Aids to embryology / by J.S. Baxter.

  • James Sinclair Baxter
Date:
1948
Catalogue details

Licence: In copyright

Credit: Aids to embryology / by J.S. Baxter. Source: Wellcome Collection.

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    termed the tunica vasculosa lentis. The vessels of the tunica begin to disappear after the seventh month of foetal life and have usually completely gone at birth. The Vitreous Humour.—The vitreous humour was formerly believed to be developed from the mesoderm which had insinuated itself into the optic cup with the development of the lens. T. H. Bryce (1908) claimed that it develops from the protoplasmic threads formed when the lens draws away from the optic cup. These threads are therefore ectodermal in origin, and they form a meshwork filling up the optic cup. The true explanation is probably that both ectoderm and mesoderm contribute to the vitreous. The embryonic vitreous contains a central canal having in it the hyaloid artery, a continuation of the artery of the foetal fissure to the posterior surface of the lens. Sclera, Cornea and Choroid.—A condensation of mesenchyme occurs as a capsule around the optic cup, which differentiates into two layers. The layer next the cup becomes highly vascular and forms the choroid. This layer becomes thickened at the margin of the optic cup to form the ciliary body. Anterior to this it fuses with the pars iridica retinae to form the iris which is therefore partly ectodermal and partly mesodermal, the musculature of the iris probably arising from the ectodermal component. The outer mesenchymal layer differentiates into the sclera and cornea. The latter, becoming transparent, has added to its structure the overlying surface ectoderm. A fluid filled cleft, the anterior chamber, forms between the posterior surface of the cornea and the anterior surface of the iris. An important drainage space, the canal of Schlemm, appears just lateral to the margin of this chamber. The Eyelids.—The eyelids develop as two folds of
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