الفهرس 
The skinOnly 14 pages are availabe for public view
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Abstract
fferent tissues and organs of three stages of eels (Angui L la vultzrrri~ 1. ) in natural environments were studied histologically, histochemically and ultrastructurally. The tissues and organs represent the skin, gills, blood, kidneys, a&renal gland, corpuscles of Stannius, alimentary canal and liver. The stages of eels which living in different habitats are the freshwater adult silver eels, the freshwater yellow eels and the seawater juvineles glass eels. Freshwater yellow eels and seawater glass eels were experimentally subjected to and adapted in reverse habitats. Comparison studies were carried out to throw some light on the effect of salinity changes on the structure of tissues and organs. THE SKIN of the eel is mainly consisted of epidermis and dermis. The thickness of the epidermis is variable in the different stages; it measures about 123.9 pm. in the silver eels, 65. i m in the freshwater yellow eels and about 18.8 pm. in the glass eels. The adaptation of the freshwater yellow eel to seawater caused an increase in -thickness of the epidermis; 90.5 pm. and an increase in number and size of both mucous sereting and alarm-sustance cells. Changes in THE GILL STRUCTUHES of different stages of the eels either in natural environments or adapted to reverse habitats are confined to changes in the abudance of the chloride cells in the primary lanrellae, Cytometrical measurements revealed differences in their size as well as in number in different salinity concenterations. Electron microscopy revealed the presence of accessory cells only in the seawater eels. Also a comparison fine structure of the chloride cells in the gills of freshwater and seawaker eels showed that the seawater chloride cells are more electron dense, containig more rru-roerous mitochcmdria, more branching pattern of cytoplasmic tubular system and appearance of additional paracellular pathways or leaky junctions for extratransport of salts between the cells and their exterior. The size of MATURE ERYTEIROCYTES and %heir nuclei of the present studied eels represent medium values among the measurement series of teleostean blood cells. The nucleocytoplasmic ratio (N/C) of the silver eels is very small, 0.9%, relatively higher in the freshwater yellow eels, 10.7% and very high in the marine glass eels, 25% . The adaptation of the normal freshwater yellow eel. to seawater showed significant differences in the measurements of -their red blood cell dimensions as well as a marked decreasing in the abundance of the immature and. dividing cells in the blood smears. The mature erythrocytes of the yellow eel adapted to seawater are significantly smaller than those of the normal freshwater ones. A slight shrinkage of the nuclei was also observed, though the nucleocytoplasmic ratio (N/C) is slightly increased and the haemoglobin surface area became smaller than those of the normal freshwater yellow eel. Moreover, a great decreasing in the ratio af the immature erythrocytes occurred, it became about 10.4% instead of 27.4% in the f restrwater. In addition, the dividing erythrocytes are disappeared from the blood smears after seawater adaptation. On %he other hand, the blood of the marine glass eels adapted to freshwater showed a contradictory results, the cells and their nuclei became larger in size, the ratio of nuclear size to cytoplasm (N./C) became smaller and the haemoglobin surface area became higher. The ratio of the immature cells increased, it became 17.8% in freshwater instead 0.1% in seawater.