The
development of the frog is a reminiscence of it, on the lines of
the embryonic law which we saw earlier. An animal, in its
individual development, more or less reproduces the past phases
of its ancestry. So the free-swimming jelly-fish begins life as a
fixed polyp; a kind of star-fish (Comatula) opens its career as a
stalked sea-lily; the gorgeous dragon-fly is at first an uncouth
aquatic animal, and the ethereal butterfly a worm-like creature.
But the most singular and instructive of all these embryonic
reminiscences of the past is found in the fact that all the
higher land-animals of to-day clearly reproduce a fish-stage in
their embryonic development.
In the third and fourth weeks of development the human embryo
shows four (closed) slits under the head, with corresponding
arches. The bird, the dog, the horse--all the higher land
animals, in a word, pass through the same phase. The suggestion
has been made that these structures do not recall the gill-slits
and gill-arches of the fish, but are folds due to the packing of
the embryo in the womb. In point of fact, they appear just at the
time when the human embryo is only a fifth of an inch long, and
there is no such compression.
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