THE ORGANS OF THE OUTER GERM-LAYER. 525 



after birth with the shedding of the downy hair, which is replaced 

 on many parts of the body by a more vigorous growth of hair. In 

 Mammals the shedding of the old and the formation of new hair 

 exhibits a certain periodicity, which is dependent on the warmer 

 and colder periods of the year. Thus they develop a summer and a 

 winter coat. Even in Man the shedding of the hair is influenced, 

 although less noticeably, by the time of year. 



The falling off of the hair is initiated by changes in the part 

 resting on the papilla and called the bulb. The cell-multiplication, 

 by means of which the addition of new corneous substance takes 

 place, ceases ; the falling hair becomes detached from its matrix and 

 its deep end looks as though it were split into shreds ; but it is still 

 retained in the hair-follicle by its closely investing sheath, until it is 

 forcibly removed or is crowded out by the supplementary hair that 

 takes its place. 



The opinions of investigators still differ concerning the manner in 

 which the supplementary hairs are developed. An especial subject 

 of controversy is the point whether the young hair is formed fi-om 

 an entirely new papilla (STIEDA, FEIERTAG) or from the old one 

 (LANGER, v. EBNER), or whether both methods occur (KOLLIKER, 

 UNNA). It seems to mo that the first view is the correct one, and 

 that the shedding of the hairs is due to the atrophy of their papillcc. 

 During this slowly occurring process of degeneration, perhaps even 

 before it begins, the substitution is initiated by the occurrence of an 

 active cell-proliferation at a place in the outer sheath of the root 

 which indeed consists of cells rich in protoplasm and by the 

 formation of a new plug, which grows out deeper into the derma 

 from the bottom of the fundament of the old hair. At the blind 

 [deep] end of this secondary hair-germ there is then developed from 

 the derma a new papilla, upon which is formed the new hair and 

 its sheaths alongside of and below the old one, in the manner 

 previously described. When it begins to increase in length, it 

 presses against the old hair lying above it, crowds the latter out 

 of its sheaths, until it falls off, and finally itself takes the place 

 of it. 



According to this account there would be a certain similarity 

 between the shedding of the hair and that of the teeth, inasmuch as 

 in both cases secondaiy epithelial processes, from which the new 

 tooth- or hair-papilla begins, arise from the primary fundament, 

 and inasmuch as the new structures by their growth displace the 

 old. 



