168 EDWARD B. POULTON. 
In deeper sections it is found that the base of the old hair, 
becoming irregular in outline, first enters (fig. 18), and then 
soon comes to an end, in the posterior wall of the outer 
root-sheath, terminating as a knob, a rounded end, or in 
diverging lines of corneous cells, which spread among the cells 
of the sheath; while the new hair passes far deeper and ends 
in a bulb. When, similarly, successively deeper sections of a 
- bundle of small hairs are examined, it is found that the pig- 
mentless bases of old hairs soon come to an end, together with 
their follicles, while the much larger coloured shafts of the still 
growing hairs pass down far deeper to end in bulbs. These 
appearances are confirmed by the examination of longitudinal 
sections, which show that the extreme ends of the bases thin 
away, become sinuous, and sometimes branch among the cells 
of the outer root-sheath. 
The probable interpretation is as follows:—AlIl the parts of 
a hair which possess the typical structure are developed froma 
bulb, which is nearly exhausted when the hair has almost 
attained its full length. The hair is, however, further pro- 
truded by the development and growth of a solid and pig- 
mentless base possessing none of the typical features of a hair, 
from the outer root-sheath, which is thus used up, the whole 
follicle being shortened and retracted towards the surface. 
When the base of the old hair has reached a certain height the 
lower blind end of the outer root-sheath begins to descend 
again, forms a new bulb and a new hair, which therefore 
ascends and passes the base of the old one. It is perhaps 
significant that each fresh hair which develops during the life 
of the individual should originate in an epidermic down- 
growth, thus repeating the development of the first-formed 
embryonic hair. 
It is, I think, improbable that the outer root-sheath which 
forms the base of the old hair, and not the typical structure of 
the shaft, should itself possess the power of originating a new 
bulb. It is more probable that the new bulb is developed 
from cells which are the genetic descendants of the old one, 
and which retain similar potentialities. 
