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 Coelenterata, 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 Gastraea 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. The ovum after the nucleus had been re-formed became the cytula, which was the ontogenetic counterpart of the amoeba. The morula, a compact mulberry-like congeries of segmentation-cells, corresponded to the synamoeba, or earliest association of undifferentiated amoeboid cells to form the first multicellular organism. The blastula, or hollow sphere of segmentation cells, usually ciliated, was reminiscent of the planaea, an ancestral free-swimming form whose nearest living relation is the spherical _Magosphaera_. The gastrula, finally, is the two-layered sac formed from the blastula, typically by invagination of its wall. It repeats the organisation of the gastraea, which is the common ancestor of all Metazoa, and finds its nearest living counterpart in the simple "sponges" _Haliphysema_ and _Gastrophysema_.[439] The ancestral line of all the higher animals begins with the five hypothetical forms of the moneron, amoeba, synamoeba, planaea, and gastraea. [Illustration: FIG. 15.--The Five Primary Stages of Ontogeny. (After Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.] We may take the following account[440] of the phylogeny of the human species, from the gastraea stage onwards, as typical of Haeckel's speculations on the evolution of the higher forms. The progenitors of man are, after the Gastraeada:--