Half-way through this movie you might ask yourself, what is that funny three-celled embryo?  How can it have three cells?  Is it normal?  If one found a single three-cell embryo in a culture of normal-looking two-cells, one would assume it was polyspermic or otherwise defective.  But what about a whole dish of them?  This apparent three-cell – or trefoil – stage is a normal feature of early development for many molluscs and annelids.  The scaphopod featured here, a predatory infaunal mollusc with a tube-shaped shell, exhibits classic polar lobe formation: the extrusion and transient partition of a large parcel of cytoplasm which is subsequently resorbed into one daughter cell.

As in this case, segregation of the polar lobe cytoplasm at the vegetal end of the embryo results in a protrusion so constricted that it appears to be a third cell.  This is remarkable for two reasons: first, the polar lobe is a mechanism for cell fate specification; second, it is a cytokinetic puzzle.  In animal cells, the cytokinetic furrow always crosses the midplane of the mitotic apparatus.  That’s how the cell ensures that the cell division plane passes between carefully-sorted sets of duplicated chromosomes.  In polar-lobe-forming cells, of course there is only a single mitotic apparatus, bisected by the “normal” furrow.  How is the furrow around the neck of the polar lobe induce?  There are several possible explanations, but all of them are speculative.

However it forms, what embryologist could resist the temptation of experimentally removing such a tidy bundle?  This experiment has been performed on several species, most famously by Clement on the snail Ilyanassa obsoleta.  The results show that the contents of the polar lobe endow its lineage with something that makes one of its descendant cells behave as the organizer for the rest of the embryo.  One interpretation would be that there is information essential for normal development contained within the polar lobe.  The nature of that information, however, remains elusive, and another possibility is that the polar lobe just makes some cells bigger, and consequently different.

Some molluscs and annelids make small polar lobes.  Others simply undergo an unequal first cleavage, without a polar lobe.  But there is good reason to think that the ancestors of both molluscs and annelids probably cleaved equally, instead of determining the organizer lineage in the first few divisions.  That means polar lobes have arisen several times during the evolution of existing molluscs and annelids.  Why?  Is there some reason, in the embryology of these groups, why many evolutionary lineages found it adaptive to speed up early cell fate specification?  And are these embryos, for some reason, predisposed to come up with this peculiar mechanism of unequal division?

— text by Katie Bennett and George von Dassow