The structure and function of the vacuoles in the eggs of Schistosoma mansoni is unknown. In this study the eggs have been immersed in serum albumin and examined by phase-contrast optics. Under these conditions the vacuoles exhibit a definite internal structure, suggesting that they may be or may have been cellular. Histochemical staining techniques to detect the nature of the vacuolar contents revealed that the vacuoles stained heavily with iodine and gave a positive PAS reaction. This suggested the presence of carbohydrate. Stains for nucleic acids, protein and lipid were not taken up by the vacuoles. The egg shell stained with o–toluidine blue and with basic fuchsin uniformly, except for a thin irregular band which might indicate a possible line of weakness. The vacuolar membranes seem to be similar to the vitelline membrane in permeability to hydroxyl ions and it is possible that these membranes have a common origin. Plasmolysis studies with salts, urea, glycerol and sucrose showed that the vacuolar membranes have about the same permeability properties to these compounds as the other egg membranes. Glycerol treatment of the eggs until equilibrium is attained, followed by transfer of the eggs to water or saline results in a hatching of the eggs. In this, a dead mira-cidium is extruded from the shell. Similar treatment of eggs equilibrated with sucrose does not result in this hatching process. It is postulated that glycerol solubilizes a factor which aids the hatching process.

I should like to thank the following: Dr S. A. Ibrahim, in whose department this work was carried out; Dr S. Dawood of the Stack Laboratory, Khartoum, who kindly made his microscope available for use; Mr J. R. Lauder for valuable discussions and suggestions; and many young patients in Khartoum Civil Hospital for stool samples and smiles.