182 EDWARD B. POULTON. 
an abbreviated development of the epidermic tube, we should 
expect its constituents to be histologically equivalent to those 
of the tube, and of nothing else. 
Different writers have taken different sides upon this question, 
some accepting such an account as that summarised by F. 
Balfour,! following Kolliker,’ that the hair develops as ‘‘a cor- 
nification of the cells of the axis of one of the.... [down- 
growing, solid, epidermic] processes, and is invested by a 
sheath similarly formed from the more superficial epidermic 
cells ;” and that given by Gegenbaur’—“ the shaft is differ- 
entiated from the invaginated epidermis by the cornification of 
its cells, while the other cellular parts of the follicle form the 
root-sheaths.” The other point of view is concisely expressed 
by Klein,t who describes the hair and inner root-sheath as 
formed by the bulb alone, and pushed up the axis of the solid 
cylinder of cells which connects the bulb with the superficial 
epidermis, and who expressly states that “the cells of the 
primary solid cylinder represent the rudiment of the cells of 
the outer root-sheath only.” 
The latter account is supported by the fact that, as soon as 
the tubular follicle has been formed, it possesses no power of 
development into a hair. This power is only present in the 
epidermic upgrowth from the bottom of the tube, which repre- 
sents, on the view expressed above, the epidermic upgrowth at 
the surface from which the primitive hair was developed. The 
walls of the tube, the outer root-sheath, have no such power, 
but, as has been previously pointed out, can only form the far 
less complex structure of the hair-base. 
In the case of successional hairs some authorities consider 
that the new bulb is formed from the outer root-sheath of the 
old, while others regard it as formed from the old bulb. Even 
on the former hypothesis there is no difficulty in looking upon 
1 «Comparative Embryology,’ London, 1881, vol. ii, p. 328. 
2 ¢« Rntwickelungsgeschichte d. Mensch, u. der hoheren Thiere,’ Leipzig, 
1879. 
3 *Comparative Anatomy,’ translation by Bell, London, 1878, p. 420. 
4 © Atlas of Histology,’ London, 1880, p. 325. 
