BIPARTITY OF THE YOLK SAC IN CYPRINID EMBRYOS

Studies on fish embryogenesis have demonstrated that, although the yolk sac in most fish is spherical or subspherical and is definitely uniform in its architecture throughout, its shape in some cyprinid species (bream, sun bass, bleak and others) changes considerably during late organogenesis. The changes involve a rapid and conspicuous division of the yolk into two parts; the proximal one occupies about three quarters of the yolk volume and is spherical, while the caudal part, a quarter of the yolk volume, becomes elongated or even cylindrical. The yolk sac walls in the caudal part were observed to pulsate rhythmically, thus preceding the pulsation of the budding heart. It may be contended that the yolk sac wall pulsation causes mixing of the internal fluids in the developing embryo, an activity and role preceding that played by the central part of the emerging circulatory system, i.e., the heart. In teleost fish, the yolk occupies more than 90% of the egg cell. Initially, during embryogenesis, the yolk is covered by a thin ectoplasm layer which, shortly after the egg has been activated and hydrated and as a result of complex changes, develops into a reception mound which will later evolve into the embryonic disc and the embryo itself. The yolk is a storage site for substances supplying the developing embryo, and later the larva - freshly out of the egg membranes - with nourishment and energy. Consequently, as the organism develops, the yolk is resorbed at an ever increasing rate. In most fish species, the yolk sac retains a more or less spherical shape until the yolk is entirely resorbed. However, occasionally, and particularly during the later stages of embryonic development and immediately after hatch, the yolk sac becomes oval, pear-shaped, or cylindrical.