And this study of
embryology brought to light many new and interesting facts. Agassiz
especially emphasized and maintained the universality of the fact
that there was a remarkable parallelism between embryos of later
forms and adults of old or fossil groups. The embryos of higher
forms, he said, pass through and beyond certain stages of structure,
which are permanent in lower and older members of the same group.
You remember that the fin on the tail of a fish is as a rule
bilobed. Now the backbone of a perch or cod ends at a point in the
end of the tail opposite the angle between the two lobes, without
extending out into either of them. In the shark it extends almost to
the end of the upper lobe. Now we have seen that sharks and ganoids
are older than cod. In the embryo of the cod or perch the backbone
has, at an early stage, the same position as in the shark or ganoid;
only at a later stage does it attain its definite position.
So Agassiz says the young lepidosteus (a ganoid fish), long after it
is hatched, exhibits in the form of its tail characters thus far
known only among the fossil fishes of the Devonian period. The
embryology of turtles throws light upon the fossil chelonians. It is
already known that the embryonic changes of frogs and toads coincide
with what is known of their succession in past ages.
Pages:
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53