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Updated: June 5, 2025
Although the fully-formed alimentary canal is thus a very elaborate organ, and although in detail it has a quantity of complex structural features into which we cannot enter here, nevertheless the whole complicated structure has been historically evolved from the very simple form of the primitive gut that we find in our gastraead-ancestors, and that every gastrula brings before us to-day.
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.
This gastrula theory, now generally accepted, is one of Haeckel's two great fundamental contributions to the evolution philosophy with the history of which his life work is so intimately linked.
All forms of animal life, from the coelenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways. We may infer, therefore, that during the pre-Cambrian periods organic evolution followed the lines thus dimly traced.
The solid foundation of this important thesis is the gastrula, the most instructive embryonic form in the animal world, which we still find in the same shape in the most diverse classes of animals.
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.
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.
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.
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