cytology: the study of the cell 147 



Thus, the electron microscope is making possible an increasingly detailed 

 analysis of the finer structures of the protoplasm, the general ground sub- 

 stance, as well as the definitive bodies which exist in the cell. Interestingly 

 enough, more success has attended the study of cytoplasm by these means 

 than of the nucleus. Thus far, it has not been possible to prepare nuclear 

 material for electron microscopy without a considerable amount of distortion 

 which tends to obscure the results obtained. 



A third center of interest among cytologists arises from the fact that 

 cytological studies can throw considerable light on the relationships of species 

 and races and can furnish clues as to the paths along which evolutionary 

 progress has been made. The nature of the evidence which cytology is able 

 to present varies with the material. In some cases, a comparative study of 

 chromosome structure will indicate relationship. Chromosomes have defini- 

 tive shapes and sizes, and it is often possible to recognize particular chromo- 

 somes under the microscope. In many genera, it is possible to compare the 

 various species or subspecies from the standpoint of chromosome structure. 

 Species which have similar or identical chromosomes so far as morphology is 

 concerned are considered to be more closely related than species whose 

 chromosomes differ in these regards. Other lines of evidence must of course 

 be brought to bear on the problem, but chromosome structure has often 

 proved to be a characteristic of great value in determining relationships. 



In other cases, relationships can be determined or confirmed by analyzing 

 the structural alterations which have occurred in the evolution of the group. 

 Thus, in certain species of Drosophila, the fruit fly, inversions of chromosome 

 segments have occurred with relative frequency. In some cases, sequences of 

 inversion can be followed, especially where a portion of a previously inverted 

 segment becomes involved in a second inversion. Dobzhansky and his students 

 (da Cunha, 1955; Dobzhansky, 1944) have been able to learn much regard- 

 ing the evolutionary history of certain species by analyzing the inversions 

 which have occurred and determining the sequences of their occurrence. In 

 other organisms, other types of structural alteration have proved of value. 

 For instance, in the evening primrose {Oenothera), the author and his stu- 

 dents (Cleland et al., 1950) have found that exchanges of segments between 

 non-corresponding chromosomes have occurred with unusual frequency and 

 it has been found possible to analyze many races from the standpoint of the 

 interchanges which have occurred. Races which show evidence, at least in 

 part, of the same interchanges are considered, other things being equal, to 

 have been derived from common ancestors in which these interchanges oc- 

 curred. The more closely races resemble one another in respect to the inter- 

 changes which have occurred in the course of their evolution, the more closely 

 related they are considered to be, and vice versa. As a result of cytological 

 and genetical techniques, the evolutionary story of the evening primrose is 



