ADDRESS BEFORE THE BRITISH ASSOCIATION. 35 



lished in 1858, Lord Lister demonstrated that the pigment granules 

 moved in the cell plasma, by forces resident within the cell itself, acting 

 under the influence of an external stimulant, and not by a contractility 

 of the wall. Under some conditions the pigment was attracted to the 

 center of the cell, when the skin became pale; under other conditions 

 the pigment was diffused throughout the body and the branches of the 

 cell, and gave to the skin a dark color. It was also experimentally 

 shown that a potent influence over these movements was exercised by 

 the nervous system. 



The study of the cells of glands engaged in secretion, even when the 

 secretion is colorless, and the comparison of their appearance when 

 secretion is going on with that seen when the cells are at rest, have 

 shown that the cell plasm is much more granular and opaque, and con- 

 tains larger particles during activity than when the cell is passive; the 

 body of the cell swells out from an increase in the contents of its plasm, 

 and chemical changes accompany the act of secretion. Ample evidence, 

 therefore, is at hand to support the position taken by John Goodsir, 

 nearly sixty years ago, that secretions are formed within the cells, and lie 

 in that part of the cell which we now say consists of the cell plasm; that 

 each secreting cell is endowed with its own peculiar property, according 

 to the organ in which it is situated, so that bile is formed by the cells in 

 the liver, milk by those in the mamma, and so on. 



Intimately associated with the process of secretion is that of nutri- 

 tion. As the cell plasm lies at the periphery of a cell, and as it is, alike 

 both in secretion and nutrition, brought into closest relation with the 

 surrounding medium, from which the pabulum is derived, it is neces- 

 sarily associated with nutritive activity. Its position enables it to absorb 

 nutritive material directly from without, and in the process of growth it 

 increases in amount by interstitial changes and additions throughout its 

 substance, and not by mere accretions on its surface. 



Hitherto I have spoken of a cell as a unit, independent of its 

 neighbors as regards its nutrition and the other functions which it has 

 to discharge. The question has, however, been discussed, whether in a 

 tissue composed of cells closely packed together cell plasm may not give 

 origin to processes or threads which are in contact or continuous with 

 corresponding processes of adjoining cells, and that cells may therefore, 

 to some extent, lose their individuality in the colony of which they are 

 members. Appearances were recognized between 1863 and 1870 by 

 Schron and others in the deeper cells of the epidermis and of some 

 mucous membranes which gave sanction to this view, and it seems pos- 

 sible, through contact or continuity of threads connecting a cell with its 

 neighbors, that cells may exercise a direct influence on each other. 



Nageli, the botanist, as the foundation of a mechanico-physiological 

 theory of descent, considered that in plants a network of cell plasm. 



