Questions and Answers about BIOLOGY

by Ken Miller and Joe Levine


(2) QUESTION: My teacher said the chicken egg isn't one big cell, but that the little white membrane looking thing that is attached to the yolk is the CELL not the whole thing.

Well, your teacher is essentially (but not totally) correct. In an unfertilized egg (the kind you buy at the grocery store), there is a small, whitish disk on one side of the yolk. This little structure is called the germinal disc, and it contains the nucleus and most of the cytoplasm of the egg cell. The yolk, however, is actually part of the cytoplasm of that cell, so that one could say that the yolk is actually a single huge cell.

You teacher is correct, however, in pointing out that the yolk itself doesn't contain the structures and organelles that we expect to find in living cells. In fact, it's little more than a mass of stored nutrition waiting for embryonic development.

Here's why your teacher's opinion is correct in an important way. If the cell is fertilized by a sperm, which happens inside the body of the hen before the shell is completely formed, the whole yolk doesn't divide. Instead, cell division is limited to the germinal disc, so that little cluster of cells known as the embryonic disc forms on top of the yolk. The embryo develops from this disk, and gradually sends blood vessels into the yolk to use it for nutrition as the embryo develops.

Q: Another student said they had always thought that white thing was the sperm.

That's not correct. No sperm are found in an egg, fertilized or not, once it has been encased in a shell and has left the body of the chicken

Q: I am confused if it is just one cell then how did the rest of the egg get there without cell division? And exactly how would that egg get fertilized since the shell can't be penetrated????? - perplexed in Kentucky

Well, "Perplexed," here's how it works: After a chicken mates with a rooster, the sperm cells that he deposits in her reproductive tract can live for more than a week. Egg cells develop in the chicken's ovary, which is where most of the proteins and lipids that make up the yolk are added to the egg. Once they are released from the ovary, egg cells are fertilized by these sperm left in the reproductive tract. The white of the egg and its shell are then added (after fertilization!) as the fertilized egg cell begins its first few rounds on cell division to produce the embryonic disk.
I hope this answers all of your questions!

Ken Miller (2/7/02)

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(A web site developed by Ken Miller and Joe Levine to provide scientific and educational support for teachers and students using our textbooks)