Abstract: Apparatus for processing tissue samples for histological examination comprises a cassette defining a chamber to receive a tissue specimen and having an opening to allow processing fluids to enter and leave the chamber and to allow, eventually, molten wax to enter the chamber for embedding the tissue sample. The apparatus incorporates a temperature-sensitive valve mechanism for closing said opening once the cassette, with the tissue specimen therein, has been immersed in a container of hot molten wax, so that the wax can be retained in the cassette, around the specimen, when the cassette is withdrawn from the molten wax container, until the wax around the specimen has solidified. The temperature sensitive valve mechanism may comprise a shape-memory element.