Which purified form of nitrocellulose is typically used for large hollow cavities and embryos?

Celloidin is a purified form of nitrocellulose that is notably used for embedding biological specimens, particularly when there are large hollow cavities or delicate structures like embryos. Its viscous nature and ability to provide excellent support for fragile specimens make it ideal for such applications.

During the embedding process, celloidin infiltrates the tissue, filling in the spaces and providing structural integrity, which is especially beneficial for specimens that might collapse or lose their shape if prepared with other mediums. Celloidin hardens upon evaporation of the solvent, offering a firm matrix that allows for fine slicing and stable handling.

In contrast, the other substances mentioned do not have the same characteristics suited for large cavities or fragile tissues. Carbowax is used for embedding but is typically more suited for smaller specimens or those that are less delicate. Gelatin is generally used for embedding softer, more histologically demanding materials rather than larger specimens. Paraffin is the most commonly used embedding medium for many types of tissues but may not encapsulate hollow cavities as effectively as celloidin does.