DEVELOPMENT OF THE SPINAL NERVES, &C. 169 



no openings from without into the throat), a transverse section 

 through the dorsal region exhibits the following features (PL 

 22, fig. A) :- 



The external epiblast is formed of a single row of flattened 

 elongated cells. Vertically above the neural canal the cells of 

 this layer are more columnar, and form the rudiment of the 

 primitively continuous dorsal fin. 



The neural canal (nc) is elliptical in section, and its walls 

 are composed of oval cells two or three deep. The wall at the 

 two sides is slightly thicker than at the ventral and dorsal ends, 

 and the cells at the two ends are also smaller than elsewhere. 

 A typical cell from the side walls of the canal is about T ^ m inch 

 in its longest diameter. The outlines of the cells are for the 

 most part distinctly marked in the specimens hardened in either 

 chromic or picric acid, but more difficult to see in those pre- 

 pared with osmic acid ; their protoplasm is clear, and in the 

 interior of each is an oval nucleus very large in proportion to 

 the size of its cell. The long diameter of a typical nucleus 

 is about ^W inch, or about two-thirds of that of the cell. 



The nuclei are granular, and very often contain several espe- 

 cially large and deeply stained granules ; in other cases only 

 one such is present, which may then be called a nucleolus. 



In sections there may be seen round the exterior of the 

 neural tube a distinct hyaline membrane : this becomes stained 

 of a brown colour with osmic acid, and purple or red with 

 haematoxylin or carmine respectively. Whether it is to be 

 looked upon as a distinct membrane differentiated from the 

 outermost portion of the protoplasm of the cells, or as a layer 

 of albumen coagulated by the reagents applied, I am unable 

 to decide for certain. It makes its appearance at a very early 

 period, long before that now being considered ; and similar 

 membranes are present around other organs as well as the neu- 

 ral tube. The membrane is at this stage perfectly continuous 

 round the whole exterior of the neural tube as well on the dorsal 

 surface as on tJie ventral. 



The section figured, whose features I am describing, belongs 

 to the middle of the dorsal region. Anteriorly to this point the 

 spinal cord becomes more elliptical in section, and the spinal 

 canal more lanceolate ; posteriorly, on the other hand, the spinal 



