388 GENERATION. 



three layers of cells, which we shall call the external, the in- 

 termediate, and the internal blastodermic membranes. The 

 earliest stages of development have been studied almost ex- 

 clusively in the chick, and the processes here observed cannot 

 be assumed 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 de- 

 scriptions of the early arrangement of cells, given by some 

 recent observers. 



A general idea of the development of certain of the im- 

 portant parts of the embryon will aid us in comprehending 

 the more minute processes and the formation of special or- 

 gans ; 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 por- 

 tion 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 en- 

 deavor 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 em- 

 bryon. Now, to adopt, with slight modifications, a simile 

 given by Hermann, 1 we may imagine a young mammal, with 

 a short, straight alimentary canal, taking no account, for the 

 present, of its glandular appendages. We take the entire 



1 HERMANN, Grundiss der Physiologic, Berlin, 1870, S. 469. 



