110 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



Fis;. 12. 



Simple isolated cells con- 

 taining productive mole- 

 cules. 



parent, between its transparent exterior and its coloured interior. Thus we 

 have the first indications of the cell-wall, and the cavity. As the enlarge- 

 ment proceeds, the distinction becomes more obvious ; the cell-wall is seen to 

 be of extreme tenuity, perfectly transparent, and apparently homogeneous in 

 its texture ; whilst the contents of the cavity are distinguished by their colour, 

 which (in the species here alluded to) is commonly either green or bright red. 

 At first they, too, seem to be homogeneous ; but a finely-granular appearance 

 is then preceptible amongst them ; and a change gradually takes place, which 



seems to consist in the aggregation of the minuter 

 molecules into granules of more distinguishable size and 

 form. These granules, which are the germs of new 

 cells, seem to be at first attached to the inner 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 ter- 

 mination of the life of the parent-cell ; but the com- 

 mencement of the life of a new generation : since every 

 one of these germs may develope itself into a cell, after 

 precisely the foregoing manner ; and will then, in turn, 

 propagate its kind by a similar process. 



123. The development of new cells within the pa- 

 rent, or what may be termed the endogenous mode 



of cell-growth, takes place in many instances on a plan which differs in no 

 respect from the preceding, except that the parent-cell does not rupture. The 

 granules it contains derive their nutriment from the surrounding fluid, which 

 is included within the cell ; by their progressive increase in size, they gradu- 

 ally fill up the whole cavity of the parent-cell ; and by a further increase, they 

 distend its wall, which becomes thinner and thinner, and at last ceases to be 

 visible around the newly-formed cluster. 



124. In other instances, however, we find that the development of new 

 cells proceeds, not from granules scattered through the whole 

 interior of the cell, but from a determinate spot or nucleus, 

 which is seen upon its wall. This nucleus is frequently formed 

 very early, by the aggregation of molecules around the original 

 granule or cell-germ, even previously to the first appearance of 

 the distinct cell-membrane ; and by Schleiden, who first ob- 

 served this process, it was thought that the body thus produced 

 was essential to the development of the new cell, whence he 

 gave it the name of cytoblast. It appears, however, from more 

 extended inquiries, that this is not the case ; and that the nu- 

 cleus is rather concerned with the subsequent operations which 

 the cell performs, than with its original development. Fre- 

 quently the nucleus does not make its appearance, until the cell 

 itself has been completely formed. It is chiefly in the higher 

 tribes of Plants, that we find these nucleated cells; the nucleus 

 in the cells of the lower Cryptogamia being usually more or 

 less expanded or diffused (as it were), through the entire cavity. 

 The destination of the several forms of cells which make up 

 the complex structure of the higher plants, is very different; 

 and their office seems in great measure to depend upon the 

 peculiar powers of the nucleus. In some instances, this body 

 seems to be the centre which attracts new deposits ; even the 

 spiral filament being probably formed by its agency. We 

 io, showing spiral have, in some of the lowest Cellular Plants, a curious fore-sha- 

 arriingementoftiic j ow i n g O f ^ ie S pi ra i vessels of the most perfect ; the green 

 nuclear particles. 



Fis. 13. 



* : < 

 



Cells of 



