, that of the pupa--and we can perhaps account for the
marvellous case of the Cecidomyia.
It has already been stated that various parts in the same individual, which
are exactly alike during an early embryonic period, become widely different
and serve for widely different purposes in the adult state. So again it
has been shown that generally the embryos of the most distinct species
belonging to the same class are closely similar, but become, when fully
developed, widely dissimilar. A better proof of this latter fact cannot be
given than the statement by Von Baer that "the embryos of mammalia, of
birds, lizards and snakes, probably also of chelonia, are in the earliest
states exceedingly like one another, both as a whole and in the mode of
development of their parts; so much so, in fact, that we can often
distinguish the embryos only by their size. In my possession are two
little embryos in spirit, whose names I have omitted to attach, and at
present I am quite unable to say to what class they belong. They may be
lizards or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these animals.
The extremities, however, are still absent in these embryos. But even if
they had existed in the earliest stage of their development we should learn
nothing, for the feet of lizards and mammals, the wings and feet of birds,
no less than the hands and feet of man, all arise from the same fundamental
form.
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