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THE AMERICAN NATURALIST [Vol. LIV 



shaft (MS, Fig'. 167) is filled with an anastomosing mass 

 of cornified filaments, which probably represent a 

 closely compressed aggregat ion of small mednllary cells 

 (Fig. 178). A type of medulla in which the component 

 cells are still preserved so that their individual nature 

 can still be seen, is shown in Fig. 147. Two divisions of 

 the continuous medulla can be readily recognized; the 

 nodose (or irregular) (Figs. 138 to 147), and the homo- 

 geneous Figs. 148 to 153). Between these two forms, all 

 sorts of intergradational varieties exist. 



The fragmental medulla (Figs. 155 to 1G6) represents 

 perhaps various stages in the reduction of this element 

 of the hair shaft structure, and seems to have been de- 

 rived from the continuous type. Where the medulla 

 seems to be lacking altogether, minute traces can still be 

 found in various portions of the hair shaft, particularly 

 in the region just below the mouth of the follicle. Struc- 

 tural indications seem to suggest that the developim-m 

 of medullas is from the discontinuous, through the con- 

 tinuous, to the fragmental, and finally, as is the case in 

 the bats, to no medulla at all. 



To prepare hairs for microscopical examination care 

 must be exercised that the reagents used in cleaning, 

 staining, etc., do not soften the cuticle, and thus distort 

 the form of the scales, or that the cover glass is not made 

 to press too heavily upon the hair, and thus flatten it out, 

 deforming both the cuticular scales and medulla as well. 



The simplest treatment for scale examination consists 

 in washing the hair thoroughly in a solution composed of 

 equal parts of 95 per cent, alcohol and ether (or chloro-. 

 form). The hair may then be dipped into pure ether, or 

 chloroform, to insure rapid drying, and when thoroughly 



for immediate examination. Some hairs, <>.,/., those of 

 sheep of most varieties, the fur hair of the camels, and 

 the protective hair of many of the bats, notably the sil- 

 very bat (Lasionycteris noctivagans), exhibit the scales 

 very well after this simple treatment. The 8x or lOx eye- 



