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:
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:
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.