. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. XIV] ORGANS FROM THE MESODERM 14" Over the anterior end of the embryo and around the phar3aix the mesoderm forms a thin layer of cells, loosely held together (Fig. 26, B). The mesoderm over the dorsal surface of the pharynx and beneath the brain plate is represented by only a single layer of somewhat scattered cells. Around the blasto- pore there is a thick layer of mesodermal cells which is thickest on the dorsal surface. In general, in the posterior region of the body the mesoderm is thicker
. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. XIV] ORGANS FROM THE MESODERM 14" Over the anterior end of the embryo and around the phar3aix the mesoderm forms a thin layer of cells, loosely held together (Fig. 26, B). The mesoderm over the dorsal surface of the pharynx and beneath the brain plate is represented by only a single layer of somewhat scattered cells. Around the blasto- pore there is a thick layer of mesodermal cells which is thickest on the dorsal surface. In general, in the posterior region of the body the mesoderm is thicker than in the middle and anterior regions. The Mesodermic Somites In the following stages of development of the embryo the dorsal ectodermal plate is lifted up and rolled in to form the central nervous system (Fig. 42). The mesoderm lying on. Fig. 42. —Cross-section through middle of embryo. M. Medullary plate. N. Noto- chord. Nc. Neural crest. PS. Primitive segment-plate. SO, SP. Somatic and sijlauchnic mesoderm. each side of the notochord changes shape somewhat during this time. It forms on each side a thick, nearly solid mass of cells, the plate of the primitive segments or segmental plate (Fig. 42). The outermost cells of this mass, those lying nearest to the dorsal surface, now show a tendency to arrange them- selves into an epithelial layer. This layer is at first continu- ous at the sides with the outer or somatic layer of cells of the lateral mesodermal sheets. The two layers of cells of the lateral mesodermal sheets (Fig. 42, SO and SP), the somatic and splanchnic layers, often show a tendency to separate and leave a cavity between them. This cavity filled with fluid. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Morgan, Thomas Hunt, 1866-1945. New York, The Macmillan company; [etc. , etc. ]
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectembryology, bookyear1
