The last chapter presented the evidence that the determination of cells in the sea urchin morula and blastula is affected by interactions occurring throughout the embryo. The present chapter will consider evidence for similar interactions occurring at the equivalent stages in other animal embryos.

Insects

As has already been made clear (p. 119), the nuclei of insect eggs at first divide without cleavage of the cytoplasm. The stages considered here (Fig. 6.1) are those when most nuclei enter the peripheral cytoplasm (the syncytial blastoderm), when cleavage furrows appear between them (the cellular blastoderm) and when the future embryonic area becomes recognisable (germ band stages). Despite the great morphological differences from the sea urchin, developmental interactions apparently determine cell fates in a similar way, and the mechanisms involved have been subjected to genetic analysis particularly in Drosophila.

Cycles of nuclear division are lengthening during these stages (e.g. see Foe & Alberts, 1983), just as cleavage cycles do in the equivalent stages of other embryos. At syncytial blastoderm stages they occur as parasynchronous waves, starting in positions which vary with the species and even sometimes amongst individuals (Miyamoto & van der Meer, 1982; Foe & Alberts, 1983; Lundquist & Löwkvist, 1983). The divisions become completely asynchronous during cellular blastoderm stages, despite the fact that the nuclei are not completely separated by the new membranes. The ingrowth of furrows apparently uses a single network of microfilament bundles extending throughout the cortex of the blastoderm (Fullilove & Jacobson, 1971; Warn, Bullard & Magrath, 1980).