912 GENERATION. 



of the groove which is subsequently developed into the neural canal. At this portion of 

 the ovuin, there is a thickening of the blastoderm, which then presents three layers, the 

 middle layer, the thickest and most important, being developed from the opposite sur- 

 faces of the external and the internal layer. We have to study, then, the changes which 

 take place in three layers of cells, which we shall call the external, the intermediate, 

 and the internal blastodermic membranes. The earliest stages of development have been 

 studied almost exclusively in the chick, and the processes here observed cannot be as- 

 sumed to represent exactly the mode of development of the human subject. For this 

 reason, we feel justified in adopting the simplest division of layers, which is into three, 

 and shall not attempt to follow the excessively minute descriptions of the early arrange- 

 ment of cells, given by some recent observers. 



A general idea of the development of certain of the important parts of the embryon will 

 aid us in comprehending the more minute processes and the formation of special organs ; 

 and this we can give without reference to the various divisions of the blastodermic layers 

 adopted by different writers. It makes very little difference, indeed, as regards our actual 

 knowledge of development, whether we restrict the external blastodermic membrane to 

 the development of the epidermis, or whether we assume that a portion of it forms the 

 walls of the neural canal. In the latter case, we simply make a thicker external layer 

 at the expense of a portion of the intermediate layer. It is the discussion of such minor 

 points as this, which depend mainly upon observations made upon the chick, that we 

 propose to avoid, in our endeavor to make the description of the first processes of devel- 

 opment as simple as possible. 



We may assume that the furrow for the spinal canal and its dilated superior portion, 

 the head, have been closed over by the union of the dorsal, or medullary plates behind. 

 At a later period, there has been a growth of the abdominal, or visceral plates, which 

 finally close over the front of the embryon. Now, to adopt, with slight modifications, a 

 simile given by Hermann, we may imagine a young mammal, with a short, straight ali- 

 mentary canal, taking no account, for the present, of its glandular appendages. We take 

 the entire body as a tube, the caliber of which is the alimentary canal, with walls formed 

 of concentric layers. Counting these layers from within outward, we have first, the 

 mucous membrane ; 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 integument. All of these layers are developed, to a greater or less degree, 

 simultaneously, from different layers of the blastodermic cells. With the view that we 

 shall adopt, the external blastodermic membrane becomes the epidermis, and the internal 

 blastodermic membrane, the epithelium of the alimentary canal. The intermediate mem- 

 brane splits into two layers; the outer layer becoming attached to the external blasto- 

 dermic membrane and forming the muscular layer of the trunk, while the inner layer is 

 connected with the internal blastodermic membrane and contributes to the formation of 

 the viscera. At a later period, the extremities are developed, as solid processes con- 

 nected with the outer layer of the intermediate membrane and covered by a prolonga- 

 tion of the external blastodermic membrane. 



Development of the Cavities and Layers of the Trunk in the Chicle. As an intro- 

 duction to a description of the development of special organs in the human subject and 

 in mammals, 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 cavities. In doing this, we shall endeavor to 

 describe the figures given by Briicke, which were photographed on wood from large dia- 

 grams, made from actual preparations, by Seboth. In this description, we shall take no 

 account of the formation of the membranes. 



Fig. 294 illustrates one of the earliest stages of development in the chick. In this 



