A text-book of embryology for students of medicine / by John Clement Heisler.

  • Heisler, John C. (John Clement), 1862-1938.
Date:
1901
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of embryology for students of medicine / by John Clement Heisler. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

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    nucleus, situated under the germinative disk; of a larger mass, the latebra, more deeply placed; and of several con- centric layers separated from each other by the yellow yolk. Such is the egg as it leaves the hen's ovary. In the begin- ning of the oviduct it is fertilized by the spermatozoa already there. After fertilization it passes into the longitudinally furrowed second part of the tube, where it receives a copious coating of albuminous material, the white of the ≫ thence it goes into the villous third part of the oviduct, where it acquires a calcareous coating, the shell; finally, passing through the fourth part of the canal, it is laid. The layer of albumen immediately surrounding the yolk is relatively dense; it is prolonged to either extremity of the egg, somewhat spirally twisted, as the chalazse. Enclosing the albumen is the thin tough shell-membrane. This con- sists of two layers, which separate at the blunt pole of the egg soon after it is laid, giving rise to the air-chamber. The shell, composed largely of lime salts, is very porous and thus readily permits of the necessary gas-interchange between the contents of the egg and the external air during incubation. Ova do not possess the remarkable vitality which is char- acteristic of spermatozoa. An unimpregnated ovum per- ishes in from seven to nine days. Oogenesis.—The formation of ova takes place throughout the greater part of fetal life and continues for a short time (two years, according to Waldeyer, Bischoif, and others) after birth. Their number is estimated to be about seventy thousand. The ovum, the direct derivative of the germinal epithe- lium covering the free surface of the ovary, is situated in the cortical part of the latter organ, being enclosed in the Graafian follicle. As a rule, each Graafian follicle or ovi- sac contains but one ovum, though sometimes two, and more rarely three are present. The Grraafian follicle, in its mature condition, is a vesicle from 4 to 8 mm. in diameter, which is surrounded by a sheath, the theca foUiculi or tunica vasculosa, consisting of a condensation of the ovarian stroma. The outer, more fibrous