THE DEVELOPMENT AND NUTRITION OF

THE FROG (XXXI-1)

The problem. All plants and animals, no matter how large they may become, begin life as a single cell. In some the cell undergoes enlargement and new cells are formed by the division of the first cell, and this growth and division of cells continues throughout the entire life of the individual. In other cases a plant or an animal may consist of a single cell throughout its whole life, but these cells are extremely small and are not ordinarily observed.

What to use. Fresh eggs of the frog or toad (these may be secured in early spring from the pools of water in which they are deposited, the toad's eggs usually appearing a little later than those of the frog), large aquarium jars, and hand magnifying glasses.

What to do. 1. Place the eggs of the frog or toad in an aquarium in shallow water. Observe them from day to day, noting early stages of development. Some of the eggs may be examined from day to day under the low power of a microscope. The following stages may be observed.

2. If the eggs are secured very soon after they are laid, it will be possible to see, with a hand magnifier or under the low power of a microscope, some which have divided into two cells, four cells, or a larger number of cells.

3. At a later period the original single egg cell has divided into such a large number that the individual cells of the mass cannot be separately distinguished by use of the magnifier.

4. The mass of cells produced by the egg becomes elongated instead of spherical. This is called the embryo.

5. The embryo continues to elongate; head and tail may be distinguished; eyes and other organs appear. Determine what the animal feeds upon at this stage and how it secures its food.

6. The young animal frees itself from the surrounding jelly, swims through the water, and usually attaches itself to floating objects or to the walls of the aquarium. At this stage it has conspicuous gills (breathing organs). Is the source of food and method of feeding the same in this stage as in the preceding one?

Questions. At what point in the division of the egg can you see the cells begin to be different from one another? How can you distinguish frogs' eggs from toads' eggs? In what kinds of situations do these animals lay their eggs? How many of the stages given in figure 170 of the text can you recognize? Upon what do adult toads and frogs feed? Are their feeding habits of any economic significance?

Suggestions for report. Record and describe all of the changes which you have observed in the development of the egg.

Reference work. Read sections 371 to 378.

Optional problems. Place the young tadpoles from the eggs you have collected in aquaria where they will thrive, and follow their development through several weeks to determine the nature, time, and order of the changes which occur as the tadpole is becoming a frog or toad. By varying the conditions in which the developing tadpoles are kept, determine the conditions which are best for their growth.

DEVELOPMENT OF THE BIRD EMBRYO DURING INCUBATION OF THE EGG (XXXI−2)

The problem. The cell from which the bird develops is embedded within the food material of the egg. When the egg is fresh this cell is so small that most people fail to notice it, but soon after the incubation of the egg has started, the growth and division of the cell is such that it may be readily observed.

What to use. Two dozen hen's eggs and an incubator.

What to do. Place at least two dozen hen's eggs in an incubator which has previously been in operation until it is regulated. Open an egg on each of the following days of the incubating period: first day, second, third, fifth, eighth, twelfth, and sixteenth. On the first day note the germ cell, or fertilized egg, lying close to the yolk, it being recognized by a reddish ring about it. In the other observations determine what changes have taken place in the eggs.

Questions. Why will eggs usually not hatch after they have been subjected to sudden and great changes in temperature ? Why will eggs not hatch after they have been in cold storage?

Suggestions for report. Prepare a set of sketches showing the general appearance of the developing bird at about five different stages between your first and last observations.

Reference work. Read section 378.

Optional problems. How do you account for the fact that in a nest of young robins the birds hatch on successive days, while in a nest of quail or domestic fowls the young birds usually all hatch on the same day?

THE YOUNG PLANT (XXXI-3)

The problem. Some plants, like some animals, never become larger than a single cell and others grow until they contain millions of cells arranged in the form of different plant organs. The beginnings of a plant are as interesting as the beginnings of an animal.

What to use. Seeds of bean, corn, clover, or peanut; sawdust, sand, or common soil; earthen pots, or small wooden boxes which may be made by pupils.

What to do. 1. Plant seeds of bean, corn, or peanut at least two weeks before it is intended to make this study. If the seeds are planted near the glass side of a properly constructed box (Fig. 46), this side being darkened by the use of a black cloth, the growth may be observed during the process. Observe the plantings from time to time and record all facts of interest as the young plants develop. When

FIG. 46

the plants have from two to four leaves, make your final study and description. At this time the young stem and roots, as well as the leaves, will have assumed definite form, also their characteristic positions.

2. Soak some seeds in water for about twenty-four hours and examine them to discover how many and what structures found in the seedling may be found also in the seed. Examine the seeds. for evidence of stored food.

Questions. What is the function of the food stored in the seed? Is this food stored in the same kinds of places in different