GASTRIEIA THEORY (Neo-Lat., from Gk. -yacrrhp, gas ter, stomach). A theory propounded by E. Haeckel, according to which the gastrula stage in the development of animals (see Em ERYOLOGY) is a recapitulation of a hypothetical common ancestor—the gastma; for just as the two-layered gastrula stage, although sometimes disguised by the presence of much yolk, is com mon in the embryological development of the Metazoa, so in their phylogenetic development there was a primitive type that was the starting point from which all the various metazoan types have developed along diverging lines. The gas trula is the type which seems to be the common one in the embryological development of the Metazoa. The hypothetical phylogenetic type, the starting-point of the Metazoa, Haeckel named `gastra.' The Gastruidme were supposed to be of world-wide distribution, and of many families and genera. The outer and inner layers of the gastrula and the gastrmea Haeckel homologized with the ectoderm and entoderm of the Metazoa. This theory, however, should not be wholly ascribed to Haeckel, for the homologies of the germ-layers had already been pointed out by Kowalewsky, Von Baer, Remak, and others. Ko walewsky concluded from his embryological re searches that the nervous layers and the ectoderm of insects and vertebrates are homologous, and that the germinal layers of Amphioxus and vertebrates correspond with those of ascidians and worms. Kowalewsky, indeed, believed "that the homologies of the general layers in the dif ferent types afford a scientific basis for compara tive anatomy and embryology, and must be recog nized as the starting-point for the proper under standing of the relationships of the types." The generalizations of Haeckel, although based large ly an such work as Kowalewsky's, are much bolder than those just quoted.
The simplest and probably the most primitive gastrula seen in vertebrate development is that of Amphioxus. The blastula, or stage that is ante cedent to the gastrula in Amphioxus, is composed of a single layer of cylindrical cells closely joined in the shape of a hollow sphere. At one place in this sphere, called the vegetative pole, the cells are larger and contain more yolk-granules than the cells of the rest of the circumference. The vegetative surface begins to flatten and then to push toward the inside of the sphere. This inpushing is termed invagination.' As the cav
ity formed by invagination grows larger, the original cleavage cavity in the sphere grows smaller, until finally it is wholly obliterated. The resulting individual is two-layered and cup shaped with one large opening to the exterior, the primitive mouth or blastopore. This double layered, cup-shaped individual is the gastrula, and its inner cavity is the primitive intestine. Neither this mouth nor the intestine is homol ogous with the mouth or the intestine of the adult animal. The two primary germ-layers of the gastrula are known as ectoderm and ento derm. The outer or ectoderm is the sensitive layer, and the inner is the nutritive layer. C. E. von Baer calls them, in view of their function, the two primitive organs of the animal body. By the separation and differentiation of cells from one or the other, or both of these layers, all subsequent development and differentiation of the body is brought about. Embryonic stages quite like this of the Amphioxus are known to exist in the Ccelenterata, some Scolecida, Echino dermata, and some Annelida, iu addition to those of the higher vertebrates.
As Huxley has pointed out, the Porifera and Ccelenterata very nearly approach the conditions of the gastrcea. The fresh-water hydra and the microhydra, for example, arc two-layered animals with a central digestive cavity surrounded by both layers and opening to the exterior at a point about the margins of which the two layers are continuous. This permanent mouth is the terminal aperture of the gastnea and serves both for the ingestion and extrusion of materials, while in the Porifera it serves as the permanent egestive opening only. Consult: Haeckel, "Die Gastramtheorie, die phylogenetische Classification des Thierreichs und die Homologic der Keim bliitter" (Jena, 1874) ; "Die Gastrula und die Eifurchung der Thiere" (Jena, 1875) ; "Ursprung und Entwicklung der thierischen Gewebe: Ein histogenetiseher Beitrag zur Gastruatheorie," (Jena, 1885), all published in Zeitschrift fur Thiermedicin; Kowalewsky, "Weitere Studien fiber die Entwicklungs-geschichte des Amphioxus lanceolatus, nebst einem Beitrage zur Homologic des Nervensystems der Wiirmer und Wirbel thiere," in Archiv far Mikroscopisehe Anatomic., vol. xiii. (Bonn, 1877) ; Biltschli, "Bemerkungen zur Gastrwatheorie;" in Morphologisches Jahr buch, vol. ix. (Leipzig, 1884).