DETERMINATION OF NEURAL CONNECTIONS
205
the threshold became increasingly higher, and after complete regeneration
the corneal reflex disappeared. After reamputation of the foot the reflex
reappeared to stimulation of the resulting blastema. This effect could
be reproduced routinely, and in some cases even a third amputation was
effective. From the fourth to the fifth month after metamorphosis no
corneal reflex from the resulting blastema was ever seen, and the regeneration of the foot also either failed or was incomplete in this period.
The result convincingly shows that a kind of "corneal-specific modulation" could not be at work in the establishment of a corneal reflex from
an atypical place. There are, however, two factors that are common both
to the cornea and to the regeneration blastema and which may suggest
an explanation from another point of view. These are the type and the
density of sensory endings. The sensory fibers of the cornea are known
to form a meshwork-like plexus in the connective tissure of the corneal
substantia propria from which the free beaded terminals supplying the
cornea arborize. These are regarded as nerve endings of a low grade
of differentiation. No histological study was made of the innervation of
the corneas of extra eyes, but from the investigation of Dijkstra (1933)
one can infer that the type of sensory endings are determined by the
innervated tissue. With the aid of a modified Bodian Protargol technique,
Singer (1949) succeeded in showing the innervation of regenerating feet.
The blastema is invaded by large bundles of fibers from which the emerging secondary and tertiary fibers penetrate the epithelium. He also
reported a hypersensitivity of the blastema from the third day after amputation. In an electron-microscope study of the innervation of regeneration blastemas, Hay (1960) showed that the terminal fibers exhibiting
bulbous enlargements enter the interfacial canals between the epithelial
cells. Thus, an intraepidermal type of innervation of a regeneration
blastema, similar to that of the cornea, seems to be established. Convincing evidence has been accumulated by Singer (1946, 1952) and
Thornton (1954) in favor of the assumption that extensive innervation
of the apical epithelial cup of a blastema is essential for regeneration.
The number of limb-innervating fibers does not increase considerably
during the animal's growth (Litwiller, 1938), and the lack of regeneration in adult amphibian limbs is due, as was proved by Singer (1952), to
the relative decrease of the innervating fibers. As a matter of fact, no
corneal reflex could be observed from a blastema beyond the time at
which the limb had lost the capacity to regenerate a foot.
The common feature of an extra cornea and a limb regeneration
blastema in eliciting a corneal reflex may, therefore, be their high degree
of sensitivity, as the consequence of a high concentration of epithelial
terminals. This suggests, on the one hand, that much more afferent im-
205
the threshold became increasingly higher, and after complete regeneration
the corneal reflex disappeared. After reamputation of the foot the reflex
reappeared to stimulation of the resulting blastema. This effect could
be reproduced routinely, and in some cases even a third amputation was
effective. From the fourth to the fifth month after metamorphosis no
corneal reflex from the resulting blastema was ever seen, and the regeneration of the foot also either failed or was incomplete in this period.
The result convincingly shows that a kind of "corneal-specific modulation" could not be at work in the establishment of a corneal reflex from
an atypical place. There are, however, two factors that are common both
to the cornea and to the regeneration blastema and which may suggest
an explanation from another point of view. These are the type and the
density of sensory endings. The sensory fibers of the cornea are known
to form a meshwork-like plexus in the connective tissure of the corneal
substantia propria from which the free beaded terminals supplying the
cornea arborize. These are regarded as nerve endings of a low grade
of differentiation. No histological study was made of the innervation of
the corneas of extra eyes, but from the investigation of Dijkstra (1933)
one can infer that the type of sensory endings are determined by the
innervated tissue. With the aid of a modified Bodian Protargol technique,
Singer (1949) succeeded in showing the innervation of regenerating feet.
The blastema is invaded by large bundles of fibers from which the emerging secondary and tertiary fibers penetrate the epithelium. He also
reported a hypersensitivity of the blastema from the third day after amputation. In an electron-microscope study of the innervation of regeneration blastemas, Hay (1960) showed that the terminal fibers exhibiting
bulbous enlargements enter the interfacial canals between the epithelial
cells. Thus, an intraepidermal type of innervation of a regeneration
blastema, similar to that of the cornea, seems to be established. Convincing evidence has been accumulated by Singer (1946, 1952) and
Thornton (1954) in favor of the assumption that extensive innervation
of the apical epithelial cup of a blastema is essential for regeneration.
The number of limb-innervating fibers does not increase considerably
during the animal's growth (Litwiller, 1938), and the lack of regeneration in adult amphibian limbs is due, as was proved by Singer (1952), to
the relative decrease of the innervating fibers. As a matter of fact, no
corneal reflex could be observed from a blastema beyond the time at
which the limb had lost the capacity to regenerate a foot.
The common feature of an extra cornea and a limb regeneration
blastema in eliciting a corneal reflex may, therefore, be their high degree
of sensitivity, as the consequence of a high concentration of epithelial
terminals. This suggests, on the one hand, that much more afferent im-