16 ELEMENTARY STRUCTURE OF ORGANIZED BODIES. 



more widely, in proportion as they are examined in animals 

 of a higher rank. As we descend in the scale of being, 

 the differences become gradually effaced. The soft body 

 of a snail is much more uniform in its composition, than 

 the body of a bird, or a quadruped. Indeed, multitudes of 

 animals are known, made up of nothing but cells in contact 

 with each other. Such is the case with most of the Infu- 

 soria, which nevertheless live and move most freely, by 

 means of little hair-like organs at their surface, that are 

 themselves merely modified cells. 



47. A no less remarkable uniformity of structure is to be 

 observed in the higher animals, in the earlier periods of 

 their existence, before the body has arrived at its definite 

 form. The head of the adult salmon, for instance, con- 

 tains all the tissues we have mentioned, namely, bone, 



cartilage, muscle, nerve, brain, 

 vessels, and membranes. But let 

 us examine it during the embry- 

 onic state, that is, while it is yet in 

 the egg, and we find that the whole 

 Fig. 3- head is made up of cells which dif- 



fer merely in their dimensions ; those at the top of the head 

 being very small, those surrounding the eye a little larger, 

 and those beneath being still larger. It is only at a later 

 period, after still further development, that these cellules 

 become transformed, some of them into bone, others into 

 blood, others into flesh, &c. 



48. Again, the growth of the body, the introduction of 

 various tissues, the change of form and structure, proceed in 

 such a manner as to give rise to several cavities, variously 

 combined among themselves, and each containing, at the 

 end of these transformations, peculiar organs, or peculiar 

 systems of organs. 



