ORIGIN OF FROG AND SOLE. 65 



shows itself as the white line unitino; wall and bars to the 

 sole. 



The greater portion of the wall is formed from the cells 

 covering the coronet. The corium of this region carries 

 numerous papilhc of varying length ; those placed low down are 

 usually longer than those above. The papilhc act as moulds, 

 upon which the horn tubes are cast, and from them the tubes 

 grow like the hairs from the papillae sunk in the corium of other 

 regions, as noted above. The epithelial cells covering the depres- 

 sions between the papilhe form horn in a fashion precisely similar 

 to those forming the horny laminae, and hence the horn tubes 

 or fibres are cemented together by a horny matrix, as may be 

 ascertained by consulting any good illustration of a section 

 through the wall. Occupying the horn tubes may be found a 

 material that has been named intratubular material ; this is de- 

 rived from the cells covering the extreme tips of the papillae on 

 the coronet, and hence the different portions of the horn form- 

 ing the wall are named tubular, intertubular, and intratubular, 

 according to their origin from the cells covering the sides of the 

 papillae, the depressions between, or the summits of the papillae. 



The horn of the frog is developed partly from the cells 

 covering the numerous papillif of the sensitive frog or from the 

 depressions between. The peculiaiity of the frog may partly 

 be accounted for by the mass derived from the non-papillated 

 regions, and partly by the secretion formed by the sudoriparous 

 glands found in the frog. Close and attentive examination of 

 these glands show that they are not true sweat glands, secreting 

 sweat as it is usually understood. The material formed, as 

 observed in the ducts of the gland tubes, resembles ear wax 

 rather than true sweat, and such a secretion would keep the 

 frog in the condition we find it, better than a watery secretion 

 subject to rapid evaporation. Moreoverr, the ragged nature of 

 the frog may be explained by the hypothesis that the horn of 

 purely cellular, as distinct from a moulded and papillated 

 origin, is present in greater abundance than in the wall, and, 

 as we shall observe later, such an explanation may be offered 

 as to the structure of the horny sole, but here no glands are 

 present. The cellular horn of the frog, as distinct from the 

 tubular horn, has a remarkable appearance on close examination 

 with the microscope. The cells are disposed in two alternating 



E 



