A large metacoel in the metencephalon opens into the myelocoel. The myelocoel is also continued into the corpora restiforma; posteriorly it joins the central canal which extends down the center of the spinal cord.
Dissection of the eye. Remove one of the eyes from its orbit, and divide it into inner and outer halves by an equatorial cut around the eyeball (not directly through it, as this tears the lens from its fastenings). Place the halves under water and observe:
In the inner half:
The posterior chamber, the cavity of the eyeball which has been opened. During life it is filled by a gelatinous substance, the vitreous humor.
The retina, a delicate yellowish-white membrane lining the interior of the eye, loosely attached to the outer coats except at the point of entrance of the optic nerve.
The choroid coat, a thin, black membrane outside the retina. It can be pulled away from the outer coat quite easily except near the optic nerve.
The sclerotic coat, the outer coat of the eye. This is composed of connective tissue having an almost cartilaginous consistency, is only slightly pigmented, and is somewhat translucent. The muscles of the eye are inserted upon the sclerotic.
In the outer half:
The ora serrata, an irregular line along which the retina ends.
The iris, a fold of the choroid extending inward like a shelf, and perforated centrally to form the pupil. Around the iris the choroid is folded radially into the ciliary processes.