I realise that mammalian eggs (such as are produced in the ovaries) are single cells. Is this also true for reptiles, birds, fish etc that lay eggs outside the body? When I buy half a dozen hen eggs in a cardboard carton, am I in fact buying six individual cells? This question has always bugged me.
Poultry eggs are a collection of many, many cells.
Eh, no, Crotalus.
Eggs are one big cell, yes. Even ostrich eggs.
Oops. Sorry. That’s what I get for thinking I know something.
Some confirmation of what Nava said here .
From the link:
Several months ago someone asked about single-cell seaweed. I posted to the thread because I’d recently read a paragraph in Calypso Log about Caulerpa taxifolia that made the claim, and I found the statement to be unbelievable. (I thought perhaps they had misspoke or worded the claim poorly.) I was amazed it’s true.
From here (emphasis mine):
Wouldn’t that take the crown from poultry ova as the ‘largest living cell’?
Well, it sort of depends on what you define as “cell”. Basically the organization is a bunch of cells with channels connecting all of their cytoplasm, so structurally it’s sort of a whole lot of cells, but it also has properties of one large cell.
The alga is probably multinucleate; a mass of protoplasm containing thousands of nuclei, and their associated chromosomes. Such coencytic structures more closely resemble The Blob which tried to eat Steve McQueen than they do a normal cell.
Man, I love these boards.
Don’t forget the giant space amoeba from Star Trek.
To elaborate, the yolk of an unfertilized egg is essentially a single cell. The cell is almost completely filled with food material, which is the yelllow substance of the yolk. On the surface of the yolk, there is a small germinal disk, which contains the cell’s cytoplasm, nucleus, and other organelles. Once fertilzed, this germinal disk divides and becomes a multicellular embryo, so a fertilized egg is no longer a single cell. The egg white (albumen), various membranes, and the shell are not part of the cell. The yolk + germinal disk of the ostrich is the largest extant cell with a single nucleus. Some extinct birds and dinosaurs had larger eggs.
Caulerpa is indeed a single cell with multiple nuclei. Since some species can get up to 3 meters long and have 200 fronds, they could potentially outweigh an ostrich egg cell.
Some fungi, notably bread molds, have a similar coenocytic multinucleate. However, I am not aware that any of these get large enough to compete with avian eggs or Caulerpa.
The site linked to does not come clearly down on the side of “the egg is a giant cell.” It says that by some definitions, it is, and by some definitions, the egg simply contains a large cell surrounded by yolk, other stuff, and a shell.
Its my understanding that the egg shell is evolutionarily cognate with the mamalian (now I can’t remember what it’s called) “bag” that surrounds an infant in the womb.
-FrL-
coenocytic multinucleate structure .
The answer given on that website is rather garbled, and I wouldn’t consider it at all authoritative. Since the yolk material and germinal disc are contained within a single cell membrane, by the usual definition they constitute a singel cell.
You are thinking of the amnion, but this is not homologous to the egg shell; birds’ eggs also have an amnion. Here’s a diagram of a typical amniote egg. (Amniotes include reptiles in the traditional sense, birds, and mammals.) Various membranes associated with the embryo include the amnion, allantois, and chorion. These mainly originate from the original egg cell. The egg white (albumen) and the egg shell are not derived from the embryo, but are secreted by the mother.
A mammalian egg is a single cell, but not a complete one. It has half a set of chromosomes, and it could never develop on its own. When it meets with a sperm, also a half cell, the fertilized zygote they form is one complete cell, not two. That complete cell is capable of developing into a blastosphere, then an embryo, then a fetus (I think I may have left out a step or three.) All that stuff will happen only if a series of conditions are just right. If they aren’t, the zygote gets sloughed off, and the female mammal is unaware that fertilization took place.
I wouldn’t phrase it that way. A mammalian ovum is certainly a complete cell; it just has half the normal number of chromosomes. Most multicellular animals have paired chromosomes, that is, they are diploid. Gametes (ova and sperm), and some insects (notably male ants, bees, and wasps) have a single set of chromosomes, and are referred to as haploid. Many fungi and algae are also normally haploid. As shown by these organisms, haploids are perfectly capable of developing and functioning as organisms, and their cells can’t be considered to be “incomplete.”
This is the source of the common joke that the “H” in Jesus H. Christ stands for “Haploid” (due to his virgin birth).
If an chicken egg is one cell, where’s all the chromosomes at?
If you crack an egg into a bowl, you’ll find a white spot on the yolk. That’s the germinal disk, and the chromosomes are inside it, along with 99% of the other organelles.
** thinking **
Does roster sperm have to get through the shell and then find a needle in a haystack inside the egg? How’s that work?
Nope. The sperm is shared with the chicken before the eggs are shelled. Roosters don’t have penises. Rather, both roosters and hens have cloacas, which are usually internal tubes that carry waste. When chickens do the nasty, they evert (turn inside out and stick out of their bodies) their cloacas. The rooster cloaca has sperm on it, and he rubs it on the hen’s cloaca, which then retreats back into her body. Then the sperm fertilize the germinal disk inside her, before the shell is formed around the egg.
Any hen has several eggs in varying stages of development inside her at once, and her body stores the sperm for a little bit to fertilize a bunch of eggs with it, not just one egg for one bout of sex.