Volume 1

Human anatomy : including structure and development and practical considerations / by Thomas Dwight ... [et.al.] ; edited by George A. Piersol.

Date:
[1918]
Catalogue details

Licence: Public Domain Mark

Credit: Human anatomy : including structure and development and practical considerations / by Thomas Dwight ... [et.al.] ; edited by George A. Piersol. Source: Wellcome Collection.

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    EARLY DEVELOPMENT. THE human body with all its complex organism is the product of the differentia- tion and specialization of the cells resulting from the union of the parental sexual elements, —the ovum and the spermatozoon. The Ovum.—The maternal germ-cell is formed within the female sexual gland, the ovary, in which organ it passes through all stages of its development, from the immature differentiation of its early condition to the partially completed matura- tion of the egg as it is liberated from the ovary. The human ovum, in common with the ova of other mammals, is of minute size, being, as it is discharged from the ovary, about .25 millimetre in diameter. Ex- amined microscopically and after sectioning, the human ovum is seen to be enclosed within a distinct envelope, the zona pellucida, .o14 millimetre in thickness, which in favorable preparations exhibits a radial striation, and hence is also named the zona radiata. ‘This envelope at first was confounded with the proper limiting mem- brane of the cell, and for a time was erroneously regarded as corresponding to the Corona radiata Zona pellucida Germinal] vesicle (nu- cleus) containing germ- inal spot (nucleolus) Zone rich in deutoplasm Zone poor in deutoplasm fading into homogene- ous peripheral zone Human ovum from ripe Graafian follicle. 10. (Nagel.) cell-wall. The nature of the zona pellucida is now generally conceded to be that of a protecting membrane, produced through the agency of cells surrounding the ovum. The substance of the ovum, the yolk, or w7tel/us, consists of soft, semifluid pro- toplasm modified by the presence of innumerable yolk-granules, the representatives of the important stores of nutritive materials present in the bird’s egg. Critically examined, the vitellus is resolvable into a reticulum of active protoplasm, or odplasm, and the nutritive substance, or dewtoplasm. At times the yolk is limited externally by a very delicate envelope, the vztelline membrane, which usually lies closely placed, or adherent, to the protecting zona radiata ; sometimes, however, it is separated from the latter by a perivitelline space. The vitelline membrane is probably absent in the unfertilized human ovum. A large spherical nucleus, the germinal vesicle, approximately .037 millimetre in diameter, usually lies eccentrically within the yolk, surrounded by the distinct nuclear membrane. Within the germinal vesicle the constituents common to nuclei in
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