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"Gastrula of a Chalk-sponge (Olynthus). B, longitudinal section through the axis: g, primitive intestine; o, blastopore; i, inner cell-layer of the body-wall; e, outer cell-layer." -Whitney, 1911

Chalk Sponge Gastrula

"Gastrula of a Chalk-sponge (Olynthus). B, longitudinal section through the axis: g, primitive intestine;…

"Diagram of a Diblastula. a, orifice of invagination (blastopore); b, archenteric cavity; c, endoderm; d, ectoderm." — Encyclopedia Britannica, 1893

Diblastula

"Diagram of a Diblastula. a, orifice of invagination (blastopore); b, archenteric cavity; c, endoderm;…

"Formation of the Diblastula of Eucope (one of the Calyptoblastic Hydromedusae) by delamination. A, B, C, three successive stages. ep, ectoderm; hy, endoderm; al, enteric cavity." — Encyclopedia Britannica, 1893

Diblastula

"Formation of the Diblastula of Eucope (one of the Calyptoblastic Hydromedusae) by delamination. A,…

"Mesoderm formed by pouches from entoderm after gastrulation. A and B, early and later stages in formation of mesoderm and coelom. a, primitive gut; bp., blastopore; coe, body cavity, formed from pockets of the archenteron; ec., ectoderm; en., entoderm; m., mesoderm; m.so, body-wall mesoderm; m.sp., visceral mesoderm; s.c., segmentation cavity." -Galloway, 1915

Mesoderm Forming

"Mesoderm formed by pouches from entoderm after gastrulation. A and B, early and later stages in formation…

"Diagrams of stages in the metamorphosis of Polygordius, a primitive annelid. Ectoderm throughout is represented as nucleated without cell boundaries; the entoderm has the cell-boundaries shown, and the mesoderm is diagonally shaded. A, gastrula; B, same with blastopore closed; C and D represent formation of stomodaeum and proctodaeum from ectoderm; E, Trochosphere stage showing formation of segments in the posterior portion; F, adult (sagittal); G, adult (transverse). a, archenteron; bp., blostopore; br, brain; c, coelom; d, dorsal; di, dissepiments; m, mesenteron; pr., proctodaeum; s.c., segmentation cavity; st, stomodaeum; v.n., ventral nerve chain; z, zone of formation of nerve segments." -Galloway, 1915

Polygordius

"Diagrams of stages in the metamorphosis of Polygordius, a primitive annelid. Ectoderm throughout is…

"Diagrams to illustrate the development of one of the simpler types of sponge: I, the egg; 2, section of 16- to 32-celled stage; 3, section of later stage, a ciliated larva (blastula); 4, gastrula; 5, section through older larva which has become attached by the end containing the blastopore. New openings break through by the coalescence and perforation of the ectoderm and entoderm, and and a form results... a, archenteron; bl., blastopore; ec., ectoderm; en., entoderm; mes., mesenchyma; s, segmentation cavity." -Galloway, 1915

Simple Sponge Development

"Diagrams to illustrate the development of one of the simpler types of sponge: I, the egg; 2, section…