THE METHODS AND SCOPE OP GENETICS 9 



as nearly as may be by cytological research. Moreover, the separation 

 of homologous contributions of the parents in the formation of germ cells 

 and the union of two homologous sets of hereditary elements for the 

 production of new individuals represent another phase of the problems 

 which have been solved by cytological research. 



Although obviously the dangers of misinterpretation in dealing with 

 fixed and stained preparations of cells or sections of cells are very great, 

 a fact which is disclosed by the diverse interpretations which different 

 investigators have given of the same phenomena and structures, never- 

 theless the importance of this field of research should not be under- 

 estimated on that account. There are several reasons for reposing confi- 

 dence in the results of cell investigations, and these come from two 

 sources; from the confirmations of the growing field of what may be 

 called experimental cytology, the observation of cell phenomena directly 

 in living cells, and from the broad general result of cytological research 

 that the mechanism which has been discovered is by nature such an one 

 as might be expected from a priori consideration of the results of 

 Mendelian investigations. The close correspondence which exists 

 between cell behavior as it is believed to exist from cell investigations 

 and hereditary phenomena as they are known to exist from Mendelian 

 investigations has given renewed confidence to students of heredity in the 

 validity of their interpretive conclusions. 



The most important progress which has been made within the last 

 decade in genetic science has been that of interpreting Mendelian phe- 

 nomena of inheritance in terms of the behavior of the cell mechanism. 

 Thus far this work has been carried to any degree of completeness in 

 only one species, the common fruitfly, Drosophila ampelophila. In the 

 extensive investigations which have been made with this species, Morgan 

 and his associates have demonstrated how close a correlation exists all 

 along the line between cell behavior and hereditary distribution of 

 characters. Certain characters are distributed independently of each 

 other, the pairs of chromosomes separate independently of each other in 

 the formation of gametes; certain characters display irregularities in 

 distribution and expression associated with differences in sex, the chromo- 

 some content of the two sexes is demonstrably different; four sets of 

 characters exist the members of which tend to remain together in trans- 

 mission in the combinations in which they occurred originally, the entire 

 chromatin material is contained in four pairs of chromosomes; and 

 finally irregularities in character distribution have been discovered, the 

 chromosome constitution and distribution in such cases are correspond- 

 ingly irregular. These facts the student will be better fitted to appreciate 

 later on; they are given here to show how the results of one method of 

 investigation are supported and strengthened by those of other methods. 



