How Do Organisms Achieve Their Basic Form? That is to ask, how does a single fertilized egg become a elephant, a tiger, a human, or any of the numerous other kinds of animals or plants we see around us?
“The earliest events leading from the first division of the egg cell to the blastula stage in amphibians, reptiles and mammals are illustrated in figure 5.4. Even to the untrained zoologist it is obvious that neither the blastula itself, nor the sequence of events that lead to its formation, is identical in any of the vertebrate classes shown. The differences become even more striking in the next major phase of in embryo formation – gastrulation. This involves a complex sequence of cell movements whereby the cells of the blastula rearrange themselves, eventually resulting in the transformation of the blastula into the intricate folded form of the early embryo, or gastrula, which consists of three basic germ cell layers: the ectoderm, which gives rise to the skin and the nervous system; the mesoderm, which gives rise to muscle and skeletal tissues; and the endoderm, which gives rise to the lining of the alimentary tract as well as to the liver and pancreas.,,, In some ways the egg cell, blastula, and gastrula stages in the different vertebrate classes are so dissimilar that, where it not for the close resemblance in the basic body plan of all adult vertebrates, it seems unlikely that they would have been classed as belonging to the same phylum. There is no question that, because of the great dissimilarity of the early stages of embryogenesis in the different vertebrate classes, organs and structures considered homologous in adult vertebrates cannot be traced back to homologous cells or regions in the earliest stages of embryogenesis. In other words, homologous structures are arrived at by different routes.” Michael Denton – Evolution: A Theory in Crisis – pg 145-146 More.
See also: Philip Cunningham on determinism vs free will