198 THE PHILOSOPHY OF BIOLOGY 



of the constituents but something quite different, 

 whereas in the Mendelian cross the characters remain 

 distinct, but one of them is patent while the other is 

 latent. In the molecule, however, the atoms are 

 regarded by the chemist as lying beside each other 

 in certain positions, and the Mendelian factors are 

 also spoken of as if they lay side by side in the germ- 

 plasm. This terminology is useful, perhaps necessary, 

 in the work of investigation, but we must not forget 

 that it symbolises, rather than describes, the results 

 of experiment. If the factors are identified with 

 certain morphological structures in the nuclei of the 

 germ-cells, obviously all the objections that may be 

 urged against the Weismannian hypothesis as an 

 hypothesis of development apply also to the Mendelian 

 hypotheses as descriptions of a physical process of 

 the transmission of morphological characters. 



It should clearly be understood what is implied 

 in the construction of such a hypothesis. Certain 

 processes are observed to take place when a somatic 

 cell divides : these processes we have regarded as 

 having for their object the exact division of all the 

 parts of the cell into two halves. This process of 

 somatic cell division is modified when a germ cell 

 divides prior to maturation (the process fitting it to 

 become fertilised). Then the cell nucleus divides into 

 four daughter-nuclei. One of these remains in the 

 cell substance which is to become the ovum, and the 

 other three, each of them invested in a minimal 

 quantity of cytoplasm, are eliminated as the " polar 

 bodies." Also the number of chromosomes in the 

 mother-cell becomes halved, so that the mature ovum, 

 or spermatozoon, possesses only one-half of the number 

 of chromosomes which are present in the ordinary 

 somatic cell. Now let the reader puzzle out for 



