. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. TYPES OF CLEAVAGE 293 of the dark pigment, while in the lower portion of each blastomere the yellow- white yolk is concentrated. As a rule, the substance of the gray crescent is found in two of the blastomeres; the four blastomeres under the circumstances are not qualitatively equal. The third or latitudinal cleavage plane is at right angles to both of the foregoing and somewhat above the equator, dividing each of the four blas- tomeres into an an
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. TYPES OF CLEAVAGE 293 of the dark pigment, while in the lower portion of each blastomere the yellow- white yolk is concentrated. As a rule, the substance of the gray crescent is found in two of the blastomeres; the four blastomeres under the circumstances are not qualitatively equal. The third or latitudinal cleavage plane is at right angles to both of the foregoing and somewhat above the equator, dividing each of the four blas- tomeres into an animal pole micromere and a larger vegetal pole macromere -ANIMAL POLE. Fig. 141. Normal development of Rana sylvatica. (A) Egg at fertilization. (B) Formation of gray crescent, sharply defined at one hour after sperm entrance. (C) First cleavage furrow meridional. (D) Second cleavage furrow meridional. (E) Third cleav- age furrows, latitudinal in position. Four micromeres above and four macromeres below. (F) Fourth set of cleavage furrows, meridional in position, although some variation may exist and vertical furrows may occur. (G-1) Later cleavage stages. Pigmented pole cells become very small, and pigmented cells creep downward over vegetative pole area. (J) Appearance of dorsal blastoporal lip. (K) Blastoporal lips spread laterally, forming a broad. V-shaped structure. Pigmented cells proceed toward blastoporal lips. (L) Yolk-plug stage of gastrulation. (After Pollister and Moore, '37.). 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 Nelsen, Olin E. (Olin Everett), b. 1898. New York, Blakiston
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