I have now said enough on these subjects.

With regard to the generation of fish, the puzzle VII Various points. may be raised, what the Cause can possibly be why neither the females of Selachian fishes are seen shedding their fetations nor the males their milt, whereas the males and females are observed so doing in the case of non-viviparous fishes. The reason is that in general the class of the Selachians is not rich in semen; and also in the females the uterus is up towards the diaphragm.^a Of course males of one class differ from males of another, and females similarly; and the fact is that the Selachians yield less semen than most. With the oviparous fishes, the males shed their milt, just as the females lay their eggs, because there is such an abundance of both; the males have more milt than the amount which suffices for copulation, because Nature prefers to expend the milt in helping to enlarge the eggs after the female has laid them, rather than in constituting the eggs at the outset. This remark is explained by what has been said both in our earlier discussion and also not long ago, viz., the eggs of birds are perfected inside the parent, but the eggs of fish outside. In a way, fish resemble the larva-producing animals, for the latter deposit a fetation which is even more imperfect still. The perfecting in both cases, birds’ eggs and fishes’, is accomplished by the male. With birds this is done within the parent animal, because a bird’s egg is perfected inside; with fishes, outside, because the egg is in an imperfect state when it is deposited outside. The upshot however is the same in both cases.