THE METHODS AND SCOPE OF 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@ prior? 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. 
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