124 Dr Barry on the Unity of Structure 



h Skin. 



t Amnion. 



fc Serous covering, resulting from the closing of the amnion at m. (See 



also g of Fig. 3.) 

 / Yolk-bag; 



m n Central line, common to all the Fundamental organs* 

 g Vascular lamina, on the Yolk-bag. 



Fig. 7. Formation of the Germinal Membrane into the Embryo of a Ver- 

 tebrated Animal. 



/> Central line, common to all the Fundamental organs. 



a Chorda vertebralis. 



b Formation-arc of the Laminae dorsales. 



c Formation-arc of the Laminae ventrales. 



d Formation-arc of the Nervous Tube. 



e Formation-arc of the Vascular Tube. 



/ Formation-arc of the Mucous Tube. 



m Ridge of the Lamina dorsalis. 

 m' That place in the Germinal Membrane out of which the ridge (m) arises. 



n Ridge of the Lamina ventralis. 

 n' That place in the Germinal Membrane out of which the ridge (n) arises. 



x Perforating formation-arc of the Eye. 



y Perforating formation-arc of the Ear. 



The manner of development seems to be as follows: * 

 The Germinal Membrane separates into two disjoined lay- 

 ers ; viz. into a Mucous or Vegetative (Plate I. Fig. 3, ***), 

 and a Serous or Animal layer (same Fig. **) ; the latter being 

 in contact with the Primary Membrane (same Fig. *), enclosing 

 the Germ and Yolk ; the former lying immediately upon the 

 Yolk itself. The Vegetative layer is afterwards seen to be com- 

 posed of two intimately united laminae ; viz. the proper Mucous 

 (Fig. 4, ***), and the Vascular (Figs. 4 and 5, h, i, k). The 

 Animal layer also, in the embryo at least, divides itself into two 

 laminae, viz. into the Skin, on the one hand (Fig. 6, 7t), and 

 into a mass containing the Fleshy layer, as well as, in vertebrated 

 animals, the Osseous, and the Nervous layers, on the other 

 (Fig. 6,#, b, c t d). This division into layers, is the " primary 1 '* 

 separation. During the course of this separation, the layers 

 become tubes, or Fundamental organs. (See Plate I, Fig. 7.) 

 There occurs, at the same time, a separation of textures, in the 

 substance of the layers or tubes ; cartilaginous, nervous, and 

 muscular substance, separating from each other ; while a part of 

 the mass becomes fluid. Some of the elementary textures 

 also, assume the form of laminae, which are subordinate to the 

 original layers ; the latter therefore, (now tubes), become the 



* We here present, in a very condensed form, Vcn Bar's observations on 

 the Vertebrata, as contained in his work " Entwickelungsgeschichte der 

 Thiere," &c. 1828, pp. 153-159, &c. : so modified, however, as to make the 

 description applicable to invertebrated animals also. 



