32 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



distinguished by their colour, which is very commonly either green or 

 crimson. At first they, too, appear to be homogeneous ; but a finely- 

 granular appearance is then perceptible ; and a change gradually takes 

 place, which seems to consist in the aggregation of the minute granules 

 into molecules of more distinguishable size and form. These molecules, 

 which are the germs of new cells, seem to be at first attached to the 

 wall of the parent cell ; afterwards they separate from it, and move 

 about in its cavity ; and at a later period, the parent-cell bursts and 

 sets them free. Now, this is the termination of the life of the parent 

 cell ; but the commencement of the life of a new brood ; since every one 

 of these germs may become developed into a cell, after precisely the 

 foregoing manner, and will then in its turn multiply its kind by a simi- 

 lar process. 



28. By reasoning upon the foregoing history, we may arrive at cer- 

 tain conclusions, which will be found equally applicable to all living 

 beings. In the first place, the cell originates in a germ or reproductive 

 body, which has been prepared by another similar cell that previously 

 existed. There is no sufficient reason to believe that there is any ex- 

 ception to this rule. So far as we at present know, every Plant and 

 every Animal is the offspring of a parent, to which it bears a resemblance 

 in all essential particulars ; and the same may be said of the individual 

 cells, of which the Animal and Vegetable fabrics are composed. But 

 how does this germ, this apparently homogeneous molecule, become a 

 cell ? The answer to this is only to be found in its peculiar property, 

 of drawing materials to itself from the elements around, and of incorpo- 

 rating these with its own substance. The Vegetable cell may grow 

 wherever it can obtain a supply of water, carbonic acid, and ammonia ; 

 for these compounds supply it with oxygen, hydrogen, carbon, and nitro- 



fen, in the state most adapted for the exercise of the combining power, 

 y which it converts them into those new compounds, whose properties 

 adapt them to become part of the growing organized fabric. Here, then, 

 we have two distinct operations ; the union of these elementary sub- 

 stances into that composite protoplasma, which seems to be the imme- 

 diate pabulum of the Vegetable tissues ; and the incorporation of that 

 product with the substance of the germ itself. 



29. The first of these changes may be, and probably is, of a purely 

 Chemical nature ; and analogous cases are not wanting, in the pheno- 

 mena of Inorganic Chemistry, in which one body. A, exerts an influence 

 upon two other bodies, B and c, so as to occasion their separation or 

 their union, without itself undergoing any change. Thus platinum, in 

 a finely-divided state, will cause the union of oxygen and hydrogen at 

 ordinary temperatures ; and finely-powdered glass will do the same at 

 the temperature of 572. This kind of action is called catalysis. A 

 closer resemblance, perhaps, is presented by the act of fermentation ; in 

 which a new arrangement of particles takes place in a certain compound, 

 by the presence of another body which is itself undergoing change, but 

 which does not communicate any of its elements to the new products. 

 Thus, if a small portion of animal membrane, in a certain stage of de- 

 composition, be placed in a solution of sugar, it will occasion a new 

 arrangement of its elements, which will generate two new products, 



