April 9, 2003
From: Kathe R. Jensen
Dear Rachel & Bill,
As you mention, there seems to be 2 types of "extra-capsular yolk" in sacoglossans: the superficial "blobs", which are also found in Elysia obtusa and I think in E. atroviridis and possibly most species of Bosellia. Unfortunately we do not know where either of the 2 types of "yolk" are produced. I think this would be a nice research topic for a student of micro-anatomy and histology. Most likely these superficial blobs are produced in the distal part of the mucus gland (which should not be spelt mucous though it is slimy (mucous) as well as slime- (mucus-)producing), and the internal band-like yolk is produced in some more central part of the reproductive system.
I cannot explain why the yolk-dots are black towards both ends of the egg mass.
I checked the picture of the Thuridilla vatae egg mass and I don't think that it has superficial spots of yolk. In Thuridilla the eggs are also coloured red or orange.
I wondered whether the large spots in the Thuridilla vatae egg mass were eggs or these 'superficial blobs' [what a wonderful scientific term] and decided that if they were eggs they were extremely large. Do you know if it is a direct developer? Any photos of eggs with peculiar 'yolk' would be very welcome.