The hatching of eggs, whether of fresh-water or salt-water fishes, presents no serious difficulties, if suitable apparatus is employed; but the rearing of fry to an advanced stage, without serious losses, is less easy, and in the case of sea-fishes with pelagic eggs, the larvae of which are exceedingly small and tender, is still an unsolved problem, although recent work, carried out at the Plymouth laboratory of the Marine Biological Association, is at least promising.