. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Evolution of Mammalian Development • Smith 125. Figure 3. Photographs of a embryo Monodelphis domestica (approximately six somites); (A) is a dorsal view and (B) is an anterior-dorsal view of same specimen. Neural crest migration occurs early relative to neural tube differentiation in marsupials. Although no closure of the neural tube has occurred, streams of neural crest have migrated into the first arch region, are migrating into the second arch region, and appear to be about to migrate into poste
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Evolution of Mammalian Development • Smith 125. Figure 3. Photographs of a embryo Monodelphis domestica (approximately six somites); (A) is a dorsal view and (B) is an anterior-dorsal view of same specimen. Neural crest migration occurs early relative to neural tube differentiation in marsupials. Although no closure of the neural tube has occurred, streams of neural crest have migrated into the first arch region, are migrating into the second arch region, and appear to be about to migrate into posterior regions. Further, at this time the hindbrain is fairly well differentiated, with recognizable rhombomeres, yet little or no development of midbrain or forebrain regions has occurred. This is quite different from the pattern seen in eutherians. Key: C, cervical region; O, otic sulcus (region of rhombomeres 5 and 6); PC, preotic sulcus (between rhombomeres 2 and 3); FB, forebrain region; 1, the first stream of neural crest, which appears to populate the first arch and frontonasal region; 2, the second stream of neural crest, which appears to provide cells to the second arch; 3, the third stream, which appears to go to the third through sixth branchial arches. from a neural tissue—the neural crest (re- viewed in Le Douarin [1982], Noden [1983, 1987, 1991], Hall [1987], and Hall and Horstadius [1988]). The relative tim- ing of neural crest differentiation serves as the earliest "decision point" in embryonic allocation to neural or to mesenchymal tis- sues. Do these differences originate with shifts in the relative timing or pattern of neural crest migration? A positive answer would support the hypothesis that this is a fundamental change in development and tliat early development is potentially plas- tic. Neural crest migration has been studied extensively in a number of nonmammalian vertebrates, particularly in the quail-chick system (, Le Douarin, 1982; Nod
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