ISO Dr Barry on the Unity of Structure 



semble each other in different animals of the Class Vertebrata. 

 Of Invertebrated animals, we shall presently speak. 



In development, germs, and even embryos, belonging to dif- 

 ferent groups of the same great Class, may long be indistin- 

 guishable ; and still longer, those that are more nearly allied. 

 But those belonging to different great Classes, begin to diverge 

 sooner ; or rather, the angle of divergence being greater, a differ- 

 ence is appreciable at an earlier period ;* and in proportion to 

 the angle of divergence in a germinal, are the structures unlike 

 in a perfect state. Just as, in a tree, those branches that have 

 been given off nearest to its root, become most widely separated 

 in their terminating twigs. 



In different Classes, development, though it proceeds in the 

 same manner, yet taking thus different directions, attain?, with 

 materials perhaps essentially the same in primordial structure, 

 very different ends (types). 



Thus it proceeds in the Vertebrata or Osteozoa, with especial 

 reference to the central portion of the nervous system ; in the 

 Arthrozoa (which include, besides the Articulata, some Zoo- 

 phytes), having for its chief object, the organs of locomotion. 

 In both of these Classes, therefore, it is the Serous or Animal 

 layer of the Germinal Membrane, that is seen first advancing ; 

 and out of this, in these two Classes, there is thus produced, a 

 very different system of organs. 



In the Gastrozoa (i. e. the Mollusca and most Zoophytes), on 

 the other hand, the organs of nutrition are especially the ob- 

 ject ; and in them, therefore, development proceeds chiefly in 

 the Mucous or Vegetative layer. 



To these priorities in development, and to the important in- 

 fluence they have on the direction which development takes in 

 other parts of the system, are referrible the leading characters 

 of Classes.f Yet it is in direction only, that development can 



The primitive trace is very different in Invertebrated animals, for ex- 

 ample, the Crustacea, from what it is in the] Vertebrata; and even among some 

 of the Vertebrata, there are observable, in this respect, no small differences ; 

 as between the primitive trace of Batrachian reptiles, and that Birds, 

 (Valentin, Entwickelungsgeschichte, &c.) 



f- Hence we cannot compare animals, belonging to different Classes, in re- 

 gard to what is called their "rank," unless we keep in view, not the degrte 

 alone, but also the direction of development. For the same reason, it is ab- 

 surd to saj, that one Class of animals can pass into another ; such, for exam- 



