Mammalia. The early development of the perichordal cartilaginous tube and rudimentary neural arches is almost the same in Mammals as in Birds. The differentiation into vertebral and intervertebral regions is the same in both groups; but instead of becoming divided as in Reptilia and Birds into two segments attached to two adjoining vertebræ, the intervertebral regions become in Mammals wholly converted into the intervertebral ligaments ([fig. 322] li). There are three centres of ossifications for each vertebra, two in the arch and one in the centrum.
Fig. 322. Longitudinal section through the intervertebral ligament and adjacent parts of two vertebræ from the thoracic region of an advanced embryo of a Sheep. (From Kölliker.)
la. ligamentum longitudinale anterius; lp. ligamentum long. posterius; li. ligamentum intervertebrale; k, k´. epiphysis of vertebra; w. and w´. anterior and posterior vertebræ; c. intervertebral dilatation of notochord; c´. and c´´. vertebral dilatation of notochord.
The fate of the notochord is in important respects different from that in Birds. It is first constricted in the centre of the vertebræ ([figs. 320] E and [321]) and disappears there shortly after the ossification; while in the intervertebral regions it remains relatively unconstricted ([figs. 320] E, [321] and [322] c) and after undergoing certain histological changes remains through life as part of the nucleus pulposus in the axis of the invertebral ligaments[201]. There is also a slight swelling of the notochord near the two extremities of each vertebra ([fig. 322] c´ and c´´). In the persistent vertebral constriction of the notochord Mammals retain a more primitive and piscine mode of formation of the vertebral column than the majority either of the Reptilia or Amphibia.
Bibliography of Notochord and Vertebral column.
(415) Cartier. “Beiträge zur Entwicklungsgeschichte der Wirbelsäule.” Zeitschrift für wiss. Zool., Bd. XXV. Suppl. 1875.
(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomie der Wirbelsäule der Amphibien und Reptilien. Leipzig, 1862.
(417) C. Gegenbaur. “Ueber die Entwickelung der Wirbelsäule des Lepidosteus mit vergleichend anatomischen Bemerkungen.” Jenaische Zeitschrift, Bd. III. 1863.
(418) C. Gegenbaur. “Ueb. d. Skeletgewebe d. Cyclostomen.” Jenaische Zeitschrift, Vol. V. 1870.
(419) Al. Götte. “Beiträge zur vergleich. Morphol. des Skeletsystems d. Wirbelthiere.” II. “Die Wirbelsäule u. ihre Anhänge.” Archiv f. mikr. Anat., Vol. XV. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. XVI. 1879 (Teleostier).
(420) Hasse und Schwarck. “Studien zur vergleichenden Anatomie der Wirbelsäule u. s. w.” Hasse, Anatomische Studien, 1872.
(421) C. Hasse. Das natürliche System d. Elasmobranchier auf Grundlage d. Bau. u. d. Entwick. ihrer Wirbelsäule. Jena, 1879.
(422) A. Kölliker. “Ueber die Beziehungen der Chorda dorsalis zur Bildung der Wirbel der Selachier und einiger anderen Fische.” Verhandlungen der physical. medicin. Gesellschaft in Würzburg, Bd. X.
(423) A. Kölliker. “Weitere Beobachtungen über die Wirbel der Selachier insbesondere über die Wirbel der Lamnoidei.” Abhandlungen der senkenbergischen naturforschenden Gesellschaft in Frankfurt, Bd. V.
(424) H. Leboucq. “Recherches s. l. mode de disparition de la corde dorsale chez les vertébrés supérieurs.” Archives de Biologie, Vol. I. 1880.
(425) Fr. Leydig. Anatomisch-histologische Untersuchungen über Fische und Reptilien. Berlin, 1853.
(426) Aug. Müller. “Beobachtungen zur vergleichenden Anatomie der Wirbelsäule.” Müller’s Archiv. 1853.
(427) J. Müller. “Vergleichende Anatomie der Myxinoiden u. der Cyklostomen mit durchbohrtem Gaumen, I. Osteologie und Myologie.” Abhandlungen der königlichen Akademie der Wissenschaften zu Berlin. 1834.
(428) W. Müller. “Beobachtungen des pathologischen Instituts zu Jena, I. Ueber den Bau der Chorda dorsalis.” Jenaische Zeitschrift, Bd. VI. 1871.
(429) A. Schneider. Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere. Berlin, 1879.
Ribs and Sternum.
Ribs. Embryological evidence on the development of the ribs, though somewhat inadequate, indicates that they arise as cartilaginous bars in the connective tissue of the intermuscular septa, and that they are placed, in Elasmobranchii and Amphibia, on the level of division between the dorso-lateral and ventro-lateral divisions of the muscle-plates. This does not appear to hold true for either Ganoidei or Teleostei. In Teleostei they are entirely below the muscles along the lines of the intermuscular septa, and this is partially true for Ganoidei, though not wholly so in Lepidosteus. They may be attached either to the hæmal (Pisces) or neural (Amphibia and Amniota) arches. The connective tissue from which they are formed is continuous with the processes of the vertebræ to which they are attached; but the conversion of the tissue into cartilage takes place more or less independently of that of the arches, although in many cases the cartilage of the two becomes continuous, the separation of the ribs being then effected by a subsequent process of segmentation (Fick, No. [431]). It is possible that the ribs of Pisces may not be homologous with those of Amphibia and the Amniota, but till the reverse can be proved it is more convenient to assume that the ribs are homologous structures throughout the vertebrate series.
In Elasmobranchii the ribs are relatively of less importance in the adult than in the embryo. By a careful examination of their early development, I have satisfied myself that the differentiation of the ribs is independent of that of the hæmal processes to which they are attached, although the differentiation proceeds in such a manner that, when both are converted into cartilage, they are quite continuous. Subsequently the ribs become segmented off from the hæmal processes. At the junction of the tail and trunk, where the hæmal processes commence to be ventrally prolonged, eventually to unite in the region of the tail below the caudal vein, the ribs are attached to short processes which spring from the sides of the hæmal arches ([fig. 317]). The ventral hæmal arches of these fishes are therefore clearly in no part formed by the ribs.