FORMATION OF THE TISSUES. 
317 
Kidneys, &c.,—destined to throw them off by Excretion. 
The greater number of these processes have already been 
treated of in more or less detail. Those included under the 
first head were considered, in a general form, in Chap. i. of 
this Treatise. Those which are comprehended under the 
second head have been dwelt-on in Chaps, v. and vi.; and 
will be again noticed, when the actions of the Nervous and 
Muscular tissues are described. And the varied actions which 
are included under the third and fourth classes, have been 
discussed in the two Chapters which precede the present one. 
We have now to enter, in more detail, into the mode in which 
the circulating fluid is applied to the Nutrition and Formation 
of the Tissues. 
Formation of the Tissues . 
379. There is sufficient reason to believe that every living 
being is developed from a germ; no organized structure being 
able to take its origin (as some have supposed) in a chance 
combination of inorganic elements. All the facts relating to 
the production of Eungi and Animalcules, which have been 
imagined to favour this doctrine, may be satisfactorily ex¬ 
plained in other ways (Veget. Phys. § 779 ; Zool. § 1213). 
Now the first structure developed from this germ, in the 
Animal as in the Plant, is a simple cell; and the entire fabric 
subsequently formed, however complex and various in struc¬ 
ture, may be considered as having had its origin in this cell. 
The cells of Animals, like those of Plants, multiply by the 
development of new cells within them; each of these be¬ 
comes in its turn the parent of others; and thus, by a con¬ 
tinuance of the same process, a mass consisting of any number 
may be produced from a single one. It is in this manner that 
the first development of the Animal embryo takes place, as 
will be shown hereafter (Chap. xv.). A globular mass, con¬ 
taining a large number of cells, is formed before any diversity 
of parts shows itself; and it is by the subsequent development, 
from this mass, of different sets of cells,—of which some are 
changed into cartilage, others into nerve, others into muscle, 
others into vessels, and so on,—that the several parts of the 
body are ultimately formed. 
380. This process of differentiation is carried to very 
different degrees in the development of the several classes of 
