SCANNING ELECTRON MICROSCOPIC AND LIGHT MICROSCOPIC OBSERVATIONS ON THE MICROPYLE APPARATUS AND THE PATHWAY OF SPERM PENETRATION INTO THE MATURED EGG IN FOUR FRESHWATER FISHES (WUCHANG FISH, GRASS CARP, SILVER CARP AND BIG HEAD)

Surrounding membranes (chorion) of the matured eggs in these four Chinese pond fishes (Megalobrama amblycephala Ctenopharyngodon idellus, Hypophthalmichthys molitrix and Aristickthys nobilis) are all tough. The sperm penetration into the matured egg is restricted to the micropylar canal of the chorion, but there is no micropylar cell above the canal.There is a funnel-like depression in front of the micropylar canal; an outer opening of the canal is situated on the bottom of the depression, so that this depression is the vestibule of the micropyle apparatus. While the micropyle itself is a canal-like structure surrounded by the chorion and opens to the surface of plasma membrane on the animal pole directly, therefore, it is called the micropylar canal. The diameters of the outer and inner openings of the micropylar canal are different from each species, the outer opening being 4 to 4.5μ M in Wuchang fish and about 3 to 3.5μ M in grass carp and silver carp. while the inner opening of the canal being always smaller than the outer one.In short, the micropyle apparatus of matured eggs in these studied fishes are all formed of one funnel-like vestibule and an outer circular opening of the micropylar canal at its bottom, while the pathway of sperm penetration into the matured egg is only the micropylar canal.