Avian spermatozoa penetrate zona-free hamster oocytes in vitro

After collection from the cloaca of adult males, budgerigar spermatozoa were incubated in modified BWW medium at pH 8.3. By light microscopy, many spermatozoa (greater than 100 per oocyte) were seen to be bound to the oolemma, and a small proportion (1-2 spermatozoa) were judged to have undergone fusion and penetration of the vitellus, as indicated by complete decondensation of their chromatin. The latter finding was confirmed at the electron microscope level by the presence of sperm nucleus, midpiece and tail in egg cytoplasm. This finding is discussed in relation to events at fertilization in the different Phyla.