. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 132 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. MESEXCHYMATOU5 STRUCTURES. As was stated on a preceding page (p. 8), the mesenchyme may arise from ectoderm, entoderm, or mesothelium, either by the separation of isolated cells, as is usually the case, or by the immigration of large masses of cells into the space (Le., the re- mains of the segmentation cavit}'^ between the other body la} ers. This immigration in large masses from the mesothelium is shown in the formation of the sclerotomes in Fig. 112, and from the ectoderm into
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 132 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. MESEXCHYMATOU5 STRUCTURES. As was stated on a preceding page (p. 8), the mesenchyme may arise from ectoderm, entoderm, or mesothelium, either by the separation of isolated cells, as is usually the case, or by the immigration of large masses of cells into the space (Le., the re- mains of the segmentation cavit}'^ between the other body la} ers. This immigration in large masses from the mesothelium is shown in the formation of the sclerotomes in Fig. 112, and from the ectoderm into the region of the head to form the gill cartilages in Fig. 138. The mesench^Tne is characterized by the fact that it never gives rise to epithelial structures,^ and as a rule, b}' the great development in it of intercellular substance, as seen in fibrous or areolar connective tissue, cartilage, bone, blood, etc. Smooth muscle tissue, however, is an ex- ception in this respect. Besides the connective tissues proper, which extend through all parts of the body, forming a sup- port and connection for tissues and organs, the mesenchyme also gives rise to most of the skeletal and circulatory structures. 1 It is possible that the epitheliam (endothelium) lining the cavities of the viscular system is of roesenchymatous origin, but ttie weight of evidence goes to show that some of it at least is ot entj-dcrmic Fig. 138. Section throngh the head of an embryo Amblysloma, showing the points, H and J/, where the ectoderm is producing the mesenchyme to form the hyoid and mandibular arches. A, audi- tory ganglion; C, coelom of man- dibular arch; C[., cnticnlar layer of ectoderm; MO, medulla oh>- longata; ^\'/, nervous layer of ectoderm; VII, facial 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 Kingsle
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