442 



SECRETION 



organ, however complex its structure may be. 

 The progress of comparative anatomy has 

 shown that neither the form nor the in- 

 ternal arrangement of the parts of a gland 

 could have any essential connection with the 

 nature of its product (see GLAND) ; since 

 even those glands (the liver and the kidney, 

 for example) in which there is the greatest 

 complexity of structure, make their first 

 appearance at the lower end of the animal 

 series, as in the early embryo of the very 

 highest, in the simplest possible form. Still 

 something was wanting to prove that the 

 structural elements immediately concerned 

 are in all instances the same ; and there 

 seemed no analogy whatever between the 

 secreting membrane of the animal and the 

 secreting cell of the plant. The doctrine 

 was first propounded by Purkinje * and 

 Schwannf, adopted and extended by Henle J, 

 and fully confirmed by the researches of 

 Goodsir$ and Bowman ||, that the true pro- 

 cess of secretion under whatever form it 

 may present itself is always performed by 

 the intervention of cells ; which, as part of 

 their own regular vital actions, select and 

 withdraw certain ingredients from the nu- 

 tritive fluids, and afterwards set them free 

 again, generally by the rupture or dissolution 

 of the cell-wall, but sometimes perhaps by a 

 simple act of transudation. Fo/ the proper 

 comprehension of this doctrine in all its ge- 

 nerality, it is necessary to give some attention 

 to the history of cell-development, as mani- 

 fested in the simplest forms of organic ex- 

 istence ; those cryptogamic plants, namely, 

 in which every cell is a distinct and inde- 

 pendent individual. 



The earliest condition of such a cell is a 

 minute molecule, which cannot be discerned 

 except under a considerable magnifying 

 power, and in which even the highest ampli- 

 fication fails to exhibit any distinction of 

 parts. When placed under circumstances 

 favourable to its development, namely, 

 when supplied with the materials of its nu- 

 trition, and stimulated by the requisite de- 

 gree of warmth, this germ increases in size; 

 and a distinction becomes apparent between its 

 transparent exterior and its coloured interior. 

 Thus we have the first indication between 

 the cell-wall and the cell-cavity. As the en- 

 largement proceeds, the distinction becomes 

 more obvious ; the cell-wall is seen to be of 

 extreme tenuity and perfectly transparent, 

 and to be homogeneous in its texture, whilst 

 the contents of the cavity are distinguishable 

 in the Algae by their colour, which is green in 

 the Chlorococci, and bright red in the Hcema- 

 tococci ; but in the simple fungi, such as the 

 Torula cerevisii y or yeast plant, they are 

 colourless. The contents of the cell-cavity 



* Isis, 1838, No. 7. 



+ Froriep's Notizen, Feb. 1838. 



t Muller's Archiv. 1838, p. 104108 ; 1839, p. 45. 



Trans, of Royal Society of Edinburgh, 1842. 



|| Art. Mucous MEMBRANE ; and Phil. Trans. 

 1842, " On the Structure and Uses of the Malpighian 

 Bodies of the Kidney." 



have no relation whatever to the material of 

 the cell-wall. Of this we have a remarkable 

 example in the cases just cited ; for whilst 

 the red and green coloured products of the 

 Algae are probably nearly related to each 

 other and to the chlorophyll of higher plants, 

 being simple ternary compounds of water and 

 carbon, the cell-contents of the yeast-plant 

 are closely allied to the protein compounds ; 

 and yet the cell-walls in both instances are 

 composed of the same material, cellulose. It 

 is evident, then, that the inherent powers of 

 the cell are not confined to the application of 

 nutrient materials to the extension of its own 

 walls, and the consequent enlargement of its 

 cavity ; but that they are exercised also in 

 selecting from (and it may be in combining or 

 modifying) the same materials, in order to fill 

 this cavity with a certain product, which may 

 be altogether different in its constitution and 

 its properties from that of which its wall is 

 composed. This latter process is as essential 

 to our idea of a living cell, as is the growth 

 of its wail; and must never be left out of 

 view when the history of cell-development is 

 being considered. 



The nature of the compound thus stored 

 up in the interior of a cell depends in part 

 upon the original inherent endowments of the 

 cell itself, derived from its germ ; and, in 

 part, upon the character of the nutriment 

 supplied to it. Thus we find that the simple 

 Algae will grow wherever they can obtain, 

 from the air and moisture around, the elements 

 of their cell-walls and of their cell-contents ; 

 which elements they have themselves the 

 power of combining into those peculiar com- 

 pounds, of which analysis shows that they 

 are composed. But out of the very same 

 materials, and under circumstances to all 

 appearance identical, the Chlorococciis manu- 

 factures a green product, and the Haemato- 

 coccus a red one. On the other hand the 

 yeast-plant, like the fungi in general, will 

 only grow where it meets with an azotised 

 compound already formed ; and from this it 

 elaborates the product which occupies its 

 cell-cavity, its cell- wall being apparently 

 formed by the same process as that of the 

 simplest Algae. It could no more vege- 

 tate, as they do, upon cold damp surfaces, 

 than they could develop themselves in a 

 solution of fermentible matter secluded from 

 the light. 



A similar variety of function is seen amongst 

 the cells, whose aggregation makes up the 

 structure of any one of the higher plants, and 

 which are all the descendants of the single 

 cell which constituted its original germ. Thus 

 we have in the green cells of the leaves the 

 representatives of the simple Chlorococci ; 

 these, under the influence of solar light, com- 

 bining the carbon which they derive from the 

 atmosphere, or from the soil, with the water 

 transmitted from the roots, and elaborating 

 these elements into a variety of new products^ 

 amongst which chlorophyll and cellulose are 

 still prominent ; but also operating upon the 

 azote which they draw from the atmosphere 



