The exact mode of formation of the diaphragm is not fully made out; the account of it recently given by Cadiat (No. [491]) not being in my opinion completely satisfactory.
Bibliography.
(491) M. Cadiat. “Du développement de la partie céphalothoracique de l'embryon, de la formation du diaphragme, des pleures, du péricarde, du pharynx et de l'œsophage.” Journal de l'Anatomie et de la Physiologie, Vol. XIV. 1878.
Vascular System.
The actual observations bearing on the origin of the vascular system, using the term to include the lymphatic system, are very scanty. It seems probable, mainly it must be admitted on à priori grounds, that vascular and lymphatic systems have originated from the conversion of indefinite spaces, primitively situated in the general connective tissue, into definite channels. It is quite certain that vascular systems have arisen independently in many types; a very striking case of the kind being the development in certain parasitic Copepoda of a closed system of vessels with a red non-corpusculated blood (E. van Beneden, Heider), not found in any other Crustacea. Parts of vascular systems appear to have arisen in some cases by a canalization of cells.
The blood systems may either be closed or communicate with the body cavity. In cases where the primitive body cavity is atrophied or partially broken up into separate compartments (Insecta, Mollusca, Discophora, etc.) a free communication between the vascular system and the body cavity is usually present; but in these cases the communication is no doubt secondary. On the whole it would seem probable that the vascular system has in most instances arisen independently of the body cavity, at least in types where the body cavity is present in a well-developed condition. As pointed out by the Hertwigs, a vascular system is always absent where there is not a considerable development of connective tissue.
As to the ontogeny of the vascular channels there is still much to be made out both in Vertebrates and Invertebrates.
The smaller channels often rise by a canalization of cells. This process has been satisfactorily studied by Lankester in the Leech[221], and may easily be observed in the blastoderm of the Chick or in the epiploon of a newly born Rabbit (Schäfer, Ranvier). In either case the vessels arise from a network of cells, the superficial protoplasm and part of the nuclei giving rise to the walls, and the blood-corpuscles being derived either from nucleated masses set free within the vessels (the Chick) or from blood-corpuscles directly differentiated in the axes of the cells (Mammals).
Larger vessels would seem to be formed from solid cords of cells, the central cells becoming converted into the corpuscles, and the peripheral cells constituting the walls. This mode of formation has been observed by myself in the case of the Spider’s heart, and by other observers in other Invertebrata. In the Vertebrata a more or less similar mode of formation appears to hold good for the larger vessels, but further investigations are still required on this subject. Götte finds that in the Frog the larger vessels are formed as longitudinal spaces, and that the walls are derived from the indifferent cells bounding these spaces, which become flattened and united into a continuous layer.
The early formation of vessels in the Vertebrata takes place in the splanchnic mesoblast; but this appears due to the fact that the circulation is at first mainly confined to the vitelline region, which is covered by splanchnic mesoblast.