Info

Identifying Animal Characteristics

Materials 3 X 5 in. note cards (20), 5 pictures of vertebrates, 5 pictures of invertebrates

Procedure

1. Working in pairs, one partner will write one different vertebrate characteristic on each of 10 note cards. The other partner will write one different invertebrate characteristic on each of 10 note cards.

2. Place the animal pictures upside down in a stack. One partner (the dealer) will shuffle and deal all the cards and turn over one animal picture.

3. The nondealer plays first by laying down as many cards as possible that describe characteristics of the pictured animal. If no card matches, the play is passed to the other player. When neither partner can play, another picture is turned up and play continues.

4. Play ceases when neither student can play or when no pictures are left.

Analysis What are the disadvantages of using only morphological characteristics to identify an organism?

Timeline

1817 Christian Pander identifies germ layers.

1828 Karl Ernst von Baer discovers mammalian eggs and the fate of neural folds and opposes preformation.

1875 Oscar Hertwig observes fertilization.

1883 August Weismann publishes germ plasm theory.

1924 Spemann and

Mangold discover the neural organizer.

1950s to present

Stem cell research begins and continues.

1969 Lewis Wolpert studies pattern formation and positional information in embryos.

1983 Researchers discover homeotic genes.

1996 Scottish researchers clone Dolly the sheep from a mammary cell.

Present Researchers find stem cells in many types of adult tissues.

Developmental Biology

People have known only since the the 1800s that eggs and sperm unite at fertilization. In a little more than a century, researchers have come to understand a great deal about how one cell divides repeatedly to form a mass of cells and how a complex multicellular organism develops from that mass of dividing cells. As in many areas of biology, the speed and sophistication of research in developmental biology has been remarkable.

Until the 19th century, biologists believed that embryos grew from very small versions of complete organisms. In 1817, a Russian naturalist named Christian Pander observed developing chicks and described the three embryonic layers now called ectoderm, mesoderm, and endo-derm. In 1828, Karl Ernst von Baer published a paper in which he concluded that the neural fold of an animal embryo gives rise to the animal's nervous system and notochord.

In 1875, German embryologist Oscar Hertwig first observed the union of the nuclei of male and female gametes at fertilization. Soon after, in 1883, another German biologist, August Weismann, laid out his germ plasm theory. This theory states that the body has germ cells, which pass along hereditary traits, and somatic cells, which do not pass along traits to new generations.

In 1924, German embryologists Hans Spemann and Hilde Mangold studied how cells "know" when to divide and what to do in a growing embryo. They hypothesized that one group of cells might "organize" the rest of the cells. Spemann and Mangold discovered that cells that were transplanted from the blastopore region of a blastula to another region of the blastula caused a new nervous system to form. Today, scientists are still searching for the signal these "organizer cells" give. Part of that search has been the study of pattern formation, or how cells respond to signals and form cells and tissues that have particular functions.

In 1969, Lewis Wolpert proposed that morphogens, or pattern-directing substances, diffuse through a body region—a limb bud, for example—and cause fingers or toes to form in a certain order.

In 1983, researchers discovered genes called homeotic genes that help control where limbs, such as the legs or antennae, grow on a fruit fly embryo.

Another recent line of research centers on stem cells. Stem cells are undifferenti-ated cells that are found in embryos and adult tissues and that can give rise to new cells. Researchers have recently discovered that adult stem cells exist in many more types of tissues than were once thought possible. A related line of research led to the birth of Dolly in 1996, the first mammal to be cloned from an adult somatic cell.

Review

1. What does the germ plasm theory state?

2. Critical Thinking Why are "organizer cells" important to the growing embryo.

3. Critical Thinking How does the production of genetic clones affect the germ plasm theory?

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www.scilinks.org Topic: Development of Mammalian Embryo Keyword: HM60398

Present Researchers find stem cells in many types of adult tissues.

www.scilinks.org Topic: Development of Mammalian Embryo Keyword: HM60398

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