, the connective
tissues, and the nerves belonging to these. In order to have a
convenient term for future use, I have named this layer the
muscle-layer" (p. 153).

The process of delamination results then in the formation of four
layers, of which the upper two (composing the "animal" or "serous"
layer) will give origin to the animal (neuromuscular) part of the body,
the lower pair to the plastic or vegetative organs. The uppermost layer
will form the external covering of the embryo, and also the amniotic
folds; from it there differentiates out at a very early stage the
rudiment of the central nervous system, forming a more or less
independent layer. Below the outermost layer lies the layer from which
are formed the muscular and skeletal systems, and beneath this
"muscle-layer" comes the "vessel-layer," which gives origin to the main
blood-vessels. The innermost layer of the four will form the mucous
membrane of the alimentary canal and its dependencies; at the present
stage, however, it is, like the other layers, a flat plate.

From all these layers tubes are developed by the simple bending round of
their edges. The outermost layer becomes the investing skin-tube of the
embryo; the layer for the nervous system forms the tubular rudiment of
the brain and spinal cord; the mucous layer curls round to form the
alimentary tube; the muscle layer grows upwards and downwards to form
the fleshy and osseous tube of the body wall; even the vessel layer
forms a tube investing the alimentary canal, but a part of it goes to
form the medial "Gekroese," or mesenterial complex, which departs
considerably from the tubular form.

When these tubes or "fundamental organs" are formed the process of
primary differentiation is complete. The fundamental organs, however,
have at no time actually the form of tubes; they exist as tubes only
ideally, for morphological and histological differentiation go on
concurrently with the process of primary differentiation.

Through morphological differentiation the various parts of the
fundamental organs become specialised, through unequal growth, first
into the primitive organs and then into the functional organs of the
body. "Single sections of the tubes originally formed from the layers
develop individual forms, which later acquire special functions: these
functions are in the most general way subordinate elements of the
function of the whole tube, but yet differ from the functions of other
sections