In general, the part of the body to form earliest is the bra_Natural Wonders

THE CHILDREN’S LIBRARY

NATURAL WONDERS

[Illustration: The Robin Moth]

THE CHILDREN’S LIBRARY

NATURAL WONDERS

EDWIN TENNEY BREWSTER

Garden City — New York

DOUBLEDAY, DORAN & COMPANY, INC.

1928

PRINTED IN THE UNITED STATES AT

THE COUNTRY LIFE PRESS, GARDEN CITY, N. Y.

PREFACE

No small part of our fundamental knowledge concerning the world of

nature has been put into shape for comprehension by children, time

out of mind. “The Swiss Family Robinson” is half natural history,

even if not always of an especially convincing kind; and science of

all sorts, good and bad together, makes up no small portion of Jules

Verne’s uncounted tales. “Cousin Cramchild’s Conversations,” if

there had been such a book, would have embodied the Victorian idea

of what every child should know about his universe; while of actual

books, we elders recall at once Abbott’s “Science for the Young,”

and the half dozen contributions to juvenile knowledge of John

Trowbridge and “Arabella Buckley.” Even the great Ostwald, within

the decade, has made a child’s book on chemistry after the old

conversational form.

In school, moreover, between his geography and his nature study, the

modern child becomes acquainted with not a little modern science,

while in most of our states a detailed acquaintance, by no means

always scientific, with his own physiology is required by law of

every public school pupil. One thing with another, today’s child of

eight or ten is supposed to know a little of physics and of biology,

together with a good deal in a general way of earth science and the

elements of human physiology.

Naturally, there are excellent texts and reading books in all these

fields. So far as I am aware, however, the present work is the first

attempt to set before young readers some knowledge of certain

loosely related but very modern topics, commonly grouped together

under the name, General Physiology. It is, in short, an attempt to

lead children of eight or ten, first to ask and then to answer, the

question: What have I in common with other living things, and how do

I differ from them? Incidentally, in addition, I have attempted to

provide a foundation on which a perplexed but serious-minded parent

can himself base an answer to several puzzling questions which all

children ask—most especially to that most difficult of them all: By

what process of becoming did I myself finally appear in this world?

How far I have succeeded with either task, I leave to the mothers

who shall read this book aloud.

E. T. B.

Andover, Massachusetts

NATURAL WONDERS

CONTENTS

Chap. I — How The Chicken Gets Inside The Egg

Chap. II — Some Other Sorts of Eggs

Chap. III — Little Fishes in The Brook

Chap. IV — Of Plants’ Eggs

Chap. V — What Little Boys And Girls Are Made Of

Chap. VI — More About Living Bricks

Chap. VII — How Much Of Us Is Alive

Chap. VIII — How We Grow

Chap. IX — How We Grow Up

Chap. X — How We Grow Old

Chap. XI — Why We Grow At All

Chap. XII — Things That Do Not Have To Be Learned

Chap. XIII — Why We Like Certain Things

Chap. XIV — Animals’ Games

Chap. XV — Some Instincts Of Chicks And Kittens

Chap. XVI — Certain Stupidities Of Animals

Chap. XVII — How We Differ From The Animals

Chap. XVIII — Something More About Speech And Thinking

Chap. XIX — Why Most Of Us Are Right-Handed

Chap. XX — Where We Do Our Thinking

Chap. XXI — Where Some Of The Animals Do Their Thinking

Chap. XXII — What Plants Know

Chap. XXIII — What Plants Can Do

Chap. XXIV — Some Plant-Like Doings Of Animals

Chap. XXV — The Five Senses And The Other Five

Chap. XXVI — Eyes

Chap. XXVII — Seeing And Believing

Chap. XXVIII — Some Other Senses

Chap. XXIX — The Sight And Hearing Of Ants

Chap. XXX — Ants’ Noses

Chap. XXXI — Some Other Eyes And Ears

Chap. XXXII — Having Senses And Using Them

Chap. XXXIII — Seeing In The Mind’s Eye

Chap. XXXIV — Ear Minds And Others

Chap. XXXV — Living Automobiles

Chap. XXXVI — Air And Fuel

Chap. XXXVII — Men In Glass Boxes

Chap. XXXVIII — Of Sugar And Other Poisons

Chap. XXXIX — Snake Venoms And Others

Chap. XL — Of Measles And Rusty Nails

Chap. XLI — The Great War

Chap. XLII — More About The Great War

Chap. XLIII — Living Apothecary Shops

Chap. XLIV — What Becomes Of The Tadpoles

Chap. XLV — Nature’s Repair Shop

Chap. XLVI — Little Monsters

Chap. XLVII — How The Animals Keep Their Tools Sharp

Chap. XLVIII — Why The Blood Is Salt

Chap. XLIX — Horses’ Fingers

Chap. L — How The Elephant Got His Trunk

Chap. LI — Something Nobody Understands

ILLUSTRATIONS

Frontispiece

The Robin Moth

Full Page Illustrations

How the Chicken Gets Inside the Egg

Seeds That Have Plumes and Wings

The Star-fish Has Eyes on His Arms; The Slug Also Has Eyes on His

Horns; The Snail Has Eyes on His Two Longer Horns

Lymph Cells or White Blood Corpuscles

Extinct Reptiles Which Look Like a Mixture of Alligator, Rhinoceros

and Kangaroo but Their Bones Were More Like the Bones of Birds

In the Text

A Sea-Urchin

Eggs of Perch After Egg Laying

Salmon with Yolk Sac

The Bean Egg Changes to a Bean Plant

Living Bricks Which Make the Skin of a Leaf

Cells of the Inner Tree Pulp

Cells of the Outer Skin of a Leaf

Cells of a Pond Scum Much Enlarged

Three Sorts of Infusoria Much Enlarged

Some Jelly-fish Grown on Stalks and Some Swim About in the Sea

The Cob Is the Mother of the Corn

Pollen Grains Much Enlarged

A Right-handed Person Has All His Thinking Spots on the Left Side of

His Brain

A Sea-anemone

More Common Infusorians, Much Enlarged

The Leaf Has a Spiral Joint on Which to Turn

Optical Illusions

Ear of a Mole Cricket on the Front Leg

Back of the Frog’s Eyes Are the Ear Drums

A Newt

The Leaves Take in Air Through Breathing Holes

In Place of Lungs, Insects Have Breathing Holes

The Minute Animal Which Causes the “Sleeping Sickness”

The Caterpillar Changes into a Moth

Accidents to Growing Fish Eggs

A Two-headed Turtle, a Crab with an Eye on One Side and a Feeler on

the Other, and a Child With Two Great Toes on Each Foot

The Fangs of a Rattlesnake

Early Man Scratched Pictures of the Mammoth on Pieces of Its Own

Bones

The Elephant Has Lost the Front of His Face

Our Single-toed Horse Has Been Made Over from a Four-toed One

How the Chicken Gets Inside the Egg

There is no more fascinating sight to be seen anywhere than an

incubator full of eggs just as the chickens begin to hatch out. You

look through the little glass window in the side and see, at first,

only rows of clean white eggs, dozens upon dozens of them, looking

as if they were all ready to go into the family ice-chest or to be

made into omelets for breakfast.

But they are not. First you begin to hear faint scratchy sounds.

Pretty soon, here and there, a hole breaks through the broad end of

an egg, and a tiny bill sticks out. The little chick is packed so

tightly into the egg that it can move only its head. So it pecks and

pecks; and stops to rest; and pecks again; and the hole in the shell

gets larger and larger; until by and by, the egg cracks open, and a

brand-new chicken draws its first long breath and looks out into the

world.

After that, the chick usually takes a long rest, for it is pretty

tired. When it feels better, it begins to move its legs and wings,

and a half-hour or more after it first began work, it gets clear of

the shell and stands up on wabbly legs, wet, bedraggled, weary, as

disconsolate looking a little object as can well be imagined.

Shortly, however, the feathers, which at first were plastered tight

to the skin, dry off and fluff out, the legs get steady, and soon

there is running about a rolypoly yellow chick, seemingly at least

twice as large as the egg which held him only an hour before. Truly

it is a wonderful sight, five hundred eggs turning into little

chicks in an incubator, for all the world like the kernels of corn

changing to pop-corn in the popper.

But wonderful as it is to see the way a chick comes out of an egg,

it is still more wonderful to see the way it gets in. A fresh,

new-laid egg has no chick inside. After it has been kept warm three

weeks, it has—all ready to come out. The question is how the chicken

got there.

Many different men have studied this question. For the most part,

they have started a dozen or more eggs at once, and then taken them

one by one and two or three hours apart, and cautiously broken them

open to see what was inside. Sometimes, however, a student of eggs

carefully cuts away the shell on one side, until he has made a hole

about the size of a ten-cent piece. Over this he cements a sheet of

glass as thin as paper, so that he can look through this tiny window

into the egg, and see the chick grow.

This is really easier than it sounds. The yolk, as everyone must

have noticed in hard-boiled eggs, does not stay in the middle of the

egg, but always floats to the upper side. The chick, too, always

forms on the upper side of the yolk; and when the egg gets turned

over, the yolk rolls round like a barrel in the water and brings the

chick to the upper side. So the chick, until it grows big enough to

be a tight fit, always lies crosswise of the egg, on the upper side

of the yolk just under the shell.

At first, of course, there is no chick at all, but only a round

white fleck hardly larger than the head of a large pin, on the side

of the yolk where the chick is by and by going to be. Before the end

of the first day after the egg is laid, this little fleck has become

somewhat oval in outline and an eighth of an inch across. Through

its center runs a whiter line, as thick as heavy basting cotton and

a sixteenth of an inch long; about half as large, that is, as an “l”

or a figure “1” in the type on this page.

This is the beginning of the chick. Only it has hardly yet begun to

be a chick, for it has as yet neither head, tail, wings, legs, eyes,

nose, mouth, heart, stomach, brain, nor any other parts. It is in

short, only a tiny line of chicken substance, which is now to begin

to be made into a chicken.

Early in the second day of incubation, the little white line begins

to get thicker on the end where the head is going to be. The brain

and spinal cord appear first; later in the day there is the first

sign of eyes and ears. At about the same time, the heart begins to

form, and the minute blood vessels to grow out into the yolk like

the first roots of a tiny plant. Before the end of the second day,

the heart has begun to beat, and the blood vessels have begun to

absorb the yolk to feed the growing chick. The yolk, in its turn,

feeds on the white; for as everybody knows, the yolk and the white

of an egg are stored up food, on which the little bird can live and

grow until it is old enough to get out of the egg and shift for

itself.

At the beginning of the third day, or a few hours before, the chick,

which has been lying on the yolk face down (only it hasn’t any face

yet), turns over on its left side. It is getting to be a big bird

now, a quarter of an inch long and as thick as a good sized pin.

Next, the brain grows rapidly; and so do the eyes, though these are

not so large as the eyes of the finest needles. Now too, the nerves

begin to form; also the lungs, the stomach, liver, and other organs

of digestion; and there are beginnings of a tail, though without

feathers.

During the fourth day, there are signs of a mouth. Legs and wings,

looking just alike, begin to bud out from the body. Another day, and

one can tell which is which; while now there appear beginnings of

the skull and of the place where the back bone is going to be.

Meantime, the little bird has become more than a half-inch

long—though it does not yet look the least bit like a bird, but more

like a large “?” mark. There is still no front to the body, and the

heart, beating merrily away, hangs out in the yolk.

With the second week, the little chicken does begin to look

something like a real bird. The bones begin to harden; while on the

tip of what has been just an ordinary nose appears a speck of chalk,

which will by and by harden into a bill. The claws begin to grow;

and there are signs of feathers, each one still enclosed in the

little transparent sac in which it forms.

At the end of two weeks, the white of the egg is all used up; and

the little bird, which has been lying crosswise of the egg, now

turns to bring its head toward the broad end. The yolk, too, is

getting small; and on the nineteenth day, the chick pulls the last

remnant into its little tummy, and begins to close over the hole. At

about the same time also, he pecks through into the large air space

which one sees in the broad end of an egg, when he eats it,

hard-boiled, at a picnic. For a week or more, he has been breathing

by means of a sort of gill, much like that of a fish, only that

instead of being on the side of the head like a fish’s it grows out

from the middle of the stomach on a long stalk and spreads over the

inside of the shell. So the chick breathes through the shell, which

is full of minute holes almost too small to be seen. But after the

last bit of yolk has been taken in, this gill shrivels up and drops

off, and the chick breathes with its lungs like the rest of us.

At the end of three weeks, there is nothing left of the egg but the

shell and a tea-spoonful of water. The chick, which began life the

size of a pin head, now fills the shell jam full, with only just

room enough to peck the hole that lets him out. On the twenty-first

day of his imprisonment, out he comes.

[Illustration: How the Chicken Gets Inside the Egg]

Some Other Sorts of Eggs

All birds lay eggs. Some are brown or white like the hen’s egg; some

are green, some buff, some blue; many are speckled. Some, like the

eggs of the screech owl, are almost as round as marbles; not a few

are so pointed at one end as to be fairly pear-shaped. The

hummingbird’s egg is the size of one’s finger tip, the ostrich egg

is as large as one’s head. But all alike they have shell and yolk

and white; and by and by, a little bird inside. Only sometimes, like

the chick, the little bird hatches out with feathers grown, and only

needs to dry off and get its breath, before it is ready to run about

and pick up a living for itself; and sometimes, like the little

robin, it has no feathers, cannot stand up on its legs, and has to

be fed by its parents, like a human baby.

Snakes and turtles have eggs also, very much like birds’ eggs. Like

these, they have white and yolk; and the little reptile grows in the

egg almost exactly like the little bird. For curiously enough the

turtles, snakes, lizards and crocodiles, tho they look so very

different from birds, are really very like them. They all have large

eggs, with large yolks; and the little animal begins at a point in

the side of the yolk, and does not, for a long time, fill the entire

egg.

Oddly enough, there does not seem to be much connection between the

size of an animal and the size of its egg. Big birds, to be sure,

have big eggs; and little birds have little eggs. But a great

crocodile, fifteen or twenty feet long and able to bite a man in

halves, is hatched from an egg no larger than that of a goose. The

little salt water minnow, or killifish, which is only as long as

one’s finger, has very large eggs, for a fish, almost as large as

small blue berries, and quite as large as the eggs of salmon and

trout which grow to be a hundred times heavier. But cod fish, which

sometimes are almost as large as a man, and the great sturgeons,

which are as long as three men and as heavy as a horse, have eggs

not much larger than the periods on this page, smaller even than

those of a tiny ant. As for the little sea creatures, star-fish,

sea-urchins, and the like (which to be sure, are quite as large as a

hummingbird or a wren) their eggs are but fine dust, which cloud the

water and are too small to be seen at all.

However, the smaller the eggs, the more of them there are, to make

up. While some birds lay only two eggs at a time, and few more than

a dozen, some fishes lay a hundred or more, the cod a hundred

thousand, and the sturgeon two or three million.

[Illustration: A Sea-Urchin]

Sometimes, when one is poking about in the brooks in the spring of

the year—as every boy and girl should do, for it is great fun—one

happens upon masses of transparent jelly half as large as one’s

head, full of tiny black dots. These black dots, which are just

about the size of the o’s in this book, are the eggs of frogs. If

instead of being in round masses, they are in long strings, a yard

sometimes in length, then they are almost always the eggs of toads;

but if they occur neither in masses nor in strings, but separately,

then they are the eggs of newts.

It seems strange that a frog should be able to lay a mass of eggs

and jelly forty or fifty times larger than the frog itself. The real

egg, however, is only the dark speck; and this when it is first laid

has only a thin coating of jelly, hardly thicker than paper and

nearly dry. As soon, however, as it touches the water, this dry

jelly begins to swell, and goes on swelling and swelling for three

hours until it is a hundred times larger than it was to start with.

These balls of frog’s eggs look, then, very much like tiny hens’

eggs with black yolks, broken into a bowl ready for cooking. They

really are not quite this; because the frog’s eggs have no shell and

no white, being simply yolk and nothing else. In fact, the only

sorts of eggs that do have white are those of birds and reptiles;

while few others have shells either. The jelly of frog’s eggs is not

“white,” because it is not meant for the little frog to eat, but to

keep other creatures from eating him. Besides this, it helps to keep

the little chap warm.

You will recall that the little chick begins as a tiny dot on one

side of the yolk, and keeps growing larger and larger until it uses

up both yolk and white and fills the entire shell. Not so the little

frog. Always, from the very beginning, it is as large as the egg. It

is the egg, in fact. You can see that the egg is dark above and

light below just as the tadpole will be, and the frog after him. At

first, however, the baby tadpole does not have any parts or members.

He gets in proper succession, eyes, ears, backbone, brain, skin,

tail, and the rest; but he does not grow any larger until he hatches

out, wriggles his way thru the jelly, and begins to eat.

At first the tadpoles are very tiny, only a quarter of an inch in

length; and they cling in tufts to the under side of the water

plants. After that, I suppose, everybody knows what happens.

There is still another curious difference between hens’ eggs and

frogs’. When a frog lays an egg, that egg is nothing else but just

egg—the little frog has not begun at all to form inside it. But when

a hen lays an egg, while there is no little creature in that either,

still the egg has already begun to get ready to turn into a chick.

Some animals go farther than this, so that when their eggs are laid,

the little creature is already formed inside, and so has only the

last part of his growing left to be done outside. Certain fishes,

certain reptiles, and various other animals besides, actually put

off laying the eggs until so late that the young is all ready for

hatching. Such eggs are laid and hatched at the same time, or even

hatched first and laid afterwards.

All the four-footed creatures which have fur and hair, horses,

cattle, dogs, cats, monkeys, and the like, manage in this way. And

because this kind of egg doesn’t get knocked about, it does not need

to have either hard shell nor thick jelly to protect it, but only a

thin skin. For this reason, and because the egg hatches a few

moments before it is laid, people are apt to miss it entirely, and

so to get the idea that these animals have no eggs at all. But they

have—one egg for each little animal.

We pretend that the bunny rabbits at Easter are hatched from the

colored Easter eggs. They really are hatched out of rabbits’ eggs.

No one notices the remnants of the rabbits’ egg, because what little

there is soon dries up to almost nothing, or else the old mother

rabbit eats it. Besides, one has all one can do to look at the new

bunnies. Nevertheless, all little animals come out of eggs, puppies,

colts, lambs, calves, kittens, every kind of living creature that is

big enough for you to see, and a good many besides that are so small

that you have to look for them with a microscope.

III

Little Fishes In The Brook

Of all eggs, the most interesting, I think, are the fishes’. Nearly

all of these are pretty small, little round whitish globules like

sugar pills. Some, like the eggs of trout and salmon which one finds

in the gravel banks of rapid streams, are as large as fair-sized

beads. Many, like the eggs of sea fish which float near the surface

of the ocean, would go thru the eye of a darning needle.

The point, however, which makes them especially interesting is that

so many of them are like tiny glass marbles. The membrane around

them is so clear, and the substance of the egg itself so

transparent, that with a magnifying glass, one can look right thru

the egg, and see the little white fleck inside grow from nothing at

all to a real fish, long enough to reach clear round the egg and lie

with its tail almost in its mouth.

Some eggs are much clearer than others. The clearest are, at first,

like clear glass, so that they can not be seen at all under water.

Soon, however, a tiny vague white spot begins to form on the lower

side. Then one can make out that the egg is covered with a rather

thick membrane, that within this is a narrow, clear space filled

with water, while within this and still smaller, floats the tiny

yolk which is the real egg that is going to become the little fish.

[Illustration: Eggs of Perch after Egg Laying.]

The white spot on the yolk is not itself fish, but only fish stuff,

which is being made ready to turn into fish by and by. The spot

grows larger and thicker, until it looks like a round dab of putty

stuck on the side of a marble. When this cap has grown until it is

about half as much in diameter as the egg itself, it thins in the

middle and thickens at the edges, until it forms a ring. A very

strange thing, thereupon, happens to this ring. It begins to grow;

and as it grows, it keeps slipping farther and farther round the

egg. Soon it has become a band round the middle of the egg. Then as

it moves along still farther to the other side of the egg, it has,

of course, to grow smaller in order to fit. So it does, and the

extra length taken in at one point in the ring, forms the body of

the little fish. The head has already begun to form from a

thickening at one side of the ring before it passed the middle of

the egg. The two sides of the ring form the two halves of the body.

But the tail being easier to make, does not grow out until much

later.

Now there is a head and a body; but the only difference is that the

head is bigger. Neither has any parts. There are no eyes, ears,

nose, or mouth in one; nor any fins, backbone, stomach, nor scales

in the other. These all appear later, much as in the chick—eyes,

ears, brain, and heart early; fins and tail, scales and the whole

front of the body not until long afterwards.

Many learned men have spent their entire lives in studying the way

in which all these various parts form in the young animal, and a

most strange and fascinating study it is, quite worth any man’s

spending his life on. If I were to tell all that is known about the

least part of one fish, the tale would fill up this entire book and

leave no room for anything else. I shall, therefore, tell about the

eyes only—partly because they are interesting and important organs;

but more because they happen to be parts of the body which form in

the same manner in all animals that have a backbone, whether they

are fishes, frogs, birds, four-footed beasts, or human beings. The

eyes with which you, my reader, are reading this page, grew in the

way I shall describe, as I have myself seen it in the egg of cod and

sea-bass.

In general, the part of the body to form earliest is the brain. Next

after that come the eyes. These begin as two buds which grow out one

on each side of the brain where the head is going to be. Each is a

hollow, bubble-like affair on a short stalk; as much as anything,

except for size, like a hollow rubber ball stuck on a pencil stub.

One would think that this hollow ball would simply change into an

eyeball; but it doesn’t, for Nature rarely does things simply.

Instead, one side of the eye-bud folds in, as you might push in a

hollow rubber ball with your finger, until it forms a cup. This cup

is the eyeball. The sides grow out until the hole narrows down to

the dark opening in the middle of the eye which we call the pupil.

Various kinds of eye-stuff grow over the edge and form the interior

parts of the eye; other tissues on the outside thicken the walls and

form the transparent cornea in front; and while the pupil is still

large, a portion of the substance which is later to become the skin,

buds into the eyeball to form the lens of the eye. The reason, then,

for this round-about process, this doubling in of the original eye

bud to make a cup, which afterwards closes down to the eyeball we

finally use, is to get various substances inside the eye, and

finally to leave a pupil for the light to enter.

[Illustration: “Salmon With Yolk Sac.”]

Thus far, like the little chick, the little fish has had no front to

its body. It lies on the yolk, curled round it like a child with the

stomach ache hugging a pillow. By and by the tail grows out free of

the yolk. The head also lifts clear, and the lower jaw has room to

form. Last of all, the sides of the body grow completely round the

yolk, and put it where it will do the most good.

Now the fish is ready to hatch. For some time it has been giving

occasional wiggles inside the egg membrane; finally it breaks thru

and floats out. It is a tiny helpless creature, still more than half

yolk. It cannot swim, but floats, belly up, and mouth wide open, not

yet able so much as to close its jaws.

From this time on, the fish grows rapidly, living on the yolk, which

grows smaller and smaller. At first the little creature floating on

its back can only give an occasional wiggle. As the yolk becomes

more manageable, the fish wiggles more. Soon it turns for a moment

on its side, then clear over; and by the time the last of the yolk

has disappeared, it is swimming right side up and has begun to eat

the still tinier water creatures which are its food. At this stage,

if it is a fresh water fish, it begins to be visible in the shallows

in schools of minute, but veritable, fishes a quarter inch long and

mostly eyes.

Of Plants’ Eggs

The plant’s egg is, of course, the seed. We commonly say that the

plant grows from the seed. And so it does. Yet this is not exactly

true either, because the ripe seed is already a little plant, folded

up tight and packed away in a hard case, like a chick inside its

shell.

If one takes, for example, an ordinary bean or a peanut, peels off

the shell and opens it carefully, it separates into two halves, held

together by a little nodule at one end. These two halves, which

together form pretty much the entire bean, are really two fat

leaves. They are the yolk of the bean egg, on which the new bean

plant is going to feed until it has grown leaves and root, so that

it can pick up a living for itself out of the earth and air.

The rest of the new plant is the little nodule which lies between

these seed-leaves. Curled up against the outside of the seed, like a

puppy’s tail between its legs, is a short fat root; while hidden

away between the seed-leaves is the next pair, tiny leaves almost

too small to see, but real leaves nevertheless.

So the bean is an egg. Not a new-laid egg, but an egg with a little

plant inside, all ready to hatch out and grow.

[Illustration: The bean egg changes to a bean plant.]

If instead of cooking and eating the bean, we plant it in the

ground, or in wet sawdust or blotting paper, it soon hatches out.

The shell drops off, the seed-leaves first take in water and swell

and then shrink away to nothing as the growing plant eats them up.

The little root grows down, the little leaves grow up, the whole

plant turns green and begins to climb the bean pole.

All seeds, then, are eggs just ready for hatching. They are like

fish eggs, however, rather than like birds’ eggs, because the little

fish and the little plant both save most of their yolk to use in

getting a start in the world after they are hatched out. But the

birds, you will recall, because they have large eggs and plenty of

room inside, keep on growing till the yolk is all gone, and then

hatch.

The little plant, as you might expect, gets inside its seed almost

exactly as the little bird or frog or fish gets inside its egg. The

“string beans” which we eat in the summer are fat pods stuffed out

with bean-stuff to be used in making seeds. There are tiny beans

inside, which are new-laid bean eggs, and so have no little plant

inside, but only bean-stuff. The little bean plant, starting from

nothing, forms one part after another, like chick and fish.

But where does the egg come from in the first place? The bean egg

forms in the pod, too small at first to be seen at all, and keeps

growing until it is big enough to begin to form the new plant. In

exactly the same way, the mother fish, and the mother frog, and the

hen, have a sort of pod inside them. First this pod stuffs and

fattens itself out with egg-stuff, like a string bean. Then some of

it turns into little eggs, too small to be seen. These grow and

grow, like the beans in the pod, while the pod shrinks away. Only

after they have grown a great deal, do they begin to form little

beans, or fish or chicks inside.

[Illustration: Seeds that have Plumes and Wings]

First of all, in short, the bean pod begins as a minute speck and

grows into a proper pod. Then the bean inside this pod begins as a

minute speck, and grows into a proper bean. Then the new bean plant

inside this bean begins as a minute speck and grows into a proper

bean plant, ready to be hatched out and shift for itself. So part of

the mother plant becomes pod, and part of the pod becomes bean, and

part of the bean becomes little growing plant. So it is with little

fishes and little birds and little rabbits and puppies and kittens

and all the rest of the little animals that you know.

What Little Boys and Girls are Made Of

“What are little boys made of, made of?

What are little boys made of?

Snaps and snails and puppy dog’s tails;

That’s what little boys are made of.

What are little girls made of, made of?

What are little girls made of?

Sugar and spice and all that’s nice;

That’s what little girls are made of.”

So says the old nursery rhyme. It has this much truth in it, that

little boys and little girls are far from being alike, and it isn’t

worth while to try to make either one over into the other. What

little boys and girls are really made of, and all other living

things as well, is a much longer story.

Oddly enough one can tell this story more simply by telling first

about little star-fish and sea-urchins, and what they are made of.

Star-fishes’ and sea-urchins’ eggs (for the two creatures are really

very much alike, for all they look so different) are much like the

eggs of fishes. They are round and transparent, and so minute that

they look like fine red dust in the water. Naturally, therefore, few

people ever see them at all.

Each of these eggs is a tiny drop of fluid substance with a very

thin skin round it. It is in fact, not unlike a toy rubber balloon,

filled with thin jelly mixed with oil, and set floating in the

water.

This then is the young egg, before there is any sign of a growing

creature inside. One would perhaps expect to see the oil and jelly

mixture change gradually into a star-fish. Instead of this, however,

this little balloon-like affair splits squarely in two, and makes

two little balloons just alike, which lie side by side and more or

less flattened against one another, like two soap bubbles blown from

the same pipe. In about a half hour, each of these balloons or

bubbles, “cells” as they have come to be called, has divided again;

so that now there are four. The four soon become eight; the eight,

sixteen. In the course of a few hours, there are hundreds, all

sticking together and all very minute; so that the whole mass looks

like the heap of soap bubbles which one blows by putting the pipe

under the surface of the soap suds.

So the first single cell of the new laid egg, small as it was, has

become several hundred still smaller. These, however, are not yet

star-fish, but only star-fish-stuff, arranged in a little pile like

a heap of bricks, and all ready to build into a star-fish.

Now if a man is building a house out of bricks, he piles the bricks

near where the house is going to be; and then he takes them, a few

at a time, and cements them into his wall. Not so the star-fish

house. This has to be built right in the living brick pile. It is as

if we dumped a heap of bricks in a field, and then each brick of its

own accord got up and went to its proper place in the house. The

little ball of cells which is the egg, begins to swell, and fold,

and move. It pushes out one part here, and doubles in another there.

The cells divide rapidly in one place, and form a thick solid bunch;

in another they spread to a thin sheet. By and by, there is a little

creature; not indeed a star-fish, but something with a stomach and

an outside skin, and between the two, certain nondescript cells,

which later on are going to make the hard skeleton and the muscles.

After this, the cells still keep on dividing, but instead of getting

smaller and smaller, they wait each time they divide till they grow

to full size again. Thus the baby star-fish grows. And by growing

fast in some places, and slowly in others, and in still others not

growing at all, it changes at length into a veritable star, altho no

bigger than a grain of sand.

All eggs change into little animals in this same way. The hen’s egg

yolk is such a cell—a thin skin filled with oil and jelly. The

frog’s egg is another, with one side colored black. The fish egg is

like the others, with an especially clear jelly that one can easily

see into. Frog eggs and star-fish eggs and sea-urchin eggs, most

sorts of eggs in fact, split fairly in two the first time they split

at all, the whole yolk divides and the little animal, from the first

moment when there is any at all, is always as large as the egg. But

birds’ eggs, most fish eggs, and some other sorts too, are so loaded

down with fat that the egg does not divide clear thru, but as I have

already explained, only at a tiny spot on one side where the jelly

is thickest. But whether this pile of minute cells which is the heap

of little animal bricks, is a small spot on the side of a large egg

or the whole of a small one, it all comes to the same thing in the

end. When the proper moment arrives, the living cell bricks move to

their appointed stations, and the new creature begins to form.

Now we know what little boys and girls are made of. They are built

of enormous numbers of these living bricks which we call cells, just

as other living creatures are. All of us, men or animals, trees,

bushes, or grass, were once, each of us, just one single round cell

which divided, and divided, and divided again, until it became a

vast number. Out of this vast number the new plant or animal builds

itself.

If it is an animal like ourselves, this body stuff, before it

becomes a body, is a round ball. A furrow doubles in along the place

where the back is to be, and becomes the spinal cord. A rod strings

itself along underneath this, and becomes the backbone. The front

end of the spinal cord grows faster than the rest, becomes larger,

and is the brain. The brain buds out into the eyes. The outer

surface of the body, not yet turned into skin, buds inward and makes

the ear. Four outgrowths come down from the forehead to make the

face. The limbs begin as shapeless knobs, and grow out slowly into

arms and legs. Sometimes these make a mistake at their ends, and

split into six fingers or toes instead of the customary five. Then

if the little creature is a human baby, the Doctor has to cut one of

these off; but if it’s a kitten we say it has double paws and will

be a good mouser—tho really I don’t suppose it makes the least

difference.

Most of our growing, then, is just the increase in numbers of these

little living bricks. There is a spot at the bottom of each

finger nail where the nail cells are dividing and pushing out the

finger nail. The white spots in the nail do not mean that one has

been telling white lies, as some people say. They come because one

happens to bruise the soft “root” of the nail where the nail cells

are new and easily hurt like the soft flesh of a little child.

When we were very much younger than we are now we had no teeth. As

biting-time drew near, the cells of the thin skin which lines the

mouth began to multiply so rapidly where the two gums touch one

another that they soon formed a thick ridge growing back into the

jaw. A little later, and this ridge continued to grow at twenty

separate points while it stopped growing everywhere else. Soon these

twenty growing points opened up into twenty pockets. From the bottom

of each pocket grew up a tooth; while from the side of each there

budded out another pocket in which, when the baby is eight years or

more old, the second teeth form. But the three back teeth in each

side of a man’s jaw, tho they come late and are the largest he has,

really belong to the first milk set, the rest of which he lost as a

child.

Even the hair grows by the division of cells at the inner end of the

little bulb which you see on the end of the hair when you pull it

out and look at it against white paper. Just between hair and skin

is a spot which is neither hair nor skin, where all the growing of

the hair is done.

So we are not built like a cement or a wooden house, but like a

brick one. We are made of little living bricks. When we grow it is

because these living bricks divide into half bricks, and then grow

into whole ones again. But how they find out when and where to grow

fast, and when and where to grow slowly, and when and where not to

grow at all, is precisely what nobody has yet made the smallest

beginning at finding out.

More About Living Bricks

The largest of these living bricks is the yolk of an ostrich egg;

since this is, of course, like all eggs before they begin to grow, a

single cell. The smallest known are certain of the bacteria and

germs which float about in the air, and are so minute that they

cannot be made out even with the strongest microscopes. All one can

see is that there is something there; something which if placed a

thousand in a row, would still not reach across a grain of dust.

Few cells, however, are as small as bacteria on the one hand, nor

anything like as large as the yolks of birds’ eggs on the other.

Many are just comfortably visible to the unaided eye. But the great

mass of cells which make up our own bodies, the bodies of other

animals, and of plants are a little too small to be made out with a

common pocket lens, tho an ordinary microscope shows them with ease.

While the egg yolk is dividing to form the first hundred or more

living bricks out of which the little animal is to be built, the

cells are all about alike, generally round except where they are

flattened against one another. As soon, however, as they begin to

move about into place to build the new animal, they begin themselves

to change. Some remain small; others grow large. Some grow out into

long strings, and become muscle fibers or nerve. At one point, many

thousands together swell up with oil and become fat. At another,

more thousands build themselves about with hard lime phosphate, and

become bones and teeth. Those which form within them little brown

granules, give the color to hair and skin. The blood is colored red

by the coin-shaped cells which float in it. In certain parts of the

eye, on the other hand, the cells have to remain perfectly clear and

colorless, else the light could not come thru and we should never

see truly.

When an animal is very young indeed, long before it is ready to

leave the egg, the whole outer surface of its body is covered with a

single layer of these cells. They are packed closely together, and

flattened against their neighbors so that the sheet of cells is not

unlike, on a small scale, the marble floor of a public building or

the block pavement of a city street. Like other living cells, these

grow, and divide. They cannot grow sidewise, for the space is

already filled; nor inward for that way lies the entire body. So

they split off a piece of their outer ends. Then they do it again,

and yet again; until the outer skin of the body, from being one

layer of cells in thickness has become many.

Only the original inner layer, however, grows and divides. The split

off ends dry up to a roundish cracker shape, grow hard and homy, and

become the thin outer skin of the body, which we run pins and

needles under, and pull off or scrape off when we “bark” our shins,

without hurting. This part of the skin is dead. It gets rubbed off

by our clothes, or soaks off in the bath tub and has to be scrubbed

off the sides. But as fast as it is removed on the outer surface, it

grows again from the living bottom layer. No matter how old one

gets, this lower layer of the skin continues to split off the outer

ends of its cells, just as it did before there was any proper skin

at all. Most parts of the body grow thruout their mass; but the skin

grows only on the inner side.

On the palms of the hands and the soles of the feet the skin grows

very rapidly and is especially horny. When one works with his hands

more than he is accustomed, the first effect is to wear the skin

thin and sore, or to pull it loose from the bottom layer and make

blisters. In the end, however, the rubbing only makes the live skin

work faster, until it builds great homy callouses that no work can

wear thru. But when our boots do not fit and rub in one spot, this

also starts up the live skin to working hard. First thing we know,

we have a corn. For a corn is only an especially hard and thick

callous, where the living skin made a mistake and grew too much in

one little spot.

Each finger nail and toe nail is a sort of corn. It grows from a

fold of skin, forming from the bottom layer like any skin, but it is

especially homy, even more horny than the hardest callous. The hair,

also, is a sort of corn. The skin doubles in to form a minute

pocket; and at the bottom of this pocket this same living under

layer of the skin grows into a narrow shaft of cells, dry and dead

and homy like skin and nails.

The horns of animals, too, are only thick hard skin. Sometimes they

have a core of bone inside, but the outside is just a special sort

of skin. Wherever we go in the body, there we find some special sort

of cell. They may be large, small, thick, thin, long, round, soft,

hard. They may build this, that, or the other thing around them.

They may have this, that, or the other thing inside. But in one way

or another the whole body, from head to heels, is built of these

cells and their products.

[Illustration: Living bricks which make the skin of a leaf. Five pairs

of these are the lips of breathing holes.]

It is the same way with the plants. They too are built of these

living bricks. Each leaf and blade of tree or grass is covered with

a sheet of colorless cells one layer deep, which one can often peel

off from the green pulp underneath. The green pulp, in turn, is a

rather loose pile a half dozen thick, of roundish brick-shaped

cells, each containing scattered grains of green coloring matter.

The solid wood of a tree is only the thick walls of long slender

cells, overlapping at the ends and packed tightly together. These

cells lie lengthwise of the tree; that is why wood splits with the

grain so much easier than it cuts across it.

[Illustration: Cells of the inner tree pulp. The rings show that the

tree is three years old.]

[Illustration: Cells of the outer skin of a leaf. At the bottom is

the mouth of a breathing hole.]

I have already said that at the time of year when the tree is

growing rapidly, these woody cells are large; but when the tree is

growing slowly, they are small. So each year there is a change from

large cells formed in the spring to smaller ones grown in the fall.

The next year, the living substance of the cell moves off to the

growing region next the bark, and leaves the old wood cells empty.

These, therefore, never change; and because the large cells and the

small ones do not look quite alike, we see the annual rings of wood

in the tree trunk, as thick as card board, which give us the light

and dark lines in our furniture and our hard wood floors. From these

one can tell, not only how old the tree is, but also what were its

good years when it grew rapidly, and what its poor seasons when it

hardly grew at all. If a drought came along any summer, or if

insects one year ate off all the leaves, that too shows in the wood.

But trees which grow in the tropics, where they keep growing the

whole year thru, do not have annual rings.

While some cells of the tree form wood and some green pigment,

others in the bark produce cork, as one can see nicely in the thin

layers of cork in the bark of an elm. The cells of juicy fruits

swell up with water, and form sugar and various flavoring matters

and pleasant acids. Where the animal cells swell up with oil and

become fat, the plant cells swell up with starch grains and become a

potato or the thick seed-leaves of a bean. But other cells form gum,

rosin, turpentine, pitch, and the various oils and the like,

pleasant or bitter, which we use for food and medicines.

So the plant, like the animal, is just a great mass of different

sorts of these living bricks, and of the various substances which

they form within and around them.

Naturally it takes millions upon millions of these living bricks to

build up the body of a man or an apple tree, still more of a whale

or one of the giant redwood trees of California. Many humbler

creatures, on the other hand, both animals and plants, contain

comparatively few. Our common green pond scums, for example, which

tho they are plants, have neither leaves nor stems nor roots, are

like single long lines of tiny green barrels set end to end. Our

common sea-lettuce is a sheet of cells only one layer thick; while

other sea-weeds and water plants are but bundles of a score or more.

Often the fewer such bricks there are, the larger they are; even at

times, to a half-inch in thickness and an inch or more in length.

[Illustration: Cells of a pond scum much enlarged. The green living

substance flowing from one to another unites to form an egg or spore.]

A vast number of plants and animals, moreover, are single cells.

Such among plants are the yeasts with which most of us make our

bread, and a few of us brew our beer. Such also are the hundreds of

different sorts of bacteria, which tho some of them are the germs of

various catching diseases, are for the most part useful enough. But

of these we shall learn more by and by. The green spots and patches

on the bark of old trees and fences, and sometimes even on damp

earth, are due to enormous numbers of minute plants, green with the

same green pigment as the leaves of the largest tree; while the

green tint of the gray lichens on rocks and tree trunks is caused by

similar single-celled plants which grow among the white fibers of

the lichen proper. Besides these, there are many like plants which

float about in fresh water, each a single cell.

[Illustration: Three sorts of infusoria much enlarged.]

The diatoms which one finds in the mud at the bottom of ditches and

mud-puddles, tho they have shells and move about, are usually

counted among plants; but the water of most ditches and puddles

swarms with amoebas, infusoria, animalcules of various sorts, most

of them large enough to be made out with the unaided eye when seen

in a tumbler against the light, and each a single cell.

Many animals, then, and many plants are just one single cell and no

more. Many others, like pond scums and sea-lettuces,—which are

plants,—and sponges and jelly-fishes,—which are animals,—are

composed of many cells, but all pretty much alike. But the animals

and plants which we know best, kittens and oak trees and horses and

grass, and the creatures we know best of all which are ourselves,

are made up of many cells, and many different sorts—skin and bark

and wood, flesh and fat and leaves and hair and all the rest, so

many that it would take half an hour merely to write them all down.

[Illustration: Some jelly-fish grow on stalks and some swim about

in the sea.]