. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 128 Bulletin Museum of Comparative Zoology, Vol. 156, No. 1. Figure 5. Three stages of development in Monodelphis do- mestica: (A) approximately 11 days of gestation; (B) approxi- mately 12 days of gestation; (C) approximately 13 days of ges- tation. Note that in each specimen the anterior part of the body is greatly advanced relative to the posterior in size and relative degree of differentiation. In particular, the forelimb is highly advanced relative to the hind limb at all stages. Comparisons Across Mammalia To as
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 128 Bulletin Museum of Comparative Zoology, Vol. 156, No. 1. Figure 5. Three stages of development in Monodelphis do- mestica: (A) approximately 11 days of gestation; (B) approxi- mately 12 days of gestation; (C) approximately 13 days of ges- tation. Note that in each specimen the anterior part of the body is greatly advanced relative to the posterior in size and relative degree of differentiation. In particular, the forelimb is highly advanced relative to the hind limb at all stages. Comparisons Across Mammalia To assess the condition at the node Mammaha, information on monotremes, the third major clade of extant mammals, is needed. Clear possession by mono- tremes of the derived elements of marsu- pial development would be parsimoniously interpreted as a shared derived resem- blance. On the other hand, resemblance of monotremes to the eutherian condition (which is shared with nonmammalian am- niotes) would further highlight the derived and specialized nature of marsupial devel- opment and reproduction. Few monotreme embryos are available for study. Most are in the Hill Collection, part of the Hubrecht Comparative Embry- ology Laboratoiy. Preliminary evaluation of some of this material indicates that inonotremes exliibit a mosaic of marsupi- allike and placentallike developmental characters. First, monotreines share with marsupials and nonmammalian amniotes many primitive characteristics of the ear- liest embiyo. For example, all develop as a flat blastodisc on a large yolk, in a man- ner that is quite distinct froin that of eu- therians (Hughes, 1993). In addition to these shared primitive characters of early development in marsupials and mono- tremes, monotremes and marsupials share some derived characters. For example, early in development in monotremes the branchial arches apparently are accelerat- ed relative to the neural tube. Somewhat later in development, mono- tremes seeiu to res
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