Nassarius fossatus is a marine snail. Snails, or gastropods, belong to the Spiralia - a large group of animals with spiral cleavage. Nassarius fossatus has unequal cleavage, which means that one of the cells at the two-cell and four-cell stage is larger than the others. The first four cells in a spiralian embryo are denoted as A, B, C and D. The D cell is the largest in unequal spiral cleavage. There are several mechanisms by which unequal cleavage can be accomplished. Nassarius does this via the so-called polar lobe, which is shown in these pictures. A polar lobe is an anucleated protuberance which forms at the vegetal pole during first, second, and sometimes subsequent cell divisions. It then fuses with one of the cells, making it larger than the others.
The top picture shows polar lobe formation during the first cell division. One can see two polar bodies. Polar bodies are the tiny sister cells of the oocyte which are produced during meiosis, contain discarded DNA and mark the animal pole of the embryo (up in the first three pictures). The opposite pole of the embryo is the vegetal pole. The two cells at the animal pole are the first two blastomeres. What looks like a third cell at the vegetal pole is the polar lobe, which at this stage is nearly completely cinched off from either blastomere. Subsequently the polar lobe fuses with one of the blastomeres (second picture from top), so that by the end of the first cell division one of the blastomeres (called CD) is noticeably larger than the AB cell (third picture from top). Polar lobe also forms at the second cell division (not shown). At the four-cell stage blastomere D is the largest, blastomere C is the second largest, while A and B cells are about the same size (bottom picture). The first three pictures are lateral views, while the bottom picture is a polar view. It is the first time I have heard of and observed unequal spiral cleavage, and I think it is remarkable. I also liked these eggs because the egg capsules they are laid in are very beautiful when viewed under the dissecting microscope (see picture by Janelle Urioste).