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NATURE AND AFFINITIES OF THE SPONGES. 179in the same specific type, would there seem to be much prospect of arrivalat a more definite result.The Metazoic nature of the sponges, in deference to the authoritativedictum of Professor Haeckel, being accepted d priori as an article of creed,it has been rendered necessary to indicate, in one and all of those diverseso-called ciliated sponge-embryos, the existence of the two primary andabsolutely essential constituents of the Metazoic embryo, the ectodermand endoderm, as produced by the segmentation and subsequent metamorphosisof a primitive unicellular impregnated ovum. Deferring for awhile the consideration of the presumed identity of this earliest or initialphase, it may be first observed that the structural type, out of the threerespective series just enumerated, which has been accepted as conformingitself most conveniently to the Metazoic formula, is exhibited by that onein which the apical pole or segment of the reproductive body is composedof more minute columnar flagellate cells, and the opposite one of largerbut simply subspheroidal elements. Here, as typically represented in thecalcareous sponge, Grantia compressa (PI. IX. Fig. 27), there certainly, atfirst sight, appears to be a remarkable structural correspondence with thesegmented holoblastic ovum of the Mammalia, Amphibia, and various fishes,including Amphioxus, and numerous higher Invertebrata in which one-halfof the primitive ovum, dividing more rapidly and abundantly, becomes convertedinto numerous minute columnar blastomeres, and the opposite half,Dividing more slowly and less extensively, into fewer larger and subspheroidalblastomeres. Out of these two elemental series, distinguishedrespectively as the epiblast and hypoblast, the future ectoderm and endodermare subsequently developed, the former from the minute columnar blastomeresor epiblast, and the latter from the larger blastomeres or hypoblast.The identity of the segmentation process in the Metazoic embryo and inthe so-called sponge-larva being so far regarded as complete, the apparentcorresponding factors in either case have also been accepted as homologousectodermic and endodermic elements. Supposing, for the time, that thesetwo structural elements could be consistently correlated, what should bethe next step?In the Metazoic embryo it invariably happens that either by theinvagination or falling inwards, as in Amphioxus, upon the primitivecentral segmentation cavity or archenteron of the hypoblast or endodermicelement, or by the encroachment upon or growing over the latter, as inthe Amphibia, of the epiblast or ectodermal element, it comes to pass thatthe endoderm is enclosed within the ectoderm, and a bilaminate structureisproduced roughly resembling the double-walled sac-like body or so-called" "gastrula of Professor Haeckel. The outer lamina or wall of this saclikebody is now the ectoderm, the inner one, closely applied to it, theendoderm. The central cavity most usually enclosed within these layersrepresents the primitive alimentary tract or archenteron, and the apertureplacing the latter in communication with the outer world the primitive analN 2