, 

 indeed, to serve in fixing the identity of criminals? 

 How is it that the cells forming the nose of a child 

 arrange themselves into an organ closely resembling 

 in every detail that of a parent, instead of assuming 

 a quite different form and size ? The clew to such 

 questions is doubtless to be sought in properties 

 possessed by the well-defined rod-shaped masses 

 of matter in the nuclei of the germ cells at the time 

 of fertilization, which are known as chromosomes. 

 These are now generally recognized to be at least an 

 important factor in hereditary transmission, as will 

 be pointed out in discussing the law of Mendelian 

 heredity and the recent developments of biological 

 science in relation to sex. But none of the facts 

 now known to us shed any real light on the way in 

 which the forces of regulation are operative in the 

 growing cells. The instances of regulation of growth 

 which I have just cited are sufficiently puzzling. 

 But they are at least referable, as already stated, 

 to an hereditary mechanism of some sort. There 

 are, however, still more enigmatical cases. I will 

 cite the case of the regeneration of the lens of the 

 eye in the tadpoles of salamanders, a case which 

 has been assumed by some biologists to be incapable 

 of mechanical explanation. If we remove the crys- 

 talline lens from the eye of the young salamander 



