Anuran fertilization: a morphological reinvestigation of some early events

Abstract

The morphological events dealing with the fertilization or activation process of the eggs of different Rana species are reported. Steps from sperm penetration to male pronucleus formation are described as well as the egg surface evolution until the second polar body formation. Depending on the species, a “fertilization body” (FB) rapidly differentiates at the site of sperm entry. This FB is either extruded or is reincorporated into the egg. In both cases a landmark persists. For the first time, it is clearly demonstrated with SEM that the cortical-granule exocytosis wave starts from the site of sperm entry. Just after the exocytosis wave occurs, the villi elongate and the movements of the pigment granules increase, reflecting a change in the egg cortex fluidity. This last phenomenon should be considered as new criteria of activation. The cell coat appears just as the cortical granule material is extruded; it is no longer possible to recognize the new membranes.