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“Just as in the construction of a building, where there is an order to the sequence of steps . . . there is an order to building animals, from the making of the basic body plan to the fine detailing of individual body parts.”

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“For example, the expression of one tool kit gene marks the future forebrain and midbrain, while a second gene marks out the hindbrain and the midbrain/hindbrain boundary.”

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"By visualizing tool kit genes in action and embryos- the gene stripes I first saw that night in Colorado- we can see the position and shapes of structures long before they actually form. The images of tool kit genes in embryos create a vivid, dynamic map of the geography of a growing embryo"

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"... so it takes about as long to make a 32- cell human embryo as to make a complete frog tadpole" (Ch. 4, Page 88).

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“The beauty of the limb is also sculpted by death. The seperation of digits in mice, chicks, and humans is due to the death of the tissue between the digits in the developing limb. Within the padlike hands and feet, these interdigital zones are marked by the expression of different tool kit genes that instruct the cells in the zones to undergo programmed cell death” pg 103

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“Tool kit genes, stirred into action by fertilization, are starting to mark out the geography of the developing embryo. While all cells in the growing embryo contain the same DNA (and the same genes), the tool kit genes become active.”

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"He showed a picture of an intact embryo, then an embryo that had been put through a blender, and finally an embryo whose blended components had been centrifuged together" (Ch. 4, Page 83).

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Frog and fly embryos are very vulnerable to predators. The sprint to complete development is a survival imperative and of the hundreds of eggs produced by a female, only a fraction will reach adulthood. Humans have a different ecology, and our development takes place in maximum security and unfolds, at least initially, at a much slower pace.

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“There is an orderly pace of somite formation, occurring about every twenty minutes in a zebrafish embryo, every hour and a half in a chicken embryo, and every two hours in a mouse embryo.”

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“The somites are initially identical in appearance, but they will give rise to distinct types of vertebrae, ribs, and musculature, depending upon their position along the head-to-tail axis” (101)

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“The somites are initially identical in appearance, but they will give rise to distinct types of vertebrae, ribs, and musculature, depending upon their position along the head-to-tail axis” (101)

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“Our tool kit genes are just pieces of our genetic material, a small fraction of the 13,700 genes in a fruit fly or the 25,000 genes of a mammal.” (Ch. 4, pg.83)

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“While the eggs of vertebrates differ greatly in size and character, from tiny mammal eggs to the enormous shelled eggs of large birds and reptiles, and the adult forms range from guppies to elephants and dinosaurs, all vertebrate embryos pass through a stage of development where they appear somewhat similar.” (Ch. 4, 98)

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Our tool kit genes are just pieces of our genetic material, a small fraction of the 13,700 genes in a fruit fly or the 25,000 genes of a mammal. Granted, we have identified many critical pieces…

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I have described the genetic tool kit for development and how its discovery was driven by the study of spectacular mutants that made the wrong number of body parts, or put a part in the wrong place..

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“In terms of our geography analogy, cells, tissues, and organs have a specific position on the globe of the embryo defined by their longitude, latitude, altitude (if projecting out from the body), and depth (within the layers of the body), as well as a “national” identity (nerve cells, liver cells, etc.)”