Formation of the first cleavage furrow in the telolecithal egg of Loligo was studied with the electron microscope. Before the actual furrow forms, a dense filamentous band develops below the plasma membrane from membrane-bounded dense bodies which appear to be Golgi-derived. The egg surface is thrown into a number of longitudinal folds which parallel the furrow and eventually become incorporated into it. These longitudinal folds contain a network of tubules and vesicles. Frequently, multivesiculate bodies are associated with the furrow and possibly give rise to the network of tubules and vesicles. Apparently part of the membrane between the two new blastomeres is derived from the surface of the longitudinal folds. The theory of furrow formation by contraction is discussed in light of the filamentous band.

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