. Comparative anatomy. Anatomy, Comparative. THE DIGESTIVE SYSTEM 205 by the hard and soft palates, which separate the mouth cavity from the narial passage above. Development of the Mouth At a relatively late stage of ontogenesis, at the anterior end of the fore-gut where the mouth is to break through, the ectoderm invaginates to form the stomodeum, the depth of which, in amniotes, is considerably increased by the expansion of the fore-brain in front and the heart behind. At the bottom of the stomodeum, ectoderm and endoderm are in contact as a two-layered membrane, which ruptures and disappea
. Comparative anatomy. Anatomy, Comparative. THE DIGESTIVE SYSTEM 205 by the hard and soft palates, which separate the mouth cavity from the narial passage above. Development of the Mouth At a relatively late stage of ontogenesis, at the anterior end of the fore-gut where the mouth is to break through, the ectoderm invaginates to form the stomodeum, the depth of which, in amniotes, is considerably increased by the expansion of the fore-brain in front and the heart behind. At the bottom of the stomodeum, ectoderm and endoderm are in contact as a two-layered membrane, which ruptures and disappears leaving no dicnccphalon epiphysis lateral telencephalic vesicle. hyomandibolar cleft hyoid arch visceral arch in Fig. 247.—Drawing to show the external appearance of the structures in the oral region of a four-day chick. Ventral aspect. (From Patten's " Embrvology of the ;) trace in the adult. Thus, in forming the mouth, the ectoderm takes the initiative, whereas the gill-slits are primarily outpocketings of the endoderm. The covering of the lips and gums is derived from the ectodermal stomodeum, while that of the rest of the mouth is endodermal. The salivary glands arise from the epithelial lining of the mouth and are generally supposed to be ectodermal in origin. The enamel of the teeth is a product of the stomodeal wall, and therefore, although within the digestive cavity, ectodermal. Evolution of the Mouth There is no doubt that the mouths of all vertebrates are homologous, the sucking mouth of cyclostomes being no exception. Cyclostome and gnathostome mouths have the same fundamental structure, development, and relations to other parts, and must therefore be considered 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 Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, H
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