To this form Haeckel gave the name of gastraea or gastrula.
The many new points of view and fresh ideas suggested by my gastraea theory and Hertwig's coelom theory led to the publication of a number of writings on the theory of germinal layers.
In the light of the gastraea theory it is hardly necessary to dwell on the defects of this earlier view and the erroneous conclusions drawn from it.
On the whole, the hydra has preserved to our day by heredity the simple structure of our primitive ancestor, the gastraea (cf.
Hence in the light of the gastraea theory we must regard the features of the amphioxus as the only and real primitive structure among all the vertebrates, departing very little from the palingenetic embryonic form.
The gastraea theory, which has its chief application here, teaches us that it is the very reverse of the truth.
As this theory, a logical conclusion from the gastraea theory, has been fully substantiated by the comparative study of gastrulation in the last few decades, we must exactly reverse the hitherto prevalent mode of treatment.
But we can only indirectly and approximately, with the aid of comparative anatomy and ontogeny, conjecture what lower forms enter into the chain of our ancestry between the gastraea and the chordula, and between this and the lancelet.
The second section, that leads from the Gastraea to the Prochordonia, is much more difficult and obscure.
The uni-axial Gastraea became sessile, and gave rise to two stems, the Sponges and the Cnidaria (the latter all reducible to simple polyps like the hydra).
The next stage to the Blastaea, and the sixth in our genealogical tree, is the Gastraea that is developed from it.
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.
This gastraeatheory is now accepted by nearly all zoologists.
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.
The gastraea theory has now convinced us that all the Metazoa or multicellular animals can be traced to a common stem-form, the Gastraea.
Its functions are represented by the simple cell-layer of the ectoderm, which the lower Metazoa have inherited from the Gastraea (Figure 1.
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.
The undifferentiated skin-layer or ectoderm of the Gastraea is the simple stratum of cells from which the differentiated sense-organs of all the Metazoa (including the Vertebrates) have been evolved.
The digestive gut is still the simple primitive gut of the gastraea (d), with a single aperture that is both mouth and anus (m).
It will be useful first to point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea.
Gastraea body has been transmitted to the phase of development which directly arises out of the Morula.
According to the fundamental law of biogeny this gastrula proves the former existence of an independent form of primaeval animal of the same structure, and this we have named primaeval stomach animal, or Gastraea (pp.
The type of these Primaeval Worms, the ancient Prothelmis, may be directly derived from the Gastraea (p.
Some of them remain at a very low stage, as does our common green fresh-water polyp (hydra viridis), which only differs from the gastraea by a few variations in tissue and the formation of a crown of feelers about the mouth.
The simplest living forms of the sponges (olynthus) and the cnidaria (hydra) only differ from this hypothetical primitive form of the gastraea by a few secondary and subsequently acquired features.
However, comparative ontogeny teaches us that all these various structures have been developed from one simple fundamental form, as I have shown in my theory of the gastraea (1872).
In regard to the first, I formulated the following principle thirty years ago in my first study of the gastraea theory: "Phylogenesis is the mechanical cause of ontogenesis.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "gastraea" in a variety of sentences. We hope that you will now be able to make sentences using this word.
