xvr] COMPARATIVE CYTOLOGY 261 



objects or artefacts; in general the technique is largely 

 empirical, and could possibly be improved if this side of 

 the subject were better known. Such examples could be 

 multiplied indefinitely, and all indicate that the purely 

 descriptive cytology of the past must give place to a more 

 experimental science in the future, and that from the union 

 of the two methods of work will be born the fuller know- 

 ledge for which we hope. 



Lastly, further investigation is desirable in a direction 

 quite different from the lines suggested above, and in a 

 branch of cytology that has been scarcely mentioned in the 

 preceding pages. At the present time the science of Com- 

 parative Cytology can hardly be said to exist, and yet, when 

 the great results are borne in mind that have been obtained 

 from the study of comparative anatomy of animals and 

 plants, it can hardly be doubted that there is an important 

 future for the comparative study of the cellular phenomena 

 in the different groups. Perhaps the chief reason for. the 

 meagreness of our present knowledge is the remarkable 

 uniformity of cytological behaviour not only in all groups 

 of animals but also in many plants. If the Protozoa are 

 excluded, the general structure of cells at rest and in division 

 is extraordinarily similar in all groups of animals from the 

 lowest to the highest, and differs only in minor points in 

 the majority of plants. Even in the Protozoa essentially 

 similar phenomena are found, though in their types of 

 mitotic division, in the reconstitution of nuclei from chro- 

 midia, and in the great complication of the cell-body if the 

 Protozoan body is in fact a cell they may show rather con- 

 spicuous deviation from the more usual type. There is, of 

 course, immense variety of cytological type in the animal 

 kingdom, but comparatively little has been done in the way 

 of correlating this variety with the classification based on 



