170 Heredity and Environment 



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 dif- 

 ferences 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- 



Protoplasm Specific. — Many differences in the material sub- 

 stances 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 Weidal test in typhoid 

 fever, the Wassermann reaction in syphilis, the reactions of im- 

 munized animals to different toxins, etc. Lillie has recently 

 shown that egg cells give off a substance which he calls "fertil- 

 izin," which can be detected only by the way in which spermato- 

 zoa react to it. No chemical or physical test can distinguish be- 

 tween 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 dif- 

 ferences are here present but no chemical methods at present 

 available are sufficiently delicate to detect them. 



The developmental test indicates that there are as many kinds 

 of oosperms as there are different kinds of individuals which come 

 from oosperms. It is one of the marvelous facts of biology- that 

 practically every sexually produced individual is unique, the first 

 and last of its identical kind, and although some of these individ- 

 ual differences are due to varying environment, others are evi- 

 dently 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, 



