. The chordates. Chordata. Reproduction 263. Fig. 217. Section transverse to the primitive streak of a chick embryo of about 15 hours' incubation. The section is taken near the middle of the length of the streak. (EC) Ectoderm; (EN) endoderm; (MES) mesoderm; (PG) primitive groove of primitive streak; (Y) yolk at inner margin of area opaca. (X 100.) (After Duval. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Rlakiston Company.) together without sharp demarcation (Fig. 212). Following gastrulation, the blastodermal layers continue to spread rapidly over the surface of
. The chordates. Chordata. Reproduction 263. Fig. 217. Section transverse to the primitive streak of a chick embryo of about 15 hours' incubation. The section is taken near the middle of the length of the streak. (EC) Ectoderm; (EN) endoderm; (MES) mesoderm; (PG) primitive groove of primitive streak; (Y) yolk at inner margin of area opaca. (X 100.) (After Duval. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The Rlakiston Company.) together without sharp demarcation (Fig. 212). Following gastrulation, the blastodermal layers continue to spread rapidly over the surface of the yolk. In so doing, the growth posteriorly causes the somewhat thickened region of the blastoporal rim to become drawn out into a long streak, the primitive streak, lying in the median line of the blastoderm (Fig. 216). Along the whole extent of this modified blasto- poral region, the ectoderm and endoderm merge without sharp demar- cation, just as they did in the earlier blastoporal walls (Fig. 217). This primitive streak is the primary seat of mesoderm formation. Rapid proliferation of cells within the substance of the thickened streak gives rise to masses of cells which move out into the space between ectoderm and endoderm (Fig. 217, MES). These masses of cells increase by continued contribution from the streak and by growth within them- selves, and soon become arranged in a layer which rapidly grows later- ally and forward from the primitive streak and always in the space between ectoderm and endoderm. This layer, like the early mesoderm of amphibians, is at first unsegmented and devoid of cavity. In the sauropsidan embryo, then, as in the amphibian, rapid growth and cell proliferation within the blastoporal rim is the primary source of mesoderm. EARLY DEVELOPMENT IN PLACENTAL MAMMALS The early development of placental mammals exhibits features peculiar to the group and more or less difficult of comparison with anything in the development of lower vertebrates. The minute
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