689 



CHAPTER XVIII, 



VERTEBRATED ANIMALS. 



537. We are now arrived at the highest division of the Animal 

 Kingdom, in which the bodily fabric attains its greatest develop- 

 ment, not only as to completeness, but also as to size ; and it is in 

 most striking contrast with the Class we have been last considering. 

 Since not only the entire bodies of Vertebrated animals, but, 

 generally speaking, the smallest of their integral parts, are far too 

 large to be viewed as Microscopic objects, we can study their 

 structure only by a separate examination of their component ele- 

 ments; and it seems, therefore, to be a most appropriate course to 

 give under this head a sketch of the microscopic characters of 

 those Primary Tissues of which their fabric is made-up, and which, 

 although they may be traced with more or less distinctness in the 

 lower tribes of Animals, attain their most complete develop- 

 ment in this group. — Since the time when Schwann first made 

 public the remarkable results of his researches, it has been very 

 generally believed that all the Animal tissues are formed, like 

 those of Plants, by a metamorphosis of Cells; an exception being 

 taken, however, by some Physiologists in regard to the Simple 

 Fibrous tissues (§ 558). The tendency of many recent investiga- 

 tions, however, has been to throw further doubt on the generality 

 of this doctrine ; and an attempt will now be made to show, that, 

 whilst the Vegetable Physiologist may rightly treat the most highly 

 organized Plant as a mere aggregation of Cells, analogous in all 

 essential particulars to those which singly constitute the Unicellular 

 Protophytes (§ 183), the Animal Physiologist does wrong in seek- 

 ing a like cellular origin for the component parts of the Animal 

 fabric ; but that he may best interpret the phenomena of Tissue 

 formation in the most complicated Organisms, by the study of the 

 behaviour of that apparently-homogeneous substance of which the 

 simplest Protozoa are made up. 



538. Although there would at first sight appear but little in 

 common between the simple body of those humble Rhizopods which 

 constitute the lowest types of the Animal series (§ 325), and the 



Y Y 



