During blastoderm formation, theDrosophilaembryo produces a large area of new membrane to accommodate the simultaneous demands of approximately four thousand newly cleaved cells. The embryo was examined with the electron microscope at various stages during cleavage in order to investigate the high membrane forming capacity of these cells. Embryos were subjected to the histochemical procedure for the demonstration of thiamine pyrophosphatase. The enzyme was present in the cisternae of the endoplasmic reticulum, Golgi bodies and the nuclear envelope. No activity could be demonstrated on the furrow surface or at the furrow tip despite closely adjacent reactive cisternae. It is concluded that the endoplasmic reticulum and Golgi bodies are not major contributors to the new surface.
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