420 OF NUTRITION'. 



within the parent vesicle ; and that, even when it disappears, it is by a resolu- 

 tion into its component granules, which may act as well separately as in appo- 

 sition. It is not to be inferred from the preceding account, that cells are ever 

 formed by the mere apposition of particles, at all in the manner of the aggre- 

 gation of molecules in a crystal ; since there appears sufficient reason to believe 

 that in this instance, as in all others, germs are present in the fluid, which 

 were prepared by previously existing cells, and which grow and develop 

 themselves in virtue of their peculiar inherent powers. 



558. We are thus brought to the second mode in which Cells may be de- 

 veloped, which is within the parent vesicles or primordial cells ; the granules 

 contained in these being apparently the germs from which they originate. 

 This is probably the mode which is always followed when a tissue previously 

 existing has to be extended or partially renewed ; the former one being adopted 

 where a structure entirely new has to be evolved, which is normally the case 

 in certain phases of Vegetable life, but is less common in Animals. The 

 secondary cells developed within a parent vesicle, originating in the granular 

 germs which it includes, at first grow at the expense of the fluid it contains, 

 and afterwards by absorbing nutrient materials through its walls. When they 

 have undergone great increase in size, they distend the original vesicle in such 

 a manner that its limits are no longer apparent. The pressure to which they 

 are subjected during their development, determines their form, as in the pre- 

 vious case. If the original cell be spherical, and the pressure be equal on all 

 sides, they also will be spherical until their sides are flattened against each 

 other, when they will become rhomboidal dodecahedrons. If, on the other 

 hand, the pressure be predominant in one direction, or there be any traction in 

 another, the newly-forming cells will be elongated in the direction of least re- 

 sistance ; and this elongation may be carried to such an extent as to impart to 

 them the fibrous character. The development of cells within cells is most 

 distinctly seen in the case of the spore or pollen-grain ; the granules contained 

 in which are clearly the germs of the cells that compose the tissue of the em- 

 bryonic structure. These cells, when fully evolved, in their turn produce 

 others in their interiors ; and in this manner a complex and extensive organism 

 may be developed from a single cell-germ. This, in fact, is what takes place 

 in the lowest plants, in which the cell-germs or reproductive granules are set 

 free from the parent vesicle, before they are themselves developed into cells ; 

 and each one of them, imbibing nutriment from the air and moisture around, 

 may ultimately evolve itself into a complete individual. In the higher Crypto- 

 gamic Plants, on the contrary, the parent vesicle or spore does not rupture, but 

 the new cells of the embryo are developed within it, at last distending its walls 

 so much that they can be no longer traced ; and it would seem as if it served 

 to elaborate for them, from the surrounding elements, the nutriment they re- 

 quire. In the flowering plants, a further supply of this nutriment is provided 

 in the ovule, where materials previously elaborated are stored up, to be absorbed 

 through the wall of the parent cell, and to be subservient to the development 

 of its contained germs. This process, although forming a part of the function 

 of reproduction, is in reality essentially the same with the ordinary nutritive 

 operations ; for in these the circulating fluid supplies the pabulum or organiza- 

 ble matter ; whilst the cells already formed contain the germs, which, with the 

 assistance of this, evolve themselves into new cells, and thus become the means 

 of the extension of the original structure. 



559. The Animal body exhibits phenomena of a character essentially the 

 same. Even in the fully-formed organism, many parts may be found, which 

 are composed, more or less evidently, of isolated cells or vesicles, analogous 

 to those of Plants ; and it has been clearly proved that, in its early condition, 

 the whole fabric has this character. In fact, it has been shown by the re- 



