These liquids, then, are nearly always congenital in animals,
but milk and sperm come at a later time. Of these latter, that
which, whensoever it is present, is secreted in all cases
ready-made, is the milk; sperm, on the other hand, is not secreted out
in all cases, but in some only, as in the case of what are
designated thori in fishes.

Whatever animals have milk, have it in their breasts. All
animals have breasts that are internally and externally viviparous, as
for instance all animals that have hair, as man and the horse; and the
cetaceans, as the dolphin, the porpoise, and the whale-for these
animals have breasts and are supplied with milk. Animals that are
oviparous or only externally viviparous have neither breasts nor milk,
as the fish and the bird.