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Rib development in chick embryos, with special emphasis on the origin of ribforming cells, has been analyzed by inserting tantalum foil blocks parallel to certain somites of the prospective rib-forming level. Intact edges of bent pieces of foil the width of 4-6 somites were inserted through the somatopleure at stage 14-16 either (1) immediately lateral to the nephrotome, ( 2 ) between proximal and distal portions of the lateral plate or (3) within the distal lateral plate. Edges of similar pieces of foil containing a triangular notch the width of 2 somites were likewise inserted through the somatopleure of stage 14-16 embryos (4) immediately lateral to the nephrotome or (5) between proximal and distal portions of the lateral plate.On the basis of the modified rib patterns studied in toto after 11 days of incubation it was concluded that the (1) original somatic mesoderm does not participate in formation of vertebml rib components, ( 2 ) original somatic mesoderm plays some role in formation of sternal rib components (ribs 3-7) but probably does not contribute chondroblxsts for this purpose, and (3) distalmost somatic mesoderm is not involved in rib formation. Consistent types of modified wings are described and also modifications of the body wall, breast and ventral feather tracts of the right side.

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