THE CELLULAR BASIS OF HEREDITY 233 



are clue to a specific organization of the germ rather than to specific 

 stimuli. 



Why does one egg give rise to a chicken and another to a duck, or a 

 fish, or a frog? Why does one egg give rise to a black guinea-pig and 

 another to a white one, though both may be produced by the same 

 parents ? Why does one child differ from another in the same family ? 

 Why does one cell give rise to a gland and another to a nerve, one 

 to an egg and another to a sperm? If these differences are not due 

 to environmental causes, and the evidence shows that they are not, they 

 must be due to differences in the structures and functions of the cells 

 concerned. 



Many differences in the material substances of cells are visible, and 

 many more are invisible though still demonstrable. These differences 

 may not be detectable by chemical or physical tests, and yet they may be 

 demonstrated physiologically and developmentally. The most delicate 

 of all tests are physiological, as is shown by the Widal test in typhoid 

 fever, the Wassermann reaction in syphilis, the reactions of immunized 

 animals to different toxines, etc. Lillie has recently shown that egg 

 cells give off a substance which he calls fertilizin, which can be detected 

 only by the way in which spermatozoa react to it. No chemical or 

 physical test can distinguish between the different eggs or spermatozoa 

 produced by the same individual, but the reactions of these cells in 

 development prove that they are different. Undoubtedly chemical and 

 physical differences are here present, but no chemical methods at present 

 available are sufficiently delicate to detect them. The developmental 

 test proves that there must be as many kinds of germs as there are dif- 

 ferent kinds of individuals which come from germs. It is one of the 

 marvelous facts of biology that every individual which has been pro- 

 duced sexually is unique, the first and last of its identical kind, and 

 although some of these individual differences are due to varying environ- 

 ment, others are evidently due to germinal differences, so that we must 

 conclude that every fertilized egg cell differs in some respects from every 

 other one. 



But are there molecules and atoms enough in a tiny germ cell, such 

 as a spermatozoon, to allow for all these differences? Miescher has 

 shown that a molecule of albumin with 40 carbon atoms may have as 

 many, as one billion stereoisomers, and in protoplasm there are many 

 kinds of albumin and proteins, some with probably more than 700 

 carbon atoms. In such a complex substance as protoplasm the possible 

 variations in molecular constitution must be well-nigh infinite, and it 

 can not be objected on this ground that it is chemically and physically 

 impossible to have as many varieties of germ cells as there are different 

 kinds of individuals in the world. 



Even with regard to morphological elements which may be seen with 

 the microscope it can be shown that an enormous number of permuta- 



