360 GENERATION. 



gard to the application of direct researches made upon the 

 chick, in which most of these early processes of development 

 have been studied, to the mammalia and the human subject. 

 We shall endeavor to describe the different layers in as sim- 

 ple a manner as is consistent with our positive knowledge, 

 omitting all points that are unsettled or seem to be of minor 

 importance. 



The blastodermic cells, resulting originally from the seg- 

 mentation of the vitellus, are first apparently split into two 

 layers, which may be termed the external and the internal 

 blastodermic membranes. According to the most recent ob- 

 servations, the main portion of the external layer, sometimes 

 called the serous layer, simply forms a temporary investment 

 for the rest of the vitellus and is not developed into any part 

 of the embryon. The internal layer, called the mucous lay- 

 er, is developed into nothing but the epithelial lining of the 

 alimentary canal. There is a thickening of both of these 

 layers at the line of development of the cerebro-spinal sys- 

 tem, with a furrow, which is finally enclosed by an elevation 

 of the ridges and their union posteriorly, forming the canal 

 for the spinal cord. 



As the spinal canal is thus developed, a new layer is 

 formed, by a genesis of cells from the internal surface of the 

 original external layer and the opposite surface of the inter- 

 nal, or mucous layer. This layer of new cells may be termed 

 the intermediate layer ; and it is from this that nearly all the 

 parts of the embryon are developed. 



To summarize the development of the layers just men- 

 tioned, we may state that the external layer is a temporary 

 structure ; the internal layer is very thin, and is for the de- 

 velopment of the epithelial lining of the alimentary canal ; 

 the most important structure is a thick layer of cells, devel- 

 oped from the opposite surfaces of the external and the in- 

 ternal layer, and situated between them, called the interme- 

 diate layer ; and it is from these cells that the greatest part of 

 the embryon is formed. 



