United States or Saint Vincent and the Grenadines ? Vote for the TOP Country of the Week !
But the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected. The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body.
The skin lost something of its chalky hue, while at the roots of the hair, as it grew, a distinct infiltration of pigment-cells was visible. And at this sight Allan rejoiced exceedingly. Beatrice did not now go much abroad with him, on account of her condition.
Many kinds of fish similarly alter their colour to suit their background by forcing forward or backward certain special pigment-cells known as chromatophores, whose various combinations produce at will almost any required tone or shade.
We may, however, according to M. Jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on sarcodic tissue. Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness.
By his experiments on the pigment-cells of a frog Lister has established the relation existing between these elements and innervation, which formerly had been supposititious. Doubtless a solution of the central control of pigmentation would confirm the best theory of the cause of leukoderma i.e., faulty innervation of the skin.
Darwin doubts not that it does slowly and by degrees by single acts, for instance, instead of by a succession of acts, aggregating into the semblance of an optic nerve certain elements in the sarcode of certain low organisms, spreading out the nerve thus formed into a network or retina, forming a number of separate pigment-cells into a homogeneous cornea, and following up these first steps by others which, how much soever more apparently complex, would cost comparatively little after the earlier and simpler ones had been taken.
In certain star-fishes, small depressions in the layer of pigment-cells are filled with transparent gelatinous matter projecting with a convex surface like a rudimentary cornea, and this, it has been suggested, may serve, not only to form an image, but to concentrate the luminous rays.
Word Of The Day