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In general, it is certain that the greater part of the cells that compose the vessels and their contents come from the mesoderm in fact, from the gut-fibre layer; it was on this account that Baer gave the name of "vascular layer" to this visceral layer of the coeloma. But other important observers say that a part of these cells come from other germinal layers, especially from the gut-gland layer.

The coeloma or body-cavity has some very important and distinctive features in the Amphioxus. The embryology of it is most instructive in connection with the stem-history of the body-cavity in man and the other vertebrates.

We should be quite unable to explain these if comparative anatomy and ontogeny did not come to our assistance. The vascular system in man and all the Craniotes is an elaborate apparatus of cavities filled with juices or cell-containing fluids. With the latter are connected the large cavities of the body, especially the body-cavity, or coeloma.

The oesophagus descends into the pectoral cavity along the vertebral column, behind the lungs and the heart, pierces the diaphragm, and enters the visceral cavity. This separation is not found in the beginning; there is at first a common breast-belly cavity, the coeloma or pleuro-peritoneal cavity.

In some there is another gland besides the liver, and this is taken to represent the kidneys. The body-cavity proper, or coeloma, which is filled with blood and encloses the hepatic gut, is very narrow in the Ascidia, as in the Amphioxus, and is here also usually confounded with the wide atrium, or peribranchial cavity, full of water.