216 The Unity of the Organism 



stances of smaller molecular weight and greater solubility 

 which are present in the more fluid phases of the system — 

 namely, the cell juices. It is important to remember that 

 change in any one of these constituent phases, including 

 the metaplasmic phases, must affect the equilibrium of the 

 whole cell system, and because of this necessary equilibrium- 

 relation it is difficult to say that any one of the constituent 

 phases, such as we found permanently present in a living 

 cell, even a metaplasmic phase, is less essential than any 

 other to the 'life' of the cell, at least when we view it from 

 the point of view of metabolism." ^" 



Biology will sooner or later surely have to take seriously 

 in hand this matter of the diff*erences between organic spe- 

 cies in all the stages of the lives of the individuals repre- 

 senting those species, the germ-cell stages with the rest. 

 There is urgent need for the extension of ordinary taxono- 

 mic investigations to the entire ontogenetic series of organ- 

 isms, the germ-cell stages included. Such work as that 

 by Dr. Th. Mortensen on the comparative larval stages of 

 echinoderms is in this direction and should be far more 

 widely prosecuted than heretofore. 



The classical systematic studies of Gustav Retzius on 

 the spermatozoa of the animal kingdom are on the whole 

 the most complete we have in this field; but it must be re- 

 membered that the differential attributes of species at this 

 level are likely to have somewhat less correlational sig- 

 nificance than similar attributes of the eggs, for the reason 

 that the sperm-cells are more highly differentiated for their 

 special environment and habits and offices. We may confi- 

 dently predict extensive researches in the future on the 

 taxonomy of germ-cells according to the frankly descrip- 

 tive standpoint, and on the promorphology of germ-cells 

 according to the morphological standpoint. 



The term promorphology has usually been restricted to 

 a very different use from that to which I have here put it, 



