. The chordates. Chordata. Reproduction 251 importance is that it gives the embryonic material increased superficial contact with the environment, thus favoring metabolism. Its prospec- tive significance lies in the fact that further development is to consist, a' Fig. 208. Cleavage of the germ-disk of the egg of a turtle (Glyptemys in- sculpta); eight-cell stage. The eggshell is not shown. About twice natural size. (A) Albumen; (C) the eight-cell blasto- derm; (Y) yolk. (After Louis Agassiz. Courtesy, Neal and Rand: "Chordate ; Philadelphia, The Rlak- iston Company.) to a lar
. The chordates. Chordata. Reproduction 251 importance is that it gives the embryonic material increased superficial contact with the environment, thus favoring metabolism. Its prospec- tive significance lies in the fact that further development is to consist, a' Fig. 208. Cleavage of the germ-disk of the egg of a turtle (Glyptemys in- sculpta); eight-cell stage. The eggshell is not shown. About twice natural size. (A) Albumen; (C) the eight-cell blasto- derm; (Y) yolk. (After Louis Agassiz. Courtesy, Neal and Rand: "Chordate ; Philadelphia, The Rlak- iston Company.) to a large extent, in the manipulation of layers of embryonic material. The adult is hollow. It has a body-cavity and other cavities. Most of its organs are hollow. The walls of the hollow structures are constituted. Fig. 209. Early blastoderm of chick; plane of section passes through center of egg. (R) Rlastocoele (subgermi- nal or cleavage cavity); (C) cells of blastoderm; (V) fluid-filled vesicles; (Y1) yellow yolk; (Y2) white yolk. (Magnified nearly 20 diameters.) (After Duval. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Rlakiston Company.) of layers—skin, epithelium, endothelium, peritoneum, muscle layers, connective-tissue layers. For the construction of such a many-layered thing, the embryo naturally proceeds as early as possible to dispose its building material in the form of 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 Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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