. Thus the nerve-tube differentiates into sense-organs, brain
and spinal cord, the alimentary tube into mouth cavity, oesophagus,
stomach, intestine, respiratory apparatus, liver, bladder, etc. This
specialisation in development is bound up with increased or diminished
growth" (p. 155). Rapid growth concentrated at one point brings about an
evagination; in this manner are formed the sense-organs from the
nerve-tube, the liver and lungs from the alimentary tube. Or increased
growth over a section of a tube causes it to swell out; in this wise the
brain develops from the nerve-tube, the stomach from the alimentary
tube. The segmentation which soon becomes so marked, particularly in the
muscle layer, is also due to a process of morphological differentiation.

At the same time that the organs of the body are being thus roughly
blocked out and moulded from the germ-layers the third process of
differentiation is actively going on. "In addition to the
differentiation of the layers, there follows later another
differentiation in the substance of the layers, whereby cartilage,
muscle and nerve separate out, a part also of the mass becoming fluid
and entering the bloodstream" (p. 154). Through histological
differentiation the texture of the layers and incipient organs becomes
individualised. In its earliest appearance the germ consists of an
almost homogeneous mass, containing clear or dark globules suspended in
its substance (ii., p. 92). This homogeneity gives place to
heterogeneity; the structureless mass becomes fibrous to form muscles,
hardens to form cartilage or bone, becomes liquid to form the blood,
differentiates in a hundred other ways--into absorbing and secreting
tissues, into nerves and ganglia, and so forth. It will be noticed that
the concept of histological differentiation is independent of the
cell-theory; it signifies that textural differentiation which leads to
the formation of tissues in Bichat's sense. The tissues and the
germ-layers stand in fairly close relation with one another, for while
certain tissues are formed chiefly but not exclusively in one layer,
others are formed only in one layer and never elsewhere. For example,
peripheral nerves are for the most part formed in the muscle layer,
though the bulk of the nervous tissue is formed in the walls of the
nerve tube; similarly blood and blood-vessels may arise from almost any
layer, though their chief seat of origin is the vessel-layer; on the
other ha