mesendodermは、最終的に中胚葉（mesoderm）と内胚葉（endoderm）に分化する能力を持っ細胞群のことのようです。原始線条から陥入する細胞は、中胚葉や内胚葉に分化するので、それらに分化する前の状態の細胞を指す言葉のようですね。

Mesoendodermとは

Mesendoderm (ME) refers to the primitive streak in mammalian embryos, which has the ability to further differentiate into mesoderm and endoderm.

Signaling Control of Differentiation of Embryonic Stem Cells toward Mesendoderm ALu Wang , Ye-Guang Chen　Journal of Molecular Biology Volume 428, Issue 7, 10 April 2016, Pages 1409-1422 Journal home page for Journal of Molecular Biology Review https://www.sciencedirect.com/science/article/abs/pii/S0022283615003538　本文有料

Mesoendodermの分子マーカー

Existence of mesendoderm that can give rise to both endoderm and mesoderm has been reported from C. elegans to Xenopus(Rodaway and Patient, 2001). In early zebrafish development, the marginal zone bordering on the vegetal margin contains precursors for endoderm as well as mesoderm. Both brachyury and Gata5, which are specific markers for mesoderm and endoderm precursors, respectively, are co-expressed in this marginal zone(Rodaway et al., 1999).

01 October 2005 Characterization of mesendoderm: a diverging point of the definitive endoderm and mesoderm in embryonic stem cell differentiation culture Shinsuke Tada, Takumi Era, Chikara Furusawa, Hidetoshi Sakurai, Satomi Nishikawa, Masaki Kinoshita, Kazuki Nakao, Tsutomu Chiba, Shin-Ichi Nishikawa

https://journals.biologists.com/dev/article/132/19/4363/42973/Characterization-of-mesendoderm-a-diverging-point