1. The distribution of polyphenol oxidase in quinone-tanning systems may be demonstrated in frozen-dried sections by incubation in 0.2 per cent, aqueous catechol at 400° C. for 15-60 minutes. A red colour develops at the enzyme site.

2. The evidence for the view that the egg-shell in trematodes, in certain cestode groups, and in turbellarians, is a quinone-tanned protein secreted by the so-called ‘vitelline’ glands, is summarized. The ‘vitelline’ cells, in addition to giving positive reactions for proteins and phenols, give a strongly positive reaction with the catechol polyphenol oxidase test.

3. The catechol technique may also be applied to whole helminths fixed in 70 per cent, alcohol, and serves as a useful whole mount stain for the shell-producing regions of the female genitalia.

4. In Mytilus the catechol technique reveals the presence of polyphenol oxidase in an ‘upper’ or enzyme gland in the foot.

5. It is suggested that in Mytilus the byssus is formed from a phenolic protein secreted from the phenol gland, which on contact with polyphenol oxidase can undergo ‘auto-quinone tanning’.

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