BI256 Chapter Notes - Chapter 8: Oocyte, Zygote, Transfer Rna
Document Summary
Discovered by spemann when he used tissue from salamanders to induce development in another egg, the dorsal lip tissue specifically. Modern discovery: this occurs by secretion of certain molecules that trigger of repress the activity of combinations of genes in near by cells. Preformation: belief that young animals were preformed in eggs and that development was simply a matter of unfolding what was already there. Concept strongly supported by most 17th and 18th century naturalist philosophers. Showed in chickens no preformation but granulated undifferentiated material that became arranged in layers. Termed epigensis meaning an idea that a fertilized egg contains building material only, somehow assembled by an unknown directing force. Presently know far more about what directs growth and differentiation. Hierarchy of developmental decisions: the sequential emergence of cellular diversity. The decision made at each stage in the hierarchy further limits developmental fate. Once cells commit to differentiation, it is said to be determined and usually irreversible.
