THE DEVELOPMENT OF HALF AND QUARTER EGGS OF ARBACIA PUNCTULATA AND OF STRONGLY CENTRIFUGED WHOLE EGGS

Abstract

1. With strong centrifugal force and the proper medium, Arbacia eggs can be separated into two half-eggs, one colorless containing oil, nucleus, clear layer, fifth (granular) layer and a little yolk; the other slightly smaller containing yolk and pigment. With greater centrifugal force, the colorless half-eggs can be separated into quarter-eggs, one perfectly clear with oil and nucleus; the other, smaller, with fifth layer (granules) and yolk. All of these half- and quarter-eggs can be fertilized, form fertilization membranes and cleave. 2. Nuclear phenomena accompanying fertilization and cleavage, quite normal, can be observed with great clearness in the colorless half-eggs. Astral rays occur only where granules are present. These half-eggs cleave regularly and form swimming blastulae and plutei, normal except for color and size. 3. The pigmented half-eggs develop with only the male nucleus which divides repeatedly, usually without cell division. Some blastulae and a few plutei developed but these eggs and larvae are not very viable. 4. Whole eggs, centrifuged till dumb-bell shaped, retain their shape if fertilized immediately, even if the fertilization membrane is removed. The first cleavage in elongate eggs is usually parallel with the stratification, in spherical eggs it is usually perpendicular to it. Slipper shaped blastulae develop from the elongate eggs, and normal plutei. 5. Clear quarter-eggs begin to cleave very slowly (after 7 hours), and usually form loose clusters of cells owing to the breaking of the fertilization membrane. 6. Granular quarter-eggs develop with only the male nucleus and a little more slowly than the normal whole eggs; cleavage is quite regular but no swimming blastulae were obtained. 7. The pigmented half-eggs can be drawn out with a micro-dissection needle, and the material is pliable and elastic; the colorless half-eggs explode when punctured, pouring out granules. The clear quarter-eggs collapse immediately when punctured, and the granular quarters pour out their granules slowly. 8. All of the half- and quarter-eggs will start to develop partheno-genetically, i.e., throw off a fertilization membrane, if treated with distilled water. Only those with a nucleus develop further, till just before cleavage.