. The chordates. Chordata. Reproduction 277 Mesoderm. The vertebrate mesoderm is at first devoid of segmen- tation and ordinarily contains no definite cavity (Fig. 215). At an early embryonic stage, the mesoderm upon either side splits into two layers: an outer lying against the ectoderm, and an inner lying against the endoderm. The two layers remain connected, however, at the upper and thicker region of the original sheet (Figs. 228, 229A). At about the same time, this dorsal and thicker zone of the mesoderm develops trans- verse fissures which divide it into a series of paired blocks (somite
. The chordates. Chordata. Reproduction 277 Mesoderm. The vertebrate mesoderm is at first devoid of segmen- tation and ordinarily contains no definite cavity (Fig. 215). At an early embryonic stage, the mesoderm upon either side splits into two layers: an outer lying against the ectoderm, and an inner lying against the endoderm. The two layers remain connected, however, at the upper and thicker region of the original sheet (Figs. 228, 229A). At about the same time, this dorsal and thicker zone of the mesoderm develops trans- verse fissures which divide it into a series of paired blocks (somites) lying symmetrically on either side of the neural tube (Figs. 228, 230). This segmentation begins in the anterior part of the embryo and progresses backward just as, in Amphioxus, the mesodermal pouches are formed successively from anterior to posterior. The process of segmentation involves only the upper part of the mesoderm. As segmentation goes on, the space between the lower thin and unsegmented layers on either side becomes wider—a space already recognizable as the coelom bounded externally by a somatopleure consisting of ectoderm and the outer sheet of mesoderm, and internally by a splanchnopleure consisting of endoderm and the adjacent layer of mesoderm. The mesodermal layers upon either side grow down to >INAL CORD •EPIMERE PRIMITIVE DUCT NOTOCHORD NEPHROSTOME. 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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