CHAPTER VII

ADAPTATIONS FOR THE PRESERVATION OF THE SPECIES

#A. METHODS OF REPRODUCTION#

1. The Simple or Asexual Method of Reproduction

_Materials._

Slides or diagrams, showing a dividing amoeba, a dividing paramecium, a dividing vorticella, reproduction in some form of sporozoa, budding hydra, gemmules of spongilla, and some species of worm as Dero or Nereis in the process of dividing.

_Definitions._

_Spore_, a cell capable of developing into a new organism.

_Asexual reproduction_, reproduction by division of the cell or body.

_Sexual reproduction_, reproduction by means of the conjugation of two reproductive cells known as the egg and sperm cells.

_Fertilization_, the fusion of the male or sperm cell with the egg or female cell.

_Ovary_, an organ producing eggs.

_Spermary_, an organ producing sperm cells.

_Cross fertilization_, fertilization in which the sperm and egg cells are produced by different individuals.

_Dioecious_, the different kinds of reproductive organs found in different individuals.

_Monoecious_, the different kinds of reproductive organs found in the same individual.

_Directions._

_Note._--Refer to your notes, if the animals mentioned in these exercises have been already studied. This exercise may be largely review.

Study the methods of reproduction in the specimens or diagrams before you. Determine first, in what respects the methods of reproduction are similar in all; second, in what respects there is a variation.

_Questions._

1. What has an amoeba gained by dividing? What powers has each new cell that the original amoeba had lost? What would have been the fate of the amoeba if it had not divided into new cells?

2. What various forms of cell division did you find?

3. What is the simplest method of reproduction?

2. The Complex or Sexual Method of Reproduction

_Materials._

Slides or diagrams showing hydra and sponge reproducing sexually. Conjugating paramecia, fertilized and unfertilized starfish eggs.

_Directions._

Identify the reproductive organs or gonads of the hydra. These are slight swellings on the surface. The one nearer to the mouth end is the spermary and that near the attached end is the ovary.

_Questions._

1. How is an egg cell produced in hydra? In general how do the reproductive cells of sponges and hydra originate?

2. What is gained by limiting the process of reproduction to special cells?

3. What is the difference between the appearance of the nucleus of the fertilized and the unfertilized egg?

4. What is the advantage of cross fertilization? How accomplished in Hydra? What reason can you suggest for the spermary's position?

5. Describe the conjugation of a paramecium.

6. Describe the process of maturation and fertilization in a starfish egg.

_Suggested drawings._

a. Diving Amoeba or Paramecium.

b. Budding Hydra.

c. Hydra showing gonads.

d. Starfish egg--fertilized and unfertilized.

_Summary_ of important points in the study of methods of reproduction:--

1. What are the two principal methods of reproduction? How do they differ?

2. Why is reproduction necessary?

#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.

1. 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").

2. Why does the latter retain its shape?

3. Why do the white and yolk not mix unless shaken or beaten together?

4. Look for the chalaza.

5. 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._

a. The egg as it appears in the saucer.

b. 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._

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 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._

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 antennæ. 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 assumed by the wrigglers.

b. 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._

1. 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?

2. What kind of mouth parts has the caterpillar?

3. 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?

4. 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.

5. 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?

6. What rudimentary structures can you identify in the pupa? What kind of mouth parts has this stage?

7. 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?

8. Describe what occurs when the pupa changes into the adult. What is the appearance of the wings in the beginning? What changes take place?

9. What kind of mouth parts has the adult? Describe.

10. Describe the antennæ.

How do the antennæ of moths and butterflies vary?

11. What is the size and appearance of the eggs? Are they laid singly or in groups?

_Suggested drawings._

a. A caterpillar, side view.

b. A pupa, ventral view.

c. An adult, dorsal view.

d. 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._

1. 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.

2. 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.

3. 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?

4. With what is a chick covered when it is first hatched? How long before feathers begin to develop?

5. How long before a chick needs food? How does it recognize it?

_Summary._

1. What are the two kinds of development?

2. What are the stages in embryonic development?

3. What two general types of postembryonic development as determined by the amount of the food supplied in the egg?

#C. PROTECTION AND CARE OF YOUNG#

Library Exercise

_Materials._

Books and diagrams showing as many methods for the care of young as possible.

_Observations._

1. To what extent is care given to the eggs and young in the case of fish? Is the number of eggs large or small? Contrast this condition with that of the robin. What general conclusion is suggested?

2. Describe the egg-laying habits of five or more of the following: grasshopper, cecropia moth, tussock moth, leaf miner, case bearer, leaf roller, sphinx moth, gall insects, ichneumon flies, spiders, earthworm. How is the developing animal protected in each case? How is food assured?

3. Contrast the method of the honeybee and the solitary wasp as to the method of caring for and feeding the young.

4. Describe the nests of five or more of the following: spider, honeybee, bumblebee, paper wasp, mud dauber, digger wasp.

5. Describe the nests of the following: stickleback fish, sunfish.

6. Describe the nests of ten birds common to your neighborhood.

7. Describe the homes of the following: woodchuck, mole, squirrel, rabbit, muskrat, prairie dog, beaver, bear.

8. In case of birds which of the parent birds builds the nest and cares for the little birds?

9. How are the young cared for in the following cases: crayfish, cyclops, pipefish, Surinam toad?

_Summary._

1. What general methods are there for protecting the young?

2. What various devices for assuring plenty of food for the developing animal?

3. What is the relation between the care given the young and the number of eggs produced?

#D. ADAPTATIONS FOR THE PRESERVATION OF THE SPECIES#

Review Questions and Library Exercise

1. Show how the sexual method of reproduction tends to produce variations.

2. What is meant by the term _heredity_?

3. What are chromosomes? What do some zoölogists believe to be the relation between these chromosomes and heredity?

4. What are dominant and recessive characters? What is meant by "Mendel law of heredity"?

5. What is meant by the term _parthenogenesis_? What are some of its advantages and under what conditions does it take place? Name some animal in which parthenogenesis commonly takes place.

6. What is sex dimorphism? Give some examples.

7. What were the experiments of Professor Loeb and others in connection with artificially fertilized eggs?

8. How do eggs vary as to the kind of shell, amount of food, size, etc.? What is the effect of the amount of food upon the rate of development? On the stage of development at which the egg is hatched?

9. Contrast præcocial and altricial birds.

10. What is the effect of ground nesting and tree nesting upon the number of eggs and the care of the young?

11. Describe the metamorphosis of grasshopper, June beetle, honeybee, dragon-fly, cicada, may-fly, ant-lion, caddis fly.

12. Compare the development of the crayfish, crab, and lobster. What names are given to the larvæ? What is the significance in the fact that the lobster hatches in the "mysis stage"?

13. What are some of the peculiar names given to the larvæ in the case of echinoderms, worms, and mollusks? Why should these have received special names?

14. Name the three primitive germ layers. State the principal organs derived from each in the higher animals.

15. What is ontogeny? phylogeny? What is the meaning of the law "The ontogeny is an epitome of the phylogeny"?

16. Who was Weissman? What important contribution did he make to zoölogy?

17. With what phase of zoölogy is the name of T. H. Morgan associated?