gallic acid derivatives to give an intense blue-black coloration. That is the 

 suggested explanation of the colour obtained when the osmium-fixed tissues 

 are treated with ethyl gallate. 



Ordinary methods of fixation and staining reveal particularly proteins 

 and nucleic acids. The osmium-ethyl gallate method stains chiefly the fatty 

 substances. It therefore provides a histological picture which is unlike that 

 given by standard methods; it closely resembles the image seen with the elec- 

 tron microscope. 



The cells in the epidermis of Rhodnius, as seen in Avhole mounts immediately 

 after feeding, before growth has begun, sho\v the nuclear membrane and a small 

 nucleous. The most conspicuous objects in the cells are the fine granular 

 or filamentous mitochondria. 



Within twenty-four hours after feeding (or injection of the moulting hor- 

 mone ecdyson) the nucleoli are noticeably enlarged and the mitochondria 

 are now contracted and sлvollen; the fine filamentous forms have disappeared 

 (Wigglesworth, 1957a). 



By two days after feeding, the nucleoli are still further enlarged and the 

 màtochondria are much more numerous. By this time the cells are increasing 

 in thickness and the protein and nucleic acid in the cytoplasm are increasing 

 in amount. 



During the height of cell division in the epidermis, \vhich occurs about 

 six days after feeding in the 4th -stage larva of Rhodnius, the difference bet\veen 

 the appearance after standard methods of staining and after the use of os- 

 mium and ethyl gallate is very striking. After fixation in Carnoy and stain- 

 ing with haematoxylin, the most conspicuous objects in the dividing cells 

 are the chromosomes. But nucleic acids do not react with osmium; the chromo- 

 somes are unstained in ethyl gallate; in the cell in metaphase the zone occupied 

 by the chromosomes appears as a pale bar. The most conspicuous objects 

 in the cell are the mitochondria (Wigglesworth, 1957b). 



In the cells of the fat body the droplets of neutral fat become coloured 

 a homogeneous deep blue black with osmium and ethyl gallate. The mito- 

 chondria are coloured a brownish grey. In the unfed or newly fed insect 

 the mitochondria appear mainly as rounded or oval objects or as short rods. 

 Within twenty four hours after feeding or the injection of ecdyson, the mito- 

 chondria are greatly increased in number and many of them are assuming 

 a filamentous form, sometimes branched. Meanwhile much new protein and 

 nucleic acid is being synthesized in the cytoplasm, but is not visible after 

 this method of staining. 



The sense cells and their axons have likewise been studied in whole mounts 

 by the osmium-ethyl gallate method (Wigglesworth, 1959a). The sense 

 cells contain conspicuous mitochondria. Over the greater part of their length 

 the sensory axons are too fine to contain mitochondria. But at intervals 



42 



