I've been doing some online reading on embryonic development lately, and I keep bumping into a strange dichotomy between two seemingly incompatible depictions of how we get triploblastic (three-layered) structure out of a blastula.
Both versions agree on roughly what a blastula is: a round group of cells formed by repeated division of the fertilized egg cell, with all of the cells arranged around the outside surface of the sphere and an acellular liquid mass enclosed inside. But what happens to it next is where the contradictions start.
In one story, some point on the surface starts dipping inward, forming a dent which continues to get deeper until it reaches the opposite side and creates a second opening. The tube this creates is the endoderm, which becomes, among other things, the digestive tract. The cells that remained on the outside of the ball during this are the ectoderm, which becomes, among other things, skin. The space between them (now shaped like a donut wrapped around the endoderm tube) will later be filled in with mesoderm, which becomes most of the other organs that aren't associated in some way with the digestive tract or skin.
In the other story, no such dent/tunnel forms. The liquid space inside the blastula soon gets divided by an additional wall/membrane of cells running through it. On one side, the acellular liquid remains; on the other side, a mass of other cells builds up, which, like the sphere's original outer layer of cells, will not become part of the new individual's body; they become placenta instead. Only that interior membrane/wall of cells, the layer separating the internal liquid space from the internal cell mass, actually ends up as the embryo. It's flat, like the end of a paddle. A process I won't get into here (but it involves a thing called a "primitive streak", in case you want to look for it) divides this flat body into three flat layers which remain stuck together: endoderm, mesoderm, and ectoderm. But the "endo" and "ecto" aren't really the inside or outside of anything; they're just flat things with a flat mesoderm between them, like a sandwich. And there's no sign of a digestive tract yet. And some videos go on from there to depict the rest of fetal development in a way that makes it look like the "endoderm" forms the belly and chest and face, while the "ectoderm" forms the back, instead of any kind of reshaping to get the endoderm inside and the ectoderm outside where their names would make sense and they could become the organs they're supposed to become.
The first version is more consistent with other information I know of, such as which organs are normally identified with which germ layers and the origin & meaning of the word "deuterostome" (the name for a high-level, large-scale clade we are members of). But I don't see much sense in figuring that the second story is entirely fictional either. So what's going on here?
Both versions agree on roughly what a blastula is: a round group of cells formed by repeated division of the fertilized egg cell, with all of the cells arranged around the outside surface of the sphere and an acellular liquid mass enclosed inside. But what happens to it next is where the contradictions start.
In one story, some point on the surface starts dipping inward, forming a dent which continues to get deeper until it reaches the opposite side and creates a second opening. The tube this creates is the endoderm, which becomes, among other things, the digestive tract. The cells that remained on the outside of the ball during this are the ectoderm, which becomes, among other things, skin. The space between them (now shaped like a donut wrapped around the endoderm tube) will later be filled in with mesoderm, which becomes most of the other organs that aren't associated in some way with the digestive tract or skin.
In the other story, no such dent/tunnel forms. The liquid space inside the blastula soon gets divided by an additional wall/membrane of cells running through it. On one side, the acellular liquid remains; on the other side, a mass of other cells builds up, which, like the sphere's original outer layer of cells, will not become part of the new individual's body; they become placenta instead. Only that interior membrane/wall of cells, the layer separating the internal liquid space from the internal cell mass, actually ends up as the embryo. It's flat, like the end of a paddle. A process I won't get into here (but it involves a thing called a "primitive streak", in case you want to look for it) divides this flat body into three flat layers which remain stuck together: endoderm, mesoderm, and ectoderm. But the "endo" and "ecto" aren't really the inside or outside of anything; they're just flat things with a flat mesoderm between them, like a sandwich. And there's no sign of a digestive tract yet. And some videos go on from there to depict the rest of fetal development in a way that makes it look like the "endoderm" forms the belly and chest and face, while the "ectoderm" forms the back, instead of any kind of reshaping to get the endoderm inside and the ectoderm outside where their names would make sense and they could become the organs they're supposed to become.
The first version is more consistent with other information I know of, such as which organs are normally identified with which germ layers and the origin & meaning of the word "deuterostome" (the name for a high-level, large-scale clade we are members of). But I don't see much sense in figuring that the second story is entirely fictional either. So what's going on here?
via JREF Forum http://forums.randi.org/showthread.php?t=263303&goto=newpost
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