Many animals lay large, very yolky eggs which develop directly to a small adult: presumably they can omit larval feeding stages because of their large food store. Often such eggs cleave partially, or ‘meroblastically’, to form the embryo in a restricted area, and then the uncleaved yolk is gradually absorbed into this embryo. Such adaptations have evolved independently in a large number of animal groups, but in this chapter we shall consider only those groups where it is the normal mode of development. In at least these cases it is the area around the animal pole which forms the embryo and the more vegetal parts which are nutritive (indeed this is the origin of the terms ‘animal’ and ‘vegetal’: see p. 10). Earlier chapters have shown for other groups that spatial differences are most pronounced along the egg's animal–vegetal axis and are very important for embryonic determination. How then does determination operate in cases where a large part of the animal–vegetal axis has an extraembryonic fate?

Cephalopods

The cephalopods are a molluscan group but show no signs of the spiral cleavage patterns so characteristic of other groups in this phylum. As noted in the first two chapters of this book the axes of bilateral symmetry are often already visible in the egg (p. 10) and fertilisation is followed by cytoplasmic streaming to form a blastodisc at the animal pole (p. 32).