. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Development of the Individual 161 great extent, they tend to form a continuous layer that is more than sufficient to make up the original surface of the sphere in this region. They are, there- fore, forced to find more standing room which is accomplished by their pushing into the segmentation cavity, with the result that a double cup, or gastrula, is formed (Fig. 112). Ectoderm^ -Endoderm. Archenteron Fig. 112. Gastrulation in the development of amphio
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Development of the Individual 161 great extent, they tend to form a continuous layer that is more than sufficient to make up the original surface of the sphere in this region. They are, there- fore, forced to find more standing room which is accomplished by their pushing into the segmentation cavity, with the result that a double cup, or gastrula, is formed (Fig. 112). Ectoderm^ -Endoderm. Archenteron Fig. 112. Gastrulation in the development of amphioxus. (From Huett- ner, Fundamentals of Comparative Embryology of the Vertebrates, copyright 1941, by permission of The Macmillan Company, publishers.) The outer layer of this cup is the ectoderm, the inner layer the endo- derm (or "entoderm" of some authors). The new cavity within the cup is termed the archenteron, or primitive digestive cavity, and its opening to the exterior, the blastopore. The latter is at the posterior end of the embryo. The inpushing (invagination) of the endoderm continues until the seg- mentation cavity is almost obliterated. Meanwhile the addition of new material in a growth region near the posterior end of the gastrula, around the blastopore, brings about an elongation of the embryo and a marked decrease in the size of the blastopore. As this elongation is more rapid. 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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