256 H. J. VAN CLEAVE 



reliable data, is of extreme value in that it forshadows a field 

 . of investigation from the viewpoint of the problem of the cytol- 

 ogy of heredity before the exact facts in the case have been 

 established. 



3. The field of cell constancy 



Martini has pointed out concisely the obstacles in the path of 

 an investigation on cell constancy, in his comment upon the dif- 

 ficulty in determining either for or against any such position when 

 extremely complicated relations exist between the component 

 parts of a highly differentiated organ. It would be little less than 

 folly to attempt a study of the cell constancy of an organ in one 

 of the higher animals before the possibilities and the limitations 

 of such a study in some of the simpler forms of life had been de- 

 termined. As a similar instance, who would have considered the 

 possibility of a regularity in the segmentation of the ovum of a 

 vertebrate, had not the fundamental principles been marked out 

 previously by the pioneers in the field of cell hneage, who chose 

 for investigation such objects as displayed the phenomena of 

 cleavage most clearly and with the least complication? Before 

 going further it may be well to consider the nature of the limita- 

 tions in the realm of cell constancy. Martini has very justly elim- 

 inated the cleavage stages of the ovum from consideration here, 

 for the cells in these stages have not as yet arrived at a condition 

 to which the term cell constancy could be applied properly, for in 

 reality the field is limited to a study of such tissues and organs as 

 have passed through the embryonic stage and are definitely differ- ' 

 entiated as physiologically functioning organs. It must be kept in 

 mind that though an organism may under normal conditions have 

 portions displaying absolute constancy, yet in those regions 

 where katabolic and secretory processes are proceeding at such 

 a rate as actually to destroy the tissues, constancy, such as is 

 found in the more stable regions which are practically free from 

 radical metabolic changes, cannot exist. Likewise injury and 

 mutilation of parts of the body with the accompanying excita- 

 tion to renewed multiplication of cells, may profoundly alter the 

 conditions even though the normal organism might display a 



