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As they exist in the cells of Spongilla the corpuscles are minute oval bodies of a bright green colour and each containing a highly refractile colourless granule.

After further contraction the outer layers of the mass become still more highly differentiated and form a distinct spore membrane, and the spore itself is now highly refractile.

As the colony increases, the granular character becomes more marked, until it seems to be made up of highly refractile granules, like a mass of particles of glass.