ADDRESS. 9 



Structure of Cells. 



A cell is a living particle, so minute that it needs a microscope for its 

 examination ; it qroM-s in size, maintains itself in a state of activity, 

 responds to the action of stimuli, reproduces its kind, and in the course 

 of time it degenerates and dies. 



Let us glance at the structure of a cell to determine its constituent 

 parts and the role which each plays in the function to be discharged. 

 The original conception of a cell, based upon the study of the vegetable 

 tissues, was a minute vesicle enclosed by a definite wall, which exer- 

 cised chemical or metabolic changes on the surrounding matei'ial and 

 secreted into the vesicle its characteristic contents. A similar conception 

 was at first also entertained regarding the cells of animal tissues ; but as 

 observations multiplied, it was seen that numerous elementary particles, 

 which were obviously in their nature cells, did not possess an enclosing 

 envelope. A wall ceased to have a primary value as a constituent part of 

 a cell, the necessary vesicular character of which therefore could no longer 

 be entertained. 



The other constituent parts of a cell are the cell plasm, which forms 

 the body of the cell, and the nucleus embedded in its ^bstance. Not- 

 withstanding the very minute size of the nucleus, which even in the 

 lai'gest cells is not more than - J^yth inch in diameter, and usually is 

 considerably smaller, its almost constant form, its well-defined sharp 

 outline, and its power of resisting the action of strong reagents when 

 applied to the cell, have from the period of its discovery by Robert Brown 

 caused histologists to bestow on it much attention. Its structure and 

 chemical composition ; its mode of origin ; the part which it plays in the 

 formation of new cells, and its function in nutrition and secretion have 

 been investigated. 



When examined under favourable conditions in its passive or resting 

 state, the nucleus is seen to be bounded by a membrane which separates 

 it from the cell plasm and gives it the characteristic sharp contour. 

 It contains an apparently structureless nuclear substance, nucleoplasm or 

 enchylema, in which are embedded one or more extremely minute particles 

 called nucleoli, along with a network of exceedingly fine threads or fibres, 

 which in the active living cell play an essential part in the production 

 of new nuclei within the cell. In its chemical composition the nuclear 

 substance consists of albuminous plastin and globulin ; and of a special 

 material named nuclein, rich in phosphorus and with an acid reaction. 

 The delicate network within the nucleus consists apparently of the nuclein, 

 a substance which stains with carmine and other dyes, a property which 

 enables the changes, which take place in the network in the production of 

 young cells, to be more readily seen and followed out by the observer. 



The mode of origin of the nucleus and the part which it plays in 

 the production of new cells have been the subject of much discussion. 



