Notes on the embryology and classification of the animal kingdom : Comprising a revision of speculations relative to the origin and significance of the germ-layers.

Preliminary.—The object of the present essay is to give, in a concise form, the actual phase which those speculations have assumed, which Ifirst put forward in an article entitled, “On the Germinal Layers of the Embryo as the Basis of the Genealogical Classification of Animals,” published in the ‘Annals and Mag. of Nat. Hist.,’ May, 1873. The points of chief importance in that article were the indication of three grades of developmental complexity in the animal kingdom—the homoblastic, limited to the Protozoa; the diploblastic, reaching no higher than the Zoophytes or Cœlentera; and the tripoblastic, embracing all the higher animals which differ from the Zoophytes built up by the modification of two primary cell-layers, in the fact that a third cell-layer appears between these two, and gives rise to muscles, body-cavity, and blood-vascular systems. The precise origin of this third germ-layer, as well as its exact relation to body-cavity...