> The sperm enters the egg, not the other way around. I think this is wrong. My understanding is that the sperm cell membrane fuses with the egg cell membrane

> The sperm enters the egg, not the other way around. I think this is wrong. My understanding is that the sperm cell membrane fuses with the egg cell membrane, forming a single container. The haploid nucleus of the sperm and the haploid nucleus of the egg then fuse. This processes is by no means fully symmetric (the egg has a tough coating on its cell membrane that must be dissolved by the sperm through the Acrosome reaction), but there is also no time when the sperm cell (delineated by its membrane) is inside the egg cell. https://en.wikipedia.org/wiki/Acrosome_reaction https://en.wikipedia.org/wiki/Acrosome_reaction My point was not that there aren't many correlated asymmetries between eggs and sperm in particular parts of the phylogenetic tree (like mammals) that can be used for distinguishing them. My point was that there seems to be no fundamental asymmetry that applies to all two-sex sexually reproducing organisms; rather, there is a continuum of degree of gamete dimorphism that ends at organisms where there is no principled distinction between sperm and egg (isogamy).