How Body Shows Remains Of Primitive Ancestors
( Originally Published 1936 )
In certain snakes of the python family it is possible to find, on the under surface of the body, two little projecting points entirely different from the rest of the scales. If one were to dissect these out in a dead specimen, he would find that they were jointed to two bones and represent the primitive hind limbs. They are, of course, perfectly useless to the snake, and in most species of snakes limbs cannot be found at all. They are, in fact, vestigial structures showing that snakes once had limbs.
Man has a number of such vestigial structures in his body, a reminder of his ancestry. During the development of a human being, of course, stages in the development of animals from simple to complex are seen all through the course. The process starts with the union of two animals—one an amoeba, the other a tailed, movable infusorium.
Later, we have a fish stage and in some people gill clefts in the neck can be found which degenerate into cysts, known as "branchial cysts," and are a real source of trouble.
In the amphibian stage there is in the corner of the eyes a little membrane which corresponds to the nictitating membrane most highly developed in birds. In man this is called the "plica semilunaris," and in some individuals they are quite highly developed.
Remindful of the amphibian also is the structure under the tongue and, in fact, when the little ducts at this place become clogged up and a cyst forms behind them it is called "ranula" which means "frog."
Reminiscences of later stages in the development are the third molar teeth, which are practically useless to us and the canine teeth which look so much like wolf teeth.
The ridges in the palate are reminiscences of the third and fourth plates of teeth as seen in fish.