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Does the occurrence of blastulæ and gastrulæ and one-celled beginnings mean that the higher animals composed of numerous and much differentiated cells have evolved in company from two-layered saccular ancestors which were themselves the descendants of spherical colonies of like cells, and ultimately of one-celled animals?
Some of them are concerned primarily with digestion, others with protection, while still others are exempt from these tasks and as sense-cells they devote all their energies to the reception of stimuli from without, or, beneath the outer sheet of cells of the two-layered body, they conduct impulses from one part of the animal to another, and thus serve as coordinating members of the community.
In Prophysema the primitive gut is a simple oval cavity, but in the closely related Gastrophysema it is divided into two chambers by a transverse constriction; the hind and smaller chamber above furnishes the sexual products, the anterior one being for digestion. The only material difference between them is that in the sponge the thin two-layered body-wall is pierced by numbers of pores.
In the struggle for life they have found shelter in the body-cavity of other animals. Modern gastraeads. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophysema is a simple cylindrical or oval tube, with a two-layered wall. The two strata of cells that form the wall of the tube are the primary germinal layers.
This will represent the two-layered "gastrula" the simplest ancestral form of the Metazoa: a form which is permanently represented in some of the lowest types; for it needs but tentacles round the mouth of the sac, to produce a common hydra.
Comparative anatomy has asserted that this is so, as we have already learned, for it finds that adult animals array themselves at different levels of a scale beginning at the bottom with the protozoa, continuing on to the two-layered animals like Hydra and jellyfish and sea-anemones, and then extending upwards to the region of the more complicated invertebrates and vertebrates.
We have first to answer the difficult and much-discussed question of the development of the Chordaea from the Gastraea; in other words, "How and by what transformations were the characteristic animals, resembling the embryonic chordula, which we regard as the common stem-forms of all the Chordonia, both Tunicates and Vertebrates, evolved from the simplest two-layered Metazoa?"
This round ovum has the same characteristic form and origin as the ovum of any other mammal. From it is developed in the same manner in all the Placentals, by repeated cleavage, a multicellular blastula. This two-layered embryonic form is the ontogenetic reproduction of the extremely important phylogenetic stem-form of all the Metazoa, which we have called the Gastraea.
The gastraea theory has now convinced us that all the Metazoa or multicellular animals can be traced to a common stem-form, the Gastraea. In accordance with the biogenetic law, we find solid proof of this in the fact that the two-layered embryos of all the Metazoa can be reduced to a primitive common type, the gastrula.
A corresponding germinal condition, the two-layered gastrula, occurs transitorily in the embryological history of all the other Metazoa, from the lowest Cnidaria and Vermes up to man. From the common stock of the Helminthes, or simple worms, there develop as independent main branches the four separate stems of the Molluscs, Star-fishes, Arthropods, and Vertebrates.
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