A text-book of obstetrics / by J. Clarence Webster.

  • John Clarence Webster
Date:
1903
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of obstetrics / by J. Clarence Webster. Source: Wellcome Collection.

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    velops, the mesoblast. Its origin in the higher vertebrates is not definitely known. In all probability it is formed both from the epiblast and hypoblast layers, appearing first under the anterior end of the primitive streak. After its differentiation as a distinct layer its future growth takes place by the independent proliferation of its own cells. It spreads laterally and posteriorly, and later, from the anterior end of the central mesoblastic layer, two expansions grow forward, curving so that their tips meet in the middle line, enclosing a crescentic space known as the pro- amnion. Soon after the appearance of the mesoblast the med- ullary groove appears in front of the primitive streak. It is formed by the development of the medullary folds, two layers of an ) ~ g y | ‘Ne g : > oy ! : ene i a t 7 | e wh i FIG, 11.—Diagrammatic surface view of rabbit's ovum of two hundred and five hours (after Tourneux). The darkly shaded area indicates the extent of the mesoderm: a, Peripheral limit of area opaca; 4, of area pellucida; c, of parietal zone; d, of stem zone; /, Hensen’s node; 2, proamnion. Xl | epiblast that begin in front of the head process and extend backward. These folds curl inward until they coalesce from before backward, forming the neural tube or canal, the original primitive streak being gradually enclosed in the canal. From this tube the whole future nervous system is developed. While the neural canal is developing a solid column of cells forms under it, derived from the hypoblast. This is the noto- chord or chorda dorsalis, the axis around which the permanent spinal column is developed. It largely disappears afterward, being represented in postnatal life only by the pulp in the center of the intervertebral disks. Extension of the Mesoblast.—The early mesoblast grad-
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