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

Abstract

With strong centrifugal force and the proper medium, eggs can be separated into 2 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. 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. 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. 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. 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. 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. 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. All of the half- and quarter-eggs will start to develop parthenogenetically, i.e., throw off a fertilization membrane, if treated with distilled water. Only those with a nucleus develop further, till just before cleavage.