The parietal and visceral mesoblast, or the two middle layers, are always of later origin, and arise through evagination or plaiting of the entoblast, the remainder of which can now be distinguished as secondary entoblast from the primary.    From Wordnik.com.  [Form and Function A Contribution to the History of Animal Morphology] Reference

Then every mesoblast contracts; the contraction deepens, till it is divided across in both directions, separating thus into four parts, then into eight, then into sixteen, and so on, till every cell is crowded with hundreds of minute mesoblasts, each containing the indication of a central dot or entoblast.    From Wordnik.com.  [The Atlantic Monthly, Volume 10, No. 61, November, 1862] Reference

The following passage gives a good idea of their views and of the phylogenetic implications involved: -- "Ectoblast and entoblast are the two primary germ-layers which arise from the invagination of the blastula; they are always the first to be laid down, and they can be directly referred back to a simple ancestral form, the.    From Wordnik.com.  [Form and Function A Contribution to the History of Animal Morphology] Reference