Fertilization and activation of sea urchin eggs in glass capillaries I. Membrane elevation and nuclear movements in totally and partially fertilized eggs

Abstract

Psammechinus miliaris eggs were fertilized in glass capillaries of small diameter, the ends of which were sealed with drops of oil. Complete membrane elevation occurred on slightly elongated (cylindrical) eggs (110–140 microns) when these eggs were held stationary. Membrane elevation could be partially or totally inhibited by friction of the egg surface against the glass during sperm penetration. In eggs which had been elongated to a greater extent, the fertilization impulse failed to cover the entire egg surface, so that the egg was left only partially fertilized. The unfertilized portions of such eggs remained fertilizable, whereas the fertilized parts underwent the surface and interior changes associated with activation. Nuclear movements were observed in eggs in various degrees of elongation. The distance over which the egg and sperm nuclei must migrate in order to meet (penetration path plus copulation path) was varied by the manner of orientation of the eggs when drawn into the capillaries. The sperm nucleus penetrated a relatively constant distance from the surface (the penetration path) irrespective of the presence or position of the egg nucleus. These initial movements of the sperm nucleus were coincident with the growth of the sperm aster. Further movements of the two nuclei (copulation path) depended on the positions of the two nuclei and could be undertaken by either or both of the nuclei, which could exert a mutual attraction even when separated by a distance eight times as great as that in the normal spherical egg. The egg nucleus was unable to migrate when surrounded by unfertilized cytoplasm, but swelling and nuclear breakdown could occur. It is therefore believed that these latter phenomena were caused by diffusing substances.