Does the mesoderm evolve from the ectoderm or the endoderm? 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?

**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