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The fact that there are still in existence various kinds of gastraeads, or lower Metazoa with an organisation little higher than that of the hypothetical gastraea, is a strong point in favour of our theory.
The Gastraea itself has originated from the simple multicellular vesicle, the Blastaea, and this in turn must have been evolved from the lowest circle of unicellular animals, to which we give the name of Protozoa. We have already considered the most important primitive type of these, the unicellular Amoeba, which is extremely instructive when compared with the human ovum.
In this fact, and the fact we have already established that the Gastraea has been evolved from the hollow vesicle of the one-layered Blastaea, and this again from the original unicellular stem-form, we have obtained a solid basis for our study of evolution. The second section, that leads from the Gastraea to the Prochordonia, is much more difficult and obscure.
Our task, therefore, of constructing man's genealogy becomes the larger aim of discovering the genealogy of the entire vertebrate stem. If we do thus follow our ancestral tree through various stages down to the lowest worms, we come inevitably to the Gastraea, that most instructive form that gives the clearest possible picture of an animal with two germinal layers.
It is an interesting fact that in the plant kingdom also the simple hollow sphere is found to be an elementary form of the multicellular organism. The botanist Schmitz gave them the name of Halosphaera viridis in 1879. The next stage to the Blastaea, and the sixth in our genealogical tree, is the Gastraea that is developed from it.
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 bilateralis, of which we may conceive the bilateral gastrula of the amphioxus to be a palingenetic reproduction, represented the two-sided organism of the earliest Metazoa in its simplest form.
As we have already seen, this ancestral form is particularly important. The actual ontogenetic development of the gastrula from the blastula furnishes sound evidence as to the phylogenetic origin of the Gastraea from the Blastaea.
As the human embryo passes through the gastrula-form like that of all the other Metazoa, we can trace its phylogenetic origin to the Gastraea. These have just the same composition and genetic significance in man as in all the other Vertebrates. From the skin-sense layer are developed the epidermis, the central nervous system, and the chief part of the sense-organs.
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