Biology of Finfish and Shellfish

4.12.1. Eggs

Eggs opaque, with a narrow perivitelline space, chorion has a purplish sheen, diameter of eggs varied from 0.25 to 0.27 mm and that of yolk mass 0, 22 to 0.24 mm. The eggs when first observed at 23.45 hours were covered with a, radiating jelly like substance which partly dissolved and became granular while being observed under the microscope, and disappeared after 3 minutes. The egg was then spherical but without perivitelline space and appeared to be still invested with jelly like substance which was transparent. A polar body was seen adhering to the surface of the egg. Within one minute the perivitelline space was formed by the elevation of the fertilization membrane and the egg assumed the definitive form. Immediately after a second polar body was seen coming out of the yolk mass and traversing the perivitelline space just below the first polar body and soon reached the surface of the egg. The first cleavage began at 00.15 hours, about 30 minutes after the extrusion of the eggs. The second cleavage took place at 00.30 hours. The cleavage continued and at 01.30 hours the blastula stage was observed. An embryonic membrane was clearly visible during the blastula stage. Gastrulation started at 02.15 hours and continued up to 02.55 hours. At 04.50 hours the embryonal mass became constricted laterally and the appendages started differentiating. By 07.45 hours all the 3 naupliar appendages could be seen as lateral thickenings which became tipped with short spine-like setae by 09.45 hours. At 13.00 hours the 3 appendages were fully formed with long setae. The embryo occupied the entire space inside the egg and the movements were restricted to sudden jerks of appendages. The furcal setae first pierced the egg membrane and the nauplius wriggled out of the egg 16 to 17 hours after the eggs were spawned.