B. Development
1. The Hen's Egg
Materials.
Hen's eggs, hydrochloric acid.
Definitions.
Germ spot, a white spot, imbedded in the "yolk." This is the point at which development begins. Yolk, the yellow portion of a bird's egg. This is a food material, rich in fat. Albumen, the white, viscous portion of a bird's egg. Chalaza, the spiral portion of albumen always seen in the bird's egg.
Directions.
Boil an egg at least ten minutes in water deep enough to cover it. Note which side is uppermost and mark this part of the shell for reference. Remove the egg and pick away the shell from about half the egg, leaving the shell on that portion which was underneath when placed in water. With a sharp knife remove this half of the egg. Note the thickness of the shell. Test its composition (use hydrochloric acid). Find the membrane lining the shell and note that at one end it separates into two parts to inclose an air space.
- What is this for? How does it change after an egg has been incubated for a week or more?
- Break an uncooked egg in a saucer. Note the germ spot. Note also the difference in the consistency of the "white" (albumen) and yellow portion ("yolk").
- Why does the latter retain its shape?
- Why do the white and yolk not mix unless shaken or beaten together?
- Look for the chalaza.
- What do you think is the use of this structure?
Weigh a fresh egg, place it in a dry atmosphere for a week, and weigh it again. Record result. Why may eggs be kept a long time perfectly fresh if coated with paraffin or if put in "water glass"?
Suggested drawings.
- The egg as it appears in the saucer.
- The egg after part of the shell has been removed.
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.
- 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?
- Contrast the blastula and gastrula as to number of cavities, number of cell layers, number of external openings.
- Suggest protozoans that resemble the egg and blastula respectively. What invertebrates resemble the gastrula in body plan?
Suggested drawings.
- Some of the cleavage stages.
- A blastula.
- A gastrula.
3. Postembryonic Development or Metamorphosis of a Mosquito
Materials.
Some specimens of the larvæ and pupæ of the mosquito, ordinarily known as wrigglers. Either specimens or diagrams of egg packets should also be provided. Mounted specimens of adult of both sexes.
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.
- Describe the appearance of the egg packet both as seen with the unaided eye and with a hand lens. Find the trapdoor.
- What is the difference between the appearance of the larva and the pupa? How do their resting positions differ?
- What does the larva do when disturbed? Describe any characteristic motions that you notice.
- Contrast the pupa with the larva under the same conditions and note any differences.
- 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.
- 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?
- Examine an adult mosquito. State the kind of mouth parts, the number and appearance of the wings, the appearance of the antennæ. How does the male and female differ in this respect?
Suggested drawings.
- A diagrammatic drawing representing the jar of water and showing the various positions assumed by the wrigglers.
- Careful drawings of each stage.
4. Postembryonic Development or Metamorphosis of a Butterfly or Moth
Materials.
The eggs, caterpillars, pupæ, cocoons, and adult of some moth or similar stages of a butterfly.
Definitions.
Prolegs, short, unsegmented appendages found in the larva. Caterpillar, the larva of a moth or butterfly. Spiracles, openings into the trachæ or breathing tubes. Tubercles, knob-like projections. Chrysalis, the pupa stage of a butterfly. Cocoons, the covering spun by the larva before changing to the pupa.
Observations.
- State the color and appearance of the larva. If tubercles or spines are present, state where. Where are the spiracles? How do you distinguish the head, thorax, and abdomen in the case of the caterpillar?
- What kind of mouth parts has the caterpillar?
- How many prolegs has the caterpillar? Of what use are they?
- How does the number of prolegs differ from that of the caterpillar in the case of the grub? maggot? currant worm?
- What is the advantage of the cocoon? What is its color, appearance, and material? Is it composed of a tough substance, or is it easily torn? Where are cocoons found out of doors? Where would you look for chrysalids? (Explain differences in locations.)
- Describe the cocoons of tussock moth, clothes moth, leaf roller.
- What is the difference between a Cecropia cocoon which contains a living pupa and one that has been parasitized? What is the appearance of a parasitized caterpillar?
- What rudimentary structures can you identify in the pupa? What kind of mouth parts has this stage?
- Which of these stages is the active stage? Which stage is quiescent? What is really going on in the quiescent stage? In which stage does the insect grow? feed? If the insect were harmful, in which stage would it do the damage? How?
- Describe what occurs when the pupa changes into the adult. What is the appearance of the wings in the beginning? What changes take place?
- What kind of mouth parts has the adult? Describe.
- Describe the antennæ.
- How do the antennæ of moths and butterflies vary?
- What is the size and appearance of the eggs? Are they laid singly or in groups?
Suggested drawings.
- A caterpillar, side view.
- A pupa, ventral view.
- An adult, dorsal view.
- A few eggs.
5. Development of the Chick
Materials.
An incubator, a brooder, a setting of eggs.
Directions.
Read carefully the directions for setting up and regulating the incubator. Remember that the temperature should average 103 degrees and should not vary more than two or three degrees above or below this. Candle the eggs from time to time and note difference in appearance, as development proceeds, especially as to transparency and size of the air space. An egg that is transparent after ten days is probably infertile and should be removed. Eggs which are developing properly will show from this time on a well developed air space, and will be quite opaque. The veins often give a spider-web appearance.
Twice each day remove the tray of eggs and allow to cool slightly. Once a day the eggs should be turned before the tray is returned to the incubator.
Questions.
- Describe the appearance of the shell when the chick is about to come out. In about how many days after you put the eggs in the incubator did you first note this change? You should watch the incubator carefully from the eighteenth day on.
- What proportion of the eggs hatched? What reasons can you suggest for the failure of some to hatch? Did any which were pipped fail to hatch? If so, break them open and see if you can discover the trouble.
- How does the chick get out of the shell? How long does it take for it to get out after the shell is chipped? How does a chick look as soon as it has come out of the shell?
- With what is a chick covered when it is first hatched? How long before feathers begin to develop?
- How long before a chick needs food? How does it recognize it?