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The space occupied by the yolk corresponds to the archenteron or primitive digestive cavity; and the opening at the end to the primitive mouth or blastopore. Ectoderm and entoderm unite around the mouth. Both the blastosphere and gastrula often swim freely by flagella.
In the middle, between the two lateral coelom-folds of the primitive gut, a single central organ detaches from this at an early stage in the middle line of its dorsal wall. This axial rod, which is the first foundation of the later vertebral column in all the vertebrates, and is the only representative of it in the Amphioxus, originates from the entoderm.
The inner layer or the invaginated part of the blastoderm, which immediately encloses the gut-cavity is the entoderm, the inner or vegetal germ-layer, from which develop the wall of the alimentary canal and all its appendages, the coelom-pouches, etc. The cells of the entoderm are much larger, darker, and more fatty than those of the ectoderm, which are clearer and less rich in fatty particles.
These longitudinal folds of the entoderm proceed from the primitive mouth, or from its lower and hinder edge. They indicate the original starting-point of the two coelom-pouches, which grow from this spot between the inner and outer germinal layers, sever themselves from the primitive gut, and provide the cellular material for the middle layer.
"No thoroughfare" is often written across paths previously followed by blood or nervous impulses, when other paths have been found more economical or effective. But where did the notochord come from? I do not know. It always forms in the embryo out of the entoderm or layer which becomes the lining of the intestine.
In higher forms it becomes of continually greater importance, until finally nearly all the organs of the body develop from it. In our bodies only the lining of the mid-intestine and of its glands has arisen from the entoderm. And only the epidermis, or outer layer of our skin, and the nervous system and parts of our sense-organs have arisen from the ectoderm.
This is shown in the fact that each entodermal cell digests its own particles of food, although the nutriment once digested diffuses to all parts of the body. Also almost any part of the animal containing both ectoderm and entoderm can be cut off and will develop into a new animal.
The cells of both layers have at their bases long muscular fibrils, those of the ectodermal cells running longitudinally, those of the entoderm transversely. The animal can thus contract its body in both directions, or, if the body contain water and the transverse muscles are contracted, the pressure of the water lengthens the body and tends to extend the tentacles.
Their earliest source is the primary body-cavity, the simple space between the two primary germinal layers, which is either a relic of the segmentation-cavity, or is a subsequent formation. Amoeboid planocytes, which migrate from the entoderm and reach this fluid-filled primary cavity, live and multiply there, and form the first colourless blood-cells.
But, broadly speaking, we may take it as a positive and important fact that in man and the higher animals the chief part of the animal organs comes from the ectoderm, and the greater part of the vegetative organs from the entoderm.
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