. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 130 BtiUetin Museum of Comparative Zoologij, Vol. 156, No. 1. ^^99 (Ji^^^JftlA â ' â '9 Kb^P t': 1'° "^'^SH ^ VK,OtiMnBQ||HW \>j^^^^^^Ba iV origins of mammalian developmental ad- aptations. DISCUSSION Developmental Plasticity and Conservation The processes that distinguish marsupial and placental maminals begin at the ear- liest point in the differentiation of tissues of the craniofacial region. They involve fundamental shifts in early patterning events, and comprise changes in a complex series of events. The
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 130 BtiUetin Museum of Comparative Zoologij, Vol. 156, No. 1. ^^99 (Ji^^^JftlA â ' â '9 Kb^P t': 1'° "^'^SH ^ VK,OtiMnBQ||HW \>j^^^^^^Ba iV origins of mammalian developmental ad- aptations. DISCUSSION Developmental Plasticity and Conservation The processes that distinguish marsupial and placental maminals begin at the ear- liest point in the differentiation of tissues of the craniofacial region. They involve fundamental shifts in early patterning events, and comprise changes in a complex series of events. These changes may be traced back to the appearance of the neu- ral plate, where at this stage large numbers of cells differentiate into migratory neural crest cells, rather than neural tissues. Within the neural tube the hindbrain dif- ferentiates early and the midbrain and forebrain are delayed. In addition, a local- ized acceleration of somitic differentiation occurs in the cervical and upper thoracic regions and a marked delay occurs in cau- dal somites. The distinction is not a simple shifting foi"ward in time, or speeding up the rate of development of a few features, nor is it due to the establishment of a sim- ple anteriorâposterior gradient of acceler- ation along the body axis. The changes in- volve multiple advancements and delays of sets of cells, tissues, and organs, within and between regions. Developmental differences between mar- Figure 7. Sections through the head of (A) Monodelphis; (B) Ornithorhynchus; (C) Mus. Specimens were chosen for ap- proximate match in the relative development of the eye. In Monodelphis the neural tube is at an early stage with no sig- nificant proliferation of the neural epithelium (although the tel- encephalon has evaginatedânot visible in this section). How- ever, at this time the maxillary, dentary, and premaxillary bones have begun ossification; cartilage is present in the ba- sisphenoid and basiocclpital regions; and muscle
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