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I know the embryos of most animals develop into 3 germ layers. However primitive animals, such as Cnidarians, only have 2 germ layers - they lack a mesoderm. Therefore the 3rd germ layer must have evolved from cells which originally differentiate to one of the two more basal germ layers.

So my question is - does the mesoderm evolve from the ectoderm or the endoderm? Meaning, during embryogenesis, which cell type is the first to differentiate from the other two - the ectoderm or the endoderm?

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In human,the mesoderm doesnot develop from the endoderm or the ectoderm.It develops from epiblast of the bilaminar germ disc stage.

During the week 3 of development, the epiblast of the disc invaginate and divide rapidly to form the cells of the endoderm.The division is rapid enough to replace the hypoblast layer.

Even after the endodermal layer has formed, the epiblast continues to divide at the region of the invagination.The divided cells migrate between the epiblast(soon to be the future ectoderm) and the newly formed endoderm to form the mesoderm.

REFERENCES:

  • Langman's Medical Embryology (13th edition) T.W. Sadler
  • website
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  • $\begingroup$ Okay. So is it correct to say that just after the endoderm develops, there are cells in the epiblast which can develop to either mesoderm cells or ectoderm cells, depending on their environment? $\endgroup$
    – Alon Navon
    Nov 26, 2017 at 22:38
  • $\begingroup$ @Alon Navon No.Nothing is environment dependent here...its all gene regulated... either mesoderm cells or ectoderm cells No, its genetically programmed to form both. $\endgroup$
    – user 33690
    Nov 27, 2017 at 12:29
  • $\begingroup$ All the cells have the same genes. Their differentiation is dependent on various chemical signals. $\endgroup$
    – Alon Navon
    Nov 27, 2017 at 12:32
  • $\begingroup$ There is some kind of 'family tree' between cell types. Some cells can only develop to bone or cartilage, but not nuerons. Based on this, some pair of {ectoderm, mesoderm, endoderm} is closer developmentally than the third one. $\endgroup$
    – Alon Navon
    Nov 27, 2017 at 12:35
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    $\begingroup$ yes.true.you must have followed that the endoderm and mesoderm are formed from the cells that migrate from the epiblast...the epiblast itself forms the ectoderm... $\endgroup$
    – user 33690
    Nov 27, 2017 at 12:40

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