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

¹ From the Stazione Zoologica, Naples, and the Biological Laboratory, Princeton University

1. The ectoplasmic layer is added on to the surface of the egg of Arbacia pustulosa on fertilization.

2. The ectoplasmic layer can be thrown off the fertilized eggs of Parechinus microtuberculatus, Paracentrotus lividus, Sphrechinus granularis, Arbacia pustulosa, and A. punctulata by centrifugal force as a ring or crescent which lies in the perivitelline space.

3. The ring is not formed in absence of calcium, but the dissolved ectoplasmic material is precipitated when the eggs are returned to sea water as refringent spherules in the perivitelline space.

4. The ectoplasmic layer is reformed on eggs with fertilization membranes and the eggs develop normally.

5. Soon after insemination (1-6 minutes), all the species studied break into many very small pieces; during the monaster stage (6-30 minutes), they form long streamers; later, elongate dumb-bells. The fertilized eggs of Parechinus and Paracentrotus break more readily at all stages than unfertilized eggs, those of Sphærechinus less readily; the fertilized eggs of all species stratify less rapidly than the unfertilized.

6. The female pronucleus is driven by centrifugal force to the light pole and the male pronucleus to the heavy pole of the elongate eggs.

7. The male pronucleus may become much larger than the female before fusion; the size of the male nucleus depends on the time before union and the density of the surrounding protoplasm.

8. An egg may be broken into two fragments, one containing the female and the other the male nucleus. Usually the former forms a monaster and does not develop, the latter an amphiaster followed by normal cleavage. In two lots of Paracentrotus eggs, the reverse took place; the division center was associated with the female nucleus and this fragment divided while the other fragment with the male nucleus did not divide.

9. The spindle is thrown to the light pole and cleavage usually comes in through its equator, perpendicular to the stratification. 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.

10. Many whole eggs centrifuged after fertilization develop normally and also many fragments thus obtained. Failure to develop is due to the lack of an ectoplasmic layer which causes the cells to fall apart, or to the absence of one or both nuclei.

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