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I am completely confused by the images circulating on the internet of human gastrulation.

First, lets see how it happens in deuterostomes. This image depicts the process:

enter image description here

(image is from Wikipedia)

From here we can conclude that blastula becomes gastrula when some of the ectoderm beecomes endoderm, the place where it goes inside becomes anus and then mesoderm is formed.

Ectoderm here is the outermost level and gastrula already has anus.

However, in this image of human gastrulatuion we see completely different things:

enter image description here (image source)

The mesoderm here is the outermost layer, ectoderm is inside and the gut cavity is formed by the separation of a part of the yolk sac. There is no anus and the posterior end of the gut is blind. Also other similar images suggest that a twins may be separated already after the mesoderm was formed (thus uniplacental twins).

The lack of agreement between the images disturbs me.

Just to point out some of the differences in the depicted processes:

  • In first image mesoderm forms after gastrulation, on the second it forms far before gastrulation
  • In the first image anus forms in the process of gastrulation, in the second image anus remains blind
  • In the first image ectoderm is the outermost layer, while in the second picture mesodem is the outermost level that encloses all, including ectoderm.
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Two suggestions: Please link to the source of the images, and please state what your actual question is. –  Michael Kuhn Feb 22 '12 at 14:35
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3 Answers 3

Human embryogenesis is much more complicated than the more general and simplified picture given in your 1st figure. For details, please study http://en.wikipedia.org/wiki/Human_embryogenesis

To point out the most distinctive features which might have led to your confusion:

  1. Before gastrulation, the blastocyst cavity is formed.
  2. Only the inner cell mass undergoes gastrulation.
  3. Mesoderm forms quiet late and spreads, thereby also forming the chorion as well as the amnion.
  4. The embryo itself is formed from a subset of all embryogenetic cells. This is shown in your 2nd figure, part C and D. Yet, tissues are still going to spread such that the final ectoderm as well as mouth and anus will be formed.
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The second picture does not show human gastrulation.

Your source talks about the development of yolk sac in human embryos, and does not claim it depicts gastrulation. In human embryos gastrulation happens about 7 days after fertilization and the development of membranes, as depicted in your second picture, begins about 15 days after fertilization.

I highly recommend the part of Chapter 11 from "The Developmental Biology", which deals with early mammal embryo development, from fertilization till formation of the body axes.

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How it turns that mesoderm is the outermost layer in the second picture and not in the first one? –  Anixx Oct 10 '12 at 16:27
    
Second picture doesn't mention mesoderm. Look closely at the titles. It's just that colour schemes are different, so one thing is blue in the first one and another in the second one. –  jkadlubowska Oct 10 '12 at 17:36
    
Mesoderm is red on the second picture. It is tissue that forms heart. –  Anixx Oct 10 '12 at 18:13
    
Also ectoderm and endoderm are shown in direct connect to each other in the second picture while on the first one they are separated with mesoderm –  Anixx Oct 10 '12 at 18:16
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The Wikipedia protostome vs. deuterostome figure depicts a "prototype" deuterostome, such as an echinoderm or an amphibian. In amniotes (reptiles and their descendants - birds and mammals), the giant amniotic egg full of yolk led to profound changes in early embryogenesis, including the disappearance of the true blastopore. (Its analogue as organizer of gastrulation is the Hensen's node, but it is dorsal and does not form the anus.) Placental mammals have secondarily lost the amniotic egg, but a substantial part of their gastrulation movements still reflect its former presence. Although in us the blastopore does not form the anus and does not even exist, we are still classified as deuterostomes because of our clear kinship with amphibians.

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This gives a third theory - that the proto-anus is homologous to neural tube! –  Anixx Aug 15 '13 at 10:03
    
By your theory how neural tube is formed in amphibians then if they have blastopore which becomes anus? –  Anixx Aug 15 '13 at 10:08
    
What is the question? –  kmm Aug 15 '13 at 14:33
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