Crossword-Solution: GASTR 5 letters, 6 clues 🏆 scrabble score: 6

Stated in this form, the "gastræa theory" of Haeckel appears to the present writer to be one of most important and best founded of recent generalisations.

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 gastræa and the chordula, and between this and the lancelet.

Thirty-six years have now elapsed since, in my _General Morphology,_ I established phylogeny as an independent science and showed its intimate causal connection with ontogeny; thirty years have passed since I gave in my gastræa-theory the proof of the justice of this, and completed it with the theory of germinal layers.

The typical embryonic form of the metazoa, as it is presented for a time by this simple structure of the two-layered body, is called the _gastrula_ ; it is to be conceived as the hereditary reproduction of some primitive common ancestor of the metazoa, which we call the _gastræa._ This applies to the sponges and other zoophyta, and to the worms, the mollusca, echinoderma, articulata, and vertebrata.

Although I laid special stress on the great morphological importance of this cavity in my _Study of the Gastræa Theory,_ and endeavoured to prove the significance of the four secondary germinal layers in the organisation of the cœlomaria, I was unable to deal satisfactorily with the difficult question of the mode of their origin.