In the Amphibia there is no structure which admits from its mode of development of a complete comparison with the sternum of the Amniota; and it must for this reason be considered doubtful whether the median structure placed behind the coracoids in the Anura, which is usually known as the sternum, is really homologous with the sternum of the Amniota[202].
The remaining Ichthyopsida are undoubtedly not provided with a sternum.
Bibliography of Ribs and Sternum.
(430) C. Claus. “Beiträge z. vergleich. Osteol. d. Vertebraten. I. Rippen u. unteres Bogensystem.” Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.
(431) A. E. Fick. “Zur Entwicklungsgeschichte d. Rippen und Querfortsätze.” Archiv f. Anat. und Physiol. 1879.
(432) C. Gegenbaur. “Zur Entwick. d. Wirbelsäule des Lepidosteus mit vergleich. anat. Bemerk.” Jenaische Zeit., Vol. III. 1867.
(433) A. Götte. “Beiträge z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergürtel.” Archiv f. mikr. Anat., Vol. XIV. 1877.
(434) C. Hasse u. G. Born. “Bemerkungen üb. d. Morphologie d. Rippen.” Zoologischer Anzeiger, 1879.
(435) C. K. Hoffmann. “Beiträge z. vergl. Anat. d. Wirbelthiere.” Niederländ. Archiv Zool., Vol. IV. 1878.
(436) W. K. Parker. “A monograph on the structure and development of the shoulder-girdle and sternum.” Ray Soc. 1867.
(437) H. Rathke. Ueb. d. Bau u. d. Entwicklung d. Brustbeins d. Saurier. 1853.
(438) G. Ruge. “Untersuch. üb. Entwick. am Brustbeine d. Menschen.” Morphol. Jahrbuch., Vol. VI. 1880.
[199] This tube consists of a peculiar form of fibrous tissue rather than true cartilage, though part of it subsequently becomes hyaline cartilage.
[200] The presence of intercalated pieces in the neural arch system of Elasmobranchii, Chimæra, etc. is probably not the indication of an highly differentiated type of neural arch, but of a transitional type between an imperfect investment of the spinal cord by isolated cartilaginous bars, and a complete system of neural arches like that in the higher Vertebrata.
[201] This view was first put forward by Lushka, and his surmises have been confirmed by Kölliker and other embryologists. Leboucq (No. [424]) however holds that the cells of the notochord in the intervertebral regions fuse with those of the adjoining tissue; and Dursy and others deny that the nucleus pulposus is derived from the notochord.
[202] The so-called sternum of the Amphibia develops in proximity with certain rudimentary abdominal ribs, and Ruge has with some force urged (against Götte) that it may be for this reason a rudimentary structure of the same nature as the sternum of the higher types.