122 Dr Barry on the Unity of Structure 



It appears also, that essentially, the manner of the metamor- 

 phosis, or metamorphoses, from a more homogeneous or general 

 structure, to one more heterogeneous and special, i. e. the man- 

 ner of development, is universally the same. 



Such a proposition seems deducible from what we know of 

 development, not only in all the Vertebrata, but in many Inver- 

 tebrated animals ; such as the Insecta, * Crustacea, ) Arach- 

 nida, J and even Mollusca ; and Von Bar seems to have 

 meant the observation to apply to animals in general, when he 

 spoke of development proceeding by " a continued elaboration 

 of the animal body, through growing histological and morpho- 

 logical separation.*" || Zoophytes themselves, so far as their de- 

 velopment extends, may also be included, as subject to the same 

 law. 



To the manner of development, we shall presently return. 



The Layer of granules, already spoken of, as having in its 

 centre the Germinal Vesicle (fig. 1. p. 120), appears, on the 

 bursting of the latter, to contribute to the formation of the 

 Germinal Membrane (Plate I. fig. 3) : though the central and 

 most important part of the latter is perhaps constituted, by the 

 contents of the Germinal Vesicle. 



The Germinal Membrane in some of the Vertebrata, is at first 

 a more or less circumscribed disk, covering only a part of the 

 Yolk, and afterwards extending itself to surround and enclose 

 the whole of it ; in others, it encloses the whole of the Yolk from 

 the first. This membrane in the Invertebrata, presents differ- 

 ences in this respect, regarding which physiologists are not quite 

 agreed.^" 



In most vertebrated animals, the Embryo is at first nothing 

 more than the exuberant growth of a part of this Germinal 

 Membrane, near its centre, (see Plate I. fig. 3.) ; i.e. in the 



* See Burmeister's Entomology, translated by Shuckard, 1836, 8vo. 



j- Rathke, liber die Bildung und Entwickelung des Flusskrebses, 1829, fob 



Herald, Untersuchungen liber die Bildungsgeschichte der Wirbellosen 

 Thiere im Eie, 1824, foL Also Rathke, in Burdach's Physiologic als Erfah- 

 rungswissenschaft. 



See Von Bar's observations on the derelopment of Snails, in his " Ent- 

 wickelungsgeschichte der Thiere," &c., 1836, 4to. 



j| L c. p. 231. 



^[ See Valentin, Entwickelimgsgeschichte des Menschen, &c. pp. 144 and 

 602-3. Also Herold. 1. c. ; and Rath**, 1. c. 



