Jeffries.] 234 [February 7, 



The epitrichial layer, or the name thus applied to the outer 

 layer of cells in reptiles and to the outer coat of pin feathers, is 

 not so easily explained. Is the name properly applied, or is the 

 resemblance in both cases to the embryonic layer due to similarity 

 of position ? To my mind both seem probably true. In the for- 

 mation of a pin feather there is a return to embryonic conditions 

 and a similar mode of growth which results in an outer coat of 

 undifferentiated cells. These cells being on the outside naturally 

 flatten and become polygonal, while later, nutriment being cut off 

 by the cells below, death and a resulting coagulation takes place. 

 I doubt if the primitive epitrichial layer, provided the epiderm 

 was ever only two cells deep, had its present final structure. 



The horn layer is common to all the purely terrestrial verte- 

 brates, that is to the Mammalia, Aves and Reptilia ; in the other 

 groups it does not exist. Its place is occupied by parenchymatous 

 cells much like horn cells in their early development. Again it is 

 of comparatively late development, so I am inclined to regard it 

 as an adaptation to, or a result of, an aerial life and consequent 

 drying of the surface ; and this the more since the eye, ear and 

 nostrils do not develop horn cells. The epitrichial layer can be 

 traced into the ears, nostrils, and perhaps eyes. 



The other layers spoken of in the descriptive part are so irreg- 

 ular in their occurrence and so clearly the result of local condition 

 that no value can be attached to them. The toothed cells 

 described by Kerbert are not of definite extent, and curiously are 

 often a product of imflammation in man. The granular cells are, 

 as already stated, simply starved transitional cells. 



The epiderm, therefore, may be regarded as primitively con- 

 sisting of a smooth mucous layer, an epitrichial layer, and per- 

 haps an intermediate layer of parenchymatous cells (transitional 

 cells) . The outer layer is lost in birds and mammals, and never 

 renewed. The middle layer becomes thickened and subject to 

 various modifications, as drying, keratinization (to coin a word), 

 and the like, and enters into the structure of all the appendages. 



The epiderm may also be considered in relation to the cutis 

 vera. If this is done the epiderm will be found to be the primi- 

 tive skin if not the true one. It is formed long before the corium 

 of the outer embryonic layer and forms all the organs of sense, 

 while the corium is a late, very variable product of the mesoblast 



