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THE AMERICAN NATURALIST [Vol. LI 



as to have required thorough and detailed investigations 

 in order to establish their reciprocal relationship. The 

 size of the cells makes difficult a detailed study of the 

 changes in the chromosomes, and they require further 

 investigation, nevertheless the possibility of distinguish- 

 ing ditTerent types of chromosome-complexes in dit¥erent 

 cells is not to be overlooked; it is easy to identify, for 

 example, in the thymus entodermal cells, hemoblasts and 

 small lymphocytes, during mitosis by their chromosome- 

 complexes. The assumption of invariability on the part 

 of the chromosome-complexes in the somatic cells requires 

 some qualification. The chromosomes of a cell and the 

 cytoplasm together embody specificity. Changes in both 

 may transform the cell so completely as to deprive it of its 

 faculty of proliferation. Erythrocytes and leucocytes in 

 the blood cell series afford examples of such final modi- 

 fications which have been gradually determined at least in 

 part by the external factors' of the environment. 



