RELATIVE INFLUENCE OF PARENTS. 239 



In an early stage of the development of the germ, 

 a blastodermic membrane is formed, from which the 

 embryo is developed. This blastodermic membrane 

 is soon separated into two layers, which are designated 

 as the external and internal layers of the germinal or 

 blastodermic membrane. "According to the most 

 recent observations, the main portion of the external 

 layer, sometimes called the serous layer, simply forms 

 a temporary investment for the rest of the vitellus 

 (yolk), and is not developed into any part of the em- 

 bryon. The internal layer, called the mucous layer, 

 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 cere- 

 bro-spinal system, with a furrow, which is finally in- 

 closed 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 layer and the opposite surface of the 

 internal 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 de- 

 veloped. 



" To summarize the development of the layers just 

 mentioned, we may state that the external layer is a 

 temporary structure ; the internal layer is very thin, 

 and is for the development of the epithelial lining of 

 the alimentary canal ; and the most important struct- 



is a thick layer of cells developed from the oppo- 

 site surfaces of the external and the internal layers, 

 and situated between them, called the intermediate 



