Jeffries.] 204 [February 7, 



Finally, as to the methods of investigation. Being fully con- 

 vinced that accurate ideas can be obtained only by long and careful 

 study, everything that offered the faintest glimmer of light has 

 been followed up. Every structure has been studied in at least 

 two different ways, in order to exclude as far as possible errors 

 introduced by reagents. 



The objects have been hardened either by freezing, by a chro- 

 mium compound, or by alcohol; mostly by the latter reagent. 

 Carmine, hematoxyline, eosin, French blue, picro-carmine, nitrate 

 of silver and chloride of gold, were the chief staining reagents 

 used. The neutral solution of a mixture of eosin and French 

 blue used by botanists has been found of great value. The active 

 portions of the tissue are stained red, while the dead or inactive 

 parts are stained various hues of blue and purple. Not only 

 this, but the blood globules, in virtue of the action of their haema- 

 globin on the blue, are stained a peculiar yellowish red. By this 

 means the capillaries are easily traced. 



Above all, every structure has been examined in situ as a 

 whole and its relation carefully noted. 



ADULT SKIN. 



The skin in adult birds has, as in all the vertebrate groups, an 

 epidermal coat, the representative of the embryonic ectodermal 

 layer. It is this epiderm in its undifferentiated parts that we 

 now have to consider, leaving the appendages and spots of pecu- 

 liar structure till later. 



The base of the epiderm is formed by a layer of cuboidal, or 

 columnar cells, resting directly upon the cutis vera, and usually 

 known as the mucous layer, or rete Malpighi ; the first name is 

 the one adopted in this paper. It has often been stated that a 

 basement membrane exists, separating the mucous layer from the 

 cutis vera, and some state that it is made up of polygonal ' cells. 

 Such a membrane I have never been able to find. By no man- 

 ner of manipulation have I been able to separate such a mem- 

 brane, even where the cells have been broken from the ends of 

 thin section. But the cells of the mucous layer and, indeed, all 

 the epidermal cells are held together by a homogeneous inter- 

 cellular substance, which forms in large part the "walls" of the 



