THE ORGAN'S OF THE OUTER GERM-LAYER. 523 



epithelial plug is different. In the latter case the papilla arises at 

 the surface of the skin and is forced down by a plug-like epithelial 

 growth ; in the former the epithelial plug first sinks into the under- 

 lying tissue and then at its deep end the hair-papilla is formed by a 

 growth of the corium. 



The question arises, Which of these two methods of development 

 is to be considered the more primitive ? In my opinion it is the 

 formation of the hair-papilla at the surface of the skin. For this is 

 unquestionably the simpler and less complete condition, from which 

 the latter is derivable and through which it is explainable. The 

 hairs sink into the underlying tissue for the purpose of better 

 nourishment and attachment. A parallel is furnished by the 

 development of the teeth. In the Selachians the latter arise (so 

 far as they are developed as protective structures in the skin) from 

 papillae which grow from the corium into the epidermis ; in Teleosts 

 and Amphibia, on the contrary, the teeth, which are found dis- 

 tributed over extensive areas in the oral mucous membrane, are 

 established deep down in that membrane, for epithelial growths in 

 the form of plugs first sink down into the connective tissue, and it 

 is only subsequently that the dental papilla? are formed by a process 

 of growth in the connective tissue at the bottom of the epithelial 

 down growth. 



Let us return after this comparison to the further development of 

 the hair ; this takes place in the same manner in both the cases 

 distinguished above. The epithelial cells which cover the papillae 

 multiply and are differentiated into two parts (fig. 292 C) ; first, 

 into cells that are more remote from the papillae, that become 

 spindle-shaped and united into a small cone, and that by cornification 

 produce the first point of the hair (ha), and secondly into cells which 

 immediately invest the papilla, remain protoplasmic, and constitute 

 the matrix the hair-bulb (hz) by means of which the further 

 growth of the hair takes place. The cells of the hair-bulb, which 

 rapidly increase by division, are added below to the first-formed part 

 of the hair, and by cornification contribute to its elongation. 



The hair in process of development on the papilla at first lies 

 wholly concealed in the skin and is enveloped on all sides by cells of 

 the epithelial plug, at the bottom of which the first trace of it was 

 formed. From this investment are formed the outer and the inner 

 sheaths of the root (fig. 292 C and D aw and iw). Of these the 

 outer (aiv) consists of small protoplasmic cells and is continuoiis 

 externally with the mucous layer of the epidermis (schl), internally 



