Failure of Lens Formation after Extirpation in the Medullary Plate

The optic cup and the lens originate from two different sources : the optic cup optic cup from an evagination of the forebrain, the lens as an invagination of the head epidermis. In the neural fold stage, the two prospective areas are located at some distance from each other. The fact that the lens is formed at the point at which the optic vesicle establishes contact with the overlying epidermis is suggestive of a casual relation between the two. Spemann was first to test this assumption experimentally. He extirpated the eye primordium in the medullary plate or optic vesicle stage and found that no lens would differentiate, although the prospective lens area was left undisturbed. He concluded that lens differentiation is induced by the optic vesicle.

Procedure:

The aim of this experiment is to remove the eye primordium without disturbing the lens forming area, which is located just outside the meduallry plate or fold.

a. Remove all membranes from a number of healthy neurulae.

b. Prepare a sterile dish with agar bottom; make a shallow grove in the agar and place an embryo in it. Use full strength culture medium.

c. With a glass needle and hair loop, cut out the area a-b-c-d as in the following figure. Make the anterior cut very close to the inner edge of the transverse neural fold. Do not cut deeply; with some experience it is not difficult to peel off the medullary plate with the tip of the needle and hair loop without injury to the roof of the archenteron.

d. Allow the wound to heal in concentrated medium (this may take an hour or longer), and transfer to another sterile agar dish, with dilute medium.

f. When the eyes have become pigmented and the cornea transparent, terminate the experiment.

g. Dissect the skin over the brain and eyes with a sturdy glass needle or fine forceps.

h. Compare and describe any deviations from normal.