We have further to note that in the embryos of Metazoa at all advanced in organization, there arises between these two layers a third-the mesoblast.
"Essays: Scientific, Political, & Speculative, Vol. I"
Herbert Spencer
I must also refrain from dwelling at length on the fact already adverted to, that the intermediate derived layer, or mesoblast, which was at the outset completely internal, originates those structures which ever remain completely internal, and have no communication with the environment save through the structures developed from the other two: an antithesis which has great significance.
"Essays: Scientific, Political, & Speculative, Vol. I"
Herbert Spencer