Reference: Biol. Bull. 167: 488-494. (October, 1984) MICROSCOPIC OBSERVATIONS OF CRYPTIC POLAR BODY PRODUCTION AND SPIRALIAN ORGANIZATION IN THE EGG OF ILYANASSA OBSOLETA MARJORIE MCCANN-COLLIER Biology Department, Saint Peter's College, Jersey City, New Jersey 07306 and Marine Biological Laboratory, Woods Hole, Massachusetts 02543 Abstract Light microscopy of semi-thin sections and scanning electron microscopy reveal a pit at the animal pole of Ilyanassa obsoleta during Meiosis I. The incipient polar body is within the pit. Sections show the spindle at an oblique angle to the egg axis, an early expression of the spiralian organization of this egg. Surface changes on the incipient polar body are similar to those described by others for the polar lobe of Nassarius reticulatus. Introduction In the marine mud snail Ilyanassa obsoleta the egg is ripe at late prophase of Meiosis I and fertilization is internal. The eggs are fertilized in the pallial oviduct, and subsequently surrounded by a viscous fluid, enclosed in capsules (usually in groups of 50-250), and deposited on an external substratum. Normally, the eggs are deposited before maturation is complete, but there is great variability in the time elapsed between fertilization and deposition — in some cases, 24 h or more (Collier, 1981). Thus, the time of deposition is unreliable in determining the stage of devel-opment, and selecting just-laid capsules does not insure that eggs pooled from these different capsules are the same fertilization age. Collier (pers. comm.) found that a reliable criterion is the appearance of a dark spot at the animal pole. This spot, which can be seen under low magnification, is found only in the freshest eggs (preceding first polar lobe formation), lessens in intensity after 20-30 minutes, and disappears before the second polar lobe forms. For many years this dark spot was thought to be the germinal vesicle and its disappearance, the breakdown of that body. Recently, light microscopy of semi-thin sections of this stage have shown differently: in reality the spot is a depression, or pit, at the bottom of which the first polar body is forming. Scanning electron microscopy has confirmed this observation. Materials and Methods Adult snails were maintained and freshly laid capsules of fertilized eggs were obtained as described by Collier (1981). Eggs were removed from the capsules and fixed either in ( 1 ) a solution of picric acid to saturation and 2% paraformaldehyde in 0.10 M NaH2P04 buffer at pH 7.3 (PAF), which is a modification of a procedure of Stefanini et al. (1967), for 2 h at room temperature, rinsed in buffer, left in lithium-saturated buffer overnight, and rinsed again in buffer, or (2) in 1% OSO4 in 0.4 M sodium acetate, pH 6.0, for 1 h at room temperature (Burgess, 1977). The PAF Received 7 May 1984; accepted 20 July 1984. 488
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