DEVELOPMENT AND MULTIPLICATION OF CELLS. 



113 



Fig. 18. 



A. 



is dependent upon nothing but a due supply of nutriment and a proper tem- 

 perature for the continuance of its growth, and for the due performance of its 

 functions, until its term of life is expired. It originates from a reproductive 

 granule, previously formed by some other cell ; this granule attracts to itself, 

 assimilates, and organizes, the particles of the nutrient fluid in its neighbour- 

 hood ; and converts some of them into the substance of the cell-wall, whilst 

 it draws others into the cavity of the cell. In this manner the cell gradually 

 increases in size ; and whilst it is 

 itself approaching the term of its 

 life, it usually makes preparation 

 for its renewal, by the develop- 

 ment of reproductive granules in 

 its interior ; which may become 

 the germs of new cells, when set 

 free from the cavity of the parent, 

 by the rupture of its cell-wall. 

 There is an important difference, 

 however, in the endowments of 

 the Animal and Vegetable cell. 

 The latter can in general obtain 

 its nutriment, and the materials for 

 its secretion, by itself combining 

 inorganic elements into organic 

 compounds. The former, how- 

 ever, is totally destitute of this 

 power ; it can produce no organic 

 compound, and we have yet to 

 learn how far its power of con- 

 verting one compound into an- 

 other may extend ; its chief en- 

 dowment seems to be that of at- 

 tracting or drawing to itself some 

 of the various substances, which 

 are contained in the nutritive fluid 

 in relation with it. This fluid, as 

 we shall hereafter see, is a mixture 

 of a great number of components ; 

 and different sets of cells appear 

 destined severally to appropriate 

 these, just as the different cells of 

 a parti-coloured flower have the 

 power of drawing to themselves 

 the element of their several colour- 

 ing matters. As far as it is yet 

 known, however, the composition 



B. 



Nostocmacrosporum: A, a long convoluted filament, 

 composed of linear series of minute cells, enclosed in 

 general mucous envelope ; B, group of shorter fila- 



of the cell-wall is everywhere the mentS) each with ils own gelatinous envelope; pro . 



Same, being that Of Proteme. It bably formed by the division of the preceding. 



is in the nature of the contents of 



the cell (as among the cells of Plants), that the greatest diversity exists ; and 

 we shall find that the purposes of the different groups of cells, in the general 

 economy of the Animal, depend upon the nature of the products they secrete, 

 and upon the length of time during which these products are retained by them. 

 127. Of the general account just given, the development of certain cells, 

 which float in the Chyle, Lymph, and Blood, may be adduced as an exam- 

 ple ; these, which are known as the Chyle and Lymph corpuscles, and as the 



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