SIMPLE ISOLATED CELLS. 



131 



that the life of an independent cell may commence in various modes), it 

 attracts to itself, assimilates, and organizes, the particles of the nutrient 

 fluid in its neighbourhood ; converts some of them into the substance of 

 its cell-wall, whilst it draws others into its cavity ; in this mariner the 

 cell gradually increases in size ; and whilst it is- itself approaching the 

 term of its life, it may make preparation for its renewal, by the deve- 

 lopment of reproductive particles in its interior, which may become the 

 germs of new cells, when set free from the cavity of the parent. In the 

 interior of most Animal cells, usually attached to some part of their wall, 

 is seen a collection of granular matter, which is called the nucleus (Fig. 

 16 a). This appears to be the centre of the vital forces of the cell ; 

 being the part through which it specially exerts its agency on the sub- 

 stances brought under its influence ; and being also the chief instrument 

 in the reproductive operation. There is reason to believe, indeed, that 

 iclear bodies may exert their vital power, and may effect the transfor- 



Fig. 16. 



Cells from chorda dorsalis of Lamprey, a, a, nnclei. 



mation or new arrangement of organic compounds, without the forma- 

 tion of a cell-membrane ; the purpose of the latter, indeed, being appa- 

 rently to bound or limit the substances drawn together by the nucleus, 

 and to cut them off from others in their neighbourhood. When the for- 

 mation of a cell is complete, and it is not destined to reproduce its kind, 

 the nucleus frequently disappears ; this is the case, for example, with 

 the Red corpuscles of the Blood of Mammalia ( 215), and also with Fat- 

 cells ( 257). So far as is yet known, however, the composition of the 

 cell-wall is everywhere the same ; being that of Proteine. It is in the 

 nature of the contents of the cell, that (as among the cells of Plants) the 

 greatest diversity exists ; and we shall find that the purposes of the dif- 

 ferent groups of cells, in the Animal economy, depend upon the nature 

 of the products they secrete, and upon the mode ini which these products 

 are given back, after they have been subjected to the action of the 

 cells. 



212. New cells may originate in one of the two principal modes : either 

 directly from a pre-existing cell ; or by an entirely new production in 

 the midst of an organizable blastema. The development of new cells 

 from a pre-existing cell, again, may take place in one of two modes ; 

 either by the subdivision of the parent-cell, or by the production of a 

 number of new cells in its interior ; the nucleus, in each case, appearing 

 to perform an important part in the process. Of the multiplication of 

 cells by subdivision, we have a characteristic example in the growth of 

 Cartilage, which repeats in adult age the process by which the develop- 

 ment of the " germinal mass" takes place at the earliest period of embry- 



