THE YOLK-SAC IN LATE EMBRYONIC-DEVELOPMENT OF THE STICK INSECT CARAUSIUS-MOROSUS (BR)

Differentiation of the yolk sac was examined ultrastructurally and cyt ochemically in late embryonic development of the stick insect Carausiu s morosus. During migration along the yolk sac, endodermal cells form a discontinuous cell epithelium, leaving wide intercellular channels b etween neighbouring cell clusters. Within the same cell cluster, cells are all joined by septate junctions. in the proximity of the proctode um region, intercellular channels are filled with numerous cell debris which are shown to derive from vitellophages undergoing cell lysis. Y olk sacs resolved by gel electrophoresis are shown to release a number of vitellin polypeptides into the culture medium. These are equivalen t in molecular weight to those present in the vitellophage yolk granul es. This observation is consistent with the evidence that the basement lamina may act as a course physical filter, retaining particles large r than colloidal thorium dioxide and allowing free percolation of pero xidase. Differentiating endodermal cells form a microvillar striated b order along the apical plasma membrane. A number of vesicular criptae were frequently seen in these differentiating endodermal cells. Electr on dense granules released by endodermal cells are suggested to play a role in vitellophage lysis and vitellin release from the enclosed yol k granules.