Cell-Theory not Sufficient for Explaining Organism 187 



proliferate, other tissues of exactly the same kind. Nothing 

 concerning the minute structure of organic heings is better 

 established than that in general tissue cells are "true to 

 kind" ; that is, once a muscle or nerve or gland cell, always a 

 muscle or nerve or gland cell, not only in the particular cell's 

 own existence but in its progeny also. Clearly this must be 

 so. Were it not, the organism would really not be an organ- 

 ism at all; it would be a riot of cells differentiated and un- 

 differentiated. 



So obviously and widely true is this that some biologists 

 have believed it deserves crystallization into a phrase com- 

 parable to omne viimm ex livo. Accordingly we have Vir- 

 chow's omjiis cellula e ccllula transformed by L. Bard into 

 omnis cellula e cellttZa ejusdem natural. But were this for- 

 mulation rigidly true, and were the tissue elements absolutely 

 unmodifiable in their individual lives, the aduTt organism, at 

 least, would certainly be held in the grip, figuratively speak- 

 ing, of its cells, and the fact might be taken as evidence of the 

 weightiest kind in support of the theory that the cells are the 

 key to all organic phenomena. 



^luch truth as there unquestionably is in this aphoristic 

 statement, researches of later years have produced conclusive 

 proof that it can stand only after receiving important modi- 

 fication. 



Looking at the problem of the deviation of the cells of an 

 organism from type in a broad way, though without presum- 

 ing: to make the classification and discussion exhaustive, we 

 find that three rather well defined classes of such deviations 

 have been observed. There are (1) cases in which tissues are 

 induced to undergo radical and more or less permanent 

 transformation by coming into close and long-continued 

 contact wuth new or differently applied influences of the 

 external world; (2) cases in which the replacement of lost 

 parts of an organism is effected through either the direct 

 transformation of tissue of one sort belonging to an intact 



