to grow in bulk, which is its chief business during the
first two years of its existence. It quadruples its birth weight. The
brain and nervous system complete their growth in mass by the end of
the fourth year. Recall the experiments of Gudernatsch working with
tadpoles, who showed that feeding with thymus produced giant tadpoles
whose metamorphosis into frogs was inhibited, while feeding thyroid
produced frogs the size of flies. Differentiation occurred without the
preliminary increase in mass usual. As differentiation and bulk thus
appear antagonistic, at least at the beginning of growth, the function
of the thymus, at a maximum during infancy, seems then to be to
restrain the differentiating endocrines, until sufficient material
has been accumulated by the organism upon which the differentiating
process may work.

After the second year, the thymus begins to shrink. That is to say,
officially its involution begins. Careful dissection will demonstrate
some thymus tissue even in a normal subject up to the fourteenth year.
This refers to the average normal, for the large thymus may continue
large and grow larger after the second year in the type of individual
designated in a preceding chapter as the thymocentric.

If the thymus retrogresses after the second year, what takes its place
as a brake upon the forward driving impulses of the other endocrines?
We have every reason for assigning that role to the pineal. It
performs its service mainly, in all probability, by inhibiting the
sex stimulating effect of light playing upon the skin. Since it is
especially a sex gland inhibitor, the thyroid and pituitary become
freer to exert their influences than under the thymus regime. And so
we find that it is after the second year that thyroid and pituitary
tendencies manifest their effects. The Pineal Era, from the second
to the tenth to fourteenth years, remains to be investigated from a
number of viewpoints interesting to the parent, the educator, and
the student of puericulture. Precocity is directly related to early
involution of the pineal. For just as the thymus involutes at the
second year, the pineal atrophies before the onset of adolescence.

Adolescence is the period of stress and strain throughout the somatic
and psychic organism because of the volcanic upheavals in the sex
glands. The history of the individual is dominated by them up to
twenty-five or so, when maturity commences in the sense of a relative
sex stabi