Fig. 15.—The Five Primary Stages of Ontogeny. (After Haeckel.)
| 1. Monerula. | 2. Cytula. | 3. Morula. | 4. Blastula. | 5. Gastrula. |
The ovum after the nucleus had been re-formed became the cytula, which was the ontogenetic counterpart of the amœba. The morula, a compact mulberry-like congeries of segmentation-cells, corresponded to the synamœba, or earliest association of undifferentiated amœboid cells to form the first multicellular organism. The blastula, or hollow sphere of segmentation cells, usually ciliated, was reminiscent of the planæa, an ancestral free-swimming form whose nearest living relation is the spherical Magosphæra. The gastrula, finally, is the two-layered sac formed from the blastula, typically by invagination of its wall. It repeats the organisation of the gastræa, 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, amœba, synamœba, planæa, and gastræa.
We may take the following account[440] of the phylogeny of the human species, from the gastræa stage onwards, as typical of Haeckel's speculations on the evolution of the higher forms. The progenitors of man are, after the Gastræada:—
1. Turbellaria.
*2. Scolecida. (Worms with a cœlom, probably represented at the present day by Balanoglossus.)
*3. Himatega. (Evolved from Scolecida by formation of dorsal nerve-tube and chorda, and resembling tailed larvæ of Ascidians.)
4. Acrania. (With metameric segmentation. Including Amphioxus.)
5. Monorrhina. (Cyclostomes.)