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1 Tokyo Metropolitan University and Misaki Marine Biological Station
Eggs of the tubularioanthomedusan, Spirocodon saltatrix, which breeds in the neighborhood of the Misaki Station from the beginning of January to the latter part of March, are spawned daily, shortly after nightfall. The change from light to darkness causes the initiation of the maturation divisions, at the completion of which the eggs are shed. Spawning can be induced by darkening in the laboratory.
In the mature eggs, the pronucleus lies in contact with the surface, at the base of a slight depression at the animal pole. These eggs are fertilizable only when spawned into sea water containing freshly shed spermatozoa.
Examination of such eggs with the phase contrast microscope shows that the fertilizing sperm always enters in the immediate vicinity of the egg pronucleus. At the sperm entrance point a tubular structure of transparent substance begins to develop around the sperm tail as soon as the head of the sperm has penetrated the egg. This continues to increase in length and diameter for about five minutes, after which a process of degeneration sets in. At the end of about fifteen minutes the tube has become balloon-shaped and is more or less rapidly disintegrating, as the tip of the sperm tail is finally drawn into the egg.
The egg pronucleus moves a short distance from the periphery toward the center of the egg. A marked increase in size between 10 and 14 minutes after the beginning of fertilization probably indicates the union of the sperm and egg pronuclei, although no astral activity is detectable.
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