. Elementary text-book of zoology [electronic resource]. Zoology. ORIGIN OF ORGANS. 53 In other instances, it may arise as a split in a solid mass of mesoblast which has been itself produced by delamination or by polar ingression. This origin is called schizo-coelic. A third origin of the coelom is found in the case in which the mesoderm arises by inultipolar ingression. In this case the ingressive cells arrange themselves in two layers to en- close the coelom, which is thus a transformed part of the archiccele. Hence this origin is called archiavlic. The same methods of origin for the archent
. Elementary text-book of zoology [electronic resource]. Zoology. ORIGIN OF ORGANS. 53 In other instances, it may arise as a split in a solid mass of mesoblast which has been itself produced by delamination or by polar ingression. This origin is called schizo-coelic. A third origin of the coelom is found in the case in which the mesoderm arises by inultipolar ingression. In this case the ingressive cells arrange themselves in two layers to en- close the coelom, which is thus a transformed part of the archiccele. Hence this origin is called archiavlic. The same methods of origin for the archenteron of diploblastic larvae can be made out. It will, however, be clearly seen that the origin of the layer itself (hypoblast or P'ig. 20.—The Origin of an The upper row shows the_ cj^ic origin by single ("dark) cells detached from parent layer (light). The middle row shows the dermic origin and the lower the tomic. mesoblast), and not that of the cavity, is the important con- sideration. We must regard the primary layers of hypoblast and mesoblast as organs, and as such they arise according to circumstances in any of the ways in which an organ may arise. Thtse may be conveniently generalised as follows:— 1. As a number of detached cells from the parent layer {cyiic). These may be diffused or localised in their origin. 2. As a layer of cells or epithelium detached from the parent layer {dermic). 3. As a hollow sac of cells invaginated from the parent sac {tomic). Organs originating in these ways from the three primary layers form together the complex organisms found. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Masterman, Arthur Thomas; Parsons, John Herbert, Sir, 1868-1957, donor; S. H. Lazarus (Firm); University College, London. Library Services. Edinburgh : E. & S. Livingstone
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