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   Manual of Human Embryology I 1910: The Germ Cells | Fertilization | Segmentation | First Primitive Segment | Gastrulation | External Form | Placenta | Human Embryo and Fetus Age | Ovum Pathology | Integument | Skeleton and Connective Tissues | Muscular System | Coelom and Diaphragm | Figures | Manual of Human Embryology 1 | Manual of Human Embryology 2 | Franz Keibel | Franklin Mall | Embryology History

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XI. Development of the Skeleton and of the Connective Tissues

By CHARLES R. BARDEEN, Madison, Wis.

Skeleton and Connective Tissues: Connective Tissue Histogenesis | Skeletal Morphogenesis | Chorda Dorsalis | Vertebral Column and Thorax | Limb Skeleton | Skull Hyoid Bone Larynx

B. Origin and Fate of the Chorda Dorsalis

The cervical region of the chorda dorsalis appears to arise from the dorsal wall of the entero vitelline sac beneath the medullary groove, although it is not probable that these cells belong primitively to the entoderm. In many mammals the tissue from which it is derived comes primarily from the mesodermal head or chordal process (see p. 47). Bonnet,^ however, has ascribed to the yolk entoderm the origin of the tissue for the anterior tip of the chorda in the dog and sheep, although the main part of the anterior portion of the chorda in these animals is derived from tissue which has become incorporated in the entoderm through fusion of the head process with the dorsal wall of the enterovitelline sac. In the human embryo, as mentioned above (p. 293), there is at a very early period a layer of mesoderm formed between the ectoderm and entoderm in the mid-sagittal plane (Fig. 219, A). At a slightly later period (Fig. 219, B and C) the mesoderm has disappeared in this region. Possibly it is incorporated with the entoderm which now beneath the neural groove presents a plate of tissue slightly thicker than the entoderm on each side. This is the chordal plate, the anlage of the anterior end of the chorda dorsalis. Kollmann (1890) has given an account of the origin of the chorda in a human embryo 2.11 mm. long with fourteen mesodermic somites. In this embryo (Fig. 220) the neurenteric canal has disappeared. Anterior to the primitive streak in the mid-sagittal plane a longitudinal ridge of cells projects dorsally from the entoderm (Fig. 220, B). This ridge of cells gradually becomes pinched oflF from the entoderm (Fig. 220, C). In a slightly older embryo described by Mall (1897) (Fig. 229) the neurenteric canal is represented by a solid column of cells. The chorda extends forward from this column of cells as far as the buCoopharjoigeal membrane. As Seessel's pocket develops, the chorda remains for a time attached to its posterior wall. This connection is lost at about the period when the buCoopharyngeal membrane is ruptured. 'Anat. Hefte, 1901.

In the embryo described by Mall the posterior end of the chorda lies opposite what is probably the eighth cervical somite. During the development of the thoracic, lumbar, sacral, and coccygeal regions of the embryo the chorda is gradually developed caudalwards. In this portion of its development the chorda is not first embedded in the entoderm and then again differentiated out, but is derived directly from the primitive streak and from the tissue which replaces the primitive streak caudalwards.


In an embryo described by His (L, 2.4 mm. long) the notochord has a distinct lumen. This is not present in older embryos.


At first there is no distinct membrane about the chorda (see Figs. 220, B, and 220, C). The cells are large, with clear protoplasm. By the end of the fourth week of development a thin structureless membrane encircles the chorda, which is now about at the height of its development. The chorda is cylindrical. The cells are polygonal and are filled with a fine granular protoplasm. During this period differentiation of the base of the skull and of the spinal column is marked by condensations in the axial mesenchyme (Fig. 231). Subsequently in the spinal region the intervertebral discs and the bodies of the vertebrae form about the chorda dorsalis. Between the chorda cells and the outer sheath of the chorda there appears an inner membrane, apparently mucoid in nature (Williams). according to Minot (1907), at the period when the axial mesenchyme begins to be differentiated into vertebras the notochord shows slight transient dorso-ventral segmental flexures. Just before ossification begins the chorda disappears in the vertebral bodies. In the intervertebral discs it becomes transformed into the tissue of the nucleus pulposus. In the cranial region the posterior and the anterior portions of the chorda dorsalis become embedded in the skeletal tissue of the base of the skull while the intermediate portion lies between the base of the skull and the dorsal wall of the pharynx (Fig. 266). Ultimately the cranial part of the chorda completely disappears. That part of the chorda which lies in the retropharyngeal tissue gives rise to numerous projections and swellings and is the first portion of the chorda to disappear. The posterior portion of the cranial part of the chorda comes to lie on the dorsal side of the basal occipital plate and disappears at the time of ossification of the basi-occipital. The anterior extremity of the chorda persists longer than the retropharyngeal portion, but usually disappears during the ossification of the base of the skull. A description of the changes undergone by the chorda is given in connection with the development of the vertebral column and skull. Traces of the cranial part of the chorda dorsalis may persist in the adult and give rise to tumors.

FiB.230b. Fie2300. Eli.230d.


FiQ. 229.— (AttCT Mall, Joliin. of Morpliology. vol. 12. 18»7. Fig. Ifl.) OuUine drawing of ■ mediki ngitul sectian of (he rnodd of Msll'ii embryo XII. Magn. 50: 1. The heavy line is the aorla. The Blriated. Am., amnion: a.. Ijorder between fore-brain and mid-brain; X. X'. exwnt of oiofure of gpinal eonai; 5. BeeBBel'ii pocket; CA., chorda; b'. b'. first and aecood bnmchial poekeU mouth: T., thyroid; II., pericardial npace: Ph., pharynx; ErU.. ■ ' '


The cvlom within the biMly is represented black. O' and 0>, fint and tl C, first and eilhth cen-ical myotomes; T, tint thoracic myotome; a., aoi I., thyroid; lliver: Ph.. pharynx; i., intestiae; ne.. neurent^ric canal: A

Entochorda

A hypochorda or entochorda arising fitwn the entoderm beneath the chorda dorsalis has been found in fishes, amphibia, birds, and reptiles, but apparently has not yet been described for the human embryo. In part the tissue of the hypochorda joins that of the chorda dorsalis. (See Ad. Reinhardt, Morphol. Jahrb., Bd. 32, 1904; Ph. Stohr, Morphol. Jahrb., Bd. 23, 1895; and S. A Ussoff, Anat. Anz., Bd. 29, 1906.)

Bibliography

The chief paper on the early development of the chorda dorsalis in man is that of Kollmann (1890). Important data concerning the chorda dorsalis are to be found in the various papers deseribmg human embryos with fourteen somites or less.

Bonnet : Beitrage zur Embryologie des Hundes. Anat. Hef te. 1897 und 1901. Eternod: Conununication sur im oeuf humain avec embryou excessivement jeune. Arch. Ital. de Biologies Vol. 22. 1895. See also Monitore Zool. Ital. Vol. 5, p. 70-72. 1894. Sur un OBuf humain de 16,3 muL avec embryon de 2.1 mm. Arch, des Sciences Phys. et Nat. Ann6e 101. 4 Periode. T. 2, p. 609-624. 1896. H y a un canal notochordal dans Tembryon humain? Anat. Anz. Bd, 16, p. 131-143. 1899.

Frassi, L. : Uber ein junges menschliches Ei in situ. Arch, f . mikr. Anat. Bd. 71, S. 667. 190a

Froriep, a. : Kopfteil der chorda dorsalis bei menschlichen Embryonen. Beitrage z. Anat. und Embryol. Als Festgabe fUr Jaccb Henle. 1882.

GiacX)minIt C. : Un CBuf humain de 11 jours. Arch. Ital. de Biologic. Vol. 29. 1898.

Heiberq, J.: Uber die Zwischenwirbelgelenke und Knochenkeme der Wirbelsaule. Mitt, a, d. Embryol. Inst, der K. K. Univ. Wien. I, S. 119-129. 1880.

His: Anatomic menschlicher Embryonen. 1880.

Janosik, J.: Zwei junge menschliche Embryonen. Arch. f. mikr. Anat. Bd. 30, S. 559. 1887.

Keibel : Zur Entwicklungsgeschichte der chorda bei Saugem. Arch, f . Anat. und Physiol. Anat. Abt. 1889. Kollmann: Die Entwicklung der chorda dorsalis beim Menschen. Anat. Anz. Bd. 5, S. 308-321. 1890. Die Rumpfsegmente menschlicher Embryonen von 13-35 Urwirbeln. Arch. f. Anat. u. Physiol. Anat. Abt. 1891.

Leboucq, H. : Recherches sur le mode de dispnrition de la chorde dorsale chez les vertebres sup^rieurs. Arch, de Biol., p. 718-736. 1880. Luschka: Die Halbgelenke, 1852; 2. Aufl. 1858. Die Altersveranderung der Zwischenwirbelknorpel. Virchow's Arehiv. Bd. 9, S. 309-327. 1856. Uber gallertartige Auswiiclise am Clivus Blumenbachii. Virchow's Arehiv. Bd. 11, S. 8-12. ia57.

Mall: A Human Embryo 26 Days old. Joum. of Morph. Vol. 5, p. 459-480. 1891. Human Coelom. Joum. of Morph. 1.S97.

MiNOT, C. S.: The Segmental Flexures of the Notochord. Anatomical Record, Amer. Joum. of Anat. Vol. 6. 1907.

MuLLER, H.: Uber das Voikomnien von Resten der chorda dorsalis bei Menschen nach der Geburt. Zeitschrift f. rationelle Med. Bd. 2. 1858.

MusGRAVE, James : Persistence of the Notochord in the Human Subject. Joum. of Anat. and Physiol. Vol. 25. 1891.

Robin, C. : Memoire sur revolution de la notoeorde. Paris, 212 pp., 1868.

RoMiTi, G. : Rigonfiamenti della corda dorsale nella porzione cervicale nelFembrione umano. Notizie Anat. Siena. 1886. Spee, Graf v.: Beobachtungen an einer menschlicben Keimscheibe mit offener Medullarrinne. Arcb. f. Anat. und Pbys. Anat. Abt. S. 159-176. 1889. Neue Beobachtungen iiber sebr friihe Entwicklungsstufen des menschlicben Eies. Arch. f. Anat. und Pbys. Anat. Abt. S. 1-30. 1896.


On the development of the notochord in the higher mammals see especially: Williams, L. W. : The Later Development of the Notochord in Manmials. Amer. Jour, of Anat. Vol. 8, p. 251. 1908.