Ultrastructure and cytochemistry study of the yolk syncytial layer in the alevin of trout (Salmo fario trutta L.) after hatching. I. The vitellolysis zone.

After hatching, the yolk syncytial layer of Salmo fario trutta may be subdivided into two zones, namely, the vitellolysis zone (containing numerous yolk platelets), and the cytoplasmic zone (where yolk platelets are rare). In the vitellolysis zone, two stages in the utilization of the yolk are observed: 1) The first stage, comprises the formation of yolk platelets from coalescent yolk by spherical cutting out and basal scission. This process seems to be achieved by the invagination of fibrillar elements into the coalescent yolk to form individual yolk platelets surrounded by a limiting membrane. 2) The second stage essentially consists of the extrusion or budding of yolk matter from a yolk platelet. Again, where the yolk matter leaves a platelet, fibrillar elements are evident and show an alkaline phosphatase activity. The platelets of the vitellolysis zone have a homogeneous content and variable diameter; they never acquire a heterogeneous and polymorphic aspect which could be interpreted as an intermediate stage in their degradation.