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558 DENTAL EMBRYOLOGY AND HISTOLOGY.

increase results in an ingrowth which forms hairs, glands, etc., is a
question which has puzzled the brains of thoughtful men since anything
has been known of the histology of tissues. The only answer that can
be made is that in so doing the cells are obeying a vital poicer which
cannot be explained from a knowledge of their microscopical character
or chemical composition, or from a knowledge of the ultimate sub-
stances into which they may be resolved, but which endows each cell
with a distinct individuality. That such is the case no one can doubt
who has for himself studied the development of tissues. If such is not
the case, why is it that the cells of the mesoblast—which in the first
instance arise almost altogether from the epiblast—form one line of
tissues, while the parent epiblast forms another entirely different ? So
far as we can make out in the early stage of the differentiation of the
mesoblast from the epiblast, there is no histological difference between
the two ; but they have each separate offices to perform, and unless
interfered with by lack of material (cell-pabulum) they go on growing
and forming until they have produced very different tissues. From the
time of the separation of the mesoblast from the epiblast, histological
differences are presented M'hicli become more and more marked at each
succeeding stage, until at last the products of their life-work show the
widest divero-ence.
Development of Hairs.—The process of the development of hairs is
somewhat more complex than that of nails, but is of a similar nature.
Fig. 304.

A, Hair-nidiment from an Embryo of Six Weeks: a. horny, and h, mucous or >ralpighian laver of
cuticle: ', basenient-niemhrane: m, cells, some of which are assuming au oblong figure, 'which
chiefly form the future hair.
B, Hair-rudimei't, with the Young Hair formed, but not yet risen through the cuticle: a, hornv,
6, .Malpishian, layer ol epidermis; r, outer, point, of the hair; A, hair-papilla; », «, commencing sebaceous follicles.
C, Hair-follicle, with hair just protruded.
Instead of being developed, like the nails, upon the surface, they are
developed inside a pouch or sac, through the mouths of Avhich thev
push their way to the surface. The fii'st appearance of the development