Recent developments in the study of the biosynthetic processes Of OOgenesis in Drosophila melanogaster (Jacob & Sirlin, 1959; King & Sang, 1959; Zalokar, 1960; Sirlin & Jacob, 1960) and a more extensive knowledge of the submicroscopic structure of the developing egg (King & Devine, 1958; Waddington & Okada, 1960) have renewed interest in the ‘mosaic’ character of the freshly deposited egg. This has long been a subject of investigation for insect embryologists, but the available experimental techniques have not provided the necessary refinement for detailed studies.

Previously, the most successful method of experimental investigation was that of partial egg irradiation with ultra-violet light (Geigy, 1931a, b). The ultraviolet was particularly suitable for this type of work because of its comparatively low penetration into protoplasm: roughly 85 per cent, absorption occurred in 50 μ at a wavelength of 300 mμ (from data of Bachem & Read, 1931, for the stratum malpighii), and stronger absorption was obtained at shorter wavelengths.

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