576 Chapter 18 • The Genetic Basis of Development CHAPTER OVERVIEW T he general body plan of an animal is a speciﬁc pat-tern of structures common to all members of its species. Such body plans may be common to many very different species: all mammalian species have four limbs, whereas all insects have six. However, all mammals and insects must, in the course of their development, differ-entiate the anterior from the posterior end and the dor-sal from the ventral side. Eyes and legs always appear in the appropriate places. The basic body plan of a species appears to be quite robust—that is, the internal genetic program produces the same body plan within a broad range of environmental conditions. We should not for-get, however, that the study of the genetic determina-tion of these basic developmental processes does not provide an explanation of the phenotypic differences be-tween individual members of a species. This chapter focuses on the processes that underlie pattern forma-
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