The mode of hatching of the egg of Fasciola hepatica

Abstract

1. 1. The egg of Fasciola hepatica Linn, was used to test three theories of the mode of hatching of trematode eggs. 2. 2. Exposure to light stimulates the miracidium to release an enzyme that digests a substance binding the operculum to the shell; it is a change in this bonding substance that permits the egg to open. 3. 3. The natural “hatching enzyme” acts from within the shell, and it has no effect on the bond when applied to the outer surface of the shell. 4. 4. Solutions of crystalline pepsin and trypsin digest the bonding substance when they are permitted to come in contact with the inner surface of the shell; diastase and hyaluronidase are without effect. That both pepsin and trypsin destroy the bond suggests the substance is a protein and the “hatching enzyme” is proteolytic in function. 5. 5. At the opercular end of the egg cavity and within the vitelline membrane lies a viscous cushion of material in the shape of a concavoconvex lens. A sequence of changes in the cushion serves as a prologue to the hatching process. During this prologue the refractive index of the cushion changes slightly at one or more points, and a few seconds thereafter the material of the cushion expands rather suddenly to twice its original volume. After a brief interval, and then, in rapid succession, the operculum opens with force as if hinged to the shell, the vitelline membrane ruptures, the cushion material flows out of the shell and usually dissipates, and the miracidium promptly follows the cushion material through the narrow opening. 6. 6. Experiments demonstrate that the escape of the miracidium from the opened egg is due primarily to the hypertonicity of the egg contents and only secondarily to the muscular activity of the larva.