Source: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A2609 Cleavage in Mammals Excerpted and modified from Developmental Biology 6thEdition, Scott F. Gilbert, available online at http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio, downloaded 10/10/2009 --CSS Mammalian eggs are among the smallest in the animal kingdom, making them hard to manipulate experimentally. The human zygote, for instance, is only 100 μm in diameter—barely visible to the eye and less than one-thousandth the volume of a Xenopus(frog) egg. Additionally, mammalian zygotes are produced in much lower numbers than sea urchin or frog zygotes, making it difficult to obtain enough material for scientific investigation. Finally, unlike frogs and sea urchins, the development of mammalian embryos is accomplished within the parent, rather than in the external environment. Only recently has it been possible to duplicate some of these internal conditions and observe development in vitro.
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