THE EGGSHELL OF DROSOPHILA-MELANOGASTER .9. SYNTHESIS AND MORPHOGENESIS OF THE INNERMOST CHORIONIC LAYER

Synthesis and morphogenesis of the innermost chorionic layer (ICL) was investigated by conventional EM methods, freeze-fracturing, tissue cu lture in Robb's medium, and EM autoradiography, Both autoradiography a nd fine structure results have shown that ICL-components are secreted prior to other chorion proteins. Their secretion starts on stage 12a b ut the first layer of ICL molecules is visible at stage 12b, Its thick ness is gradually increased during the next stages, taking first, a bi laminar form along with the inner endochorion, Later, at the end of ch oriogenesis, ICL is detached from the endochorion and takes its final thickness and configuration, consisting of a 3-dimensional crystal, ab out 40 nm thick. The isolated ICL in conditions of air water interface is a monolayer crystal 10 nm thick. Studies on chorion mutants showed that the amount of protein secreted by the follicle cells is independ ent to the process of crystallisation. These data show how a proteinac eous extracellular substance is gradually assembled to form a 3-D crys tal and how it can be organised to perform functions such as the physi ological resistance of the insect eggs against water loss or water upt ake, whenever they are laid on substrates with extreme environmental c onditions, These functions are performed by ICL in conjunction with th e underlying wax layer.