Abstract
Normal oocyte development and the effects of brain extract injections on egg laying and on oocyte ultrastructure have been investigated in two populations of Helisoma: an egg-laying population (H. duryi) and a non-egg-laying, overwintering population (H. trivolvis). Oocyte development in H. duryi resembles that in other pulmonate molluscs. Endocytotic profiles occur in maturing oocytes but not in young (previtellogenic) oocytes. Injected horseradish peroxidase (HRP) and ferritin can be found in the follicular space and between follicle cells 2–3 h after injection. Ultrastructural evidence indicates that these tracers are taken up by maturing oocytes, but not by young oocytes or follicle cells. In non-egg-laying H. trivolvis large oocytes lack endocytotic profiles although a follicular space and many yolk granules may be present. Injection of extracts of dorsal bodies or suboesophageal ganglia from H. duryi induces egg laying in H. trivolvis within a few days. Injection of dorsal body extract into non-egg-laying animals also induces the appearance of many endocytotic profiles in large oocytes. It is suggested that endocytotic uptake of yolk components is stimulated by dorsal body extract in these animals.
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