a. Zygotes are totipotent, having the potential to develop into every kind of adult tissue.

b. Smaller cells produced after division of the zygote have limited developmental potential, though they have not, in general, lost any DNA.

c. In 1952, Briggs and King experimented on frog embryos (Figure 21.1) to show that nuclei from cells in even late stages of development still retain their entire complement of genes.

d. Steward isolated single carrot cells (Figure 21.2) and showed that developing plant cells also keep their entire genome.

e. Methods 21.1 covers some of the issues surrounding the selection of organisms for development and cloning research.

a. An embryonic cell becomes determined, as its fate is sealed by internal molecular events, before it is differentiated.

b. Cells become determined generally at very early stages of development; the time varies with the organism.

c. E. B. Wilson, in 1892, demonstrated determination using Nereis eggs (Figure 21.3).

d. Nereis cells show mosaic development, where each blastomere has a distinctive role in the development of the organism.

e. In some embryos, the fates of even regions of the undivided egg are determined, as demonstrated by Conklin in 1905, using tunicate zygotes (Figure 21.4).

f. A fate map shows what each region of a zygote will become.

g. Cells destined to become a particular structure are call presumptive, as in presumptive epidermis.

h. Mosaic development is an example of an autonomous process, where the development of each cell is determined entirely by its own internal information.

i. Cell-interactive processes are those causing the cell's developmental path to be determined by factors in its environment.

j. Methods 21.1 covers the cell-interactive processes of the roundworm.

a. Frogs and some other vertebrates have regulative development, where their early embryonic cells are not yet determined.

b. Figure 21.5 shows a fate map of an early frog embryo.

c. Figure 21.6 shows an experiment showing regulative development, where a patch of presumptive epidermis is cut out and exchanged with a patch of presumptive nervous tissue.

d. Identical twins demonstrate regulative development, since each grows from a half of a single embryo.

e. Beatrice Mintz produced allophenic mice with two phenotypes (Figure 21.7) by dissociating early blastulas into blastomeres and mixing cells from two embryos.

a. Cells fates become increasingly restricted as cell division continues in the growing embryo.

b. Cells gradually come to follow genetic subprograms that limit them as time goes on.

c. Figure 21.8 shows vertebrate mesodermal stem cells, which can either have limited fates, as they become committed, or pluripotent fates, as they are able to differentiate into a variety of cells of a certain type (e.g. blood cells).

d. Mintz used her allophenic mice to determine that relatively few cell divisions are needed to produce an adult; these cells are said to have been allocated to their fate (Figure 21.9).

e. Mintz's analysis suggests that only 34 fur pigment cells are allocated during development, and that each grows into a subclone to produce the fur.

a. As stem cells proliferate, typically one daughter cell remains a stem cell (Figure 21.10) while the other differentiates.

b. Plant tissues show this type of development, where some stem cells remain in meristems that never fully differentiate.

c. The meristematic layer of woody plants (Figure 21.11) lies between the xylem and phloem, and allows the plant to continue growing.

d. Terminal differentiated cells (e.g. nerve cells, blood cells) may never divide again; other cells (e.g. liver cells) retain the capacity to divide and to grow tissue when needed.

a. Differentiation often depends on embryonic induction, during which a tissue inducer causes neighboring cells to develop in a certain direction.

b. Hans Spemann discovered embryonic induction in the 1920s, working with amphibian embryos.

c. The amphibian embryo's gray crescent (Chapter 20; Figure 21.12) develops into the dorsal lip of the blastopore.

d. Spemann and Hilde Mangold transplanted the dorsal lip from a blastopore into the blastocoel of early gastrulas and got host embryos developing a second nervous system and head.

e. This experiment revealed that the donor tissue induced some host tissue to develop into structures it would otherwise not have become; the dorsal lip came to be known as the organizer of the embryo.

f. The dorsal lip forms the chordamesoderm, the dorsal surface of the archenteron, which will become notochord and mesoderm.

g. Chordamesoderm induces a central portion of ectoderm to form the neural plate, which rolls into the neural tube.

h. This induction was shown to occur in response to proteins diffusing from the chordamesoderm into the nearby ectoderm.

a. Primary induction involves an interaction between primary tissue layers.

b. Secondary induction occurs at later stages of development.

c. Two kinds of cell-cell interactions thus occur in embryos:

d. The vertebrate optic cup (Figure 21.13) induces the overlying epithelium to develop a lens through an instructive process.

e. The target tissue of an instructive process must be competent, which means not restricted along another line of development.

f. Mesenchyme generally instructs epithelium, and it also instructs the form of organs derived from the gut tube (Figure 21.14).

a. Jacob and Monod's operon model for gene regulation (Section 18.2) can be generalized as a model for differentiation.

b. A general model (Figure 21.15) shows genes in regulatory circuits, in which genes are turned on and off by signals presented by either space or time.

c. Selector genes in this model work sequentially in either space or time, by using logical connections.

d. Regulatory proteins (Section 19.7) bind to promoter and enhancer regions in various combinations to either promote or restrict transcription (Figure 21.16).

a. Quadruped limbs all grow similarly from limb buds (Figure 21.17), which have an essential ridge of ectoderm over a mesoderm core.

b. The mesoderm has been shown to be the component that gives instructions, as demonstrated by Lewis, Summerbell, and Wolpert in various transplanting experiments (Figure 21.18).

c. Their experiments also showed that the limb bud develops according to an internal clocklike mechanism, one consistent with the generalized model (Figure 21.15).

a. Cells differentiate on the basis of positional information that specifies each cell's position in the body.

b. Position-determining substances called localized determinants are distributed in the egg.

c. These determinants locally induce or repress different blocks of genes.

d. Thus, a cell knows its position in the body by responding to materials in the egg cytoplasm.

e. The materials that guide the cell's positional information are thought to be in the egg cortex, as evidenced by Harvey's experiments on sea urchin eggs.

f. The development of Drosophila melanogaster has been intensely studied and is examined in detail here:

g. The genetics of Drosophila development has also been analyzed in detail; many genes that provide positional information are known and show paths of development very clearly.

h. Figures 21.22, 21.23, and 21.24 illustrate the action of several known genes involved in Drosophila development.

i. Homeotic genes are those whose mutants can seriously disrupt development, and include the Antennapedia complex for the anterior half of the fly (Figure 21.25).

j. The bithorax complex determines the form of segments from the mid-thorax region through the abdomen; a fly with mutations in all three genes of this complex will have four wings (Figure 21.26) rather than the usual two.

a. Homeotic mutants have been shown to have a common sequence of 180 nucleotide pairs called a homeobox.

b. The homeobox encodes a sequence of 60 amino acids called the homeodomain, which enables the protein product to bind specifically to a DNA sequence.

c. All homeobox genes regulate development by binding to certain DNA sequences and turning nearby genes on or off.

d. Homeoboxes were discovered in Drosophila and have now been found in virtually every other animal, including humans.

e. Genes containing homeoboxes are called Hox genes, and the mammalian Hox genes are very similar to the insect Hox genes (Figure 21.27).

f. The discovery of the Hox genes and their similarities across various animal groups has indicated a deep commonality in the basic developmental paths of animals.

g. Chapters 24, 34, and 35 will cover the development of various animal body plans and will show them to be the result of a flexible, but basic, genetic mechanism.

a. Homeotic mutants in plants have been found, and are illustrated here using the snapdragon (Figure 21.28).

b. In this plant, the apical meristem becomes an inflorescence meristem, which produces bracts, or small leaves.

c. A floral meristem develops between each bract and the stem, and a flower eventually grows from this tissue.

d. Two kinds of homeotic mutants can change this pattern.

e. The evolution of plant anatomy can be enlightened by studies of homeotic plant genes.