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Updated: May 18, 2025
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.
Each, starting from the condition of a simple nucleated cell, becomes a cell-aggregate; and this passes through a condition which represents the gastrula stage, before taking on the features distinctive of the group to which it belongs. Stated in this form, the "gastraea theory" of Haeckel appears to the present writer to be one of most important and best founded of recent generalisations.
In all these stems the gastrula recurs in the same very simple form. The elaborate alimentary canal of the higher animals develops ontogenetically from the same simple primitive gut of the gastrula. This gastraea theory is now accepted by nearly all zoologists.
It will be useful first to point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea. Its bilateral and tri-axial type indicates that the Gastraeads the common ancestors of all the Metazoa divided at an early stage into two divergent groups. Thus arose the typical bilateral form, which has three axes.
We shall easily see that the gastraea theory is thus able to throw a good deal of light, both morphologically and physiologically, on some of the chief features of embryonic development, if we take up first the consideration of the chief element in the animal sphere, the psychic apparatus or sensorium and its evolution.
This important fact justifies us in concluding, in accordance with the biogenetic law, that their ancestors also were phylogenetically developed from a similar stem-form. This ancient stem-form is the gastraea. The gastraea probably lived in the sea during the Laurentian period, swimming about in the water by means of its ciliary coat much as free ciliated gastrulae do to-day.
Like the latter, the human gastrula and that of all other mammals must be regarded as the ontogenetic reproduction of the phylogenetic form that we call the Gastraea, in which the whole body is nothing but a double-walled gastric sac. We already know from embryology the manner in which the gut develops in the embryo of man and the other mammals.
We must grant, however, that in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty namely, that there was a long series of very different and totally extinct forms between the Gastraea and the Chordaea.
In explaining the phylogenetic origin of the gastraea in the light of this ontogenetic process, we may assume that the one-layered cell-community of the blastaea began to take in food more largely at one particular part of its surface. Natural selection would gradually lead to the formation of a depression or pit at this alimentary spot on the surface of the ball.
Just as the countless species of the Metazoa do actually develop in the individual from the simple embryonic form of the gastrula, so they have all descended in past time from the common stem-form of the Gastraea.
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