Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the third German edition by Edward L. Mark. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    destitute of cells; these layers, undergoing chemical metamorphosis, become i-espectively the membrana, elastica anterior and the mem- brane of Descemet. The interna] endothelium of the cornea is developed at an extra- ordinarily early epoch in tlie Chick. For as soon as the structureless sheet pre%dously mentioned (fig. 269 h) has attained a certain thick- ness, mesenchymatic cells proceeding from the margin spread them- selves out on its inner surface as a single-layered thin cell-membrane. With this begins also the formation of the anterior chamber of the eye. For the thin fundament of the cornea, which at first lay in immediate contact with the front surface of the lens, now becomes somewhat elevated from the latter, and sepai'ated from it by a fissure-like space filled with fluid (humor aqueus). The fissure is first observable at the margin of the secondary optic cup, and spreads out from this region toward the anterior pole of the lens. The anterior chamber of the eye does not, however, acquire a greater size and its definite form until the development of the iris. Two opposing views exist concerning the origin of the structureless sheet which has been described as constituting the first fundament of the cornea in the Chick. According to Kessleh it is a product of the secretion of the epidermis, whereas the corneal corpuscles migrate in from the mesenchyma. In his opinion, therefore, tlie cornea is composed of two entirely different fundaments. According to Kollikeb, on the contrary, all its parts are developed out of the mesenchyma, and the homogeneous matrix simply outstrips the cells in its growth and extension. In Mammals (fig. 266) the conditions differ somewhat from those of the Chick; iov as soon as the lens-vesicle in Mammals is fully constricted olf, it is already enveloped by a thin sheet of mesenchyma ill) with few cells, which separates it from the epidermis. The thin layer is rapidly thickened by the immigration of cells from the vicinity. Then it is separated into two layers (fig. 267), the pupillar membrane {tv) and the fundament of the cornea [h). The former is a thin, very vascular membrane lying on the anterior surface of the lens; its network of blood-vessels communicates on the one hand posteriorly with the vessels of the vitreous body, together with which it constitutes the tunica vasculosa lentis, and on the other anastomoses at the margin of the optic cup with the vascular network of the latter. The fundament of the cornea is first sharply delimited from the pupillary membrane at the time when the anterior chamber of the eye (Jc) is formed as a narrow fissure, which gradually increases in extent with the appearance of the iris,