. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Evolution of Mammalian Development • Smith 129. Figure 6. Embryos of (A) Monodelphis; (B) Mus; (C) Gallus; and (D) Chelydra. Note that in (B) through (D) the forelimb bud (FL) and hind limb bud (HL) are approximately the same size; in (A) the forelimb bud is massive, whereas the hind limb is not yet at the bud stage. Further note that in (B) through (D) the telencephalon (T), as well as the other regions of the brain are recognizable as distinct swellings; no such divisions yet exist in (A). Finally note that the bran
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. Evolution of Mammalian Development • Smith 129. Figure 6. Embryos of (A) Monodelphis; (B) Mus; (C) Gallus; and (D) Chelydra. Note that in (B) through (D) the forelimb bud (FL) and hind limb bud (HL) are approximately the same size; in (A) the forelimb bud is massive, whereas the hind limb is not yet at the bud stage. Further note that in (B) through (D) the telencephalon (T), as well as the other regions of the brain are recognizable as distinct swellings; no such divisions yet exist in (A). Finally note that the branchial arches and frontonasal region (N) are massive in (A), and relatively small in the other taxa. and present in the nasal and basicranial re- gions; bone is present in the dentary, pre- maxilla, and maxilla; and the tongue mus- culature has differentiated (see Smith [1994, 1997]). The Omithorhynchus em- bryo is intermediate between these con- ditions, although it is more similar to the eutherian than inetatherian condition. The major subdivisions are present in the neu- ral tube and proliferation of the neuroep- ithelium is well underway in both the tel- encephalon and diencephalon, yet like eu- therians no cartilage, bone, or muscle is present. Therefore, monotremes do not exhibit the same degree of advancement of cranial musculoskeletal tissues as mar- supials. Until more monotreme material is ob- tained and analyzed, the issue of the con- dition at the node Mammalia is obscure. Monotremes apparently share many prim- itive characters with marsupials, as well as some derived features of early develop- ment. However, indication also exists that aspects of the developmental trajectory of monotremes resemble that of eutherians (and nonmammalian amniotes). Under- standing the mosaic of patterns is essential to our efforts to reconstruct the phyloge- netic relations of mainmals and model the. Please note that these images are extracted from scanned page images that may have be
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