572 BALDWIN SPENCER AND GEOEGINA SWEET. 



arrangement is seen to be as follows : — The line of nuclei 

 representing the cuticle of the hair is directly continuous with 

 that of the inner root-sheath ; of this fact we feel satisfied, 

 after long examination of a very large number of well-pre- 

 served sections, and it may be pointed out that if, phyloge- 

 netically, the hair be regarded as a process from the surface 

 of the epidermis, which was developed subsequently in a tube, 

 and still later in a solid follicle, then this relationship is exactly 

 what would normally obtain ; for the cavity of the hair follicle 

 being regarded, ex hypothesi, as formed originally as a de- 

 pression of the surface at the base of which the hair arises, it 

 is perfectly natural for the cuticular layer which lines the de- 

 pression to be directly continuous on the one hand with that 

 on the hair which arises in the depression, and on the other 

 with the cuticle on the general surface of the body ; indeed, 

 any other relationship seems to be difficult to understand. 



In connection with this it may be noticed that the outer 

 root-sheath is in connection partly with the medulla and partly 

 with the cortex. During the earlier stages of development the 

 outer sheath is formed of a single layer of cells continuous 

 with the layer which forms the stratum Malpighii of the 

 epidermis. The layers which lie morphologically to the outside 

 are modified in the follicle to form the inner root-sheath, which 

 becomes strongly corneous, except in the region of the bulb, 

 where it, as well as the elements corresponding to the stratum 

 Malpighii, is in close relationship to the dermic papilla, — the 

 source of nutriment, — and here they retain their soft proto- 

 plasmic nature. The stratum lucidum is, during life, constantly 

 replenished, as the outer part of the epidermis is worn off, by 

 proliferation of deeper-lying elements, and at a certain stage 

 of development there takes place a similar proliferation of the 

 elements of the outer root-sheath, and hence it is only natural 

 that, in the region of the bulb, these newly formed elements 

 should give rise to a portion of the layer corresponding to the 

 one to which they would have belonged in the surface epidermis. 



The continuity of the cuticle of the inner root-sheath and 

 that of the hair is a matter of great importance, as it implies 



