Sites occupied by multilocular brown fat in the adult hamster are occupied by unilocular cells in very young animals. Immature brown fat cells are laid down in the unilocular cell matrix at 3 to 5 days of age. White fat in the hamster does not develop from cells closely resembling mature brown fat.

Two fluorescent materials have been localized in the adult firefly light organ by fluorescence microscopy. One of these is located in photocyte granules, has a maximum emission between 510 and 540 nm, is more fluorescent in basic than acidic solution, and is unstable in ultraviolet light, phosphomolybdic acid, and potassium permanganate. It is thought to be luciferin. The fluorescence of this material is very dim in untreated fireflies but increases substantially following sustained light emission induced by synephrine or prolonged electrical stimulation. It is subcellular location and chemical state of the chemicals involved. The light-producing reactions in the firefly involve 1) the combination of a substrate, D-luciferin, with an enzyme, luciferase, in the presence of ATP, and 2) the subsequent oxidation of luciferin by molecular oxygen. The enzyme luciferase appears to he associated with large cytoplasmic granules isolated from lantern photocytes (1), but the location of the light-emitting compound, luciferin, has not been described. Since luciferin is naturally fluorescent, the present study was initiated to try and localize it in intact light organs by means of fluorescence microscopy. During this study a fluorescent compound believed to he luciferin was localized in photocyte granules. A second fluorescent material was also discovered in the dorsal layer of the light organ. described above. After removal, light organs were quenched in isopentane cooled with liquid nitrogen, and then either dried in vacuo for 2 days (freeze-dried), or placed in acetone .tt 80 C for at least 1 week (freeze-substituted. Freeze-d ned light organs were vacuumembedded in paraffin. Freeze-substituted light organs were brought to room temperature in acetone and then either cleared in xylene and embedded in p.sraffin or cleared in propylene oxide and embedded in Epon. The results obtained with the two freezing methods were equivalent, 5(3 most tissues were prepared by the simpler freezesubstitution technique. Sections were cut at 1-8 am and attached to slides by warming. The use of water for spreading the sections was .svoided. Sections that ss'ere to he treated with various chemic.sls were dep.sr sffinized with xylene and hydrated in a standard series of alcohols. H.slf of the sections on each slide were then tre.sted with the This seition ,ind the one in Figure 1 are from the freeze-suhstituted lantern of .i synephrine-injected firefly.

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