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In the unarticulated stem-forms of the Chordonia, which we have called the Prochordonia, the two coelom-pouches, and therefore also the muscle-plates of their walls, were not yet segmented. This segmentation of the muscles was the momentous historical process with which vertebration, and the development of the vertebrate stem, began.
The double cervical cavity of the Amniotes is very interesting, both from the anatomical and the evolutionary point of view; it corresponds to a part of the hyposomites of the head of the lower Vertebrates that part of the ventral coelom-pouches which comes next to Van Wijhe's "visceral cavities" below.
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.
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.
So also the two coelom-pouches of the head in which they lie are still separated by a broad space. It is not until the permanent body of the embryo develops and detaches from the embryonic vesicle that the separate lateral structures join together, and finally combine in the middle line.
At first the chorda only consists of a single row of large entodermic cells. It is afterwards composed of several rows of cells. In the Ascidia-larva, also, the chorda develops from the dorsal middle part of the primitive gut, while the two coelom-pouches detach themselves from it on both sides. The simple body-cavity is formed by the coalescence of the two.
Simultaneously with the construction of the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the coelom-pouches, and the mesoderm proceeding from their wall. These processes also take place with characteristic simplicity and clearness, so that they are very instructive to compare with the vermalia on the one hand and with the higher vertebrates on the other.
Immediately after their formation the two coelom-pouches of the Amphioxus are divided into several parts by longitudinal and transverse folds. They have a different future above and below. The upper or dorsal segments, the episomites, lose their cavity later on, and form with their cells the muscular plates of the trunk.
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