ular ingrowth is the lens. The remainder of the
eye-structures are of mesoblastic origin, except the superficial
epithelium of the cornea. The retinal cup is not complete at first
along the ventral line, so that the rim of the cup, viewed as in
Figure 1, r., is horseshoe shaped. -Hence the optic nerve differs from
other nerves in being primitively hollow.- In all other sense organs,
as, for instance, the olfactory sacs and the ears, the percipient
epithelium is derived, from the epiblast directly, and not indirectly
through the nervous system. These remarks apply to all vertebrate
types.

Section 41. The supposition, that the general characters of the
rabbit's ovum were stamped upon it as an heritage from a period when
the ancestors of the mammals were egg-laying reptiles, is
strengthened by the fact that the two lowest and most reptile-like of
all the mammalia, the duck-billed platypus and the echidna, have
been shown to depart from the distinctive mammalian character, and
to lay eggs. And, in further confirmation of this supposition, we find, in
tracing the mammals and reptiles back through the geological record,
that in the Permian and Triassic rocks there occur central forms
which combine, in a most remarkable way, reptilian and mammalian
characteristics.

Section 42. In conclusion, we would earnestly recommend the
student to see more of embryological fact than what is given him here.
It is seeing and thinking, much more than reading, which will enable
him to clothe the bare terms and phrases of embryology with coherent
knowledge. In Howes' Atlas of Biology there is a much fuller series of
figures of the frog's development than can be given here,
and they are drawn by an abler hand than mine can pretend to be.
There is also an Atlas d'Embryologie, by Mathias Duval, that makes
the study of the fowl's development entertaining and altogether
delightful. Such complete series as these are, from the nature of the
case, impossible with the rabbit. Many students who take up the
subject of biology do so only as an accessory to more extended work
in other departments of science. To such, practical work in
embryology is either altogether impossible, or only possibly to a very
limited extent. The time it will consume is much greater, and the
intellectual result is likely to be far less than the study of such plates
as we have named.

_The Theory of Evolution_

Section 43. We have now considered our types, both fro