288 SHULL— GERMINAL ANALYSIS [April 23, 



common properties, and thus finally for an approximation to the 

 nature of the genes which determine them. 



The most hopeful directions of approach in the effort to learn 

 more of the true inwardness of the unit-characters, are those of 

 chemical analysis and experimental cytology. As applied to unit- 

 characters, these are almost untouched fields at present, though sev- 

 eral investigators have made a beginning. Miss Wheldale, espe- 

 cially, has made a hopeful beginning in the investigation of the 

 chemistry of anthocyanin colors which have continually exhibited 

 typical Mendelian behavior. Several unit-characters which have 

 been recognized and described heretofore only in terms of color- 

 factors, now seem to be capable of description in terms of a chro- 

 mogen (present in all sweet peas and stocks investigated), and of 

 activators, peroxidases, peroxides, and reducers, thus making the 

 various colors " the result of definite oxidation stages of the chromo- 

 gen." Riddle has come to much the same conclusion in regard to 

 the nature of the melanin colors, from a consideration of the work 

 of Bertrand, Gessard, Spiegler and others. 



In experimental cytology there seems to have been nothing done 

 as yet, which can throw light on the nature of those unit-characters 

 involving the structure and size of parts. How are the number 

 and direction of cell-divisions that shall take place in any cell-lineage 

 determined? Are these also referrable to the presence of definite 

 chemical substances or to definite configurations of protoplasmic 

 molecules? To these questions I believe no satisfactory answer is 

 now possible, but that these processes are controlled in many in- 

 stances by characteristics possessed by the germ-cells, rests upon 

 aboundant experimental evidence. 



While waiting for further information from the chemist and the 

 cytologist, there is still abundant room for the work of the experi- 

 mental breeder. Owing to the characteristic distribution of the 

 genes at the time of germ-cell formation already described, Men- 

 delian hybridization provides a partial analysis of the germ-plasm, 

 and thus gives some insight into the constitution of living proto- 

 plasm. It is of great importance that such analysis be continued 

 until all the unit-dift'erences of plants and animals have been studied. 



