Identifier: pedigreeofmanoth00haec (find matches)
Title: The pedigree of man : and other essays
Year: 1903 (1900s)
Authors: Haeckel, Ernst Heinrich Philipp August, 1834-1919
Subjects: Man Evolution Man Evolution
Publisher: London : A and H.B. Bonner
Contributing Library: ASC - York University Libraries
Digitizing Sponsor: York University - University of Toronto Libraries

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astric cavity (d)and oral opening (o) of the Gastrula. In all true animals or Metazoa, in the course of theadvancing development of the individual, appears anembryonic form that can be referred to such a Gas-trula condition. This is, however, always wanting inthe Protozoa. The cup-shaped, ovoid Gastrula (Fig. 61) contains asimple, hollow cavity, the digestive or gastric (g\ andthis opens by a mouth that serves for the ingestion offood (p). The wall of this cavity is composed of two 314 SENSE ORGANS. different cellular layers, the so-called primitive layersof the blastoderm. The inner, mucous layer, or endo-derm (i) effects the nutrition and assimilation of thebody, and from it are formed the organs of digestion.The outer serous layer or ectoderm, on the other hand,(e) is of special interest to us at present. For thesensory cells that compose it, have to do with theknowledge of the external world, and as the integu-ment of the Gastrula represent a sense-organ in itssimplest form. A . B
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Fig. 61. Gastrula of a calcareous sponge (Olynthus). A, seen fromwithout B, in longitudinal section, e. External embryonic)ayer (epidermal layer or ectoderm). i. Inner layer(mucous layer or endoderm). g. Primitive alimentarycanal or gastric cavity, o. Oral aperture. In all the Metazoa, not only do the cells that later onmake up the skin, develop from this external layer, butthe cells that form the nervous system and the rest ofthe sense-organs, have the same origin. Nerve-cellsand sense-cells are alike, therefore, primarily deriva-tives of epidermal cells, and Remak was perfectlyaccurate when, thirty years ago, he spoke of the integu- SENSE ORGANS. 315 mental covering of the embryo with its double wall asa sensory layer. Whilst most of the lower animals pass very rapidlythrough the Gastrula-stage in their development; thereare, at the present time, some lower animals that struc-turally are but little above this stage. Such permanentGastrula-forms are met with in the Gastroeada (Phys

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