of animals; others, like the weevils, feed on the hard, dry substance of seeds and grains; others, like the grasshoppers and caterpillars, eat green leaves; and still others eat other insects. The alimentary canal of each of these kinds of insects differs more or less from that of the other kinds. The specialization of the alimentary canal depends then upon the necessity for a large food-digesting and absorbing surface, and on the complex treatment of the food. The character of this specialization in each case depends upon the special kind or quality of food taken by the animal in question.

45. The mutual relation of function and structure.-The structure of an animal depends upon the manner in which the life processes or functions of the animal are performed. If the functions are performed in a complex manner, the structure of the body is complex; if the functions are performed in simple manner, the body will be simple in structure. With the increase in degree of the division of labor among various parts of the body, there is an increase in definiteness and extent of differentiation of structure. Each part or organ of the body becomes more modified and better fitted to perform its own special function. A peculiar structural condition of any part of the body, or of the whole body of any animal, is not to be looked on as a freak of Nature, or as a wonder or marvel. Such a structure has a significance which may be sought for. The unusual structural condition is associated with some special habit or manner of performance of a function. Function and structure are always associated in Nature, and should always be associated in our study of Nature.

CHAPTER V

THE LIFE CYCLE

46. Birth, growth and development, and death.-Certain phenomena are familiar to us as occurring inevitably in the life of every animal. Each individual is born in an immature or young condition; it grows (that is, it increases in size), and develops (that is, changes more or less in structure), and dies. These phenomena occur in the succession of birth, growth and development, and death. But before any animal appears to us as an independent individualthat is, outside the body of the mother and outside of an egg (i. e., before birth or hatching, as we are accustomed to call such appearance)—it has already undergone a longer or shorter period of life. It has been a new living organism hours or days or months, perhaps, before its appearance to us. This period of life has been passed inside an egg, or as an egg or in the egg stage, as it is variously termed. The life of an animal as a distinct organism begins in an egg. And the true life cycle of an organism is its life from egg through birth, growth and development, and maturity to the time it produces new organisms in the condition of eggs. The life cycle is from egg to egg. Birth and growth, two of the phenomena readily apparent to us in the life of every animal, are two phenomena in the true life cycle. Death is a third inevitable phenomenon in the life of each individual, but it is not a part of the cycle. It is something outside.

47. Life cycle of simplest animals.-The simplest animals have no true egg stage, nor perhaps have they any true

death. The new Amabæ are from their beginning like the full-grown Amoeba, except as regards size. And the old Amaba does not die, because its whole body continues to live, although in two parts-the two new Amabæ. The life cycle of the simplest animals includes birth (usually by simple fission of the body of the parent), growth, and some, but usually very little, development, and finally the reproduction of new individuals, not by the formation of eggs, but by direct division of the body.

48. The egg. In our study of the multiplication of animals (Chapter III) we learned that it is the almost univer

FIG. 38.-Eggs of different animals showing variety in external appearance. a, egg of bird; b, eggs of toad; c, egg of fish; d, egg of butterfly; e, eggs of katydid on leaf; f,egg-case of skate.

sal rule among many-celled animals that each individual begins life as a single cell, which has been produced by the

fusion of two germ cells, a sperm cell from a male individual of the species and an egg cell from a female individual of the species. The single cell thus formed is called the fertilized egg cell, and its subsequent development results in the formation of a new individual of the same species with its parents. Now, in the development of this cell into a new animal, food is necessary, and sometimes a certain amount of warmth. So with the fertilized egg cell there is, in the case of all animals that lay eggs, a greater or less amount of food matter-food yolk, it is called-gathered about the germ cell, and both germ cell and food yolk are inclosed in a soft or hard wall. Thus is composed the egg as we know it. The hen's egg is as large as it is because of the great amount of food yolk it contains. The egg of a fish as large as a hen is much smaller than the hen's egg; it contains less food yolk. Eggs (Fig. 38) may vary also in their external appearance, because of the different kinds of membrane or shells which may inclose and protect them. Thus the frog's eggs are inclosed in a thin membrane and imbedded in a soft, jelly-like substance; the skate's egg has a tough, dark-brown leathery inclosing wall; the spiral egg of the bull-head sharks is leathery and colored like the dark-olive seaweeds among which it lies; and a bird's egg has a hard shell of carbonate of lime. But in each case there is the essential fertilized germ cell; in this the eggs of hen and fish and butterfly and cray-fish and worm are alike, however much they may differ in size and external appearance.

49. Embryonic and post-embryonic development. Some animals do not lay eggs, that is they do not deposit the fertilized egg cell outside of the body, but allow the development of the new individual to go on inside the body of the mother for a longer or shorter period. The mammals and some other animals have this habit. When such an animal issues from the body of the mother, it is said to be born. When the developing animal issues from an egg

which has been deposited outside the body of the mother, it is said to hatch. The animal at birth or at time of hatching is not yet fully developed. Only part of its development or period of immaturity is passed within the egg or within the body of the mother. That part of its life thus passed within the egg or mother's body is called the embryonic life or embryonic stages of development; while that period of development or immaturity from the time of birth or hatching until maturity is reached is called the post-embryonic life or post-embryonic stages of development.

50. First stages in development.-The embryonic development is from the beginning up to a certain point practically identical for all many-celled animals-that is, there are cer

FIG. 39. First stages in embryonic development of the pond snail (Lymnæus). a. egg cell; b, first cleavage; c, second cleavage; d, third cleavage; e, after numerous cleavages; f, blastula (in section); g, gastrula, just forming (in section); h, gastrula, completed (in section).-After RABL.

tain principal or constant characteristics of the beginning development which are present in the development of all many-celled animals. The first stage or phenomenon of development is the simple fission of the germ cell into halves (Fig. 39, 6). These two daughter cells next divide so that there are four cells (Fig. 39, c); each of these divides, and this division is repeated until a greater or lesser num