. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 504 DEVELOPMENT OF PRIMITIVE BODY FORM gastrula and later embryo to the stage where a continuous antero-posterior coelomic space is formed, comparable to that found in the vertebrates. The mesoderm of the late gastrula of Amphioxus is present as a dorso- median band of cells on either side of the notochord, and together with the notochord, occupies the dorsal area or roof of the archenteron as mentioned previously. In the region of the blastopore,
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 504 DEVELOPMENT OF PRIMITIVE BODY FORM gastrula and later embryo to the stage where a continuous antero-posterior coelomic space is formed, comparable to that found in the vertebrates. The mesoderm of the late gastrula of Amphioxus is present as a dorso- median band of cells on either side of the notochord, and together with the notochord, occupies the dorsal area or roof of the archenteron as mentioned previously. In the region of the blastopore, the two mesodermal bands diverge ventrally and occupy the inner aspect of the lateral walls of the blastopore (fig. 190F, G; 247B). At about the time of blastoporal closure, the two mesodermal masses of cells, located along the lateral lips of the blastopore, are retracted dorsally, where they come to lie on either side of the notochord (fig. 247C). In this position the two bands of mesoderm and the notochord continue to form the dorsal region or roof of the archenteron until approxi- mately the time when the embryo is composed of 13 to 14 pairs of meso- dermal segments or somites (fig. 249C). (See Hatschek, 1893, pp. '31, 132; Willey, 1894, p. 115; Conklin, '32, p. 106.) When the embryo reaches a stage of development wherein 15 to 16 pairs of somites are present, the notochord and mesoderm have separated entirely from the entoderm (fig. 249D). At about this period the neurenteric canal between the metenteron and the neural tube disappears (fig. 249C, D). ENTEROCOEU. MESODERM NCHNOCOEL SPLANCHNOCOELS FUSE HORIZONTAL SEPTUM Fig. 250. Differentiation of somites in Amphioxus. (A and B from Conklin, '32; C, E, and F after Hatschek, 1888 and 1893; D from MacBride, 1898; all figures are modi- fied.) (A) Somites shortly after separation from entoderm. (B) Later stage, the somites grow ventrally. (C) Somitic wall begins to differentiate into a thickened, dorsal, myotomic area, located
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