Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1904
Catalogue details

Licence: In copyright

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

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    be induced to conclude that the coelom was probably evolved from lateral pouches of the gut and that the mesoderm is therefore derived from the primitive endoderm. Where pole-cells occur the cavity of the alimentary canal is small in proportion to the thickness of its wall, and the pole-cell might be looked on as a solid pouch. Fig. 39. Two stages in the early development of a common fresh-water mollusc, Planorbis, to show the origin of the mesoderm cells x 320. From Rabl. The ectoderm cells are deeply shaded, the endoderm cells are unshaded. A. Young stage in which the endoderm has not begun to be invaginated ; it is a lateral optical section. B. Older stage, optical section seen in front view; the endoderm cells are invaginating, and the two mesoderm cells are seen on each side. 1. Mesoderm or pole-cells; in B, each has budded off another mesoderm cell. In most Coelomata the mesoderm forms by far the greatest portion of the body, and it may be roughly stated that the mesoderm gives rise in the fully developed animal to “ the muscles, the bones, the connective tissue, both arteries and veins, capillaries and lymphatics, with their appropriate epithelium/’ and to the ex- cretory and generative organs. In the Porifera and in the more complex Coelenterata, where the thin structureless lamella of Hydra has swollen into a bulky mass of jelly such as we find in a Medusa, cells begin to wander into it from the ectoderm, and thus a kind of tissue is formed to which the name mesoderm has unfortunately been applied. We see that its origin and nature are quite different, although both in Sponges and Coelenterata it may give rise to the skeleton, and in
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