1. Experimentation (or observation) would show development from an undifferentiated mass.
2. In any given cell, only certain genes are activated. Other genes are turned off.
3. Morphogen gradients establish biochemically distinct regions and thus signal the activation of certain genes and/or certain developmental processes. Genes dictate protein development.
4. Answers will vary.
5. As an adult, C. elegans only has 959 somatic cells. Humans have trillions.
6. Homeotic genes lead to abnormal development.
7. From youngest to oldest: zygote, morula, blastocyst, gastrula, embryo, fetus.
8. See pages 217 and 218.
9. A trophoblast cell becomes part of (or gives rise to) an extraembryonic structure; an inner cell mass cell becomes part of (or gives rise to) the embryo. The ectoderm gives rise to the integument and the nervous system; the mesoderm gives rise to the blood and the muscle, skeletal, and reproductive systems; the endoderm gives rise to the organs and linings of the digestive, respiratory, and urinary systems.
10. Developmental foundations are being laid during the first eight weeks.
11. Life span is the theoretical limit or maximum age of an organism. The life expectancy is the probable length of time a given organism will live.
12. One answer is adoption studies. Other answers are possible.
13. Free radicals are not destroyed.
TO THINK ABOUT
1. Epigenesis. (One could theoretically argue, however, that if the instructions weren't there, you couldn't have preformation either because the individual couldn't grow or mature.)
2. The first question has to be, "What type of oocyte?"
3a. Transplant the dorsal lip of the blastopore from one embryo onto another embryo.
b. Remove the nucleus from a frog ovum. Remove the nucleus from a somatic cell and place this nucleus into the frog ovum.
c. Manipulate (or transplant) cells that have already differentiated.
4. Answers will vary. Numerous examples are given in the text.
5. It may be too late; the ovum may have already died.
6. If the placenta ceases to function, the child will die.
7. Answers will vary.
8. Answers will vary.
9. Answers will vary.
10. Technically, aging begins before birth. For specific points in the aging process, see page 232.
11. Answers will vary.
12. Answers will vary.
13. Answers will vary.
14. Answers will vary.
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