. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Development of the Individual 169 Segmentation Cavity In reptiles and birds the results of segmentation are still further modified by the relatively enormous amount of yolk present. The nucleus of the ferti- lized egg undergoes the usual mitoses, but the new cell boundaries fail to be extended so as to include the great sphere of yolk material. The result is a patch or disc of crowded blasto- ^dS@S&£&z~ Animal Pole meres of unequal size at the
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Development of the Individual 169 Segmentation Cavity In reptiles and birds the results of segmentation are still further modified by the relatively enormous amount of yolk present. The nucleus of the ferti- lized egg undergoes the usual mitoses, but the new cell boundaries fail to be extended so as to include the great sphere of yolk material. The result is a patch or disc of crowded blasto- ^dS@S&£&z~ Animal Pole meres of unequal size at the ani- mal pole, the larger cells with incomplete boundaries being at the periphery (Fig. 121). Al- though earlier even the central cells were not separated from the yolk below, they have by this time split off leaving a space, the seg- mentation cavity, between them and the yolk (Fig. 122). In the amphibian egg, in which the segmentation cavity has walls of unequal thickness, a slight invagination to form a groove appears on one side at the point where the upper thin wall passes over into the lower thick wall of the vegetal pole. The small cells of the upper lip (on the animal pole side) of the groove now proliferate rapidly, and the lip begins to grow down. m— Yolk Vegetal Pole Fig. 120. A hemisected blastula of the frog. (From Huettner, Fundamentals of Compara- tive Embryology of the Vertebrates, copy- right 1941, by permission of The Macmillan Company, publishers.). 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 Walter, Herbert Eugene, b. 1867; Sayles, Leonard Perkins, 1902-. New York : Macmillan Co.
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
