A Civic Biology Part 27

Blood enters the right auricle from all parts of the body; it then contains considerable carbon dioxide; the blood entering the left auricle comes from the lungs, hence it contains a considerable amount of oxygen. Blood leaves the heart through the ventricle, which thus pumps some blood containing much and some containing little oxygen. Before the blood from the tissues and lungs has time to mix, however, it leaves the ventricle and by a delicate adjustment in the vessels leaving the heart most of the blood containing much oxygen is pa.s.sed to all the various organs of the body, while the blood deficient in oxygen, but containing a large amount of carbon dioxide, is pumped to the lungs, where an exchange of oxygen and carbon dioxide takes place by osmosis.

In the tissues of the body wherever work is done the process of burning or oxidation must take place, for by such means only is the energy necessary to do the work released. Food in the blood is taken to the muscle cells or other cells of the body and there oxidized. The products of the burning--carbon dioxide--and any other organic wastes given off from the tissues must be eliminated from the body. As we know, the carbon dioxide pa.s.ses off through the lungs and to some extent through the skin of the frog, while the nitrogenous wastes, poisons which must be taken from the blood, are eliminated from it in the kidneys.

Change of Form in Development of the Frog.--Not all vertebrates develop directly into an adult. The frog, for example, changes its form completely before it becomes an adult. This change in form is known as a _metamorphosis_. Let us examine the development of the common leopard frog.

[Ill.u.s.tration: Development of a frog. 1, two cell stage; 2, four cell stage; 3, 8 cells are formed, notice the upper cells are smaller; in (4) the lower cells are seen to be much larger because of the yolk; 5, the egg has continued to divide and has formed a gastrula; 6, 7, the body is lengthening, head is seen at the right hand end; 8, the young tadpole with external gills; 9, 10, the gills are internal, hind legs beginning to form; 11, the hind legs show plainly; 12, 13, 14, later stages in development; 15, the adult frog. Figures 1, 2, 3, 4, 5, 6, and 7 are very much enlarged.

(Drawn after Leukart and Kny by Frank M. Wheat.)]

The eggs of this frog are laid in shallow water in the early spring. Ma.s.ses of several hundred, which may be found attached to twigs or other supports under water, are deposited at a single laying. Immediately before leaving the body of the female they receive a coating of jellylike material, which swells up after the eggs are laid. Thus they are protected from the attack of fish or other animals which might use them as food. The upper side of the egg is dark, the light-colored side being weighted down with a supply of yolk (food). The fertilized egg soon segments (divides into many cells), and in a few days, if the weather is warm, these eggs have each grown into an oblong body which shows the form of a tadpole. Shortly after the tadpole wriggles out of the jellylike case and begins life outside the egg. At first it remains attached to some water weed by means of a pair of suckerlike projections; later a mouth is formed, and the tadpole begins to feed upon algae or other tiny water plants. At this time, about two weeks after the eggs were laid, gills are present on the outside of the body.

Soon after, the external gills are replaced by gills which grow out under a fold of the skin which forms an operculum somewhat as in the fish. Water reaches the gills through the mouth and pa.s.ses out through a hole on the left side of the body. As the tadpole grows larger, legs appear, the hind legs first, although for a time locomotion is performed by means of the tail. In the leopard frog the change from the egg to adult is completed in one summer. In late July or early August, the tadpole begins to eat less, the tail becomes smaller (being absorbed into other parts of the body), and before long the transformation from the tadpole to the young frog is complete. In the green frog and bullfrog the metamorphosis is not completed until the beginning of the second summer. The large tadpoles of such forms bury themselves in the soft mud of the pond bottom during the winter.

Shortly after the legs appear, the gills begin to be absorbed, and lungs take their place. At this time the young animal may be seen coming to the surface of the water for air. Changes in the diet of the animal also occur; the long, coiled intestine is transformed into a much shorter one. The animal, now insectivorous in its diet, becomes provided with tiny teeth and a mobile tongue, instead of keeping the h.o.r.n.y jaws used in sc.r.a.ping off algae. After the tail has been completely absorbed and the legs have become full grown, there is no further structural change, and the metamorphosis is complete.

[Ill.u.s.tration: At the left is a hen's egg, opened to show the embryo at the center (the spot surrounded by a lighter area). At the right is an English sparrow one day after hatching.]

Development of Birds.--The white of the hen's egg is put on during the pa.s.sage of the real egg (which is in the yolk or yellow portion) to the outside of the body. Before the egg is laid a sh.e.l.l is secreted over its surface. If the fertilized egg of a hen be broken and carefully examined, on the surface of the yolk will be found a little circular disk. This is the beginning of the growth of an _embryo_ chick. If a series of eggs taken from an incubator at periods of twenty-four hours or less apart were examined, this spot would be found at first to increase in size; later the little embryo would be found lying on the surface. Still later small blood vessels could be made out reaching into the yolk for food, the tiny heart beating as early as the second day of incubation. After about three weeks of incubation the little chick hatches; that is, breaks the sh.e.l.l, and emerges in almost the same form as the adult.

[Ill.u.s.tration: The embryo (_e_) of a mammal, showing the absorbing organ in black, the branch-like processes which absorb blood from the mother being shown at (_v_); _ct_, the tube connecting the embryo with the absorbing organ or placenta.]

Development of a Mammal.--In mammals after fertilization the egg undergoes development within the body of the mother. Instead of blood vessels connecting the embryo with the yolk as in the chick, here the blood vessels are attached to an absorbing organ, known as the _placenta_. This structure sends branch-like processes into the wall of the _uterus_ (the organ which holds the embryo) and absorbs nourishment and oxygen by osmosis from the blood of the mother. After a length of time which varies in different species of mammals (from about three weeks in a guinea pig to twenty-two months in an elephant), the young animal is expelled by muscular contraction of the uterus, or is born. The young, usually, are born in a helpless condition, then nourished by milk furnished by the mother until they are able to take other food. Thus we see as we go higher in the scale of life fewer eggs formed, but those few eggs are more carefully protected and cared for by the parents. The chances of their growth into adults are much greater than in the cases when many eggs are produced.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American Book Company.

Bigelow, _Introduction to Biology_. The Macmillan Company.

Cornell _Nature Study Leaflets_. Bulletins XVI, XVII.

Davison, _Practical Zoology_, pages 185-199. American Book Company.

Hodge, _Nature Study and Life_, Chaps. XVI, XVII. Ginn and Company.

Sharpe, _Laboratory Manual_, pp. 195, 204-209. American Book Company.

ADVANCED

d.i.c.kerson, _The Frog Book_. Doubleday, Page and Company.

Holmes, _The Biology of the Frog_. The Macmillan Company.

Jordan, _Fishes_. Henry Holt and Company.

Morgan, _The Development of the Frog's Egg_. The Macmillan Company.

Needham, _General Biology_. Comstock Publishing Company.

XVII. HEREDITY, VARIATION, PLANT AND ANIMAL BREEDING

_Problems.--To determine what makes the offspring of animals or plants tend to be like their parents._ _To determine what makes the offspring of animals and plants differ from their parents._ _To learn about some methods of plant and animal breeding._ _(a) By selection._ _(b) By hybridizing._ _(c) By other methods._ _To learn about some methods of improving the human race._ _(a) By eugenics._ _(b) By euthenics._

SUGGESTIONS FOR LABORATORY WORK

_Laboratory exercise._--On variation and heredity among members of a cla.s.s in the schoolroom.

_Laboratory exercise._--On construction of curve of variation in measurements from given plants or animals.

_Laboratory demonstration._--Stained egg cells (_ascaris_) to show chromosomes.

_Laboratory demonstrations._--To ill.u.s.trate the part played in plant or animal breeding by (_a_) selection.

(_b_) hybridizing.

(_c_) budding and grafting.

_Laboratory demonstration._--From charts to ill.u.s.trate how human characteristics may be inherited.

HEREDITY AND EUGENICS

Heredity and what it Means.--As I look over the faces of the boys in my cla.s.s I notice that each boy seems to be more or less like each other boy in the cla.s.s; he has a head, body, arms, and legs, and even in minor ways he resembles each of the other boys in the room. Moreover, if I should ask him I have no doubt but that he would tell me that he resembled in many respects his mother or father. Likewise if I should ask his _parents_ whom he resembled, they would say, "I can see his grandmother or his grandfather in him."

This wonderful force which causes the likeness of the child to its parents and to _their_ parents we call _heredity_. Heredity causes the plants as well as animals to be like their parents. If we trace the workings of heredity in our own individual case, we will probably find that we are molded like our ancestors not only in physical characteristics but in mental qualities as well. The ability to play the piano or to paint is probably as much a case of inheritance as the color of our eyes or the shape of our nose. We are a complex of physical and mental characters, received in part from all our ancestors.

[Ill.u.s.tration: Variations in the Catalpa caterpillar. (Photographed, natural size, by Davison.)]

Variation.--But I notice another thing; no boy in the cla.s.s before me is _exactly_ like any other boy, even twins having minute differences. In this wonderful mold of nature each one of us tends to be slightly different from his or her parents. Each plant, each animal, varies to a greater or lesser degree from its immediate ancestors and may vary to a very great degree.

This factor in the lives of plants and animals is called _variation_.

Heredity and variation are the cornerstones on which all the work in the improvement of plants and animals, including man himself, are built.

The Bearers of Heredity.--We have seen that somewhere in every living cell is a structure known as a nucleus. In this nucleus, which is a part of the living matter of the cell, are certain very minute structures always present, known as _chromosomes_. These chromosomes (so called because they take up color when stained) are believed to be the structures which contain the _determiners_ of the qualities which may be pa.s.sed from parent plant to offspring or from animal to animal; in other words, the qualities that are inheritable (see page 252).

The Germ Cells.--But it has been found that certain cells of the body, the egg and the sperm cells, before uniting contain only half as many chromosomes as do the body cells. In preparing for the process of fertilization, half of these elements have been eliminated, so that when the egg and sperm cell are united they will have the full number of chromosomes that the other cells have.

If the chromosomes carry the determiners of the characters which are inheritable, then it is easy to see that a fertilized egg must contain an equal number of chromosomes from the bodies of each parent. Consequently characteristics from each parent are handed down to the new individual.

This seems to be the way in which nature succeeds in obtaining variation, by providing cell material from two different individuals.

Offspring are Part of their Ancestors.--We can see that if you or I receive characteristics from our parents and they received characteristics from their parents, then we too must have some of the characteristics of the grandparents, and it is a matter of common knowledge that each of us does have some trait or lineament which can be traced back to our grandfather or grandmother. Indeed, as far back as we are able to go, ancestors have added something.

[Ill.u.s.tration: COMPARISON OF s.e.xUAL AND As.e.xUAL CELL REPRODUCTION]

Charles Darwin and Natural Selection.--The great Englishman Charles Darwin was one of the first scientists to realize how this great force of heredity applied to the development or evolution of plants and animals. He knew that although animals and plants were like their ancestors, they also tended to vary. In nature, the variations which best fitted a plant or animal for life in its own environment were the ones which were handed down because those having variations which were not fitted for life in that particular environment would die. Thus nature seized upon favorable variations and after a time, as the descendants of each of these individuals also tended to vary, a new species of plant or animal, fitted for the place it had to live in, would be gradually evolved.

Mutations.--Recently a new method of variation has been discovered by a Dutch naturalist, named Hugo de Vries. He found that new species of plants and animals arise suddenly by "mutations" or steps. This means that new species instead of arising from very slight variations, continuing during long periods of years (as Darwin believed), might arise very suddenly as a very great variation which would at once breed true. It is easily seen that such a condition would be of immense value to breeders, as new plants or animals quite unlike their parents might thus be formed and perpetuated. It will be one of the future problems of plant and animal breeders to isolate and breed "mutants," as such organisms are called.

[Ill.u.s.tration: Improvement in corn by selection. To the left, the corn improved by selection from the original type at the right.]

Artificial Selection.--Darwin reasoned that if nature seized upon favorable variants, then man, by selecting the variations he wanted, could form new varieties of plants or animals much more quickly than nature. And so to-day plant or animal breeders _select_ the forms having the characters they wish to perpetuate and breed them together. This method used by plant and animal breeders is known as _selection_.

Selective Planting.--_By selective planting we mean choosing the best plants and planting the seed from these plants with a view of improving the yield._ In doing this we must not necessarily select the most perfect fruits or grains, but must select seeds from the _best plants_. A wheat plant should be selected not from its yield alone, but from its ability to stand disease and other unfavorable conditions. In 1862 a Mr. Fultz, of Pennsylvania, found three heads of beardless or bald wheat while pa.s.sing through a large field of bearded wheat. These were probably _mutants_ which had lost the chaff surrounding the kernel. Mr. Fultz picked them out, sowed them by themselves, and produced a quant.i.ty of wheat now known favorably all over the world as the Fultz wheat. In selecting wheat, for example, we might breed for a number of different characters, such as more starch, or more protein in the grain, a larger yield per acre, ability to stand cold or drought or to resist plant disease. Each of these characters would have to be sought for separately and could only be obtained after long and careful breeding. The work of Mendel (see page 257) when applied to plant breeding will greatly shorten the time required to produce better plants of a given kind. By careful seed selection, some Western farmers have increased their wheat production by 25 per cent. This, if kept up all over the United States, would mean over $100,000,000 a year in the pockets of the farmers.

Hybridizing.--We have already seen that pollen from one flower may be carried to another of the same species, thus producing seeds. If pollen from one plant be placed on the pistil of another of an _allied_ species or variety, fertilization _may_ take place and new plants be eventually produced from the seeds. This process is known as _hybridizing_, and the plants produced by this process known as _hybrids_.

[Ill.u.s.tration: In hybridizing, all of the flower is removed at the line (_W_) except the pistil (_P_). Then pollen from another flower of a nearly related kind is placed on the pistil and the pollinated flower covered up with a paper bag. Can you explain why?]

Hybrids are extremely variable, rarely breed from seeds, and often are apparently quite unlike either parent plant. They must be grown for several years, and all plants that do not resemble the desired variety must be killed off, if we expect to produce a hybrid that will breed more plants like itself. Luther Burbank, the great hybridizer of California, destroys tens of thousands of plants in order to get one or two with the characters which he wishes to preserve. Thus he is yearly adding to the wealth of this country by producing new plants or fruits of commercial value. A number of years ago he succeeded in growing a new variety of potato, which has already enriched the farmers of this country about $20,000,000. One of his varieties of black walnut trees, a very valuable hard wood, grows ten to twelve times as rapidly as ordinary black walnuts. With lumber yearly increasing in price, a quick growing tree becomes a very valuable commercial product. Among his famous hybrids are the plumcot, a cross between an apricot and a plum, his numerous varieties of berries and his splendid "Climax" plum, the result of a cross between a bitter Chinese plum and an edible j.a.panese plum. But none of Burbank's products grow from seeds; they are all produced _as.e.xually_, from hybrids by some of the processes described in the next paragraph.

The Department of Agriculture and its Methods.--The Department of Agriculture is also doing splendid work in producing new varieties of oranges and lemons, of grain and various garden vegetables. The greatest possibilities have been shown by department workers to be open to the farmer or fruit grower through hybridizing, and by budding, grafting, or slipping.