The diagram above shows the early cleavages of the embryo of the scaphopod mollusc, Dentalium. You may notice a few peculiarities: the first cleavage is asymmetric, producing a cell called AB and a larger sister cell, CD. Before the second division, CD makes a large bulge, called a polar lobe, and it almost looks like it's a three-cell stage—this is called a trefoil embryo, and can look a bit like Mickey Mouse. The second division produces an A, a B, a C, and a D cell, and there's that polar lobe, about as large as the regular cells, so that it now resembles a 5-cell embryo. What's going on in these animals?
Continue reading "Polar lobes and trefoil embryos in the Precambrian" (on Pharyngula)