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 Fungi 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 



