2. Early Embryonic Development of an Egg

_Materials._

Slides or diagrams, showing various stages in the development of some animal through the gastrula stage.

_Definitions._

_Cleavage stages_, two, four, eight, sixteen cells, etc., arising by repeated division, starting with the egg.

_Blastula_, a hollow sphere, the wall of which is composed of a single layer of cells.

_Gastrula_, a stage formed from the blastula by pushing in one side of the latter, so as to form a more or less cup-shape structure.

_Observations._

1. Is there any considerable difference between the size of the egg and the size of the blastula and gastrula? Has development taken place by an increase of size or by an increase of complexity?

2. Contrast the blastula and gastrula as to number of cavities, number of cell layers, number of external openings.

3. Suggest protozoans that resemble the egg and blastula respectively. What invertebrates resemble the gastrula in body plan?

_Suggested drawings._

a. Some of the cleavage stages.

b. A blastula.

c. A gastrula.

3. Postembryonic Development or Metamorphosis of a Mosquito

_Materials._

Some specimens of the larvae and pupae of the mosquito, ordinarily known as wrigglers. Either specimens or diagrams of egg packets should also be provided. Mounted specimens of adult of both s.e.xes.

_Definitions._

_Postembryonic development_, the changes taking place in the development of an animal after birth or hatching.

_Larva_, the active feeding stage. It is the first stage in postembryonic development, and follows the gastrula stage.

_Pupa_, usually a resting or quiescent stage. It is the stage following the larva stage.

_Observations._

1. Describe the appearance of the egg packet both as seen with the unaided eye and with a hand lens. Find the trapdoor.

2. What is the difference between the appearance of the larva and the pupa? How do their resting positions differ?

3. What does the larva do when disturbed? Describe any characteristic motions that you notice.

4. Contrast the pupa with the larva under the same conditions and note any differences.

5. Where do you think the external openings of the respiratory organs of the larva and those of the pupa are located? Give reasons for your answer.

6. Into what does the pupa change? Where must the pupa be at this time? Is it easier for the pupa to stay at the surface or at the bottom?

7. Examine an adult mosquito. State the kind of mouth parts, the number and appearance of the wings, the appearance of the antennae.

How does the male and female differ in this respect?

_Suggested drawings._

a. A diagrammatic drawing representing the jar of water and showing the various positions a.s.sumed by the wrigglers.

b. Careful drawings of each stage.

4. Postembryonic Development or Metamorphosis of a b.u.t.terfly or Moth

_Materials._

The eggs, caterpillars, pupae, coc.o.o.ns, and adult of some moth or similar stages of a b.u.t.terfly.

_Definitions._

_Prolegs_, short, unsegmented appendages found in the larva.

_Caterpillar_, the larva of a moth or b.u.t.terfly.

_Spiracles_, openings into the trachae or breathing tubes.

_Tubercles_, k.n.o.b-like projections.

_Chrysalis_, the pupa stage of a b.u.t.terfly.

_Coc.o.o.ns_, the covering spun by the larva before changing to the pupa.