. Text-book of embryology. Embryology. 364 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. Fig. 174, B, the cavity was perfectly continuous towards the head end, while posteriorly it was still in the form of isolated vacuoles. The cells which form the rudiment of the dorsal aorta are from their coarsely-yolked character clearly derived ultimately from the primitive endoderm, but the question remains whether they are derived from the definitive endoderm directly or through the inter-. FlG. 174.—Portions of horizontal sections through Polypterus larvae of stages 21+ (A) and 25 (B) showing the rudiment o
. Text-book of embryology. Embryology. 364 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. Fig. 174, B, the cavity was perfectly continuous towards the head end, while posteriorly it was still in the form of isolated vacuoles. The cells which form the rudiment of the dorsal aorta are from their coarsely-yolked character clearly derived ultimately from the primitive endoderm, but the question remains whether they are derived from the definitive endoderm directly or through the inter-. FlG. 174.—Portions of horizontal sections through Polypterus larvae of stages 21+ (A) and 25 (B) showing the rudiment of the aorta in longitudinal section. A, aortic rudiment; vae, vacuoles. mediary of the mesoderm. Such a section as that shown in Fig. 175 indicates that the latter is the case. The definitive endoderm shows a perfectly sharply defined surface, clearly marked off from the aortic rudiment, while the mesoderm of the sclerotome on the other hand is continuous with the aortic rudiment. We may say therefore with high probability that the aortic cells are derived from the sclerotome. A remarkable feature has been noticed in the development 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 MacBride, E. W. (Ernest William), 1866-1940; Kerr, John Graham, 1869-; Heape, Walter, 1855-1929. London : Macmillan
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