The Evolution of Man — Volume 1 by Ernst Heinrich Philipp August Haeckel
page 162 of 358 (45%)
page 162 of 358 (45%)
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forms of animal life; these remain unicellular throughout life. To
this group belong the amoebae, gregarinae, rhizopods, infusoria, etc. As their whole organism consists of a single cell, they can never form germinal layers, or definite strata of cells. But all the other animals--all the tissue-forming animals, or metazoa, as we call them, in contradistinction to the protozoa--construct real germinal layers by the repeated cleavage of the impregnated ovum. This we find in the lower cnidaria and worms, as well as in the more highly-developed molluscs, echinoderms, articulates, and vertebrates. In all these metazoa, or multicellular animals, the chief embryonic processes are substantially alike, although they often seem to a superficial observer to differ considerably. The stem-cell that proceeds from the impregnated ovum always passes by repeated cleavage into a number of simple cells. These cells are all direct descendants of the stem-cell, and are, for reasons we shall see presently, called segmentation-cells. The repeated cleavage of the stem-cell, which gives rise to these segmentation-spheres, has long been known as "segmentation." Sooner or later the segmentation-cells join together to form a round (at first, globular) embryonic sphere (blastula); they then form into two very different groups, and arrange themselves in two separate strata--the two primary germinal layers. These enclose a digestive cavity, the primitive gut, with an opening, the primitive mouth. We give the name of the gastrula to the important embryonic form that has these primitive organs, and the name of gastrulation to the formation of it. This ontogenetic process has a very great significance, and is the real starting-point of the construction of the multicellular animal body. The fundamental embryonic processes of the cleavage of the ovum and |
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