The differentiation of the primary envelope of oocytes of the seahorse (Hippocampus erectus) and the pipefish (Syngnathus fuscus) has been investigated by techniques of light- and electron microscopy. The developing oocytes have been divided into four stages according to size. Oogonia are designated as stage I; stages II and III are oocytes; stage IV represents mature eggs. The primary envelope which is produced by the oocyte is initially a tripartite structure; for convenience of description, the portions are referred to as zones 1, 2, and 3, respectively. Zone 1 first appears as a homogeneous substance at approximately the middle of the long axis of each microvillus. Zone 2 is immediately beneath zone 1 and consists of an extremely electron-opaque granular component. Zone 3 is subjacent to zone 2; it is the largest and most complex of the three. Zone 3 consists of an amorphous material organized in a reticular-like network. Staining procedures indicate that the envelope is composed of a glycoprotein. Just before the oocyte matures there is a structural alteration in zones 2 and 3. Zone 2 becomes a compact, dense layer and zone 3 becomes multilaminate. Subsequent to these changes, zone 1 degenerates. The classification of egg envelopes is discussed, and comparisons are made between the primary envelope of the teleosts investigated and the primary envelopes of other species.

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