ABSTRACT
THE SPIRAL type of cleavage is sufficiently defmite to allow one to recognise the existence of a common pattern in several groups of animals which are not otherwise very closely related (molluscs except cephalopods, nemerteans, platyhelminths, annelids); but overlying the basic similarities there are very many variations in detail, both in the early cleavages and in the types oflarvae which are produced by early development. To keep within the limits of space available here, it will be necessary to attempt a ruthless simplification.
