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Zona-free hamster eggs have been fertilized in vitro with boar spermatozoa in a medium enriched by arginine-3H and the sites of localization of newly synthesized arginine-3H–labeled proteins have been investigated using fine-structure autoradiography. It was confirmed that such proteins are synthesized during fertilization and that they accumulate to a notable degree in decondensing sperm chromatin as well as in the chromatin of the female pronucleus and also of the second polar body. A similar process did evidently take place also in defective pronuclei, characterized by a core of a still condensed chromatin and by remaining nuclear membrane. In such male pronuclei the highest concentration of the label was seen just on the border of the condensed chromatin, on the expected site of nuclear protein exchange. It is supposed that, at least in this experimental system, any morphologically detectable sperm decondensation is accompanied immediately by a shift from sperm basic nuclear proteins to other nuclear proteins.

In the course of in vitro fertilization of zona-free hamster and pig eggs by boar and guinea-pig spermatozoa it was observed that homologous and heterologous eggs fused together, forming cell hybrids between two or more cells. The fusogenic activity was attributed to spermatozoa and this was the hypothesis tested. The fusogenic activity (coinciding with sperm penetration activity) was dependent on the duration of sperm preincubation, which may be regarded as capacitation in vitro. Fusion occurred only after 3 hr of sperm preincubation and a narrow optimum was detected at 4–4.5 hr. Fusion of eggs was also dependent on sperm concentration. A relatively high proportion of fusions was observed at a sperm concentration of 4.0 × 104 per ml and an optimum was attained at a concentration of 5.0 × 105 per ml. The first fusions were observed at 90 min after semination. After 3 hr more than a half of the eggs reacted, and by 20 hr of incubation 80% of ova were fused. The fusability of eggs was tested and found to occur at 14 hr after ovulation. The fusion process was also studied using transmission electron microscopy. It is supposed that the process of egg fusion may be caused either by a similar mechanism to sperm-egg fusion, or by products released during the sperm acrosome reaction.