Heliocidaris Crassispina Research Articles

RE-FERTILIZATION OF THE FERTILIZED EGGS OF THE SEA URCHIN

1. If the fertilized eggs of sea urchins (Strongylocentrotus pulcherrimus, Pseudocentrotus depressus, Heliocidaris crassispina, Temnopleurus toreumaticus) are inseminated again after removal of the fertilization membrane and washing with a Ca-Mg-free medium for a few minutes, re-fertilization can take place. Spermatozoa not only penetrate into the previously fertilized eggs but also take part in the formation of the mitotic figure, so that the cleavage of re-fertilized eggs is irregular and characteristic of polyspermy.2. Re-fertilization can occur without removing the membrane mechanically if the eggs have been treated with a Ca-Mg-free medium immediately after the first insemination.3. Re-fertilization can take place at any stage of the first cell division and even at the 2-cell stage.4. It is justifiable to consider that there is a sperm-excluding substance on the surface of fertilized eggs. This substance is stable in sea water but is easily lost in Ca-Mg-free media.

Sperm entrance in echinoderms; observed with the phase contrast microscope.

1. Phase contrast microscope observation of sperm penetration into the eggs of five species of sea urchins—Mespilia globulus, Clypeaster japonica, Strongylocentrotus pulcherrimus, Pseudocentrotus depressus, Heliocidaris crassispina—and one starfish—Asterina pectinifera—have shown that total engulfment of the spermatozoan including all of the tail, is the rule in these species.2. In every case the fertilization cone makes its first appearance after contact has been established between the sperm head and the living egg cytoplasm, and persists in normal cases for from 5 to 15 minutes after the beginning of sperm penetration. Subjection of the eggs to some experimental conditions—heat, cold, nicotine solution—causes abnormal production and persistence of the fertilization cones.3. Engulfment of the sperm tail in these species is usually complete within 2-4 minutes after the beginning of penetration. No significant change in engulfment time could be induced by exposing the eggs to the abnormal physiological conditions tested—heat, cold, aging, exposure to polyspermy-producing agents.4. In the two species with the most transparent eggs (Mespilia and Clypeaster), the movements of the sperm head within the cytoplasm can be clearly observed, from the time it passes the cortex until it becomes quiescent, which approximately coincides with the final engulfment of the tail. Progress of the spermatozoan appears to be effected by a combination of autonomous movement and streaming of the egg cytoplasm.5. About four minutes after the beginning of fertilization, the clear outline of the sperm head is lost and the sperm aster begins to develop. From this time the middle piece of the spermatozoan can be found lying in the cytoplasm, and traced as it moves together with the sperm aster toward the center of the egg. So far as could be determined, it takes no active part in the first cleavage.6. In one case a sperm head, which had entered an egg close to the exceptionally excentric egg pronucleus, was attracted to and held motionless against the pronuclear membrane, within about a minute and a half after entering the cytoplasm, while its tail was still largely outside the egg. This, however, produced no variation in the subsequent process of engulfment of the tail and formation of the sperm aster.