FIRST DEyELOPMENT OF THE EMBRYON. 389 



body as a tube, the caliber of which is the alimentary canal, 

 with walls formed of concentric layers. Counting these lay- 

 ers from within outward, we have first, the mucous mem- 

 brane ; next, the muscular coat of the intestine ; then, the 

 visceral serous membrane, the parietal serous membrane, the 

 muscles of the trunk with the bones ; and finally, the integu- 

 ment. All of these layers are developed, to a greater or less 

 degree, simultaneously, from different layers of the blasto- 

 dermic cells. With the view that we 'shall adopt, the exter- 

 nal blastodermic membrane becomes the epidermis, and the 

 internal blastodermic membrane, the epithelium of the ali- 

 mentary canal. The intermediate membrane splits into two 

 layers, the outer layer becoming attached to the external 

 blastodermic membrane and forming the muscular layer of 

 the trunk, while the inner layer is connected with the inter- 

 nal blastodermic membrane and contributes to the formation 

 of the viscera. At a later period, the extremities are devel- 

 oped, as solid processes connected with the outer layer of the 

 intermediate membrane, and covered by a prolongation of 

 the external blastodermic membrane. 



Development of the Cavities and Layers of the Trunk in 

 the Chick. As an introduction to a description of the devel- 

 opment of special organs in the human subject and in mam- 

 mals, it will be found very useful to study the first stages of 

 development in the chick, by which we can get an idea of 

 the arrangement of the different blastodermic layers, and the 

 way in which they are developed into the different parts of 

 the trunk, with the mode of formation of the great cavi- 

 ties. In doing this, we shall endeavor to describe the figures 

 given by Briicke, which were photographed on wood from 

 large diagrams, made from actual preparations, by Seboth. 1 

 In this description, we shall take no account of the forma- 

 tion of the membranes. 



Fig. 37 illustrates one of the earliest stages of develop- 



1 BBUCKE, Vorlesunffen uber Physiologic, Wien, 1873, Bd. ii., S. 276. 



