From the Gastræa theory there followed at once two consequences, (1) that ectoderm and endoderm, invagination-cavity (Urdarm) and gastrula-mouth (Urmund or Protostoma), were, with all their derivatives, homologous, because homogenous, throughout the Metazoa, and (2) that the descent of the Metazoa had been monophyletic, since all were derived from the ancestral Gastræa. Huxley's suggestion (supra, p. 208) that the outer and inner layers in Cœlentera were homologous with the ectoderm and endoderm of the germ was thus fully confirmed and greatly extended.
The great importance of the Gastræa theory lay in the fact that it linked up, by means of the biogenetic law, the germ-layer theory with the doctrine of evolution. It supplied an evolutionary interpretation of the earliest and most important of embryogenetic events, the process of layer-formation. Upon the Gastræa theory or its implications were founded most of the phylogenetic speculations which subsequently appeared.
Upon the Gastræa theory Haeckel based a system of phylogenetic classification which was intended to replace Cuvier's and von Baer's doctrine of Types. This took the form of a monophyletic ancestral tree. Its main outlines are given on p. 290 in graphic form, combined and modified from the table on p. 53 of the 1874 paper and the genealogical tree given in the Kalkschwämme.[434]
Monophyletic Genealogical Tree of the Animal Kingdom, based upon the Gastræa Theory and the Homology of the Germ Layers.
The scheme is in many respects an interesting and important one. The great contrast between the Protozoa, or animals with neither gut nor germ-layers, and the Metazoa, which possess both structures, is for the first time clearly brought out. The derivation of all the Metazoa from a single ancestral form, the Gastræa, leads to the conclusion that the types are not distinct from one another as Cuvier and von Baer supposed, but agree in the one essential point, in the possession of an archenteron (Lankester, 1875), and an ectoderm and endoderm which are homologous throughout all the Metazoan phyla. Finally, in the separation of the sponges, Cœlenterata and Acœlomi as animals lacking a body cavity or cœlom[435] from the four higher phyla, which are essentially Cœlomati, there is contained the germ of a conception which later became of importance.
Somewhat similar views as to the importance of the germ-layer theory for the phylogenetic classification of animals were published by Sir E. Ray Lankester in 1873.[436] He distinguished three grades of animals—the Homoblastica, Diploblastica, and Triploblastica. The first included the Protozoa, the second the Cœlenterata, the third the other five phyla, distinguished by the possession of a third layer, the mesoderm, and a "blood-lymph" cavity enclosed therein. He used the germ-layer theory to prove the essential unity of type of all the Triploblastica.
The Gastræa theory gave point and substance to the biogenetic law, and enabled Haeckel to state much more concretely the parallelism existing between ontogeny and phylogeny. He was able to assert that five primordial stages, each representing a primitive ancestral form, recurred with regularity in the very earliest development of all Metazoa.[437] These were the monerula, cytula, morula, blastula, and gastrula (see [Fig. 15]). The monerula was the fertilised ovum after the disappearance of the germinal vesicle;[438] it was the equivalent of the primordial anucleate Monera which are the ancestors of all animals.