Photoreactivation of Pyrimidine Dimers Generated by a Photosensitized Reaction in RNA of Insect Embryos (Smittia Spec.)

Abstract

Insect eggs are large cells, ranging from a few tenths to several millimeters in length. The egg cell is surrounded by a vitelline membrane, and an outer shell, the chorion. The yolk-rich and opaque endoplasm contains glycoproteid spheres, lipid droplets, and glycogen particles, whereas the superficial periplasm is yolk-free. After fertilization or parthenogenetic activation, the nucleus undergoes a series of mitotic divisions within the yolk endoplasm. This period is referred to as intravitelline cleavage, although the egg cell is not cleaved. Rather, the embryo develops in a plasmodial state, containing eventually hundreds of energids, i.e. nuclei with jackets of cytoplasm. During nuclear migration stages, most energids move into the yolk-free periplasm, where they become enclosed by infoldings of the plasmalemma of the egg cell. The resulting blastoderm cells may be eventually separated from the yolk endoplasm, or may remain connected to it by cytoplasmic bridges. While cellularization is in progress, nuclear divisions may continue. Following this period of nuclear proliferation and the formation of an apparently homogeneous layer of blastoderm cells, regional differentiation begins. Many of the blastoderm cells build the originally unsegmented germ anlage while the remainder form the embryonic membranes. After gastrulation and segmentation, the embryo reaches the germ band stage which already reflects the basic organization of the larva. Recent descriptions of early Drosophila embryogenesis have been given by Fullilove and Jacobson (1978), Turner and Mahowald (1976), and Zalokar and Erk (1970). For reviews on insect development see Counce and Waddington (1972, 1973).