. Analysis of development. Embryology; Embryology. Fig. 112. Comparative lateral diagrams of pro- spective areas in the early gastrula and diagrams of gastrulation stages. White: prospective nervous system. Stippled: prospective chorda. Oblique lines: future somites. Shaded: extraembryonic area (yolk sac). TB, tailbud material; Ent, entoderm. The boundary between chorda and entoderm in the Fundulus diagram is purely conventional; experi- mentally it has not been specified. The asterisk in the gastrulation series represents the migration of a group of cells on the germ ring into the somite meso
. Analysis of development. Embryology; Embryology. Fig. 112. Comparative lateral diagrams of pro- spective areas in the early gastrula and diagrams of gastrulation stages. White: prospective nervous system. Stippled: prospective chorda. Oblique lines: future somites. Shaded: extraembryonic area (yolk sac). TB, tailbud material; Ent, entoderm. The boundary between chorda and entoderm in the Fundulus diagram is purely conventional; experi- mentally it has not been specified. The asterisk in the gastrulation series represents the migration of a group of cells on the germ ring into the somite mesoderm. (From Pasteels, '36a; Oppenheimer, '36b, '37.) tion of material to lower layers is accom- panied by marked convergence of both in- vaginating and superficial material to the dorsal midline of the future embryo. Hence, prospective areas for nervous system, chorda and other mesoderm extend at first some dis- tance laterally to the position they will later occupy. The convergence movements in the mesoderm are summarily indicated by migration of the asterisk in the lower figures. Contrary to what might be expected a priori, the trout blastodisc, which has so much centrifugal growth and stretching to accomplish, likewise undergoes much more marked convergence movements. Meso- dermal trunk material in Fundulus lies originally closer to the embryonic shield than it does in Salmo. In both, ultimately, the ventral lip enters the tailbud. For the avian embryo, we have almost no comparative material for this period; our information derives almost exclusively from the egg of the domestic fowl. Surface dia- grams are presented in Figure 113 for the stages corresponding to those just indicated for teleosts. What emerges from the some- times conflicting studies is that the following order of gastrulation takes place: (1) The hypoblast is formed by a process of sporadic delamination from the posterior region of the pellucid area; this process is not shown in the diagrams of Figure 113. (
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