Section 40. We may note here the development of the eye. This is shown in Figure 4, Sheet 24. A hollow cup-shaped vesicle from the brain grows out towards an at first hollow cellular ingrowth from the epidermis. The cavity within the wall of the cup derived from the brain is obliterated, [and the stalk withers,] the cup becomes the retina, and -its stalk- [thence fibres grow back to the brain to form] the optic nerve. The cellular 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_