1. Over 98% of a dose of [14C]leucine injected into the circulation of Octopus vulgaris is removed from the blood during the first hour.

2. There is a rapid accumulation of labelled protein in the ovaries of maturing animals within 2 h of injection. Within 5-7 h the ovaries contain nearly 40% of the injected label in protein form.

3. Removal of the optic glands prevents this accumulation of protein.

4. There is little labelled protein in the livers of either control or maturing animals at any time; but a slow, steady accumulation occurs in their blood.

5. The level of labelled protein appearing in the blood of acutely ovariectomized, maturing females is no higher than in controls; and when blood protein from ovariectomized animals is injected into normal maturing females it is not taken up by the ovaries.

6. The labelled protein which accumulates in the blood is probably haemocyanin. Preliminary experiments indicate that the branchial glands, which are already believed to be a site of haemocyanin synthesis on morphological grounds, show a high rate of protein synthesis and release.

7. Isolated ovarian follicles in a liquid medium synthesize protein at a rate somewhat lower, but comparable with, the apparent in vivo rate.

8. The combined evidence from these experiments indicates that in Octopus yolk proteins are formed within the ovary-probably by the follicle cells-rather than being synthesized elsewhere and transported through the blood, as in arthropods and vertebrates.

9. The optic gland gonadotropin is essential for maintenance of protein synthesis during secondary vitellogenesis and the follicle cells are a likely site for its action during this stage of development.

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