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Updated: May 23, 2025
Had it done no more than give to Protozoa and Protophyta that cell-form which characterizes them had it done no more than entail the cellular composition which is so remarkable a trait of Metazoa and Metaphyta had it done no more than cause the repetition in all visible animals and plants of that primary differentiation of outer from inner which it first wrought in animals and plants invisible to the naked eye; it would have done much towards giving to organisms of all kinds certain leading traits.
When we come to the higher Metazoa, in which the sensory functions and their organs are more advanced, we find a division of labour among the ectodermic cells. Groups of sensitive nerve cells separate from the ordinary epidermic cells; they retire into the more protected tissue of the mesodermic under-skin, and form special neural ganglia there.
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.
At the lowest root of the common genealogy of the Metazoa stand the Gastraeadae and Spongidae; their whole body consists, in the simplest case, solely of a round digestive sac, the thin wall of which is formed by two layers of cells the two primitive germinal layers.
From coenobia or social unions of these afterwards arose the lowest histones, multicellular plants and animals. By the sure help of the three great empirical "records of creation," palaeontology, comparative anatomy, and ontogeny, the history of descent now leads us on step by step from the oldest Metazoa, the simplest pluricellular animals, up to man.
He tells us that large numbers of the lower animals have no nervous system, though they exercise the functions of sensation and will like the higher animals. In the unicellular Protozoa, which do not form germinal layers, there is, of course, neither nervous system nor skin. But in the second division of the animal kingdom also, the Metazoa, there is at first no nervous system.
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.
We may, therefore, conclude that the ontogenetic blastula is the reproduction of a very early phylogenetic ancestral form, and that all the Metazoa are descended from a common stem-form, which was in the main constructed like the blastula. In many of the lower animals the blastula is not developed within the foetal membranes, but in the open water.
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.
Romanes writes: "Professor Weismann has shown that there is throughout the metazoa a general correlation between the natural lifetime of individuals composing any given species, and the age at which they reach maturity or first become capable of procreation." This, I believe, has been the conclusion generally arrived at by biologists for some years past.
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