June 15, 2008
I've been reading The Canon by Natalie Angier, which she calls a "whirligig" tour of science. It's not 100% clear to me what she means by whirligig, but I might go with "giddy" (#). Angier has a style that features a lot of wordplay; think, dunno, Anthony Lane, maybe, but about science, not movies. The book tours physics, chemistry, biology, astronomythe usual suspects. It's an exuberant piece of writing, although I'm not sure I'd hand it out as an introductory textbook.
I didn't want to let the book slip back to the library unmentioned, and as an inducement for you to check it out, I wanted to set something down. In the chapter on biology, I learned something that I had never thought about and that I found quite surprising. Here 'tis. (I'll note that this is an unusually subdued bit out of the book, but it was one of the more remarkable things I learned.)
Nucleated or not, cells consist of three defining ingredients, and it so happens that one type of bioentity that fulfills the criteria is the egg. An egg has an outer membrane, a viscous cytoplasm that in the edible egg we call the yolk, and a set of genes only half the number of genes needed to spawn an offspring, and half the number of genes found in other body cells of the egg bearer, but a gene set nonetheless. An egg, then, before it merges its DNA with a gene set supplied by a sperm and starts developing into an embryo, is a single cell, and that goes for the egg you can see well enough to scramble. Yes, believe it or not, an unfertilized chicken egg of the kind you buy at the grocery store is a single cell, although strictly speaking it’s the cheery, marmalade-colored yolk of the egg that is bounded by the plasma membrane and thus qualifies as the cell proper. The translucent, whippable, protein-rich "egg white," the hard outer shell of calcium chloride, and the thin, slippery membrane lining the shell are all bonus coats added on later, as the yolk makes its way down the mother’s cloaca. Still, chicken yolks are no joke, and they keep getting ever more jumbo even as we fret over the wisdom of eating any eggs at all.
Did you know that? Not me, man. Angier continues with a Fun Fact:
The largest egg in the world, and thus the largest cell in the world, is the ostrich egg, which measures about eight by five inches and weighs three pounds with this extracellular shell, two pounds without. (Interestingly, the ostrich egg is also the smallest bird egg relative to the size of its mother, amounting to only 1 percent of the female ostrich’s body mass. The she-birds most deserving of every mother’s pity are the kiwis and hummingbirds, which lay eggs that are 25 percent as big as they are—the equivalent of a woman giving birth to a thirty-pound baby.)Dang.