THE ORIGIN OF THE EYEBALL
THE IRIS:
The mesenchyme situated on the anterior surface of the lens condenses to form the
pupillary membrane. The two layers of neuroectoderm forming the edge of the optic cup,
having covered the ciliary muscle, now extend onto the posterior surface of the pupillary
membrane. These structures fuse to become the iris. The sphincter and dilator muscle of the
pupil are derived from the pigmented cells of the neuroectoderm. The mesenchyme forms
the connective tissue and blood vessels of the iris. Pigment cells derived from the
neuroectoderm penetrate the sphincter muscle and enter the connective tissue. The opening
in the central part of the iris becomes pupil. In its earliest development, the pupillary
membrane to begin with is attached to the edges of the pupil. Later, as the result of a split in
the mesenchyme the pupillary membrane begins to separate from the iris but remains
attached to the front to it. At about the eighth month the pupillary membrane starts to
degenerate and eventually disappears. Fine fibrillary remnants often persist even after birth.
THE IRIS:
The mesenchyme situated on the anterior surface of the lens condenses to form the
pupillary membrane. The two layers of neuroectoderm forming the edge of the optic cup,
having covered the ciliary muscle, now extend onto the posterior surface of the pupillary
membrane. These structures fuse to become the iris. The sphincter and dilator muscle of the
pupil are derived from the pigmented cells of the neuroectoderm. The mesenchyme forms
the connective tissue and blood vessels of the iris. Pigment cells derived from the
neuroectoderm penetrate the sphincter muscle and enter the connective tissue. The opening
in the central part of the iris becomes pupil. In its earliest development, the pupillary
membrane to begin with is attached to the edges of the pupil. Later, as the result of a split in
the mesenchyme the pupillary membrane begins to separate from the iris but remains
attached to the front to it. At about the eighth month the pupillary membrane starts to
degenerate and eventually disappears. Fine fibrillary remnants often persist even after birth.
