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STUDENT OBJECTIVE

Students observe development of echinoderms, ribbon worms, and chickens. The early developmental stages in the sea star are studied and compared with more complex patterns of development. Students experimentally observe fertilization and early cleavage in sea urchins.

EQUIPMENT	AMOUNT
	(Class of 24 with 8 groups)
Incubator, 37oC Dissecting microscope Compound microscope Refrigerator	1/lab 1/student 1/student 1/lab
MATERIALS
Prepared slides (sharing possible to reduce costs) Sea star, developmental stages (CBS#31-1126)* Cerebratulus sp., developmental stages (CBS#30-9556)* Sea urchins, live in pre-breeding condition; check with supplier Ward’s #87W9030 or Marinus, Inc. Syringes, 5 cc Slides, glass and depression Coverslips, glass, #1 medium square Pasteur pipettes Scissors Dissecting needles Forceps Paper toweling Beaker, 50 ml and 20 ml Petroleum jelly Chicken eggs, fresh, 33 and 72 hours old (CBS#13-9290)* Finger bowls Plastic wrap Rubber bands Developmental stages of chick—models, photographs, or plastic whole mounts (CBS#31-1700)* Bunsen burner Glass rod, 5 mm _ 10 cm Aquarium	1/student     12/lab 8/lab 24/lab 1/2 oz/lab 30/lab 8/lab 16/lab 16/lab 2 pkg/lab 8 + 8/lab 1 each/group 2/group Display 1/group 1/group 2/lab

*Please refer to the Appendix for name and address of supplier.

SOLUTIONS

Avian Ringer’s solution
Seawater
0.5 M potassium chloride (KCl)
Formalin
3.5% saline

PREPARATIONS

Several Weeks before Lab

Place an order for the sea urchins to arrive two to three days before the lab. Ward’s supplies an informative booklet with an order. Request that adequate seawater be shipped with the order. A local source for fertilized chicken eggs is preferred; however, supply houses do have them. Timing is critical in delivery of eggs. Check with the developmental biologist on the staff.

One Week before Lab

1. Potassium chloride preparation:

   0.5M KCl 9.3 g KCl/250 ml distilled water

   Dissolve salt in water.

2. Avian Ringer’s solution preparation:

9.0 g NaCl
0.4 g KCl
0.2 g CaCl₂
0.2 g NaHCO₃

Three Days before Lab

Fertilized fresh chicken eggs (8) should be placed in the incubator to supply 72-hour embryos. Place a pan of water at the bottom of the chamber to provide high humidity. Turn the eggs once or twice and spray with water.

Two Days before Lab

If time does not permit students to examine and handle the sea urchins in class, harvest gametes according to technique illustrated in the laboratory manual (fig. 12.2). Undiluted sperm will be viable for two days if kept in the refrigerator. Eggs should be kept in a small amount of seawater in the refrigerator.

33 Hours before Lab

1. Fertilized fresh chicken eggs (8) should be placed in incubator to supply 33-hour embryos.
2. All slides, suspensions, and equipment for observation of sea urchin fertilization should be refrigerated before lab use.

One Day after Lab

1. The various stages of student preparations of sea urchins should be preserved with formalin.
2. If the primitive streak is observed, preserve the fertilized egg with formalin.

NOTES

1. Marine aquariums should be kept in a room with a constant temperature (21^oC). Light should be diffused; avoid bright artificial light or direct sunlight.
2. The aquarium should be checked daily and dead sea urchins removed.
3. Check out the links for this lab topic at http://auth.mhhe.com/biosci/genbio/dolphin/ You will find useful materials for developing your lab introduction or summary, and in some cases, you may want to tell students to connect to a particular site for further information.

ANSWERS TO CRITICAL THINKING QUESTIONS

1. Frog — moderate amount of yolk provides sufficient nutrition for development.

   — gas exchange occurs directly with the environment through the gelatinous coat.

   — nitrogenous wastes also diffuse directly into the environment.

   Chicken — nutrition is provided by the large amount of yolk, surrounded by a yolk sac that slowly
   digests the yolk and makes the nutrients available to the embryo.

   — gas exchange occurs through the chorion, an extraembryonic membrane lining inside of
   the shell.

   — nitrogenous wastes are stored in the allantois, a sac produced from the allantoic
   membrane. This membrane also provides a respiratory surface.

   Humans — nutrient uptake, gas exchange and waste removal is all performed by the placenta, a
   complex organ arising from the growth of chorionic villi into the surface of the uterus.

2. Identical twins arise from the splitting of a single zygote. Fraternal twins result from two zygotes. Early human enbryonic cells are obviously totipotent through the two-celled stage because identical twins occur. Totipotency ceases soon after the two-cell stage.
3. The size of eggs reflects the amount of stored material used to fuel development. Bird eggs (and reptiles) because of the shell are closed systems. Sufficient material must be packaged into the egg to fuel development from the zygote to the hatchling stage which resembles the adult except for size. Most invertebrates (and amphibia) have larval stages in their life cycles. The fuel that must be pre-packaged in the egg is needed only to support development to an intermediate stage which is self-feeding. The larval stage is able to gather its own food which fuels further development to the adult stage. The transition from larva to adult usually involves a major metamorphic event. Mammal eggs are small, yet newly born mammals resemble the adult stages. The placenta allows the developing embryo to obtain nutrients from its mother throughout the developmental period.

SUPPLEMENTAL MATERIALS