to begin, but it is certain that the secretion soon stops if the milk is
not drawn from the glands by the sucking action of the offspring, or the
artificial imitation of that action. A cow that is not milked or milked
incompletely ceases to give milk. When the stimulus ceases, lactation
ceases. The pressure of the secretion in the alveoli causes the cells to
cease to secrete, much in the same way that pressure in the ureters
injures the secretory action of the renal epithelium. In the earliest
Mammals we may suppose that the young were born in a well-developed
condition, for at first the supply of milk would not have been enough to
sustain them for a long time as their only food. We must also suppose that
the mother began to cherish the young, keeping them in contact with her
abdomen. Then being hungry they began to suck at her hair or fur. The
actual development of the milk glands in Marsupials has been described by
Bresslau [Footnote: Stuttgart, 1901.] and by O'Donoghue. [Footnote:
_Q.J.M.S._, lvii., 1911-12.] The rudiment of the teat is a depression or
invagination of the epidermis from the bottom of which six stout hairs
arise. The follicles of these hairs extend down into the derma, and from
the upper end of the follicle, _i.e._ near the aperture of the
invagination, a long cellular outgrowth extends down into the derma,
branches at its end, and becomes hollow. These branches are the tubules of
the future milk gland. Another outgrowth from the follicle forms a
sebaceous gland. Later on the hairs and the sebaceous glands entirely
disappear, and the milk gland alone is left with its tubules and ducts
opening into the cavity of the teat. This is clear evidence that the milk
gland was evolved in connexion with hairs, and was an enlargement of
glands opening into the hair follicle, but it is difficult to understand
why a sebaceous gland is developed and afterwards disappears. This would
seem to indicate that the milk gland was not a hypertrophied sebaceous
gland, but a distinct outgrowth, which however had nothing to do with
sweat glands.