. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. REPRODUCTION AND DEVELOPMENT 273 lip of the blastopore becomes differentiated into a plate that sinks downward to form a groove. This plate of ectodermal cells is destined to form practically the entire nervous system (Fig. 149). The boundaries of the groove close over and fuse, leaving the future nerve elements as a tube, under the ectoderm and surrounded by mesoderm. The dilations of the anterior end of this neural tube to THYROID VISCERAL POUCHES TRACHEA STOMACH PANCREAS INTESTINE. -MOUTH
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. REPRODUCTION AND DEVELOPMENT 273 lip of the blastopore becomes differentiated into a plate that sinks downward to form a groove. This plate of ectodermal cells is destined to form practically the entire nervous system (Fig. 149). The boundaries of the groove close over and fuse, leaving the future nerve elements as a tube, under the ectoderm and surrounded by mesoderm. The dilations of the anterior end of this neural tube to THYROID VISCERAL POUCHES TRACHEA STOMACH PANCREAS INTESTINE. -MOUTH LIVER YOLK STALK ALLANTOIS ANAL PLATE Fig. 185.—The primitive digestive system of the chick embryo after 4 days' in- cubation of the ^gg. (After Patten: Embryology of the Chicly, published by P. Blakis- ton's Son and Company.) form the basic portions of the brain have already been described (p. 217), as have also the origin of the ganglia of the spinal cord and of the sympathetic nervous system, and other parts of the adult vertebrate nervous mechanism. The Alimentary Canal. The endoderm in invertebrates be- comes rapidly differentiated into a functional alimentary tract. In diploblastic forms and in the more primitive triploblastic animals, for example the Platyhelminthes, it does not acquire a second open- ing. But in the more advanced invertebrates and in all chordates a contact forms between the ectoderm and the anterior end of the. 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 Buchanan, James William, 1888-. New York, London, Harper & brothers
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