The Presence of Centrioles in Artificially Activated Sea Urchin Eggs

The origin of the cleavage center or centriole of fertilized eggs has been of a controversial nature for many years. There is much evidence which shows that in cases where meiosis has been completed and the egg is in the pronucleus stage when fertilization takes place (sea urchins, tunicates, some nemertines and amphibians) the centriole of the fertilized egg is derived from the sperm middle piece, the mature egg seeming to have no active center of its own. The sperm-derived centriole then acts as an initiator or organizer for the asters and spindle of the mitotic figure and continues to duplicate itself in subsequent divisions. An excellent example of this type of behavior was observed by Boveri (1888) in eggs of the sea urchin, Echinus. He was able to show that when the sperm center outstrips the nucleus in its migration in the fertilized egg, it forms an amphiaster with the chromosomes of the female pronucleus, leaving the male pronucleus behind. There are, however, conditions under which the egg, without mediation from the spermatozo6n, may be made to produce all the structures required for division. For example, in artificial parthenogenesis the egg can produce asters and, under optimal conditions, a complete division figure (see reviews by Wilson, 1924, and Tyler, 1941). Clearly, these figures cannot derive from the sperm. Since (in eggs which have completed meiosis before fertilization takes place) the fate of the formerly existing egg centriole is unknown, there has been much speculation (Wilson, 1924; Briggs and King, 1959) as to whether the centers of artificially produced asters derive from this centriole or whether they arise de novo. Studies by E. B. Harvey (1936) on artificial activation of enucleated halves and quarters of sea urchin eggs strongly suggest that the asters formed under these conditions have been produced by centrioles with de novo origin. On the other hand, there is convincing evidence that normally (i.e., in cells undergoing mitosis) the centriole is a truly self-duplicating structure (Pollister, 1933; Cleveland, 1957). It would seem unlikely that such a structure would also have the ability to arise de novo. For this reason, it has been suggested (Brachet, 1957) that artificially produced cytasters do not arise from true centrioles at all, but may arise from any cytoplasmic granule. Recent electron microscopic studies which show a highly organized structure of the centriole, both in vertebrate material (de Harven and Bernhard, 1956) and in marine eggs (Harris, 1961; Rebhun, 1960), stimulated the present preliminary investigation, for they offered the opportunity to determine whether the asters produced by artificial parthenogenesis might have at their center similar structures.