se of development. Roux distinguishes in all development two
periods, in the first of which the organ is formed prior to and
independent of its function, while in the second the differentiation and
growth of the organ are dependent on its functioning. Latterly (1906 and
1910) Roux has distinguished three periods, counting as the second the
transition period when form is partly self-determined, partly determined
by functioning. As this conception of Roux's is of the greatest
importance we shall follow it out in some detail.

The idea was first elaborated in the _Kampf der Theile_ (1881), where he
wrote:--"There must be distinguished in the life of all the parts two
periods, an embryonic in the broad sense, during which the parts
develop, differentiate and grow of themselves, and a period of completer
development, during which growth, and in many cases also the balance of
assimilation over dissimilation, can come about only under the influence
of stimuli" (p. 180). There is thus a period of self-differentiation in
which the organs are roughly formed in anticipation of functioning, and
a period of functional development in which the organs are perfected
through functioning and only through functioning. The two periods cannot
be sharply separated from one another, nor does the transition from the
one to the other occur at the same time in the different tissues and
organs.

The conception is more fully expressed in 1905 as follows:--"This
separation (of development into two periods) is intended only as a first
beginning. The first period I called the embryonic period [Greek: kat'
exochen] or the period of organ-rudiments. It includes the 'directly
inherited' structures, _i.e._, the structures which are directly
predetermined in the structure of the germ-plasm, as, for instance, the
first differentiation of the germ, segmentation, the formation of the
germ-layers and the organ-rudiments, as well as the next stage of
'further differentiation,' and of _independent_ growth and maintenance,
that is, of growth and maintenance which take place without the
functioning of the organs.

"This is accordingly the period of direct fashioning through the
activity of the formative mechanism implicit in the germ-plasm, also the
period of the self-conservation of the formed parts without active
functioning.

"The second period is the period of 'functional form-development.' It
includes the further differentiation and the maintenance in th