EFFECTS OF CENTRIFUGAL FORCE ON THE ECTOPLASMIC LAYER AND NUCLEI OF FERTILIZED SEA URCHIN EGGS

Abstract

The ectoplasmic layer can be thrown off the fertilized eggs of Parechinus microtuber-culatus, Paracentrotus lividus, Sphaerechinus granularis, Arbacia pustulosa, and A. punctulata by centrifugal force as a ring or crescent which lies in the perivitelline space. The ring is not formed in absence of calcium, but re-fringent spherules are precipitated when the eggs are returned to sea water. The ectoplasmic layer is replaced on eggs with fertilization membranes and the eggs de- velop normally. The [female] pronucleus is driven by centrif-ugal force to the light pole and the [male] pronucleus to the heavy pole of the elongated eggs; the [male] pronucleus may become much larger than the [female] before fusion. An egg may be broken into 2 fragments, one containing the [female] and the other the [male] nucleus; only one fragment de-velops. The spindle is thrown to the light pole and cleavage usually comes in perpendicular to the stratifi-cation. If centrifuged just before cleavage, the cleavage plane may come in parallel with the stratification and in no relation to the new position of the spindle. Many whole eggs centrifuged after fertilization (fertilization membranes removed) develop normally, and many fragments thus obtained. Some lack an ectoplasmic layer and the cells fall apart.