A Guide for the Study of Animals Part 54

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

3. What is the simplest method of reproduction?

2. The Complex or s.e.xual Method of Reproduction

_Materials._

Slides or diagrams showing hydra and sponge reproducing s.e.xually.

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 Amba 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 princ.i.p.al 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.

_Alb.u.men_, the white, viscous portion of a bird's egg.

_Chalaza_, the spiral portion of alb.u.men 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 sh.e.l.l for reference. Remove the egg and pick away the sh.e.l.l from about half the egg, leaving the sh.e.l.l 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 sh.e.l.l. Test its composition (use hydrochloric acid). Find the membrane lining the sh.e.l.l and note that at one end it separates into two parts to inclose an air s.p.a.ce.

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" (alb.u.men) 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 gla.s.s"?

_Suggested drawings._

a. The egg as it appears in the saucer.

b. The egg after part of the sh.e.l.l has been removed.