. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. EMBRYOLOGY. 13 teron in connexion with the exterior. A similar, but less well defined proctodeum (fig. 7, p) arises at the hinder end of the digestive tract. Thus the anterior and posterior ends of the alimentary canal are ectodermal, the middle region entodermal, in origin. Metamerism.—In the adult, various parts, essentially like each other, are repeated one after another—are metameric. The list includes, among others, muscles, nerves, blood-vessels, vertebras, ribs, etc. There is much evidence to show that m
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. EMBRYOLOGY. 13 teron in connexion with the exterior. A similar, but less well defined proctodeum (fig. 7, p) arises at the hinder end of the digestive tract. Thus the anterior and posterior ends of the alimentary canal are ectodermal, the middle region entodermal, in origin. Metamerism.—In the adult, various parts, essentially like each other, are repeated one after another—are metameric. The list includes, among others, muscles, nerves, blood-vessels, vertebras, ribs, etc. There is much evidence to show that metamerism had its origin in the mesothelial structures and has been secondarily impressed on other svstems. \. FIG. 8.—Stereogram of a later stage than fig. 6, showing the segmentation of the meso- thelium. The approach of the walls of the coelom (c), dorsal and ventral to the alimentary canal, to form the mesenteries is shown, al, alimentary canal; em, epimere ;/&, forebrain; hb, hind brain; hm, hypomere; m, myotome; mb, midbrain; mm, mesomere; me, metaccele; myc, myocoele; n, nervous system; nc, notochord; s, stomodeal region; so, sp, somatic and splanchnic layers; st, sclerotome. The mesothelial ccelomic pouches, as left above, are near the dorsal side of the embryo. With growth they gradually extend downward on either side and tend to enclose the whole archenteron, and upward on either side of the notochord and spinal cord (fig. 8). The fates of the different parts of the mesothelial walls warrants the recognition of three horizontal regions or zones in the walls of each ccelom. These are a dorsal muscle-plate zone (epimere, em), a lower or lateral-plate zone (hypomere, hm), and a middle-plate zone (mesomere, mm) between them. All three of these occur in the trunk, but only the epimere is well developed in the anterior part of the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - col
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