40 ELEMENTARY STRUCTURE OF ORGANIZED BODIES. 



46. The tissues above enumerated differ from each other 

 more widely, in proportion as they are examined in animals 

 of a higher rank. As we descend in ihe 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 to be made up of nothing but cells 

 in contact with each other. Such is the case with the 

 polyps; yet they contract, secrete, absorb, and repro- 

 duce; and most of the Infusoria move freely, by means of 

 little fringes on their surface, arising from a peculiar kind 

 of 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 not only all the tissues we have mentioned, namely, 



bone, cartilage, muscle, nerve, brain, 

 and membranes, but also bloodves- 

 sels, glands, pigments, &c. Let 

 us, however, examine it during the 

 embryonic state, while it is yet in 

 Fig. 8. the egg, and we find that the whole 



head is made up of cells which differ merely in their dimen- 

 sions ; those at the top of the head being very small, those sur- 

 rounding the eye a little larger, and those beneath being still 

 larger, (Fig. 8.) 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. 



