114 



ON THE ELEMENTARY PARTS OF THE HUMAN FABlilC. 



Colourless corpuscles of the Blood, have no single nucleus, but contain seve- 

 ral scattered particles, each of which seems to be a reproductive granule ; and 

 they emit these by the bursting or liquefaction of their wall, a change which 

 may be effected in them at any time, by the application of chemical reagents. 

 The granules thus set free appear to float in the current of fluid, and to be in 

 their turn developed into cells at the expense of the materials it affords. The 

 exudations of the plastic or organizable matter of the blood, which are thrown 

 out upon inflamed or wounded surfaces, appear to contain some of these gra- 

 nules ; for similar cells are speedily developed in these exudations, giving rise, 

 in their turn, to new generations, when their own term of life is ended. 



128. In general, however, we find the cells of Animal tissues furnished with 

 a nucleus ; and this may be formed, as in Plants, either at an early stage of 

 the development of the cell, by the aggregation of minute molecules around 

 the original granular germ (which germ seems to be the nucleolus of some 

 authors) ; or after the cell has attained its full size. The nucleus, where it 

 exists, appears to be the chief instrument in the functions of the cell; the 

 cell-membrane probably having little else, than the mechanical office of 

 bounding or limiting the contents of the cell. In some cells the function is 

 restricted to the attraction of certain constituents, by which the cavity of the 

 cell is filled. These constituents may be of a nature to give solidity and 

 permanence to the texture; thus, the cells of the Epidermis are strengthened 

 by a deposit of horny matter, those of Shell by the deposit of carbonate of 

 lime; those of Bones and Teeth by a mixture of mineral and earthy matter, 

 &c. Or they may be of a fluid nature, readily passing into decomposition, 

 and destined to be retained only for a short time; being given up again by 

 the rupture or liquefaction of the cell-wall, as in the case with the cells of 

 Glandular structures in general. Now such cells do not usually reproduce 

 themselves, but successive crops of them are generated as fast as required 

 from other sources; and the function of their nuclei appears to be limited to 

 their chemical agency upon the materials which they select. It would seem, 



in fact, as if the direction of thenisus 

 or power of the cell to this object, 

 prevented the exercise of its repro- 

 ductive powers ; and where we find 

 these last most strongly manifested, 

 it is usually observable that the cell 

 performs little or no other duty. 



129. In the endogenous develop- 

 ment of Animal cells, the nucleus 

 seems always to perform an important 

 part, where it has a distinct existence. 

 In many cases, the multiplication can 

 be clearly perceived to take place, by 

 the division of the nucleus into two 

 or more portions ; each part giving 

 origin to a new cell. This seems to 

 be the case, for example, in the ordi- 

 nary production of Cartilage-cells; for 

 on examining sections of cartilage that 

 is undergoing rapid extension, we find 

 groups of cells, in all respects corre- 

 sponding with those of the simple 

 cellular plants, which can be seen to 

 increase in the same way. Thus in 

 Fig. 19, which represents a section of one of the branchial cartilages of the 



Fig. 19. 



Section of branchial Cartilage of young Tadpole ; 

 o, b, c, intercellular substance ; rl, single nucleus; e, 

 nucleus dividing into two ; //', e' t two nuclei in one 

 cell, formed by division of single nucleus;/", second- 

 ary cell, forming around nucleus g-/ h, two nuclei 

 within single secondary cell ; i, three secondary 

 cells, within one primary cell. 



