428 THE POPULAR SCIENCE MONTHLY 



E a factor for the extension of black or brown but not of yellow. 



Plate found that all of these factors except the last, E, are also 

 involved in the production of the coat colors of mice. Baur has recog- 

 nized more than twenty different factors for the color and form of 

 flowers in the snap-dragon, Antirrhinum. 



These factors are probably complex chemical substances which pre- 

 serve their individuality in various combinations, just as groups of atoms 

 or radicals do in chemical reactions; they may be dropped out or added, 

 substituted or transposed, just as chemical radicals may be in chemical 

 compounds. To this extent they maintain continuity and indepen- 

 dence, but they are not absolutely independent, for they react upon one 

 another as well as to environmental changes, so that the characters of 

 the developed organism are the results of all these reactions and inter- 

 actions. 



Inheritance Factors and Germinal Units 



It is obvious that there must be things in germ cells which corre- 

 spond to the inheritance factors ; furthermore, these things must be ma- 

 terial particles even though they be only atoms or molecules and their 

 combinations or dissociations. And yet there are many students of the 

 phenomena of heredity who know little about germ cells and to whom 

 all parts of a cell are hypothetical structures, to whom "chromosomes 

 are articles of faith," and who protest rather violently against any at- 

 tempt to find the factors of inheritance in any of the structures of the 

 germ cells. And yet it is perfectly evident that if there are inheritance 

 units they must exist in the germ cells as discrete particles, even if they 

 are only molecules, by whose associations or dissociations in response 

 to intrinsic or extrinsic conditions the various characters of the devel- 

 oped organism arise. It is certainly legitimate to ask what the germi- 

 nal elements are which correspond to inheritance factors. 



There was a time when the cell was the ultima thule of biological 

 analysis and when the contents of cells were supposed to be " perfectly 

 homogeneous, diaphanous, structureless slime." Then the nucleus was 

 discovered within the cell, then the chromosomes within the nucleus, 

 then the chromomeres within the chromosomes, and there is no reason 

 to suppose that organization ceases with the powers of our present 

 microscope. With every improvement of the microscope and of micro- 

 scopical technique, structures have been found in cells which were un- 

 dreamed of before, and it is not probable that the end has been reached 

 in this regard. We know that cells contain nuclei and chromosomes 

 and chromomeres, centrosomes and plastosomes and microsomes, and 

 we know that some of these parts differ in function as well as in struc- 

 ture. And there is no reason to doubt that if we had sufficiently power- 

 ful microscopes we should find still smaller and smaller units until we 

 came at last to molecules and atoms. 



