1.— PHYSIOLOGY. 173 



appear that the niaiutenance, for a time at least, of the irritability of proto- 

 plasm and its restoration when it has disappeared — but not irreversibly — 

 does not require the presence of the nucleus. It may require oxygen or 

 the presence of certain ions, but this may merely mean that the labile 

 state of the protoplasm has been uspet by the products of the cell's activity, 

 and removal of these will restore it to its irritable condition. 



We have no evidence that irritability as a manifestation of what we 

 call life is more than the possession of extremely labile structures, sensitive 

 to minute environmental changes. The nucleus, on the other hand, is 

 essential to the continuous life of the cell and its growth. It appears also 

 to determine very largely the magnitude of the respiratory processes 

 which occur in it at rest, though not necessarily the excessive respiration 

 observed in the recovery from functional activity. Can we therefore go 

 so far as to say that the nucleus is the seat of those synthetic activities of 

 the cell which appear to depend on its living character rather than on its 

 irritability, or have we to regard the protoplasm as equally li\'ing so that 

 it is able to reproduce itself and in addition bring about such syntheses 

 as those of fat and glycogen ? 



There is something to be said in favour of the idea that the protoplasm 

 is not living in the sense in which the nucleus is, and therefore is less likely 

 to be the seat of certain synthetic processes. It is, I think, quite a tenable 

 view that protoplasm is made up, largely but not entirely of combinations 

 of amino-acids such as we find in the proteins, and that it is synthesised 

 by the necleus to serve special as well as certain general requirements. 

 These special requirements must and do vary greatly with each type of 

 cell. 



Consider the mammalian erythroblast. The principal substance in its 

 protoplasm is the protein hsemoglobin. It is doubtful whether this cell 

 exists except to produce haemoglobin. When it has matured the nucleus 

 degenerates and disappears leaving the red blood corpuscle. Along with 

 this the respiratory activity of the cell practically disappears too, and there 

 is not much discoverable in it except haemoglobin. It is certainly now 

 not living in the sense in which its progenitor, the erythroblast, was. Is 

 it not reasonable to suggest, therefore, that the production of haemoglobin 

 as the erythroblast grows and matures is a function of the nucleus and not 

 of the protoplasm of this cell ? All semblance of further synthesis of 

 haemoglobin certainly disappears when the nucleus goes. The adoption of 

 such a view does not imply that having produced the protoplasm of a cell 

 the nucleus has nothing more to do with it. We know that in some 

 indefinable way the nucleus in most cells controls the structure of the 

 protoplasm and maintains its lability, but we have no knowledge as yet 

 of the mechanism by which this is brought about. 



In emphasizing the great similarity between the growth of protoplasm 

 and the sjTithesis of proteins, other examples in addition to that of haemo- 

 globin come to mind. The cells of the Malpighian layer of the epidermis 

 have to produce the protein which eventually becomes the keratin of the 

 stratum corneum. This would appear to be one of their chief functions. 

 When it is done the cell dies and the nucleus becomes functionless. The 

 production of collagen by cells of the connective tissue and of milk pro- 

 teins by the cells of the mammary gland are instances of the continuous 



