Things you learn
The difference is in the way that a fertilized egg turns into an embryo. In radial cleavage – the upper set of figures – a fertilized egg divides so that cells stay aligned in a cube. This is the basic template for the deuterostomes, a group that includes humans starfish and sea urchins.
In spiral cleavage, the cells divide at an angle, building a spiraling pyramid of cells. The lab manual we're using treats spiral cleavage as a trait only found in the protostomes, which include insects, earthworms, mollusks and other treyf (yes, I know that some locusts are kosher). Systematists reading this may be shouting at their screens about Lophotrochozoa and Ecdysozoa, but I don't care right now. We present both options to the students, but focus on morphological characters.
A student asked me a very smart question: since there are lots of animals that aren't protostomes or deuterostomes (our lab manual doesn't put roundworms, flatworms or jellyfish in either group), and they all have some sort of cell division, what path do they take? The lab manual allows "not applicable," but that's a cop-out.
So I pulled out my dusty developmental biology book, and it was easy enough to find out that flatworms and roundworms have spiral cleavage. That isn't surprising, because most biology books put roundworms into the protostomes. But the developmental biology book didn't even talk about the cnidaria, a group that includes jellyfish, hydras, corals, and a host of other neat animals.
A little time with Google (while my student waited) brought me to this article from American Zoologist, which says:
In most metazoans, a stereotypical species- and often phylum-specific cleavage program ensues after fertilization. Virtually all cnidarians lack such a regular cleavage program. In fact, cnidarian development often involves "chaotic" cleavage in which no two embryos in the same spawning will exhibit an identical cleavage pattern.This didn't really surprise me, but I never knew it before. This tells me that there's a fairly amorphous form of cell differentiation in basal metazoans, and the more derived metazoans developed either some sort of radial cleavage or some sort of spiral cleavage. Signaling between cells presumably determines how cnidarians develop, what my developmental biology book calls "the 'American' style; there is a great deal of mixing between cells, and each cell is determined by who its neighbors are." The contrast is a "'European style'; that is, each cell is determined by who its ancestors were." Deuterostomes tend to follow the American style, while protostomes tend to follow some version of the European style. We could feel very proud of our cells, but by far the more successful group is the protostomes.
Learning something new is always exciting, but learning something new because a student asked a smart question is especially thrilling.
Nerds reading this are free to use the comments to crow about bilateral cleavage in tunicates and cephalopods and rotational cleavage in mammals.