ELECTRON MICROSCOPIC OBSERVATION ON THE YOLKFORMATION OF OREOCHOMIS MOSSAMBICUS

The oocyte growth in Oreochromis mossambicus is morphologically identified into fourstages.Oocytes in stage 1 are in their primary growth.Oocytes in stage 2 are distingui shed bythe appearance of yolk vesicles(or cortical alveoli).Oocytes in stage 3 are vitellogenic oocytesaccumulating yolk protein. Stage 4 represents postvitellogenic oocytes which have completedthe accumulation of yolk protein.The fomation of yolk globules was arialysed.The oocytesincorporate a large quantity of yolk precursor protein, vitellogenin,by micropinocytosis fromthe blood stream,and construct yolk globules during exogenous vitellogenesis.In theprevitellogenic oocyte,mitochonria and multivesicular bodies comprise the main componentsof Balbian's vitelline body(or yolk nucleus).The concentric multi-laver membranes,formedby modification ofmitochondria, are the accumulating sites for exogenous yolk protein.Thecomplete yolk globule is composed of three parts:limiting membrane,central crystalline body(or bodies)and,between them,the non-crystalline area.