. Comparative anatomy. Anatomy, Comparative. 2l6 COMPARATIVE ANATOMY If an opinion were to be based upon the relation of the sternum in Necturus and of the ontogenesis of the mammalian sternum alone, we should have to conclude that the sternum arose from paired segmented cartilages formed near the mid-ventral line independently both of the girdle and of the ribs. Under the circumstances, and until more decisive evidence is discovered, suspension of judgment is necessary. In the reptiles the sternum is converted into a metameric structure composed of a series of sternebrae and connected with th
. Comparative anatomy. Anatomy, Comparative. 2l6 COMPARATIVE ANATOMY If an opinion were to be based upon the relation of the sternum in Necturus and of the ontogenesis of the mammalian sternum alone, we should have to conclude that the sternum arose from paired segmented cartilages formed near the mid-ventral line independently both of the girdle and of the ribs. Under the circumstances, and until more decisive evidence is discovered, suspension of judgment is necessary. In the reptiles the sternum is converted into a metameric structure composed of a series of sternebrae and connected with the ribs as in Fig. 173.—Diagrams showing the development of the primordial skull. Since this organ develops primarily beneath the brain as a support the figures represent the ventral aspect. (A) Early stage, before the appearance of cartilage. The notochord is seen lying along the nerve cord as far forward as the hypophysis. The three sense-organs, nose, eye, and ear, have already appeared. (B) This stage shows the trabeculae (l), the parachordals (p), and the capsules around the sense-organs. (C) In this the trabeculae, the parachordals, and the nasal and otic capsules have fused into a single mass, the primordial skull, or chondrocranium. The anterior end of the notochord is imbedded in this. The cartilaginous capsule of the eye remains free to allow the necessary movements of the eyeball. (From Wilder's "History of the Human Body," Courtesy of Henry Holt and Co.) The mammalian sternum differs little from that of reptiles. It is divided into the same three elements as those of reptiles and man, pro- meso-, and meta-sternum. The Skull. There are two chief parts to the skull, which have different origins and a different history. One of these is the cranium or brain-case, together with the bones of the face except the two jaws. The other is the visceral skeleton, that is to say, the two jaws, the hyoid bone, the ear bones, and the cartilages of the larynx. T
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