Sperm fertilizing egg. Colored scanning electron micrograph of a single sperm fertilizing a human egg. The human female produces usually a single large egg or ovum (at lower frame) for fertilization, whereas the male releases some 300 million much smaller sperm (spermatozoa) which travel with whiplash movements of their tail through the uterus to reach the egg. Only a few hundred sperm survive this journey and encounter the egg in the fallopian tube, and only one sperm can actually fertilize the egg. The sperm succeeds in penetrating the egg's wall to fuse with the egg nucleus. When this happens the egg membrane forms a barrier to other sperm. Magnification: unknown.