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)
Just for any of you who haven’t read this post on Pharyngula, go take a look. For this geologist, it’s a facinating read.
What signaling molecules set up spiral cleavage patterns? How do they differ from those that set up cleavage patterns in bilateral organisms?
As I read this it sounds like a test question, but I really don’t know.
Delta Pi Gamma (Scientia et Fermentum)
Update