oblastica, 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:--

1. Turbellaria.
*2. Scolecida. (Worms with a coelom, probably represented
at the present day by _Balanoglossus_.)
*3. Himatega. (Evolved f