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The micromeres give rise to the ectoderm; each of the sixteen macromeres, after budding off a small mesoblast cell, passes on as endoderm.

Hertwig says that all the cells of the epiblast, hypoblast, mesoblast, and of the later derivatives of these primary layers, receive identical portions of germplasm by means of doubling nuclear divisions.

The segmentation is of the mesoblast to begin with, and appears later behind the mouth, the part anterior to this becoming the prostomium of the adult.

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.

It is due to an arrest of development, whereby the closure of the primary medullary groove and the ingrowth of the mesoblast to form the spines and laminæ fail to take place.