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BIOLOGY

CONN

LIBRARY
G

BIOLOGY

AN INTRODUCTORY STUDY

FOR USE IN COLLEGES

HERBERT W. CONN, PH.D.

I '
PROFESSOR OF BIOLOGY IN WESLEYAN UNIVERSITY

SILVER, BURDETT AND COMPANY

BOSTON NEW YORK CHICAGO

BIOtOfiY

PREFACE

THIS work is intended to serve as an introduction to the
study of botany and zoology. It has been for some time recog-
nized that there is a series of laws and principles which relate
both to animal and plant life, and another series of important
facts which refer to the relations of animals and plants to
each other. In helping to a comprehension of nature, these
interrelations are really of more significance than the detailed
study of certain animals and plants. But with the tendency
shown frequently in our educational system, to divide biology
into zoology and botany, there is danger that these fundamental
truths and interrelations be neglected, since a consideration of
them belongs strictly neither to zoology nor to botany.

To students of the age of those in secondary schools, the
study of such concrete facts as the description of animals and
plants is most attractive; and for them, courses in elementary
botany and zoology are eminently appropriate. But to students
of the greater maturity of college grade, the study of the funda-
mental biological laws is more stimulating and better calculated
to develop the thinking powers. It is, therefore, the author's
belief that the proper way for older students to begin the
study of the great department of biology is to consider the
fundamental principles relating to both animals and plants,
before either of these groups is studied in detail. After the
student turns his attention more particularly to zoology or
botany, he is likely to be engrossed in the details of the life
and structure of animals and plants, and so almost inevi-
tably neglects the broader fundamental laws which should
correlate the phenomena of life as one science. Unless, there-
fore, the foundation principles of biology be studied as an
introductory course, it is very probable that they will be
neglected. For this reason, this text has been provided as

iii

544529

ir PREFACE

an introductory survey of the laws which apply to both ani-
mals and plants, and those principles which coordinate and
correlate them. It is hoped that it may have some influence
in developing the study of the fundamental principles of biology
as an introductory course, thus supplanting the old custom of
plunging the student at the outset more specifically into zoology
or botany.

It is designed that this work shall be an elementary study
of biology, on a par with, and parallel to, elementary physics
and chemistry. Logically it should follow, rather than pre-
cede, these two sciences, although it may be taken simultane-
ously with them. Its place in a curriculum should, therefore,
be about the same as that of elementary physics and chemistry;
and as developed in the following pages, it belongs to the be-
ginning of college work.

In preparing these pages, it has been recognized fully that
a certain amount of laboratory work is necessary in order that
the student may properly understand biological phenomena.
It is also appreciated that, with the present development of
the teaching of biology and the present equipment of many of
our institutions, it is frequently impossible to introduce any
extended laboratory work, on account both of insufficient equip-
ment and lack of time in the already crowded courses of study.
For this reason, the chapters have been arranged so that, where
necessary, they can be used without the accompanying labora-
tory demonstrations. Although this is an undesirable method
of studying biology, the author believes that the biological
principles covered in the following pages may be comprehended
in a fairly satisfactory manner, even though the student does
not have the opportunity of making the laboratory tests. It
is hardly necessary to state, however, that as much practical
laboratory work as possible should accompany the study of
the text. For this reason, outlines of the correlative laboratory
work have been added at the end of the chapters. In all cases
where laboratory work is possible, students should be required

PREFACE 7

to make careful drawings of the objects studied. Wherever
time permits, the laboratory work outlined here should be
expanded by instructors. (For more detailed laboratory di-
rections than can be given here, reference should be made to
the many excellent handbooks of zoological and botanical lab-
oratory work, a few of which are mentioned in the brief bibli-
ographies at the close of the chapters.)

In place of the ordinary index there will be found at the
close of the book a glossary-index. In it are given brief defi-
nitions of all the technical words used in the book, with deri-
vations and with page references. To make this more valuable
as a reference glossary, some common biological words which
do not chance to be used in the text are defined. These are
easily recognized from the fact that they have no page numbers.

H. W. CONN.

CONTENTS

APTER PAGE

I. THE SCOPE OF BIOLOGY . . . . . 1
The New Biology and the Old. The Funda-
mental Properties of Living Things. Chemical
Composition of giving Tissues. Origin of
Life. The Biological Sciences: Morphology.
Physiology. Zoology and Botany.

II. CELLS AND THE CELL THEORY . . , . 26
Organisms. The Cell as the Unit of Organic
Structure. Cell Structure. Cell Substance or
Protoplasm. The Nucleus. The Centrosome.
The Cell Wall. Cell Functions. History of
the Cell Doctrine: 1. The Early Conception
of the Cell (1839-1861). 2. Protoplasm and
the Mechanical Theory (1861-1885). 3. The
Nucleus and its Significance (1880 to the
present). What is Meant by Protoplasm.

III. UNICELLULAR ORGANISMS 52

Animals: AmcebcL Paramedwn. Plasmo-
dium Malarice. Chilomonas. Pandorina. In-
termediate Organisms: Peranema. Euglena.
Plants: Pleurococcus. Saccharomyces — Yeast.
• Bacteria.

IV. CELL MULTIPLICATION AND THE CELLULAR

STRUCTURE OF ORGANISMS .... 85
Cell Division or Karyokinesis. Unicellular
and Multicellular Organisms. Penitillium, a
Simple Multicellular Plant. Other Species of
Molds.

vii

viii CONTENTS

CHAPTER PAGE

V. THE CASTOR BEAN, A COMPLEX MULTICELLU-

LAR PLANT . . .103

The Castor Bean (Ricinus Communis). Gross
Structure. Structure of the Stem. Structure
of the Root. Structure of the Leaf. Repro-
ductive Organs.

VI. THE PHYSIOLOGY OF A TYPICAL PLANT . .126
Photosynthesis or Starch Manufacture. Me-
tastasis. Photosynthesis and Metastasis Con-
trasted. Miscellaneous Functions of Plant
Life.

A VII. MULTICELLULAR ANIMALS! HYDRA FuSCA . . 138

General Life Functions of Animals. Animal
Biology. Hydra Fusca, a Simple Multicellu-
lar Animal. The Relation of the Whole Or-
ganism to its Different Parts.

VIII. MULTICELLULAR ANIMALS: THE EARTHWORM

(Lumbricus) 155

Anatomy. Microscopic Anatomy, orJEttstology.

X IX. MULTICELLULAR ANIMALS: THE FROG (Rana).

GENERAL DESCRIPTION 175

X. THE PHYSIOLOGY OF AN ANIMAL . . t . 204
Physiology of the Earthworm.

L THE DIFFERENCES BETWEEN ANIMALS AND
PLANTS: THE MUTUAL RELATIONS OF OR-
GANISMS 217

The Differences between Animals and Plants.
Contrast between the Activities of Animals
and Plants. The Mutual Relations of Organ-
isms. Nature's Life Cycle.

CONTENTS ix

PAGH

REPRODUCTION : SEXUAL AND ASEXUAL METHODS 238

General Types of Reproduction. Reproduction
in Unicellular Organisms. Reproduction in
Multicellular Organisms. Division without
Cell Union. Multiplication by Cell Union.
The Union of the Sex Bodies or Fertilization.
The Relation of the Chromatin to Heredity.
The Purpose of the Union of the Sexes.

DISTRIBUTION OF SEXUAL AND ASEXUAL

METHODS. ALTERNATION OF GENERATIONS 262

Summary of the Methods of Reproduction.
Origin of Sex Union. Distribution of Asexual
Reproduction. Distribution of Sexual Repro-
duction. Reproductive Bodies or Reproductive
Cells. Cross Fertilization the Rule. Alterna-
tion of Sexual with Asexual Methods of Repro-
duction.

XIV. DEVELOPMENT OF THE FERTILIZED EGG . . 280

Embryology and Metamorphosis. Embryology
of the Frog.

XV. THE SOURCE AND NATURE OF VITAL ENERGY . 292

Matter and Energy. The Conservation of
Energy. The Transformation of Energy. The
Living Organism as a Machine. The Life of a
Plant. The Life of an Animal.

XVI. THE MECHANICS OF THE LIVING MACHINE . 303

Details of the Action of the Machine. Vital
Force or Vitality. Summary. What is Life?

x CONTENTS

CHAPTER PAGE

XVII. THE ORIGIN AND DEVELOPMENT OF ORGANISMS:

HEREDITY AND VARIATION" . . . 325

The Origin of the Living Machine Not Ex-
plained. The Forces Which Have Produced
Organisms. Conformity to Type. Divergence
from Type.

XVIII. THE ORIGIN OF THE LIVING MACHINE: ADAP-

TATION; THE FORCES OF ORGANIC EVOLU-
TION 342

Adaptation. The Theory of Evolution.

XIX. CLASSIFICATION AND DISTRIBUTION . . . 364

Classification (Taxonomy) . The Significance
of Classification. An Outline of the Classi-
fication of the Living World. Distribution
of Animals in Space and Time. Distribution
of Organisms in Time: Paleontology.

GLOSSARY-INDEX

387

BIOLOQY

CHAPTER I
THE SCOPE OF BIOLOGY

THE NEW BIOLOGY AND THE OLD

BIOLOGY is often described as the most recent of the sciences,
despite the fact that it was one of the first to be studied. Four
centuries before Christ, animals were dissected and described
by Aristotle, and from that time on, the study of living things
has never ceased. In the last half century, however, the study
of vital phenomena has assumed a new aspect. Formerly
animals and plants were studied only as objects to be classified
and named; now they are studied as objects to be explained.

Progress of Scientific Thought. — This new method of bi-
ological study is only another expression of man's changed
attitude toward all natural phenomena. In early times, people
imagined that all the phenomena of nature which they could
not understand were produced by gods. One god caused the
winds; another the motions of the sun and stars. Gradually
these conceptions have been changed by the attitude of modern
science. First, the motions of the heavenly bodies were ex-
plained under the general law of gravitation. Then, the mys-
terious phenomena of fire and of electricity were comprehended
under the laws of chemistry and physics. Later, the various
changes on the earth's surface, such as the formation of moun-
tains, of valleys, of rivers, and of plains, were explained as the
result of the ordinary forces of nature.

In all this there has been a progress in one direction, namely,
toward the explanation of natural phenomena by natural
forces. The most recent of the natural phenomena to be
studied with this end in view, are those associated with living

2 BIOLOGY

animals and plants. The question whether the activities of
animals and plants can be explained by the same forces found
elsewhere in nature, and the attempt to answer this question
in the affirmative, form the basis of the new science of biology.
Modern biology is thus something more than the study of
animals and plants as dead objects to be collected, named,
and classified. It is a study of animals and plants in action;
as living beings to be related to their environment. It is this
attempt to explain life processes which may be said to have
raised biology to the rank of a new science.

THE FUNDAMENTAL PROPERTIES OF LIVING THINGS

Distinction between the Living and the non-Living. — Since
biology (Gr. bios = \ife -\-logos = discourse) is the science of living
things, we must first ask how living things may be distinguished
from non-living. While it is a comparatively easy matter to
recognize the distinction, it is difficult to draw it sharply.
Indeed, some biologists are of the opinion that no rigid line
can be drawn, and that there are some states of matter which
are halfway between the living and the non-living. Whether or
not this be so, it certainly is true that between most forms
of matter which we call alive and those which we call non-
living, there is a marked and recognizable difference, although
it may be difficult to define it accurately. Four or five fun-
damental properties are characteristic of life:

1. Activity. — The most noticeable difference between the
living and the non-living is in the presence or absence of spon-
taneous activity. If we wish to find out whether any given
body is alive, we watch it carefully to see if it shows any power
of independent activity, and if it does so, we call it alive. If
the object, a seed for example, seems to be perfectly dormant,
we may put it under conditions in which, if alive, it will
develop activity. If it then begins to grow into a plant we say
that the seed was alive at first but dormant. If, however, it
fails to show any power of developing into a plant when placed

THE SCOPE OF BIOLOGY 3

in proper conditions, we conclude that the seed is not alive.
Hence the best criterion that we have for separating the living
from the non-living is to determine whether or not the body
in question either shows any signs of independent activity or,
when put under proper conditions, may be made to show any
signs of such activity.

Automatic activity. — The simple fact of showing activity is,
however, not enough to serve as a criterion of life. Other
things besides living beings have the power of activity. A
watch, or a locomotive, or a steam engine certainly shows
activity, and yet none of these is alive. There is, however,
one distinction between the activity of such machines and
the activity of a living organism. Machines show activity
only when they are started into action by some outside in-
fluence; while a living organism develops activity from its
own internal, independent power. With this modification,
the first criterion that we have for distinguishing the living
from the non-living is the power of developing automatic
activity, and only objects possessing this power do we speak
of as being alive.

2. Death. — The fact that living things show automatic activ-
ity has a converse side. This activity may cease, the object-
losing its power of showing spontaneous activity. This consti-
tutes the phenomenon spoken of as death. To define either
life or death has proved a puzzle to both science and philosophy.
For our purpose, however, they can be fairly well defined as
follows: By life, we mean the possession of the power of show-
ing spontaneous, automatic activity; by death, we mean the
disappearance of this power. Why an animal or plant, when it
dies, loses this power, we do not know. In some cases it is
undoubtedly because the complicated machinery which com-
poses the body is injured and consequently cannot work
properly. This we find true also in the case of ordinary ma-
chines. If a locomotive should burst its cylinders, it would no
longer be able to run. If a watch has its mainspring broken,

4 BIOLOGY

it is thrown out of adjustment and consequently does not
show activity. So in regard to living things; the inability to
show further activity may undoubtedly be attributed to the
fact that the machinery is out of order. If, for example, the
beating of the heart ceases for any length of time, life activity
must cease, because life activity is dependent on the circulation
of the blood. Thus, in many cases we know positively that
death comes from the breaking down of the machine. Whether
death means anything more than the breaking down of the
machine; whether anything is lost which can be called the
life force, is one of the questions over which philosophy and
biology have puzzled for long years, and upon which they
have not reached any definite conclusion.

3. Growth. — All organisms disintegrate by oxidation and
waste. When a piece of wood reaches the required temperature
to unite with the oxygen of the air, it burns. Waste products
appear as gases and ashes, and the wood disappears. In a
similar way, by union with oxygen the living body is being
constantly converted into waste products which are given off
from the body as excretions. As a result the organism is
constantly disintegrating. This would inevitably result in the
disappearance of the organism if it were not for the opposite
power of reintegration, or growth.

All living things have the power of growing, and no object
that is not alive has this power. It is true that, under some
circumstances, crystals may increase in size, and this is some-
times referred to as a growth of the crystals; but it is a totally
different kind of growth from that which we find in living
things. In the case of the crystal, the new material is simply
laid upon the outside of the old, layer after layer, and the
apparent growth is really an increase in size, by the process of
accretion. In the growth of the living organism, material is
taken inside of the body, and there it is transformed into
compounds like those of the living organism which has ab-
sorbed it. Thus the living organism increases from within, —

THE SCOPE OF BIOLOGY 5

a type of growth spoken of as intussusception (Lat. intus =
within + suscipere = to take up). With this understanding of
growth we can state that nothing grows except living things.
As the result of their activities, living things are constantly
wasting away; but by growth they repair and keep pace with
their own wastes and remain in a practically constant condi-
tion, in spite of their ceaseless activity. In time, however,
the disintegrating tendencies surpass the powers of repair, and
the organism dies of old age.

4. Reproduction. — The power of reproduction is found only
in the realm of the animate world, for only a living organism
can produce another like itself. Inanimate things cannot
reproduce their kind.

As a result of this power of reproduction, held in common
by all things possessed of life, there is a constant replacement
of the individual, a constant wearing out and death, a constant
rebirth and growth, the new organism ever replacing the old as
it disintegrates and disappears. There is a constant tendency
to undergo cyclical changes present in all manifestations of life.

5. Consciousness. — Consciousness is characteristic of some
living bodies, but is probably not universal among them, for it
is practically certain that life occurs in many places without
consciousness, although some theorists have endeavored to
argue that all forms of life, even the plants, have a very dim
form of consciousness. This is very doubtful, and we cannot
regard consciousness as universally characteristic of life.
Wherever consciousness is found, however, it indicates the
presence of life, and thus may be deemed one of the most
important signs of life.

CHEMICAL COMPOSITION OF LIVING TISSUES

Chemical Elements in Living Tissues. — Although there is a
large variety of chemical compounds found in living animals and
plants, nevertheless there is a certain uniformity among them.
All animals and plants are made up primarily of a small num-

BIOLOGY

her of elements, nine chemical elements being ordinarily pres-
ent in living things, four of which predominate, while the other
four are present only in small quantities. They are as follows : —

Oxygen, a colorless, odorless gas, forming about one-fifth
of the atmosphere.

Carbon, a solid at ordinary temperatures. Charcoal, graphite,
lampblack, and diamond are examples of almost pure carbon.

Hydrogen, a gas, the lightest of all known substances and
highly inflammable.

Nitrogen, a colorless, odorless gas which comprises about
four-fifths of the atmosphere.

Sulphur, phosphorus, calcium, iron, and potassium consti-
tute the other chemical elements that are found in living

Oxt/ye/j

Mitroq

tfydroqen

FIG. 1. — DIAGRAM SHOWING THE RELATIVE PROPORTIONS

OF THE CHIEF ELEMENTS MAKING UP A LIVING BODY

things. Only very small amounts of these elements are present,
although calcium is found in animals in considerable quantities
in the bone. Figure 1 shows diagrammatically the relative

THE SCOPE OF BIOLOGY 7

proportions of the chief chemical elements in the animal body.
Oxygen, carbon, hydrogen, and nitrogen constitute about
98 per cent of the animal body and not far from the same pro-
portion of the composition of the body of most plants. These
four elements also constitute by far the largest proportion of
the material present in the earth's crust; so that the living
body is made of the same materials that are most abundantly
present in the inanimate world around us.

Chemical Compounds in Living Tissues. — It is perfectly
evident that the elements enumerated do not exist in the
living body as uncombined elements. Two or more of them
are always united as chemical compounds to form a substance
different from either of them. The chemical compounds that
are present in the bodies of animals and plants are of an endless
variety; but a few general types are most widely present and
may be regarded as the fundamental compounds of living
things. These compounds are important, since they enter
into the food of all animals. They are as follows: proteids,
carbohydrates, fats.

Proteids. — Proteids are extremely complex substances, com-
posed chiefly of the elements: carbon, oxygen, hydrogen, and
nitrogen, but containing also in small proportions sulphur
and the other elements that have been enumerated above.
They are by far the most complex substances in living things;
that is, in a proteid molecule, there are present more chemical
atoms than are found in a molecule of any other substance
existing in the animal body. The exact chemical composition
of proteids is not known and it suffices for our purpose to
state, that they are composed of a highly complex combina-
tion of the elements we have mentioned, so united that hun-
dreds of atoms are probably always combined to make a mole-
cule. Some idea of their complexity may be obtained from
the fact that one chemist gave as a formula for egg-albumen,
C2o4H322N52O66S2 (a formula too complicated to have any real
meaning); and indeed, no two chemists agree upon the chem-

8 BIOLOGY

ica! composition of any proteid. The following are the best-
known proteids: albumen, the white of an egg; myosin, the
lean part of the meat; casein, the curd of the milk; gluten,
the sticky substance in flour; legumen, a similar sticky material
present in peas and beans. Besides these, there are many
other proteids present in animal and plant tissues. Living
tissue is almost entirely proteid in character.

Sources of proteids. — Since living things are made up
largely of proteids, we next inquire into the source of these
proteids. As will be noticed later, green plants can combine
the gases of the air with the water and certain minerals obtained
from the soil, and thus manufacture their own proteids. Animals
and colorless plants (fungi) are totally unable to manufacture
proteids from inorganic compounds. Hence it follows that
animals and the colorless plants depend upon the green plants
for their proteids, which is simply another way of stating the
fact that animals require plants for their food. Although unable
to manufacture proteids, colorless plants and animals are, how-
ever, able to modify them more or less, having the power to
transform one kind of proteid into another. If, for example,
an animal is fed with the white of an egg, it can transform
this proteid into the proteid of muscle, thus changing albu-
men into myosin. Since animals are unable to manufacture
muscles from any substances but proteids, it follows that they
are obliged to have proteids in their diet.

Carbohydrates. — Starches and sugars are the best-known
examples of carbohydrates. They are much simpler than
proteids, consisting of only three chemical elements: carbon,
oxygen, and hydrogen. These elements are combined in mole-
cules with the following formulas : C6Hi0O5 (starch) and C6Hi2O6
(sugar). There is quite a large number of starches and sugars,
differing from each other in some respects, but these formulas
are typical of their general nature. It will be seen from the
formulas that the difference between the molecules of starch
and sugar is in the presence, in sugar, of H^O in addition

THE SCOPE OF BIOLOGY 9

to the group contained in the starch molecules. H^O is a
molecule of water; and hence we say that if a molecule of water
is added to a starch molecule, it will convert it into a sugar
molecule. It must not be understood, however, that this
can be done by simply adding water to starch, for the two
will not combine. There are methods (see page 306), however,
by which they can be made to combine, and under these cir-
cumstances starch can very easily be converted into sugar.

Among the different types of sugars, there are two of espe-
cial importance. One of these is grape sugar, also called glu-
cose or dextrose. These three names are closely related, al-
though not exactly identical. The formula for these is also
CeH^Oe. The other type is cane sugar, obtained from sugar
cane or the sugar beet. The formula for this is C^H^On,
which, as will be noticed, is nearly, but not quite, twice the
formula of the grape-sugar molecule. By the addition of a
molecule of water it is possible to break a molecule of the cane
sugar into two molecules of the grape-sugar type, according
to the following equation: Ci2H22Oii+H2O = 2C6Hi206. This
is commonly spoken of as inverting the sugar.

Sources of carbohydrates. — Carbohydrates come almost wholly
from the vegetable world. Green plants manufacture starch
in their leaves by combining the carbon dioxid gas which
they absorb from the air with the water which they absorb
from the soil. This starch is very easily converted into
sugar within the plant, and then carried to various parts
where it may be stored, either in the form of starch or sugar.
It is subsequently used by the plant as food, or, if the plant
is consumed by animals, it serves as their food. So far as
known, there is no other source of carbohydrates in nature
besides the green plants, and as all animals and all plants
consume carbohydrates, it is plain that the whole living world
is dependent upon the green plants for carbohydrates.

Hydrocarbons (Fats). — Good examples of fats are found in but-
ter, in mutton tallow, in lard, in olive oil, etc., and in many other

10 BIOLOGY

food products. Fats contain the three elements, carbon, oxygen,
and hydrogen, in this respect agreeing with the carbohydrates.
They are, however, considerably more complex than carbohy-
drates, a molecule of fat containing more atoms, as is shown by
the formula C5iHio4O9, which represents a common fat. When
treated by a simple chemical method, fats are broken up into two
substances, one of which is called glycerine and the other a fatty acid.

Sources of fats.— ¥ at can be manufactured by either ani-
mals or plants out of other foods. If an animal is fed upon
proteids or carbohydrates, it can manufacture fat from them;
and plants are able to make fat out of the food materials
which they absorb from the air and water.

The table on page 11, which illustrates the composition of a
few of our common foods, shows that our ordinary diet con-
tains a fair proportion of each of these three foodstuffs. It
will also be seen from this table that the largest proportion
of proteids comes from animal foods, while the largest pro-
portion of carbohydrates comes from plant foods.

>/ ORIGIN OF LIFE

Perhaps no feature of modern biology is more important
than the acceptance of the theory that every living thing
comes from a living source. All living animals and plants
with which we are familiar to-day have originated from pre-
viously existing life. The living animal comes from the egg
that was produced by another living animal; the plant comes
from a seed that was produced by another living plant. But
the question of the primal origin of life is sure to intrude itself
upon our minds, and we are forced to ask whether living things
can be, or ever have been produced by any other means. Did
there ever occur, or does there occur in the world to-day, a
spontaneous generation of life? In other words, did a living
thing ever arise from some source which was not alive? So
far as our knowledge of nature is concerned, there are no means
of starting new life except from previously existing life.

THE SCOPE OF BIOLOGY

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12 BIOLOGY

Spontaneous Generation or Abiogenesis. — This idea of spon-
taneous generation, or abiogenesis (Gr. a = without + bios =
life -f genesis = generation), has been before the scientific world
for centuries. The ancients in the time of Aristotle, and for
centuries later, had no especial question in regard to the matter,
and took it for granted that living things did come from in-
animate matter. Virgil tells us of bees coming from the flesh
of bullocks; Ovid recounts that slime begets frogs; and many
centuries afterwards, we read that water produces fishes and
that mice can come from old rags. Although to-day these
ideas seem nonsensical, once they appeared perfectly logical.

Experiments of Redi. — This idea that life could come from
non-living matter was held without question during the earlier
centuries, and indeed until about the 17th century. In 1680
an Italian named Redi made an observation which led him to
what was at that time a rather startling conclusion. It had pre-
viously been observed that fly maggots made their appearance
in decaying flesh, and it was taken for granted that they devel-
oped spontaneously. Redi noticed flies hovering over meat,
and demonstrated by experiments that if the flies were kept
away by simply tying paper over a bottle containing the meat,
maggots could never develop in it. A little further study
proved that the flies laid eggs on the meat which developed
into fly maggots. From this observation he drew the far-
reaching conclusion that spontaneous generation did not occur
and that all living things came from living ancestors.

This conclusion started a dispute which lasted for two cen-
turies and was not fully settled until about 1875. For the
conclusion of Redi, that all living things came from living
ancestors, was vigorously disputed by the adherents of the old
idea that life could arise spontaneously. Many ingenious
experiments were devised to settle the question. It did not
take, long to prove that so far as the larger animals and plants
were concerned, the conclusion of Redi was correct. But just
at this time the newly invented microscope was beginning to

THE SCOPE OF BIOLOGY

show a world of invisible life, and in the various bottles and
flasks used in these early experiments, a large number of micro-
scopic forms of life appeared in spite of all attempts made to
prevent their entrance. Although in a piece of meat no fly
maggots developed unless flies had previous access to the meat,
innumerable microscopic forms of life did appear in it, in spite
of all efforts to exclude them, even when the meat was care-
fully and hermetically sealed. Some of the early experimenters
naturally concluded that these microscopic forms of life ap-
peared spontaneously, while others insisted that these little
organisms had found entrance into the sealed vessels from the
outside, in spite of all precautions taken to keep them out.
Great ingenuity was shown in devising experiments for settling
this question. The results obtained by different experimenters
were in great conflict for over two centuries, and apparently
equally good evidence
was found both for and
against the belief in
spontaneous genera-
tion.

Needham and Spal-
lanzani. — The general
method used by the
experimenters was to
place meat, hay infu-
sions, cheese, etc., in
flasks, and then by
boiling to attempt to
kill all life in the ma-
terial, and later, by
sealing hermetically,
to guard against the entrance of any form of microscopic life from
without. But even under these conditions it was frequently found
that microscopic life made its appearance in the sealed vessels ;
Fig. 2. It proved very difficult to be sure that nothing was left

FIG. 2. — APPARATUS USED BY SCHWANN IN
EXPERIMENTING ON SPONTANEOUS GENERA-
TION

Steam produced by boiling passed out through the
tube, but upon cooling was drawn in again through the
heated coil, which sterilized it.

14 BIOLOGY

alive in the material after boiling, — i.e., that it was sterile,
— and to be sure that the sealing was effectual. Two names
especially connected with this dispute were Needham, in 1749,
and Spallanzani, in 1777. Needham believed firmly in spon-
taneous generation, while Spallanzani insisted that the micro-
scopic organisms that appeared in these experiments were
either there originally and not killed by the boiling to which
the material had been subjected, or had found their way into
the solutions through microscopic cracks left by the imperfect
sealing.

Pasteur and Appert. — In the middle of the last century the
French scientist, Pasteur, carried out a series of experiments
and attained results which conclusively disproved the theory
of spontaneous generation. But the long debated question
would not be settled even then. It is a curiously interesting
fact that, while scientists were disputing over this matter, the
question had, for practical purposes, actually been settled by
Appert, who in 1831 had discovered the method of preserving
animal and vegetable foods by the means of heat and sealing, —
the method used by the canning industries of the present day.
But the significance of this practical discovery was not appre-
ciated, and the dispute continued even after Pasteur's work,
the advocates of spontaneous generation continuing as insistent
in their claims as ever. The settlement of the question
was not reached until the English physicist, Tyndall, devised
a new and ingenious method of experimenting which so satis-
factorily guarded all sources of error that criticism was silenced.
Indeed, so convincing were his experiments that his conclusions
have practically never been questioned.

Tyndall's Experiments. — Briefly, Tyndall's method of ex-
perimenting was as follows: An airtight box was constructed,
rectangular in shape and provided at either end and in front
with glass windows. Into the top of this box passed small
glass tubes which had been thrown into several curves, through
which the air was allowed to enter freely; Fig. 3 a.

THE SCOPE OF BIOLOGY

Recognizing that the great source of error in these experi-
ments was due to the germ-bearing dust of the air, Tyndall
attempted to free the
air from dust by coating
the inside of the curved
tubes with glycerine, to
entangle the dust particles
of the air as it passed up
and down the series of
curves into the box. This
method proved to be suc-
cessful, for experiment and
microscopic study showed
that no dust passed be-
yond the second curve of
the tubes. The interior of
the box was also coated
with glycerine, so that the
dust particles which either
settled to the bottom or

FIG. 3. — APPARATUS USED BY TYNDALL

For description see text.

floated against the side or top of the box would be caught in the
glycerine. In this way Tyndall argued that he could obtain,
in time, air perfectly free from germ-bearing particles.

He did not wish to begin an experiment until the air in the
box was absolutely free from dust, and in order to determine
this point the two glass windows at the end of the box were
used. A ray of light was thrown through the box, in at one
window, and out through the other. Thus, any dust particles
that remained floating in the air of the box would be illumined
and made clearly visible through the window in front. At
first there were many dust particles to be seen floating in the
air; but after the box had remained quiet for several days,
the ray of light was invisible as it passed through the box,
proving that no floating dust particles were present to be
illumined. When this condition was reached Tyndall assumed

16 BIOLOGY

that the air was sterile, that is, pure, so far as any floating
particles were concerned, and that his box was ready for
experiment.

At the bottom of the box were a series of tubes whose mouths
opened into the box but whose lower ends projected below;
Fig. 3 b. By means of the long tube, c, which could be
moved to and fro (since it passed through a rubber diaphragm,
d), all the test tubes could be filled successively with any of
the solutions with which he wished to experiment. In these
tests, Tyndall used various materials: old meat, old cheese,
hay infusion, etc., besides many other substances that pre-
vious experimenters had used in their attempt to settle the
question. After filling the tubes with these various materials,
they were heated to a temperature sufficiently high to destroy
all life that they might have contained in the beginning. This
was easily done, since the lower end of the tubes projected
below the level of the box and could be very easily put into
a bath of oil or brine, and heated to any desired temperature.
Any steam or vapor that might arise from the open end of
the test tube would pass into the box and readily find exit
through the glass tube at the top. Upon cooling, a fresh
supply of air would be drawn back into the box through the
curved tube a, but, as already indicated, no dust particles
would find entrance. Having thus, by heat, killed any living
organisms that might be in the solutions to be tested, he
again set the boxes aside and watched day by day to see
what would happen. Since everything was clearly visible to
the eye, it was possible to determine very quickly and surely
whether any living organisms developed in the test tubes.

Tyndall's care in his experiments was so great that they were
quite beyond criticism. His experiments showed the cause of
previous errors and explained why there had been such con-
flict in the earlier experiments. He demonstrated among other
things that some forms of life, called spores, might remain
alive in boiling water for some time. This conclusion had been

THE SCOPE OF BIOLOGY 17

previously reached by others; but Tyndall proved definitely
that while a temperature below boiling is sufficient to kill
active germs, the spores stand a temperature of boiling for a
long time, and hence boiling does not sterilize liquids. Since
previous experimenters had assumed that all life was destroyed
by boiling, they had been contented with the simple boiling
of the liquid to eliminate any organisms that might have been
there originally. If, therefore, any of these resisting spores
chanced to be in their solutions, they would subsequently
develop; and from this fact the experimenter might reach the
erroneous conclusion that the living organisms coming from
these spores developed spontaneously. Tyndall carefully
eliminated all of these errors and established the following
important conclusions. No evidence for spontaneous genera-
tion exists and the success of an experimenter in obtaining
any evidence of spontaneous generation is in inverse propor-
tion to the care with which he performs his experiments.

This statement has stood almost unquestioned by biol-
ogists since it was first promulgated in 1875; and during
the last thirty years the work of thousands of experimenters
in the science of bacteriology has only confirmed the accuracy
of Tyndall's conclusion.

We must accept the fact that whenever any living animal
or plant, no matter how small, makes its appearance in a
solution, originally there was present in this solution a living
germ which started the development of the organism by the
process of ordinary reproduction and growth. At the present
time, therefore, there is no shred of evidence that, under any
conditions which we can produce, life can arise spontaneously.

The Primal Origin of Life. — The conclusion that spontaneous
generation does not occur to-day, leaves unanswered the ques-
tion of the primal origin of life. It has been a disappointment
to biologists to be obliged to admit that they can find no evi-
dence for the theory of spontaneous development, since at some
period in the history of the world, life must have made its

18 BIOLOGY

appearance for the first time. In an early period of the world's
history, the earth was a hot, molten mass, and under these
conditions no living matter could exist. It follows, then, that
life must have made its appearance after the earth had suffi-
ciently cooled. Biology, in endeavoring to explain life by
natural forces, has been eager to believe that in these earlier
conditions of the world the first living thing may have ap-
peared as the result of natural law. The fact that biologists
have almost universally accepted Tyndall's conclusion that
no evidence for spontaneous generation exists, is thus a testi-
mony, both to the truth of this conclusion and to the honesty
of the scientists who have accepted it. They would have much
preferred a conclusion of the opposite kind. The majority of
biologists, however, believe it to be logically necessary to as-
sume that at some time in prehistoric ages, the first living
thing appeared from a source which was not living. While
accepting the fact that abiogenesis does not occur at the present
day or under present conditions, biologists still claim that we
have no means of knowing what may have occurred under
different conditions in earlier eras of the world's history. Thus,
the problem of the primal origin of living matter still remains
unsolved.

THE BIOLOGICAL SCIENCES

Since the science of biology deals with all living matter, it
might broadly be defined as the study of life in all its phases.
With this comprehensive definition, biology can be made to
cover nearly the whole field of human knowledge — most sciences
and even philosophy — including not only everything which
relates to the life of man, but all that concerns the life of the
animal and plant world as well. But for practical convenience
in study, the field of biology is usually restricted to a group of
definitely related sciences, — the so-called biological sciences, —
and although within this group there are to be found many
ill-defined boundary lines, and much overlapping and division

THE SCOPE OF BIOLOGY 19

into sub-groups, the sciences which compose it may be enumer-
ated as follows: morphology, with its sub-groups: anatomy,
histology, taxonomy, distribution, structural embryology; and
physiology, with its sub-groups: physiology proper, functional
embryology, psychology, ecology, and sociology. (See reference
chart, p. 21.)

MORPHOLOGY

Morphology (Gr. morphe = form + -logia = discourse) is that
branch of biology which deals with the structure and form of
animals and plants. It may be divided into five sub-heads:

1. Anatomy (Gr. ana = up + temnein = to cut) is the study
of all of the grosser structure of animals and plants, that can be
seen and dissected without the aid of the microscope.

2. Histology (Gr. histos = a web + -logia) is the study of the
minute structure of animals and plants which is disclosed only
by the aid of the microscope. It is sometimes called microscopic
anatomy and deals chiefly with cell structure.

3. Taxonomy (Gr. taxis = arrangement + nomos = law) is
the study of the relations of the organisms to each other and
includes the classification of species.

4. Distribution is the study of the geographical distribution
of organisms at the present time, and also their distribution in
the past as disclosed by geology; to the latter study is given the
name paleontology.

5. Embryology (Gr. embryon = an embryo + -logia) is the
study of the development of the organism from the egg to
the adult life. It is also called ontogeny (Gr. on (ont) = be-
ing + -geneia = producing) in distinction from phytogeny (Gr.
phylon = race -\--geneia = producing), the development of the
race.

PHYSIOLOGY

Physiology (Gr. physis = nature + -logia) is the study of the
activities or functions of organisms. Its scope may be best
understood by its division into sub-heads:

20 BIOLOGY

1. General physiology. Physiology deals primarily with the
functions of the different organs. Correctly used, it should
include the functions of all animals and plants. Since, however,
human physiology has been so much more studied than that of
other animals, the term physiology usually refers to mankind.
When the study extends to other animals or to plants, it is des-
ignated respectively as animal physiology and plant physiology.

2. When embryology concerns itself with the activities of
the embryo, it then belongs to the domain of physiology.

3. Psychology (Gr. psyche = soul + -logia) is the study of
the functions of the brain. It includes not only the study of the
human brain but the brain activities of other animals as well,
under the term comparative psychology.

4. Ecology (Gr. oikos — house -f -logia) is the study of the
relations of organisms to their environment. This includes their
relations to inanimate nature as well as to animate. The term
ecology is now more widely applied in relation to plants than to
animals. Ecology includes sociology (Lat. socim = a compan-
ion + Gr. -logia), which is the study of the interrelations of ani-
mals of the same species. This, however, is chiefly confined to
the human race, the term sociology usually referring to mankind.
There are, however, some animals like ants, bees, etc., that have
social relations, and the term sociology might be extended to
them.

ZOOLOGY AND BOTANY

The general term zoology includes any of the biological sci-
ences when studied in their relation to animals, and the general
term botany, when they are studied in their relation to plants.

THE SCOPE OF BIOLOGY

BIOLOGY

The science of living things

MORPHOLOGY

The science of form

PHYSIOLOGY

The science of function

Anatomy

General Physiology

The study of

The study of func-

gross structure

tions of organs

Histology
The study of mi-
nute structure

includes

Embryology
(functional)

The study of the
activities of the

Taxonomy

embryo

The classification

N §

Ecology

of species

The study of the

Distribution

i?5.

relation of organ-
isms to their en-

The geographical

vironment

and chronological

relation of organ-
isms

Sociology
The study of

interrelations

Embryology

of animals

(structural)

of the same

The study of de-

species

velopment from
the germ

Psychology
The study of

brain functions

22 BIOLOGY

LABORATORY WORK WITH ORGANIC COMPOUNDS

PROTEIDS

Albumen. — Separate a little of the white from the yolk of an egg and
dilute with three times its quantity of water. With this solution make
the following tests:

1. Place a little of the albumen solution in a test tube and boil, noting
that a precipitate appears; that is, the albumen coagulates. Repeat this
test, heating the albumen in a test tube in a water bath, determining,
by a thermometer placed in the test tube, at what temperature the coagu-
lation occurs.

2. Add a little strong HNO3 to some of the albumen in a test tube.
A precipitate appears. Boil, and the precipitate will turn yellow. Allow
it to cool and add enough ammonia to neutralize the acid and it will turn
a deep orange. This is known as the xanthoproteic test for proteids.

3. To a weak solution of albumen add a few drops of NaOH and a
few drops of a 1% solution of CuSO4; heat gently and the solution will
turn blue if ordinary proteids are present, but if peptones are present it
will show a reddish color.

Gluten. — Place some flour in a large piece of cheesecloth, and gather-
ing up the edges of the cloth, wash thoroughly in a pail of water. Much
of the bulk of the flour will wash away, but the gluten will finally be left
in the cloth, as a sticky mass that will not wash out.

Remove a little of the wash water from the pail in a test tube and add
a few drops of iodine to it. If it turns blue it will indicate the presence
of starch.

Casein.— Add a little 2% HCL to a few c. c. of milk. A curd will form
whioh can be separated from the liquid by allowing it to drain through
cheesecloth. The curd is the proteid, casein.

Myosin. — Soak some chopped beef in cold water for half an hour;
stir and filter through cheesecloth. Boil the filtrate, and a mass of myosin
will appear, which was dissolved in the cold water but is coagulated by
heat.

Fibrin. — This is a proteid formed from blood. It may be obtained by
collecting freshly drawn blood and stirring it immediately with a piece
of wire gauze for about ten minutes. A mass of fibrin will collect on the
wire, and it will be found that the blood will not subsequently clot, the
removal of the fibrin preventing it.

CARBOHYDRATES

Starch. — Rub up a little starch (potato starch is best) in an evaporating
dish with a considerable quantity of water. Place a few drops in a teat
tube and add a little iodine solution. The starch will turn blue.

THE SCOPE OF BIOLOGY 23

Examine a little of the starch water under a microscope. Sketch some
of the starch grains. Make a thin section of a bit of potato with a razor
and examine under a microscope, noting the starch grains. Add a little
iodine solution and again examine with a microscope.

Boil the starch water over a flame. As it comes near to the boiling
point the mass will become thick and pasty (starch paste), due to the
bursting of the starch grains by heat. Place a little under the microscope
and look for grains. Add to the paste a little iodine and it will turn a
brilliant blue.

Test for Sugar. — Put a little glucose or dextrose in a test tube con-
taining a considerable quantity of water. Add to this a few drops of
weak H2SO4 and a few drops of NaOH; boil. The presence of sugar is
determined by the appearance of a brownish red precipitate, which goes
through a series of color changes, but finally remains as a brownish red
sediment at the bottom of the tube.

FATS

One of the simplest tests for fat is to place the material in which the
fat is supposed to be upon a sheet of common paper. The paper will be
rendered transparent by absorbing the oil of the fat-containing tissue.

Fat Emulsion. — Fat has the property of being readily divided into
minute particles which, when mixed with water, float in the liquid, form-
ing what is known as an emulsion. Place a few drops of olive oil in a
test tube half full of water. The oil will rise to the top of the water and
appear as a clear yellowish layer. Put a finger over the mouth of the
test tube and shake vigorously. The whole contents of the tube will
turn a milky white, and upon being allowed to stand the milkiness will
remain for a long time. Eventually, however, the fat again separates
from the water. This milky appearance is produced by the fact that the
fat has been divided into minute particles that float through the water
and refract the light in such a way as to give a white color. This is called
an emulsion.

Examine, under a microscope with a high power, a drop of milk, noting
that it is an emulsion.

BOOKS FOR REFERENCE
Recent Works in General Physiology and Psychology

CONN, Advanced Physiology, Silver, Burdett & Co., New York.
DRIESCH, The Science and Philosophy of the Organism, A. & G
Black, London.

24 BIOLOGY

FOSTER, Text-book of Physiology, Macmillan Co., New York

JAMES, Psychology, Vols. I and II, Henry Holt & Co., New York.

JUDD, Psychology, General Introduction, Chas. Scribner's Sons, New
York.

McCABE, The Evolution of Mind, A. & C. Black, London.

MEYERS, Text-book of Experimental Psychology, Longmanns, Green &
Co., London.

PILLSBURY, The Essentials of Psychology, Macmillan Co., New York.

TIEGERSTADT, Text-book of Physiology, Appletons, New York.

TITCHNER, Text-book of Psychology, Experimental Psychology, Vols. I
and II, Macmillan Co., New York.

VERWORN, General Physiology, Macmillan Co., New York.

WUNDT, Physiologische Psychologic, Vols. I-III, Outlines of Psy-
chology (translation), Engelmann, Leipzig.

Important Works Bearing upon Abiogenesis

SWAMMERDAM (1637-1680), Bibel der Natur, 1652.

REDI, Esperinze interno alia generazione degl' insetti, 1688.

LEEUWENHOEK (1632-1723), Arcana naturae, 1695.

NEEDHAM, Observations upon the Generation, Composition, and De-
composition of Animals and Vegetable Substances, London, 1749.

BONNET (1720-1793), Considerations sur les corpes organises, 1762.

SPALLANZANI (1729-1799), Physicalische und mathematische Abhand-
lungen, 1769.

SCHULTZE (1825-1874), Vorlaufige Mittheilung der Resultate einer
experimentellen Beobachtung iiber generatio sequivoca, Gilbert's Annalen,
1836.

SCHWANN (1810-1882), Vorlaufige Mittheilung, betreffend Versuch iiber
die Weingahrung und Faulnis, Gilbert's Annalen, 1837.

PASTEUR (1822-1895), Comptes Rendus, Vol. 50, 1861.

BASTIAN, The Beginnings of Life, 1872.

TYNDALL (1820-1893), Philosophical Transactions, 1876, 1877; Floating
Matter in the Air, 1882.

Useful Laboratory Manuals

ANDREWS, Practical Course in Botany, American Book Company,
New York.

THE SCOPE OF BIOLOGY 25

CALKINS, Protozoology, Lea & Febiger, New York.

COLTON, Zoology, Descriptive and Practical, D. C. Heath & Co.,
Boston.

DAVIDSON, Practical Zoology, American Book Company, New York.

HAWK, Physiological Chemistry, P. Blakiston's Son & Co., Philadelphia.

LONG, Text-book of Physiological Chemistry, P. Blakiston's Son &
Co., Philadelphia.

MELL, Practical Laboratory Methods, Macmillan Co., New York.

PARKER and PARKER, Practical Zoology, Macmillan Co., New York.

PAYNE, Manual of Experimental Botany, American Book Company,
New York.

PRATT, Invertebrate Zoology, Ginn & Co., Boston, Mass..

RICHARDS and WOODMAN, Air, Water, and Food, Wiley & Son, New
York.

ROCKWOOD, Laboratory Manual of Physiological Chemistry, F. A.Davis,
Philadelphia.

SHARPE, Laboratory Manual in Biology, American Book Company,
New York.

CHAPTER II

CELLS AND THE CELL THEORY
ORGANISMS

ONE characteristic feature of living matter is that it is not
indefinitely distributed around the world, but is always asso-
ciated in distinct units or individuals. In other words, there
is no life apart from individuals. These units always contain
different parts, each with a distinct function. This is very
evident among well-known animals and plants. The human
body possesses a heart, a stomach, a brain; and a tree has
roots, leaves, flowers, etc. These different parts are called
organs ; and because it possesses organs, a living being is called
an organism. While it is true that practically all living things
do have organs, some of the lowest are so small that no organs
have yet been found in them, as for example, bacteria; see
Fig. 7. It is probable, however, that these do have organs
if we were only able to see them; at all events, the term organ-
ism is extended to all living things whether they possess evi-
dent organs or not.

From the word organisms is coined the adjective organic, that
is, pertaining to organisms. Organic substances have been pro-
duced by living beings, while inorganic substances have no con-
nection with living things. Bone, muscle, wood, sugar, coal, etc.,
are organic; while stones, water, and air are inorganic. Nearly
all organic substances contain carbon and are capable of being
burned, while inorganic substances usually contain no carbon.

THE CELL AS THE UNIT OF ORGANIC STRUCTURE

The slightest familiarity with the larger well-known animals
and plants shows not only that they are made up of different
organs, each with its definite duty to perform, but also that these
organs are composed of different parts, each having its specific

CELLS AND THE CELL THEORY

function. The stomach has its muscles and its secreting glands;
the foot has its muscles, bones, tendons, ligaments, nerves, etc.
The different kinds of substance which form the organs are
known as tissues, and usually each tissue contains only one kind
of material and has but one kind of duty to perform. For
example: muscles, bones, glands, nerves,
and tendons, each represent a distinct
tissue; each has its special function in
the organ, and each is different from the
other. Muscles have the power of con-
traction, bones are for support, etc.

By studying these different tissues un-
der the microscope we shall find that
they, too, are made up of minute parts,
called cells, and that in most instances iNG CARTILAGE TISSUE
each cell is essentially like all the other
cells of the same tissue. This may be shown by examining
Figures 4 to 6, in which several kinds of tissue appear, each made

FIG. 4. — CELLS FORM-

FIG. 5. — CELLS FORMING BONY TISSUE

up of a large number of independent, similar cells. These cells
represent the ultimate units to which the analysis of the struc-

BIOLOGY

ture of living things has been carried at present; for while each
cell is made up of parts, life as a whole seems to be found only
where we have the whole structure of the cell developed. In
other words, the cell is the simplest form in
which life occurs, and is, in this sense, the ulti-
mate unit of living structure. While an organ
may contain many different kinds of cells, each
tissue is, as a rule, made of but one kind of cell.
The cells of the bone, for example, are all essen-
tially alike, and so, too, are the cells of muscles
and glands. The different cells in the same tissue
may differ in shape and size; but these differences
are only superficial ; fundamentally the cells form-
ing a single tissue are alike. Therefore, if we de-
fine a cell as the ultimate unit in the analysis
of living structure, we may define a tissue as an
aggregate of similar cells, all having similar func-
tions; see Figs. 4, 5, and 6.

While the form, structure, and size of cells
present an almost endless variety, in both the
animal and plant worlds, nevertheless, all cells
have in common certain general parts. Thus we
may speak of the structure of a cell in general, recognizing that
all living cells of both animals and plants, in spite of their
differences, conform essentially to the type of an ideal cell.

FIG. 6.— CELLS

FORM ING
MUSCLE TIS-
SUE FROM
THE INTES-
TINE WALL

CELL STRUCTURE

The description given below is not that of any particular cell,
but rather that of a typical or ideal cell. Though a cell exactly
like that described will not be found, it resembles closely the
cell which forms the egg of certain animals, and in essential
structure is like all cells found in animals and plants.

Structure. — The cell consists of four primary parts, some of
which may be absent: —

CELLS AND THE CELL THEORY

1. The protoplasm, or cell substance, a liquid making up the
bulk of the cell.

2. The nucleus, a rounded body within the cell substance.

3. The centrosome, a small body near the nucleus.

4. The cell wall, an outer covering which holds the cell
substance. (The cell wall and centrosome are sometimes
absent.)

Size. — There is much variation in the size of cells. Some of
them are extremely minute.
Bacteria, which are sometimes
not more than 1/50,000 of an
inch in diameter, are probably
cells (Fig. 7), although we do
not yet know positively that
they contain a nucleus and cen-
trosome. At all events the
yeast, which is only a little
larger than a bacterium (about
1/3000 of an inch), is a typical
cell, possessing a nucleus, cell
wall, and cell substance; see
Fig. 32. At the other end of
the scale we find giant cells,

FIG. 7. — BACTERIA VERY HIGHLY
MAGNIFIED

Showing the complex internal structure with
bodies supposed by some to be nuclei. At o
one cell shows what resembles karyokinetic
division.

FlG. 8. NlTELLA

A, about natural size, showing nodes
and internodes; B, one of the inter-
nodes more magnified. The part en-
closed by brackets, between the two
rows of leaves, is a single cell.

which may be an inch in length, as in the case of a small
plant known as Nitella (Fig. 8), or larger still, as in the egg

BIOLOGY

FlG. 9. — A DIAGRAM OP AN IDEAL CELL
I, linin;
m, microsomata;

as, centrosphere ;
ch, chroma tin;
cr, centrosome;
cw, cell wall;
cy> cytoplasm;
/, fibers;
ky, karyoplasm;

nm, nuclear membrane;
n, nucleus;
no, nucleolus;
p, plastids;
v, vacuole.

stance) has been given. This material
within the cell wall, or lie in the form

of the ostrich, which is
really a single cell. As
a rule, however, cells
are microscopic in size.
Shape. — A cell is usu-
ally more or less spher-
ical (Fig. 9), although
it may be distorted by
pressure or irregular
growth.

CELL SUBSTANCE OR
PROTOPLASM

The material which
composes the active
part of the cell appears
like a mass of more or
less transparent jelly to
which the name pro-
toplasm (Gr. protos =
first + plasma = sub-
may fill the entire space
of a thin layer next to

FIG. 10. — CELL OF RPIROGYRA

cl, cell chlorophyll; cs, cell sap; cw, cell wall; n, nucleus; p, protoplasm.

the cell wall, the rest of the space being filled by a watery
liquid; Fig. 10.

CELLS AND THE CELL THEORY

Structure. — When protoplasm is examined under the micro-
scope it is not found to be a homogeneous jelly, as was at first
thought, but to have an intricate structure which is only partly
disclosed by the microscope; Fig. 11. The exact structure of
this cell substance has not been fully determined, and there
are at least three different theories to explain its microscopic
appearance.

The Reticular Theory. — One school of scientists describes pro-
toplasm as an extremely minute network of fibers forming a sort
of sponge, in the meshes of which
there is found a moving liquid;
Fig. II A . This is the so-called
reticular or fibrillar theory of pro-
toplasmic structure.

The Foam Theory. — Another
school explains the appearance
of protoplasm as due to a mass
of minute bubbles, like soapsuds
on a small scale; and insists that
what appear to be fibers are only
the delicate lines separating the

bubbles from each other; Fig. 11 B. This is the foam theory of
protoplasmic structure.

The Granular Theory. — Still a third theory suggests that the
protoplasm consists of an indefinite number of minute, living,
moving granules, arranged in lines resembling fibers or in various
other figures. This is the granular theory of protoplasmic
structure.

Between these theories the scientists have not reached any
conclusion, although the first two have been more generally
accepted than the last. It is quite possible, and even probable,
that all of the theories may have a certain amount of truth in
them, and that protoplasm does not in all cases have the same
structure. It is certain, however, that protoplasm always shows
a structure and is not a homogeneous body. In most cases

A B

FIG. 11. — DIAGRAMS ILLUSTRAT-
ING THEORIES OF PROTOPLASM
A, the Fibrillar; B, the Foam.
(Dahlgren and Kepner.)

32 BIOLOGY

two, and frequently three, distinct substances are discernible
in it.

1. A mesh work (reticulum) resembling fibers.

2. A liquid (cytoplasm) occupying the meshes of the net-
work.

3. Minute bodies (microsomata) (Gr. micros = small -f-
soma = body) scattered along the branches of the network,
regularly or irregularly, and frequently moving to and fro in the
cell.

Activity of Protoplasm. — If living protoplasm be studied under
the microscope, it will frequently show a type of motion called
streaming. This is due to minute granules constantly circu-
lating in a more or less definite or indefinite fashion within
the cell. Whether all protoplasm will show such motion we do
not know, but apparently whenever this substance is actually
alive this motion is present. Possibly this may not be true of
protoplasm that is known as dormant, but it is almost certainly
true of all active cells.

THE NUCLEUS

Lying within the cell substance there is a smaller body,
usually of an approximately spherical shape, called the nucleus
(Lat. nucleus = nut); Fig. 9n. This is a structure of extreme
complexity. It is, as a rule, bounded by a delicate nuclear mem-
brane nm, which holds the contents and separates them from
the surrounding cell substance. Within this membrane may be
found a jelly-like mass, very similar to, if not identical with, the
cell substance outside, and also included under the term pro-
toplasm. To distinguish these two parts of the protoplasm,
that inside of the nucleus is called karyoplasm (Gr. karyon =
nut + plasma = substance) or nucleoplasm, while that outside
is called cytoplasm (Gr. cytos = cell + plasma) ; Fig. 9 ky and cy.
In addition to karyoplasm, however, there are other distinct
parts in the nucleus. Delicate fibers run through it called linin
fibers (Fig. 90, and a small rounded body known as the nucle-

CELLS AND THE CELL THEORY

olus (Fig. 9 no) is usually present and is sometimes very promi-
nent. The significance of this nucleolus is at the present day
unknown.

The most remarkable substance in the nucleus is a material
known as chromatin (Gr. chroma = color); Fig. 9 ch. It has
received the name chromatin from the fact that it has a special
affinity for certain staining reagents, the chromatin material in
the nucleus being the first thing to absorb the color and become
stained. By special methods the chromatin may be stained and
the rest of the nucleus left unstained. The latter is sometimes
called achromatin (a = without + chroma = color). By this
special process of staining it is possible to show the chromatin
in prepared specimens, although in the living cell the chromatin

FIG. 12. — NUCLEI, SHOWING THE DIFFERENT APPEARANCES OP
THE CHROMATIN (VARIOUS AUTHORS)

is so transparent as to be practically invisible. Chromatin
occurs in a great variety of forms in different nuclei. Some of
these are shown in Figure 12. It is sometimes diffused irregu-

34 BIOLOGY

larly through the nucleus; it may be in the form of stars, or a
long coiled thread, or it may appear as isolated threads, or as
threads interlaced, etc. Whatever its form, it always has the
power of absorbing coloring material and is probably always of
the same general, chemical composition. The nucleus controls
the cell activities, and the chromatin forms the most impor-
tant part of the nucleus.

THE CENTROSOME

Near the nucleus in many cells may be found a minute body
(Fig. 9cr) known as the centrosome (Gr. centron = center +
Gr. soma = body), which is usually present in the cells of
animals, where it seems to have an important function in con-
trolling the multiplication of the cell. The centrosome is
usually lacking in the cells of the higher plants. Frequently
two centrosomes are found near together, and sometimes they
are surrounded by a clear area, which is designated as the
centrosphere. At one time the centrosome was considered of
great importance in the life of the cell, from its prominent role
in cell division; but since it has been discovered that some cells
have none, while others have several, its significance as an
essential element in cellular structure has been doubted.

THE CELL WALL

One of the functions of the cell substance in many cells is to
secrete around the cell a material of harder consistency than the
protoplasm, the cell wall. Some cells have no cell wall; for
example, the animal shown in Figure 13 is a cell devoid of a cell
wall; and in many other animal cells the wall is either very
slight or entirely lacking. From this, it is evident that the cell
wall cannot be regarded as an essential part of the cell. In
nearly all vegetable tissues, the living protoplasm secretes a
membrane of greater or less consistency, and the same is also
true of many animal cells. The cell wall may be made of a
variety of different materials, In plants it is sometimes of wood,

CELLS AND THE CELL THEORY

or of a material allied to starch and known as cellulose. Again
it may be composed of lime, or made up of a hornlike substance,
as in the case of the
cells that secrete the
finger nails, or the horns
of animals. The cell
wall is not alive, being
simply a secretion of
the living cytoplasm.
The cell walls may be
very thin, or entirely
absent as in Figure 13.
In other cases they may
be very thick and form
a tissue principally
composed of cell wall,
with only scattered bits
of b /ing protoplasm in
the midst of a great

ma-ss of secreted wall substance. This is especially true in the
case of the cartilage, as shown in Figure 4. The shape of a
cell is usually determined by the shape of its cell wall. Figure 14
shows a number of cells and gives an idea of the various shapes
ihe cell wall may assume.

\| Since the cell wall is lifeless and has only the function of sup-
port, the cell contents alone being alive, it follows that any
organism may contain both living and lifeless material. Among
plants the lifeless material may far surpass the living in bulk.
In a tree, for example, most of the trunk, roots, and branches
are made of the dead walls of cells which were formerly filled
with living protoplasm. In a large tree only a thin layer of cells
directly under the bark, the cells found in the leaves, buds,
and some cells in the roots, are actually alive. In animals a
much larger proportion of the body cells are alive, the bulk of
the muscles beirg living protoplasm ; but the skin, hair, cartilage,

FlG. 13. — A SINGLE-CELLED ANIMAL

ACTINOPHRYS
A cell without a cell wall.

BIOLOGY

F.IG. 14. — SHOWING CELLS OF VARIOUS SHAPES

All except C are plant cells; C, a ciliated cell from the oesoph-
agus of an animal. (From various authors.)

CELLS AND THE CELL THEORY 37

and bone contain in a marked degree lifeless cell walls from
which the living matter is either wholly withdrawn, as in the
hair, or remains only in a relatively small amount, as in
bone and cartilage.

Other Substances in a Cell.— Cells may contain other bodies
than those already described, which cannot be regarded, how-
ever, as essential to cell life, since they are not characteristic
of all cellular structure. Some of these are called plastids (Fig.
9 p) , and seem to grow and divide and to be handed on from
one cell generation to the next. Examples of such plastids are
the chlorophyll bodies in plant cells, or vacuoles in some animals.
Other bodies included in cells are purely passive bodies which
seem to be functionless, inert, excreted substances, not growing
and not handed down from generation to generation.

CELL FUNCTIONS

The cell with its protoplasm and nucleus contains all of the
parts that are necessary for life, and, so far as we know, nothing
simpler than a cell is capable of carrying on all the functions of
life. If this be true, we are justified in saying that the ideal cell
we have been describing is the simplest bit of structural machinery
that can manifest all the functions of life. All living organisms,
animals and plants alike, are either single cells (unicellular) or
complexes of cells (multicellular), and the life of the organism
as a whole is thus the combined life of its individual cells.

Definition of a Cell. — To sum up, then, we may say: A cell is
a combination of a bit of protoplasm (cytoplasm) with a nucleus,
and it is the simplest structure known to show the phenomena
of life.

HISTORY OF THE CELL DOCTRINE

The development of the cell doctrine may, for convenience, be
divided into three periods : —

1. The early conception of the cell, 1839 to 1861.

38 BIOLOGY

2. The discovery of protoplasm and the development of the
mechanical theory of life, 1861 to about 1885.

3. The discoveries of the functions of the nucleus and its
relations to reproduction and heredity, from about 1880 to the
present.

While these periods are not sharply marked off from each
other, they do represent different epochs in the development of
the conception of the nature of the cell.

i. THE EARLY CONCEPTION OF THE CELL (1839-1861)

The Formulation of the Cell Theory, 1839. — It was not
definitely proved until about 1839 that the tissues of animals
and plants were composed of cells, although cells were first
described in 1665 by Robert Hooke. A microscopic study of a
piece of cork showed him that it was made up of large numbers
of minute compartments which reminded him of the cells of a
monastery. Hence he gave them the name of cells, which they
still bear. Miscellaneous observations followed at intervals in
the next two centuries. In 1833 Brown described the nucleus
as a constant part of the cell. In the years 1838 and 1839 two
Germans, Schwann and Schleiden, one studying animals and
the other, plants, advanced the theory that the tissues of all
animals and plants were made up of these independent units,
to which they still gave the name of cells. These observations
formulated the so-called cell doctrine.

The Original Conception of the Cell. — It was first supposed
that the cell wall was the most essential part of the cell in con-
trolling the processes of life and separating die contents of the
cell from the surrounding medium. This conception did not
last long, for it was soon seen that there were many cells that
did not have cell walls. In these early days the existence of a
nucleus was not realized as of much significance.

The Origin of Cells. — In the beginning it was supposed that
cells were like crystals and developed from a cytoblastema as

CELLS AND THE CELL THEORY

crystals form in a supersaturated solution of sugar, the cytoblas-
tema being described as a complex, supersaturated solution
formed by the living body. This theory did not last many years,
however, because it was shown that
cells arise only from other cells. Even
as early as 1846, Schultze and others
proved that cells have no other origin
except from previously existing cells.
Starting with an egg, which is easily
demonstrated to be a single cell (Fig.
15 A), and then carefully studying its
development, it can be shown that
its growth is by the method of re-
peated division and sub-division (Fig.
15 £, C, D, E, F) until the single-
celled egg gradually becomes the
many-celled adult. Although the
cells become very numerous, they all
arise by the process of division from
the original egg cell. For many years,
however, it was considered possible
for a cell to arise in some other way
than by division of the original egg
cell; and even as late as 1880 discus-
sions took place as to whether "free
cell origin" was possible. By this ghowing how & single.celled egg
term was meant the origin of cells (A),bydiv»onCBtoO),grow»iiito

a many-celled animal.

from any source except from a previ- ^ endoderm;

ously existing cell. In time this ques- ec' ectoderm-

,. ,,1 i • ,! A- j F and £ show side folding inward

tlOn Was Settled in the negative, and to form what becomes the digestive
. . . , tract.

we are now certain that cells never

arise except from the division of earlier cells, and that all the
cells of an adult animal body, though there may be millions,
have arisen by the process of division from the original egg,
which was in itself the single cell from which the life of the

FIG. 15. — THE DEVELOP-
MENT OF THE EGG OF A
SEA-URCHIN

40 BTOLOG\

individual started. Figure 15 shows how the single cell divides
and continues to divide to produce the many cells of the
adult organism.

2. PROTOPLASM AND THE MECHANICAL THEORY ( 1861-1885 )

The Discovery of Protoplasm. — In 1839 Purkinje first recog-
nized under the name "sarcode" the contents of the animal cell;
H. Von Mohl in 1846 applied the term protoplasm (Gr. protos =
first + plasma = substance or form) to the viscid, granular sub-
stance found in plant cells. Cohn in 1850 claimed not only the
identity of animal and plant protoplasm but contended that it
was the seat of vitality, — the basis of life. In 1861 Max Schultze
established Cohn's theory and extended the meaning of the
word protoplasm to include all living matter. This was a new
conception and at once placed the doctrine of biology upon a
new basis.- If it could be proved that the cell substance, which
is the living material in all cells, is always alike, it would show
that life could be reduced to one fundamental basis. The name
protoplasm had been given to the living substance in the animal
embryo and then to a similar material in the cells of plants; but
it was Schultze who identified it with the living material of
animal cells and extended the name to apply to this universal
life substance. With this new conception, he defined a cell as
a mass of protoplasm surrounding a nucleus, and thus placed the
keystone in the arch of the protoplasmic theories.

Schultze's conception of protoplasm was somewhat expanded
and made more significant by Professor Huxley in 1866. Hux-
ley, giving to it the name "physical basis of life," drew far-
reaching conclusions as to the significance of the phenomenon
that we call life, based upon this universal physical substance.
He argued that the properties of life are simply characters of
this protoplasmic substance, just as other properties are char-
acteristic of water; and that life represents no distinct entity,
but is simply a name applied to the combined properties of
this remarkable chemical compound, protoplasm. This started

CELLS AND THE CELL THEORY 41

a long search for a chemical explanation of life phenomena.
In accordance with this idea, life was looked upon as merely
representing a special manifestation of chemical and physical
forces; it was argued that there was no more reason to speak of
vitality as a special property possessed by living things, than to
speak of aquosity as a special property possessed by the chemical
compound water.

The Mechanical Theory of Life. — Based upon this conception
arose a large number of interesting speculations, and the discus-
sions during the next twenty-five years resulted in a develop-
ment of the mechanical theory of life. It was argued that, if
life is merely a name given to the properties of protoplasm, and if
chemists could manufacture the chemical substance protoplasm,
they could thus create life, i.e., living protoplasm. Chemistry
was at this time advancing with prodigious strides, and chemists
were making more and more complex substances, and new com-
pounds which had hitherto been considered beyond their reach.
Many of the substances, which had previously been supposed to
be produced only by living processes, were, one by one, manufac-
tured synthetically in the chemist's laboratory. From this the
further assumption and confident prediction was made that the
time would come when it would be possible to manufacture a bit
of protoplasm by purely chemical means; and then it would fol-
low, if the mechanical theory of life were correct, that this bit of
protoplasm would necessarily be alive and scientists would thus
be able to manufacture a living thing. This was the essence of
the mechanical theory of life which largely dominated discussion
of biology for a quarter of a century.

General Properties of Protoplasm. — With this idea of pro-
toplasm as the basis of life, a large amount of study was given
to this interesting material. Since it is alive, it has of course
all the properties of life. If we look upon protoplasm as the
physical basis of life, we may in one sense say that its proper-
ties are as varied as are the properties of living things, since
the characteristics of living things are based upon the charac-

42 BIOLOGY

teristics ot their protoplasm. If the characters of mankind are
dependent upon the properties of its protoplasm, it follows
that the protoplasm that makes up the cells in man must
differ as much from the protoplasm that makes up the cells
of a plant as mankind differs from the plant. There will be,
then, is many varieties of protoplasm as there are varieties of
living beings in the world. But apart from these detailed charac-
ters, we find that the substance protoplasm, using this term now
to refer to the general life substance of the cell, has a few charac-
teristics that are present in all forms of protoplasm whether
animal or plant. In other words, all forms of living matter
possess certain general properties, which are frequently spoken of
as the general characters of protoplasm. They are as follows : —
I. Chemistry of Protoplasm. — Various attempts were made
in earlier years to determine the chemical composition of pro-
toplasm. The chemical elements out of which it is made are
easily found to be carbon, hydrogen, oxygen, nitrogen, sulphur,
and some other substances in small quantities. For a time it
was supposed to be a definite chemical substance with a definite
formula, and attempts were even made to give the number of
atoms present in a molecule of protoplasm We now know that
such attempts were necessarily futile. Protoplasm is not a
chemical compound but a mixture of a variety of different com-
pounds. The fibrillar network, the liquids, the microsomata,
and the chromatin are certainly all different from each other,
and it is manifestly impossible to speak of the chemical composi-
tion of protoplasm as a whole. We can safely say that proto-
plasm contains proteids, but beyond this, little of significance
has yet been determined. Since it is in a very unstable condi-
tion, constantly undergoing changes, its chemical composition
cannot be constant. Moreover, the chemical nature of living
protoplasm is doubtless different from the same material when
dead, and since any chemical tests are sure to result in its death,
it is impossible to determine the composition of the material
when alive.

CELLS AND THE CELL THEORY 43

2. Irritability. — All forms of living protoplasm have the power
of reacting when stimulated. This phenomenon is called irrita-
bility and is produced by the action of a large variety of external
ibrces upon the protoplasm itself. Any external force which
serves to produce a reaction in the protoplasm is spoken of as
a stimulus. Almost any kind of stimulus has the power of
affecting protoplasm: mechanical, thermal, electrical, and chemi-
cal. Stimuli all "have their effect upon protoplasm and all pro-
duce certain reactions within it. Protoplasm is, in short, irri-
table to almost any external stimulus. While the different
forms of protoplasm show different degrees of irritability to
various stimuli, they have certain general reactions in common.
The activity of protoplasm increases directly with the heat to
a certain point, and then decreases, and finally ceases altogether
if the temperature continues to rise.

Although some forms of protoplasm are much more irritable
to mechanical stimuli than others, nevertheless, all types of pro-
toplasm are influenced by external, mechanical force. Various
other factors,— light, chemism, gravity, etc.,' — mentioned upon
pages 57, 58, stimulate protoplasm. Various organic, internal
changes stimulate it as well. If the protoplasm is improperly
nourished it produces a condition that is in general known as
hunger, and this excites the irritability of protoplasm. The same
thing is true if there is insufficient water within the protoplasm,
producing an irritation called thirst. Protoplasm is also destroyed
by various chemicals called poisons, like chloroform, corrosive
sublimate, etc.

3. Conductility. — An irritation produced in any one part of a
bit of protoplasm is rapidly conducted throughout the whole
mass, a phenomenon known as conductility. In an ordinary
cell, this phenomenon of conductility does not have very much
meaning, because the bit of protoplasm is too small; but some
cells possess long protoplasmic fibers extending from their
bodies; and then this function of conducting impulses from one
end of the protoplasm to the other becomes of considerable

44 BIOLOGY

importance. For instance, a nerve fiber, even in the higher
animals, consists of a long bit of protoplasm extending from
the cell body; see page 169. The phenomenon of conductility
in this case is of great significance because it may carry an im-
pulse from the outer end of these nerves (the periphery) to the
cell body in the brain, or it may carry one that started within
the body rapidly outward to the periphery. This phenomenon
of conductility, therefore, forms the primary function of the
nerves. It is this function that makes it possible for a stimulus
applied to the outer part of the animal to be carried rapidly over
the animal so as to produce a response in other parts of the body.
4. Assimilation. — All protoplasm has the property of taking in
food material, changing its chemical nature and converting it into
new protoplasm by assimilation; a process which may result in
growth. This process is probably always a constructive one;
i. e., it builds more complicated materials out of simpler ones.
Different kinds of protoplasm have this power developed to a
widely different extent. Some cells assimilate and grow with
great rapidity, with the result that they multiply rapidly; other
cells seem to have lost much of this power of assimilation in their
adult life, and are able only to replace the worn-out parts of
their own structure. In the higher animals, for example, the
cells are all capable of rapid assimilation, growth, and reproduc-
tion in youth, but many of them nearly or wholly lose this power
after the animal has reached adult life. The nerve cells in the
brain and spinal cord, for example, seem largely to have lost
this property of assimilation, for they are unable to grow after
they have once reached the adult form, although able to repair
their own wastes. Later in life, nearly all the cells in the body
lose this power, a condition characteristic of old age. Speaking
generally, this power of assimilation and growth is most active
at the very beginning of the life of a cell; it continues for a
period with a gradually declining vigor and finally comes to an
end, starting vigorously again as the result of the process of
reproduction.

CELLS AND THE CELL THEORY 45

5. Reproduction. — Reproduction is the direct result of assimi-
lation; for assimilation produces growth, and growth in the end
results in division. All forms of reproduction take the form of
division.

The four properties, irritability, conductility, assimilation, and
reproduction, have been described as belonging to protoplasm;
and the mechanical theory of life has centered around this con-
ception. But in a sense it is misleading to call them properties
of protoplasm, unless in the term protoplasm we include all of
the contents of a cell, the nucleus as well as the cell substance.
A living cell shows these general properties; but the living
cell consists of protoplasm and nucleus, both of which are neces-
sary in order that all the functions mentioned should be shown.
The material frequently called protoplasm, i. e., the substance
outside of the nucleus, does not show all these functions. We
ask, therefore : What are the functions of the nucleus and proto-
plasm as distinct from each other? To draw a sharp line
between them is not possible at present.

3. THE NUCLEUS AND ITS SIGNIFICANCE (1880 TO THE
PRESENT)

In the early study of the cell the nucleus was looked upon as
an unimportant part, and in all of the early discussions its sig-
nificance was generally neglected. From about 1880 the modern
microscope and modern methods began to be directed towards
the nucleus, and a series of marvelous and unexpected results
were obtained, leading to the recognition of the nucleus as perhaps
the most important part of the cell, and as possessing a structure
of wonderful complexity and marvelous properties. The struc-
ture of the nucleus has already been outlined and may be seen in
Figure 12. These figures are enough to disprove any idea that
either cytoplasm or nucleoplasm can be considered a definite
chemical substance. They indicate clearly that in the simplest
life unit, we are not dealing with a homogeneous compound but
with a complex structure and a mechanism of delicate adjust-

BIOLOGY

ment. This has been made even more evident and brought to
a point beyond discussion by a study of the functions of the
nucleus.

A nucleus is necessary to the complete life of a cell. Among
the unicellular animals are some cells large enough for experi-
menters to cut to pieces in order to study the different functions
of the fragments. These experiments are very difficult and deli-
cate, but they have been carried on by a number of investigators
independently, who have demonstrated the following facts: If
a cell is cut to pieces in such a way that each piece contains a
fragment of the nucleus, ^each fragment is capable of carrying
on independently all life functions. Each can feed, grow, and

FIG. 16. — STENTOR.
A SINGLE-CELLED
ANIMAL; n, THE

LONG NUCLEUS

FIG. 17. — SHOWING HOW THE STENTOR,
WHEN CUT INTO TWO PIECES ALONG
THE LINE AB, DEVELOPS INTO TWO
COMPLETE ANIMALS

multiply, and seems to be lacking in none of the essential func-
tions of life; Figs. 16 and 17. If, however, the animal is cut to
pieces in such a way that some of the fragments contain
pieces of the nucleus, while others contain none, the frag-

CELLS AND THE CELL THEORY

ments act in totally different ways. Those that contain nu-
clear material are able to redevelop lost parts, to carry on their
life processes and to grow and multiply as usual; the fragments
that contain none of the nucleus, although they can move around
and apparently maintain life for a while, are unable to feed, or at
least to assimilate their food ; they are
unable to grow and unable to multiply;
Fig. 18. They have thus lost the most
essential features of life, since they
have lost the constructive power by
which protoplasm can assimilate and
grow. These experiments, repeated
many times over, show that the com-
plete life of a cell is impossible with-
out the presence of a certain amount
of nuclear material, but if nuclear
matter is present, the cell can carry
on its complete life, even though
the nucleus is itself cut into many
pieces. Such experiments, of course,
demonstrate very conclusively that
life functions cannot be carried on by
protoplasm alone, but only by proto-
plasm in combination with nuclear
substance.

The Nucleus in Heredity. — It is well to anticipate here one
further fact that demonstrates the great significance of the
nucleus and chromatin. As we shall notice on a later page,
nearly all animals and plants show a form of reproduction in
which cells from two different individuals, male and female,
combine. This is known as sexual reproduction or fertilization.
When this union takes place, it is not the whole cells that com-
bine but only the nuclei; or still more accurately, it is the
chromatin material of the cells that combines rather than the
whole nuclei. The reconstructed cell contains chromatin ma-

FIG. 18. — STYLONYCHIA.
A SINGLE-CELLED ANIMAL

If cut along the lines AB and
CD, only the middle piece con-
tains any nuclear matter; this
alone develops into a complete
individual, the other fragments
soon dying; n, the two nuclei.

48 BIOLOGY

terial from both of the cells which entered into the combination.
Now inasmuch as, after this combination, the offspring which
arises from the cell thus formed by the union of the two parental
cells inherits characteristics from both parents, and inasmuch
as the only part of the original sex cells which enters into the
union is the chromatin, it follows that the chromatin material
itself is the bearer of heredity, and that in these little chromatin
threads, minute as they are, there must be a complexity suffi-
cient to contain the features of inheritance that are handed on
from generation to generation.

These facts give at least some idea of the separate properties
of cell substance and nucleus. The cell substance by itself has
the functions of irritability and conductility ; but not of assimi-
lation, growth, or reproduction. These latter functions can be
carried on only when a nucleus is present.

WHAT IS MEANT BY PROTOPLASM

It has become evident by this time that the original con-
ception of protoplasm has quite disappeared. Indeed, if we
ask to-day just what is meant by protoplasm, the question
becomes very difficult to answer. We can no longer look upon
it as simply the jelly-like substance within the cell in which the
nucleus lies embedded, for it is evident that although this
substance has the properties of irritability and conductility, it
does not have the properties of assimilation and growth. If we
wish still to call protoplasm the physical basis of life, we must
extend the term to include the nucleus as well as the sub-
stance outside of the nucleus, since without the nucleus,
protoplasm is unable to carry on life processes. If, however,
we include, in this term protoplasm, the centrosome, and the
nucleus with its chromosomes, it becomes evident that proto-
plasm has quite lost its original significance. It is no longer
the homogeneous substance, and can no longer be looked upon
as a chemical compound, but is on the other hand a mechanism
with a number of distinct, though closely correlated parts.

CELLS AND THE CELL THEORY 49

The explanation of its activities can no longer be regarded as a
chemical problem simply, but must be in a measure a mechanical
problem as well. This conception totally alters the significance
of the phrase "the physical basis of life" and puts the prob-
lem of the mechanical theory upon a decidedly new footing.

To-day biologists are gradually giving up the use of the term
protoplasm as confusing and misleading, replacing it by more
definite terms which refer directly to the different parts of the
cell. So now we find coming into general use the terms cytoplasm
and karyoplasm (see page 32) to cover what was formerly called
protoplasm. Both cytoplasm and karyoplasm are necessary
and must act together in order to show the general characters
of life. That reproduction may occur, the chromatin, and per-
haps the centrosome also, are requisite.

The mechanical theory is no longer tenable in the form in
which it was originally advocated and discussed. That position
has been necessarily abandoned since the studies of more recent
years have demonstrated that protoplasm is not a homogeneous
substance and cannot be regarded simply as a chemical com-
pound. It is, on the contrary, a very complex mixture of sub-
stances, forming a complicated machine in which the parts are
most intricately interrelated and adjusted. While chemical forces
may be regarded as sufficient to manufacture almost anything
in the way of chemical compounds, they are not adapted to the
manufacture of such a mechanism as living protoplasm has
been proved to be. This change in the attitude of biologists
has been brought about mainly through the minute study of
the nucleus and the constantly increasing recognition of its
great importance in the life of the cell.

Are There Life Units Simpler Than Cells?— As we have
learned, the cell is by no means a simple structure but a compli-
cated mechanism. The question inevitably arises whether the
cell is the simplest structure that can manifest life or whether it
may not be analyzed into simpler units. This is one of the
puzzling and unsettled problems of biology. Certainly some of

50 BIOLOGY

the most minute living things (certain bacteria) seem to possess
a body in which there is no definite nucleus, but in which the
chromatin matter is more or less scattered without being aggre-
gated into a nuclear mass, and this has led to the suggestion
that perhaps the simplest life unit may be an excessively minute
granule of chromatin with delicate fibrils extending from it, and
that a cell is a combination of many of these minute elements.
Other facts disclosed by the minute study of many animal cells,
with very high magnifying powers and under special conditions,
have pointed to a similar conclusion. As a result there has been
advanced recently a theory that the cell is far from the simplest
unit of life, and that it can be analyzed into a great number of
minute elements called "chromidial units," each made of a
granule of chromatin with fibers of linin radiating from it.
According to this theory the whole cell is made of a network
of linin fibers with granules at the nodes, each granule thus
representing a life unit far simpler than a cell. This has been
called the "protomitomic network." This protomitomic theory
is as yet only a matter of speculation, and its chief interest
to-day is in the fact that it suggests that the cell may be far
from the simplest unit manifesting life. Whether this new
suggestion be established or not, it seems certain that the
manifestation of life requires the presence of three elements:
(1) chromatin material, (2) delicate fibrils radiating from it, and
(3) of a liquid material in which the other parts are embedded.
As yet we know of nothing simpler than a combination of these
three that is able to manifest all the properties of life.

LABORATORY WORK ON CELLS

A satisfactory study of cells requires familiarity with the microscope and
considerable skill in microscopic methods. Little can be wisely undertaken
by elementary students, beyond the examination of prepared specimens,
properly stained, which should be furnished by the instructor. Drawings
should be made by the student in all cases. The cellular structure of animal
tissues may be studied in the following preparations: —

Blood. — A small drop of frog's blood in a little normal solution (.9%

CELLS AND THE CELL THEORY 51

NaCL) examined with a 1/6 inch objective, will show blood cells, the red
cells having nucleii.

Cartilage. — Mounted sections of cartilage will show nearly rounded cells,
embedded in a very thick mass of cell wall, the thickened cell wall forming
the intercellular substance, or basis of the cartilage.

Bone. — Mounted sections will show cells lying in irregular spaces, within
a hard secreted mass of intercellular substance in which mineral salts have
been deposited.

The cellular structure of plants may be studied by the following prepara-
tions:—

Cork or wood sections show plant tissue made of numerous cells of varying
shape. In these sections the cell walls only appear.

A section of a growing root tip. Longitudinal sections of Podophyllum,
which are particularly good, should be furnished. These sections, if properly
stained, will show the cell contents as well as the cell walls. The protoplasm
and nucleus may be seen and drawn. In particularly good specimens,
stained with iron haematoxylin, the chromatin in the nucleus may be seen
with an oil immersion, 1/12 inch objective.

For the study of protoplasm Spirogyra is a favorable object. The student,
after studying the normal specimen, should treat it with a little glycerine,
which will cause the protoplasm to shrink away from the cell wall so that
it can be seen.

The movement of the protoplasm within the cell is best seen in the long
internodal cells of Chara or Nitella. It may also be seen in the stamen hairs
of Tradescantia.

Ci'iary motion may be studied best by cutting off a bit of the edge of the
gill of a fresh-water clam, and examining with a high-power objective. It
may also be shown by scraping the roof of a frog's mouth with a scalpel
and mounting the scrapings in a little normal fluid.

BOOKS FOR REFERENCE

WILSON, The Cell in Development and Inheritance, Macmillan Co.,
New York.

BAILEY, Text-book of Histology, Wm. Wood, Philadelphia, Pa.

STOHR, Text-book of Histology, P. Blakiston's Son, Philadelphia, Pa.

DAHLGREN and KEENER, Principles of Animal Histology, Macmillan
Co., New York.

MELL, Biological Laboratory Methods, Macmillan Co., New York.

HERTWIG, Die Zelle und Die Gewerbe, Gustav. Fischer, Jena.

CALKINS, Protozoology, Lea and Febiger, Philadelphia, Pa.

BERNARD, Some Neglected Factors in Evolution, G. P. Putnam's Sons,
New York.

CHAPTER III

UNICELLULAR ORGANISMS

IN order to become familiar with the general properties of
living things, we will study the structure and functions of some
of the simplest organisms. Those that are studied in this chap-
ter are all microscopic, and belong to the group of unicellular
organisms sometimes called animalculae.

ANIMALS

The first organisms to be studied are undoubtedly to be

regarded as animals.

AM(EBA

Size and Shape. — The Amoeba (Gr. amoibos — changing) is
a microscopic animal found both in fresh and salt water. The
most common species averages about 1/100 of an inch in diam-
eter, but the size varies in different species. With perseverance
they may be discovered in nearly all bodies of water where there
is mud and slime. One of the best methods of procuring them
for study is to collect water plants (Ceratophyllum) or even pond-
lily leaves, and to place them in dishes of water until they decay.
After a couple of weeks or so a brown scum appears and an
examination of this scum usually shows Amoebce in abundance.

Under the microscope the Amoeba is seen to be a single cell
without definite form, the same animal undergoing constant
changes in outline. Lobes are thrust out first in one direction
and then in another (Fig. 19), and as soon as one lobe is protruded
the contents of the body begin to flow into it and may continue
to flow until the whole body substance has passed into the lobe,
other lobes being formed in the meantime. By a continual pro-
trusion of such lobes and the flowing of the body into them, the
Amoeba has a slow motion. These lobes are thus used as organs
of locomotion and are called pseudopodia (Gr. pseudos = false -f
pous = foot).

UNICELLULAR ORGANISMS

There has been considerable speculation as to the forces which
produce pseudopodia, and various attempts have been made to
explain them by purely physical forces. It has been suggested
that they are due to the adhesion of the sticky substance of
which the animal is made, to the object upon which it rests.

FIG. 19. — AMCEBA PROTEUS

A, the animal in its natural condition; B, an animal that has swallowed a long filamentous
plant; C, the animal in the state of division.

cv, contractile vacuole; ex, remains of undigested food;

ec, ectoplasm; p, protoplasm.

en, endoplasm;

Another suggestion is, that the pseudopodia are due to changes
in surface tension produced by the currents in the body as they
flow to and fro. Still another theory seeks to explain the forma-
tion of pseudopodia by stereotropism (Gr. stereos = a solid +
trope =a turning), the attraction of a solid body for living
tissue, which is supposed to cause the body of the animal to
flow from one point to another of the surface upon which it
rests. There is also the theory of chemical attraction.

54 BIOLOGY

However, the production of these pseudopodia cannot be
satisfactorily explained by any of these means; enough careful
study of the Amceba in motion has been made to show that the
pseudopodia may be thrust out in any direction, either horizon-
tally or vertically; and when thrust out vertically they may be
bent forward until they come in contact with the surface on
which the animal rests and then become attached. Their motion
has to be explained by an active power of the living substance.
This power on the part of the living substance has been called
contractility, and it cannot be explained as due to any physical
force like surface tension, adhesion, or chemical attraction, but
is due rather to active contraction which must be regarded as
a general function of the protoplasm of a living cell.

Structure. — The body of Amoeba is made up of a transparent
mass of protoplasm, in which there may be distinguished an
outer clearer layer, called ectoplasm (Gr. edos = outside +
plasma), and an inner, more granular mass called endoplasm
(Gr. endon = within + plasma). No very definite line can be
drawn between them, the difference being due chiefly to the
presence of granules in the interior and their absence from the
outer layer. These granules are in motion, slowly circulating
within the animal, and thus showing the existence of currents
in the protoplasm. When the pseudopodia are protruded, the
first change is the protrusion of a lobe of the ectoplasm; after
which the granules can be seen flowing into the lobe until
finally the whole of the endoplasm may flow into the extruded
lobe. Many of these granules represent food in various stages
of digestion, some of them being digested food and others un-
digested refuse. Among them may be found drops of clear
liquid with a bit of digested food in their center.

Besides these granules, two more definite bodies are always
found. One (Fig. 19 ri), the nucleus, is a small rounded body
near the center of the animal, but not fixed in position, since it
moves with the protoplasmic current. This is one of the struc-
tural parts of the animal, not, like most of the granules, merely

UNICELLULAR ORGANISMS 55

extraneous material, and is always present in the living animal.
The other body commonly found is the contractile vacuole (Lat.
vacuus = empty) (Fig. 19 cv). This is a clear, pulsating drop,
at one moment appearing as a good-sized sphere, and the next
contracting and disappearing, to reappear again. It is thought
that when it contracts, its contents, which are liquid, are forced
out of the Amoeba's body through minute openings that appear
in its sides. These pulsations, which are fairly regular, plainly
indicate the performance of some important function.

Assimilation and Growth. — When the Amceba comes in con-
tact with a small plant or other bit of food, the pseudopodia
flow around and over it so that the food is taken bodily inside
the animal. The food may be taken in at any point on the
surface of the Amoeba's body, though more frequently it is
engulfed by the anterior pseudopodia. As shown in Fig. 19 B,
particles of food longer than the whole animal may be ingested.
After a time the bit of food thus ingested begins to show signs
of disintegration. It loses its sharp outline and becomes slowly
softened and dissolved. This change is produced by the action
of certain fluids which the animal secretes, and is a process of
digestion. The nutritious portions become in time absorbed
by the protoplasm and converted into new Amoeba substance;
the last process being assimilation. The refuse finds its way
eventually to the surface of the animal, a temporary opening-
appears and the Amoeba crawls away, leaving the refuse behind
it; Fig. 19 ex. Any part of the body may thus serve for the
ingestion of food or the ejection of refuse, although the food is
commonly taken in at the anterior end, and the refuse ejected
from the posterior end.

Respiration. — Amoeba is not only carrying on a process of
assimilation, by which new substances are built up, but is also
at the same time carrying on a process of disintegration, by
which the complex substances are broken down. This latter
is based upon oxidation or union with oxygen. As the result of
oxidation there is always formed carbon dioxid gas (C02) as a

5C BIOLOGY

waste product, which must be eliminated. The Amoeba is,
therefore, obliged to absorb oxygen gas from some source and
to eliminate carbon dioxid gas. This process of absorbing and
eliminating gases is known as respiration. In the Amoeba there
appear to be no special respiratory organs, although possibly
the contractile vacuole performs this function. But the body
of the animal is so small that special respiratory organs are
unnecessary, since gas is readily absorbed directly through the
surface of the body from the water in which the animal lives,
and carbon dioxid is as readily eliminated into the water. A
respiratory function is thus developed, but no distinct respira-
tory organs. The elimination of carbon dioxid gas, since it is
the getting rid of a waste product of metabolism, is not only
part of the function of respiration, but belongs also to the func-
tion of excretion.

Excretion. — As the result of this disintegration there arise
in the Amoeba disintegration products which are waste materials
and must be eliminated from the body. These products are
primarily three : carbon dioxid gas, water, and a product contain-
ing nitrogen, and related to urea which is excreted by the kid-
neys of higher animals. The function of getting rid of these
waste products is called excretion. In Amoeba the gas and
the water are • excreted directly into the surrounding water,
either through the general surface of the body or by the contrac-
tile vacuole. The urea is probably eliminated by the contractile
vacuole.

It should be clearly recognized that the elimination of the un-
digested portions of the food, mentioned on page 55, is not
excretion. These undigested parts of the food, though sometimes
called "excreta," have never become part of the Amoeba's body
and are simply foreign bodies that have been rejected as useless.
True excretion, on the other hand, always refers to the elimina-
tion of the products of dissimilation.

Relation to Water. — Protoplasm requires water for its activi-
ties. Ordinary active living matter contains 60% to 80% of

UNICELLULAR ORGANISMS 57

water, and some forms of protoplasm much more, certain
organisms containing over 95%. When dormant, protoplasm
may remain alive with a far smaller percentage, dried seeds
containing as little as 8%. Some animals also may be dried
(dessicated) and still retain their vitality for a long time. This
is true of many of the microscopic, unicellular animals and also
of some of the higher types (e. g., Hydatina; see Fig. 116). In
all such cases life activities are suspended but will be resumed
when the animal imbibes water.

Irritability. — The Amoeba has no sense organs nor does it
have any nervous system. It is difficult or impossible to deter-
mine positively whether it has any conscious sensations, but
it certainly has the power of reacting when stimulated, thus
showing that it possesses irritability.

Reaction to contact (Thigmotropism) (Gr. thigma = touch +
trope = a turning). — If the moving Amoeba is touched by a solid
object, the part touched draws away from the object, new
pseudopodia being thrust' out in another direction. If, however,
the object be a particle of food, the animal is differently affected
and the pseudopodia flow around it so as to engulf it.

Reaction to chemicals (Chemotropism) (Gr. chemesa = chemis-
try + trope ) . — If certain chemicals are brought in contact
with the Amoeba, it moves off in some other direction. Sugar,
lactic acid, sodium chloride, and many other substances have
this effect.

Reaction to heat (Thermotropism) (Gr. thermos = heat -{-trope).
—The activities of the Amoeba are directly dependent upon tem-
perature. At a temperature of freezing, no activities are mani-
fest. If the temperature is raised the activities begin and become
more active with the increase in temperature up to a certain
point, about 85° F. If warmed still more, they become less
active, and when heated to about 90° F. the activities cease en-
tirely. At about 105° F. the protoplasm is coagulated and the
animal killed. If a warm or hot object is brought near an
active Amosba the animal moves away from it.

68 BIOLOGY

Reaction to light (Phototropism) (Gr. photos - light -f- trope).—
If a strong light is directed upon an Amoeba from one side, it
will move away from the light. A strong, white light may cause
the animal to stop moving.

Reaction to electricity (Electropism) (Eng. electro -f- Gr. trope).—
If an electric current is passed through an Amoeba, it contracts
on the side of the positive pole of the current and moves toward
the negative pole.

In all these cases the Amceba reacts to a stimulus. But there
are other things which are irritable and react to a stimulus in
a purely mechanical fashion. Gunpowder is also irritable, since
it will react to heat with an explosion. A locomotive is irri-
table, since it will react to a touch upon its throttle valve. The
Amoeba certainly reacts in a more complex and more varied
manner, but the question inevitably arises whether the action
may not be simply that of a bit of machinery responding to its
appropriate stimulus. There is no definite answer to this ques-
tion that can yet be given.

Reproduction. — As the Amoeba by assimilation converts its
food into new protoplasm, it inevitably increases in size. If
this went on without interruption there would be no limit to
the size of the animal. But after growing for a time, a constric-
tion appears in the middle of the body which deepens until it
finally divides the animal into two parts; Fig. 19 C. Each -of
the resulting parts is like the other and each like the original,
except in size. It is the nucleus that seems to take the lead in
this process of division, which is one of great complexity. This
will be described in the next chapter, for it goes through the
complicated series of changes known as karyokinesis (Gr.
karyon = nucleus + kinesis = movement) described on page 85.
As a result of this division there arise two animals, evidently
alike, each of which now moves away and lives an independent
life. This method of reproduction, by which the animal divides
into two practically equal parts, is called fission.

A second method of reproduction sometimes occurs in

UNICELLULAR ORGANISMS

Amoeba. This is very unusual, however, and has been seen by
only one observer (Sheel). In this method the animal draws in
its pseudopodia, assumes a spherical form and secretes around
itself a thin shell called a cyst. Inside this cyst the nucleus
divides into many parts, some five or six hundred nuclei thus
finally arising by division. After this the rest of the substance
divides so that each nucleus finally becomes surrounded by a
little protoplasm, the contents of the cyst coming thus to con-
sist of some hundreds of little bodies, each with its nucleus.
Eventually the cyst bursts and the little cells escape, each being
now a minute Amoeba, which has only to grow, to be like the
original. This method of reproduction is also evidently a divi-
sion. It is a type of division called spore formation. The whole
process takes two and a half to three months, and the condi-
tions which bring it about are unknown.

FIG. 20. — SINGLE-CELLED ANIMALS RELATED TO AKKEBA

A, Difflugia, an Ama>ba-\ike animal with a shell made of pebbles; B and C, Podophrya
and Acineta, animals with stiff protruding tentacles of protoplasm; /, food; D, Arcella, an
Amoeba-like animal with a secreted shell.

PARAMBCIUM

Paramedum can usually be found in the same localities as
Amoeba and can easily be obtained by allowing lily pads to
decay in a dish of water. A quantity of living organisms soon

BIOLOGY

appears in the scum that forms on the surface, and among them

may be seen some minute white specks, just visible to the naked

eye, each one of which is a Parame-
dum; Fig. 21.

Like the Amoeba, it is a single cell,
and like the Amoeba also, it is made
up of protoplasm consisting of an
outer, somewhat clear ectoplasm and
an inner, more granular endoplasm.
The Paramedum has a body which,
although flexible, is somewhat rigid
and elastic, and, unlike the Amoeba,
always tends to preserve a definite
form. It is elongated, somewhat
blunter at one end than the other,
and in its motion carries the blunt
end forward. The protoplasm has
no power of protruding pseudopodia,
and the animal therefore does not
change its shape like the Amoeba.
Upon one side, posterior to the
middle of the body, there is a groove
extending obliquely backward. This
is the oral groove (og), at the bottom
of which there is an opening leading
to a short tube which extends through
the ectoplasm into the endoplasm.
The opening is the mouth, and the
tube is known as the oesophagus or
gullet; oe.

Locomotion. — The whole of the outer
surface of the animal is covered with
numerous, fine, threadlike projections

of protoplasm, protruding from the ectoplasm into the water.

These are called cilia and are capable of rapid motion back

FIG. 21. — PARAMECIUM

AURELIA

ci, cilia;

cv, contractile vacuole;

ex, excreta;

m, mouth;

mic, micronucleus ;

mn, macronucleus;

mb, membranella;

oe, 03sophagus;

og, oral groove.

UNICELLULAR ORGANISMS

and forth. Ordinarily in life, they are directed somewhat back-
wards, and as a result of this position, when they beat back and
forth they cause the propulsion of the animal forward through
the water with a uniform motion. When the cilia are directed
forward, their beating back and forth will cause the animal to
move backward. At the same time with their back-and-forth
motion they beat slightty to one side, causing the animal to
rotate slowly on its long axis as it moves either forward or back-
ward. Exactly how these cilia are able to move is not known,
but a power of automatic vibration is always characteristic of
these organs. Lining the tube called the oesophagus, leading
from the mouth, there are special cilia, longer than the rest and
united to form a vibrating membrane known as membranella ;
Fig. 21 mb. The function of this mass of fused cilia is to guide
the food from the mouth down through the oesophagus into the
body cavity. The direction in which the cilia point, and con-
sequently the direction of the motion they produce, are affected
by a variety of external condi-
tions, for the Paramecium, like
the Amoeba, is irritable and its
motions are regulated by the
surrounding conditions.

Structure. — The ectoplasm of
the Paramecium is somewhat
clearer than the endoplasm,
but it contains large numbers
of minute threadlike organs
known as trichocysts (Gr. trix
= hair + cystis = bag); Fig.
22 tr. These may be discharged
from the animal; and they ap-
pear to be organs of offense or

defense, since they apparently contain a small quantity of poison
by which the animal may kill or paralyze its prey or its enemies.
On the very outside of the ectoplasm is an extremely thin mem-

FlG. 22. — A BIT OF THE OUTER
EDGE OF THE PARAMECIUM
(HIGHLY MAGNIFIED)

(Modified from Maier.)

CM, cuticle;
ec, ectoplasm;
en, endoplasm;

fv, food vacuole;
tr, trichocyst.

62 BIOLOGY

brane known as the cuticle (cu), through which the cilia pro-
trude. This is ordinarily invisible and can only be seen under
special conditions. It is a protective covering which makes the
body a little more resistant than it otherwise would be. The
endoplasm fills the rest of the body and is very highly granular,
containing large numbers of food masses in various stages of di-
gestion. The nucleus is double, showing a large macronucleus*
(Fig. 21 win), and near it a small micronucleusf, mic. These
two bodies lie close together near the mouth and hold fairly
constantly their relative positions in the body of the animal.
Two contractile vacuoles (cv) are found in the common species
of Paramedum, one at each end. These vacuoles connect
with the different parts of the body by a number of minute
radiating canals, six or ten in number, which extend in all direc-
tions. Certain liquids are, apparently, poured into these canals
from the living protoplasm and through them flow into the
vacuoles, which increase in size until they reach a certain mag-
nitude and then suddenly contract and discharge their contents
to the exterior, probably through minute openings. The con-
traction of the vacuoles is fairly regular, varying in rapidity
with the temperature; the two vacuoles do not contract simul-
taneously, but alternate with each other. These organs, as in
the case of the Amoeba, are probably associated with the func-
tion of respiration and excretion.

Assimilation and Growth. — The food of the Paramedum con-
sists chiefly of minute bacteria. These are driven into the
mouth by the action of the cilia, and by the membranella in the
ossophagus, and then guided down the oesophagus to its inner
end. Here the bacteria collect in a little drop of water. The
oesophagus then contracts and pinches off this little drop con-
taining the bacteria, and thus forms what is called a food vacu-
ole, which enters into the general mass of the endoplasm and
follows the movement of the protoplasm around the body.
The digestive juices are secreted and gradually digest the bac-
* Gr. macros = large. f Gr. micros = small.

UNICELLULAR ORGANISMS 63

teria, the nutritious portions of which are absorbed by the body
and assimilated into new Paramecium substance. The un-
digested refuse portion is eventually discharged at the posterior
end of the body on one side. There is no perrrmnent opening
here, but whenever material is to be rejected a temporary
opening appears, at the point shown at Fig. 21 ex, and the refuse
material is discharged into the water. The process by which
food is used, including the absorption of oxygen and the excre-
tion of waste products, as well as the oxidation of the food
itself, is essentially identical with that in the Amoeba. As the
result of the process, the food material is eventually assimilated
into new Paramecium substance, and the animal grows, increas-
ing in size until it is ready for reproduction.

Irritability. — Paramecium is totally lacking in sensory organs
or in a nervous system, but like the Amceba it reacts to a variety
of stimuli. If an injurious stimulus is applied to one side of it,
the animal will reverse its cilia and move away from the irritat-
ing stimulus. It may move backward or it may turn its forward
end in any direction and move off to one side. It is attracted
by certain chemical stimuli and repelled by others. It is
affected by heat in the same way as the Amoeba. It is slightly
affected by an electric current, but is not affected by ordinary
light, although the so-called ultra-violet rays have an influence
upon it. These various reactions give to Paramecia an appear-
ance of conscious sensation, and it appears as if they had the
power of volition to enable them to avoid irritating or unpleasant
conditions. But the facts do not necessarily prove this, for it
is possible that these reactions are ^only mechanical responses
to stimuli, such as might be found in other machinery. The
responses, however, are so complicated, and so resemble those
of truly conscious animals, that it leads one to suspect that they
are actually conscious functions.

Reproduction. — The ordinary method of reproduction of the
Paramecium is by division (fission) similar to that of Amoeba,
although it is more complicated, since the animal is more com-

BIOLOGY

plex in structure. The first step in the process is the elongation
and division of the micronucleus into two parts, one of which
comes to lie at each end of the animal;
Fig. 23. This is followed by a similar
elongation and division of the macronu-
cleus. The oesophagus produces a little
bud which develops into a new oesopha-
gus, and then this and the old one move
apart, so that the latter advances to the
front part of the body, and the former
lies in the posterior part. A new mem-
branella develops in the oesophagus. Two
new contractile vacuoles make their ap-
pearance, one just in front of, and 'one
just behind, the middle line of the body.
Meantime a constriction has been mak-
ing its appearance, which gradually deep-
ens, cutting the animal into two parts
by a cross division. The two halves
thus produced separate from each other
and swim away to live an independent
life. It should be noted that in this reproduction each of the
important parts of the animal divides, so that each of the two
new individuals has a part of each organ which the original Para-
medum possessed. This multiplication by division may go on
almost indefinitely if the animal is properly fed and placed
under favorable conditions. Ordinarily it will occur about
once in twenty-four hours, although the frequency may vary,
becoming greater or less with varying conditions of food and
temperature. A continuous reproduction of this kind has been
followed for over 2500 successive divisions. Whether it can go
on indefinitely if the conditions were favorable is not known.
It is known, however, that under ordinary conditions this power
of reproduction gradually becomes less and less, and finally
tends to disappear altogether. It is believed that in nature

FIG. 23. — PARAME-
C1UM IN PROCESS OF

DIVISION

m, mouth; mac, macro-
nucleus; mic, micronucleus.

UNICELLULAR ORGANISMS 65

this disappearance of the power of multiplication and the nat-
ural disappearance of the race is prevented by the occurrence
of another process known as conjugation (Lat. con = together
+ jugare = to join).

Conjugation. — Two individual Paramecia come together and
place themselves side by side, adhering to each other as shown in
Fig. 24 a. They do not actually fuse together, but remain at-
tached. The micronucleus in each undergoes a series of changes
which results in its dividing into several parts, three of which
degenerate and disappear; c. Soon the fourth divides again into
two, one of which is slightly larger than the other; d. The
smaller part resulting from this last division passes over into
the other of the two conjugating individuals, the two animals
thus exchanging nuclear matter with each other, as shown by
the arrows in d. This small piece of the micronucleus, thus
exchanged by each individual, unites in each case with the larger
piece of the nucleus remaining in the other individual, and the
two combine to form a new nucleus, a fusion nucleus, shown
at /. The animals now separate, each of them carrying off in
itself a bit of the micronucleus from the other individual. The
old macronucleus next disintegrates and disappears (f), and the
fusion nucleus divides into eight parts (g) , three of which soon
degenerate. One of the five that are left remains as a micro-
nucleus, while the other four become macronuclei, at h. At this
stage of the process each Paramedum has one micronucleus and
four macronuclei. Next the micronucleus divides into two,
and the entire animal divides at once into two separate parts,
giving one-half of the micronucleus to each part. This gives two
individuals, each with a micronucleus and two macronuclei ; i to
k. The process is again repeated, the micronucleus and the
whole animal, except the macronuclei, dividing; the result is two
more individuals, each containing one micronucleus and one
macronucleus; I to m. This brings the animal back to its original
condition, and now the ordinary process of fission begins and
may go on again indefinitely, both micro- and macro-nuclei

BIOLOGY

FIG. 24. — CONJUGATION OF PARAMECIUM

ption of the various stages, see text; m
cleus; n, the micronucleus. (Modified from Maupas.)

For the description 9f the various stages, see text; m, in all cases represents the macronu-
"~ed fron

UNICELLULAR ORGANISMS 67

dividing with each subsequent cell division. Apparently the
purpose of this conjugation is an interchange of the material
present in the micronucleus; for it will be seen that after con-
jugation each of the resulting animals contains nuclear material
derived from the micronucleus of the other individual as well
as from its own

The Life Cycle of Paramecium.— We usually think of the life
history of higher animals as marked off in definite life cycles.
For example, from the egg of the hen develops the chick, which
grows into an adult hen and produces another egg and thus
starts the process over again. Such a life cycle we speak of as
comprising a single generation, and by the term individual we
refer to all the stages of the life of the organism between one
point in the cycle and the next similar point. When we attempt
to think of the Paramecium in a similar way, we find the case
so modified that the terms are somewhat difficult to apply.
But still in the Paramecium we can recognize a life cycle some-
what similar to that of other organisms. We shall learn in a
later chapter that the life of an animal like a hen begins with
a single cell, which, dividing by a process similar to that we have
just studied in the Paramecium, gives rise to a large number of
cells; see Fig. 15. These, however, remain attached to form
the individual which we speak of as the chick, which grows into
the hen, and which is thus composed of large numbers of cells.
This individual continues a separate existence and eventually a
single cell is separated from it to form another egg and to start
the process over again, in a new individual.

Now if we compare these facts with those just seen in the
Paramecium, we shall find that the life cycle of the Paramecium
is as follows: Starting in the cycle at the point where two ani-
mals separate after conjugation, there begins a series of cell
divisions which rapidly increases the number of cells. The cells
at once separate from each other, become perfectly independent,
swimming apart as quite isolated animals, In this respect the
development of the Paramecium differs very markedly from

68 BIOLOGY

that of the higher animals where the cells remain attached. But
the process of division is the same and may continue for a long
time. Eventually, however, as we have already seen, this
power of division by the simple process of fission becomes ex-
hausted, and the multiplication tends to die out. We can per-
haps compare this with the old age of a larger animal, for in old
age we find division becoming less and less vigorous, until it
finally ceases altogether and the whole generation of cells dies.
Among the larger animals, to prevent the extermination of the
race, a single cell, an egg, is set aside to start the process over
again, thus beginning the new cycle. In the case of the Parame-
tium, after the ordinary reproduction has gone on for a long time
it becomes impaired in vigor and seems to be started over again
by this process of conjugation. The process of conjugation,
therefore, corresponds to reproduction by an egg in one of the
larger animals or plants. Hence one life cycle of the Parame-
dum lasts from one period of conjugation, through all the nu-
merous successive divisions by ordinary fission, until again the
conjugation occurs to start a new cycle. One generation, then,
consists of all the members that arise between one conjugation
and the next; and inasmuch as these animals may multiply
almost indefinitely by ordinary division, it is evident that one
generation of Parameda may consist of thousands of organisms
scattered over a wide territory. It is evident, therefore, that
the term individual in the case of the Paramedum cannot have
the same significance that it has with the higher animals, since
the individual of one of the higher animals would correspond to
a combination of all of the different Parameda that arise from
the division of any single cell that comes from a process of con-
jugation, until again it enters into a process of conjugation with
another cell. Conjugation thus starts a new generation or a
new individual.

We do not know how long a time may elapse between two
successive conjugations in the case of a Paramedum, nor do we
know the conditions which bring about the process. We are

UNICELLULAR ORGANISMS 69

even ignorant as to its exact purpose, although it apparently
appears to be a process necessary to reinvigorate the race and
prevent it from dying out under the ordinary conditions of
environment. The process is evidently closely associated with
sex reproduction in the higher animals and plants, which is to be
taken up in a later chapter. We may even speak of the youth
and maturity of a Paramecium; by the term youth meaning the
period of rapid cell division that follows conjugation, and by
maturity and old age, the period of slower cell division that
appears later in the life cycle of the animal. Possibly we may
say that the animal eventually dies of old age, by which we
would mean that unless conjugation occurs the process of simple
division is brought to an end by exhaustion. Whether old age,
and therefore conjugation, are necessary in the life history of
Paramecium is not yet settled. Experiments have seemed to
show that under proper conditions fission may go on almost
indefinitely, certainly up to 2500 cell divisions, without the
necessity of conjugation, or without seeming to produce any
impairment in the power of division. In the normal life of the
individual it appears that conjugation is required, however, by
some of the conditions of life. Paramedumy therefore, has a
definite life cycle, although we do not know its possible length
or the conditions which modify it.

PLASMODIUM MALARIA

As an example of a still more minute animal, we will study the
malarial organism, Plasmodium malarice, which lives in the hu-
man body. Human blood contains minute circular disks known
as red blood corpuscles (see page 192), within which the malarial
organisms may be found in persons who are suffering from
malaria, or chills and fever. The organism first appears as an
extremely minute body (Fig. 25 a), in shape somewhat like
the Amoeba, though much smaller. It increases in size as
shown by the successive figures a to e. After reaching a size
which nearly fills up the red blood corpuscles, it breaks up

BIOLOGY

^T^Afc* £F f

FIG. 25.— THE LIFE HISTORY OF THE MALARIAL ORGANISM

This is shown in two cycles, the upper one taking place in the human
red blood corpuscles, and the lower one in the mosquito. For description
ol the individual stages, see text. (From various authors.)

UNICELLULAR ORGANISMS 71

into twelve to sixteen small spores, as is shown; / to g. The
blood corpuscle now breaks to pieces and the spores are liberated
into the liquid blood h. Each may then make its way into a
new corpuscle and repeat again the history as already described.

Although this animal in its general structure and shape is
much like the Amoeba, its habits are totally different. While
growing in the red blood corpuscles of the human body, it pro-
duces the disease which is known as malaria, chills and fever,
or fever and ague. The period when the chills occur corresponds
to the time when the blood corpuscles have broken up and the
spores are liberated into the blood. The organism may continue
to repeat the above history time after time in the blood of the
same person, the spores after being liberated entering into new
corpuscles, and again repeating their life cycle almost indefinitely
and prolonging the disease. There are three different species of
the malarial organisms, distinguished by the different length of
time required for their life cycles. The most common form takes
48 hours, a second species takes 72 hours, and a third is irregular.

By the method of reproduction above described, this organism
may multiply inside the blood of one person but is unable to
pass to a second individual. Malaria is therefore not communi-
cable as long as this process alone is repeated. But after a time,
for some unknown reason, the organisms in the corpuscles assume
two different forms shown in Figure 25 at g to i. One of them
grows into a large rounded mass, while the other develops sev-
eral long motile, thread-like bodies, which become detached. No
further change occurs unless the patient is now bitten by a cer-
tain kind of mosquito (Anopheles). If the blood of a patient
is swallowed by this mosquito, the malarial organisms undergo
a new series of changes. The thread-like bodies become de-
tached from the mass that produces them, and one of them unites
with one of the larger rounded masses, j and k. -This union is
regarded as a sex union (see Chapter XII), the larger rounded
mass being the female cell (or egg) and the thread-like body the
male cell (or sperm) in the sexual union. After the thread-like

72 BIOLOGY

body penetrates the egg, the nucleus it contains unites with the
nucleus of the egg, shown at k and I. After this union the com-
bined mass grows rapidly in size, I to o, and eventually breaks
up into an immense number of minute spores, p, greatly in
excess of those found at the stage g in human blood. These
minute spores lodge in the salivary glands of the mosquito, and
are ejected into the blood of the person bitten by the mosquito.
Thus a new human individual is inoculated with the spores,
which find their way into the blood corpuscles of the new victim
and produce the disease. It is not the most common mosquito
(Culex) that is concerned in this history, but one that is ordi-
narily less abundant, a species called Anopheles. From these
facts it follows that malaria will not occur in any locality unless
this particular mosquito is present; and further, that only the
mosquitoes which have previously bitten malarial patients will
be able to carry the infection.

It will thus be seen that the malarial organism passes through
two stages in its life cycle, reproducing itself in each by the
production of spores, though the spores are of two different
kinds; and that at one stage there is a union of cells of unequal
size, which may probably be regarded as a true sex union. All
stages of its life are passed within the bodies of other animals,
and it is thus wholly parasitic. The three different species of
the malarial organism have similar life cycles, though differing
slightly in details.

The malarial organism passes through two stages, in its life
cycle, each in different animals. Such a complicated history,
in which there is more than one distinct stage, is known as a
metamorphosis (Gr. meta = beyond + morphe = form). Many
other animals have a metamorphosis, one of the best-known
examples being that of the butterfly, which passes through
the well-known states of egg, caterpillar, cocoon, and butterfly.
Another example is the frog; see page 286. A metamorphosis
is thus found both among higher animals and also among the
lowest.

UNICELLULAR ORGANISMS

CHILOMONAS

This is an example of a still more minute organism found very
abundantly the world over in water among decaying leaves.
From Figure 26 it will be seen that its structure
is extremely simple. It has a slightly elongated
oval body, with a little depression at one end,
at the bottom of which food is taken into the
animal, the depression serving as a mouth. There
are no internal indications of organs, except a
small nucleus. At one end are two filaments
called flagella (Lat. flagellum = a whip), which
have the power of lashing to and fro. By means
of their lashing the Chilomonas is driven through
the water. Chilomonas multiples by simply dividing FIG. ' 26. —

into two, essentially in the CHILOMONAS

A very mi-

same manner as Amceba. nute, flagellate,

unicellular ani-
mal, found in
PANDORINA stagnant water.

Pandorina is an animal very similar
in its general structure to Chilomonas,
except that it is made up of a number
of cells grouped together, instead of
a single individual body; Fig. 28 A.
The method by which this group is
formed is simple. The animal starts
as a single cell, which divides, but

IVJ. *-' I . -1. VY V/ kJJLJ.-* VJTJJAJ * i.imJ-i-.**-' - -I • • . . i • J_ 1 /*

ANIMALS, RELATED TO after division the parts, instead of
CHILOMONAS separating at once, remain attached,

A,Gymnodinium; B,Ceratium. i 11 • /• • i

and there arises a group of sixteen

cells attached together. They secrete a little mass of jelly
around themselves and the flagella projecting through this
jelly enable the whole spherical mass to be rotated as a
unit. The individual members are somewhat independent of
one another, but are attached so as to form one single unit.
Such a group is called a colony.

FlG. 27.— TWO SINGLE-CELLED

BIOLOGY

Multiplication. — Reproduction of Pandorina is of two kinds:
1. Each of the cells of the colony divides into sixteen parts,

which, however, remain
attached together, mak-
ing a cluster of sixteen
groups of sixteen cells
each. Then the whole
colony breaks up, and
each group of sixteen cells
forms a new colony liv-
ing independently of the
others; Fig. 28 B. Thus,
by simply dividing, the
original colony produces
sixteen others.

2. By the second
method of reproduction
a conjugation occurs.
The cells of a colony
break into either sixteen
or thirty-two parts, and
then the whole mass
breaks to pieces, each cell
separating, not only from the colony but from its sister cells.
Among the hundreds of cells thus formed some are smaller than
others; Fig. 28 C and D. After swimming around for a while
one of the smaller and one of the larger cells unite with each
other; Fig. 28 E and F. The combined mass then secretes a red
shell or cyst about itself and remains dormant for a time, show-
ing no signs of motility, H. Later, however, it resumes its
activity and may divide into two or three parts, which then
escape from the cyst and swim around for a time as single cells,
called swarm spores, /. Eventually each divides into sixteen
cells which remain together, forming a new colony like the,
original, J

FIG. 28. — PANDORINA, A COMMON FRESH-
WATER, COLONIAL, UNICELLULAR ANIMAL

A, the animal in its adult condition.

B, showing the method of reproduction by simple
division, each cell dividing into sixteen parts and
the whole colony breaking up into sixteen colonies.

C to J shows the successive stages of reproduction
accompanied by conjugation ; C, the larger of the unit-
ing cells; D, the smaller ones;E, their conjugation;
//, the dormant condition within the cyst. For de-
scription, see text.

UNICELLULAR ORGANISMS

INTERMEDIATE ORGANISMS

The organisms thus far described are always classed as ani-
mals. We will now study two similar organisms, which stand
midway between animals and plants.
They are closely related, and yet one
of them is not infrequently classed as a
plant, while the other is almost always
placed with the animals.

PGRANEMA

Peranema is a microscopic organism
found in stagnant fresh water; Fig. 29 A.
It is elongated and tapers slightly in
front. At the narrower end, which is
carried forward in locomotion, there
projects a long motile flagellum, by
the motion of which the animal is moved
through the water. At the base of this
flagellum is an opening in the animal,
constituting a mouth, leading into a
short cesophagal tube. At the bottom
of this tube is a peculiar little rod-shaped
organ, which apparently serves as a suck-
ing organ for seizing food. Near by is
a clear contractile vacuole. The proto-
plasm of which the body is made is ex- FIG. 29.— Two SINGLE-
tremely flexible, and the animal, instead CELLED ORGANISMS RE-
of retaining its shape, shows a variety
of irregular wavelike contractions pass-
ing from end to end. A nucleus is
present, and the animal moves either mother respects they are much
by the motion of its flagella or by

creeping somewhat after the fashion of the Amoeba. As it
possesses a mouth and an resophagal tube, it lives on solid

SEMBLINGBOTH ANIMALS

AND PLANTS

A, Peranema; B,

76 BIOLOGY

food and thus resembles Paramecium and the other animals
already described.

EUQLENA

Euglena (Fig. 29 B) greatly resembles Peranema in shape and
structure. Like the Peranema, it has an elongated body, taper-
ing, however, at both ends. One end carries a long, motile flagel-
lum by means of which the animal moves through the water. It
is made up of flexible protoplasm and goes through a series of
contorted motions similar to those seen in Peranema. One or
more contractile vacuoles are found near the base of the flagel-
lum. The animal moves about either by its flagellum or by the
creeping motion noticed in Peranema. It has also a reddish
"eye spot" near the front end.

Evidently, these two organisms are very closely related. In
two respects, however, there is a striking difference, which has
led to the classification of the Euglena by some biologists among
the plants instead of among the animals. The Euglena probably
possesses no true mouth and does not take in solid food, though
this is disputed. Moreover, this animal is green, and since
green coloring matter is one of the distinctive characters of
plants, its presence in Euglena has led to much controversy
regarding the classification of this organism. Peranema with
its mouth and the animal habits should evidently be classed
with the animals, whereas Euglena, with its green color, would
naturally be classed with the plants; and yet their similarity
would lead to classing them together. A further consideration
of this subject will be given in a later chapter.

PLANTS

Although there is a difference of opinion in regard to the
classification of Euglena and Peranema, there is none in regard to
the organisms which are now to be described. The following
organisms are always recognized as plants, although some of
them, for reasons that will be given later, have certain charac-

UNICELLULAR ORGANISMS

ters that have caused biologists, in the past, to group them with
animals. Modern scientists, however, are unanimous in opinion,
grouping the following organisms among the plants.

PLEUROCOCCUS

Pleurococcus appears like a green stain, growing in abundance
upon damp tree trunks, fence posts, or even damp rocks. Upon
scraping off some of the material
and examining it with a microscope
it is found to consist of a great num-
ber of small green cells. These
(Fig. 30) are spherical, and contain
no visible internal organs except a
nucleus. The cells are found massed
together into irregular bunches,
but are not really attached together.
As they grow in size they divide by
fission in two parts, each of which
divides subsequently, the new in-
dividuals sometimes remaining at-
tached, to form irregular masses
which are easily shaken apart. No
other method of reproduction is
known. It is possible that this little
plant is really a stage in the life of
some higher plant whose develop-
ment is not yet known, since it

has been shown that some of the more complex plants have a
stage in which they are simple green cells like Pleurococcus.
Concerning this organism, however, nothing is known positively
except that it occurs abundantly in damp places and, so far
as known, has no other phase of its life than that already
noticed.

FIG. 30. — PLEUROCOCCUS

a, a single cell; 6, one showing
division by fission; c, a later stage
of division. The plant in its grow-
ing condition is bright green.

BIOLOGY

SACCffAROM YCES— YEAST

The yeast is a plant slightly smaller than Pleurococcus but
resembling it in its general shape, although it differs in some

important respects. It is made
up of single cells, usually slightly
oval in shape, although some-
times they are elongated and
occasionally spherical; see Fig.
31. These organisms are ex-
tremely minute in size, not
being more than 1/4000 of an

FIG. 31.- YEAST CELLS inch in diameter. They are so

showing budding and formation of groups small that almost no internal

structure can be seen, although

each one of them possesses a nucleus and a small vacuole

which is not contractile;

Fig. 32. As each of these

bodies possesses a nucleus,

it is a cell, and thus we see

that the yeast is made up

of clusters of single cells.
Reproduction. — The

method of reproduction of

yeast is by the growth of

buds on the side of the old

cell. The bud appears first

as a swelling, which grows

until it is the size of the FIQ 32>_YEAST CELLS MORE HIGHLT

original cell, and may then MAGNIFIED AND WITH INTERNAL

break away and become an STRUCTURE SHOWN

independent cell (Fig. 32), n, the nucleus;

v, the vacuole;

Of Several Of them may re- *» shows spores in the spore sac or ascus.

_ • 11 u J j. 4-1^.^ t The figures show that in budding the nucleus

mam attached together tor divide8j Kone portioil of it pa8Sing*into the bud

some time, forming a group and the other

of more or less independent cells. This process is called budding.

UNICELLULAR ORGANISMS 79

A second type of reproduction sometimes occurs in some
species of yeast. Under conditions not yet clearly understood,
the contents of a yeast cell breaks up into two, three, or four
parts which become surrounded by thick walls; Fig. 32 s. These
are called spores, or ascospores, because held in an ascus (Gr.
ascus = sac) or sac, and eventually they are liberated by the
breaking of the sac. Each spore is tnen capable of starting a
new series of generations of ordinary yeast cells. The spores
can resist drying and therefore serve to protect the yeast from
adverse conditions.

A comparison of Figures 30 and 31 will show that yeast and
Pleurococcus greatly resemble each other in structure; but there
is one important difference between them, for Pleurococcus is
green and yeast is colorless. This difference in color makes
a very great difference in their life; see page 131. Whereas
Pleurococcus may grow luxuriantly upon a fence post, and even
bare rocks, feeding upon the gases of the air, yeast is unable to
live and grow unless it is fed upon some organic matter, like
sugar. While yeast cells may be found widely distributed in the
air, in the soil, and in the water, they grow only where they find
organic food to eat, and chiefly in solutions containing sugar, like
fruit juices, etc. Elsewhere, in the soil or air, while they may be
alive, they are dormant.

The chief function of yeast in nature is to convert sugars into
carbon dioxid and alcohol. Sugar is produced in great quanti-
ties by various fruits and vegetables, and is eventually attacked
by the numerous yeasts that are floating in the air. After the
yeasts have acted upon it, the sugar disappears and in its place
can be found a gas, carbon dioxid (CO2), and a liquid, alcohol
(C2H6O). This is called fermentation, and it is used extensively
in the fermentative industries which produce alcoholic beverages,
like beers, wines, ales, brandies, etc. The fermentation by yeasts
is also made use of in the raising of bread. The yeast growing
in the midst of bread dough produces bubbles of carbonic acid
gas which cause the solid heavy dough to become light and

BIOLOGY

spongy. The bread made from such dough is full of holes, and
is more palatable and digestible than bread cooked from dough
that has not been rendered light and porous (i.e., unleavened
bread). In the case of bread raising and beer making, the yeast
as a rule is intentionally planted in the material which is to be
fermented. In the making of wines or the making of cider,
yeast is not planted. In these cases, the grape juice or the apple
juice is allowed to stand undisturbed, and the yeasts that are
floating around in the air, known sometimes as "wild yeasts,"
have an opportunity of getting" into the juices, where they grow
and produce fermentation. Thus although no yeast has been
added to these materials, the fermentation is brought about by
yeast exactly as if the yeast had intentionally been added.

BACTERIA

The simplest of all known living organisms are the Bacteria.
These consist of the extremely minute organisms shown in Figure

33. Some of them are spher-
ical, some are in the form of
short rods or long threads,
and some spiral. They are so
minute that practically no

o oo cooo

FIG. 33.— BACTERIA

A, rod-shaped form, Bacillus or Bacte-
rium; 1, Diphtheria bacillus; B, spiral forms,
Spirillum; C, spherical forms, Coccus; 2,
Streptococcus; D, the method of multiplica-
tion by division ; E, the formation of spores, s.

FlG. 34. A DIAGRAM SHOWING THE

RELATIVE SIZE OF THE POINT OF
A FINE NEEDLE AND BACTERIA

The small dots at the tip of the needle
represent bacteria.

internal structure can be seen. Some of them are not more
than 1/50,000 of an inch in diameter; see Fig. 34. Some

UNICELLULAR ORGANISMS

FIG. 35. — BACTERIA WITH FLAGELLA

A, flagella are distributed over the whole
body, a condition called peritrichic; B, flagella
grouped together in cluster at one end, called
Jophotrichic; C, a single flagellum, monolrichic*

bacteria (see Fig. 35) have minute flagella, which by lashing to
and fro cause them to move. Beyond the points shown in the
figures, there is very little to
be said concerning the struc-
ture of bacteria.

Reproduction. — Bacteria
all multiply, by fission, each
dividing into two parts,
which again divide when
they have grown to the
size of the parent cell.

Spore Formation. — Some
species of bacteria produce
spores in the following man-
ner: After growing for a
time by division the contents of a single bacterium collect into
a rounded mass which becomes surrounded by a hard resisting
wall; see Fig. 33 E. This is set free by the breaking of the bac-
terium that holds it and is then capable of starting a new series
of generations. This clearly resembles the ascospore formation
in yeast, except that there is no actual multiplication of indi-
viduals, one bacterium giving rise to one spore only. The
spores have resisting walls and are able to stand drying and
a fairly high degree of heat. Their function is thus that of
protecting the race from destruction by drying and heat rather
than that of multiplication, the latter function being performed
by the process of simple division.

Bacteria are very widely distributed in nature. They are
found in the air, in the soil, in all bodies of water, and, in fact,
practically everywhere. They play an extremely important
part in the life processes of nature through their relation to all
forms of putrefaction, decomposition, and decay. The bacteria
are important agents in maintaining the continued fertility
of the soil, making it capable of producing crops year after
year. A few species live as parasites within human bodies

*Gr. peri = around
Gr. lophos = tuft
Gr. monus = one

trix = hair.

& BIOLOGY

and th«ee of animals. These are pathogenic bacteria or disease
germs. They cause many of our most serious contagious
diseases like typhoid fever, tuberculosis, diphtheria, blood poison-
ing, etc. Thus, although they are extremely minute, bacteria
are agents of great importance in the world. It is hardly
possible to imagine anything more simple in structure, but
at the same time of greater importance, than bacteria.

LABORATORY WORK

The best method of obtaining material for laboratory work is to place
in a number of glass jars or shallow dishes pond-lily leaves, leaves of other
plants, algae of various kinds, or any other decaying organic material
from ponds and ditches. Fill the dishes with water and allow them to stand
undisturbed from one to several weeks. Various kinds of microscopic
organisms will appear in the different dishes, from which the desired organ-
ism can be chosen.

Amoeba. — A brown scum will usually appear in a few days on the surface
of the water covering the decaying organic material which is likely to contain
Amoebae. When this scum is scraped from the leaves and studied under a
1/6 inch objective it will usually disclose small specimens of Amoeba. The
animals should be studied alive and without any special treatment, since
they are sufficiently transparent, and slow enough in their movements to
show all the points in their anatomy, and nearly all the features mentioned
in the text may be seen without difficulty.

Paramedum. — These may be found in abundance in the scum from the
decaying pond weeds after they have been left for a week or more. Many
white, moving bodies, just visible to the naked eye, will be found in a drop
of this scum, which should be studied with a 1/6 inch objective. The chief
difficulty in studying them is due to their constant motion; various methods
of holding them quiet may be used. A bit of filter paper under the cover
glass will sometimes hold the individuals quiet in its meshes, or they may be
held quiet under a cover glass by supporting it on a small bit of paper, just
thick enough to hold them without crushing them. The animals are
to be studied alive, and a little patient examination of several specimens
will usually show most of the points of structure mentioned in the text.
To bring out the nucleus, a very weak aqueous solution of methyl green
should be run under the cover glass. If the solution is not too strong it will
stain the nucleii green, before affecting the rest of the organism. Animals

UNICELLULAR ORGANISMS 83

in the state of division may readily be found. Conjugation, however, is
rare and cannot be studied by a class.

The other unicellular animals mentioned in Chapter II may be commonly
found with Amceba and Paramedum. They cannot always be obtained,
however, and the student will often be obliged to omit them. Euglena
should not be omitted, however, if any appear in the dishes of decaying
pond weeds.

Pleurococcus. — The best method of obtaining this for studyis to find some
fence post or log which is covered with a green growth. This material
scraped from the wood will usually prove to be a mass of Pleurococci. No
special method of study is needed except to place a small quantity in a
drop of water and study with a 1/6 inch objective. The structure can be
readily seen and cells may be found showing division by fission.

Yeast. — A cake of ordinary compressed yeast furnishes excellent material.
A small quantity should be rubbed with a little water in a watch glass. A
minute drop of this material diluted still further in water, and studied with
a 1/6 inch, will show the structure of the yeast except the nucleus, which
can only be made out by special methods. Many cells showing buds may
be found in a fresh yeast cake. Such a yeast preparation usually contains
grains of starch, which may be distinguished from the yeast by running
a little iodine solution under the cover glass, which will turn the starch blue.
The starch has nothing to do with the yeast, being added to the cake to
give it body. A few drops of the yeast emulsion should be planted in several
large test tubes containing a fermentable liquid. Pasteur's solution is best,
but a little diluted molasses will serve. Pasteur's solution contains the
following ingredients : —

Water 837.60 c. c.

Grape sugar 150 gms.

Ammonium tartrate .......... 10 "

Potassium phosphate 2

Calcium phosphate .2 "

Magnesium sulphate .2 "

1000

If these tubes are placed in a warm place, 80° to 90° F., fermentation will
soon begin, and after a few hours bubbles of CO2 may be seen rising through
the liquid. After 12 hours a little of the scum or the sediment will show the
actively growing yeast. This growing yeast should be carefully compared
with the fresh, dormant yeast in the yeast cake.

Bacteria, — Only a little work can be done without special methods which
are complicated and difficult. Bacteria may be shown, however, as follows:

84 BIOLOGY

Spread a bit of any decaying matter (the decaying pond weeds will do very
well, or a bit of tartar scraped from the teeth) in as thin a film as possible
upon a slide, dry in air or fix by heat by passing it twice through a gas flame.
When thoroughly dry flood the slide with a solution of fuchsin or methylene*
blue and allow to stain for two to five minutes. Then wash the stain off in
running water, and place a cover glass over the stained mass on the slide.
The bacteria appear under a high power objective as minute stained dots,
or short rods. They are much smaller than yeast cells, and are only just
visible with a 1/6 inch objective. Higher powers are needed to study
them.

BOOKS FOR REFERENCE

BRONN, Klassen und Ordnung des Thierreichs, C. F. Winter, Leipzig.

DAVIDSON, Practical Zoology, American Book Company, New York.

HEGNER, Introduction to Zoology, Macmillan Co., New York.

HEGNER, College Zoology, Macmillan Co., New York.

HERTWIG, Manual of Zoology, translated by Kingsley, Henry Holt
& Co., New York.

JORDAN and PRICE, Animal Structures, D. Appleton Company, New
York.

MARSHALL, Microbiology, P. Blakiston's Son & Co., Philadelphia.

PARK, Pathogenic Bacteria and Protozoa, Lea & Febiger, New York.

PARKER, Elementary Biology, Macmillan Co., New York.

PARKER and HASWELL, Text-book of Zoology, Macmillan Co., New
York.

PRATT, Invertebrate Zoology, Ginn & Co., Boston.

*Methylene blue solution is made as follows: —

Saturated alcoholic solution of methylene blue . . . 15 c. c.

Potassium hydrate (1 : 10,000) 50 c. c.

To make a 1 : 10,000 solution of KOH, add 1 c. c. of a 10% solution ,to
99 c. c. of water and then add 5 c. c. of this to 45 c. c. of water.

CHAPTER IV

CELL MULTIPLICATION AND THE CELLULAR
STRUCTURE OF ORGANISMS

BEFORE undertaking the study of the multicellular organisms
we must study in detail the process by which cells multiply.
We have already seen that the Amoeba, Paramecium, and other
single-celled animals and plants have the power of dividing.
Indeed all active, growing cells have the power of multi-
plying by division. Although division seems a very simple
process, in reality it is unexpectedly complex. The internal
changes in the cell during division have been made out only
by long study. While they differ in many small details,
all cells agree in certain broad general facts. The process
known as karyokinesis or mitosis (Gr. mitos = thread) is alike
in outline in most cells and is as follows : —

CELL DIVISION OR KARYOKINESIS *

The Resting Cell. — In Figure 36^4. will be seen a cell in
the condition of rest, before it has passed into the stage of
division. It will be noticed that the centrosome is in the form
of two minute granules, and that the chromatin inside of the
nucleus is in the form of a diffused network. No other factors
need concern us at the present time.

1. Prophase. — The first stage in the division involves both
the nucleus and the centrosome. In the nucleus the chromatin
assumes the form of a long thread sometimes known as the
spireme. This condition, however, is only preliminary to the
breaking up of the thread into a number of short pieces which
are called chromosomes (Gr. chroma = color + soma — body) ;
Fig. 36 B. The number of chromosomes which arise in the
nucleus varies with different organisms but is constant for each
species of organism and is always an even number. In the

*As here described karyokinesis applies chiefly to animal cells.
85

BIOLOGY

type represented in Figure 36,
is invariably four.

FIG. 36. — DIAGRAM SHOWING THE

SUCCESSIVE STAGES IN THE PROCESS
OF KARYOKINESIS

A, the resting cell before it enters into the
process of cell division; H, the completed proc-
ess after the cell has divided into two parts;
ce, the centrosome; ch, the chromatin. For
description of the different stages, see text.

spindle, known as the equatorial
chromosomes and the separation

the number of chromosomes

The second part of the
first stage consists of the
separation of the two gran-
ules of the centrosome as
shown at B. As these parts
separate, they are seen to
be connected by fibers form-
ing what is called the spin-
dle. The granules continue
to move away from each
other until they finally
come to lie at opposite
poles of the nucleus, form-
ing the amphiaster (Gr. am-
phi = both -f aster = star)
as shown at D. They are
still connected by the fibers
of the spindle, which now
pass into the nucleus itself;
the nuclear membrane in
the meantime has disap-
peared. At the end of this
phase the chromosomes
have assumed a position
midway between the two
granules, lying on the mid-
dle of the spindle, and at
right angles to the line con-
necting them, at E. They
thus form a sort of plate be-
tween the two poles of the
plate. The formation of the
of the centrosomes may take

CELL MULTIPLICATION 87

place simultaneously, or one of them may precede the other;
the relative order of these changes varies and is a matter of
no especial importance.

2. Metaphase. — The second stage in cell division is a very
important one and is really the key to the process. Each of
the chromosomes splits lengthwise into two identical halves,
which at first are parallel, as at D. This splitting of the chro-
mosome into identical halves is for the purpose of dividing
equally the chromatin material, so that the two cells which
are to arise from the original cell may each contain one-half
of the chromatin rods of the original cell. The fact that the
chromosomes split lengthwise is of significance, for it is mani-
fest that if the thread splits lengthwise, the two halves will
be essentially identical, while if it should divide crosswise,
the two halves would not be necessarily alike. In the equa-
torial plate, at E, these eight chromosomes become slightly
flattened and are drawn more closely together.

3. Anaphase. — In the third stage, the two halves of each
chromosome begin to move apart. As shown at F, four of
the chromosomes move away from the equatorial plate toward
each of the two centrosomes. There is little doubt that the
minute fibers which connect the poles of this spindle are con-
cerned in the separation of these chromatin threads, though
exactly how they work is not known. Finally, the separate
halves of the chromatin thread are brought close to the minute
granules lying at the two ends of the spindle, at G.

4. Telophase. — The last stage in the division simply com-
pletes the process, for the essential feature of division has
already occurred. The chromatin threads, which have come
to lie near the pole of the spindle, now combine and form a
network, at G, much like that present in the original nucleus,
and a nuclear membrane forms around this mass of chromatin
material at H. The minute granule within the center of the
spindle pole is divided in two, either now or later; and thus
a complete nucleus is produced with a centrosome beside it,

88 BIOLOGY

containing two granules, at H; this nucleus is an exact repe-
tition of the one with which we started. Meantime a division
plane forms, passing through the cell midway between these
reconstructed nuclei, and the division of the cell into two
parts is now completed. There are thus produced two cells,
identical with each other and identical with the original cell,
each with similar chromatin material, since each contains
half of the original chromosomes. By this process, therefore,
the chromatin of the nucleus is continuous from one cell gen-
eration to another.

It will be evident that the essential purpose of this cell
division is the splitting of the chromatin material into identical
halves. It would seem much simpler for the cell to divide
immediately into two parts without this long process; but this
might not make the two parts equivalent. In order that they
may be equivalent, the cell adopts the complicated process
of karyokinesis. In the case described, the two final cells
are practically of equal size; but even in instances where the
cells finally produced are of very unequal size (Fig. 121),
the amount of chromatin in each is the same. Since, therefore,
the essential purpose of this process of karyokinesis is the
splitting of the chromatin, it is evident that this material
must be of extreme significance in the life of the cell. When
we combine this knowledge with the fact mentioned in Chapter
II, that the cell can carry on its life processes only when it
has nuclear material, it becomes manifest that the nucleus,
instead of being a negligible part of the cell, is really the cen-
tral feature of its life.

Nuclear Division without Cell Division. — As a rule, almost
immediately after the nucleus completes its division, the body
of the cell divides so that a cell does not contain more than
a single nucleus for any length of time. Occasionally, however,
the division of the cell body is delayed and the nucleus divides
a second time, and perhaps several times, before the cell body
divides, the result being one mass of protoplasm containing

CELL MULTIPLICATION

several nuclei. In most instances the division of the cell is
simply delayed and takes place later, so that finally the con-
dition of a single nucleus in each cell is resumed. This occurs
in the dividing egg of insects, for example. In some instances,
however, the cell body does not divide at all, and the continued
division of the nucleus produces a connected mass of proto-
plasm with many nuclei. This occurs, for example, in some
molds shown in Figure 42 E, in which there is no sign of cell
division, although there are many nuclei. Such a condition
is called a syncytium (Gr. syn = together + cytos = cell) and
is sometimes described as acellular. This multicellular state
with incompleted cell division is rare, for in most instances
the division is completed promptly.

Amitosis. — While division by karyokinesis is the common
method of cell division among all organisms, there are some
instances where cells divide without
going through these stages. This is
most likely to occur in the old age of
the cell when its vitality begins to de-
cline. In these cases, the nucleus di-
vides directly ; sometimes being simply
pinched into two parts (Fig. 37), some-
times being compressed into a middle
plate which divides into two halves
and then separates, and sometimes
forming two nuclear membranes in-
side of the original membrane which
then ruptures and permits the escape
of the new nuclei. In these cases,
it frequently happens that, though
the nucleus divides, the cell body
does not divide, so that 'there re-
sults a cell with more than one nucleus. This process of di-
vision is called amitosis (Gr. a = without + Lat. mitos = thread),
and it is thought to indicate a decline in the vigor of the cells.

FIG. 37. — DIAGRAM SHOW-
ING THE PROCESS OP
NUCLEAR DIVISION BY
AMITOSIS
(Modified from Wheeler.)

BIOLOGY

UNICELLULAR AND MULTICELLULAR ORGANISMS
All of the organisms thus far studied have been made up
of single cells, each cell being independent and capable of
carrying on all life processes within itself, although many of them
are quite complex, having several organs and much variety;
see Fig. 38. In contrast to these unicellular organisms we
shall find organisms made up of large numbers of cells (multi-
cellular organisms). All of the larger and higher animals
and plants in the world are made up of great numbers
of cells, each having the same general structure as the uni-
cellular organisms we have already studied. These larger
organisms begin their life as single cells and become multi-
cellular by the division of their cells into many parts.
There is no doubt that the ~ multicellular organisms of the

world must have been de-
rived originally from the
unicellular organisms.

Intermediate Types.—
While the organisms de-
scribed in the last chapter
are called unicellular, there
are some of them to which
this term cannot be ap-
plied with strict accuracy.
Pandorina (Fig. 28), for
example, consists of a
group of sixteen cells at-
tached in a spherical,
gelatinous mass. Each of
these masses of sixteen
cells has been derived
from a single cell by divi-
sion. It is a question

whether this organism should be called unicellular or multi-
cellular. It is certainly made up of more than one cell; but

FIG. 38. — BURSARIA. ONE OF THE LARG-
EST AND MOST COMPLICATED OF THE
SINGLE-CELLED ANIMALS

/, food;
m, mouth;

mb, membranella;
mac, macronucleus.

CELLULAR STRUCTURE OF ORGANISMS 01

on the other hand the cells are all alike, are all capable of
carrying on the various functions of life, and may be more
or less independent of each other.

Vorticella and Carchesium. — Other examples of types inter-
mediate between unicellular and multicellular forms are shown
in Figures 39 and 40. The Vorticella, shown at Figure 39 A,

FIG. 39. — Two SPECIES OF UNICELLULAR ORGANISMS

Showing the formation of colonies. A, a single-celled Vorticella; B, the process of divi-
sion; C, a single cell of Carchesium; D, a colony of Carchesium, produced by the incom-
plete division. Vorticella always separates after division, but Carchesium remains attached
as shown at D,

cv, contractile vacuole;
oe, O3sophagus;
m, mouth;

mac, macronucleus;
mic, micronucleus.

is unquestionably a single-celled animal, bell-shaped and pos-
sessing cilia, a mouth, oesophagus, vacuole, and a macro-
and micronucleus; the whole is attached to a stalk containing
a muscle which enables it to contract. This single cell divides
in a normal manner (B) and after division the parts separate

BIOLOGY

to become independent animals. In Figure C is shown another
cell much like Vorticella, possessing the same shape and similar
organs. In this animal, after the cells divide, they do not
separate but remain attached to a common stalk, and subse-
quently divide again and again, the result being a group of
similar cells connected by a branching stalk, D. This animal
is named Carchesium, and such a cluster is called a colony.
In this colony the members are independent, each carrying
on for itself all of the functions of life and each contracting
and expanding by itself independently of the rest. A third
species is found resembling Carchesium except in one respect.

In this animal, Zoo-

\ \ ' ' i •' '•/;/// / v thamniwn, there is a

1 ' ' 'i/ &&•••/ / ^^

common muscle ex-
tending through the
stalk and its branches.
When this muscle con-
tracts, all the members
of the colony contract
simultaneously.

These three animals
are evidently closely re-
lated; but Vorticella is SL
true unicellular animal,
Carchesium a cluster of
independent cells at-
tached together, and

Zoothamnium a similar colony in which the members are not
wholly independent but have a vital connection.

There are many other animals which are in a similar way
made up of colonies of cells, alike in structure and function.
Several of these are sho\/n in Figure 40. In all cases the ani-
mals start their life as single cells which become colonies by
the method of incomplete division. All these are commonly
classed among unicellular animals and called Protozoa (Gr.

FIG. 40. — COLONIES OF UNICELLULAR OR-
GANISMS MADE UP OF SEVERAL CELLS
ATTACHED TOGETHER

A, an animal with its pseudopodia protruding; in
the other specimens only the shell is visible. These ani-
mals belong to the group of Forminifera, whose shells
form chalk cliffs and limestone rocks.

CELLULAR STRUCTURE OF ORGANISMS

protos = first + zoon = animal), although they are not strictly
unicellular.

The same principle is illustrated by many of the lower plants,
of which a single example will be given.

Ulothrix. — One of the common fresh-water pond scums,
found everywhere in ditches by the roadside, is made of a
green plant, Ulothrix; Fig. 41. Ulothrix consists of a long,
slender thread formed by a row of nearly cylindrical cells,
placed end to end; Fig. 41 A. The individual threads are
barely visible to the naked eye. In each one of these cells
rnay be seen green coloring matter, chlorophyll (Gr. chloros =
green + phyllon = leaf), and each cell contains a nucleus.
The cells are identical from one end of the thread to the other,
differing only slightly in
size, and each of them is
capable of carrying on
all the functions of life
independently.

The reproduction
in Ulothrix is interest-
ing; and, like some or-
ganisms already studied,
it shows two quite
distinct methods. The
first and simplest is as
follows: The contents of
one of the cells breaks up
into several parts, which,
after a time, escape upon
the bursting of the
plant's cell wall. As they
come out, each is seen to
be provided with four little flagella and is thus enabled to swim.
They are called zoospores (Gr. zoon = animal) ; Fig. 41 a. After
swimming for a time they settle down, lose their cilia, and

FIG. 41.— PLANTS MADE UP OF COLONIES
OF SINGLE CELLS

A, Ulothrix. a, shows the process of multiplica-
tion by the formation of zoospores; b to /, show the
formation of sex cells, their conjugation with each
other; g, their subsequent division into spores;
h, a single spore which grows into a new thread, like
the original shown at large A. B, Pediastrium.

94 BIOLOGY

each begins to develop into a new filament like that from
which it originated. The growth into the new filament is by
division; the cells after dividing remain attached together in
the form of a long chain.

The second method of reproduction is by conjugation and
reminds us of that in Pandorina. In this case, the contents
of some of the cells break up into a large number of parts
instead of a small number, and these, by the bursting of the
cell wall, are finally liberated into the water ; Fig. 41 c.
They are then found to possess two flagella, instead of four
like the zoospores, and by means of these they swim around.
These small spores are, however, unable to grow into new
threads.* After the spores have been swimming about foi
some time they come in contact, as shown in Figure 41 d, and
fuse together, the fusion being identical with that already
described in Pandorina; see page 74. There are thus formed
conjugation spores known as the zygospores (Gr. zygon = yoke).
These zygospores, after a time, produce by division several
more spores which, upon becoming free, soon begin to divide
and grow into new filaments like those with which we started ..
This kind of reproduction is very similar to that of Pandorina
and clearly suggests the sexual reproduction which occurs in
higher organisms.

In the organisms thus described, we have examples which
cannot properly be called unicellular, nor on the other hand
can they be called multicellular; each one of these cells carries
on by itself all the functions of the organism, whereas in multi-
cellular organisms, as we shall presently see, the different cells
have different functions to perform, and the cells that make
up the individual are not all alike as they are in the forms
already described. We must look upon the Pandorina and
Ulothrix as intermediate between the unicellular and the
multicellular forms. In this way they illustrate the general

* Sometimes, however, they do grow into very short threads which are
much smaller than the original.

CELLULAR STRUCTURE OF ORGANISMS 95

biological principle that sharp lines dividing groups can hardly
ever be drawn, and it is almost always possible to find inter-
mediate forms connecting widely separate types.

True Multicellular Organisms. — Multi cellular organisms are
always made up of more than one cell; but the fact that they
consist of many cells is not enough to define them accurately.
A brief account of the manner in which multicellular organisms
develop will explain the meaning of the term. In all cases
they begin as a single cell, Which may be either an egg or a
spore. This cell divides into two parts, these into four, and
so on, the number of cells increasing indefinitely; but after
dividing, the cells remain attached instead of separating.
After a while some of the cells assume a variety of types, i.e.,
they become differentiated in form and function, and play
different parts in the life of the organism. Such a differentia-
tion of cells occurs in all true multicellular organisms. Hence
we may define a multicellular organism as one composed of many
cells which show a differentiation in structure and function.

With this differentiation of cells, tissues appear for the first
time. Cells with similar structure and function are commonly
grouped together, to form a tissue. The cells with special
contractile power, for example, form muscle tissue; cells with
power to secrete bone form bony tissue; and those in which
conductility and irritability are particularly developed are
grouped together to form nervous tissue; and so on. Tissues are,
of course, impossible among unicellular organisms, but univer-
sal among multicellular organisms.

With the multiplication of cells and their differentiation,
there also appears the formation of true organs. Among the
unicellular animals and plants there may be certain parts
of the cell, like the mouth and nucleus, set apart for certain
functions, and these are, to be sure, cell organs. But they are
not organs in the sense in which the term has been used among
the multicellular animals, where groups of cells, usually of
various kinds, are aggregated to form distinct parts with

96 BIOLOGY

definite functions, so that an organ contains several tissues
grouped together to form a complex structure.

In the study of multicellular organisms, which follows in the
later chapters, it will be seen that some of them have only a
few simple organs, while others have many complex organs.
Those which are of simple structure and have few organs we
call low organisms, while by high organisms we refer to those
whose structure is complex.

PEN1CILLIUM, A SIMPLE MULTICELLULAR PLANT

As an example of a multicellular plant with very slight
complexity, we will study one of the common molds, which
may be found growing upon almost any moist food the world
over. It may usually be obtained in abundance by placing
a bit of bread or a slice of lemon in a dish, covering it so that
it will be kept from drying, and allowing it to remain in a
warm place for a few days. The object will soon become
covered with a mold (Penidllium) which after a day or two
assumes a greenish-blue color. This organism is somewhat
difficult to study under the microscope because it is so massed
together that special methods have to be taken for preparing
the specimens. The best method is to plant some of the spores
upon a little jelly which has been hardened on a glass slide,
and then study the spores under the microscope every day
and notice the method by which they sprout and eventually
form the complete plant.

Structure. — The structure of Penidllium may best be under-
stood by studying Figure 42. It is made up of a mass of deli-
cate, branching threads, extending in various directions. These
threads are white or colorless and very minute. In the com-
mon species of Penidllium they are hardly visible to the naked
eye, although in some species of molds they are slightly larger,
and in others they are large enough to be plainly seen. These
threads, which are known as the mycelium (Gr. mykes = fun-
gus), have the function of assimilation, and absorb nourish-

CELLULAR STRUCTURE OF ORGANISMS

ment from the substance upon which the molds are growing.
Although the threads are very delicate, they can by growth
force their way through the substance upon which they are
feeding until they penetrate into the bread, or slice of lemon,

FIG. 42. — VARIOUS MOLDS

A, a colony of Penicillium, showing the fruiting spore-bearing masses arising from the
mycelium; B, a bit of the colony more highly magnified; C, one of the fruiting masses, form-
ing spores; D, a colony of Mucor; E, the sporangia of Mucor, with the spores emerging, and
showing also the mycelium below not divided into cells; F, a bit of the colony of Asper-
gillus, showing a third method of formation of spores.

or decaying apple, for some distance, and the material thus
becomes permeated with the mycelium. Careful study of the
threads of this mycelium with a high magnifying power shows
that they are made up of many cells. Cross partitions divide
the threads at intervals and separate the consecutive cells;
Fig. 42 B. The contents of each cell include protoplasm
and a nucleus. There is no differentiation of the cells, all in

98 BIOLOGY

the mycelium being essentially alike, although a single plant
may contain hundreds of these cells in its growing, branching
mycelium.

Reproduction. — The only noticeable differentiation of cells
that is seen in Penidllium occurs after the plant has grown
for a few days and is ready for multiplication. There may then
be seen arising from the mycelium minute branches that extend
vertically into the air instead of growing horizontally over the
surface of the object upon which the mold is nourishing itself.
These rise from the mycelium, simply as branches, and are
known as aerial hyphae (Gr. hyphe = web) ; Fig. 42 B and C.
The ends of these hyphae branch into a number of finger-like
processes, which extend vertically, parallel with each other, as
shown at C; after a time these branches divide by constriction
into rows of minute balls. These little spheres eventually break
off from the plant and then, blown by the wind, are scattered
far and wide. Each of them is capable, under proper conditions
of jnoisture and temperature, of developing into a new plant.
They are evidently spores, this particular kind of a spore
being named conidia (Gr. konis = dust). The conidia are
bluish in color and they cause the mold, which is at first white,
to assume a distinct blue tinge, giving to this plant its common
name of blue mold. They are extremely light and may be
blown for a long distance before settling to the ground. When-
ever they do settle upon any moist place they germinate;
each spore produces a new thread which in the course of a
few days becomes a new, branching mycelium and thus forms
a new mold. The conidia produced by a single plant are very
numerous and so light that they may be carried for a long time
in the air. Indeed, the air is at all times more or less filled
with them, in summer and winter alike; and it follows that
any moist material which will furnish them with food, like
bread, or pieces of lemon, or the surface of any fruit, if exposed
to the air for a short time, will be sown with these little spores,
and in a few days will begin to show signs of molding. So

CELLULAR STRUCTURE OF ORGANISMS 99

widely scattered are these floating mold spores that it is hardly
possible to expose any moist organic substance even, for a
few minutes, without its becoming inoculated with some of
them and showing, a few days afterwards, the growth of molds
upon its surface.

Penidllium has a second method of multiplication which is
rarely seen. It occurs only under special conditions which are
not understood, and it has not been observed by many bota-
nists. It consists in the formation of minute sacs, within which
spores are formed, usually four or eight in number. These
sacs are known as asci and the spores are ascospores. Even-
tually the sacs burst, the spores come out and are then capable
of developing into new plants. This method of forming spores
is evidently similar to that already described in Yeast (see
Fig. 32 s), and shows that yeast is closely related to the
molds. The same method of spore formation is found in a
large number of other plants (lichens, cup fungi, etc.) and is
used as a basis of classification for a class of Fungi called As-
comycetes (Gr. ascus = sac + mykes = fungus). It must be
no^ed, however, that not all of the molds form spores in this
way. The one shown in Figure 42 D has a method of repro-
duction by conjugation.

OTHER SPECIES OF MOLDS

Molds are very abundant in all parts of the earth wherever
there is much moisture, and any bit of organic material left
to itself will be sure to show signs of their growth in course
of time. Many species of molds, which to the naked eye
closely resemble each other, may be distinguished by careful
microscopic study. In all cases the plant is a branching,
colorless mycelium, similar to that described in Penidllium.
In a few species, however, the mycelium is not divided into
cells by partitions, as in Penicillium, but the whole thread
forms one continuous mass called a syncytium; Fig. 42 E.
The chief method by which the molds are distinguished from

100 BIOLOGY

each other is not by the structure and shape of the mycelium,
but rather by their method of producing spores. Penidllium
is one of the more common, but there are many other species
in which the spores are produced by different methods. Three
of these methods of spore formation are shown in Figure 42
C, E, and F. In some cases the spores are formed in a sac
called a sporangium, as at E. In others they are borne upon
a globular head, not inclosed in a sac; see F. Other species
show various methods; but in all cases the method of spore
formation is quite distinctive, and a careful microscopic study
of the different forms makes it possible to separate them into
species according to their methods of producing spores.

Molds play a very important part in the life processes in
nature. The term mold is not a proper scientific designation
for these plants, but a popular name, covering a variety of
plants of similar form and structure, but with many different
botanical relations. That they belong to different groups is
proved by the fact already mentioned that they have different
methods of reproduction, some of them forming ascospores,
while others form spores by a process of conjugation, which,
as we shall learn later, is a type of sexual reproduction.

LABORATORY WORK

The laboratory work that can be done by an elementary class upon
karyokinesis is very limited. Mounted preparations should be furnished
by the instructor. For this purpose the young growing root tips of Podo-
phyllum are excellent. If these are collected in the spring and carefully
preserved, sectioned, and stained in iron haematoxylin, they will show all
stages of cell division. Longitudinal sections are. best, and they should be
studied with a 1/12 immersion objective to make out the details. By
patient study of a few sections thus prepared the various steps in karyoki-
nesis may be made out.

If the instructor can furnish other examples of dividing cells the student
should make comparisons. Many tissues of animals and plants may be
utilized.

CELLULAR STRUCTURE OF ORGANISMS 101

Carchesium. — Colonial forms of Vorticella-\ik.e organisms, either Car-
chesium or Zoothamnium, may usually be found in aquaria in which various
fresh- water plants are kept. The dishes which have been prepared for the
culture of Amceba and Paramecium will frequently show them. If they are
obtainable they should be studied. No special methods are necessary, the
colonies being small enough to be placed under a cover glass and studied
alive. Staining with methylene green is useful to bring out the nuclei.

Ulothrix or Spirogyra. — One of these forms should be studied as an
example of filamentous plants. Either of them may be found in
ponds or ditches by the roadside. They are to be studied without any
special preparation, the fresh form showing most points perfectly well.
The shape of the cells and of the chlorophyll bodies should be noticed.
The nucleus may usually, though not always, be seen without any treat-
ment. A little glycerine added underneath the cover glass will cause the
protoplasm to contract from the cell walls. Staining with methylene green
will show the nucleus if it has not been seen without this. If material is
at hand to show the conjugation, it is desirable to have the student study
threads of conjugating Spirogyra and compare with the conjugation of
Paramecium described in the text. The reproduction of Ulothrix by for-
mation of spores is so difficult to obtain that it is impractical to furnish
material to a class for study.

Penicillium and Other Molds. — Molds may be easily obtained by allowing
bits of lemon, banana, bread, etc., to remain for a few days in a closed jar
in a warm place. The general appearance of the molds can be studied on
the surface of these articles. For a more careful study it is necessary to
study the colonies growing from spores. A simple method is as follows:
Prepare a culture medium from dried beans by placing a pint in about twice
as much water as is necessary to cover them. Allow to stand 12 hours and
add enough water just to cover the beans. Then strain off the liquid from
the beans and filter. To the filtrate add 1% of agar and boil so as to
completely dissolve the agar. Place the material in test tubes, about 10
c. c. in each, and plug the mouths of the tubes with cotton. Place in a wire
basket and sterilize by steaming for three-quarters of an hour on three
successive days. To use this culture medium, melt several of the tubes
of agar and pour each into a petri dish, allowing the agar to harden. When
thoroughly hard, remove with a platinum needle a minute quantity of
the spores, which appear on the mold on the lemon or bread, and just touch
the surface of the agar with the spore-laden needle tip in several places.
This will sow the spores. Place the petri dish (covered to prevent drying)
in a warm place. This dish may then be studied from day to day by
putting it under a microscope, and the sprouting of the spores, the

102 BIOLOGY

growth of mold colonies, and their production of spores can be followed
in detail. Several kinds of mold will usually start to growing on the lemon
etc., and may be distinguished by their color. The different species
will show differences in spore formation. Sketches of the colonies and their
method of spore formation should be made. The type which will be most
commonly found are Penicillium, Aspergillus, and Mucor; Fig. 42.

CHAPTER V

THE CASTOR BEAN, A COMPLEX

PLANT

THE plants hitherto mentioned do not
possess flowers and belong to what are
called the flowerless plants or Cryptogams
(Gr. cryptos = concealed + gamos = mar-
riage). As an example of the higher
multicellular plants we will describe one
of those producing true flowers, i. e., one
of the flowering plants or Phanerogams
(Gr. phaneros = open + gamos) . For this
purpose we will study the castor bean.

THE CASTOR BEAN (RIC1NUS COMMUN1S)

The castor bean (Ritinus communis)
is the plant from which castor oil is ob-
tained; it is also used as an ornamental
foliage plant on account of its large,
beautiful leaves. Other plants may serve
for this study, but this one illustrates
especially well the structure of the higher
plants. The seeds may be obtained at
seed stores and will readily sprout in
moist sawdust.

GROSS STRUCTURE

Figure 43, which represents a young
seedling of the castor bean about two
weeks old, illustrates the general struc-
ture of other multicellular plants, since
the higher plants are essentially alike in

103

MULTICELLULAR
P-.

FIG. 43. — A YOUNG

SEEDLING OF THE
CASTOR BEAN, THREE
WEEKS OLD

s, the stem; r, the roots;
f, expanded seed leaves;
p, permanent leaves.

104

BIOLOGY

this respect. It consists of a stem connecting two expanded
surfaces, the one ending in the leaves, and the other dividing
under the soil into fine rootlets which bear root hairs. Plants
obtain their food partly from the air and partly from the
soil, and this explains why they expand their branches into
leaves in the air, and their roots into root hairs in the soil. The
stem of the plant serves chiefly as a connection between the
leaf and the root and as a support for the branches and leaves.

STRUCTURE OF THE STEM

The structure of the stem may best be understood by begin-

cross section of a young
stem, shortly after it has

ning with the examination of
ep

emerged from the seed;
see Fig. 44. *

Fundamental Cells. -
The bulk of the stem
consists of a mass of ap-
proximately round cells,
which are called funda-
mental cells, p. These
cells are largest toward
the center of the stem
and grow smaller toward
the outer edge. The large
cells in the center form the pith. On the outer edge of the
stem is a single layer of small rounded cells forming the epi-
dermis (Gr. epi = upon + derma = skin), ep. Just beneath
the epidermis are several irregular rows of cells, larger than
the epidermal cells, known as the cortex (Lat. cortex = bark),
co. At this stage the cortex on its inner edge is not very sharply
marked off from the cells which fill the center of the stem,
and form the pith.

Fibrovascular Bundles. — A short distance within the cortex
will be found several groups of especially marked cells, }b,

FIG. 44. — A SECTION ACROSS THE STEM

OF THE SEEDLING

fb, the fibrovascular bundle; co, the cortex;
ep, the epidermis; p, the general fundamental
cells.

THE CASTOR BEAN

105

known as fibrovascular bundles (Lat. fibra — fiber + vas =
vessel). In the young stem there is a row of eight to ten of
these groups, arranged to form a ring a short distance beneath
the epidermis. The bundles do not actually touch each other,
but the cells of the pith and
the cortex are connected. v; i-.i < \9°

Structure of a Fibrovas-
cular Bundle.' — Figure 45
shows a highly magnified
view of a cross section of
one of these fibrovascular
bundles. It consists of three
parts : —

1. Running across the
middle are several rows of
small thin-walled cells
known as the cambium
layer, c (Lat. cambire= to
exchange). These cells are
full of active protoplasm
and are the chief growing
cells of the stem.

2. On the inside of this
layer, and therefore toward
the pith, is the xylem, x (Gr.
xylon = timber), a somewhat

FlG. 45. A HIGHLY MAGNIFIED SEC-
TION OF A FIBROVASCULAR BUNDLE

s, sieve cells;
t, tracheids;
x, is the xylem;
ph, the phloem part of
the bundle;

a, accompanying cells;

c, cambium layer;
co, cortex;

d, ducts;

pa, parenchyma;

st, stereome cells.

triangular mass of cells, the walls of which are thicker than
those of the cambium. Among them may be seen at least two
kinds of cells; one of small size but with very thick walls
forming the tracheids (Gr. trachea = windpipe) or wood cells,
t, and the other of larger size with relatively thin walls,
forming the ducts or vessels, d.

3. On the outside of the cambium, and therefore toward
the epidermis, is a somewhat irregular mass of cells called the
phloem (Gr. phloios = inner bark), ph, within which may be

106

BIOLOGY

cost

seen four kinds of cells. There are a few large cells called sieve
cells, Sj and near them some small cells called the accompany-
ing cells, a. Other cells still smaller and with thin walls form
the parenchyma (Gr. para = beside + en = in + chein = to
pour), pa, and a few cells, with very thick walls, are called
the stereome cells (Gr. stereos = solid), st. The cells of the
cambium do most of the growing; as they multiply they pro-
duce new cells both on their inner and their outer edge, causing
the bundles to increase in thickness by additions between
the xylem and the phloem.

Figure 46, a longitudinal section through a bundle, shows
the real shape of the cells. The cambium layer is composed

of slightly elongated

? ' f /? &• SP f P ce^s w^h scluare ends.

Each of these cells con-
tains protoplasm and a
prominent nucleus, dif-
fering in this respect
from the majority of
the cells of the bundle,
which are empty arid
represent only the cell
walls from which the
protoplasm has been
removed. The xylem
cells of the bundle,
forming the wood
proper, show several
types. The large ducts
have peculiarly marked cell walls. Some of them show rings
forming thickenings on the inside of the cell wall, or the
thickenings may take the form of a spiral, sp. Other ducts
show dots or pits and various peculiar markings, d. The
smaller cells, the tracheids, t, are much narrower than the
ducts, but have relatively thicker walls. Some have square

Phloem Xylem

FIG. 46. — LONGITUDINAL SECTION OF A
FIBRQVASCULAR BUNDLE

a, accompanying cells;

c, cambium cells;
co, cortex;

d, ducts;
p, pith;

s, sieve cells;

sp, spiral ducts;

st, sterome cells;

t, tracheids or wood cells.

THE CASTOR BEAN

107

ends and others have ends tapering to a point, the cells dove-
tailing to form the hard, resisting part of the stem; the phloem
outside the cambium layer also contains several kinds of cells.
Some of them are large and have oblique ends which are per-
forated by apertures that place one cell in communication
with the next above and below. Because of these openings,
these cells are called sieve cells. It is through these cells that
the food supply is transported through the plant from the
leaves. Close to the sieve cells are smaller cells, the accom-
panying cells, a, which are long and slender. The phloem
also contains many rather narrow cells with square ends called
parenchyma cells, and a few small, short cells with very thick
walls known as stereome cells.

The same longitudinal sec-
tion shows that the pith, p,
is made of short, square cells
with very thin walls. Evi-
dently the pith is a soft tis-
sue and the strength of the
stem is due to the hard and
resisting fibrous cells in the
bundles. Outside of the
bundles, directly beneath the
epidermis, it will be seen that
the cells of the cortex, co, are
much like those of the pith,
hardly longer than they are
broad, with thin walls and
square ends.

The relation of the fun-
damental cells to the fibro-
vascular bundles is better
shown in Figure 47, which shows how the bundles extend
through the stem and strengthen it. The bundles evidently
consist of very different material from that found in the pith

FIG. 47. — PERSPECTIVE VIEW OF A
PIECE OF A YOUNG STEM, SHOW-
ING THE FIBROVASCULAR BUNDLE
EXTENDING LENGTHWISE IN THE
STEM FOR SUPPORT

fb, fibro vascular bundle;
P, pith.

co, cortex;
ep, epidermis;

108

BIOLOGY

and cortex. They are mostly long, narrow cells with com-
paratively thick walls, which are hardened by the deposition
of woody substance. The name fibrovascular is appropriately
applied, since they are principally made up of fibers mixed
with vessels. The strength of a stem depends upon the density
of these bundles, and the thickness of the walls of the tracheids.
Of all this mass of cells only a few are filled with living
protoplasm. The cambium cells are always alive and the sieve
cells may contain protoplasm. The other cells contain proto-
plasm when they first form, but when they are fully grown
most of them are only the empty cell walls. This is particularly
true of the wood cells of the xylem. Protoplasm is more usually
found in the phloem and the cortex than in the true wood.

Arrangement of Bundles in an Older Stem. — An examination
of a slightly older stem shows that the bundles increase in

width and finally fuse.
In Figure 48 it will be
particularly noticed
that the cambium layer
of one bundle has grown
until it comes in con-
tact with the cambium
layer of the next, and
thus forms a cambium
ring extending around
the stem a short dis-
tance within the cortex
separating the outer
portion of the stem,

which is now called the phloem or bark, from the inner part,
the xylem, or wood proper. Later the other parts of the
bundles fuse, forming a complete ring of woody tissue and
a complete ring of bark separated by the cambium.

Remembering that this cambium layer is made up of actively
growing cells, it is easy to see how a stem of this kind may

fo c co

FIG. 48. — CROSS SECTION OF AN OLDER
STEM, SHOWING CAMBIUM FUSED TO FORM
A COMPLETE RING, C

(In other respects as in Fig. 43.)

THE CASTOR BEAN

109

FIG. 49. — DIAGRAM SHOWING THE METHOD
BY WHICH THE CAMBIUM LAYER PRODUCES
WOOD CELLS ON ITS INSIDE AND BARK
CELLS ON THE OUTSIDE

be, the cells of the bark;
c, cambium cells;
we, the wood cells.

increase in size. As the cells of the cambium layer divide,
new cells are formed between the bark and the wood of the old
bundles. Some of these new cells are formed inside of the
cambium layer, and outside of the xylem, as shown diagram -
matically in Figure 49.
Other cells are formed
on the outside of the
cambium and inside

of the old phloem
layer. These new
cells soon assume the
form of new wood
cells, new tracheids or
ducts on the inside;
while those outside
the cambium assume

the form of sieve cells, parenchyma, etc. It thus comes about
that the plant is producing new wood cells in the form of a
layer outside the old wood ring, and new phloem cells in a
layer inside the old phloem ring. The wood grows by addi-
tions upon its outer surface and the bark by additions to its
inner surface. Since the cambium forms a complete ring,
this method of growth evidently will produce a complete ring
of wood around the stem, and since the cambium cells con-
tinue to produce new cells during the whole of their active
life, they will continue to add new layers of wood on the out-
side of the old wood. The wood ring, which at first is only
a thin layer just inside the cambium, becomes thicker and
thicker as the growth continues. As it becomes thicker, the
stem, of course, increases in diameter, and, since the cambium
always remains on the outside of the wood, the stem may keep
increasing in size as long as the cambium cells are able to de-
velop new cells to be deposited as wood cells on the outside
of the old wood. In the same way the cambium deposits
masses of cells on the inner side of the phloem of the bundles,

110

BIOLOGY

and the bark also increases in thickness by growing on its
inner side. This growth is, however, not so vigorous as is that
of the wood, and the bark does not increase in thickness so
much as does the stem. Since too the new cells of the bark
are deposited on the inner side, the older parts of the bark
must stretch to cover the increasing diameter of the growing
stem. When a stem becomes of considerable size the outer
bark will be found to be rough and broken by the expansion
of the stem which it covers.

Some plants, which have but one year's growth, form a
single ring of wood as described, and die at the close of the
season. Other plants, like large trees, do not die, but live
year after year; and each year the cambium layer adds new
masses of cells outside of those previously existing. In plants
that live in regions where the climate changes with the seasons,

the cells formed by the
cambium layer are larger
at certain seasons of the
year than at others. In
temperate regions, the
wood cells formed in the
spring are larger and
relatively thinner walled
than those formed later
in the season. During
the winter, growth ceases
entirely; but as soon as
spring comes again, a

FIG. 50. — SECTION ACROSS AN EXOGENOUS

STEM OF FOUR YEARS' GROWTH, SHOW-
ING THE FOUR RINGS OF WOOD

new layer of large cells
will be deposited on the
outside of the last ring
that was deposited in
the fall. The result of

this is a series of rings easily recognized when a cross section
of a stem is made; Fig. 50. Since each ring indicates ordi-

fe, bark;

c, cambium layer;

w, wo<xl ring.

THE CASTOR BEAN

111

narily a year's growth, the age of the plant may be determined
by counting the number of rings. Such rings are rarely visible
in the bark, although the bark also increases in thickness by
layers added to its inner side.

From this description, it is evident that the growing part of
the stem is the cambium layer and that the stem of the plant
is capable of continuing its life only as long as this cambium
layer is intact. What is known as girdling a tree consists in
cutting a ring through the bark around the tree in such a way
as to destroy entirely the bark and the cambium layer; this
effectually kills the tree because the cambium layer is destroyed,
and unless there is a connection of living cambium between
the roots and the leaves, the life of the plant cannot be main-
tained. It is also evident why the bark may be stripped
away from the wood
of the tree so readily.
The inner edge of the
bark comes next to the
cambium; the cambium
cells are thin- walled,
full of soft protoplasm
and easily broken, and
hence the bark is easily
separated from the rest
of the tree at this point.

Medullary Rays. -
The cells in the vascular
bundle extend up and
down the stem. There
are, however, other
cells that run horizon-
tally, extending from
the center to the outer
edge. These form what are called medullary rays (Lat. medulla
= marrow); see Fig. 51. They probably serve for the trans-

$ ph

FlG. 51. — A PARTLY PERSPECTIVE VIEW,
SHOWING THE RELATION OF THE PARTS
IN THE STEM OF AN OAK

c, the cambium layer;
7», medullary rays;
ph, phloem;

s, stereome cells;
x, xylem.

112

BIOLOGY

ference of the material from the outer part of the stem toward
the center, or the reverse. This type of stem is called an
exogenous stem (Gr. exo = outside + genes = a producing) , a
name given to it from the fact that it grows by the addition
of new layers of wood upon its outer side. Such a stem may
increase enormously in thickness; some trees live for many
hundreds of years and become several feet in thickness.

There is, however, another type of stem which has a different
arrangement of the fibrovascular bundle. This is shown in

cross section in Figure 52,
which represents a corn-
stalk. In this section there
is no ring of wood, the fibro-
vascular bundles are scat-
tered irregularly through
the stem, and there is no
bark or true pith. More-
over, closer examination of
these fibrovascular bundles
shows that they do not
have any distinct layer of
cambium cells. As a result,

they have no growing layer and are not capable of increasing in
size. Such a stem is known as an endogenous stem (Gr. endon
= within + genes), and belongs to a type of plants, like the
grasses and bamboos, that grow tall and slender. Their stems
are only a little larger at the bottom than at the top and do not
materially increase in diameter. This type of stem forms a totally
different group of plants from the first, differing in many respects
in their leaves and flowers, as well as in their stem structure.

STRUCTURE OF THE ROOT

The structure of the root of the castor bean resembles that
of the stem, with some noticeable differences. A cross section
shows that the cortex is very much thicker than it is in the

FIG. 52. — CROSS SECTION OF
ENDOGENOUS STEM

ep, the epidermis; /, the fundamental cells;
fb, the fibrovascular bundles scattered indefi-
nitely through the stem.

THE CASTOR BEAN

113

stem, and there is also a layer of cells on the inner side of the
cortex known as the endodermis; Fig. 53. Within this are
the fibrovascular cells fused
together and showing little
definition into cambium
layer or fibrovascular bun-
dles. The pith is reduced
to a few cells in the center
of the root. The tip of the
root is always small and
delicate, yet it must force
its way through the hard

co, the cortex;
ep, epidermis;
en, endodermis;

fb, fibrovascular bundle;
rh, root hairs.

To protect them the

'en ep

FIG. 53. — CROSS SECTION THROUGH

A SMALL ROOT .
soil. The end of the root
contains delicate, thin-
walled, growing cells, which
would be injured in pushing their way.
tips of the roots are covered with what is known as the

root cap; Fig. 54. This is a
mass of rather hard corky
cells which covers the deli-
cate growing cells and pro-
tects them from injury as the
root pushes its way through
the compact soil.

On the outside of the root-
lets, chiefly near their ends, are
the most important structures
connected with the root, the
root hairs; Figs. 55 and 56.
They are very delicate threads
which grow out of the side
of the root and radiate from
it into the soil. Figure 56
shows a more highly magnified view of some of these hairs,
showing that it is a single cell arising from the epidermis of

FIG. 54. — A SEC-
TION THROUGH
THE TIP OF A ROOT

Showing the root cap, c.

Showing the
abundance of
root hairs.

114

BIOLOGY

FIG. 56. — CROSS SECTION OF
A MINUTE ROOT

Showing the relation of the root hairs to
the cells of the root.

the root. The root hairs are present in immense numbers
on the fine, delicate growing root tips, and grow in all direc-
tions into the soil. They are
thus brought into close contact
with particles of soil and serve
the plant as an organ for
absorbing water. All of the
nutrition that a plant derives
from the soil is drawn through
these root hairs, which are
closely connected with the
cells on the interior of the
root; so that liquids absorbed
by the hairs pass readily into
the substance of the root

itself. From here they pass from cell to cell, and eventually
find their way to all parts of the plant. The root hairs, con-
stituting the absorbing organ of the plant, are of great func-
tional value. If a plant is forcibly pulled out of the soil, all of
the root hairs are torn from the root and left attached to the
particles of the earth. If, however, the whole plant is removed
from the ground and the soil is carefully washed from the roots,
the root hairs may be found still attached to the rootlets, and
may show grains of sand attached to the root hairs.

STRUCTURE OF THE LEAF

A complete leaf consists of three parts: The broadly ex-
panded blade; the contracted stem or petiole; and two little
appendages called stipules attached to the base of the petiole
where it is connected with the stem. The stipules are not
present in all leaves and are not found in the castor bean.
Running from the top of the petiole out into the blade are a
series of fine veins; in some plants they run in a parallel direc-
tion (parallebveined leaves), and in others they branch profusely
into many small twigs (netted-veined leaves).

THE CASTOR BEAN

115

Minute Structure of the Leaf. — A section across the petiole
of a leaf shows a structure similar to that found in the stem
of a plant, except that there is no regular ring of fibrovascular
bundles and no cambium layer. In this petiole may be seen
several fibrovascular bundles separated from each other; and
if these are traced down to the stem from which the petiole
of the leaf arises, they will be found continuous with the fibro-
vascular bundles of the stem. Followed into the blade of the
leaf, these bundles are found to pass out into it and form the
veins. Thus the veins of the leaf are simply an extension of
a few of the fibrovascular bundles that come from the stem.
Being hard and tough, they give sufficient rigidity to the leaf
to support the softer parts, which are the active portions of
the leaf structure.

Microscopic Structure of the Blade. — A cross section through
the blade of the leaf is most instructive, since it is in the blade

ep-

FIG. 57. — CROSS SECTION OF A BIT OF THE BLADE OF A LEAF

cl, chlorophyll bodies;

ep, epidermis;

fb, fibrovascular bundles;

m, mesophyll cells;
p, palisade cells;
st, the stomata.

of the leaf that the most important function of plant life is
carried on. Upon its upper and under surface there are single
layers of cells, the epidermis; Fig. 57 ep. These are made of
small, irregular cells, closely compacted together and possessing

116

BIOLOGY

a hard cell wall forming a layer that is impervious to liquids

and even to gases. They form a covering of the leaf which

prevents the entrance of water, and
protects it from too great a loss of
water by evaporation. Through the
epidermis are numerous openings
known as stomata (Gr. stoma =
mouth), st, that serve as breathing-
pores. If a bit of the epidermis is
stripped from the leaf, it will ap-
pear as shown in Figure 58. The
cells of the epidermis are irregular
in shape, due to the irregular growth
of the leaf, and among them are nu-
merous pores. Each pore is sur-
rounded by two crescent-shaped cells,
guard cells, so related to each other
that the pore itself lies between the
two crescent cells. The guard cells
are capable of expansion and con-
traction under different conditions.
As they expand, they straighten out

and close the opening of the stomata; and when they contract

they shorten slightly and the opening

of the stomata is enlarged; Fig. 59.

In this way they can change the size

of the breathing pores of the plant

and thus regulate the amount of air

that passes in and out of the leaf.

These stomata occur in the epidermis

of the petiole and all over the leaf, less

abundantly on the upper side than on

the under side. In the leaves of water oc< suard cells-

plants, however, the stomata are chiefly on the upper side of the

leaves, where they are in contact with the air when the plant

FIG. 58. — THE EPIDERMIS
SHOWING THE STOMATA

A, from the leaf blade; B, from
the petiole.

FIG. 59. — DIAGRAMMATIC
CROSS SECTION OF A

STOMA

THE CASTOK BEAN 117

floats on the surface of the water. The shape of the stomata
and guard cells varies slightly in different plants, but their
structure is always essentially like that seen in the Figures
58 and 59.

In the middle of the leaf may be seen cross sections of the
veins, which are typical fibrovascular bundles (Fig. 57 /&),
composed of essentially the same kind of cells that we have
found in the bundles of the stem. The rest of the substance
of the leaf is filled with a loose mass of cells which are the
active cells of the plant. Immediately under the upper epi-
dermis is a layer of slightly cylindrical cells forming a fairly
definite row. These are called the palisade cells; Fig. 57 p.
They contain minute granules (chloroplasts) of green coloring
matter called chlorophyll (Gr. chloros = green -f phyllon =
leaf), cl, and each contains protoplasm and a nucleus. Below
the palisade cells are other cells more irregular in shape and
more loosely packed. In this part of the leaf these cells are
called mesophyll cells (Gr. mesos = middle + phyllon = leaf),ra,
and their shape is so irregular and they are so loosely packed
that many air spaces communicating with the exterior through
the stomata are left between them. These mesophyll cells
are filled with active protoplasm and crowded with chloro-
plasts (Gr. chloros = green + plastos = molded) . The intimate
connection which these chlorophyll-bearing cells have with
the air that enters through the stomata is evident from Figure
57, and is a matter of extreme significance, since these cells
extract from the air the food from which the plant manufac-
tures starch, the first step in the production of food for all
animals and plants; see page 129.

The epidermis of the leaves of some plants has various
other structures. Not infrequently it is prolonged into hairs
of various shapes and sizes; sometimes these hairs have a
little poison at their ends and then they constitute nettle hairs.
The general function of the hairs is to protect the plant from
injury by small insects and other animals.

118

BIOLOGY

REPRODUCTIVE ORGANS

The organs which are designed for reproduction are widely
different in different groups of plants. Among the higher
plants this function is carried on by specially modified branches
known as flowers. Although the greatest variety is shown
among the flowers of different plants, when compared they
are readily seen to have the same general structure. The fol-
lowing description is not that of the flower of the castor bean,
or of any other plant, but an ideal description of a typical
flower, and in a general way applies to the flowers of all the
higher groups of plants.

General Structure of a Flower. — A flower is always borne
at the end of a stem; even although it appears to come from

the side, when carefully exam-
ined it is found to be really
on the end of a short, unde-
veloped stem arising from the
side of the larger one. Indeed,
a flower is itself a short stem
bearing usually four rows of
leaves; Fig. 60. The stem of
the flower is called the pedun-
cle, p; at its top it is fre-
quently slightly enlarged to
bear the several rows of
leaves, this enlargement being
known as the receptacle, r.
The flower itself is composed
of four rows of leaves so
closely attached to each other
that they appear to arise at the same point of the stem;
careful study, however, shows that in all complete flowers
the four different kinds of leaves are produced one row above
the other.

FIG. 60. — DIAGRAM SHOWING THE
PARTS OF AN IDEAL FLOWER

r, receptacle;

a, the anther;
car, carpels;
p, peduncle ;
pi, petal;

r, recepta
s, sepals;
st, stamens.

THE CASTOR BEAN

lid

The Calyx composed of Sepals.— The lower row, which is
on the outer side of the flower, is made up of small parts which
are usually green and leaf-like in appearance. This row is
known as the calyx, and the leaves of which it is composed
are called sepals.

The Corolla composed of Petals.— Just above and within
the calyx, in an ordinary flower, is a second row of leaves,
usually larger than the calyx and of some brilliant color. This
row of leaves is known as the corolla and the individual
leaves as petals, pi. It is these colored
petals that give the flower its brilliancy,
and their function seems to be to attract
the insects, that are useful to the flower
in producing cross fertilization; see page
267. The calyx and corolla together are
sometimes known as the perianth (Gr. peri
= around -f anthos = flower). In some
flowers either the calyx or the corolla may
be lacking, and in others both may be lack-
ing. When only a single row of leaves is
found in the perianth, it is customary to call
it a calyx, irrespective of its shape and color,
and such plants are usually spoken of as apeta-
lous (Gr. a = without + petalon = a leaf).

The Stamens. — Within the petals is a FIG. 61.— THREE
third row of leaves, the stamens, st, which,
however, have almost wholly lost their re-
semblance to leaves. Each of these consists
of a delicate stem, called the filament (Fig.
61), at the top of which are little sacs, usu- of splitting open to dia,
ally two in number, which are known as the
anther, a. Within these sacs are. produced large numbers
of spores, the spores in this case being called pollen; Fig. 62.
The stamens are usually as many as the petals, although in
some flowers there are two or three times as many, and in

STAMENS WITH
D I F F ERE NT
FORMS OF AN-
THERS

a, showing methods
splitting open tc
charge the pollen.

120

BIOLOGY

FIG. 62. — DETAILS OF AN

ANTHER OF A FLOWER

A, section across the anthers,
showing the four cavities with the
pollen, p, enclosed; B and C, pollen
grains; n, nucleus; sp, the pollen cell
or microspore.

others some of the stamens disap-
pear. Some flowers are entirely
without stamens and are spoken
of as imperfect flowers.

Carpels. — Within the stamens is
the fourth and last row of leaves.
In this case the parts have lost all
resemblance to leaves and in ordi-
nary flowers they would never be
thought of as corresponding to
leaves, unless carefully examined.
The parts of this inner row are
known as carpels (Gr. carpos =
fruit); Fig. 60 car. Each carpel
consists of three portions, a lower,
somewhat expanded portion known
as the ovary (Fig. 63 ov), and above
this a more or less elongated, slen-
der part, called the style, s, whose
upper, slightly roughened surface
is known as the stigma, st. These
three parts form what is commonly
called the pistil. It frequently
happens that the number of car-
pels is less than that of the calyx,
corolla, or stamens. Moreover, the
carpels are often so fused together
that it is impossible to count dis-
tinctly the separate carpels of which
it is composed. When this occurs,
there is found in the center of the
flower what is known as a com-
pound pistil, i. e., a pistil made of

A, a pistil made up qi a smgie

Several Carpels fused together; See carpel; B, a compound pistil made

up of three carpels; s, the style; st,

Fig. 63 B. But it is usually ' "

A B

FIG. 63.— PISTILS

A, a pistil made up of a single

the atigma; ov, the ovary.

THE CASTOR BEAN

121

easy to perceive this condition in the pistil and to determine
the number of carpels of which it is made. The pistil shown
at Figure 63 B is evidently made up of three carpels, with
fused ovaries, but remaining more or less separated from
each other above. In some cases the style and stigmas, as
well as the ovaries, are fused together, and it is more diffi-
cult to determine the number; but even in these cases we
can easily distinguish in a compound pistil the number of car-
pels of which it is composed, by counting the number of rows
of seeds in the ovary, there being usually one row of seeds
for each of the carpels in the compound ovary.

In some flowers the carpels are entirely absent, and such a
flower is called an imperfect flower. A perfect flower is a
flower that has both sta-
mens and pistils, and such
a flower is capable of pro-
ducing seeds. An imper-
fect flower is one in which
either the stamens or the
carpels are lacking, and
such flowers are not alone
capable of producing
seeds.

Within the ovary are
found the true reproduc-
tive bodies. These at first
appear as several rounded
masses called ovules (Fig.
64), within each of which
is a single minute spore
cell, s, corresponding to
"the spores which form the
pollen. This spore never
leaves the ovule, but undergoes a series of changes within
the ovary which result in the production in each ovule of

FlG. 64. — A LONGITUDINAL SECTION OF
A PISTIL IN DIFFERENT STAGES OF
DEVELOPMENT

A, showing the immature ovules with the en-
closed spore, s; B, the older ovules, containing an
egg, e; C, the ripened ovary with the seeds, sd,
each containing a young embryo plant.

122 BIOLOGY

one or two eggs, e. As these spores produce eggs, which
are the female reproductive bodies, we may speak of them as
female spores. Older botanists, before their real
— -*\£ nature was understood, called them by the name
of embryo sacs. The small spores (pollen) pro-
duced in the anther, on the other hand, are
spoken of as male spores, inasmuch as their
function in reproduction is that of the male.*

Fertilization. — The pollen grains, or male spores
from the anther, are carried by some means to
the stigma of the stamen. They are sometimes
carried by insects, sometimes by wind, or by
various other means. The stigma on the top
of the pistil is usually rough and sticky, and the
pollen grains readily adhere to it. In this posi-
tion, the pollen grows and a long tube arises
from each pollen grain and pushes its way down
through the style and within the ovary; Fig.
65 pt. This tube is the pollen tube. In the
FIG. 65. — meantime the female spore in the ovary has pro-
LONGITUDI- duced the egg. The pollen tube is attracted to

NAL SECTION ., -, ~ ,? ., ,. • -,i

the egg, and finally its tip comes in contact with
CARPEL it- Inside of this pollen tube is found one or
showing the more special cell nuclei which are carried in the
?achedntop'the tip of the growing tube and finally pass into the
SKIS poT egg, fusing with it. This latter process is called
whichbehap's fertilization.

The The Seed. — After the egg, which is a single
cell, has fused with the contents of the pollen
tube, it divides, and in a few days produces a
little multiceilular plant. This plant, while still in the ovary of
the pistil, develops a stem and one or more leaves; Fig. 64 sd.

*The pollen because of the small size, is also called a microspore, and
the spore in the ovary, being larger, is called a megaspore or macrospore.
The significance of this we shall notice in a later chapter.

THE CASTOR BEAN

123

FIG. 66. — LONGITUDINAL SECTIONS

OF THREE SEEDS

Showing the enclosed young plant or embryo, e,
and food, /. In the middle figure, the food is deposited

After a few days it stops growing and becomes surrounded
by a hard shell, and is now known as a seed; in this form,
protected by its shell, it may remain dormant for some
time. If any seed is carefully examined it will be found to
contain a little plant, or
seedling, with a stem and
one or more leaves; Fig.
66. The leaves inside of
the seed are known as cot-
yledons, and while they
are true leaves they are
different in shape and
structure from the leaves
which this same plant
is to produce later when
the seed has germinated;
see Fig. 43.

There is also deposited in the leaves of the embryo; in the two other figures

the food is around the embryo.

in the seed, either around

the seedling or within it, a quantity of food upon which the
young plant can feed during the first few days of its life, before
it can feed itself from the soil.

This whole process of fertilization, growth into a little plant,
and the development of the shell around it to form a seed,
occurs within the pistil of the flower. The flower in the mean-
time withers and the ovary increases in size to accommodate
the growing seeds. Eventually, the fruit is broken open
(dehiscence) and the seeds drop out. When this occurs the
duty of the flower is over and all its parts decay, leaving the
plant without flowers until the next season. From this de-
scription, it will be seen that there are in the flower at least
four different kinds of reproductive bodies: the male spores,
or pollen; the female spores, or embryo sac; the eggs which
develop from the female spores and finally grow into seeds;
and the male nuclei inside the pollen tube which fuse with the

124 BIOLOGY

egg. The relation of these different bodies to one another and
to the general process of reproduction will be considered in a
later chapter.

LABORATORY WORK ON THE CASTOR BEAN

Seeds may be obtained at almost any seed store. For the study of the
seeds they should be soaked over night in water, which will soften them so
that the outer covering may be removed and the seed readily dissected.

The study of the plant structure should be made from young seedlings.
Soak the beans in water over night and then plant them in a box containing
moist sawdust, covering the box with a piece of glass to prevent evapora-
tion. Place the box in a warm place and water the seeds daily, keeping
the sawdust quite moist. The seeds will sprout quickly and at varying
periods of growth plants may be removed, the sawdust washed from their
roots, and the plants studied as a whole.

For the study of the stem both cross sections and longitudinal sections
should be made with a sharp razor, the piece of the stem to be sectioned
being held between two bits of pith which are hollowed out to receive them.
These sections may be mounted in water and studied directly, without
any further preparation. Some points can be seen more satisfactorily by
the use of various stains. It is best to begin with the study of a young
seedling about two inches high, and to follow with older plants which will
show the growth of the fibrovascular bundles and their fusion into a ring.
All of the points mentioned in the text should be studied.

The study of the root is made in the same way. To obtain root hairs,
it is better to sprout sunflower seeds by placing them, after soaking in
water, between two layers of blotting paper in a covered dish, which should
be kept moist and warm. After two or three days the rootlet of the young
seedling will show a mass of root hairs. They should be examined through
a lens without disturbing the seedling, and then one of the rootlets should
be placed in a watch glass in water and examined with a microscope.

The epidermis of the leaf may be studied by stripping off with fine forceps
a bit of the epidermis from the upper or under side of a leaf. Any plant
will serve for this, and it is well to examine the epidermis of several different
plants. The study should be made with a high power. The internal
structure of a leaf must be made by cross sections. These are very diffi-
cult to make, and prepared, stained sections should be furnished by the
instructor.

The stems of other plants showing annual rings of growth should alsc
be studied in both cross and longitudinal sections. Twigs of the pine,

THE CASTOR BEAN 125

apple, or oak which show about three years' growth are satisfactory. The
wood is hard to cut and is apt to injure the razor. It may be softened by
soaking the stem in a mixture of equal parts of alcohol and glycerine. The
stems should remain in this mixture for several days at least, and may be
left in it for months without injury, and be ready for section at any time.
For the study of a flower any simple wild-flower may be used to show the
general relations of the reproductive organs. A common Trillium is an
excellent example. The grosser anatomy of the flower should be studied;
sections should be made through the ovary both of a young flower and, if
possible, of the fruit after the flowering is completed, in order to show the
chambers of the ovary and the seeds with their attachments. The pollen
should be examined with a microscope.

BOOKS OF REFERENCE

ANDREWS, Practical Course in Botany, American Book Company,
New York.

ATKINSON, College Botany, Henry Holt & Co., New York.

BERGEN and CALDWELL, Practical Botany, Ginn & Co., Boston.

CALDWELL, Plant Morphology, Henry Holt & Co., New York.

COULTER, BARNES, and COWLES, Text-book of Botany, American Book
Company, New York.

CURTIS, Development and Nature of Plants, Henry Holt & Co., New York.

DUGGAR, Plant Physiology, The Macmillan Co., New York.

GANONG, Plant Physiology, Henry Holt & Co., New York.

MACDOUGALL, Plant Physiology, Henry Holt & Co., New York.

STEVENS, Anatomy of Plants, P. Blakiston's Son & Co., Philadelphia

STRASBURGER, NOLL, SCHENCK, and KARSTEN, Text-book of Botanyr
The Macmillan Co., New York.

CHAPTER VI
THE PHYSIOLOGY OF A TYPICAL PLANT

IN order to carry on its life a plant must have an income of
matter and energy. The problem of energy will be reserved for
a later chapter: only a consideration of the relation of plants
to their food and its utilization will be given here.

Plant Foods. — The income of an ordinary green plant is de-
rived partly from the air and partly from the soil. It consists
of:-

1. Carbon dioxid (CO2), absorbed from the air by the leaves.

2. Water (H20), absorbed from the soil by the root hairs.

3. Nitrates or other nitrogen salts, absorbed from the soil by

the root hairs.

4. Phosphates, potash salts, and other minerals, in small

amounts, absorbed from the soil by the root hairs.

The carbon dioxid and water are absorbed by the plant in
enormous quantities and constitute by far the largest proportion
of their foods; the soil minerals, although absolutely necessary,
are needed only in small quantities. Roughly speaking, the
amount of material absorbed from the soil is represented by the
ashes that are left after a plant is burned. All of the minerals
are dissolved by the waters in the soil and absorbed in this form
by the root hairs.

Ascent of Sap. — Since the foods are obtained through organs
situated at the opposite ends of the plant, in order that they
may be utilized they must be brought together, and since it is
in the leaves that they are utilized, the water, containing the
dissolved minerals absorbed by the roots, must be carried up the
stem to the leaves. This ascent of sap is going on constantly
during the activity of the plant and its rapidity is proportional
to the activity of the processes going on in the leaves and buds.

126

PLANT PHYSIOLOGY 127

The method by which the sap is carried up the stem is only
partially understood; there are several factors concerned. One
factor is osmosis. The water from the soil is absorbed by the
root hairs principally through the physical force of osmosis, a
force which is capable of causing some substances to pass, even
against resistance, through the thin-walled root hairs, while
others are rejected. An osmotic pressure is thus produced in
the root, due to the absorption of liquids from the soil, and this
forces a current up through' the stem.

A second factor is the absorptive power of protoplasm. Liv-
ing protoplasm has a strong avidity for water and absorbs it
until it is saturated. If a plant were in absolute equilibrium,
each bit of protoplasm would absorb all the water that it could
obtain and a condition of rest would soon appear. If, however,
a cell loses any of its liquid, it will have at once a stronger
demand for water than before, and will tend to draw it away
from neighboring cells that are more nearly saturated. Hence
in a plant there will be a constant flow of water from saturated
parts to those less saturated. In an ordinary green plant there
are several processes that use up the water, all of them especially
active in the leaves and growing buds at the top of the plant.
These are as follows: —

1. Water is being used in the leaves to manufacture starch.

2. New protoplasm is being made in the leaves and in the
growing buds, and this new protoplasm demands water.

3. Water constantly evaporates from the leaves through the
stomata (transpiration). The extent of this evaporation varies
greatly with the warmth and dryness of the air and also with
the extent to which the stomata are opened. When there is
abundance of water in the plant, the stomata are widely open
and evaporation is rapid; but when the water is insufficient
these pores partly close and evaporation is checked. On a warm
day when the air is dry the evaporation is increased, but in a
cool damp atmosphere it is lessened.

A third factor is capillarity; this is the same force that

128 BIOLOGY

causes oil to rise in the wick of a lamp. To what extent this
contributes to the flow of sap is uncertain.

These factors combine to produce a lack of water at the
top, and an excess in the roots, which produces a conse-
quent tendency of the liquids in the plant to flow upward;
the total result being a flow of the liquids from soil to root, from
root to stem, and through the stem to the leaf and bud. The
rapidity of this ascent of sap is directly proportional to the ac-
tivity in the leaves and buds, since this determines the extent
to which the water is used up. In warm bright sunshine the
life processes in the leaves are vigorous, the stomata open, and
the sap rises rapidly. At night the current is decreased, and in
winter the processes practically cease, to be revived again
when the warm sun of spring makes it possible for the cells in
the leaves and buds to resume their activity. It is known that
the water rises chiefly in the large ducts of the fibrovascular
bundles, the spiral and ringed ducts serving for this purpose.
It does not flow, however, in the cavities of these ducts, but
rather in their walls, passing from cell to cell within the thick,
but evidently porous, walls.

While these factors partly account for the rise of sap, they do
not explain the actual force which lifts the water, rising as it
does to the tops of the tallest trees. This is difficult to explain.
It is generally thought to-day that the three forces above men-
tioned are sufficient for the process: (1) Osmosis: this forces
the water from the soil, through the root hairs into the roots,
and probably from cell to cell within the plant, up through the
root and stem to the top of the plant. (2) Capillarity: this
force causes liquids to rise inside of small spaces, and
must play some part in the rise of water in the plant.
(3) Avidity for water: the demand for water of the protoplasm
at the top of the plant, above explained, is doubtless an
active agent also in producing the flow of water from cell to
cell up the plant. Whether these forces are sufficient to explain
the ascent of sap we do not know; but at all events the plant

PLANT PHYSIOLOGY 129

possesses no distinct circulatory organs, and it is believed that
tnese physical forces are sufficient to account for the lifting of
water from the soil to the leaves and buds.

Transfer of Substances Downward. — It is evident that there
must be a transfer of material downward as well as an ascent
of sap. As we shall presently notice, plants are engaged in
making starch in their leaves, and this starch is certainly carried
to all parts of the plant, since it may be stored in the under-
ground parts. The starch in a potato, for example, is made in
the leaves and hence it must be carried downward. The method
by which the material is carried from the leaves downward is
even less understood than the ascent of sap, although osmosis
is undoubtedly one of the factors. It is known, however, that
the starch is first changed to sugar and then dissolved in the
liquids of the plant. It is also known that these materials then
descend, not in the same cells in which sap is ascending, but
in the large sieve cells of the bark (see Fig. 46) , which are the
cells chiefly concerned in the downward current. Since the bark
is needed for this downward passage of food, we see another
reason why the cutting of the bark away from the tree for a
short distance, girdling, will in time kill the plant, since the food
materials made in the leaves cannot then be carried to the roots
and they will die for lack of nourishment.

PHOTOSYNTHESIS OR STARCH MANUFACTURE

By the process just described, the water, with the dissolved
minerals, is brought to the chlorophyll cells in the leaves. These
same cells are also in direct contact with carbon dioxid which
is in the air and is brought into the leaf through the stomata.
The chlorophyll-containing cells have the wonderful power of
causing the carbon dioxid obtained from the air, and the water
obtained from the soil, to combine with each other chemically
to form a new product. The transformation is represented by
the following equation : —

(Starch)

130

BIOLOGY

It must not be understood that this equation is an accurate,
statement of what occurs, for we do not know the details of the
building of starch from carbon dioxid. There is no doubt that
the process is far more complex than is. indicated by this simple
equation. The building of CO2 and H2O into starch is not done
by a single step as here represented, but in all probability by
several steps. Moreover, the starch molecule is by no means a
simple molecule as the formula C6Hi0O5 indicates, but some
multiple of this formula; how high a multiple we do not know,
but probably with many times this number of atoms in the
molecule. The above equation represents the ratio of the atoms

but not their actual number. While
the details of the method by which the
complex molecule of starch is formed
are not yet known to us, we do know
that the essential features represented
by this equation — namely, that C02
and H2O are combined, that starch is
manufactured, and that oxygen is set
free — are in the main correct. This
process is called photosynthesis (Gr.
photos = light + synthesis = composi-
tion), and it is the only known method
by which starch can be manufactured,
chemists having hitherto been unable
to make it by any artificial means.

From the above equation it will be
seen that while carrying on photosyn-
thesis, a plant is using up carbon dioxid
and at the same time liberating oxygen
and producing starch. The oxygen is
liberated in the form of a gas which
passes from the plant into the atmos-
phere. The liberation of oxygen may be easily demonstrated
by placing some kind of green water plant in a dish of water

FIG. 67

Showing a method of demon-
strating that a plant while grow-
ing eliminates oxygen gas. The
plant is a green water plant, and
the bubbles which arise from it
and collect in the tube prove to
be oxygen.

PLANT PHYSIOLOGY 131

and placing it in the sunlight. Minute bubbles of gas will soon
make their appearance on the plant, which will rise through the
water and pass off into the air. If these bubbles are collected in
an inverted funnel (Fig. 67) and tested chemically, the gas
proves to be oxygen. All green plants liberate oxygen when
growing in sunlight, a process that is exactly the reverse of the
respiration of animals, which absorb oxygen gas and liberate
carbon dioxid gas.

Photosynthesis is the foundation of all life, since the life of
all animals as well as plants depends upon starch. Its relations
to various external conditions are as follows : —

Chlorophyll. — Photosynthesis is dependent upon chlorophyll
and hence occurs in green plants only. Moreover, in these
plants, photosynthesis occurs only in those cells that contain
chlorophyll, and thus chiefly in the palisade and mesophyll cells
of the leaf, although it may take place in other cells if they
contain chlorophyll.

Sunlight. — Photosynthesis is dependent upon sunlight and
therefore never occurs in plants unless they are in the light.
The vigor of the process is dependent also upon the intensity
of the sunlight. It is most active in direct sunlight, less so in
diffused daylight, and stops entirely when light is withdrawn.

Carbon Dioxid. — Photosynthesis is dependent upon the
presence of carbon dioxid. Those plants which live in the air
will always have plenty of carbon dioxid, since the air contains
this gas. Water plants depend upon the gas dissolved in water.
The dependence of photosynthesis upon carbon dioxid can be
shown if a green water plant is placed in sunlight in ordinary
water, when bubbles of gas (oxygen) arise from it, showing the
presence of photosynthesis. If, however, this plant be placed
in a dish of boiled water which has been cooled, the bubbles do
not arise from its leaves, showing that photosynthesis does not
occur. Boiling the water drives off the carbon dioxid dissolved
in it, and the plant, having no carbon dioxid at its command,
cannot carry on photosynthesis.

132 BIOLOGY

Temperature. — Photosynthesis is dependent upon tempera-
ture. Even though the sunlight be brilliant, if the temperature
be below freezing photosynthesis cannot go on. It can, how-
ever, take place in temperatures very slightly above freezing,
and will continue from this point up to moderately high tem-
peratures. At higher temperatures, 120° to 130° F., the process
stops. The temperature at which photosynthesis goes on most
rapidly, the optimum temperature, varies with different plants,
depending upon the structure of the plant itself. Some plants
are so constructed that they can grow only at moderately low
temperatures, and others only at high temperatures. In some
of the arctic plants, photosynthesis, as well as all the other
functions of the plant, goes on very readily when the tempera-
ture is not much above freezing, whereas in tropical plants
photosynthesis does not occur unless the temperature is high.

METASTASIS

Photosynthesis may be spoken of as food manufacture, for
the starch thus made is later utilized for the life processes of
the plant. The use of this starch as food is generally spoken of
under the term metastasis (Gr. meta = beyond + histanai =
to place). This is too complicated a process to be described
here in detail, and only a few of the main features will be briefly
explained.

As already stated, the plants take in through their root hairs
not only water but a number of ingredients dissolved in it.
Among these are nitrates, phosphates, potash, and various other
substances in smaller quantities. All of these substances are
carried up through the plant and distributed so that each living
cell may receive some of this dissolved material. The starch,
formed chiefly in the leaves, as we have seen is converted into
sugar, chiefly in the night, and then transported through the
plant in the sieve cells of the bark. The living cells in the various
parts then take the water and minerals brought with the ascend-
ing sap, and the sugars brought from the leaves, and by changes

PLANT PHYSIOLOGY 133

of complex but unknown nature cause them to combine within
the cell protoplasm into new substances.

These new substances are of many varieties. The most im-
portant among them is the class of compounds which we have
already learned to call proteids. Proteids contain chiefly the
elements carbon, oxygen, hydrogen, and nitrogen, and are built
out of the nitrates and other minerals absorbed from the soil,
in combination with the sugars brought to them from the leaves.
Proteids are not the only substances manufactured in the plant
cells. Fats are produced which may be stored away in the plants
or used for other purposes. Wood is also made and deposited
around the protoplasm, forming the walls of the wood cells.
Numerous other substances are produced which we need not
mention, for the end result is the growth of all parts of the
plant which increases in size as these new substances are formed.
In all cases, however, the starch made by the leaves is the foun-
dation of the new substances made. Starch is always used up
and the plant can grow only so long as it has starch at hand in
abundance. This process of using starch and making other
substances is known as metastasis.

One of the results of the use of starch for any of these purposes
is a combination of part of its carbon with oxygen, forming CO2.
This is a process similar to the respiration of animals, and the
CC>2 is in plants, as in animals, a waste product which must be
excreted. It is thus seen that plants carry on two opposite
processes. By photosynthesis CO2 is utilized, starch is formed
and 0 is set free; by metastasis O is used, starch is destroyed and
C02 is set free. During the ordinary life of a plant in daylight,
although both processes are going on simultaneously, photo-
synthesis is much more vigorous than metastasis, and much
more starch is made by the plant than is used, so that oxygen
is constantly eliminated. Photosynthesis, since it takes place
only in sunlight, can occur only in the daytime, while metastasis,
requiring no sunlight, can go on in the night. The process of
metastasis goes on fully as well, and certain phases go on better,

134 BIOLOGY

in the darkness than in the light. As a result, green plants in
sunlight and in the daytime give off a surplus of oxygen, while
in the night they are giving off carbon dioxid but no oxygen.
Oxygen gas is a material that is utilized by animal life, while
carbon dioxid gas is a waste product of animals as well as plants.
Hence it has been said that, in the daytime plants are useful in
a living room, while in the night-time they are harmful. There
is really no foundation for this claim, since the amount of carbon
dioxid given off by a few plants in a room is so slight that it is
of no practical significance in its bearing upon animal life. In
nature, however, the plant and animal life balance each other;
while animals absorb the oxygen given off by plants, they them-
selves give off carbon dioxid that is utilized by plants; and thus
the condition of the atmosphere is kept practically constant
so far as concerns its content of both oxygen and carbon dioxid.

In general, plants manufacture far more starch than they
need for their own life. The surplus is stored in some form as
starch, sugar, fat, proteid, or some other material, and upon this
surplus the whole animal world is nourished.

All ordinary green plants carry on this process of photosyn-
thesis. Fungi, illustrated by bacteria, yeasts, molds, mushrooms,
etc. (Figs. 32, 34, 42), all agree in lacking the green chlorophyll
and are for this reason sometimes called colorless plants. Since
they have no chlorophyll they are unable to carry on the process
of photosynthesis, unable to utilize the energy of sunlight and
manufacture starch. But they must have energy as well as
green plants for their life, and are therefore dependent upon the
latter for their food. The Fungi are commonly found growing
and feeding upon organic foods, and are quite unable to utilize
the minerals of the soil and the gases of the air. They are
usually found, therefore, in the midst of masses of decaying
organic refuse, on dead tree trunks, in manure heaps, growing
from rotting leaves, etc. They feed upon the remains of past
generations of green plants, having, as we shall see later, a very
important part to play in nature's food cycle.

PLANT PHYSIOLOGY

135

PHOTOSYNTHESIS AND METASTASIS CONTRASTED

The relation between these two functions of plant life may
be better understood by the following contrast: —

PHOTOSYNTHESIS
Takes place only in green cells.
Takes place only in light.

C02 is absorbed and used up
and oxygen given off.

Carbohydrates are formed.
The plants grow in weight.

The energy of sunlight is stored ;
see Chapter XV.

METASTASIS
Takes place in all living cells.

Takes place equally well in
darkness.

Oxygen is absorbed and used
and CO2 given off.

Carbohydrates are destroyed.

The plants lose weight, but may
increase in size.

The stored energy of sunlight is
liberated and used.

The forces concerned in starch making and the building of
proteids and other materials are ordinary chemical and physical
forces. While we cannot cause these particular chemical com-
binations to occur in our laboratories, and do not understand
them fully, we do know enough about them to prove that they
belong to the ordinary forces of chemical affinity. In starch
making the atoms are combined in ordinary proportions, and
there is no reason for thinking that any other factors are con-
cerned besides those of chemical affinity.

MISCELLANEOUS FUNCTIONS OF PLANT LIFE

Besides the processes of photosynthesis and metastasis, the
only other prominent function of plant life is reproduction.
The two functions of motion and coordination, which are very
prominent in animal forms, are very slightly developed among
plants.

136

BIOLOGY

Motion. — The most striking distinction ordinarily recog-
nized between animals and plants is the absence of the power
of motion in plants and its presence in animals. This distinc-
tion, however, is by no means a sharp one, for motion is not
wholly lacking in plants. Many of the lower types of plants
are capable of locomotion. This is confined largely to the micro-
scopic forms, and in some plants it is present only in their re-
productive spores. For example, Ulothrix (see page 93) is a
motionless organism in its ordinary adult form, but produces
reproductive spores, called zoospores, which swim rapidly in the

water. Among other
microscopic plants,
locomotive power is
found even in the adult
life of the animal.
This is true of Osdl-
laria, Diatoms, and
some other organisms ;
Fig. 68. Among the
higher plants no active type of locomotion is found, although
many of them are constructed in such a way that they may be
carried to and fro by motile animals. Even among the highest
plants, however, a certain amount of motion is developed in the
different parts of the plant. Among the flowers of the highest
groups of plants, motion is developed in certain parts ot the
flowers for the distribution of pollen. In most of the highest
class also, careful study has shown that the leaves are constantly
in a state of slow motion, waving to and fro during the growth
of the plant in sunlight. Of course the leaves are almost always
moved by the wind, but quite independently of air currents they
have a motion of their own which can be detected by a careful
recording apparatus. It is thought that this motion is due
principally, perhaps entirely, to the unequal evaporation of
water on different sides of the stem. At all events it is so slight
that it can hardly be considered true motion, and it certainly

FIG. 68. — THREE PLANTS HAVING

THE POWER OF MOTION

A and B, Diatoms, which move readily through water;
C, Osdllaria, which simply waves back and forth.

PLANT PHYSIOLOGY 137

is not locomotion. In addition to this, some plants have the
peculiar property of closing their leaves in the night. The leaves
droop and close themselves in such a way as to present a small
surface for evaporation. This motion is sometimes spoken of
as the sleep of plants. It is not developed in all, but it is more
common than has generally been believed.

Thus, while it is believed that plants do not as a rule possess
the power of motion and, except in the lowest forms, no power
of locomotion, it is not absolutely true that motion is lacking
in the vegetable kingdom. Speaking in general, however, plants
are characterized by absence of motility.

Coordinating Functions. — Plants have nothing whatever that
corresponds to a nervous system in the sense of possessing nerves
or nerve fibers which coordinate the different parts of the body.
There is practically no coordination between the functions carried
on in the different parts of the plant. True sensory functions are
also lacking from plants. In a general way the protoplasm of
plants, as well as that of animals, is sensitive. All protoplasm
reacts under certain stimuli and is therefore sensitive. Moreover,
there are some of the higher plants which react so quickly and so
strongly to certain stimuli that they are spoken of as sensitive
plants. In the common so-called sensitive plant a touch upon
the leaf will cause the leaf to close, and a slight touch of the
branch will cause all the leaves on that branch to droop. Such a
condition, however, is very unusual among plants, and in these
cases it is incorrect to speak of the plants as sensitive in any
proper" sense. There is no reason for thinking that the plant has
any sensation, i.e., any true consciousness; and all that is meant
by being sensitive in these cases is a quick ability to respond to
an external stimulus.

CHAPTER VII

MULTICELLULAR ANIMALS: HYDRA FUSCA
GENERAL LIFE FUNCTIONS OF ANIMALS

THE life of animals is much more complicated than that of
plants and the animal body is correspondingly more complex.
It will make the study of multicellular animals more intelligible
if at the outset we notice certain general functions of life that
are exhibited by all higher animals. They are as follows: —

Alimentation (Lat. alimentum = food). — The process of
food getting is called alimentation. The organs concerned in
it are those that take food into the body, those that digest it,
and finally those that absorb it into the circulating medium.

Circulation. — The process by which food and other ingredients
are transported through the body is called circulation. Usually
it is brought about by a circulating medium called the blood, by
a series of tubes in which the blood is carried, known as blood
vessels, and by a pump, or heart, designed to keep the blood in
motion. In some of the smaller animals this system of organs
is far simpler, neither blood vessels nor a heart being present;
but some form of circulation is always found.

Respiration. — The chief chemical process in the animal body
is oxidation, i.e., the combination of the food with the oxygen.
For this purpose, oxygen gas must be absorbed by the blood.
As a result of the oxidation of the food another gas (CO2). arises,
which is also taken up by the blood and must be eliminated,
since it is a waste product. The function by which these two
gases (O and CO2) are absorbed and discharged is called respira-
tion. Respiration is thus a gas exchange that takes place be-
tween the body and the surrounding medium.

Metabolism (Gr. meta = beyond + ballein = to throw). —
The foods taken into the body are eventually combined with
the oxygen taken in by respiration and as a result new products

138

HYDRA FUSCA 139

arise, some of which are useful, while others are waste products.
The result of the combination of food with oxygen is, that a
certain amount of force is liberated in the same way that heat
is liberated from coal when it is burned. This force varies
according to the amount of activity of the animal life. The
whole process of chemical change by which the food is used is
called metabolism. Two distinct phases of it may be recognized :
anabolism (Gr. ana = up) , the process by which complex sub-
stances are built out of simpler ones; and katabolism (Gr. kata =
down), the process by which complex substances are torn down
into simpler ones. In animals the latter are more extensive than
the former.

Excretion. — The function of getting rid of the waste products
of metabolism is called excretion. These products are no longer
valuable but act as a direct poison to the body if allowed to
remain. These waste products are solid, liquid, or gaseous.
The gases are excreted by respiration, as just described. In
higher animals the liquids are carried off by the lungs, by the
skin, and by special organs called kidneys. It must be remem-
bered that excretion does not refer to the passage from the
intestines of the undigested food. This undigested food has
never become part of the body and its passage from the intes-
tines is not strictly excretion. There is apt to be confusion in
the use of the terms, as the undigested food which passes through
the intestines frequently goes by the name of excreta. In the
strict sense, however, the excreta or faeces are not excretions.

Motion. — Practically all animals possess some power of mo-
tion and have special organs adapted for bringing it about.

Support. — The living parts of an animal (protoplasm) are
made up of a soft, jelly-like substance, too non-resistant to
have the power to hold any particular shape. If the animal is
small the resisting power of the jelly may be sufficient to preserve
its shape; but in large animals it is necessary to have some hard
support for holding the soft parts. This hard supporting sub-
stance may be in the form of a skeleton or shell.

140 BIOLOGY

Coordination. — The numerous activities of the animal body
are brought into harmonious action for a common purpose.
The function by which they are related to one another is known
as coordination (Lat. con = together + ordinare = to regu-
late), and the system of organs that produces this coordination
is generally spoken of under the name of the nervous system.

Reproduction. — This is the function of producing new individ-
uals like the old, which prevents the species from disappearing
from the earth.

The nine functions thus outlined are necessary to the life of
all animals. In a few of the lower animals, some of these func-
tions are very slightly developed; and in quite a number of
smaller animals we do not find any special system of organs
devoted to some of these functions. For example, many small
animals have no skeleton, and some of the very simple ones
have no organs that can properly be called a coordinating sys-
tem, since all of the functions of the animal take place in one
small cell where no coordination is needed. But speaking in
general, all animals, high or low, carry on all these functions.

ANIMAL BIOLOGY

In our consideration of animal Biology we shall study three
animals, chosen to illustrate different grades of structure.
Hydra will be an example of one of the simplest multicellular
animals; the earthworm, an animal of moderate complexity;
and the frog will be an example of the more highly complex
types.

HYDRA FUSCA: A SIMPLE MULTICELLULAR ANIMAL

General Description. — The brown Hydra is a very common
water animal and may be found in almost any pond on the under
side of lily pads or pond weeds. Here it may be seen as a small
reddish body, just large enough to be visible. Our common
Hydra ( Hydra fused) is of a brown color, but another common
species (Hydra viridis} is bright green. If the animal, still

HYDRA FUSCA 141

attached to the lily leaf, be removed from the pond, placed in
a dish of water and left undisturbed for a time, it will slowly ex-
pand and assume the form represented in Figure 69 A. It shows
then a slender body about a quarter of an inch or less in length,
attached at one end to some other solid object. At the other
end it bears a crown of tentacles, which in the brown Hydra
are from five to ten in number, and in the green Hydra are from
five to twelve. These tentacles are very delicate, hairlike bodies,
which may be expanded to considerable length, as at A, but when
contracted, shrink into minute knobs hardly big enough to be
seen. Indeed, the whole body of the Hydra is extremely con-
tractile, and though when undisturbed it may be a half an inch
or more in length, on being disturbed it will contract into a
small body no larger than a pinhead; see Fig. 69 B. Hydra
seems at first to be a stationary animal, although it can move
its tentacles slowly to and fro in the water. A careful examina-
tion, however, shows that it has some power of motion; the ani-
mal, creeping by means of its base, can move slowly over the
object upon which it is fastened. Occasionally also it moves
by turning end over end. It first attaches its tentacles to the
object to which its base is attached. Then the base lets go its
hold and is moved over and fastened again in another spot.
The tentacles let go their hold and the animal straightens up.
The movement is not unlike that of a boy turning a handspring.

Structure. — In the midst of the crown of tentacles is a little
conical projection, on the top of which is a mouth. This is
star-shaped rather 'than circular, and opens into a cavity which
fills the whole of the body of the Hydra and even extends into
its tentacles. This cavity is the digestive cavity and is called
the gastrovascular cavity; see Fig. C.

Hydra is a true multicellular animal, made up of many thou-
sands of cells which are not alike but show a considerable differ-
entiation and have a division of labor among them. All of these
cells, however, are arranged into two layers, one on the out-
side called the ectoderm (Gr. ectos = outside + derma = skin), ec

14?

BIOLOGY

FIG. 69. — HYDRA

A, an animal in its expanded form; B, the same animal contracted; C, a diagram of the
longitudinal section of the animal, showing the internal structure; D, an epithelio-muscle
cell; E, a bit of the body wall highly magnified showing the two layers of the body; F, a
digestive cell; G, one of the nematocysts with its thread extruded; H, a second type of
nematocyst with the coiled thread within the sac; /, nematocyst of the third type with its
thread extruded; J, a bit of the tentacle, very highly magnified, showing the batteries of the
nematocysts; K, two of the secreting cells of the basal disk.

en, cnidocil;
ec, ectoderm;
en, endoderm;

m, mouth;
mes, inesogloea;

o, ovary;
s, spermary.

HYDRA FUSCA 143

(Fig. C), and one on the inside called the endoderm (Gr,
endon = inside), en. These two layers are found throughout the
body, both the ectoderm and endoderm extending into the
tentacles to their very tips. Hydra is thus a double sac with
no space between its two layers. The layers of ectoderm and
endoderm are not in actual contact with each other, but are-
separated by a thin supporting layer known as a mesogloea
(Gr. mesos = middle + gloia = glue), mes. By means of this
intermediate layer, the ectoderm and endoderm are very firmly
attached to form one solid mass, forming a body wall made up
of two layers of cells.

Ectoderm. — The ectoderm is made of two chief kinds of cells.
The first of these is the epithelio-muscle cells; Fig. D. These
are in the shape of cones, with their broad ends outward and
their tapering ends toward the mesoglcea. At the tapering ends
some long fibers protrude which extend over the body of the
animal next to the mesogloea. The great contractility of Hydra
is due to these fibers. The second type of cells is the interstitial
(Lat. inter = between + sistere = to stand) cells. These are
found between the first cells and are somewhat smaller than the
epithelio-muscle cells. They are chiefly interesting because
they produce a very peculiar type of organ possessed by Hydra
known as the nematocysts (Gr. nema = thread + cystis = sac) .
The nematocysts, or stinging cells, are little sacs scattered all
over the outside of the body of the animal, especially in the
tentacles. Each of these is an oval sac, one side of which is
pushed inward like the finger of a glove inverted into its palm;
Figs. G, H} and 7. This inverted portion is in the form of a
long thread, much longer than the diameter of the sac, and is
wound up in a long coil inside of it; Fig. H. Besides this
thread the sac contains a liquid. The peculiarity of these
cells is that under a proper stimulus the minute thread may be
inverted from the sac as shown in Figure G. This inverted por-
tion when discharged carries with it a small quantity of poison,
and thus each thread serves as a little poison dart. The thread

144 BIOLOGY

is not shot away from the animal, but only protruded to its
length. If any small animal with thin skin comes in contact
with the Hydra, some of these threads are discharged, the
animal is hit by them, paralyzed by the poison, and then
transferred to the Hydra's mouth by means of its tentacles.
In Hydra these cells are so small that they cannot pierce the
human skin and their sting cannot be felt; in some allied animals,
like the jellyfishes or sea nettles, these cells, although the same
in structure as those of the Hydra, are much larger and may pro-
duce a severe sting. After the thread is once discharged, it can-
not be withdrawn again into its sac; the cell thus becomes use-
less. It is necessary, therefore, for Hydra to be constantly
replacing them, and new nematocysts are constantly growing
from the old interstitial cells. The special cell that produces
the nematocyst is known as the cnidoblast. This is simply one
of the interstitial cells which has for its function the production
of these stinging sacs.

In the brown Hydra there are three kinds of nematocysts.
The larger one, G, is somewhat pear-shaped, and when its thread
is protruded it has, close to the base of the thread, two or three
slender barbs projecting backwards. When the thread is dis-
charged from the cell these barbs are ejected first. It is thought
that their function is to pierce the skin of the animal into which
the poison is to be ejected. Close to the base of the thread is a
minute little organ called the cnidocil (Gr. cnide = thistle) whose
function is unknown ; Fig. G, en. It has been supposed that it
helps to discharge the cell as a trigger does a gun. This is doubt-
ful, for it is known that the cell is most easily discharged by
changing the internal pressure, rather than by any mechanical
touch upon this cnidocil. The second of the nematocysts in
the Hydra, H, is smaller but more elongated. The thread when
discharged is very different in shape, lacks the projecting barb,
and, relative to the size of the sac, is much longer. The third
cell is smaller still, I, oval in shape, and contains a thread that
when discharged always coils up in a spiral form. It is thought

HYDRA FUSCA 145

that this spiral coiling is to enable the animal to adhere to the
minute spines or hairs of its prey by coiling around them in a
corkscrew fashion.

The nematocysts are scattered all over the body of Hydra
except in its base. In some parts, especially in the tentacles,
they are grouped into little bunches which project from the side
and form tubercles; Fig. J. These little clusters are spoken of
as batteries.

The Basal Disk. — The base of Hydra is different from the
rest of the body. It secretes a sticky substance by means of
which the animal attaches itself to an object. This base has
the power of causing the animal to glide very slowly over the
object upon which it is attached, though the exact method by
which this motion is produced is not known. In this part of the
body the nematocysts are lacking, and the epithelio-muscle cells
not only have muscle fibers but some of them have the func-
tion of secreting a cement, and differ in appearance from those
of the rest of the body; Fig. K.

Endoderm. — The endoderm is about twice as thick as the
ectoderm and contains cells of two kinds, known as the digestive
cells and the secretory cells. The digestive cells are long and
cup-shaped, and have, extending from their base next to the
mesoglcea, fibers of contractile substance. At their inner or
free end they bear two lashing flagella; Fig. F. It is interest-
ing to note that the free end of these cells may be protruded in
the form of pseudopodia, much like those already seen in the
Amoeba, and that they are able to take into their bodies small
solid particles of food which are then probably digested within the
cells of the body itself. Thus Hydra has a function of digestion
similar to that of the Amoeba, being able, to a certain extent,
to take inside of its digesting cells solid particles of food and to
digest them (intracellular digestion). The chief digestion, how-
ever, is carried on by the other cells, the secretory cells. These
are smaller than the digestive cells and lack the contractile
fibers at their base. They produce a secretion which is discharged

146 BIOLOGY

from their free surface into the cavity of the body, and is thus
poured upon the food which is taken into the mouth and lies
free in the gastrovascular cavity (intercellular digestion).

Hydra has thus, in addition to a method of digestion which
resembles that of the Amoeba, the power of producing a digestive
secretion, which is poured upon the food in the general cavity,
only the nutritious portions of the food being absorbed after
digestion. This method of digestion, which is peculiar to the
higher animals, is, in the Hydra, combined with the simple
method of digestion characteristic of the PROTOZOA; and in
this respect the Hydra represents a transition stage between the
unicellular animals and the higher, multicellular forms. The
function of the hairlike flagella on the endodermal cells appar-
ently is to keep in circulation the liquids present in the body and
thus to aid in bringing the digestive juices in contact with the
food which lies in the cavity. This is the only trace of a circu-
latory system that the Hydra possesses.

Nervous System. — According to recent investigation, it seems
that Hydra possesses a very simple nervous system, so delicate,
however, that it requires special methods of study; and very
little is known about it. There is a series of nerve cells near
the mouth and another near the base of the animal, and these
are connected with excessively delicate fibers passing over the
body. There are sensory cells on the surface layer that are
probably connected with the nerve cells, and some of the nerve
cells apparently send nerve fibers to the contractile fibers of the
epithelio-muscle cells. This system is, however, very simple
and rudimentary, and is of interest chiefly as the simplest type
of nervous system found among animals.

Growth and Budding.— The food of Hydra consists mainly
of minute water animals which are captured by means of its
tentacles. The tentacles are protruded into water, and small
animals, coming in contact with them, are paralyzed by the
discharge of the nematocysts. The tentacles then transfer the
food to the mouth. It is pushed into the gastrovascular cavity

HYDRA FUSCA 147

and then, by the contraction of the body wall, forced downward
to the basal end of the cavity. Here it is mixed with the diges-
tive juices of the animals and slowly digested. In time the diges-
tive parts are dissolved and absorbed into the cells that form
the body wall and are assimilated. After all the nutritious por-
tions have been digested and absorbed from the food particles,
the undigested refuse is then ejected from the mouth by a sudden
contraction of the body and opening of the mouth, which throws
the ejected portions some distance from the animal. As the
result of digestion and assimilation, the animal grows.

After it reaches a certain size, rarely more than one-half an
inch in length, the further growth shows itself in the formation
of buds which appear on the sides of the old individual; see
Fig. A. These buds rapidly increase in length, and after a
time a circle of minute secondary buds can be seen at their tips.
These secondary buds are rudimentary tentacles, for they in-
crease in length till eventually they become new tentacles. In
the middle of the circle of tentacles thus formed a small opening
makes its appearance, which forms a new mouth at the end of the
growing bud. After a time the bud itself separates from the body
of the animal from which it grew and floats off by itself as an
independent individual, identical in structure with the one from
which it came, though somewhat smaller. In this way the
Hydra reproduces itself indefinitely by budding (gemmation) as
long as it has sufficient food and proper conditions for feeding
and growth. If the conditions are favorable two or more buds
may be seen arising from the same individual, and occasionally
a secondary bud may be found arising from the side of the bud,
even before it has broken away from the animal that produced
it. In the case of the Hydra, however, these buds do not remain
attached very long, but always separate; so that we never find
the animals grouped together in great masses. While we may
find one Hydra with one, two, or three buds, this is the extent
of group formation. In closely allied animals, however, the
budding may go on almost indefinitely, and groups are formed

148

BIOLOGY

containing hundreds of members, all having arisen from the
original by budding. This occurs among the hydroids which
are common at the seashore, examples of which are shown in
Figures 70, 72, and 73. In such colonies the individual mem-
bers are called zooids.

Polymorphism. — In the colonies of hydroids shown in Fig-
ure 71 the members of the colony are all alike. It not infre-
quently happens, however, that when one of these hydroids

produces a colony by budding,
the members (zooids) assume

FIG. 70. — PARYPHA

An animal related to Hydra, but forming
colonies by budding.

FIG. 71. — ALCYONIUM. AN ANI-

MAL RELATED TO HYDRA
WHICH FORMS COLONIES

The individual members, which have
arisen by budding, are imbedded in a
lime base; p, one of the members of the
colony more highly magnified.

forms unlike each other. In Figure 72 will be seen a colony with
two types of members; one of them possessing tentacles and
adapted for feeding, and the other without tentacles but develop-
ing the reproductive bodies inside of a case. One of these mem-
bers is known as the nutritive zooid, nz, and the other as the
generative zooid, gz. In some other types of hydroids the mem-
bers which arise by budding assume even a greater variety of
form. In the colony shown in Figure 73 there is a complicated

HYDRA FUSCA

149

colony made up of at least five different types of members or
zooids. Among them may be found members adapted to feed-
ing, n; others having purely sensory functions, called tentacu-
lar zooids, t; some adapted for reproduction, g; others in the form
of bells with muscles which enable them to move about, called
the swimming zooids, sw; and
finally at the top of the colony

FIG. 72. — CAMPANULARIA

A colony of Hydroids showing a differenti-
ation into feeding zooids, nz, and generative
zooids, gz; e, e, eggs in different stages of
development; e' the young embryo extruded
into the water.

. 73. - A SIPHONOPHORE

An animal showing a high condi-
tion of polymorphism; /, the floating
zooid; g, the generative zooid ; n.the
nutritive zooid; sw, the swimming
zooid; t, the tentacular zooid.

a single one develops as a gas bladder, /, which enables the
animal to float in the water. All of these combine to form
a colony. Where several different types are found arising by
budcfeng from the same original stock the condition is spoken
of as polymorphism (Gr. polus = many + morphe = form).
Polymorphism is best illustrated in simple organisms, being well
developed among the animals related to the Hydra; but the samp

150 BIOLOGY

principle is found in a less developed extent in some of the organ-
isms with a higher structure, though nowhere do we find it so
highly developed as among the hydroids. Where polymorphism
is developed the whole colony acts as a unit, and the colony,
therefore, may be compared to a more highly complex organ-
ism with its various organs. Polymorphism always arises as
the result of asexual growth and not by sexual reproduction,
and when it occurs the members of the colony always show a
differentiation in function as well as in shape and structure.

Regeneration of Lost Parts. — Hydra has a wonderful power
of reproducing lost parts. If it is cut into two pieces, each
part will develop the part that it has lost and becomes a
new Hydra. Indeed, it may be cut into a large number of frag-
ments, and every fragment is capable of growing and developing
into a new form like that of which it was originally a part.
If the small conical projection containing the tentacles is cut
off from the rest of the Hydra, each piece will develop the part
that it has lost. The animal may be split lengthwise into two
or four parts and each will become a perfect animal. If a head is
split in two and the parts slightly separated, each will develop
its crown of tentacles and a two-headed animal will result. If
an animal is turned wrong side out, it will adjust itself to new
conditions and a perfect animal will soon be produced. This
power of regenerating lost parts is found in many of the lower
animals, but in no place is it better developed than in Hydra.
In the higher animal the power of regenerating lost parts eventu-
ally disappears entirely. It is very evident that this power must
be of considerable advantage to the animal in the struggle for
existence. In Hydra the power is so extraordinarily developed
that a piece of the animal not more than one-hundredth of an
inch in length is capable of reproducing all of the parts that are
lacking and developing into a new animal. In some cases the
new animal is produced by a multiplication of cells from these
pieces, so that a fair-sized animal is developed; while in other
cases the cells and fragments are remolded into new icdividuals

HYDRA FUSCA 151

which are like the original in shape but much smaller in size.
Some of the experiments described were originally performed
long ago, by Trembley in 1740 ; but they have since been
confirmed by other investigators.

Sexual Reproduction. — By the method of budding Hydra
may multiply indefinitely as long as it has plenty of food. It
has also a second method of reproduction by a true sexual proc-
ess. Under certain, not well-understood, conditions the animal
produces outgrowths on its side, shown in Figure 69 C, which
are the sexual glands, — ovaries, o, and spermaries, s. Within
them are produced special cells, called eggs and sperms, which
unite with each other in a manner similar to that seen in the
cells of Pandorina (page 74). The significance of this repro-
duction will be noticed in a later chapter.

Hydra, as will be seen from the above description, possesses
the systems of alimentation, metabolism, motion, and reproduc-
tion. Circulation is wanting; respiration is carried on through
the general surface of the cells; no excretory system is found, each
cell probably excreting its waste products directly into the
water; support is unnecessary in such a small animal; the rudi-
ments of nerves suggest the beginning of a coordinating system.

THE RELATION OF THE WHOLE ORGANISM TO ITS DIFFER-
ENT PARTS

With the appearance of multicellular organisms we also find
that the entire animal has -now a life more or less independent
of the life of its parts. The multicellular animal or plant lives
a life as a complex, and in addition each cell has a life of its own;
so that we can distinguish, in a multicellular animal, a life of
the organism as a whole and a life of its separate cells. It is
possible for the death of the organism as a complex to occur
while the individual cells still remain alive. It is true that in the
multicellular organism each of the individual cells is dependent
upon the activity of the whole to keep it properly nourished and
supplied with the necessary conditions of its life. The different

152 BIOLOGY

cells that make up such organisms are not independent and can-
not live long except when related to the other cells that make up
the multicellular organism. Nevertheless, there is a certain
amount of independence in the individual cells, especially among
plants and some of the lowest animals; for in these we may
remove only a comparatively small number of cells from the
whole organism and these cells will still retain their vitality, still
continue their power of growth, and under proper circumstances
develop more cells which eventually become exactly like the
animal from which they were obtained. This is especially true
of Hydra, which can be cut into many pieces, each piece retain-
ing the power of independent life, and in time becoming an inde-
pendent and well-developed animal. In such low organisms the
life of the organism as a complex has not wholly destroyed the
independence of the individual parts. This is more or less true
throughout the whole of the plant kingdom. Among most higher
plants as well as the lower, small pieces separated from the
parent plant will not die at once, but may, if put under proper
conditions, develop into fully grown individuals like those from
which the fragments were obtained.

With animals, however, it is only among the lowest and
simplest forms that a piece, containing a relatively small
number of cells, can be separated from the rest and still be
capable of developing into a new organism like the original, as
in the case of the Hydra. As we pass to the higher animals
this power of regeneration disappears, and among almost all
animals, even of comparatively low structure, the independent
life of the parts is lost, so that when one portion is removed
from the complex that makes up the animal it no longer retains
its power of life and growth. But even in these cases and among
the highest animals, we do find that some parts may have more
or less independent life when separated from the organism of
which they are a part. In an animal like a frog, for example,
the heart may be totally removed from the body and it will still
keep up its life for many hours when put under proper condi-

HYDRA FUSCA 153

tions, long after the frog itself has been killed. More remarkable
is this power in the case of a turtle, for here even when the animal
has its head entirely cut from the body, and the rest of
the animal destroyed, the heart, if removed and kept under
proper conditions, will keep on beating for at least two days.
Still more remarkable is it to find that in the air passages of the
turtle there are ciliated cells which have a special power of motion ;
Fig. 14 C. During all the life of the turtle these cilia are in a
state of active motion, and after the turtle is dead the cilia
may continue moving for as long as two weeks. We thus see
that among the higher organisms the death of the animal as a
whole does not necessarily involve an immediate death of all its
parts. The individual parts are, of course, closely dependent
upon each other, and, at least in the higher organisms, the life
of neither is capable of being long maintained without the other;
but the life of the individual cell may frequently continue some
time after the life of the organism as a whole has been brought
to an end.

From this it follows that the term death may have a different
meaning in different connections. In speaking of the death of an
animal, we may refer, and usually do refer, to the death of the
animal as a whole, which means the destruction of the compli-
cated mechanism that forms the animal organism. But we may
also refer to the death of the individual parts, and in this case
the exact time when the animal comes to its death is difficult
to state. The animal as a whole may die on one day, while
some of its parts may remain alive at least two weeks. In
such instances it is not easy to say when death occurs. Never-
theless, it is customary to refer by this term, not to the death
of the individual parts or the individual cells which make
up the animal, but to the destruction of the organism as a
whole, which causes it to cease to act as a unit. Usually,
therefore, death refers to the breaking down of the mecha-
nism of which an organism is composed so that its parts do
not act together.

154 BIOLOGY

LABORATORY WORK

Hydra. — Almost any pond will furnish Hydra, which may be found
clinging to the under side of pond-lily leaves. If such leaves are placed in
a dish of clean water, the Hydra will detach themselves from the leaves and
cling to the side of the dish. For study, a specimen is to be detached from
the dish, placed in a watch glass containing a little water, and examined
under the microscope with a low-power objective. The general structure
and motion of the animals may easily be seen. For the cellular structure
of the body, stained, mounted sections should be furnished the student by
the instructor. For a study of the nematocysts, a bit of the tentacles of a
brown Hydra should be cut off with delicate scissors and placed on a slide
in a small drop of water. A cover glass is placed upon the drop and gently
pressed. This will crush the tentacle and cause many of the nematocysts
to discharge their stinging hairs. The nematocysts may also be made to
discharge their stinging hairs if a little weak acetic acid is added .to the
water. A careful examination with a |-inch objective will show all three
kinds of nematocysts, both discharged and undischarged. For comparison
of Hydra with other Hydroids, preserved and mounted specimens should
be furnished by the instructor, some of which should show colonies and
others jellyfishes.

BOOKS FOR REFERENCE

COLTON, Zoology, Descriptive and Practical, D. C. Heath & Co.r
Boston.

PRATT, Invertebrate Zoology, Ginn & Co., Boston.

MARSHALL and HURST, Practical Zoology, G. P. Putnam's Sons, New
York.

CHAPTER VIII

MULTICELLULAR ANIMALS: THE EARTHWORM (LUM-

BRICUS)

THE earthworm is an extremely common animal the world
over, being found buried in moist earth in practically all parts
of the world. There are numerous species, differing from each
other in minor details, but agreeing in their fundamental
structure. The animals vary in size from those an inch or two
in length, to some which are nearly a foot; and one species
is reported two feet in length. Earthworms are of practical
importance in stirring up the soil. They are constantly en-
gaged in bringing soil from below to the surface, and depositing
it at the mouths of their burrows. By this slow but constant
action they are of much value to agriculture, constantly renew-
ing the surface soil.

ANATOMY

Shape of Body. — Examined externally, the earthworm is an
elongated animal, more or less cylindrical in shape, tapering,
however, at the two ends; Fig. 74. The head, or anterior
end, is more tapering than the other, the blunter one being
the posterior end. One side of the animal is lighter colored
than the other and slightly flattened, the opposite side being
more rounded. When the animal is in its natural position
on the surface of the ground, the flat side is kept undermost
and the rounded and darker-colored side uppermost. We thus
have an anterior and a posterior end, a ventral and a dorsal
surface, and, consequently, a right and left side to the animals.
The animal is, therefore, bilaterally symmetrical.

Segments or Metameres. — The body of the earthworm is
divided into a number of rings (Fig. 74) called segments or
metameres (Gr. meta =• after -f meros = part). The number
is not constant, being greater in the older and larger animals

155

156

BIOLOGY

than in the younger ones, and increasing with age. Most
of these rings are alike in shape and size,
but a few of them differ slightly from the
others. The first one at the anterior end
is not a complete ring, but a minute pro-
jection which is known as the prostomium
(Gr. pro = before + stoma = mouth). It is
slightly movable and is the most sensitive
part of the animal. The second segment is
not a complete ring, but rather in the form
of a horseshoe, with the open part of the
horseshoe above,
and with the
prostomium lobe
fitting into the
opening as shown
in Figure 75.

Underneath
the prostomium
and over the sec-
ond segment is
an opening, the
mouth, m. The
third segment is
a complete ring,
but rather small,
and from this
point backwards
the segments are

all alike in shape, increasing slightly in size
until a maximum is reached, and from this
point remaining essentially the same in size
and shape to the posterior end of the body.
A short distance back from the head there
is a series of rings, from the twenty-eighth

FIG. 74. — AN
EARTHWORM,
FROM BELOW AND
FROM THE SIDE

a, the anus; m ,
mouth; od, opening of
oviduct; sr, opening of
seminal receptacles; v,
opening of vasdeferena;
a, aetse.

FIG. 75. — THE FIRST THREE
SEGMENTS OF THE EARTH-
WORM

A, from the front; B, from the
side; p, the prostomial lobe; m, the
mouth.

THE EARTHWORM

to the thirty-fifth segments, known as
the clitellum (Lat. clitellce = saddle) ;
Fig. 74. These segments are larger than
elsewhere and have a thicker wall and
special functions. At the extreme pos-
terior end the segments become smaller,
and the last one has an opening which
is the posterior opening of the diges-
tive tract, the vent or anus, a. Be-
cause of this ringed structure the
earthworm belongs to a class of animals
called Annulata (Gr. annulus = ring).
Structure of the Body. — The body
of the earthworm can be compared to
a tube within a tube; Fig. 76. The
outer tube is called the body wall, 6,
and the inner tube the alimentary
canal or the digestive tract. Between
the body wall and the digestive sys-
tem is a space filled with a liquid, this
space being a true body cavity or coelom
(Gr. koilos = hollow), c, differing thus
from Hydra, that has no coelom. The
body cavity is not, however, an open
space extending from the anterior to
the posterior end, but is divided by
partitions into a series of chambers,
with a chamber for each segment. The
partitions are called septa (sometimes
called dissepiments) . There are minute
openings through each septum, so that
the liquid that fills the body cavity
may pass through; thus the different
chambers are in communication with
each other.

FIG. 76. — DIAGRAM SHOW-
ING THE ANTERIOR END
OF THE EARTHWORM CUT
LENGTHWISE THROUGH A
VERTICAL MEDIAN LINE

6, body wall; od, oviduct;

c, coalom;
cr, crop;
g, gizzard;
m, mouth;
mu, muscles;
n, nephridia;
o, ovary;

of, (esophagus;
ph, pharynx;
s, septa;
sp, spermary ;
sr, seminal recep-
tacles ;
t, typhlosole.

158 BIOLOGY

The Alimentary Canal. — The alimentary canal (enteron) is
a straight tube extending from one end of the animal to the
other, without any convolutions. It does, however, show
several distinct regions. The mouth opens into a slightly
swollen section known as the throat or pharynx, ph. The
pharyngeal walls are muscular, with a radiating series of
muscles that pass outward to be attached to the body wall,
mu. The contraction of these muscles will cause an expansion
of the pharynx and convert it into a sucking organ by means
of which the animal draws food into its mouth. Behind the
pharynx the canal contracts into a straight gullet or oesopha-
gus, oe, which continues back to the fifteenth segment. Here
it enlarges into a thin-walled crop, cr, which is followed in
the fifteenth and seventeenth segments by a second enlarge-
ment with thicker walls, called the gizzard, g. Beyond this
the intestine extends in a straight line to the anal aperture
or vent. The intestine is not a simple cylindrical tube but has
its dorsal side folded inward to form a longitudinal ridge known
as the typhlosole (Gr. typhlos = blind + solen = tube), ty (Fig.
81), whose purpose seems to be only to increase the amount
of interior surface within the intestine.

Circulatory System. — The circulatory system consists of two
parts, the blood system and the codomic fluid.

The blood system. — A series of tubes or vessels containing
blood comprises the circulatory system. The blood of the
earthworm is red, a very unusual condition among lower ani-
mals. The red color is due to a substance called haemoglobin
(Gr. haima = blood + Lat. globus = globe), which is dissolved
in the liquid part of the blood, and is not contained in the
corpuscles, as it is in the frog and higher animals. This blood
is kept in constant motion in the vessels, forced along by their
contractions. The chief vessels and the direction of the blood
current are shown in Figure 77 and they are as follows: —

Running anteroposteriorly, just above the alimentary tract,
is a large longitudinal dorsal vessel, dv, with muscular walls.

THE EARTHWORM

159

These muscles produce waves of contraction, which, arising at
the posterior end, force the blood forward. In the posterior
half of the body small branches pass from this tube into the
intestine, ei, supplying its walls, and the blood then enters
a rather large vessel in the typhlosole, from which it passes
back by short tubes, ai, into the dorsal vessel. The greater
part of the blood in the dorsal vessel flows forward to the seg-
ments 6-11, where five large circular vessels arise from it,
ht, which pass around the sides of the body to enter a sub-
intestinal vessel, w, also extending lengthwise and lying be-

FIG. 77. — DIAGRAM SHOWING THE CHIEF BLOOD
VESSELS OF THE EARTHWORM

an, the anterior end;

po, the posterior end of the body;

dv, dorsal vessels;

cv, circular vessels;

ei, efferent intestinal;

(Bourne and Benham.)

ai, afferent intestinal;
ht, hearts;

snv, subneural vessel;
vv, ventral vessel.

neath the intestine. These circular vessels are called hearts,
since they contract, and force the blood downward into the
ventral vessel. When reaching the ventral vessel, part of the
blood flows forward, in front of the hearts, and part of it back-
ward. From this ventral vessel branches arise which pass out
into the body wall and into other organs supplying the body
generally with blood. After passing through the organs of
the body wall, etc., the blood is collected into another set of
vessels which pass into a third longitudinal vessel lying under
the nerve chord, the subneural (Gr. neuron = nerve), snv
Through this it flows toward the posterior end. In the intestinal
region there arises from the subneural vessel, in each segment,
a circular vessel, cv, which passes up around the body to empty

160 BIOLOGY

into the dorsal vessel, dv, thus bringing the blood back again
into the dorsal vessel. There are numerous other small ves-
sels, some of which are shown in Figure 77, but the chief ones
are those that have been described.

The blood is forced onward by the contraction of the walls
of the dorsal vessel and the hearts, which are provided with
valves preventing any back flow when the contractions occur.
The course of the blood is rather indefinite and the pure and
impure blood are not distinctly separated from each other, as
in higher animals. There are no true arteries or veins, and no
true hearts. This blood is associated With respiration, and also
carries nourishment from the absorbing organs in the intestine
to the active tissues, and carries waste products from the active
cells to the excreting organs.

Coelomic or Perivisceral Fluid. — The chambers of the body
cavity are filled with a fluid called the ccelomic or perivisceral
(Gr. peri = around -fLat. viscera = internal organs) fluid, which
serves also as a circulatory medium. The food that is absorbed
makes its way into the body cavity and is partly absorbed
by this fluid. This liquid is forced irregularly backward and
forward through the cavity of the body by the motions of
the animal, and the nutritious parts of the food which are dis-
solved in it are thus directly carried to and fro and brought
in contact with the living tissues of the body, that are bathed
in this liquid. There is no distinct circulation of this fluid, and
it cannot properlybe called a circulatoryfluid. It does, however,
have some of the functions of the blood, since it carries to and
fro a part of the material absorbed from the digestive tract.
It corresponds more closely to the lymph of higher animals.

Respiration. — The earthworm has no distinct respiratory
system, but the blood vessels in their circulation in the skin
are brought into a very close proximity with the air. Gases are
readily exchanged through the thin skin, and respiration is
carried on easily without any special respiratory organs except
the minute blood vessels that lie beneath the skin.

THE EARTHWORM

161

Excretory System. — Most of the excreted matter (with the
exception of gases) is passed to the exterior by a series of tubes
known as nephridia (Gr. nephros = kidney), one pair in each
segment. Each of them (see Fig. 78) consists of a long tube,
which begins in a segment of the body cavity as a minute
funnel-shaped opening, i, and then passes through the septa, s,
to the segment immediately behind. In the posterior segment,
the tube is coiled back and forth in three distinct loops that
differ in structure and function. Eventually the distal end
passes through the walls of the body to the exterior, by a
lateral opening, e, in each
segment. These nephridia
are very delicate organs and
can only be made out by
very careful study with a
magnifying glass. Their
function in excretion is as
follows: The funnel opening

in the anterior segment is „ __

.... ... FIG. 78. — A NEPHRIDIUM, COMPLETE

guarded with cells provided

with cilia, and some of the
coils are also lined with cilia.
The movements of these cilia produce currents in the liquid
in the tube and force the liquids through the tube to the
exterior. As a further result of the action of these cilia, solid
particles of waste material, which may be floating in the
ccelomic fluid, are forced into the tube and then through
the tube, passing through its coils and finally reaching the
exterior through its opening. The coiled walls of the tube
are made up of thick active cells which are well supplied with
blood vessels. These are secreting cells and resemble gland
cells. They have the power of extracting waste products from
the blood and excreting them into the tube which they surround.
The materials enter the duct of this nephridium and are slowly
forced along by the ciliary current, and finally carried to the

i, incurrent opening;
e, excurrent opening;
s, septa.

162

BIOLOGY

exterior. These nephridia have as their primary function the
removing from the body of the waste products containing
nitrogen, related to urea. Their function is thus similar to
that of the kidneys of the higher animals, and indeed their
structure is not unlike the kidneys of some of the ver-
tebrates.

The Coordinating or Nervous System. — The nervous system
consists of a central system and a peripheral (Gr. peri = around

Fia. 79. — DIAGRAM SHOWING THE NERVOUS SYSTEM

IN THE FRONT END OF THE BODY

eg, cerebral ganglia;
com, commissures;
m, mouth;

oe, oesophagus;
pr, prostomium;
v, ventral cord.

(Shipley and MacBride.)

+ pherein = to bear) system, the latter composed of a large
number of nerves passing from the central system into the
various regions of the body.

The central system. — 1. The cerebral ganglia. These are
two nerve knots or ganglia, sometimes called the brain, united
together and lying above the pharynx in the anterior part of
the body cavity; Fig. 79 eg. From them, extending down-
ward and backward, a pair of cords or commissures (Lat.
committere — to join together), com, pass around the phar-
ynx and unite with each other below on the ventral side

THE EARTHWORM 163

of the pharynx a short distance behind the mouth. 2. The
ventral cord. When the two commissures have united they
form a cord which passes to the posterior end in the median
line of the body, closely attached to the body wall beneath
the intestines; this is the ventral cord, v. In each segment
the cord is slightly enlarged to form what is called a ganglion;
see Fig. 80 vc. At the posterior end of the body this cord
becomes smaller and finally terminates.

The peripheral system. — The nerves which form the per-
ipheral system are numerous. From the cerebral ganglion
two large nerves arise, which soon divide into many branches
and pass forward to the prostomium, giving it a very large
nerve supply and making it a very sensitive organ; Fig. 79.
From the commissures extending around the oesophagus arise
the nerves that supply the second and third segments of the
body. From the ventral cord in each of the segments, from
the fourth to the posterior end of the body, there arise three
pairs of nerves. Two pairs arise from the ganglionic enlarge-
ment and one pair from the sides of the ventral cord behind
the septum that separates each segment from the next.

Reproductive System. — The only method of reproduction in
the earthworm is by sexual process.* The two sexes are, how-
ever, combined in the same individual, so that the earthworm
is what is called an hermaphrodite; see page 251.

Female reproductive organs. — In the thirteenth segment
there is a pair of small glands called ovaries, situated on the
ventral side of the body cavity close to the middle line; Fig.
80 ov. In the same segment is the opening of a funnel which
leads into a short tube passing through the septa into the next
posterior segment. Here it is slightly enlarged to form an egg
sac, and from the sac a small duct extends through the body
wall to the exterior, opening upon the ventral surface of the
fourteenth segment. These ducts are the oviducts, od, and
through them the eggs produced by the ovary pass to the

*The earthworm has a slight power of regeneration of lost parts, but this power is far less
developed than in Hydrg,, If it is cut into two pieces two individuals are formed.

164

BIOLOGY

exterior. The openings of the reproductive organs may be
seen in Figure 74.

Male reproductive organs. — In the tenth and eleventh seg-
ments there is a pair of glands, the spermaries, sp, in which
are formed the male reproductive elements. In these two seg-
ments their position corresponds to the position of the ovary
in the thirteenth segment. They are very small glands and
can only be seen by microscopic examination. Behind each

of these sperm glands
is a funnel-shaped, cili-
ated opening, leading
into a tube which
passes through the
septa into the next seg-
ment, where it is slightly
coiled, and then passes
backward. The tubes
from the two sperm
glands on each side
unite with each other
in the twelfth segment
to form a single duct,
which passes back
through the septa to the
fifteenth segment, where
it opens through the
body wall to the exte-
rior. This duct is known
as the vas deferens
(Lat. vasa = vessel -f-
deferens = carrying

FIG. 80. — DIAGRAM SHOWING THE REPRO-
DUCTIVE SYSTEM OF THE EARTHWORM
The numbers represent the number of segments,
seminal receptacle
seminal vesicles;

es, egg sac; sr, seminal receptacles;

ne, nephridia; ' '

OB, ovary;
od, oviduct;
sp, spermaries;

vc, ventral nerve cord;
vd, vasdeferens.

down); Fig. 80 vd. In the ninth, tenth, and eleventh seg-
ments are large sacs known as seminal vesicles, sv, which serve
as a storehouse for the secretion of the sperm glands, before
these secretions pass to the exterior through the vas def-

THE EARTHWORM 165

erens. At the junction between the ninth and tenth, and
between the tenth and eleventh segments, may be found two
pairs of white sacs, each opening to the exterior by an opening
at the junction line between the segments. These are the
seminal receptacles, sr, and their function is to receive the
secretions from the seminal glands in copulation.

Copulation and Egg Laying. — Although the earthworm is an
animal producing both male and female elements in the same
individual, the habits of the animal are such that there is no
fertilization of the egg by the sperm of the same individual
that produces the egg, but a cross fertilization always occurs
between two individuals. At the breeding season, which is
early in the summer, two individuals place themselves side by
side with their heads in opposite directions, and by means of the
secretions from the glands in their skin there is formed a slimy
covering that holds the two individuals in close contact (copu-
lation). In this position, each transfers sperm material (see
Chapter XII) from its sperm glands into the seminal receptacles
of the other, after which they separate. During copulation,
or immediately afterwards, a secretion is produced by the cli-
tellum, which forms a band around the animal that extends
from the twenty-eighth to the thirty-fifth segment of the body.
At the close of copulation, after the animals have separated,
this band is gradually pushed forward until it finally slips off
over the head. As the band passes forward over the fourteenth
segment a certain number of eggs are extruded into it from the
oviduct; and when it passes over the ninth and tenth segments
some of the sperm material from the seminal receptacles is
also ejected into it. As it passes off over the head it closes
up by its own elasticity. Inside of this band the eggs of each
individual are thus mixed with the sperm from the other indi-
vidual and cross fertilization occurs. This case holding the eggs
and sperms is now known as a cocoon, and within it the
eggs develop into earthworms. The cocoons are deposited in
the soil and may be found early in the summer.

.00

BIOLOGY

MICROSCOPIC ANATOMY OR HISTOLOGY

The body of the earthworm is made of large numbers of
cells of great variety in form and structure. The cellular
structure in some of the organs of the body can readily be
made out under the microscope, but in others the cells can
be seen only by special methods. The most important features
of the histology are as follows: —

Body WalL — The body wall contains several layers; Fig.
81. On the outside a very thin cuticle covers the whole body,

FIG. 81. — DIAGRAM REPRESENTING A

OF THE EARTHWORM'S BODY

of the nephridium;

end;

perforated, however, by numerous openings through which the
various secretions pass. Inside of the cuticle is a somewhat
thicker layer of cells mainly cylindrical in form, known as

THE EARTHWORM

167

the epidermis, ep. Some of the cells are sensory cells; others
have the power of secreting a slimy material which keeps the
surface of the animal moist, and these are called gland cells;
Fig. 82. Under the epidermis is a layer of circular muscles,
cm, extending around the body, each muscle in the form of
a very long, slender fiber, tapering at both ends. Extending
around the bodj* as they do in a
circular direction, their contraction
will tend to constrict the body and
reduce its diameter. Under this is
a thicker layer of muscles, running
lengthwise, the longitudinal mus-
cles, Im. These are arranged in
bundles and in a cross section they
appear to radiate like a feather, but
each longitudinal muscle fiber has
the same structure as the circular
muscles. By their contraction the
animal's body is shortened. Under
the longitudinal muscles is an ex-
tremely delicate layer of flat cells
forming a thin membrane bound-
ing the body wall on the surface lying next to the body
cavity. This is the peritoneal (Gr. peri = around + letnetn =
to stretch) epithelium, per.

Eight delicate bristles, called setae, extend through the mus-
cle layers of the body wall and protrude through the skin,
Fig. 81 s. They are arranged in four groups, two in each
segment, and are attached by several minute muscles on the
inner end. By means of these the setae may be slightly ex-
truded, or moved to and fro so that the tips may be directed
forward and backward. If the earthworm is pulled gently
through the fingers, the projecting setae may be felt as a slight
roughness on the skin.

Motion. — The motor system of the earthworm is extremely

Fie. 82. — HIGHLY MAGNIFIED

SECTION OF THE SOX OF
THE EARTHWORM

168 BIOLOGY

simple and crude, consisting only of the two layers of muscles,
longitudinal and circular, and the seta. The method of its
action is as follows: By the contraction of the circular muscles
the diameter of the body is reduced, and, inasmuch as the body
cavity is filled with the perivisceral liquid, and liquids are
incompressible, the contraction of the diameter of the body
must necessarily increase its length. The ends are thus pushed
apart; but the setae pointed backward act as anchors, and
the pushing of the two ends of the body apart will tend to push
the head forward, the rest of the body remaining practically
stationary. After the contraction of the circular muscles the
longitudinal muscles are contracted, thus shortening the length
of the body and at the same time increasing its diameter.
As the body shortens, the tail is pulled forward toward the
head, the setae again serving as anchors to prevent the body
from moving in the wrong direction. Thus by alternately con-
tracting the circular and longitudinal muscles, the head is
pushed forward and the tail is pulled up to the head. If the
earthworm wishes to move backward, it needs only to contract
the muscles connected with the setae and to point them for-
ward, when they will serve as anchors to prevent the body from
being pushed forward; and the alternate contraction of the
two layers of muscles will make the animal move backwards.
This alternate contraction of the muscles does not occur the
whole length of the body at once, but sections -may contract
or relax, causing waves of contraction to extend from one end
of the animal to the other. This method of locomotion is very
inefficient for an animal living on a flat surface, and the earth-
worm is only able to move slowly upon the ground. In his
underground burrows, however, where the animal nearly fills
up the burrow, the method of locomotion is much more efficient
and enables the animal to move with considerable rapidity.

Alimentary System. — As shown in Figure 83, the alimentary
canal consists of five layers. On the very inside next to the
cavity of the intestine is a layer of epithelial cells (Gr. epi =

THE EARTHWORM

169

upon + thele = nipple), ep, which secrete the digestive fluids
and also aid in the absorption of the food. Just outside of
these is a layer of blood vessels, v. A third layer consists
of circular muscle fibers extending around the intestine, cm,
and outside of this is a layer of longitudinal muscles, Im. A
fifth layer on the outside consists of a thick coat of cells known
as chlorogogen cells, c. These cover the intestine with a
thick layer on its outer surface and also form the substance
of the typhlosole, which, as
shown in Figure 81, lies
within the cavity of the in-
testine. The function of the
chlorogogen cells is not
known, though it is probable
that they have something
to do with the absorption FIG. 83. — MAGNIFIED VIEW OF A
of food and possibly have a SECTION OF THE ALIMENTARY CANAL
function of secretion. On
either side of the oesophagus
in the tenth, eleventh, and
twelfth segments are three
pairs of white bodies known as calciferous glands (Lat. calx
= lime -\-ferre = to bear), producing a lime secretion which
is poured into the intestine. Its function is probably to reduce
the acidity of the food, although very little is known about
these glands or their uses.

The Nervous System. — The microscopic study of the nerv-
ous system of the earthworm, as well as of all higher animals,
has shown that while there are several kinds of cells in ijb,
the chief ones, and probably the only ones possessing nervous
functions, are large cells called neurons.

Neurons. — A single neuron of the earthworm is shown in
Figure 84 A. It has a rather irregular rounded body, with a
prominent nucleus, and from it arises a long process, much
longer than appears in the figure. Side branches of this proc-

c, chlorogogen cells;

cm, circular muscles;

ep, epithelium, lining the canal;

Im, longitudinal muscles;

v, blood vessels.

(Modified from Sedgwick and Wilson.)

170

BIOLOGY

ess may be seen near the cell body. Other much shorter
processes arise also from the cell body and divide quickly
into branches. The long fiber is called the axon or the nerve
fiber, and the other branching projections are called dendrites
(Gr. dendron = tree). Sometimes the axons at their outer or
peripheral end break up into numerous branches known as

A, a single neuron; B, a section of the ventral surface, showing the nerve
cord and its connection with the muscles and dermis.

arb, arborization of an afferent nerve; «/, sensory nerve fiber;

mf, motor fiber; so, sensory organ.

mn, motor nerve cell;

arborizations (Lat. arbor = tree), arb. In such a neuron im-
pulses enter the cell body through the dendrites and pass out
through the axon.

Similar neurons make up the nervous system of all animals
which have been carefully studied. In shape the neurons are
quite varied (Fig. 85), but in all cases there is a cell body
with one or more branching processes arising from it; and an
axon fiber of varying length extends outward from the cell.

Vast numbers of these neurons are aggregated together to
make the nervous system of the earthworm. The cerebral gan-.
glia contain them in great numbers, and the many nerves shown
in Figure 79 are formed chiefly of bundles of the axons of the
neurons, whose cell bodies are either in the ganglia or at the

THE EARTHWORM

171

outer ends of the nerves, in the prostomial lobe, etc. The

ventral cord also is a mass of neurons, and since it is simpler

than the brain it

may be more easily

understood and will

illustrate better the

relation of neurons

to the rest of the

body.

The ventral cord.

— A cross section of

the cord shows it

to be covered on

the outside by a

thin layer of epi-
thelium, the perito-
neum, inside of

which is a muscular

sheet containing a

few blood vessels;

Fig. 86. Near the dorsal surface of the cord are three clear rods

running lengthwise,
called giant fibers,
g, containing nerve
fibers. The cord
itself is really two
cords fused to-
gether. Embedded
in the cord may be
seen many large
cells, which are the
bodies of the neu-
rons making up the

cord; and extending out to form the nerve fibers which arise

from the cord are the axons of these neurons.

FIG. 85. — NEURONS OF VARIOUS TYPES

FROM HIGHER ANIMALS

A, a complex of neurons from the cerebrum; B and C,
neurons from the cerebellum; D, a single neuron from the
cerebrum.

FIG. 86. — MAGNIFIED SECTION OF THE VENTRAL
CORD OF THE EARTHWORM

g, giant fibers;
n, neurons;

nf, nerve fibers;
v, blood vessels.

172 BIOLOGY

The relation of these neurons to the body may be seen from
Figure 84. Most of the cells which appear so prominently
in the cord have connections as shown at mn. Each has a
complex of dendrites which branch in, the substance of the
cord, and a single axon which passes out through the nerve
to be finally distributed to the muscles. These neurons send
impulses to the muscles and are called motor cells. Some send
their axons to muscles on the same side, as shown in the figure,
and others send theirs across the cord to the muscles on the
other side of the body. These axons are known as efferent
(Lat. ex = out + ferre — to bear) nerve fibers. Some of the
axons do not pass out of the cord, but simply connect dif-
ferent parts of the cord itself.

The neurons which carry impulses from without toward the
center are called afferent (Lat. ad = to + ferre = to bear)
neurons. These never have their neuron bodies within the
cord but somewhere outside it. Many of them take their
origin in special cells called sense cells (Fig. 84 so), which are
sensitive to certain external stimuli. The impulses excited in
the cell pass over the axon to the ventral cord. Where the
axon enters the cord it breaks up into numerous branches, or
arborizations, arb, which spread out in the cord itself. The
impulses entering by the axons may pass from the arboriza-
tions to the dendrites of the motor cells and excite them to
action. Hence a stimulus applied to the skin may produce a
movement.

The sense organs. — The cells at the end of the afferent
nerves constitute the sense organs, and they are so constructed
as to be influenced by different external forces. The earthworm
has no eyes, although some of its sense organs appear to be
slightly affected by a bright light. They have no ears and no
sense of sound, though they are very sensitive to a slight jar.
They have a sense of taste, located in the mouth, and also a
sense of smell. None of the sense organs is visible to the naked
eye, but they may be seen by microscopic study. The end of the

THE EARTHWORM 173

prostomial lobe is the most sensitive part of the body; here
the sense cells are most abundant and here the nerve supply is
the largest; Fig. 79.

LABORATORY WORK ON THE EARTHWORM

Only large specimens should be used. These can be purchased from
dealers in natural history supplies or they may be collected by searching
with a lantern on a dark night, when they may be found stretched out
on the ground and thus readily collected. A little care and experience
is needed to do this without disturbing them, for they are very sensitive
to the slightest jar and quickly retreat into their burrows.

The specimens should first be studied alive, if possible, to see the con-
traction of the dorsal blood vessel and the contractions of the body in
locomotion. The setae may be felt by drawing the body gently through
the fingers, and they can be examined under a lens.

If the worms are to be dissected, or preserved for future use, they should
be treated as follows: Place the worms in a shallow dish with wet filter
paper torn into shreds. The animals will swallow it and as it passes
through the alimentary canal it will carry the dirt from the canal. This
part of the process is not necessary unless microscopic sections are to be
made. If they are to be kept simply for dissection, they can be preserved
at once as follows: — •

Place a number of worms in a shallow dish with just water enough
to cover them. Add a few drops of alcohol, and, after a few moments,
add a little more. Continue adding the alcohol gradually until the ani-
mals have become motionless and relaxed. This process should take at
least two hours. Then transfer them to a large shallow dish containing
50% alcohol, straightening the animals out, and laying them side by side.
After an hour replace the 50% alcohol with 70%; after a few hours change
again to a fresh lot of 70% alcohol. Finally the animals are to be placed
in 90% alcohol. It is important to keep them straight in this final hard-
ening fluid, and this may be done by laying them out on rather stiff paper,
without touching each other, and rolling them, putting about a dozen in
each roll. This will hold them in proper shape, and the rolls may be
stored in tall jars and will keep indefinitely.

Animals so preserved will serve either for microscopic sections or for
dissection. Sections should be made by the instructor and, after stain-
ing, should be mounted and furnished the student for study.

For dissection, the animal should be placed, under water, in a tray
containing dissecting wax. The anterior end is pinned down and then,

174 BIOLOGY

with fine scissors, an incision is made along the dorsal median line, from
the head to the posterior end of the body. The body is then opened and
the walk pinned out so as to disclose the internal parts. This should all
be done under water. If carefully performed the internal parts may be
easily worked out, a lens being used to show the smaller parts. To show
the nervous system and the nephridia the alimentary canal should be
cut through, behind the gizzard, and carefully dissected away in front.
There will then be no difficulty in making out all the organs except the
ovaries and spermaries. The ovaries may be found by careful study
with a lens, but the spermaries cannot be found without special methods.
The contents of the seminal vesicles and the ovaries should be examined
with a microscope. One of the nephridia should be removed and studied
with a low magnifying power.

For the study of the histology, sections should be furnished by the in-
structor. Animals preserved as above described are in good condition
for sectioning. They should be embedded in paraffin and stained in picro-
carmine. Sections through various parts of the body should be studied,
and these should include at least sections through the cerebral ganglia,
through the aortic arches, and through the posterior parts of the body show-
ing the typhlosole. The study of these sections with both low and high
powers will show the chief features of the microscopic anatomy. More
detailed study of the histology is hardly feasible with elementary classes.

CHAPTER IX

MULTICELLULAR ANIMALS: THE FROG (RANA)
GENERAL DESCRIPTION

THE body of the frog is composed of a head and a trunk,
but there is neither neck nor tail. The wide mouth extends
far back to the end of the head. On the upper side of the head
in front are two nostrils (nares) that open directly through
the bones of the skull into the mouth. Farther back on either
side of the head are the eyes, provided with two loose folds
of skin which serve as eyelids. The upper lid is immovable,
but the lower can be brought up over the eye for protection.
It is called the nictitating membrane (Lat. nictare = to wink),
is semi-transparent, and does not prevent sight wholly when
closed. Behind the eyes are two round flat surfaces, which
are membranes stretched over a shallow cavity in the skull.
They are the tympanic membranes (Lat. tympanum = drum)
and serve to collect sound waves and transfer them to the
ears which lie within the head. The part of the body behind
the anterior appendages or arms is called the abdomen, and
the cavity within, which holds the stomach and intestines,
is the abdominal cavity. The organs of the abdomen are
sometimes called viscera.

Of the two pairs of appendages, the fore legs are provided
with only four toes, while the hind legs have five toes con-
nected by a web. The hind legs are much longer than the
fore legs and are the chief organs used in locomotion. The
rest of the body is smooth, gradually tapering behind and end-
ing abruptly just above the attachment of the hind legs. Near
the posterior end of the body on the dorsal side is a good-sized
opening, the cloacal aperture (Lat. cloaca — sewer), which
serves as the common outlet of the intestine, the kidneys,
and the reproductive organs.

175

176

BIOLOGY

The whole body of the frog is covered with a smooth skin,
which is always moist and is abundantly supplied with blood
vessels, especially under the arms and on the side of the body.
The skin is everywhere loosely attached to the underlying
flesh and in certain rather large areas is not attached at all,
large spaces being thus left between
it and the flesh. These are lymph
spaces and are filled with a clear liquid
called lymph. When the skin is ex-
amined microscopically, it is found to
be made of two layers; Fig. 87. The
outer layer, the epidermis, ep, is thin,
while the inner layer, the dermis, d, is
quite thick. The epidermis is made
of several layers; the cells of the inner
layers are large, rounded, growing cells,
while the outer ones are flattened and
lifeless. The epidermis increases in
thickness from its inner side, and is
constantly wearing away on its outer
side. The dermis is a mass of con-
nective tissue fibers, among which lie
glands, blood vessels, nerves, and

numerous pigment (Lat. pingere = to paint) cells which give
the color to the skin.

The Skeleton. — The frog has an internal bony skeleton.
An internal skeleton is the most distinctive characteristic
of the highest animals. Animals with such a skeleton are
called vertebrates, a group comprising fishes, amphibians,
reptiles, birds, and mammals. No other animals except verte-
brates possess true bones. This bony skeleton gives support
to the softer parts, gives form to the body, serves to attach
the muscles, and enables them to produce the movements of the
animal. The skeleton is made of about ninety articulated bones,
i. e.t united together at the joints. Some of these form mov-

FIG. 87. — SECTION
THROUGH THE SKIN OF

THE FROG

ep, epidermis;
d, dermis.

(Modified from Howes.)

THE FROG 177

able joints, in which a movement of the bones produces a
movement of the body. In other joints the bones are firmly
grown together forming the immovable joints. The bones of
the skull, for example, are so firmly fused that they
appear as a single bone; and the bone of the forearm (Fig.
88 r-u) is really made of two bones fused together. Two dis-
tinct parts of the skeleton may clearly be seen: (I). the axial
skeleton, consisting of the skull and spinal column; (2) the ap-
pendicular skeleton, which forms the support for the arms
and legs.

The axial skeleton. — The spinal column is composed of
nine separate bones called vertebrae; Fig. 88 B. Each ver-
tebra consists of 9 centrum, c, and a neural arch, na, the
arch inclosing the neural foramen (Lat. foramen = opening).
From each side of the arch a process of bone extends laterally,
called the transverse process (Lat. trans = across -f vertere =
to turn) , tr. On the front and back of each vertebra are two
smooth surfaces where the successive vertebrae rest upon
each other, i. e., articulate (Lat. articulus = joint). They are
the articular processes, or zygapophyses. In their natural
position the nine vertebrae are joined together by their centra,
the posterior surface of one touching the anterior surface
of the next; Fig. A. The neural foramina are thus placed
opposite each other, and all together form a tube which in-
closes the spinal cord. The surfaces of the centra fit by a
ball-and-socket joint, each of the first seven vertebrae having
a ball on the posterior and a socket on the anterior surface,
while the eighth is concave on both surfaces, and the ninth
is convex on both surfaces. The nine vertebrae are much
alike, but can be distinguished from each other. The first
has no transverse process, while the centrum of the ninth has
two convex posterior surfaces, and very large transverse proc-
esses. From the posterior surface of the last vertebra a long
slender bone extends backward to the end of the body, the
urostyle (Gr. oura — tail + stylos = pillar); Fig. A, ur. The

178

BIOLOGY

FIG. 88. — THE SKELETON OP THE FROG

THE FROG 179

FIG. 88. — THE SKELETON OF THE FROG

A, one-half of the skeleton shown from above.

as, astragalus; mt, metatarsals;

c, carpals; mx, maxilla;

ca, calcaneum; na, nasal;

cr, cms; o, opening for nerve;

e, ethmoid; p, parietal;

ex, exoccipital; ph, phalanges;

/, frontal; pr, premaxilla;

fe, femur; r-u, radio-ulnar;

fm, foramen magnum; sq, squamosal;

it, ilium; t, tarsals;

hu, humerus; tr, transverse process;

me, metacarpals; ur, urostyle.

B, a vertebra from the end and from above.

c, centrum;

na, neural arch;

tr, transverse process.

C, the skull shown from below.

con, occipital condyle; pt, pterygoid;

«, Quadrate;
*>. vomer.

D, skull shown from the side.

ex, exoccipital; g, quadrato-jugal;

mx, maxilla; qu, quadrate;

m, mandible; sq, squamosal.
na, nasal;

E, the hyoid bone.

F , the shoulder girdle shown from below.

co, coracoid; pr, precoracoid;

gc, glenoid cavity; sc, scapula;

h, humerus; st, sternum.
ost, omosternum;

(of, the pelvic girdle shown from the side.

ac, acetabulum; is, ischium;

il, ilium; pu, pubis.

180 BIOLOGY

spinal cord extends into it, but soon passes out through two
small openings, on either side, o, as two small filaments. This
bone represents the tail found in allied animals (salamanders).
The frog has no ribs and the transverse processes end abruptly
at a short distance from the centrum.

The skull. — In front the first vertebra is articulated with
the skull, and the neural canal is continued into the skull
through a large opening, called the foramen magnum (Lat.
foramen = hole), fm. Inside the skull is a large cavity hold-
ing the brain, the cranial cavity. The skull itself is com-
posed of thirty-two bones, rigidly fused together to form a
solid structure. These bones, which are shown and named
in Figure 88 A and C, may be divided into three groups : 1 . The
cranial bones, which form the roof, walls, and floor of the cranial
cavity. The floor is made of the basioccipital and the para-
sphenoid, ps; the walls are made of the parietals, p, the otic
bones, and the exoccipitals, ex; and the roof is made of the
supraoccipitals and the frontals, /. 2. The facial bones, which
form the face. These are the nasals, na, the premaxillas, pr,
and the maxillas, mx, above, and the vomers, vo, below. 3. The
branchial (Lat. branchice = gills) skeleton. This part of the
skeleton is made primarily of two V-shaped arches, lying below
the cranium with the open part of the V above, next to the skull ;
but the original relation of the V-shaped arches has become so
modified that it is difficult to recognize. The first of the arches
is the lower jaw or mandible; Fig. Z>, m. The closed part
of this arch is in front where the two halves come together.
At the back the two halves spread apart and pass backward
to the point where the jaw articulates with the cranium at q.
The lower jaw is from this joint held attached to the cranium
by two chains of bones. One of them is made of the quadrate
(Fig. D, qu), and the squamosal, sq, these two forming what
is sometimes called the suspensorium. The other chain is
made of two bones lying below the cranium, the pterygoid
(Fig C, pi), and the palatine, pa. These are firmly fixed to

THE FROG 181

the cranium below. The joint is also attached to the max-
illa by a little bone called the quadrato-jugal ; Fig. D, q.
Although in the adult frog these chains of bones are firmly
attached to the cranium, they are at first free from it, and are
really the upper parts of the arches below, rather than a part
of, the cranium proper. The second arch is very rudimentary,
only a small part of it being left in the frog. It is called the
hyoid arch. Although in some animals this is also a well-
developed V-shaped arch, all that is left of it in the frog is a
flat plate, made partly of bone and partly of cartilage (Fig.
E), which is so loosely attached to the skull that it is usually
lost in prepared skulls. In the living frog it lies underneath
the larynx, to which it gives support and rigidity. It is attached
to the skull only by ligaments, without any bony connection.

When the skull begins to form in the young frog the parts
are soft, and only, as development proceeds, does true bone
form. Part of the skull forms first as cartilage, a material
that is harder than membrane but softer than bone. Later
within this cartilage the mineral matter is deposited, forming
true bone, and the bones thus formed are consequently called
cartilage bones. These are the octipitals, palatines, pterygoids,
and the mandibles. The other bones are formed first as mem-
branes rather than cartilage. Within the membrane the mineral
bony matter is laid down, and bones developing in this manner
are known as membrane bones. The membrane bones are
ihefrontals, parietals, parasphenoids, squamosals, nasals, vomers,
premaxilla, and the maxilla.

At its posterior end the skull is articulated with the first
vertebra by means of two rounded, smooth surfaces which fit
into two corresponding smooth depressions on the upper sur-
face of the first vertebra. The articular projections are called
the occipital condyles; Fig. C, con.

Appendicular skeleton. — Each appendage consists of a girdle
and the appendage proper. The shoulder girdle is a girdle
of bones surrounding the body just back of the head, and

182 BIOLOGY

holding the arm in position. It is shown from below and
flattened out in Figure F. Each half consists of a scapula, sc
(the dorsal part of which is made of cartilage), a coracoid,
co, a precoracoid and a clavicle fused together, pr. At the
place where the coracoid and the scapula come together is a
smooth cavity into which the end of the arm articulates, called
the glenoid cavity, gc. In its natural position the scapula
is bent over the back, with the coracoids touching each other
in the middle line below on the ventral side of the body. Be-
hind and in front of them are two pieces of bone, the omo-
sternum, ost, and the sternum, st. These two bones are re-
garded as a part of the axial skeleton.

The arm proper consists of the humerus (Fig. A, hu),
the radius and ulna fused together, r-u, six wrist or carpal
bones, c, and five fingers, of which the first is rudimentary.
Each finger is composed of a metacarpal, me, and several
phalanges, ph. The posterior appendages have a pelvic girdle,
made of three pairs of bones, all united into one in the adult.
One of them, the ilium, is long and runs forward to the trans-
verse process of the last vertebra; Fig. A, il. At its posterior
end each ilium joins the other two bones, the pubis (Fig. G,
pu), and the ischium, is. At the point where the three bones
meet there is a rounded cavity for the attachment of the leg,
the acetabulum, ac. The pubes and ischia of the two sides
of the body are fused together on the middle line, below the
urostyle. The leg consists of a femur (Fig. A, fe), and the
cms, cr, which is really composed of a tibia and fibula fused
together. Following the crus are the bones of the foot, consist-
ing of two slender bones, the astragalus, as, and calcaneum, ca;
then come two extremely small tarsal bones, t, and finally
a series of metatarsals, mt, and phalanges, ph.

Muscular System. — Most of the bones of the skeleton are
more or less movable one upon the other at the articulations.
The muscles which move them are numerous and complicated.
Each muscle is an elongated mass of contractile tissue, which

THE FROG

183

ab, adductor brevis;
al, adductor longus;
am, adductor magnus;
d, deltoid;
ec, extensor cruria;
gc, gastrocnemius;

FIG. 89. — THE MUSCLES OF THE

FROG FROM BELOW, THE SKIN
BEING REMOVED

The more important muscles are
named as follows: —
o, obliqus;
p, pectoralis;
r, rectus abdominis;
ri, rectus internus major;
a, submaxillaris;

st, sartorius:

t, triceps;

to, tibialis anticus;

tp, tibialis posticua;

vi, vast»s ipt«rnus.

184 BIOLOGY

is usually attached at the ends to two separate bones, the
term origin being applied to the attachment nearest to the
center of the body, and insertion to the attachment the farthest
from the center; muscles pull in the direction of their origin.
Since these muscles are numerous and attached to the bones
at different places, they pull upon the bones in different direc-
tions and produce a great variety of movements. Figure 89
shows the chief muscles of the frog. The names given to them
are the same as those applied to the corresponding muscles
in man.

Joints or Articulations. — Where two bones come together
they form a joint. In some cases the bones are so rigidly grown
together that there is no motion between them, thus forming
the fixed joints, like those that are between the bones which
form the skull. In other places the bones are freely movable,
forming the movable joints. All the movements of the body
are produced at the joints. The bones at these joints are so
connected that, while they are held firmly together, they are
at the same time freely movable. The ends of the bones are
generally more or less rounded, the end of one bone fitting
into a rounded depression on the other. The ends of the bones
are also covered by a layer of cartilage, which is quite smooth
so as to prevent friction. This structure makes it possible
for one bone to move upon the other without difficulty. All
friction is eliminated, and movement of the bones is rendered
easier, by a secretion of fluid which is poured into the joint
from the synovial glands. This is called the synovial fluid.
To prevent the bones from being pulled apart they are held
together by bands of white connective tissue called ligaments.
These are tough but flexible, and are attached to the two bones
that form the joint. They are long enough to make the mo-
tions of the bones free, but short enough to hold them in posi-
tion and prevent their being pulled away from each other by
slight strains. The bones are held firmly in position by the
muscles. The muscles which move the bones usually have

THE FROG 185

their origin on the bones above the joint, and their insertion
on the bone below. The muscles end in bands of connective
tissue called tendons that extend down over the joint to the
insertion on the lower bone. The muscles are always tightly
stretched in the body and always pulling upon the tendons.
As a result the tension upon the tendons holds the two bones
of the joint in firm contact. Outside of the muscles and tendons
is the skin. The joint thus consists of smoothly moving bones,
which are moistened by synovial' fluid, held in position by
tightly drawn tendons, prevented from being pulled apart by
ligaments that protect them against strains, and moved by
muscles.

The freedom of motion in the different joints varies with
the shape of the bones at the joints. In some of the articula-
tions, one bone ends in a ball which fits into a rounded socket
of the other bone. In this type, the ball-and-socket joint,
the bones are freely moved in any direction. The joint at
the hip and that of the shoulder are examples of this type.
In other joints the form of the bones is such that motion is
possible only backward or forward. These are called hinge
joints, and are illustrated by the joints at the elbow, the knee,
the wrist, and by the separate joints of the fingers and toes.
In some joints one bone moves around the other as on a pivot.
No good examples of this are found in the frog, but in the human
body the motion of turning the head, or turning over the hand
so that the back or the palm is uppermost, are excellent illus-
trations. It is evident that the movements of the body are
dependent upon the free motion of the bones at the joints,
and that the growing of the bones together at a joint, anchy-
losis as it is called, will destroy all power of motion.

Alimentary Canal. — The wide mouth (oral opening) leads
into a very large cavity, the buccal cavity. There are teeth
on the maxilla,. premaxilla, and vomer (Fig. 88 C, D), which
are of use for holding, but not for masticating food. On the
floor of the mouth is, a large muscular tongue, attached to

186

BIOLOGY

.oe

the base of the mouth in front, and free behind. Owing to
this attachment, the back part of the tongue can be thrown
out of the mouth for a considerable distance, serving as an
important organ for capturing insects. Just back of the tongue
on the floor of the mouth is a narrow slit, the glottis, leading
into a tube, which passes to the lungs. Behind the glottis

is a larger opening leading to the
oesophagus, and hance to the stom-
ach. The nostrils open in the mouth
through the roof in front (internal
nares); and a pair of openings in the
back part of the roof leads to the ears,
the eustachian openings.

If a slit be made through the skin
and flesh of the abdomen, passing
forward on the ventral middle line
through the sternum, and the body
opened, most of the internal organs
can be seen. The oesophagus passes
directly backward about halfway to
the end of the body, where it ex-
pands into a large chamber, the
stomach (Fig. 90 st), which extends
obliquely across the body towards
the right. The lower part of the
l-» d stomach is called the pylorus ; this
passes down into a small tube which
FIG. 90.— THE ALIMENTARY makeg a u_shaped bend, called the

TRACT OF THE FROG ^fo 4 and then formg gey_

blt bladder; oe, oesophagus;

el, cloacal cavity; p, pancreas; eral COllS Which Constitute the UltCS-

d, duodenum; sp, spleen; .

gb, gall bladder; at, stomach; 11116, in. r inally the tube paSSCS into

in, intestine; r, rectum.

a large but short chamber, the rectum,

r, which communicates with the exterior through the cloacal
opening. In front of and to the right of the stomach is the large
several-lobed liver. This secretes a liquid called bile, which

THE FROG

187

passes by a duct into the gall bladder, gb, where it is stored
for a while and from which it later passes through the bile
duct into the duodenum, close to the pyloric end of the stomach.
In the bend of the U formed by the duodenum and the stomach,
is a slender, yellowish body, the pancreas, p, which empties into
the duodenum by the pancreatic duct opening close to the
bile duct. The lining of the whole alimentary canal is called
the mucous membrane.

The whole intestine is slung in position by a thin sheet of
membrane, which passes around the intestine and then be-
comes attached to the abdominal wall. This is the mesentery,
and is really a fold of a large membrane that completely
lines the body cavity, the peritoneum.
The relations of the peritoneum, mes-
entery, and intestine are shown dia-
grammatically in Figure 91. In the
mesentery are many nerves and nu-
merous blood vessels which carry
nutrition from the intestine. The
mesentery surrounds not only the in-
testine, but also the liver and the
pancreas. In its folds below the
stomach is a rounded red body, the
spleen; Fig. 90 sp.

Circulatory System. — The circula-
tory system of the frog, like that of
the earthworm, consists of blood in-
closed in a network of blood vessels; but it is a more definite
system and the blood flows in a more regular course. It con-
sists of a true heart, and blood vessels.

The heart is situated beneath the shoulder girdle, in front
of the liver and is surrounded by a thin sac, the pericardium
(Gr. peri = around + cardia = heart). The heart itself is
made up of a sac divided into three chambers, the walls of
which are masses of muscles. The fibers of these muscles run

FIG. 91. — DIAGRAM REP-
RESENTING A CROSS SEC-
TION OF THE BODY

bw, body wall;
in, intestine;
mes, mesentery;
per, peritoneum.

188 BIOLOGY

in every direction, so that when they contract (systole) the
heart is diminished in size and the blood that is in the cavities
is squeezed out; when they relax (diastole) the heart expands
again and the blood flows into it. Figure 92 shows a diagram
of the heart structure, cut open so as to show the interior of
the cavities. At the anterior end are two cavities, the right
and the left auricles, ra and la; the right, which receives blood
from the body, being much larger than the left, which receives
blood from the lungs. These two chambers are separated
by a partition. At the lower side of the auricles each opens
into the ventricle, v, the third and largest chamber below.
The openings between the auricles and ventricle (shown by the
arrows in Fig. 102), are guarded by valves, which are flaps of
membrane, so situated that they allow blood to flow readily
from the auricle into the ventricle, but close up at once if the
blood starts to flow back into the auricle, as it would do when
the ventricle contracts, did not these valves block the passage.
The ventricle is a large chamber, partly divided by partitions.
Leading out of it in front is a large blood vessel. This
extends forward, on the ventral side of the heart, and
at the anterior end of the heart it divides into two arteries,
one turning to the right and one to the left. The large
vessel is called the bulbus arteriosus, ba, and its two
branches are the aortae, ad. This bulbus receives the blood which
is forced out of the heart when it contracts. Within it, and
at the beginning of the aorta?, are valves which control the
flow of the blood, as will be described on a later page. On the
dorsal side of the heart is a large thin-walled chamber, the ve-
nus sinus (Fig. 92 B, vs), into which open the veins that bring
the blood back from the body. This sinus opens into the right
auricle, which thus receives all the blood which flows back to
the heart from all parts of the body, except the lungs. The
blood from the lungs empties into the left auricle by two small
veins, one from each lung; Fig. 92 pv.
The blood vessels ramify all over the body in a very complex

189

FIG. 92. — DIAGRAM OF THE CIRCULATION OF THE FROG

The veins are represented in shaded lines and those entering the heart from
in front are shown only on one side. A, the general circulation shown from
below.

a, abdominal vein;
ao, aorta;
6, brachial;
ba, bulbus arteriosus;
ca, cceliac axis;
co, carotid artery;
CM, cutaneous artery;

hv, hepatic vein;
in, intestine;
jv, jugular vein;
I, lingual artery;
la, left auricle;
pro, pre vena cava;
ptc, post vena cava;

pu, pulmonary artery;
pv, portal vein;
ra, right auricle;
rp, renal-portal vein;
sp, spermary;
v, ventricle.

B, The heart from below, showing the venus sinus cut open.
ao, aorta; puv, pulmonary vein; ptc, post vena cava;

la, left auricle; pvc, pre vena cava; vs, venus sinus,

ra, right auricle;

190 BIOLOGY

system. The arteries, which take blood away from the heart,
are thick-walled and elastic; while the veins, which bring it
back again, are thin-walled. The distribution of the chief
blood vessels is shown in Figure 92. The bulbus arteriosus
soon divides into two branches that turn backward and finally
unite with each other in the abdomen beneath the stomach.
These two branches, the aortae, ao, give off in their course
many vessels, the chief of which are the lingual to the tongue,
I, the carotid to the head, co, the brachial to the arms, 6,
and the coeliac axis to the organs of the abdomen, ca. After
dividing many times into smaller and smaller branches, the
arteries finally break up into an immense number of minute
thin-walled vessels called capillaries (Lat. capillus = hair).
These are microscopic, but of great importance, since all of
the interchanges between the blood and the tissues of the body
take place through them; Fig. 101. The blood, after passing
through the capillaries, enters again into a series of vessels of
constantly increasing diameter and finds its way back to the
heart. These larger returning vessels are veins, and they
unite with others, until finally a few large veins are formed
which empty into the sinus; Fig. 92 B. The blood vessels
thus form a closed system, and the blood that leaves the heart
returns without leaving the vessels. The blood that goes to
the intestine by the coeliac axis, ca, passes through two series
of capillaries before again entering the heart. It first passes
into capillaries in the intestine, where it receives nutriment
absorbed from the food; then it is collected into a large vein,
the portal vein, pv, which enters the liver, and breaks up into
another system of capillaries; then, by the way of the hepatic
vein (Gr. hepar = liver), hv, it enters into the large posterior
vena cava (Figs. A and B, ptc), which leads to the venus sinus.
This system of veins and capillaries in the liver is called the
portal system. Part of the blood that goes to the legs also
has a double system. It first enters the capillaries in the
muscles of the legs, and on its way back a part of it passes

THE FROG 191

through the kidneys, where it is again broken up into capil-
laries. This is the course of the blood which returns from
the leg through the renal-portal vein (Fig. 92 rp), but the rest
of the blood from the legs is diverted to an abdominal vein, a,
which enters the heart without passing through the liver.
Both the liver and the kidneys have their own supply of blood
from the aorta, as well as that received from the veins.

The vessels thus far described are called the systemic cir-
culation, in distinction from the pulmonary circulation, which
is the circulation in the lungs. The lungs are elastic bags
(Fig. 92), capable of much expansion when inflated with air,
but collapsing if the air is removed. They are connected with
the mouth by the larynx, which opens at the base of the tongue
through the glottis. Through the glottis and the larynx air
is taken into the lungs to purify the blood. The arteries which
supply the lungs, the pulmonary arteries, pu, arise from the
main arteries near the heart. From each of these an artery
is given off to the skin under the arm, the cutaneous, cu. Since
in the lungs the blood is purified by the oxygen of the air,
and through the skin it is purified by the oxygen in the water,
the frog can live either in the water or in the air, i. e., it is
amphibious. The blood that is purified in the lungs enters the
heart again by a pulmonary vein, puv, which flows into the left
auricle. The pure blood in the left auricle is thus kept separate
from the impure blood in the right auricle, but as soon as the
auricles contract the blood of both auricles is forced into the
single ventricle, and intermingles. Although the blood in the
ventricle is really mixed, still the blood upon the right side of it,
since it received blood directly from the right auricle, will con-
tain more impure blood than that on the left side, which is
connected directly with the left auricle. The pure and impure
blood are kept partly separate by muscular partitions ex-
tending irregularly through the ventricle.

The blood is composed of a colorless liquid, called the plasma,
in which float two types of corpuscles. The larger, the red

192

BIOLOGY

corpuscles (erythrocytes) (Gr. erythros = red + cytos = cell) (Fig.

93 re), are oval in shape; their red color is due to the haemo-
globin, which is in the cor-
instead of in the
as in the case of

rc .

, . 7:L .••. I* ~

in. mm

FIG. 93. — BLOOD OF THE FROG, HIGHLY
MAGNIFIED

lu, leucocytes or white corpuscles;
rc, red corpuscles.

puscles,
plasma,

the earthworm. The white
corpuscles (leucocytes) (Gr.
leukos = white + cytos =
cell), lu, are smaller than
the red corpuscles, and are
able to force themselves
through the walls of the
capillaries, and wander in-
definitely through the tis-
sues. There is a third type
of very minute bodies in the
plasma, called platelets, of which little is known.

Lymph System. — Besides the blood vessels, the frog has a
system of much smaller lymph vessels in the skin, the intestine,
and other parts of the body. These are thin walled and filled
with a colorless liquid, the lymph, and are so delicate the,t
they can be seen only in specially prepared specimens. In
places these vessels are connected with spaces between the
tissues, lacunae, and with the large cavities of the body. In
the intestine the lymph vessels receive a special name, the
lacteals. Lymphatic glands are found in connection with the
lymph vessels, and in the frog there are also two pairs of lymph
hearts, whose contraction propels the lymph in its circulation.
The Nervous System. — The nervous system consists of: (1)
The cerebrospinal axis, (2) The cranial nerves, (3) The sympa-
thetic system.

Cerebrospinal Axis. — The brain and spinal cord are on
the dorsal side of the animal, within the neural canal and the
cavity of the skull; Fig. 94. The brain consists of several
distinct parts. Beginning in front they are as follows: The

THE FROG

103

olfactory lobes, ol, the cerebral hemispheres, ce, the thalamen-
cephalon, th, the optic lobes, op, the cerebellum, cb, and the
medulla oblongata, m. The cere-
bellum is very small, and the me-
dulla appears to be only an en-
larged continuation of the spinal
cord. In the latter there is a
large triangular cavity, roofed
over by a thin membrane con-
taining blood vessels (choroid
plexus) . The cavity is called the
fourth ventricle, and it commu-
nicates with other cavities in the
brain. On top of the thalamen-
cephalon is a small body, the
pineal gland or the epiphysis, pi.
The under side of the thalamen-
cephalon is produced into a
process directed backward, the
infundibulum, which ends in
another small body, the pituitary
body or the hypophysis.

The cerebrum and thalamen-
cephalon together constitute the
forebrain, the optic lobes form
the mid-brain, and the cerebel-
lum and medulla form the hind-brain. The relative devel-
opment of these different parts varies widely in different
animals, and in the higher vertebrates the cerebrum becomes
much the largest part of the brain, this development reaching
its maximum in the human species.

From the posterior part of the medulla the spinal cord, sp,
extends through the spinal column, tapering to a minute fila-
ment, which extends a short distance into the urostyle. The
brain and spinal cord are covered by two membranes, an outer

FIG. 94. — THE CENTRAL
NERVOUS SYSTEM

Shown in position in the skull and
spinal column.

cb, cerebellum; op, optic lobe;

ce, cerebrum; pi, pineal body;

m, medulla oblon- sp, spinal cord;

gata; th, thalamen-

ol, olfactory sac; cephalon.

194 BIOLOGY

tough one called the dura mater, and a more delicate, inner
membrane, the pia mater.

The Craniospinal Nerves. — Twenty pairs of nerves arise
from the brain and cord, — ten from the brain, and an equal
number from the cord. Those from the brain, the cranial
nerves, supply the organs of special sense and the muscles
and other organs of the head, the heart, lungs, and stomach.
They are as follows: —

1. The olfactory nerves, from the olfactory lobes supplying
the nasal cavities.

2. The optic nerves. These two nerves arise from the optic
lobes, cross each other to form the optic chiasm, and then
each passes to the eye on the opposite side of the head.

3. Motor ocularis, supplying the muscles of the eye.

4. Patheticus, supplying the muscles of the eye.

5. Trigeminal, supplying the sides of the head (sensory).

6. Abducens, supplying the muscles of the eye.

7. Facial, supplying the sides of the head (chiefly motor) ,

8. Auditory, supplying the ear.

9. Glossopharyngeal, supplying the pharynx and the tongue
(sensory).

10. Pneumogastric, supplying the larynx, the heart, and the
stomach.

From the spinal cord arise ten pairs of spinal nerves, one
between the skull and the first vertebra, and one between
each vertebra and the next; Fig. 95. The first supplies
the tongue (motor); the second and third unite to form the
nerve going to the arm, the brachial nerve (Lat. brachium =
arm); the fourth, fifth, and sixth supply the middle region
of the body; and the seventh, eighth, and ninth unite by cross
branches to form the sciatic plexus (Lat. plectare = to braid),
from which arise the nerves that supply the leg, the sciatic
nerve, which is the largest in the body; the tenth nerve supplies
the region of the urostyle. Each nerve arises from the cord
by two roots, of which the anterior root carries impulses away

THE FROG

195

from the brain (efferent fibers), and the posterior root carries
impulses toward the brain (afferent fibers).

The Sympathetic System. — In the abdominal cavity, lying
on each side of the spinal column, is a chain of minute nerve
ganglia, ten in number, which are also
connected with the spinal nerves; Fig.
95 sy. These constitute the sympa-
thetic system. From these two chains
of ganglia minute nerves are given off,
chiefly to supply the intestine, the kid-
ney, and the other organs of the ab-
domen. Although connected with the
spinal nerve, the sympathetic system is
quite distinct and has special functions.

The microscope shows that the nerv-
ous system, like that of the earthworm,
is composed of an enormous number of
neurons, each with its cell body, dendrites,
and axon. These are massed in the
brain and cord, and there are many also
in the ganglia outside of the cord. They
are so situated that part of them carry
impulses to the center, and part of them
carry them in the reverse direction.
Their numbers are greater and their re-
lations more complex than those of the
earthworm.

The Sense Organs. — At the periph-
eral end of all of the sensory nerves
are found very complicated organs, con-
structed so as to be affected by certain
external stimuli. When they are stimu-
lated impulses start from them and pass

over the afferent nerves to the brain, where they become
sensations. They are the sensory organs and are as follows: —

FII

vn.

FIG. 95. — DIAGRAM
SHOWING THE RELA-
TION OF SYMPATHETIC
SYSTEM TO THE
SPINAL NERVES

The sympathetic chain of
one side only is shown. The
spinal nerves are indicated by
Roman numerals; sy, sympa-
thetic nerve ; syg , sympathetic

ified from Parker.)

196

BIOLOGY

Olfactory organs. — Just within the nostrils are two little
cavities occupied by the olfactory sacs. In these sacs the
olfactory nerves are distributed, ending in delicate nerve cells,
which are sensitive to odors; Fig. 96 A.

Cornea

umor\Iris

FIG. 96. — TERMINATION OF
SENSORY NERVE CELLS

A, Olfactory cells; B, cells in
the retina, sensitive to the light,
showing the rods on the left and
cones on the right.

(Dogiel and Gaupp.)

-071

FIG. 97. — DIAGRAMMATIC CROSS SECTION OP
THE EYE OF THE FROG

ch, choroid coat;

I, the suspensory ligament;

on, optic nerve; (Retzius.)

, retina;

:, sclerotic coat.

The eyes. — The eyes are large, spherical organs, planned
after the structure of the vertebrate eyes in general. Figure 97
is a cross section of an eye showing the important parts. It
is a spherical chamber, the walls of which are opaque, except
in front, where they are transparent, and act like the dark
chamber of a camera. The walls of the chamber are made of
several layers. In the very front is the cornea, presenting a
transparent curved surface. The back part, comprising about
two-thirds of the chamber wall, is made of three layers. On
the outside is a sclerotic coat, sc, composed of fibrous tissue
and cartilage; next to this a thin coat containing pigment,
the choroid, ch, and inside of this a still thinner retina, r, which

THE FROG 197

is the sensitive part of the eye. At the back of the chamber
is an opening through which the optic nerve enters, on. After
entering the eye the nerve spreads out on the retina, where it
is affected by the light entering the eye. The chamber of the
eye is divided into two parts by a large spherical, transparent,
crystalline lens, held in position by several bands of fibers,
shown at I. Anteriorly the lens is partly covered by an opaque
membrane, really a continuation of the choroid, which grows out
from the wall of the chamber on all sides. This is the iris, and
it covers the outer part of the lens, except in the middle, where
the lens is not covered. This opening is the pupil, and serves
to allow light to enter. The iris contains pigment cells, which
give the eye its color. Each of the two chambers of the eye
is filled with a transparent fluid. That lying between the
cornea and the lens is the aqueous humor, and that back of the
lens, which is rather more solid, is the vitreous humor. The
retina, which lines the eye chamber, is an extremely complicated
organ, made of hundreds of thousands of end organs of sensi-
tive nerves. It is a complex of neuron bodies, dendrites, and
axons (Fig. 96 B)t and is highly sensitive to the light, which
is focused upon it by the lens. Attached to the ball of the eye
are six muscles, by means of which it can be rotated in any
direction:

The ears. — The frog has no external ears. Just back of the
eyes are two rounded, flat depressions, each formed by a mem-
brane which covers the real ear. If the thin skin which covers
this area be removed, a rather tough, flat membrane will be
found beneath, which is the tympanic membrane proper. This
membrane extends over a shallow conical cavity, called the
tympanum or ear-drum. This cavity connects below with
the mouth through the eustachian tube. Extending across
this is a slender bar of bone and cartilage, called the columella.
This is attached to the membrane at one end and connected
with the inner ear at the other, and transmits vibrations of
the membrane to the inner ear, the real organ of hearing.

198

BIOLOGY

FIG.

\. THE INTERNAL EAR

OF THE FROG

an, auditory nerve;
c, semicircular canals;

(Retzius.)

s, saccule;
u, utricle.

The inner ear, which is :he true sensory end organ of the audi-
tory nerve, lies embedded in the bones of the skull. Its general
appearance may be seen from Figure 98. It is quite a compli-
cated organ, and the auditory nerve enters in and finally

terminates in delicate endings,
which are readily stimulated by
the vibrations brought from the
exterior through the membrane
and the columella. The canals
shown at c, the semicircular ca-
nals, have a function related to
balancing the body and keeping
it in an upright position, i. e.,
equilibrium.

Other senses. — The sense of smell
is located in the nostrils. These
openings lead into little olfactory

sacs just within the bones, and the air which enters them
passes through the bones into the mouth by openings on the
roof of the mouth called
internal nares. The ol-
factory nerve is expanded
in the olfactory sac,
where vapors that may
be in the air affect it.

The sense of taste is
situated on the tongue,
within which are end or-
gans sensitive to liquids.
Only substances dis-
solved in liquids are ca-
pable of affecting these
end organs; Fig. 99.

The touch and pressure senses are located in the skin. Scat-
tered over the body generally are numerous end organs, which

FIG. 99. — SECTION OF TONGUE OF FROG

n, nerve ending; nc, nerve cells; ne, nerve trunk.
(Gaupp.)

THE FROG 199

form the termination of the sensory nerves. They are of different
kinds, and doubtless have different functions, but all are associ-
ated with what is in general called the sense of feeling or touch.

The Excretory Organs. — Lying in the back part of the ab-
domen near the legs are two flat, rather oval bodies, one on
either side of the middle line, the kidneys; Fig. 92. Each is
abundantly supplied with blood vessels, a fact which indicates
important functions. Microscopic study shows them to be made
of many coiled tubes, similar to the nephridia of the earth-
worm. These tubes remove excreted products from the blood
which passes through them. From the outer side of each a
small duct, the ureter, passes backward toward the cloaca,
where it empties into the bladder (Fig. 90 bl) , a rather large two-
lobed sac, which may be filled with the urine secreted by the
kidney, or may collapse when empty. It opens into the cloacal
chamber, and its contents are discharged through the common
cloacal opening. (In man a special duct, the urethra, leads
from the bladder to the exterior.)

Reproductive Organs. — The two sexes in the frog are in
separate individuals, thus differing from the condition found
in the earthworm. The male may be distinguished externally
by a thick pad on the under side of its thumb, which is rather
large in the spring, but small at other seasons of the year.
The spermaries are found in the male at the upper end of the
kidneys; Fig. 92 sp. They are two in number, rounded or
oval in shape, and of a light yellowish color. Attached to them
are usually several branching masses of yellow fat. The sperm
produced in the spermaries are carried through some delicate
ducts into the kidney. These ducts, the vasa efferentia, pass
through the kidneys and empty into the ureters, which lie
on their outer edge. The ureters in the frog thus serve for the
exit of both the kidney secretion and the secretions from the
spermaries. These ureters are, in some species of frogs, en-
larged into a small sac just at the point where they enter the
cloacal chamber, and in these sacs the sperms are stored until

200

BIOLOGY

the frog is ready to discharge them at the time of copulation.
These sacs are called seminal vesicles. Some species of frogs
do not have such vesicles.

In the females the ovaries are situated in a position cor-
responding to that of the spermaries in the male; Fig. 100 ov.

During the late spring and
summer they are rather small,
slightly folded, leaf-like or-
gans, not much larger than
the spermaries, though differ-
ing in shape. In the fall of
the year the eggs in these
ovaries begin to grow, causing
the ovaries to become greatly
expanded. During the fall the
ovaries are usually greatly
swollen and completely fill
the abdominal cavity, almost
concealing the other organs.
The oviducts that carry the
eggs to the exterior are not
directly connected with the
ovaries. They are very much
coiled tubes, o, lying beside the
kidneys, each ending at its an-
terior end in a funnel-shaped
opening. From this opening
the tube passes backward
beside the kidneys, and, after
making many coils, finally
opens into the cloacal chamber at the back. Just before its
termination it is swollen into a rather large, thin-walled cham-
ber, the uterus, ut, in which the eggs may be stored for a time
after passing through the oviducts before the final egg laying.
These long ducts vary greatly in size at different seasons,

FIG. 100. — REPRODUCTIVE ORGANS
OF A FEMALE FROG, ATTACHED TO
THE KIDNEYS

ki, kidneys;

o, oviduct;

ov, ovary filled with eggs;

ut, uterus;
c, cloacal
chamber.

THE FROG 201

being small in the summer, but enlarging with the enlargement
of the ovaries, and swelling greatly in the early spring pre-
paratory to egg laying. In the walls of the oviducts are numer-
ous little glands, whose function is to secrete material around
the egg to form the shell or other protective covering. They
are nidamental glands (Lat. nidus = a nest).

It will be seen that the sexual organs and the kidneys are
very closely connected. They lie close together, have a com-
mon opening, and in the male the same duct, the ureter, serves
for the exit of the sperms and the urine. A similar close rela-
tion is found in other vertebrates, and a study of the develop-
ment of the animals shows that their ducts are originally
derived from the same organ in the embryo. The two systems
together are known as the urogenital system. In the frog
this system opens to the exterior with the intestine by the
single common cloacal opening. In higher animals they may
have separate openings.

LABORATORY WORK UPON THE FROG

For a detailed dissection of the frog, reference must be made to some
of the numerous laboratory manuals. The brief general directions given
below will be sufficient to illustrate the topics discussed in the text, and
at least this amount of laboratory work is necessary to make the text
properly intelligible.

If the specimens are obtained alive they should first be killed with
chloroform, and, while still fresh, all of the points in the external anatomy
should be made out. Note should be made of the following: head; body;
.absence of tail; the loose skin, attached, however, at certain points; arms;
numbers of fingers; legs; number of toes; web between the toes; mouth;
nostrils; eyes with eyelids; ears; cloacal opening. Open the mouth and
note tongue; glottis; gullet.

The dissection of the organs of the abdomen can best be made with a
freshly killed specimen, but it may be done satisfactorily with animals
preserved in alcohol or formalin. The dissection of the brain and spinal
cord should always be made upon animals preserved in alcohol, since
these organs are too soft to handle in fresh specimens. A mounted skele-
ton of the animal should be at hand for study and comparison with the
animal under dissection.

202 BIOLOGY

The order of dissection given below is so planned as to make it pos-
sible to do practically all of tlie dissection upon a single specimen. The
specimen may be preserved in formalin and the work carried out at leisure.
If the order given is followed, it is possible to have a large class working
at the same time, and, when the work is finished, all of the important
parts in the anatomy will have been made out, except the skull and the
shoulder girdle, these having of necessity been destroyed in opening the
body and in exposing the brain. If frequent references are made to the de-
scription of the frog given in the text, the brief description here given
will be sufficient to make a satisfactory dissection.

Open the frog by a median ventral incision, made with scissors, ex-
tending from the legs forward to the sternum, cutting through both skin
and flesh. The blunt end of the scissors is then to be thrust under the
sternum, and this girdle of bones is to be cut through. This will make
it possible to open the abdomen, pinning out the flaps of the abdominal
walls and the arms so as to expose the organs of the abdomen. If the frog
is a freshly killed specimen, all of the subsequent study of the viscera
should be made with the animal immersed in water. If the frog is a pre-
served specimen, this is not so necessary.

The organs of the abdomen may now be studied. The following parts
should be made out without any further dissection, being disclosed simply
by pushing the organs one after the other to one side, and they may bo
examined conveniently in the following order: liver; heart; large arteries
around the heart; veins entering the heart; stomach; intestine; gall blad-
der; rectum; mesentery, which contains blood vessels that may be
traced to the liver.

In opening the body, if the specimen is a fresh one, there is danger
that some of the blood vessels may be cut, making it difficult or impos-
sible to follow the blood vessels. In order to work out the blood vessels
satisfactorily, it is necessary to have an injected specimen. These may
be bought of dealers in natural history supplies, or the injection may be
done by the instructor.

If the specimen is a female, the body cavity will, at certain seasons of
the year, be filled with an enormously expanded ovary, filled with eggs.
In order to make out the other abdominal organs, these must be removed
carefully, so as not to injure the other parts. After they have been
removed there will appear lying on either side of the back part of the
abdomen the very much enlarged oviduct, showing as a much coiled
tube. This should also be removed, note .being made of its connection
with the cloacal chamber behind. If the ovary is not thus enlarged, or
if the specimen is a male, it is not necessary to remove the reproductive
organs to show the other features.

THE FROG 203

With the organs all in position, now make out the rectum; the bladder;
the spleen; the cloacal chamber; the kidneys; the spermaries; the ovaries
and oviduct in the female, and the spermaries and vas deferens in the male.

Remove the heart, liver, stomach, and intestines. This will disclose
the lungs; the two systemic arteries uniting to form the dorsal aorta,
which should be traced to where it divides to supply the legs; the nerves
to the arms; nerves to the back; three large nerves arising from the back-
bone and extending toward the legs, and finally uniting to form the sciatic
plexus, from which arise the large nerves entering the leg. By lifting up
the aorta gently, delicate branches of the sympathetic system may be
seen and traced to their ganglia.

One leg of the animal should be dissected to make out the muscles,
nerves, bones, and joints. The muscles should be separated from each
other and traced to their origin and insertion, special notice being taken
of the long tendons extending from the lower muscles down to the toes.
In the joints, note the freedom of motion of the bones; the tendons, which
extend over them; the rather loose ligaments that unite the bones; the
readiness with which the bones come apart when the ligaments are cut;
the smooth surfaces of the ends of the bones; and the cartilage that covers
their ends. (If there is time for more careful dissection, reference must be
made to laboratory guides on the dissection of the frog.) Clean all of the
soft parts from the bones of the leg, separating and identifying each bone.

Examine the eyelids; the iris; the pupil. Make an incision through
the iris and remove the lens; note the cavity of the eye behind the lens.
Cut an incision through the tympanic membrane, noting the shallow
cavity beneath it, the tympanic cavity, the bony columella extending
across it to the skull. A bristle thrust into the bottom of this cavity will
enter the mouth through the eustachian tube.

Remove with a knife a bit of the -flat bone on top of the skull, exposing
the brain; and then, with forceps and scissors, break away the bone so
as to expose completely the brain and spinal cord down the back to the
urostyle, taking care not to injure the soft parts. Identify all parts of
the brain as described on page 193.

The skeleton should be studied from another specimen. Remove all
the soft parts from the skeleton, separating all the bones. Clean and iden-
tify each (Fig. 88), and compare with the mounted skeleton.

BOOKS FOR REFERENCE

ECKER, The Anatomy of the Frog, The Macmillan Co., New York.
HOLMES, Biology of the Frog, The Macmillan Co., New York.
MARSHALL, The Frog, The Macmillan Co., New York.
MORGAN, Development of Frogs' Eggs, The Macmillan Co., New York.
GUYER, Animal Micrology, University of Chicago Press, Chicago.

CHAPTER X
THE PHYSIOLOGY OF AN ANIMAL

HAVING now studied the structure of multicellular animals,
we will consider briefly the functions of the various organs, with
special reference to the frog.

Alimentary System. — The primary purpose of the alimentary
canal is the digestion and absorption of food. The food of
animals is always organic, since animals are unable to utilize
mineral substances upon which plants subsist. Animals feed
upon the substances manufactured by plants: starch, the first
product of photosynthesis, may serve animals for food, and the
same is true of sugar, fats, and proteids. These foods are usually
in a solid form when taken into the animal's mouth, and in order
to be of any use they must pass from the alimentary canal into
the blood vessels. Solid food is incapable of passing through the
intestinal walls, and must be changed so that it can be dissolved
in the liquids of the alimentary canal, a process called digestion.
Digestion is brought about by digestive fluids which are secreted
by digestive glands within the alimentary tract. The frog has
no salivary glands such as man possesses, and the first digestive
glands are in the walls of the stomach. These are microscopic
in size and are called gastric glands. They are present in large
numbers and secrete the gastric juice, which is poured directly
upon the food after it reaches the stomach. A second digestive
gland is the pancreas, lying just below the stomach and pouring
its secretion, the pancreatic juice, by a special duct into the
intestine, close to the stomach. This is mixed with the food
just as it leaves the stomach and after it has been acted upon by
the gastric juice. By the action of these two digestive fluids the
solid foods are changed in their nature and rendered partly
soluble. They are then dissolved in the intestinal liquids, becom-
ing a thick, rather slimy mass of dissolved material. The
different foods eaten by the animal are subject to different

204

THE PHYSIOLOGY OF AN ANIMAL 205

changes under the influence of the separate digestive fluids,
those secreted by the stomach producing a different kind of
digestion from those of the pancreas; but all aid in rendering
the food soluble.

Absorption. — The food is driven through the alimentary canal
by the muscular contractions of its walls. These muscles are in
two sets, one extending lengthwise and the other running around
the intestine in a circular direction. By their contraction waves
of constriction pass along the intestine, forcing the food slowly
along. This peculiar writhing motion of the intestine is spoken
of as peristalsis (Gr. peri = around + stalsis = a compression).

As the food is pushed through the intestine its digestion and
solution is completed and it begins to pass through the walls
of the intestine into the surrounding blood vessels. As the
intestinal contents pass onward more and more of the nutriment
contained in the food is absorbed from it and enters the blood.
The undigested and useless portions of the food pass on and even-
tually, in the form of faeces, are voided through the cloacal
opening.

Circulation. — The food absorbed into the blood is now carried
over the body in the blood. The liquid part of the blood, the
plasma, is the circulating medium, the red and white corpuscles
having special functions. The red corpuscles (erythrocytes) ,
which are by far the most numerous, give the blood its red color
and are associated with respiration. The white corpuscles (leu-
cocytes) , of which there are several kinds, have various functions,
one of which is the removing of foreign bodies from the body
and protecting it from the attacks of microscopic germs, or other
irritating substances that may enter the tissues. The white
corpuscles with this power are called phagocytes (Gr. phagein =
to eat + cytos) ; they are able to leave the blood vessels, by
forcing their way through the walls of the capillaries; Fig.
101 leu. They then migrate among the tissues and collect at
any part of the body to guard it from an attack.

The blood is kept circulating through the vessels by means of^

206

BIOLOGY

FlG. 101. — A SINGLE CAPILLARY

Showing the corpuscles being forced through its
walls.

re, red corpuscles;
leu, leucocytes;

leu, a leucocyte that is forcing its way through the
walls of the capillary into the surrounding tissues.

the heart, which acts as a pump. In the frog's heart there are
three chambers and the circulation is as follows: The blood
which enters the heart from the body, which is impure blood,

is received first into the
venus sinus (Fig. 92 B,
vs), and from here it en-
ters the right auricle;
Fig. 102 ra. At the
same time pure blood
enters from the lungs
and skin, and is received
in the left auricle. Now
the two auricles con-
tract and force the
blood into the single
ventricle v, through the
openings indicated by
the arrows in Figure

102. The ventricle thus receives both pure and impure blood,
the pure blood being poured into its left side and the impure
blood into its right side. These two kinds of blood are partly
mixed, excepft for a fraction of a second, when they are sep-
arate from each other. They are kept from mixing too quickly
by several muscular bands stretching from the walls of the
heart. But almost at the same instant that the ventricle is
filled it contracts, and its contained blood is forced into the
large artery, the bulbus arteriosus. This artery, as will be seen
from Figure 102 ba, opens on the right side of the ventricle and
consequently will receive first the blood which entered the ven-
tricle from the right auricle, which is impure blood. Thus impure
blood passes first into the arteries, to be followed by mixed blood
and finally by the purer blood that comes from the. left side of
the ventricle, and hence from the left auricle. With each con-
traction of the heart there enters the arteries first a little impure
blood, then a little mixed blood, and finally a little pure blood.

THE PHYSIOLOGY OF AN ANIMAL

207

ao-

ba-

From Figure 102 pu, it will be seen that the first branch of
the artery passes to the lungs. In the bulbus arteriosus are
valves so arranged that the first blood passing from the heart
with each beat goes to the lungs; after these are partly filled
the next blood passes through the blood vessels shown at
ao, down to the arms and to the lower parts of the body; and
finally the last of the blood that comes out with each beat of
the heart passes up into
the head through the ar-
tery, co. Thus the most
impure blood passes to
the lungs, where it is pu-
rified, the mixed blood
goes to the lower parts
of the body, and the
purest blood goes to the
head and brain. The
separation of pure and
impure blood in the frog
is not complete, but the
arrangement just de-
scribed is such as to send
the most impure blood
to the organs which pu-
rify it, and the purest
blood to the brain where
the purest blood is
needed. The two auricles are separated from the ventricle by
valves, va, opening mechanically in one direction, in such a way
that when the heart beats the blood is forced onward and
never backward.

The blood passes out through the arteries and is carried by the
numerous branches into the various parts of the body, the small
branches breaking up finally into minute twigs called capillaries,
that are distributed in great abundance in every active organ.

FlG. 102. — A DIAGRAM OF THE HEART OF
THE FROG TO EXPLAIN ITS CIRCULATION

no, aorta, passing to the posterior part of the body;

ba, bulbus arteriosus;

co, carotid artery going to the head;

la, left auricle;

pu, pulmonary artery passing to the lung;

ra, right auricle;

v, ventricle;

va, valves separating auricles and ventricle. The

arrows show the passage from the auricles to the

ventricle.

(Modified from Parker and Haswell.)

208 BIOLOGY

While passing through these capillaries, the food materials,
absorbed by the blood from the alimentary canal, and the gas
absorbed from the lungs, pass from the blood into the tissues
where they are needed. In this way the food and oxygen are
supplied to the active tissues of the body. At the same time
waste products, which have been produced in the active tissues,
are returned to the blood, so that the blood, after passing through
the capillaries, goes back to the heart as impure blood. After
reaching the heart the impure blood goes to the lungs, where
part of its impurities are passed off into the air.

Lymph System. — A part of the circulatory system is called the
lymph system. As the blood is flowing in the capillaries some
of the liquid plasma soaks through the walls of the capillaries
out into the tissues. When it reaches the tissues it is no longer
called blood but lymph, and is a colorless clear liquid which
bathes every living cell. This lymph contains, dissolved in it,
the nutriment absorbed from the intestine; and, since it now
actually bathes the living cells, these can take from it directly
the nourishment they need for their activities. Into this lymph
the living cells also excrete all the waste products that have
resulted from their life processes, the lymph receiving all the
wastes of the body. The gases, which comprise part of this
waste, pass at once into the blood by diffusion; but the other
materials remain dissolved in the lymph and finally reach the
blood by the following course: The lymph gradually collects
in tiny spaces, lacunae, scattered over the body, and from these
flows into little vessels connecting with each other, called lymph
vessels. These small vessels unite together to form larger ones
and the larger vessels finally empty into the veins. The vessels
around the front end of the body converge to two minute sacs
lying deeply imbedded near the third vertebra; and the vessels
in the hind part of the body converge into similar sacs situated
over the hips, near the lower end of the urostyle. These four
sacs have muscular walls and pulsate, and are called lymph
hearts. When they beat they force the lymph into the veins

THE PHYSIOLOGY OF AN ANIMAL 209

which lie near them and with which they are connected. In this
way the lymph, which originally came from the blood plasma
by dialyzing through the walls of the capillaries, returns into the
blood; thus all the secreted products from the living cells pass
into the blood, either directly as in the case of gases, or indirectly
by passing first into the lymph and then emptying with the
lymph into the blood vessels.

NOTE. — A similar lymphatic system is found in all higher animals, but
its course is different from that in the frog. In man, for example, the
lymph rises by diffusion through the capillaries, and collects in lacunae
and lymph vessels in a similar manner. But there are no lymph hearts.
The lymph vessels unite to form quite large vessels, and all eventually
empty into the large veins in the neck. There are two chief trunks of these
vessels, one bringing the lymph from the upper parts of the body and
emptying into the right jugular vein, and the other, a much larger one,
bringing the lymph from the lower parts of the body and from the alimen-
tary canal and flowing up through the thorax, to empty finally in the left
jugular vein. This latter lymph vessel is called the thoracic duct.

Respiration. — The impure blood from the heart passes through
the pulmonary artery to the lungs (Fig. 92), a part of it going
into a small branch, the cutaneous, cu, which carries it to the
skin. The lungs are air sacs connected with the mouth. Just
back of the tongue we have already noticed the glottis, which
is a slit leading into a small cavity holding the vocal cords,
whose vibrations cause the various sounds produced by the
animal. This cavity is the larynx and it lies just under the
throat. At its inner end it opens at once into the lungs, since
the frog has no windpipe (trachea) such as is found in animals
with long necks, like man. The air enters the lungs through the
larynx and, filling them, comes in close contact with the blood,
which is distributed in finely divided capillaries in their walls.
The blood that goes to the skin through the cutaneous artery
is distributed in fine capillaries and brought into close con-
tact with the oxygen which is dissolved in the water in which
the animal lives.

The haemoglobin, which gives the red color to the red cor-

210 BIOLOGY

puscles, absorbs large quantities of oxygen as the blood is flow-
ing through the lungs and skin. The oxygenated blood then
passes from the lungs back to the heart and is pumped out
through arteries to the tissues. Here the red blood corpuscles
give up their oxygen, and at the same time the blood absorbs
carbon dioxid (CO2) from the tissues. When the blood, there-
fore, leaves the capillaries on its journey back to the heart, it
has left behind its oxygen and taken in its place carbon dioxid,
which it gives up when it next reaches the lungs or the skin,
at the same time taking up oxygen. The process of respiration
is therefore a system of gas exchange.

Metabolism. — In the living tissues the food and oxygen are
chemically combined, an oxidation of the food taking place.
The chemical changes that occur are numerous and result in
the formation of new materials for the body, producing growth ,
development of muscular activity, and all of the other phenom-
ena of life, and finally resulting in the appearance of waste
products. The waste products are chiefly three : (1) a gas, carbon
dioxid (CO2) ; (2) a liquid, water (H20) ; (3) a solid, called urea
(CON2H4), which contains the nitrogen. Although the urea
is solid under all ordinary conditions, it is dissolved in the liq-
uids of the body, since it is soluble in water, and is therefore in a
state of solution while in the body. These three waste products
are not only valueless but distinctly harmful, and it is necessary
for the body to get rid of them. The series of chemical changes
which finally results in waste products is called metabolism.

Excretions. — The elimination of the waste products of me-
tabolism is known as excretion. The carbon dioxid gas passes
into the blood, and when the blood reaches the lungs the gas
diffuses from the blood into the air. The waste water also passes
into the blood and is passed off from the body through the kid-
neys, the lungs, and the skin. The urea finds its way into the
blood, and as the blood flows through the kidneys (Fig. 92),
they take the urea from it. They then pass it through their
ducts dissolved in the urine, and it goes to the bladder and

THE PHYSIOLOGY OF AN ANIMAL 211

then passes to the exterior with the faeces, the one cloacal
opening serving, in the frog, for the exit of undigested food
as well as for the urine.

Support. — The skeleton serves to support the softer part of
the body.

Motion. — The motion of the frog is accomplished by the
muscles. The muscles are numerous, and each has its own
special attachment to the bones; Fig. 89. Every muscle pos-
sesses the power of shortening, but has no other function;
and the ordinary muscles are attached to two bones in such a
way that when the muscle shortens one bone is moved upon
another. All the motions of the body are produced by the short-
ening of the different muscles. Many of the muscles are in
pairs, one of each pair serving to bend a joint, the flexor, and
the other straightening it, the extensor. The details of their
actions we cannot consider here, but it will readily be seen that
with the many muscles present in the frog's body a great variety
of motions can be produced. The selection of the proper mus-
cles to produce any desired motion is a complicated process,
some motions indeed requiring the orderly selection of a large
number of muscles, which must act together in perfect harmony.
This power of selecting the muscles and causing them to act in
unison and in harmony with each other is called coordination.

The Coordinating System. — The nervous system of the frog
controls all other functions. As already seen, it consists of (1) a
central system, the brain and spinal cord; (2) the peripheral
system, the latter composed of the nerves distributed over the
body, and the various end organs of the nerves. The central
system is really the center of activity, and the nerve fibers are
merely paths for conducting impulses from one part of the body
to another. Some of the end organs at the outer ends of the
nerves receive impulses from the brain; others receive them from
the exterior and transmit them to the nerves to be carried to the
brain. The brain corresponds to the central station of a telephone
system, which is connected with all parts of the city by wires hav-

212 BIOLOGY

ing at their ends the individual telephones which may receive com-
munications from the central system or send messages to it.
So the central nervous system contains the intelligent, originat-
ing force, and being in communication with every part of the
body, controls all of the functions in such a way that they act in
harmony. This central system has a series of efferent nerves,
by which it sends messages outward, and a series of afferent
nerves, by which messages are brought inward to the brain.
The most important of the latter are the sensory nerves.

Sense organs. — Each sensory nerve ends in a sense organ, so
formed that it is excited by definite external stimuli. One of
them, the ear, is stimulated by vibrations of the air; another,
the eye, by vibrations of ether; others by a slight pressure or
touch; others by heat; others again, by chemical substances,
producing taste; and others by vapors in the form of gases,
causing the sense of smell. Figure 96 shows the microscopic
structure of some of these sensory end organs. In each case
the end organ is started into activity by an external stimulus,
and when thus excited an impulse starts from it over the nerve
fiber and passes to the central part of the nervous system. In
the central system, the stimulus produces what we call a sensa-
tion, and this gives the brain a knowledge of what is going on
at the outer end of the nerve. Sensation never occurs until the
impulse reaches the brain. From these sensations the brain
obtains information as to what is going on in different parts of
the body, and upon this information, bases its knowledge and
regulates the activities of the body.

Reflexes. — The nervous system is made up of a mass of
neurons whose connections with each other are inconceivably
complex. These neurons, with their long axons, unite in har-
monious activity the different organs of the body, and they do
this by virtue of the fact that their axons, though distributed
all over the body, all converge in the central system, where they
can be associated together by the numerous neuron bodies that
compose these central ganglia; Fig. 85. The courses taken by

THE PHYSIOLOGY OF AN ANIMAL

213

these impulses after reaching the centers are complex in the
extreme, and quite beyond our power to follow. They are ac-
companied by sensations and by whatever of consciousness
the animal possesses, and they *

control the life and motions of
the animal. The simplest of
these connections may produce
motion without any conscious-
ness on the part of the animal.
This is shown diagrammat-
ically in Figure 103. Some ex-
ternal stimulus excites one of
the sense organs in the skin, s,
and starts an impulse in the
nerve fiber, which then travels
quickly through the axon to its
inner end, c, in the cord. The
impulse then passes out of the
axon through the arborizations,
at c, to the neighboring den-
drites of other neurons. These
neurons may be motor cells, m,
by which is meant that their

axons, e, extend outward and

FlG. 103. A DIAGRAM ILLUSTRAT-
ING A REFLEX ACTION

terminate in muscle fibers, as

at mu. Hence the impulse that

enters them, after passing out

over the axon, eventually

reaches a muscle fiber, and

causes the muscle to contract.

Such an action may take place

without the impulse going to the brain, and would therefore

not involve any consciousness or any sensation, for these

latter functions occur in the brain only. Hence the animal

might move if touched by an irritating object, without any

An impulse that starts from the sense
organs in the skin, s, passes to the spinal
cord through the afferent nerve, a. Upon
reaching the center, at c, the impulse may
pass over to the motor cell, TO, from whence
it passes downward through the efferent
nerve, e, to the muscle fibers, mu. Part
of the impulse from the c may pass up
through the fiber, a, to the brain and pro-
duce sensation.

214 BIOLOGY

necessary consciousness on its part, as actually happens in
sleep, for example. Such an action is called a reflex act, a
name derived from the idea formerly held that the impulse
starting from the sense organ was simply reflected back after
reaching the cord. Although we know to-day that the impulse
is not simply reflected back, but is profoundly modified in the
cord, the name reflex is still retained for this type of reaction.

Although a reflex act is not necessarily accompanied by con-
sciousness or sensation, this is not always the case. From the
diagram (Fig. 103), it is evident that the impulse, on its arrival
in the cord, may not all pass into the motor nerve cell, but some
of it may pass up through the fiber, a, toward the brain, and this
part of the impulse, when it reaches the brain, will give rise
to a sensation. The action that follows might still be the
reflex, or it might be a truly voluntary one, started by the brain
as the result of the sensation. Reflexes play a very large part
in the life of all animals. Even in our own life many of our
actions are thus reflexly performed without any special volition.

Reproduction. — The eggs of the frog are only developed at
certain seasons of the year. Late in the spring and early in the
summer the ovaries are small, but toward the end of summer
and in the fall the eggs begin to develop and cause the ovaries
to expand until they almost fill the body cavity. When the
frog goes into the dormant condition of hibernation (Lat.
hibernare = to pass the winter), the female is usually greatly
distended with the swollen ovary, and in this condition the
winter period is passed. The oviducts have also enlarged and
elongated, and remain so during the winter, while the animal
is buried under ground. With the opening of the spring the frog
emerges and resumes its active life, and in a few weeks reaches
what is called the breeding season, which means the season
for the discharge of the sexual products. As this season ap-
proaches the eggs break out of the ovary and fall into the
abdominal cavity. The funnel-shaped opening of each oviduct
is provided with vibratile cilia (Fig. 100), and, probably by

THE PHYSIOLOGY OF AN ANIMAL 215

their action, the eggs are swept into the opening, and then
slowly pass down through the coils of the oviduct toward the
uterus. As they pass along they are covered with a gelati-
nous substance, which is secreted from the glands in the
walls of the ducts and forms a layer around the eggs. When
the eggs reach the uterus they are stored there for a time until
the animal is ready to lay her eggs.

With the approach of the breeding season the spermaries of
the male also become very active and secrete sperm fluid. This
passes down the ducts to be stored in the seminal vesicles, where
it remains until the period of copulation.

At the breeding season the male frog fastens himself to the
female, who is about to lay her eggs, and remains firmly attached
to her until she lays them, remaining thus attached for days or
even for weeks in some cases (copulation). After the eggs are
laid the male leaves the female and pays no further attention
to her. When the eggs are laid they are rather slowly passed
from the body by the cloacal opening, and at the same time
the male ejects the sperm fluid from his body over them. The
sperms themselves penetrate the jelly and eventually enter the
eggs, producing fertilization. After the eggs are thus laid the
ovaries and the oviducts contract and in a short time shrivel to
a size much smaller than that which they had at the 'breeding
season. This diminished size continues until late in the summer,
when the ovaries begin to increase in size again with the growth
of the ova, in preparation for the next breeding season.

The eggs of the common frog are always laid in water and
at first form a rather small mass of eggs with their surrounding
j elly . But the j elly quickly absorbs the water and swells to many
times its original size, inclosing each egg in a thick layer. This
jelly appears to have two purposes. It is a protection to the
eggs from the attack of birds and perhaps other enemies. It
seems also to have the power of absorbing the sun's rays and
holding them back from too great radiation, the result being that
the egg is kept warmer than it would be without the jelly. This

216 BIOLOGY

hastens the development, since its rate is dependent on tem-
perature. Our common frog lays its eggs in irregular masses,
which may be found in abundance in the spring months around
pools of fresh water. The toad has somewhat similar breeding
habits, but lays its eggs in long strings. Inside the jelly the
fertilization of the eggs is completed and the development
begins, and here the young remain until they are ready to hatch
as young larvae.

PHYSIOLOGY OF THE EARTHWORM

The organs of the earthworm are much simpler than those of
the frog. Some of the systems of organs found in the frog are
apparently absent in the earthworm. There are, for example,
no lungs nor other special organs devoted to respiration; there
is neither heart nor system of bones for support. But although
some of these systems of organs appear to be absent, their
functions are not lacking. In other words, the earthworm has
exactly the same functions of life as the frog, but carries them
out in a simpler way. Respiration is carried on through the
skin; the motions of the animals are confined to a writhing
motion made by the muscles of the body wall ; the circulation
of the blood is produced by the contraction of the blood vessels
instead of by a heart; excretions are carried on through the skin
and also by the nephridia. In short, the earthworm has the
same general functions as the frog, only they are carried out on
a simpler scale and by a simpler series of organs. Since its
organs are simpler, we speak of the earthworm as having a
lower organization than the frog.

CHAPTER XI

THE DIFFERENCES BETWEEN ANIMALS AND PLANTS:
THE MUTUAL RELATIONS OF ORGANISMS

THE DIFFERENCES BETWEEN ANIMALS AND PLANTS

IF we confine our attention to the larger organisms, the
differences between plants and animals are very evident; but
when we turn our attention to some of the lower members of
each group, the differences are less evident and most of them
disappear. A castor bean and a frog are very unlike, but
Peranema and Euglena (Fig. 29) are so similar that it is
hardly possible to say whether either of them is an animal
or a plant.

In their life functions, too, the higher plants and animals
differ widely. Most of the general functions of animal life
are possessed in a modified form by plants also; but since
some functions are possessed by animals alone, a division of
functions into two categories is frequently adopted.

Vegetative functions are those possessed by both animals
and plants. They are chiefly associated with food and growth,
and are: alimentation, circulation, respiration, excretion, and
reproduction.

Animal functions are those possessed by animals and not
by plants. They are motion and coordination.

Both animals and plants have vegetative functions, but they
are carried on quite differently in the two groups, resulting in
a radically different type of life in animals and plants. The
study already made of the biology of organisms enables
us now to ask intelligently, What is the difference be-
tween animals and plants? Although it is fairly easy to see
the difference between a tree and a dog, when we come to
extend the comparison to smaller and lower organisms it
becomes more and more difficult to determine any distinc-

217

218 BIOLOGY

tions between the two kingdoms. Indeed, when we analyze
the subject to its limit, we find it impossible to draw any sharp
line separating animals and plants, for there are some living
things which show so few characteristics of either kingdom
that we cannot determine with accuracy whether they belong
to one group or the other. It is possible, however, to draw a
general distinction between the two, and from this general
distinction we can derive certain other secondary differences,
which are more evident.

The Fundamental Distinction. — The primary distinction be-
tween animals and plants is in the process of photosynthesis.
The plant kingdom alone has the power of utilizing the rays
of the sun and manufacturing starch out of carbon dioxid and
water: animals never have this power. From this primary
distinction arise several other minor points of difference, more
or less sharply separating these two groups.

Secondary Differences. — A . Color. — Plants which have the
power of photosynthesis are provided with the green coloring
matter, chlorophyll. Animals, on the other hand, are not pro-
vided with this coloring matter.

B. Motion. — Since animals live upon solid foods, they have
to search for it, and they are, as a rule, provided with motion.
Plants, on the other hand, having no need to search for their
food, since they find it in the atmosphere and soil, have not,
as a rule, developed the power of motion.

The various methods of motion developed by animals may
be summarized as follows: (1) Amoeboid movement, as found
in Amoeba, by means of lobes of the living protoplasm. It is
confined to unicellular organisms. (2) Ciliated and flagellated
motion, produced by vibratile, hairlike processes of the proto-
plasm. Cilia are moderately short processes, and where found
are usually present in large numbers. They are found in
many unicellular animals and also in multicellular forms.
Even the highest animals have cilia on the cells lining the air
passages and various other ducts. Flagella are longer than

DIFFERENCES BETWEEN ANIMALS AND PLANTS 219

cilia, and occur only in small numbers on any cell, one or two
being the usual number. Higher animals do not have true
flagella, except in their sperms; see page 250. (3) Muscular
Movements. — In all animals above the unicellular forms cer-
tain cells, or parts of cells, become specially modified for con-
traction, thus becoming muscles. These develop into a system
which produces the many types of locomotion possessed by
animals.

While plants as a rule are stationary, a few of them possess
independent motion. Spores of many plants possess flagella
or cilia; some of the lowest show amoeboid motion, and some
have methods of motion not yet understood, like Diatoms and
Oscillaria; Fig. 68. Among higher plants movements of different
parts of the leaves, stamens, etc., are not uncommon. No
muscles are developed, however, in plants, the motions being
due to slow changes in the protoplasm, which are not well
understood. An independent locomotion is unknown among
any plants except those of the lowest orders.

C. Sensitiveness. — In order to distinguish their food, ani-
mals have developed sensitiveness and sensations. Plants not
needing to distinguish food so accurately have not developed
much sensitiveness.

D. Structure. — As a rule animals have their bodies con-
densed into a small compass, and are provided with an opening
for taking in food, — the mouth, — which is connected with a
digestive system. Typical plants, since they feed upon gases
and water, which are distributed everywhere, have their bodies
widely expanded into branches, leaves, and root hairs, in order
to come in contact with a large amount of air and soil. They
never have any mouths, since they do not take solid food,
and consequently have no digestive system.

The Income and Outgo of Animals and Plants. — An animal
has an income as follows : —

Proteids, obtained from animal or vegetable food, but all
originally derived from green plants.

220 BIOLOGY

Hydrocarbons (fats), derived from both animal and vege-
table food.

Carbohydrates, derived chiefly from vegetable foods.

Water.

Oxygen, taken from the air by the respiratory organs into
the blood.

Salts, of various kinds in the foods.

The outgo of an animal consists of: —
Carbon dioxid, excreted from the respiratory organs.
Water, excreted from the skin, kidneys, and some other
organs.

Urea, excreted by the kidneys or their equivalents.
Proteids, eliminated in the reproductive bodies.
Salts, in various excretions.

After an animal has reached its full growth, the income and
the outgo practically balance. With some animals this period
of equilibrium lasts a long time, perhaps for years. With
others, growth may continue until death comes, in which case
there is never any period of actual balance.

The income of a plant consists of: —
Carbon dioxid, from the air.
Water, from the soil.
Minerals, from the soil.

The outgo of a plant consists of: —
Oxygen, from the leaves.
Water, from the leaves.

Carbon dioxid, from the leaves and other parts.
Proteids and various other substances, eliminated with dead
leaves, branches, seeds, and other reproductive bodies.

No Sharp Distinction between Animals and Plants. — The
criteria above given are ordinarily sufficient to distinguish
between animals and plants, and will separate typical forms;

DIFFERENCES BETWEEN ANIMALS AND PLANTS 221

but when we come to consider low types, some or all of these
distinctions disappear. There are, for example, many plants
which have no chlorophyll (molds, toadstools, etc.), and hence
have no power of photosynthesis; but they are, nevertheless,
clearly plants, for no one would for an instant think of con-
fusing them with animals, even though they do not contain
chlorophyll. Some plants have independent motion, while
some animals are stationary. Some plants are sensitive. The
distinction of shape applies only to the higher organisms; for
among the microscopic forms no distinction can be seen be-
tween the shape of animals and plants, some animals having
no mouth, and some plants, as well as animals, having their
bodies condensed rather than expanded. Thus it appears
that each of the distinguishing characters separating animals
and plants breaks down when we come to apply it closely to
some of the low forms of life ; until we have to admit that there
is no absolute criterion separating the two kingdoms.

Nevertheless, there is rarely any real difficulty in making
the distinction. It is true that there is a difference of opinion
as to whether a few of the very low forms should be called
animals or plants; but when we take all of the above facts into
consideration, it is only in a few instances that we are unable
to say positively that any given organism is either animal or
plant. Most of the difficulty is confined to the microscopic
forms which are among the lowest organisms, and the fact that
among these there is no absolutely fixed line between the two
kingdoms is of special significance as suggesting the origin of
the two kingdoms from a common starting point by a process
of evolution.

Organisms which possess chlorophyll, and consequently nour-
ish themselves by photosynthesis, are sometimes said to be holo-
phytic (Gr. holos = whole + phyton = plant) . In contradistinc-
tion, organisms which have no chlorophyll and must depend
upon others for sustenance are called holozoic (Gr. zoon = ani-
mal). Animals are practically all holozoic, and green plants

222 BIOLOGY

holophytic; but many plants are holozoic, a condition which is
true of all Fungi.

Protozoa and Protophyta. — Both plants and animals may
be found among unicellular organisms, the unicellular animals
being known as PROTOZOA (Gr. protos = first + zoon = ani-
mal), and unicellular plants as PROTOPHYTA (Gr. phyton =
plant) ; see Chapter III. Among such organisms there is some-
times a difficulty in distinguishing between animals and plants,
since any structure of a distinctive character is lacking. Even
here, however, the majority of forms group themselves in
one of the two kingdoms, so that they can readily be separated.
There are, however, a few forms which prove a puzzle. Euglena
(Fig. 29 B)y for example, has green chlorophyll, and is thus allied
to the vegetable kingdom (holophytic); but it has also the power
of motion, a mouth, and a red eye-spot. Peranema (Fig. 29 A)t
however, which is clearly allied to Euglena, has no chlorophyll
and no plant characters (holozoic). We may, with equal justice,
call both animals or both plants, or perhaps one an animal and
one a plant. The bacteria (Figs. 33-35) represent another group
which has been difficult to classify clearly ; and for many years
after they were first studied there was a considerable difference of
opinion as to where they belonged. They have a method of
life much like that of animals, but their general structure,
their multiplication, their division to form long chains, and an
occasional formation of spores, are points much more like
plants, especially the Fungi. Continued study of the organisms
has finally led to the conclusion that bacteria must be regarded
as plants rather than animals, associated with the group of
Fungi, and considered as resembling yeast and molds.

A few such organisms as these are the only ones that present
much difficulty in distinguishing between animals and plants,
and even these can be called animals or plants with a consid-
erable degree of certainty. While no sharp line can be drawn,
the difficulty of separating them is really not very great, and
even among unicellular forms it is rare that we cannot deter-

DIFFERENCES BETWEEN ANIMALS AND PLANTS 223

mine satisfactorily whether to call an organism an animal or
a plant.

Metazoa and Metaphyta. — With the multiplication of cells
and their differentiation we find that the distinction between
animal and plant at
once becomes marked,
until there is no longer
any similarity between
them. Indeed, in all
organisms above the
Protozoa and the
Protophyta, the two
kingdoms are sharply
separated, all multi-
cellular organisms be-
ing either so distinctly
like plants or like ani-
mals that the difficulty
of distinguishing them
disappears entirely.
From this point up,
plants usually not only
possess chlorophyll,
but also show a gen-
eral structure which
indicates that they
are adapted for the
absorption of gases
from the air, of water
from the soil, and for

the purpose of carrying on the process of photosynthesis; while
animals have a structure of body adapted for taking only solid
or liquid food. The difference in the shape of the animal
and plant body becomes so well marked that there is no longer
any confusion between them. Even though we find large

FIG. 104. — DROSERA, A CARNIVOROUS PLANT

Small insects are captured by the hairs on the leaves,
digested, and, in a measure, assimilated by the plant.

224 BIOLOGY

groups of plants that have quite lost their chloropnyll (toad-
stools, molds, etc.), there is no longer any difficulty in deter-
mining that they are to be grouped with plants rather than
with animals, in spite of their not having any green color-
ing matter. When, too, we find a plant like the sundew (Fig.
104), which captures insects by means of the hairs on its leaves,
and digests and assimilates them, we call it a "carnivorous
plant" (Lat. caro (carnis) = flesh + vorare = to eat), but do not
confound it with animals. The Metazoa (Gr. meta = after -f
zoon = animal) and Metaphyta (Gr. phyton = plant) are sharply
distinct.

CONTRAST BETWEEN THE ACTIVITIES OF ANIMALS AND

PLANTS

The similarities and differences between animals and plants
may be better understood if their properties are contrasted
with each other in regular order. The following contrasts
illustrate the distinction between these two groups : —

1. Alimentation. — In animals this system consists of a
mouth, stomach, intestine, and digestive glands; food is taken
into the body either as a solid or a liquid. In plants the system
is poorly developed, consisting of root hairs for taking in liq-
uids, and stomata for absorbing gases, but having no digestive
organs. The foods absorbed are either liquids or gases, but
never solids.

2. Circulation. — In animals circulation is brought about by
a heart and blood vessels, or something corresponding to them.
In plants the water absorbed from the roots ascends the stem,
and passes out into the leaves by a process known as the ascent
of sap, and the materials formed in the leaves are dissolved
and eventually diffused throughout the plant, passing down-
ward in certain of the cells of the stem. There are no real
blood vessels, no heart, no blood, and no definite circulation.

3. Metabolism. — In animals metabolism is essentially destruc-
tive. The animal uses as food organic compounds like carbo-

DIFFERENCES BETWEEN ANIMALS AND PLANTS 225

hydrates, fats, and proteids; combines them with oxygen, and,
as a result, produces as waste products carbon dioxid, water,
and urea. The foods are broken to pieces, and the energy
thus liberated is utilized; see Chapter XV. In plants the proc-
ess is primarily constructive, but there is in plant life both
a constructive and a destructive metabolism. By the former
the plant uses carbon dioxid, water, and nitrates, which
are combined in the plant to form organic substances, like
starches, proteids, etc., and in the combination solar energy
is stored away. As an excretion, there are produced oxygen
and water. The destructive process of plants is essentially like
that of animals: the compounds built up by the first process
are destroyed by the second. The total amount of construction
in green plants is greater than the amount of destruction,
and therefore the green plants store away organic products
which may subsequently be utilized by plant life.

4. Respiration. — Animals usually have lungs or gills filled
with blood ; they always absorb oxygen, and eliminate the car-
bon dioxid. In plants the respiration is carried on through the
stomata of the leaves; when carrying on photosynthesis, plants
absorb carbon dioxid and eliminate oxygen; when not carry-
ing on photosynthesis, the gas absorbed is oxygen and the gas
liberated is carbon dioxid.

5. Excretion. — In animals carbon dioxid is excreted from
the lungs, water from the skin and kidneys, and urea from
the kidneys. In plants there is no well-developed excretory
system, although gases are excreted through the stomata,
and certain other substances may pass out through the bark
or through the roots into the soil.

6. Motion. — The muscles of animals develop a high degree
of motion. In plants motion is very rarely developed, although
it is not wholly lacking, some plants being well supplied with
motile power. They do not, however, have muscles; and when
they have motion, they use other forms of mechanism.

7. Support. — Animals usually have a skeleton of shell or

236 BIOLOGY

bone, either internal or external. In plants the supporting
structure is, as a rule, developed better than in animals, and
consists of the great mass of wood or other resisting material
found throughout the plant.

8. Coordination. — All animals, except the unicellular forms,
have a nervous system, usually centering in the brain, which
brings into coordination the various functions of life. In plants
there is no coordinating system and practically no coordination
of the different parts. Each part of the plant may live its life
to a considerable degree independently of the others.

9. Reproduction. — The reproductive processes of animals and
plants are very similar. Both produce eggs and sperms, and
have a sexual reproduction; and in both there may be repro-
duction by an asexual method, although in animals the asexual
reproduction is less common than in plants. In the higher
animals the power of asexual reproduction is lost, while in
even the highest plants the process of asexual reproduction has
commonly been retained. In the higher members of both
groups, sexual reproduction by eggs and sperms is universal.

THE MUTUAL RELATIONS OF ORGANISMS

The close relation of organisms to each other is evident,
since all animals, as well as all colorless plants, are dependent
upon green plants for their food. They vary greatly, however,
in their methods of obtaining their food.

Food Habits

Plants may be divided into three groups, according to their
methods of obtaining their food : —

1. Autophytes (Gr. autos — self + phyton = plant). — Plants
which are not dependent upon organic foods, but are able to
take care of themselves by subsisting upon the minerals from
the soil, together with the gases from the air, are called auto-
phytes. These include the green plants (holophytic) only; and
strictly speaking, even these plants are in a measure dependent

THE MUTUAL RELATIONS OF ORGANISMS 227

upon others. The minerals that they absorb from the soil
are available for plant life only after the bacteria and other
soil organisms have acted upon them, the fertility of the soil
depending upon its microscopic life. The autophytes, however,
do not need organic food, and in this respect are much more
independent than the other two groups.

2. Saprophytes (Gr. sapros = rotten + phyton = plant). —
Plants which feed upon the dead bodies of other organisms are
saprophytes. The plants usually included under this head
are the Fungi* These constitute the scavengers of the world,
and may be found everywhere in the soil or in bodies of water,
where they consume whatever excretions of animals or plants
there may be; or live upon dead roots, leaves, and branches;
they live, indeed, upon various dead materials that have been
derived either from animal or plant life. Such organisms are
almost universally distributed over the earth, and they cause
all decay and putrefaction, these two processes being the result
of the destruction of organic material by Fungi. This class
of organisms is ever at work around us, consuming the bodies
of dead animals and plants.

3. Parasites. — Plants which live upon and feed upon other
living organisms are parasites. In such cases we call the
organism upon which they feed the host. Parasitism is very
common among both plants and animals, nearly every species
having special parasites feeding upon it. As a rule, the para-
sitic plants lack chlorophyll and belong to the group of Fungi.
Both saprophytes and parasites are holozoic.

Animals have similar relations, although in some respects
they are more complicated. No animals live a life quite inde-
pendent of organic food, like the autophytes, since they
lack chlorophyll. The great majority of animals are called
free-living, but they feed upon dead organic material (vegetable
or animal food), and in this respect resemble saprophytes.
Quite a large number of animals also feed upon a living host,
and are consequently parasites.

"Under Fungi are included bacteria.

228

BIOLOGY

Symbiosis

Among both animals and plants, however, we not infre-
quently find different individuals associated and living in
mutual relations which may or may not be those of parasite
and host. The term symbiosis (Gr. sun = with + bios = life),
which may refer to either animals or plants (literally meaning
living together), is applied to a variety of relations where two
organisms live in close relation to each other, and is in con-
trast to free-living conditions where organisms live separately
from others. The purpose of symbiosis is not always the same.
Sometimes it is to the mutual advantage of both members;
sometimes it is to the advantage of one and the detriment of
the other, in which case it becomes parasitism. In accordance
with the relation of the two members of the group, svmbiosis
may be divided into several types as follows : —

Helotism. — In helotism (Gr. Helot = a slave) one organism
is enslaved by the other; neither is especially injured, but

one does the work for the
other. Among animals the
best example of this is
among the slave-making
ants, where one species of
ants makes a slave of an-
other species, the slave do-
ing all the work for the
slave-maker. Both indi-
viduals carry on their life
in satisfactory fashion, and
neither is particularly in-
jured by the relation.

Mutualism. — When both
members of a group obtain
an advantage from associa-
tion, it is called mutualism. As examples of this, may be
mentioned the relation between domesticated animals, like

FIG. 105. — AN EXAMPLE OP
MUTUALISM

A hermit crab cr, lives in the shell of the
snail, sn, and an anemone, an, fastens itself to
the outside of the shell. Both animals are bene-
fited.

THE MUTUAL RELATIONS OF ORGANISMS

229

dogs, and the human race. Among lower animals the associa-
tion of a hermit crab with a sea anemone is an illustration;
Fig. 105. Here the anemone gains an advantage from being
carried to and fro, while the
hermit crab is protected by
the nematocysts, which, as
in Hydra (page 144), are
abundant on the tentacles
of the anemone, and which
by their poison protect the
crab from the attack of
enemies.

Mutualism is rather more
common among plants than
animals. An example is the
common gray mosses (Li-
chens) that grow on rocks FIG. 106.— CLADO NIA\ A COMMON LICHEN,
or tree trunks; Fig. 106. GROWING ON ROCKS

The microscope shows that , At B is show,n th.e y°un?. mycelium, beginning

to grow around a single cell of the green alga.

this plant is a combination

of a fungus and a green plant; Fig. 107. In this association

the green plant carries on
photosynthesis, furnish-
ing starch for both itself
and the fungus; on the
other hand, the fungus
furnishes, for the benefit
of the green plant, a
lodging place and a con-

siderable quantity 01
carbon dioxid and water,

FlG. 107. A MAGNIFIED SECTION OF

A LICHEN

Showing that it is made up of a fungus, m, and which it Collects from
cells of a spherical, green plant, a.

tion, ^therefore, seems to be one of mutual advantage. An-
other example is a group of bacteria which grows within

230 BIOLOGY

the little nodules on the roots of plants like peas and beans.
If the roots of peas, beans, clover, or similar plants, be care-
fully removed from the soil, they will usually be found covered
with little nodules ranging in size from the head of a pin to a
large pea. These are found to be produced by bacteria which
enter the roots and grow and multiply in their tissues. But
the association is mutually advantageous. The bacteria are
useful in collecting nitrogen from the air which the pea utilizes
for its own benefit; and, on the other hand, the bacteria get
the benefit of a lodging place and nourishment in the roots
of the tubercle, and therefore are themselves benefited by the
association.

Commensalism. — In commensalism (Lat. cum = with +
mensa = table) the two organisms live together without notice-
able advantage or disadvantage to either. As an example,
may be mentioned the small crab that lives in the oyster shell,
doing no injury to the oyster and gaining no special advantage.
Various vines which cling to trees offer another example.
Some of these vines force their rootlets into the tissues of the
tree and do it injury; these are true parasites. But other
vines simply use the tree for the support of their weak, climb-
ing stem, and neither plant is particularly benefited or injured
by the other, except that the vine is enabled by its climbing
habit to send its leaves up into the sunlight.

Parasitism. — In parasitism the mutual relationship is such
that one individual is benefited at the expense of the other.
The host is always injured, while the parasite is benefited.
Among parasites we recognize two types.

Ectoparasites. — Parasites that live upon the outside of their
host are ectoparasites. As a rule, they are not very harmful,
though they may be so. Among them are some in which a
parasitic life is only a part of their existence. The mosquitoes
live chiefly upon various juices, but occasionally suck the blood
of human beings. In a second class, like the bedbug, the animals
live wholly upon the nutrition from their host, but do not

THE MUTUAL RELATIONS OF ORGANISMS

231

attach themselves to the host permanently. A third type, like
the lice, lives wholly upon its host and has no life apart from it.
While these ectoparasites may be trouble-
some, they are not especially injurious,
except when they transmit disease germs.
Endoparasites. — Parasites that live
within the body of the host are endo-
parasites. They are numerous and pro-
duce far more mischief than ectoparasites.
Among them are those that produce vari-
ous deadly diseases like trichinosis (Fig.
108), tuberculosis, diphtheria, etc.

The Effect of Parasitism

Parasitism occurs among both animals
and plants. The number of species of
parasites is very great, but cannot be
estimated. Nearly all species of animals
and plants have their own parasites, and
some have several species of parasites
infesting them. For this reason it is
sometimes stated that there are at least
as many species of parasites as there
are species of non-parasitic organisms.
The effect of the parasitism upon both
host and parasite is profound, but natu-
rally quite different.

Upon the Host. — The parasite usually
injures the host and is then spoken of as
pathogenic (Gr. pathos = disease + -geneia
= producing). The amount of injury varies widely. In some
cases, the parasite produces disease and even the death of
the host. Trichina is a parasitic worm (Fig. 108), which occa-
sionally causes trichinosis in man, resulting sometimes in death.
Certain flies occasionally make their way into the skull cavities

'A B

FIG. 108.— TRICHINA

A, a single worm showing
its internal anatomy; B,
worm coiled up in a bit of
muscle of pork. If uncooked
pork containing these worms
is eaten they are set free in
the intestines and a case of
trichinosis results.

232

BIOLOGY

of cattle, producing serious and fatal brain disease. Malarial
organisms (Fig. 25) live as parasites in hiiman blood and pro-
duce malaria. Various parasitic bacteria produce serious dis-
eases in man, as typhoid fever, tuberculosis, diphtheria, etc.
The same is true of plants. The various wilts, rusts, and blights
are serious plant diseases, frequently spreading from plant to
plant, and producing death and destruction of the host. AH
are produced by parasites growing in the plant tissues. Fungi
of various kinds are the cause of the greater number of plant

FIG. 109.— A WILLOW
LEAF ATTACKED BY
"MILDEW" CAUSED
BY A PARASITIC

FUNGUS

FIG. 110.— THE "BITTER ROT"
OF CURRANTS

Produced by the parasitic fungus
(Gleosporium) . Most of the currants
have dropped from the stem and the
rest are rotted.

diseases; Figs. 109 and 110. In other cases, the effect upon
the host is far less serious. Some parasites may live upon a
host without seriously affecting it. For example, a number of
bacteria live in our intestines; they may be called parasitic, since
they dwell within a living host; but instead of being injurious,

THE MUTUAL RELATIONS OF ORGANISMS 233

some of them are beneficial to our life, and therefore are not
true parasites, according to the definition given above. Between
these two extremes are many intermediate grades. As a yule,
parasitism injures the host, and indeed, strictly speaking, para-
sitism is a term that should only be used when one animal or
plant feeds upon another, to the distinct detriment of the latter.
Upon the Parasite. — The effect of parasitism upon the
parasite itself is no less profound than its effect upon the host,
but it is of a totally different nature. The general effect of
parasitism is to cause degradation of the parasite. It is a
general law of living nature that any organs which are not used,
inevitably begin to degenerate. If an animal becomes a parasite
upon another, it shows a general tendency to lose many of
its original characters. For example, the tapeworm has become
parasitic in the intestines of animals. Here it finds its food
already digested by the digestive juices of the host; it has thus
no need of a mouth, of digestive organs, or of any power of
motion; and, in conformity with the above law of nature,
having no need of these functions, it has lost them. The tape-
worm has thus become degraded to a very simple organism,
without digestive organs and with all of its systems of organs
reduced to the lowest possible condition. Thus, parasites,
depending as they do upon their host for their nourishment,
lose their power of independence and become degraded. This
is a biological law of great significance, — the law that failure to
use any function results in its loss, — running through the whole
scale of nature. It is exemplified in the human race in numer-
ous aspects of civilized life, where one class of people depends
upon another. In our highly organized cities this principle of
loss of power as a result of disuse is as well illustrated as it is
among animals, since in the city individuals are so mutually
dependent that each one has practically lost his ability to live
by himself unaided by others. The principle of the loss of
function by disuse is one of the most fundamental and sig-
nificant of the laws of nature.

234 BIOLOGY

NATURE'S LIFE CYCLE

Construction and Destruction. — From a general survey of
the facts which have thus been explained, it will be seen that
there is a grand cycle in nature, in which the life of animals
and plants is concerned. All organisms need food, and the only
explanation of the fact that the food supply has not long since
been exhausted is the fact that the same materials have been
used over and over again, passing from plants to animals and
from animals to plants. The chemical processes going on
in the living world are of two types: those of construction
(synthetical), by which complex substances are built out of
simple ones; and those of destruction (analytical), by which
the complex materials are reduced to simpler ones. Green
plants growing in sunlight manufacture starch out of the
simple ingredients which they extract from the soil and the
air, utilizing sunlight as a source of energy for this purpose.
Though they are building up these materials primarily for their
own life, they build more than they need, so that there is a
large surplus. This surplus is utilized by animals and by the
colorless plants. It is taken into their bodies as food, and serves
them as a source of energy, as well as material out of which
they can manufacture new substances, and grow. Eventually
the material is broken to pieces in the animal body and reduced
once more to a simpler condition. In this way animals utilize
as food a part of the surplus manufactured by green plants,
consuming the surplus of proteids, starches, etc. But other
materials made by the plants, like wood and leaves, do not
so readily serve as food for animals. These materials must
usually be broken down into simpler compounds, or the sub-
stance of which they are made would not get into a condition
where it could again be utilized. This seems to be the special
function of the Fungi.

The Significance of the Fungi in Nature. — Special emphasis
must be given to the significance of the Fungi in these de-
structive processes. In order that nature's processes may

THE MUTUAL RELATIONS OF ORGANISMS 235

continue indefinitely, all kinds of material that have been
built up into organic compounds by the green plant must be
pulled to pieces again so as to be brought back into the simple
condition in which the future generations of plants can utilize
them. While animals use and break down much of the pro-
teids, starches, and fats, there are some substances that ani-
mals cannot utilize, and the Fungi are necessary to reduce
these substances to a simpler condition. Bacteria everywhere
in nature are constantly feeding upon many kinds of organic
substances, but primarily upon those that contain proteids or
other nitrogenous compounds. The yeasts have a special re-
lation to sugar; most of the sugars made by plants, and not
otherwise used, are consumed by yeasts in fermentation and are
thus brought back to the original condition of carbon dioxid and
water. Bacteria and yeasts as well as animals thus feed upon the
same substances. But there is other material of harder nature,
like wood and leaves, which does not serve as food for animals
nor to any great extent for bacteria or yeasts. The molds,
mushrooms, and tree fungi seem to be especially designed by
nature to attack these hard materials and reduce them to a
condition in which they can be destroyed. These larger fungi
consist of a mycelium of delicate, branching threads. If one
of these plants starts to grow on the trunk of a tree, the my-
celium pushes its way through the bark and in among the wood
fibers, and eventually grows through the whole substance of
the tree, the part visible on the outside of the trunk being
only the spore-producing portion that has come to the surface
to distribute the spores to other trees. The mycelium, while
growing within the wood, produces certain substances which
soften the wood and in time disintegrate it, i. e., cause it to
rot. A tree attacked by one of these Fungi in time becomes
soft and so changed in its chemical nature that it can be utilized
as the food of some insect. Eventually the trunk of the tree
is converted largely into a soft, pulpy mass, until finally it is
wholly decomposed. Its carbon and hydrogen unite with

236 BIOLOGY

oxygen, forming CO2 and H2O, which pass off into the air or
sink into the soil, while the other ingredients are incorporated
with the substances of the soil to form food for the next genera-
tion of plants.

The Fungi thus have the extremely important function of
bringing back into a primitive condition much of the material
manufactured by plants which otherwise could not readily be
disposed of. When we consider that bacteria are nature's
agents for decomposing proteids, that the yeasts act in a similar
way upon carbohydrates, and that the larger Fungi attack the
great mass of vegetable material which is otherwise beyond
the reach of animal life, we can see that the group of Fungi is
of immense significance in nature. They form a connecting
link between the products of one generation of plants and the
next. Without their agency, organic material — proteids, fats,
starches, leaves, woods, etc. — would accumulate, and in time
vegetation would cease, because the earth would be covered
with the remains of past generations, which would crowd life
out of existence. The Fungi thus act as scavengers, cleaning up
the surface of the earth and rendering nature's processes con-
tinuous by ever returning to the soil the ingredients upon which
subsequent generations can feed.

The Food Cycle Complete. — Thus, as the result of the action
of the Fungi and of animals, all of the surplus starch and sugar,
all the fat, proteids, wood, and cellulose, and indeed all other
materials which have been built up by the constructive processes
of plants, are eventually broken down, and in the end reach
a condition like that from which they started. Carbon dioxid
and water are produced, as well as nitrates and certain other
mineral salts. The carbon dioxid, being a gas, flies off into the
air to join the store of this gas in the atmosphere; the water
evaporates or sinks into the soil; and the nitrates and other
mineral ingredients also find their way into the soil. These
ingredients, again within reach of plant life, are seized by the
green plants and built up into a new generation of plants to

THE MUTUAL RELATIONS OF ORGANISMS 237

make new starch, sugar, proteids, etc. The ingredients which
feed one generation of plants may, after combination in the
plant body, nourish a generation of animals, eventually return-
ing to the same conditions as those from which they started.
The cycle is thus complete, and there need be no danger of
exhaustion of the food supply as long as it is possible for the
same materials to be used over and over again by green plants,
animals, and fungi

CHAPTER XII

REPRODUCTION: SEXUAL AND ASEXUAL METHODS
GENERAL TYPES OF REPRODUCTION

THE process by which reproduction is brought about is always
fundamentally the same. In spite of all of the numerous
modifications of the method in different animals and plants,
they are all reducible to some form of division; the original
animal or plant divides itself into parts, each of which is ca-
pable of growing into an individual like the one from which it
came.

The numerous varieties of reproduction may be grouped
together under two general types. In one of these the original
organism divides itself directly into two or more parts by
simple division. In the other the division is always compli-
cated by the union of two parts with each other. In the latter
case certain cells of the original organisms unite with each
other, and the union is followed by a rapid division of the cells.
The two types of reproduction are, therefore, (1) Division un-
accompanied by cell union and (2) Division accompanied by
cell union. The type of division in which cell union is found
is often spoken of as sexual reproduction, and the uniting cells
are the sex cells; the type in which the division is not accom-
panied by cell union is called asexual reproduction.

REPRODUCTION IN UNICELLULAR ORGANISMS
Simple Division. — All of the single-celled animals multiply
by the process of simple division; Figs. 19, 23. A careful
study of the internal changes that are going on in the celk
during this reproduction shows that they are essentially
identical with those described on pages 85-89. In other words>
there is a division of the chromatin material in the nucleus,
followed by the formation of two nuclei, which again is fol-
lowed by the division of the cell into two parts. After having

238

SEXUAL AND ASEXUAL REPRODUCTION 239

thus divided and separated from each other, each of the indi-
viduals grows until it is ready to divide, and so the process
goes on repeating itself. In most unicellular plants, the method
of reproduction is essentially the same. Figure 30, for example,
shows the reproduction in Pleurococcus, and Figure 33 in ordi-
nary bacteria. These latter plants are so small that we cannot
determine the internal changes that are going on, but can only
see that the individuals elongate and then divide in the middle,
into two parts. Recent study, however, seems to suggest that
the changes are essentially like those occurring in the Amoeba,
and at all events the process of reproduction is nothing more
than the process of division.

The reproduction of yeast by budding (gemmation) is only
a modification of division; Fig. 32. The internal changes are
essentially like those in the reproduction of the Amoeba or
Paramecium; the first step is the division of the nucleus into
two, one of which passes out of the original cell into the bud,
while the other remains in the original cell. Thus, when the
two cells separate, each has a nucleus that has come from
the original nucleus, and, while the details of the process are
somewhat different, it is as truly a cell division as in the other
examples. Nearly all of the unicellular animals and plants
show one of these two methods of reproduction; see Fig. 111.

Reproduction by Spores. — When the organism breaks up
into many parts, they are called spores. Examples of this
we have already noticed among the unicellular organisms. In
the yeast (Fig. 32 s), spores are formed within the yeast cells
under some conditions; and Figure 25, which shows the life
history of the malarial organism, indicates that one part of
its history, namely, the cycle in the human blood, is an illus-
tration of spore formation. In the malarial Plasmodium the
spore formation which occurs in the human blood alternates
with a second type of spore formation in the body of the mos-
quito. This last process is, however, associated with celi
union, as shown in Figure 25 j. Among unicellular animals

240

BIOLOGY

spore formation is unusual, except in cases where it alternates
with a cell union, as in Plasmodium. Among bacteria there is

a spore formation of a peculiar kind.
Here, as shown in Figure 33 E, each
bacterium produces a single spore only,
instead of several, and the spore forma-
tion is really not a form of multiplica-
tion. The cells formed are, however,
called spores, although their function
seems to be to resist adverse condi-
tions rather than to reproduce the or-
ganism. They have resisting walls and
are capable of developing into new in-
dividuals, thus agreeing with other
spores except in the fact that one only
is produced in a cell.

Reproduction by Cell Union among
Unicellular Organisms. — The process
of cell division among single-celled or-
ganisms may continue for a long time,
producing an indefinite series of off-
spring. Whether in any case this kind of division can really
go on indefinitely we do not positively know. There are some
organisms like yeast and bacteria, in which we have reason
for suspecting that cell division may go on indefinitely if proper
conditions can be maintained, and in which, up to the present,
no trace of any other kind of reproduction has been found.
It is believed by some that even animals like Paramedum,
which conjugate occasionally, may, if proper conditions be
maintained, go on dividing indefinitely. Whether this is true
or not, it is certain that under ordinary conditions cell division
in time becomes slower, and in Paramedum it has a tendency
to come to an end, unless it is reinvigorated in some way.
In nature such an invigoration is brought about by a fusion
of cells with each other as already described; see Fig. 23, page 64.

FIG. 111. — GONIUM. AN
ORGANISM FORMED OP
SIXTEEN CELLS UNITED

BY JELLY

A, view from the side; B,
view from above and showing
the method of reproduction by
division of each cell into sixteen
parts, which separate to form
new colonies.

SEXUAL AND ASEXUAL REPRODUCTION 241

It is probable that in most other unicellular organisms a
similar cell union occurs under some conditions. As already
described, it occurs in the malarial organism in the cycle that
takes place in the mosquito; Fig. 25 j. The cell union that
takes place is a true sex union, since there is a clear dis-
tinction of male and female cells. While such a union of cells
has by no means been found in ail unicellular organisms, it
has been found in many, and we know that it is quite widely
distributed. The studies of recent years particularly have
shown one large group of unicellular organisms called the
Sporozoa, which live as parasites on various hosts, and show
a cell union resembling that of malaria. Another example of
this will be given here in illustration of the phenomenon of cell
union among single-celled animals.

Monocystis. — In the earthworm may be found living a
unicellular parasite known as Monocystis. This animal (see
Fig. 112 A) is a single elongated t cell possessing a nucleus, but
with no other visible organs. The animal has no locomotor
organs, although it does have a slight power of motion. Its
method of reproduction involves a cycle, in which a cell union
alternates with a formation of spores without cell union, but
in a complicated manner. When ready to multiply, two indi-
viduals fuse with each other and become surrounded by a
covering or cyst; Fig. B. Inside of this cyst both of the indi-
viduals divide. First the nucleus divides into many parts
(see Fig. C), and later the rest of the protoplasm divides and
collects around the pieces of the divided nuclei, thus making
many small cells. Now the new cells from one of these indi-
viduals unite in pairs with the cells from the other. This step
occurs within the cyst, but is shown separated from it in Figure
Dj a, by and c, and it constitutes the cell union proper. When
the cells fuse together their nuclei unite, forming a single nu-
cleus, c, called the fusion nucleus, which divides into eight
parts, at e, after which the whole cell divides into eight elon-
gated cells (see /) known as sporozoites. Meantime a hard

242

BIOLOGY

shell is produced around the eight sporozoites and the whole
cluster of eight is called a sporoblast. All of this has occurred
within the original cyst, which has by this time become filled
with a large number of these sporoblasts, each with its eight

FIG. 112. — MONOCYSTIS, SHOWING METHOD OP REPRODUCTION

A, the full-grown animal; B, two individuals enclosed in a cyst; C, the division of the
nucleus into a number of parts, the protoplasm not yet divided; D, successive stages of the
fusion of thfe cells which result from division of the two animals in C; F, the cyst containing
numerous sporoblasts; G, shows the sporoblast breaking open to allow the spores to emerge,
which develop into adult animals. The stages represented in G occur only when the animal
•caches another earthworm.

sporozoites within; see Fig. F. Eventually the cyst breaks
open, allowing the contents to escape. Later these sporcblasts
themselves break open and the individual sporozoites come but
ready to grow into new animals like the original Monocystis;
Fig. G. These latter stages do not occur unless the sporozoites
find their way into another earthworm, where they live as
parasites until ready to multiply again. The sporozoites are

SEXUAL AND ASEXUAL REPRODUCTION 243

evidently spores, but they arise from the division of a mass
resulting from the fusion of two reproductive cells; and to
distinguish them from other spores they are called sporozoites.
By comparing this history with that of the malarian Plasmo-
dium, it will be evident that the spores of the latter, which are
formed in the body of the mosquito, must be sporozoites, since,
like those just described, they arise from the breaking up of
the mass of the two cells which have united by cell union.
Monocystis as here described shows no spores which correspond
to those that appear in the human blood; Fig. 25 g and h.

REPRODUCTION IN MULTICELLULAR ORGANISMS

Multicellular organisms have the same two general types of
reproduction as found in the unicellular; namely, simple divi-
sion, and division accompanied by cell union.

DIVISION WITHOUT CELL UNION

Multiplication by Simple Division. — Simple division among
multicellular organisms is more common among plants than
among animals; and excellent examples of it are familiar to all.
Many of the lower plants, like liverworts, multiply by the for-
mation of buds called gemmae, which break away from the
original, and form new plants. Even among the higher plants
the same general method is found. If one of the branches of
a weeping-willow tree is broken off and stuck into moist ground,
it will take root and grow into a new tree. Indeed, we can
cut the branches of a willow into practically as many pieces
as we wish, and find each one is capable of taking root and
growing into a new tree. The same thing is true of most ordi-
nary plants, for, with a few exceptions, trees and smaller plants
may be reproduced indefinitely by breaking off their branches
and putting them into the proper conditions for taking root.
While a few plants fail to show this power, it is a character
found very commonly in the vegetable kingdom. Many plants
normally multiply upon a similar principle. The strawberry

244 BIOLOGY

plant, for example, sends out branches which grow for some
distance, and then their tips strike root into the ground and
a new plant springs up, united with the old one at first by a
connecting branch; Fig. 113.

Among animals this method of reproduction is not so common
as in plants and is confined to the lower species. One example
has been already described in Hydra; see page 146.

Reproduction by division is evidently closely related to the
power of replacing lost parts. Hydra may be divided into many

FIG. 113. — REPRODUCTION IN A STRAWBERRY PLANT BY DIVISION

pieces, each capable of producing all of its lacking parts; but
this power is retained in diminishing degree as we go from
lower to higher animals. The earthworm does not ordinarily
multiply by simple division, but if it is cut into two pieces by
accident, each will develop the lost parts and two animals
will result. In some worms, related to the earthworm, this
method of multiplication by division, each piece developing all
of the lost parts, is a normal method of reproduction; Fig. 114.
Reproduction by Spores. — Reproduction by means of spores
is also found among the multicellular organisms, especially
among the multicellular plants. A few illustrations of it are
the following. —

SEXUAL AND ASEXUAL REPRODUCTION

245

Examples of spore formation in molds have already been
described (page 97), two methods having been mentioned.
In Mucor (Fig. 42 E) the spores are pro-
duced within a sac called a sporangium,
while in Penidllium (Fig. 42 A) they are
only the ends of branches, growing in the
air. The latter are called conidia to dis-
tinguish them from spores formed in spo-
rangia. The nature and function of spores
and conidia are the same.

Another well-known illustration of the
same is the common puffball. This is a
colorless plant, growing from a mycelium
which lies chiefly below the surface of
the ground. At certain seasons of the
year there arise from the mycelium,
rounded knobs which
rapidly increase in size.
They may grow as
large as a walnut or
an orange, and in some
species they reach a
diameter of a foot, or
even two feet ; see Fig.
115. Within this great
mass the contents di-
vide into millions of spores, and after they
have been properly matured an opening
appears at the top and the spores emerge in
the form of a fine dust. The slightest touch
upon the puffball will throw masses of dust
into the air, from which arises the name puff-
ball. This dust consists of millions of minute spores, each of
which can become a new plant.

This power of producing spores is widely distributed among

FlG. 114. TWO SEG-
MENTED WORMS,
WHICH MULTIPLY BY
ASEXUAL METHODS

A, Autolytus, multiplying
by division; B, Syllis, multi-
plying by budding, the buds
growing from the side and
breaking away to form new
individuals.

FlG. 115. A PUFF-
BALL SHOWING
THE SPORES PRO-
TRUDING FROM
THE OPENING

246 BIOLOGY

plants, occurring in the lower as well as in the higher. Even
in the flowering plants the pollen of a flower is really a mass
of spores, although their relation to the growth of the plant is
different from that of the spores to the puffball, since they do
not grow immediately into a plant like the one that produces
them.

Among the multicellular animals, the production of spores
is not found. There is, however, in a few animals a method
of reproduction, called parthenogenesis, which in some respects
resembles spore formation. The essential differences between
reproduction by spores and that by eggs is that a spore
grows into a new organism without being united with a
sperm, i. e., no fertilization is required (see page 267), while
an egg must combine with a sperm in order to be capable of
growing into a new organism. Some organisms, however,
produce eggs that can grow without fertilization. Among the
best-known examples of this is the honey bee. The female
bee produces true eggs, some of which unite with sperms, while
others develop without such union. The individuals produced
from the unfertilized and from the fertilized egg are different,
the fertilized eggs producing worker bees or females, and the
unfertilized eggs producing males (drones). So far as can foe
seen the eggs are alike, the only difference between the eggs
that produce workers and those that produce males being that
one is fertilized and the other not. This phenomenon of the
development of eggs without fertilization is called partheno-
genesis (Gr. parthenos = virgin + genesis = creation) . It re-
sembles reproduction by spores only in the fact that it consists
of a single cell developing into an adult without the neces-
sity of union with a sperm; but the reproductive bodies are
identical with eggs, and it is usually described as reproduction
by eggs which do not require fertilization.

Parthenogenesis occurs in a variety of animals with vari-
ous complications. Where it occurs it is most common to
have such a parthenogenetic reproduction alternate, with more

SEXUAL AND ASEXUAL REPRODUCTION

247

or less frequency, with sexual reproduction. In the microscopic
Animal Hydatina, for example (Fig. 116), found in fresh water,
the eggs commonly produced, called summer eggs, develop
without fertilization into new females, which rapidly mature
and produce more similar eggs that develop in the same way.
This may go on for a long time, under proper conditions for
hundreds of generations, without any
males making their appearance. Even-
tually, however, under conditions not
yet understood, males make their ap-
pearance and the females produce eggs
of a different kind, called winter eggs,
which are incapable of developing with-
out being combined with sperms by the
sexual process. Here, then, partheno-
genesis seems to be the normal method
of reproduction, sexual reproduction
alternating with it at unknown and un-
certain intervals. The reasons for this
alternation, and the conditions that de-
termine the one or the other method,
are not yet understood.

FIG. 116.— HYDATINA. A

MICROSCOPIC ORGANISM
THAT MULTIPLIES BY
PARTHENOGENESIS

ex, excretory system;
gl, gland;
m, mouth;
st, stomach.

MULTIPLICATION BY CELL UNION

Conjugation. — In all animals above
che unicellular forms, and in most
plants, cell union is found as a factor
in reproduction. Among a few plants

of the lower orders the cells which unite are alike. In
Mucor, for example, besides the spore formation mentioned
on page 97, a union of cells sometimes takes place; Fig. 117.
As shown in Figure A, special lateral threads grow out from
the ordinary mycelium of the mold, and these come in contact
with each other at their tips. After they touch each other
single cells are divided off from each, B, which fuse with

248

BIOLOGY

each other, as shown at C. This fused mass is called a zygo-
spore (Gr. zygon = yoke + spora), z. It enlarges, becomes
covered with a hard case, D, and breaks away from the plant
that produced it. It may then remain dormant for a long time,
but eventually it sprouts, E, and grows into a new plant.
In this case the two cells that unite are, so far as the micro-
scope discloses, alike, and the plants that produce them appear
identical. But careful study has proved that there is a differ-
ence in the uniting plants, shown not in their shape, but in
their uniting powers. It has been found that there are two

D E

FIG. 117. — CONJUGATION OF MUCOR

Successive stages being shown in A toE; x and y are threads from different plants, which
unite by conjugation; z, the zygospore; at E the zygospore has sprouted to form a new
plant.

types of Mucor, differing only in their power of uniting with
each other. For example, in Figure A, the two different my-
celium threads are marked x and y. It is found that while
outgrowths of x can unite with outgrowths of ?/, they can
never unite with other outgrowths of the mycelium x. There
are thus two different types of plants, each capable of uniting
with the other, but not capable of uniting with outgrowths from
itself. This reminds us of sex union, where an egg will unite
with a sperm but not with another egg. It cannot be called
true sex, however, since there are no distinguishable differences

SEXUAL AND ASEXUAL REPRODUCTION

24S

FIG. 118.— EGGS. A,
OF 'AN ANIMAL; B,

<fF A PLANT

between the uniting bodies. It is thought to be a first step
toward the true sex which is developed in higher plants. Since
the uniting bodies in Mucor are, so far as can be seen, alike,
the union is called conjugation.

Among multicellular animals conjugation
is unknown, true sex union being always
found instead.

Fertilization or Sex Union. — The eggs
of all organisms consist of single cells which
have prominent nuclei; Fig. 118. Eggs are
usually rounded in shape, although they
may vary. In size they range all the way
from eggs that are too
small to be seen with-
out the microscope, up
to the size of the ostrich
egg. The size of the egg
is by no means propor-
tional to the size of the animal that pro-
duces it. The human egg, for example, is
microscopic, and the egg of the hen is gigan-
tic in comparison. In large eggs, like those
of the hen or the ostrich, the bulk of the
egg is made of food material, sometimes
called yolk, or deutoplasm (Gr. de^teros = sec-
ond + plasma = substance), deposited within
the eggshell for the nourishment of the
young which is to be developed. The egg
has a thin cell wall which is known as the
vitelline membrane.

The eggs of animals are produced in
organs called the ovaries; Fig. 119. They
are situated in different parts of the body in different animals,
and their sole function is to produce eggs, which are then car-
ried to the exterior through ducts called the oviducts. As can

FIG. 119. — DIA-
GRAMMATIC SEC -

TION OF THE
OVARY OF AN
ANIMAL

cflls

Showing the origin of
from the ordinary
, ov ova.

250

BIOLOGY

be seen from Figure 119, the egg is really a single cell, like the
other cells of the body in structure, though larger in size. As
the egg passes along the oviduct it is not infrequently sur-
rounded with a mass of yolk and a shell; neither the yolk nor
the shell is an essential part of the egg, the yolk being a food
material for the nourishment of the embryo, and the shell be-
ing a covering to protect the egg after it has left the body.

Plants also produce eggs similar in structure to those of
animals (Fig. 118 B), though the organs that produce them
are not called ovaries.*

Sperms. — Sperms are extremely minute cells which must
unite with the eggs in order that the latter may be capable of

further development. Sperms are by
no means uniform in shape. As a
rule, each consists of a minute head
and a motile tail, whose lashing move-
ments propel the sperm through
liquids until the sperm is brought in
contact with the egg. Figure 120
shows the sperms of a number of ani-
mals and plants. There is great vari-
ety among them, and, while some of
them are provided with tails, others
are not, and, although usually motile,
the sperms of some animals are sta-
tionary. The sperms of animals are
produced in special glands called sper-
maries or testes. In the frog and
earthworm the position of these sperm
glands is shown in Figure 80. The sperms are passed from
the spermary into ducts, commonly known as the vasa defer-
entia, which carry them to the exterior. These ducts may be

* It will be noticed that the ovary of an animal is quite different from
the ovary of a flower, since the latter does not produce eggs nor have
oviducts; see page 273.

A B

FIG. 120. — VARIOUS FORMS

OF SPERMS

A, B, C, D, and E, sperms of
animals; F, of a fern; G, of a liver-

wort.

(Various authors.)

SEXUAL AND ASEXUAL REPRODUCTION 251

very short or they may be long and coiled. Sperms are much
smaller than eggs, the sperm being always microscopic. Plants
also produce sperms (Fig. 120 G), though they do not come
from spermaries or special sperm ducts; see page 271.

Males, Females, and Hermaphrodites. — When reproduction
in animals or plants is brought about by eggs and sperms,
the process is spoken of as sexual reproduction and the uniting
bodies, the eggs and sperms, are sex bodies. The glands that
produce them are the sexual glands, or gonads, and the ducts
that conduct the bodies to the exterior are the sexual ducts.
Among animals, it is most common to have one individual
produce either spermaries or ovaries, but not both, and the
individuals are then spoken of as males and females.*

In some animals, however, as already seen in the earthworm,
the same individuals may produce both spermaries and ovaries.
Such individuals are spoken of as hermaphrodites. Among
animals hermaphrodites are found chiefly among the lower
orders, very few being found among the higher. Among plants,
however, both hermaphroditic and separate sexed conditions
are common; hermaphroditic plants are called monoecious
(Gr. monos = one + oikos = house), and the separate sexed
plants dioecious (Gr. di- = twice -f- oikos). In the higher
flowering plants the relation of the sexes is peculiar, and com-
plicated by what is called alternation of generations, to be
described later.

THE UNION OF THE SEX BODIES OR FERTILIZATION

The union of the egg and the sperm is called fertilization,
and the moment when the egg and the sperm unite is the
beginning of the life of the new individual. This process of
union of the sex bodies is peculiar and of extreme significance.
In the description which follows, the successive changes which
occur are described without reference to any particular spe-

*The sign $ is used to denote the male sex, 9 to denote the female sex,
and $ to denote hermaphrodites.

252 BIOLOGY

cies. Essentially the same series of events occurs in all animals
where a fertilization takes place, although the order of events
is not always the same. In a previous chapter we have seen
that in all animals, when the chromatin of the nucleus breaks
into chromosomes before division, the number of chromosomes
is always the same in all cells of the species. In order to
illustrate the process of the origin and union of the sex cells,
we will describe the process in an animal that has four
chromosomes, meaning by this that all of the cells of the ani-
mal (except the germinal cells to be described) contain four
chromosomes at the time when cell division takes place.

Origin of the Egg (Oogenesis). — The egg is simply one of
the ordinary cells of the ovary. During the early life of the
animal, the cells in the ovary increase by the ordinary process
of cell division, with nothing especial to distinguish it from
the cell division of the other cells. In all cases, the cells are
about the ordinary size and all contain the normal number of
four chromosomes. This process continues indefinitely during
the early life of the animal, until it is ready to produce eggs.
When this time comes, some of the cells of the ovary begin
to increase greatly in size, and become in a short time very
much larger than the ordinary cells, not only than the cells
of the body generally, but much larger than all of the other cells
in the ovary. This increase in size is due largely to deposition
in the egg of food material which is to serve as nourishment
for the young that is subsequently to develop from the egg.
At the time the egg increases in size, a peculiar change takes
place in the chromosomes within the nucleus. By a series
of divisions, this chromatin divides into a number of chromo-
somes which is always double that found in the ordinary cells
of the animal. In our illustration, instead of four of these chro-
mosomes, there are eight. These chromosomes always assume
at this stage the arrangement in groups of fours, such as is
shown in Figure 121 A. There is thus produced a large pri-
mary egg (Gr. don = egg-fci/tos), called an oo'cyte, containing

SEXUAL AND ASEXUAL REPRODUCTION

253

an immense amount of food
number of chromosomes
that are found in the ordi-
nary cells of the animal.
This doubling of the chro-
mosomes is the last step in
the formation of the oocyte.
Maturation of the Egg. —
At the stage shown in Figure
121 A, the egg is not yet
mature, i. e., is not yet ready
to unite with the sperm; it
must first pass through a
further series of changes
spoken of as the maturation
of the egg. The nucleus ap-
proaches the edge of the
egg and divides into two
parts, one very large and
one very small, each retain-
ing four of the chromosomes
present in the original nu-
cleus; Fig. 121 B-D. It will
be noticed that in this divi-
sion the chromosomes do
not split* as they do in or-
dinary cell division (see
page 87), but that each
of the two nuclei formed
contains half of the original
eight. One of these nuclei
is pushed out of the egg
as a small protrusion
shown at D; the other one
a short period of rest the

yolk in it, and with double the

FIG. 121. — DIAGRAM SHOWING THE
MATURATION AND FERTILIZATION OF

AN EGG

Stages A to G represent maturation; H and
I the fertilization; J, the egg after it has di-
vided; /, female pronucleus; ra, male pronu-
cleus; p, polar bodies; sp, a typical flagellate
sperm more highly magnified.

remains within the egg. After
process of division is repeated,

254 BIOLOGY

the two nuclei once more dividing into two parts without any
splitting of the chromosomes, each of the four nuclei thus con-
taining two of the original chromosomes. Half of the nucleus
still within the egg is extruded, while the other half remains
within; Fig. 121 E, F. The nuclei which are thus extruded from
the egg are called polar cells, p, and have no further function,
since they have nothing to do with the individual which is to
arise from the egg. They are rejected products and soon dis-
appear. After the nuclei have divided the second time, the
nucleus remaining within the egg, with its two chromosomes,
once more passes toward the center of the egg and is called
the female pronucleus; Fig. G, f. The egg is now ready to
unite with the sperm. The egg, in other words, has become
mature, this process of the extrusion of the three small nuclei
being the essential feature of the process of maturation.

The Origin of the Sperm (Spermatogenesis) . — The origin of
the sperm is essentially similar to that of an egg, differing,
however, in one rather important point. As in the ovary,
the ordinary Cells in the sperm glands, during the early life
of the animal, continue their growth and division by the process
of simple cell division, with the normal method of the division
of the chromosomes. When, however, the sperms are about
to be formed, the cells of the spermary undergo a change simi-
lar to that described in the formation of the egg, except that
they do not materially increase in size. In each of these cells,
called a spermocyte (Gr. sperma = germ + cytos), the number
of chromosomes doubles itself, producing a number identical
with that found in the oocyte; Fig. 122 7. The chief difference
between this spermocyte and the oocyte at the corresponding
stage is that, whereas the egg has greatly increased in size by
the deposition of the food, the cell which is to form the sperm
does not increase in size.

The next step in the development of the sperm is the divi-
sion of this cell into four parts. This step corresponds clearly
with the division of the egg cell into four parts, as shown in

SEXUAL AND ASEXUAL REPRODUCTION 255

Figure 122 // to ///. In this case, however, the division does
not produce one large and three small cells, but four cells of
equal size, each one of which receives two of the chromosomes.
It is evident, therefore, that one of these cells is equivalent to

FIG. 122. — DIAGRAM SHOWING A COMPARISON BETWEEN THE MATURA-
TION OF AN EGG, B, AND THE FORMATION OF THE SPERMS, A
Stages / to IV in series A and B correspond with each other.

one of the cells developing in the maturation of the egg, at
least so far as concerns its nuclear matter and its chromosomes,
differing, however, in the amount of cell substance that may
be present. In the further development we find another point
of difference in the fact that each one of these four cells de-
velops into a perfect, functional sperm. In the maturation of
the egg, three out of the four cells are thrown away and take
no further part in the functions of the animal; in the develop-
ment of the sperm, however, each one of the four cells arising
from the divided spermatocyte cell becomes a typical sperm;
Fig. IV. It is evident from this that a sperm must be regarded,
so far as concerns its nuclear matter, as equivalent to a matured
egg, and equivalent also to each of the three discarded cells
which have been thrown away in the maturation of the egg.
Both the sperm and the egg contain half the normal number
of chromosomes.

An examination of a matured sperm shows the structure
-'ndicated in Figure 121 sp. It consists of a head, which is

256 BIOLOGY

sometimes rounded, but more commonly elongated. A careful
examination of this head shows that it contains an equivalent
of the two chromosomes originally present in the matured egg.
The spermatozoan head is therefore really a nucleus. Just
back of the head is a short piece known as the middle piece,
which contains a centrosome. This is the smallest part of the
sperm. The third part of the sperm is the tail, which is usually
rather long and motile, and whose only function is to produce
motion of the sperm and thus bring it in contact with the egg.
The sperms of some animals, however, have no motile tail and
are brought into contact with the egg by other means.

The important conclusion to be drawn from this description
of the origin and structure of eggs and sperms is, that they
are essentially equivalent to each other. Even though the egg
is very large and the sperm is very small, and though the egg
is motionless and the sperm is commonly endowed with motion,
so far as concerns, their most essential parts they are identical.
Each contains one nucleus, with chromosomes equivalent to
half the amount present in the ordinary cells of the organisms
from which these cells were derived; each may contain a centro-
some, though this is not always found. The eggs contain food
upon which the young embryo feeds, and the sperm possesses
a tail by which it can swim; but these are secondary features,
and in essential characters the egg and sperm are identical.

Entrance of the Sperm into the Egg. — When the sperms are
mature they are excreted through the ducts of the spermaries
to the exterior. If not excreted into the water, as is frequently
the case with water animals, a quantity of liquid is sometimes
excreted with them, in which the cells can swim by their motile
tail. All organisms have some method by which the sperms
and eggs are brought together. Sometimes both of them are
thrown in large numbers into the water and depend upon
chance currents to bring them together. Among many of the
higher animals there are developed special. copulatory organs,
whose function is to bring the eggs and sperms together. Among

SEXUAL AND ASEXUAL REPRODUCTION 257

the endless series of animals and plants may be found great
variety in the manner by which this is accomplished; but in
all cases some efficient device is found for bringing the egg
and sperm into contact.

The egg and the sperm have a strong attraction for each
other, so great that when brought into each other's proximity
the sperm will be attracted to the egg and attach itself.
The head of the sperm then buries itself in the egg, as shown
in Figure 121 G, m, the tail being left on the outside, but the
centrosome being carried in with the head. The tail has no
further function. This entrance of the sperm into the egg
may occur either before or after the changes in the egg that
have been described as maturation. If the sperm enters before
the egg is fully matured it remains in the egg in a dormant
condition, and is now known as the male pronucleus, until
after the egg has been brought into the condition above de-
scribed as mature, with its chromosomes reduced to half their
normal number. If the sperm does not enter the egg until
after the egg is mature, the further changes which bring about
fertilization occur at once.

Fertilization. — After the sperm has entered the egg and the
egg has become matured, the nucleus of the egg and the sperm
head (the two pronuclei) approach each other; Fig. 121 H.
What brings them together is not exactly known; apparently,
in some cases, the centrosome seems to have something to do
in bringing the two nuclei in contact, and without much doubt
they have an attraction for each other. At all events the egg
and the sperm are soon brought together and finally fused
with each other, forming a single fusion nucleus. This fusion
is the fertilization proper (sometimes called impregnation).
Since the egg nucleus contains two chromosomes and the
sperm head, or male nucleus, also contains two, when these
two unite the fusion nucleus evidently contains four of them,
and thus the number of chromosomes is restored to the same
number as that possessed by the ordinary cells of the body

258 BIOLOGY

of the animal. Whether the centrosome that is brought in
by the sperm and that which comes from the egg have any-
thing to do with the subsequent history of the fertilized egg,
is uncertain. In some cases it is certain that the ^centrosome
of the original egg disappears, and the only one that remains
is the one brought in by the sperm. In plants, as we have
already learned, there are no centrosomes at all, and from
these facts it would seem to follow that the centrosome can
not have very much to do with the process of fertilization.

From the facts given it is evident that the fertilized egg con-
tains material from both parents. The female parent furnishes
the bulk of the food in the egg upon which the young is to be
nourished; and it also furnishes two chromosomes. The male
parent has also furnished two chromosomes, and in some cases
a centrosome, but none of the food material. The only thing
which the two sexes have furnished in common is chromatin
material, and it is especially interesting to note that both the
male and the female parent furnish chromosomes in equiva-
lent amounts.

Unless an egg is fertilized by a sperm it has no power of
subsequent growth. Most of the ordinary cells of the animal
body are capable of a certain amount of development, but the
egg cell if unfertilized soon dies, undergoes decomposition and
disappears. The sperm cell also is unable to undergo any
development by itself. Therefore, the fusion of an egg and
a sperm is necessary, in this type of reproduction, for the
development of a new individual.

It may sometimes happen that more than one sperm is
brought into the vicinity of an egg. When this occurs, in
most cases there is some device by which the entrance of
more than a single sperm into the egg is prevented. In some
kinds of eggs, it is, however, not unusual for more than one
sperm to enter the egg, but when this occurs only one of them
unites with the egg nucleus, the others having no further
function in the process. If in any case more than one sperm

SEXUAL AND ASEXUAL REPRODUCTION 259

does unite with the egg nucleus, abnormal results arise and
no proper embryo develops. In the vast majority of cases,
however, the single sperm unites with the single egg nucleus,
and all other sperms that chance to be present have nothing
to do with the development, but soon disappear.

THE RELATION OF THE CHROMATIN TO HEREDITY

The facts just mentioned show us that the chromatin must
play a very important part in the transmission of characters
from parent to offspring. It is a demonstrated fact that
both the male and the female parents can transmit their
characters equally to their offspring. It follows that both
parents would probably transmit an equal amount of heredi-
tary substance to the next generation. The process of fertili-
zation just described shows that the only parts contributed
by the male parent to the fertilized egg are the centrosome
and the chromosomes. Hence whatever the male parent trans-
mits to its offspring must be contained either in the centrosome
or the chromosomes. But the female parent does not contribute
any centrosome to the combined egg, and it should be remem-
bered that in plants there is no centrosome. The female does
contribute an amount of chromatin equal to that which the
male contributes, namely, in the case described, two chromo-
somes. This fact proves that the chromosomes must certainly
contain hereditary material. These chromosomes are extremely
minute, far below the reach of the human vision and only
seen with a high-power microscope and by special microscopic
methods. It seems almost incredible that there can be in
such a small compass the traits of characters which an indi-
vidual transmits to its offspring and which the offspring in-
herits from its parents. But the facts described seem to be
capable of no other interpretation, and we are therefore justi-
fied in saying that the chromatin material is the bearer of
heredity. This does not necessarily mean that other parts
jf the egg and sperm may not have some share in heredity.

260 BIOLOGY

The methods of maturation and fertilization differ somewhat
in different animals and plants, but in all cases where there
is the union of the egg with the sperm it is essentially as above
described. The normal number of chromosomes is first doubled
and then reduced to one-half that which the ordinary cells
of the organism originally contained. The mature sperm also
contains half of the normal number of chromosomes; and thus,
when the egg and the sperm finally fuse, the nucleus of the
fertilized egg is always brought back into the original condi-
tion with the normal number of chromosomes, which is evi-
dently always an even number; see page 85.

It may seem a little strange that the egg should exclude
and throw away as useless such a considerable part of this
chromatin material, which must be of such great value. The
reason is not difficult to see. If the egg did not throw away
some of its chromatin material, it could not combine with the
sperm without the chromatin material in the combined egg
being doubled in quantity. If, for instance, in the case de*
scribed, the egg and sperm should retain their normal number
of chromosomes, then, after the egg and sperm united, the
nucleus of the fertilized egg would contain eight instead of
four, and all of the subsequent cells would necessarily contain,
eight. If the process were repeated at the next reproduction
the number would again double and thus the amount of the
chromatin material in each cell would become greater, genera-
tion after generation. To keep the number of chromosomes
the same in successive generations, both the sperm and the
egg throw away some of their chromatin to make room for
an equal amount brought in by the other cell at fertilization.
Why the number is first doubled before being reduced is not
clear.

THE PURPOSE OF THE UNION OF THE SEXES

Since sex union is almost if not quite universal among
animals and plants, it is evident that the process must be
one of very great significance. One of its purposes is very

SEXUAL AND ASEXUAL REPRODUCTION 261

evident. Inasmuch as the chromosomes contain the substance
which transfers the hereditary traits, it follows as a result of
this cell union that the individual that is to arise from the
fertilized egg will inherit traits of character, not from one but
from two parents. This will naturally produce a greater
variety in the offspring. If an individual arose simply as a
result of the division of a single parent, it would be expected
that it would have a tendency to show a much greater like-
ness to its parent than if it arose from the fusion of cells from
two parents, each of which possessed its own individual char-
acteristics. Thus, as a result of this sexual union, there will
be introduced into the offspring a tendency toward variety,
which would hardly be expected if they were produced always
by the non-sexual methods of simple division. It is believed
by biologists that one purpose of sex union is to produce
variety among organisms, i. e., to introduce what is technically
called variation. The importance of variation will be discussed
later; here it will be sufficient to say that upon the phenomena
of variation is based the whole problem of the evolution of
animals and plants, and therefore, without this phenomenon
of sex union, the evolution of animals and plants could hardly
have taken place, at least not in the form in which it has oc-
curred in the actual history of living things.

The process thus becomes intelligible. Sex union brings
about the combination in the offspring, of the qualities of two
parents, and thus produces a succession of generations which,
though much alike, still show a certain amount of variation
among themselves and hence a variation from the ancestral
type.

CHAPTER XIII

DISTRIBUTION OF SEXUAL AND ASEXUAL METHODS.
ALTERNATION OF GENERATIONS

SUMMARY OF THE METHODS OF REPRODUCTION

REPRODUCTION in all animals and plants is the result of divi-
sion, but according to whether the division takes place with
or without cell union, we have the two following types: —

1. Asexual reproduction. — Asexual reproduction is division
without cell union. Under this head there are at least four
different methods.

A. Division by fission.

B. Division by budding.

C. Division by spore formation.

Each of these three types of reproduction is found among
unicellular as well as multicellular organisms.

D. Parthenogenesis, or reproduction by eggs without fer-
tilization.

2. Sexual reproduction. — Sexual reproduction is division
preceded or accompanied by a union of cells, the uniting cells
being called gametes. According to whether the uniting cells
are alike or unlike, we find tw9 types.

A. Conjugation. — When the uniting cells are microscopically
identical with each other, the process is conjugation. In these
cases there are neither eggs nor sperms, and the cell resulting
from their union is a zygospore.

Conjugation has apparently for its purpose the reinvigora-
tion of the process of cell division, since, after two individuals
have united, cell division begins to take place more rapidly.
After many generations of simple cell division the process
tends to become slower, and conjugation then may occur to

262

DISTRIBUTION OF REPRODUCTIVE METHODS 26S

reinvigorate the process. Conjugation occurs chiefly among
the unicellular organisms. It is found also among some multi-
cellular plants, but in no multicellular animals.

B. Fertilization, or sex union proper. — When the uniting
cells are unlike, one being much larger than the other, their
union constitutes fertilization, or true sex union. The larger
of the two uniting bodies is the egg and the smaller the sperm.

ORIGIN OF SEX UNION

Conjugation and sex union are evidently closely allied. In-
deed, some organisms show a type of reproduction that is
halfway between conjugation and true sex union, and give
us an idea as to what was probably the origin of sex. We
have already studied Pandorina (Fig. 28), in which we found
an animal multiplying by the union of two similar cells;
but the two cells, although similar, are not exactly alike. Both
are rounded cells, both provided with flagella which enable
them to swim; but one is a little larger than the other, and
when union occurs it is always that of a larger with a smaller
cell. Whether this is a true sex union or a conjugation it is
difficult to decide.

A step further in the line of sex differentiation is found in
Eudorina. This organism is much like Pandorina, and is
composed of a cluster of rounded flagellate cells, inclosed in
jelly; Fig. 123 A. They multiply by a method of simple divi-
sion as does Pandorina (shown at A), and in addition they
multiply by cell union. In the latter case the cells break up
into many small parts, after which there is a union of cells.
But here the uniting cells are very unlike. Some of the cells,
shown at C, D, E, break up into a large number of small flagel-
late cells, of an elongated shape. The other cells of the colony
do not divide, but slightly enlarge and remain spherical. Even-
tually one of the small flagellate cells comes in contact with
one of the rounded ones and the two unite. Here there is a
plain suggestion of egg and sperm, and consequently of a true

264

BIOLOGY

sex union. Only one more step is needed to have a typical
sexual reproduction. In Eudorina all of the cells of the colony
share in the reproductive process. If only a few of the cells
of the colony should thus develop into sex cells, leaving the
colony to live an independent life, even after the sex cells have
been extruded, there would be a typical sexual reproduction.

FIG. 123. — EUDORINA

A, showing the asexual reproduction by division. C, D, and E show some cells which are
dividing into numerous flagellate gametes. These unite with the larger cells in B, which
are thus also gametes.

Such a condition is found in the multicellular plants and ani-
mals generally.

Prom such data as these it is evident that the probable
origin of sexual reproduction has been something as follows:
The first method of reproduction was by simple division, but
the independent individuals acquired the habit of fusing with
each other, as we have seen in the case of the Paramecium,
this fusion reinvigorating the life power of the fused individual.
Next there was probably a tendency for the cells to break up
into many parts which subsequently united with each other,
the parts being at first all alike. The next step seemed to be
for some of these cells to contain more food than the others
and become larger; this led to the larger cells having less power
of motion, while the smaller ones retained the power. Next

DISTRIBUTION OF REPRODUCTIVE METHODS 265

the larger cells lost their swimming flagella and were brought
into contact with the smaller cells only by the motions of
the latter, which still retained their flagella. Lastly, most
of the cells of an organism ceased to have any share in
reproduction, being simply concerned in the life of the colony.
Some of the cells in such a colony, however, assumed as their
part the process of uniting with others, and thus carried on
the functions of reproduction. These cells still continued to
differentiate into large and small cells, the large ones becoming
eggs and the small ones remaining as sperms. From this
time on the function of reproduction is independent of the
functions of the life of the colony, and the individual exists
apart from its offspring. From all of this it appears that con-
jugation is the first step in the direction of sex union, and
that conjugation must therefore be regarded as a form of sex
union, although the sexes have not been sharply differentiated
in any true case of conjugation.

DISTRIBUTION OF ASEXUAL REPRODUCTION

Among plants asexual reproduction is nearly universal,
all of the lower plants, and nearly all the higher ones, being
able to multiply by some form of budding or division. Par-
thenogenesis is also fairly frequent. Among animals multi-
plication by budding or division is also widely distributed.
It is universal among the unicellular animals, and is a common
method of multiplication among such lower forms as Hydra
and its allies. As we pass to higher animals this power dis-
appears. It is found among some worms, and one group of
animals related to the vertebrates (Tunicata) forms colonies
by budding, which may break up and become several colonies,
this constituting a modified kind of reproduction. In no other
higher animals does asexual reproduction occur. The modified
type of asexual reproduction which is called parthenogenesis
is found among some of the higher animals, being fairly com-
mon even among insects.

266 BIOLOGY

DISTRIBUTION OF SEXUAL REPRODUCTION

Sexual reproduction, using this term to include conjugation,
is very widely distributed among organisms and, indeed, is
possibly coextensive with life. It is true that there are many
forms of unicellular animals and plants in which it has never
been shown to occur; but in many cases this is due to incom-
plete knowledge. With increasing knowledge, more and more
of the unicellular organisms are known to go through the proc-
ess of cell union under some conditions. Even some of the
longest known and best studied organisms (Amoeba) have
been recently shown to undergo conjugation. Among some
of the unicellular forms, too, there occurs a true sexual union.
In the malarial organism, for example, there is at one stage
in the life history a union of two unlike cells, which are regarded
as male and female, and a similar differentiation of uniting
bodies has been found in many other single-celled organisms.
The continued discovery of new examples of sexual union or
conjugation, among the lower organisms previously supposed
not to have this power, has led to a belief that a union of cells
in reproduction may be a universal characteristic of all life,
even though there are still many of the lower animals and
plants in which it has not been found. This conclusion is
as yet by no means proved and may not turn out to be strictly
true. In all groups of animals above the unicellular types,
sexual reproduction, by the union of true male and female
cells, is universal, and in the higher groups it is the only method
of multiplication known to occur.

REPRODUCTIVE BODIES OR REPRODUCTIVE CELLS

This term refers to the parts which are separated from the
bodies of animals or plants, and capable of growing into new
individuals. Sometimes they are multicellular fragments, like
the buds of Hydra or the gemmce of a plant; but in such cases
the term reproductive body is not usually applied to them.
In the large majority of cases the bodies formed for reproductive

DISTRIBUTION OF REPRODUCTIVE METHODS 267

purposes are single cells which are capable of developing into
new individuals, and hence the term reproductive cells better
describes them. Of these reproductive cells we recognize the
following kinds : —

Spores: single-celled reproductive bodies, capable of growing
into new organisms without uniting with a sperm.

Eggs, or ova: large, stationary cells, which grow into new
individuals only after uniting with a sperm.

Sperms: minute, usually motile cells, which must unite with
an egg to enable it to develop.

Parthenogenetic eggs: large, stationary cells, resembling, or
identical with, eggs, but able to develop without union with
a sperm.

The name gametes (Gr. gamete = wife or husband) is fre-
quently applied to the cells that unite with each other in cell
union. This term, therefore, includes eggs and sperms, and also
the uniting cells in conjugation where no distinction of sex
is seen.

CROSS FERTILIZATION THE RULE

Cross Fertilization. — In ordinary sexual reproduction the
rule is that a single sperm unites with a single egg. When
the sexes are separate, as in the frog, this will always result
in the fertilization of an egg from one individual with a sperm
from another. As we have seen, some animals produce both
eggs and sperms, and might fertilize their own eggs. But
usually there is some device to prevent this. In the earth-
worm, although both eggs and sperms are produced by each
individual, in copulation there is an interchange of sperm
fluid, in such a way that the eggs of each individual are sub-
sequently fertilized by the sperms from the other. This is
called cross fertilization. In most cases where both male and
female organs are produced in the same individual, there is
some device by which cross fertilization is insured. In the

268 BIOLOGY

common flowers both male and female organs are developed
in each flower, but there is almost always some means which
prevents the flower from self-fertilization and insures cross
fertilization. In a few animals and plants, it is true, self-fertil-
ization appears to be the rule, but it is very unusual.

It appears that the reason why cross fertilization is so com-
monly found, is that it results in more or stronger offspring.
Experiments carefully carried out in plants have shown that,
in many cases at least, the offspring resulting from cross fertil-
ization are more vigorous than those coming from close fertil-
ization. In animals there is less evidence at hand on the
subject, but here, too, it has been recently shown that, in some
cases at all events, cross fertilization is more productive of a
vigorous progeny. Apparently, then, cross fertilization is based
upon a fundamental law.

Hybrids. — On the other hand, it is necessary that the sperm
that unites with the egg shall come from another individual
not too unlike the one that produces the egg. If the egg be-
longs to one species of animal or plant and the sperm to another
species, they are not likely to unite at all. If two different spe-
cies are crossed the rule is that there is no offspring, or that,
if there is offspring, they will themselves be incapable of pro-
ducing young. Such an individual is known as a hybrid,
and frequently hybrids are sterile. It was at one time sup-
posed that they were always sterile, a conclusion that was
based largely upon the fact that the mule, which is a hybrid
between a horse and an ass, is well known to be incapable of
breeding. But most careful study of both animals and plants
has shown many instances where hybrids are fertile, so that
the sterility of hybrids is by no means a fixed rule. In general,
however, in order to produce the most vigorous offspring it is
necessary that the eggs of one individual should be fertilized
by sperms from another individual of the same species, but
not too closely related. Close inbreeding has a tendency to
foster weakness.

REPRODUCTION: ALTERNATION OF GENERATIONS 269

ALTERNATION OF SEXUAL WITH ASEXUAL METHODS OF
REPRODUCTION

In many plants, and in some animals, there is a regular
alternation in the methods of reproduction, i. e., that with
sex union and that without sex union. This is commonly
spoken of as the al-
ternation of genera-
tions. One of the
simplest and most
easily understood ex-
amples is in that of
the common fern.

Life History of the
Fern. — At certain
seasons of the year,
usually in the fall,
there appear upon the
under surface of the
fern leaves, or fronds,
which grow every-
where by the road-
side, little rounded
disks known as son;
Fig. 124 B. They
are sometimes cov-
ered by a little scale
called an indusium.
A study of these
disks with a micro-
scope shows that they
are made up of a number of little sacs, containing minute
reproductive bodies; Fig. D. When mature the sacs burst
and the reproductive cells are thrown out into the air. If
they fall upon some surface where they have the proper tem-
perature and moisture, they begin to grow at once; and since

FIG. 124. — COMMON FERN

A, the fern attached to its root-stock; B, the back
of two leaflets, showing the sori ; C, a leaflet more highly
magnified showing the sporangia within the sori; D, one
of the sporangia still more highly magnified discharging
spores.

s, sori;

sp, spor.es;

spg, sporangia.

270

BIOLOGY

they are thus capable of growing immediately into new plant!
without being united with sperms, we know that they must
be spores and not eggs, since eggs require fertilization before
they will develop. The sacs that contain them are sporangia,
spg. This method of reproduction is therefore evidently an
asexual method.

When these spores develop they do not, however, grow
into a plant like the original fern, but each grows into a very

x//

FIG. 125 — THE LIFE HISTORY OP THE FERN

A and B, sprouting spore; C, prothallium full grown; D, section of an archegonium; E,
archegonium at a later stage, showing the ovum, o, and the sperm, spm, entering to fertilize
the ovum; F, section of an antheridium at an early stage; G, an antheridium at a later stage,
discharging sperms; H, the young fern, /, growing from its prothallium.

small, flat, green leaf (Fig. 125 A to C), which clings closely
to the ground, as shown at H , usually not growing to more
than one-quarter inch in diameter, and frequently even less.
It has no stem, but on the under surface are a few delicate
hairs called rhizoids, which grow downward, fastening the

REPRODUCTION: ALTERNATION OF GENERATIONS 271

plant to the soil and giving it nourishment. It is called a
prothallium (Lat. pro = before + ihallus = branch) and one
would never suspect that this little plant had anything to do
with the fern which produced it. We rarely see the prothallia
of the fern, not because they are not abundant, but because
they are so small and grow so closely to the ground that they
do not attract attention. They may be found without much
difficulty, however, by carefully searching for them. One of
the easiest places to find them is on the outside of the moist
earthen flower pots in a greenhouse where ferns are abundant.
After the prothallium has reached its full growth an exami-
nation of its under surface with a microscope shows that it
in turn is getting ready to carry on a process of reproduction.
On the under surface may be found several little projections
(Fig. C), too small to be visible to the naked eye but clearly
made out with the microscope. They are of two kinds, one
lying among the rhizoids near one edge of the leaf, an, and the
other lying near the other edge, some' distance from the rhi-
zoids, ar. The latter are slightly elongated, with an opening
at the free end, and a little canal extending down the middle:
they are called archegonia (Gr. arche- = beginning + gonos =
birth) ; Figs. D and E. At the base of each archegonium is a
single egg, o. The other protuberances, lying near the edge of
the leaf among the rhizoids, are called antheridia (Gr. antheros
= flowery) ; Figs. F and G. They are more rounded in shape,
not so long as the archegonia, and their contents are quite
different. Instead of containing a single egg, the whole con-
tents of an antheridium divides up into a large number of
parts. Eventually an opening makes its appearance at the end
of the antheridium, and these minute bodies emerge and prove
to be sperms, spm (sometimes called spermatozoids). The
fern prothallium grows only on moist surfaces and clings
so closely to the ground that in times of rains or heavy dew
its under surface is likely to be covered with water. Each
sperm bears a tuft of swimming flagella, which lash to and fro

272 BIOLOGY

and enable them to swim in the water, which moistens the
under surface of the prothallium. In thie moisture they swirn
in all directions, and some of them come to the mouths of the
archegonia. When this occurs there is an attraction between
the egg at the bottom of each archegonium and the sperm which
has reached its top; the sperm swims to the egg and fuse:-
with it, i. e., fertilizes it.

After the egg has been fertilized by the sperm, it is endowed,
like any other fertilized egg, with the power of growth. It
soon begins to divide, grows rapidly, and develops in the course
of time into a little plant which, by continued growth, becomes
the fern with which we are familiar and like that with which we
started the history; Fig. H, f. Thus we see that the common
fern grows from a fertilized egg, and that the spore produced
by the fern grows, not into a fern at first, but rather into a
prothallium.

The life history of the fern is thus an alternation of two
different stages and two different methods of reproduction.
There is first the fern proper, which, since it produces only
spores, is the asexual stage of the plant, and is called the sporo-
phyte (Gr. sporos + phyton = plant). The second stage is the
prothallium, which, since it produces eggs and sperms, is the
sexual stage. This is called the gametophyte (Gr. gamete -f-
phyton) stage, since it produces gametes. Each of the thou-
sands of spores of the fern is capable of producing a single
prothallium. The single egg at the bottom of each arche-
gonium is capable of developing a single fern, and since there
are several archegonia on each prothallium, a prothallium is
thus theoretically able to produce several ferns. Usually,
however, only one of the eggs becomes fertilized by a sperm,
therefore only a single fern develops from a prothallium. Some-
times two eggs may grow, and occasionally three may develop,
so that two or three little ferns may sometimes be found grow-
ing from a single prothallium.

Alternation of Generations in a Flowering Plant. — In a

REPRODUCTION: ALTERNATION OF GENERATIONS 273

common flowering plant there is an alternation of generations,
based upon the same principle as that just described in the
fern; but it is so obscured by certain modifications that it is
extremely difficult to understand. The difficulty lies in three
facts: (1) Two kinds of spores are produced instead of one, as
in the fern; one of them becomes the female gametophyte,
; producing the equivalent of the archegonium of the prothallium
Vth its egg, while the other becomes a male gametophyte,
producing the equivalent of the antheridium of the prothallium
with its sperms. (2) Both of these gametophytes have become
very much reduced in size and are only distinguishable by micro-
scopic examination with special methods. (3) These two
gametophytes grow attached to the plant that produces the spores
instead of detached from it, as does the gametophyte of the
fern. If these differences be kept in mind the alternation of
generations in the flowering plant is plain. It is as follows: —
We usually speak of the flower as containing sexual organs,
the stamens being spoken of as the male and the pistil as the
female organs. When the pollen is carried to the pistil it has
commonly been spoken of as fertilizing the stigma, the infer-
ence being that the pollen is the male cell and actually fertil-
izes the female cell in the pistil. When the flower is studied
by modern methods, however, it is found that in reality it is
not a sexual plant at all, and does not produce sexual organs.
The stamens are not male organs and the pollen is not a male
cell; the pistil itself produces no eggs. The pollen is really a
mass of spores, called microspores. In the pistil, as already
noticed (see Fig. 64), are several ovules and inside of each
ovule is a single large cell, formerly called the embryo sac, but
now known as a macrospore ; Fig. 126 sp. The flower thus
produces large numbers of microspores and a smaller number
of macrospores, which together correspond to the spores of
the fern. These cells are known to be spores rather than
gametes, since they do not unite with each other. That
ihe pollen is a spore rather than a sex cell is proved by

274 BIOLOGY

the fact that it will grow into a new plant without being
united with another cell. The macrospore is also proved to

be a spore by the same fact, since it
also grows into a new plant without be-
ing fertilized. Since the flower-bear-
ing plant thus produces spores instead
of eggs and sperms, it is clearly a
sporophyte rather than a gametophyte,
and it corresponds to the fern frond
rather than the fern prothallium. It
FIG. 126. — MAGNIFIED differs from the fern, however, in that
SECTION OF THE YOUNG it produces two kinds of spores instead
OVULE, o, OF A FLOWER- of one. This condition is spoken of as
ING PLANT heterosporous (Gr. heteros ••= other +

sp, macrospore; n, its nucleus. ^^ in digtinction from the homo.

sporous (Gr. homos = alike) condition of the fern.

If we now try to follow out a comparison between the flower
and the fern, we should expect that the flower spores would
germinate at once into gametophytes, just as the fern spores
germinate into the prothallium, and that the gametophytes
would produce the real sex organs with sperms and eggs. Since,
however, there are two kinds of spores, we might expect two
kinds of gametophytes to grow from them instead of one kind,
as in the fern. This actually occurs, only the two gametophytee
are very small and rudimentary. The macrospore never gets
out of the pistil but, in the midst of the pistil tissue, develops
quickly into a tiny growth that represents a gametophyte
stage, and this soon produces what corresponds to an arche-
gonium with its egg; Fig, 127. All this occurs early in the
life of the flower, before any pollen has been brought to the
pistil, and consequently before fertilization can have occurred.
It is simply the germination of a spore to form a gametophyte.
The pollen, too, goes through its history, growing very slightly
but sufficiently, to show that it develops into a gametophyte
in its turn. This occurs either before it has left the anther

REPRODUCTION: ALTERNATION OF GENERATIONS 275

that produced it, or after it has been ca-ried to the pistil.
The growth of the pollen, as well ar> its i ^semblance to the
gametophyte, is so slight that
it was not recognized for years
after plants had been carefully
studied But it is now known
thai the pollen does, at least
in some of the higher plants,
develop sufficiently to show the
gametophyte stage and then
produces what corresponds to
antheridia; Fig. 128 g. The
pollen tube which grows down
through the style of the pistil
(Figs. 65 and 127 pi), in a way
corresponds to the antheridium ;
and inside it are small cells, or
nuclei of cells, m, that corre
spondtoand have thesame func-
tion as sperms. In other words,
the pollen does not correspond
to a sperm, but is simply a spore
that grows into a male gameto-
phyte, which itself produces the
equivalent of sperms.

It is thus seen that inside
of the pistil one kind of spore

grows into a female gametophyte and produces eggs, while on
the stigma the other kind of spore grows into a rudimentary
male gametophyte and produces the equivalents of sperms.

Following farther the comparison with a fern, the next step
is the fertilization of the egg of the female gametophyte by
the sperm of the male gametophyte. In the flower this fusion
is accomplished as follows: The pollen tube "(Fig. 1281?) is an
outgrowth from the male gametophyte, and pushes its way

FIG. 127. — A SECTION OP AN

OVULE AFTER THE SPORE HAS
GROWN INTO THE FEMALE GAME-
TOPHYTE

G, the gametophyte; e, egg; pt, a pollen
tube pushing its way through the style to
fertilize the egg; m, is the male nucleus in
the pollen which corresponds to the sperm
and fertilizes the egg.

276

BIOLOGY

down the style until it reaches the ovule at the bottom of the
ovary; see Figs. 65 and 127 pt. In this ovule the female
gametophyte has formed, and has by this time produced what
corresponds to archegonia with their eggs; Fig. 127 e. The tip
of the pollen tube approaches the egg and finally comes in
contact with it. Inside o* the pollen tube are nuclei which
represent the sperms; Fig. 127 m. As we have noticed on
page 257, when the fertilization of an egg occurs it is only the
nuclei of the cells that fuse, so that the nuclei in the pollen

FIG. 128. — DEVELOPMENT OF THE POLLEN

A, a single pollen grain or microspore; B, the cell divided into two; C, the pollen, which
has produced a rudimentary gametophyte at g; D, a later stage with the gametophyte g
still more rudimentary; E, the pollen developing the pollen tube. The nucleus m
divides later into two nuclei representing sperms.

tube represent all of the important parts of a sperm. When
the pollen tube comes in contact with the egg it allows these
nuclei to escape into the egg, where one of them fuses with the
nucleus of the egg, thus producing the actual sex union.

The fertilized egg is now endowed with powers of growth
and begins at once to develop into a new plant. Again follow-
ing the comparison with the fern, we shall expect that the plant
which comes from the fertilized egg must be the sporophyte,
which in this case is, of course, the plant that produces the
flowers. The egg develops at once, growing quickly into a
tiny plant with a stem and one or two leaves. This occurs
while the egg is still retained in the ovary of the flower that
produced the spores. After a time this plant stops growing

REPRODUCTION: ALTERNATION OF GENERATIONS 277

and becomes surrounded by a hard shell, inside of which it
remains dormant for an indefinite period. This forms the
seed, which thus appears to be a little sporophyte surrounded
by a shell, and it remains dormant until later when it can be
placed under proper conditions for germination; Fig. 66. It
develops its spores, of course, after it has grown large enough
to produce flowers.

It is thus seen that the flowering plant has an alternation
of generations as truly as does the fern, only in the flowering
plant the sex stage, the gametophyte, is very small, while the
asexual stage is very large. The plant with which we are
familiar is in the sporophyte stage, and the pollen and the
single cell inside its ovule are its spores. These develop into
tiny growths that correspond to the gametophytes and are
developed within, or attached to, the sporophyte that produced
the spores, i. e., in the ovary or attached to the stigma. But
tiny as they are, they produce the equivalents of eggs and
sperms, which subsequently fuse by true fertilization. The real
fertilization of the plant, then, is the fusion of the male cell
contained in the pollen tube with the egg contained in the
ovule. The term fertilization, which has been commonly ap-
plied to the transfer of the pollen to the stigma, is a misnomer,
and is largely given up, the term pollination being substituted
instead.

Alternation of Generations among Animals. — An alternation
of generations also occurs in the animals known as hydroids,
animals related to the Hydra. The fresh-water Hydra, as
described in Chapter VII, multiplies by budding; but as fast
as the buds are produced they break away from the original
animal and become independent. In the marine Podocoryne,
the buds do not break away but remain attached to form a
colony, made up of large numbers of individuals; Fig. 129.
The individuals are partially independent of each other and.
if broken apart are capable of living independent lives. This
stage of the life of the animal, since it has an asexual multi-

278

BIOLOGY

plication by budding, is the asexual stage, and is comparable
to the asexual stage of the fern above described (the fern
proper). It differs from the fern, however, in the fact' that ?t
does not produce new individuals by spores, but by budding.
After a colony reaches a certain stage in its growth, some
buds arise which differ in shape from the others. These (Fig.
129 gb) are rounded, and eventually break away from the

FIG. 129. — A COLONY OF HYDROIDS (PODOCORYNE), SHOWING AN ALTER-
NATION OF GENERATIONS

The feeding animals have tentacles; gb, the generative buds, which eventually break away
and become medusae; m, medusa; mo, mouth of the jellyfish; ov, ovaries.

Colony and assume an independent existence. These free buds
now become bell-shaped individuals of clear, transparent
jelly, and are known as jellyfishes or medusae, m. The jelly-
fishes have muscles which enable them to swim and travel
for long distances in the ocean. As they have a mouth
and digestive cavity they can procure their own food, and grow,
frequently attaining considerable size after separating from the
original colonies; some species, indeed, assume a size very
much larger than the animal that produced them. After hav-
ing lived this free life for a time, each becomes sexually mature,
developing sexual glands, either ovaries or spermaries; Fig. 130 g.
The sex bodies become mature, and are extruded into the water,

REPRODUCTION: ALTERNATION OF GENERATIONS 279

where they float around until the eggs and the sperms come
in contact and fuse, producing a typical fertilization. The
jellyfish itself, after it has extruded the sex bodies, has no
further function, and dies. The
egg, however, now grows into
a new colony like the origi-
nal. This jellyfish is evi-
dently the sexual stage in the
development of the hydroid,
and corresponds to the sexual
stage in the development of
the fern (the prothallium).

The alternation of a sexual
with a non-sexual method is
far more common among
plants than among animals.
It is developed in all plants
except the lower orders, even
the flowering plants, as we
have just seen, having such an
alternation. Among animals,
however, alternation of genera-
tions is found only in the lower orders. It is common among
the Hydroids, and a modified form of it occurs in one of the
higher animals (Salpa) ; but among the great majority of ani-
mals, when sexual reproduction is developed, the non-sexual
method is totally lost.

FIG. 130. — A FULL-GROWN

JELLYFISH
m, mouth; g, gonads.

CHAPTER XIV

DEVELOPMENT OF THE FERTILIZED EGG

EMBRYOLOGY AND METAMORPHOSIS

BY the term embryology is meant that part of the life his-
tory of the animal or plant which begins with the fertilization
of the egg and continues up to the time when a developed
animal is formed, ready to emerge from the egg as a free-living,
independent individual. When it hatches from the egg it is
sometimes like the adult, except in size; but sometimes it is
unlike its parents and goes through a further series of changes.
In this case we speak of these later stages as constituting the
larval history or a metamorphosis (Gr. meta = beyond + mor-
phe = form). The development of animals from the egg to
the adult stage, embryology and metamorphosis, has proved
to be an especially interesting phase of biological study, and
has received much attention in the last fifty years. The em-
bryology of different animals and plants differs widely, but
certain fundamental laws and rules are found to apply to all
alike. In this introductory study it is only possible to give
a few of the fundamental principles, using a single animal as
an illustration. For this purpose will be described the de-
velopment of the frog, which, although peculiar in some
respects, will illustrate the important laws both of embryology
and metamorphosis. The embryology of plants has also been
studied rather extensively, but has not hitherto yielded so
many interesting lessons as the embryology of animals.

EMBRYOLOGY OF THE FROG

1. Segmentation. — The life of an individual frog may be
said to begin the instant that the nucleus of the egg fuses with
the head of the sperm (Fig. 121 H), the time of fertilization
being thus a starting point of a new life. This fertilization of

280

DEVELOPMENT OF THE FERTILIZED EGG 281

an egg nucleus seems to endow it with renewed power. The

nucleus of the egg previous to fertilization has lost its power of

division, and if left to itself, eventually dies

and disappears; but after fusing with the

sperm the combined nucleus shows a rein-

vigorated power of growth. It begins almost

at once to divide in two parts (Fig. 132 A);

the process of the division of the nucleus fol- A £j

lowed by the division of the cell is identical

with that described on page 85. As a result

of this division there are produced two cells, FlG 131

each with a centrosome, each with its PRODUCTIVE
nucleus, which contains the same number of CELLS OF
chromosomes as the fertilized egg nucleus. FROG
Moreover, at the beginning of the division, sp^r'megg; B' the
each chromosome is split lengthwise, and half
of each chromosome passes into each of the two nuclei
of the two new cells. Each of the two cells thus contains
chromatin material from each of the chromosomes of the
fertilized egg, and since these chromosomes come partly from
the male and partly from the female parent, it follows that
one-half of the chromatin in each cell is derived from the
male, and one-half from the female parent. Hence, each cell
will contain inherited traits from each parent. This first divi-
sion of the egg is soon followed by a second, which produces
four cells, and in this division the same process is repeated,
the chromatin material being again split up so that each of
the four cells (Fig. 132 A) contains chromatin material from
both parents. This process now goes on, the cells dividing
again and again, until the original egg has divided into a large
number of small cells, each cell probably containing chromatin
material from both parents. This process of segmentation or
cleavage is the first step by which all animals and plants begin
their life history, the egg in all cases dividing after a similar
manner into a large number of cells.

282

BIOLOGY

FIG. 132. — DIAGRAM REPRESENTING THE DEVELOPMENT OF THE PROG

A, eight stages of the segmentation of the egg; B, section of the egg showing the beginning
of the differentiation of ectoderm from endoderm; C, sections at a later stage, showing the
growth of the ectoderm over the endoderm; D, section after the germ layers are formed;
«c, ectoderm; en, endoderm; mes, mesoderm; E, surface view of a young embryo showing
two branchial slits, brc; F, surface view of an older embryo; G, diagrammatic, longitudinal
sections of the stage F ; H, a later stage. In these diagrams the ectoderm is in black, meso-
derm, with dotted shading, and endoderm without shading.

br, the brain; I, liver; nc, notocord; sp, spinal cord.

ht, heart; n, nervous system; s, sexual duct;

(Various authors.)

DEVELOPMENT OF THE FERTILIZED EGG 283

2. Differentiation. — Although the cells at the outset are
much alike, they soon begin to show differentiation. In Fig-
ure 132 B it will be seen that the upper cells are smaller
than the lower ones, and the contents of the larger cells
are quite different from those of the smaller. The difference
thus shown early in the development of the egg marks the
distinction between those cells which will eventually form the
alimentary canal and those which will form the other parts
of the body. As the development goes on and the number
of cells in the embryo increases more and more, greater and
greater differences are found among them (Figs. C and D),
so that one group of cells after another becomes set apart by
differences in structure and functions, until finally, when the
animal has reached the adult form, it is not only composed of
innumerable cells, but these cells have assumed a great variety
of shape and function. This process of gradual change of
shape and function of cells which were originally alike, is spoken
of under the name of differentiation. A similar change occurs
in all multicellular animals and plants; for, after segmentation
of the egg, there always follows a differentiation of cells.

3. The Formation of Germ Layers. — After the cells have
multiplied until they have become quite numerous, they begin
to arrange themselves in three groups. Soon there appears an
outer layer, an inner layer, and a -middle layer, known respec-
tively as ectoderm, endoderm, and mesoderm. These are
shown in Figure 132 D, which represents a later development
in the frog. The method by which these three layers are formed
is shown diagrammatically in Figure C. It may briefly be
described as the growing of the mass of the smaller, ectoderm
cells, around and over the larger, endoderm cells, so as finally
to bring the larger cells upon the inside of the embryo, surrounded
by the smaller ones. Meantime there has grown from the
outer and inner layers a third mass of cells, the mesoderm,
that pushes its way between the other two, thus partly filling
up the space between the outer and inner layers; Fig. D. The

•284 BIOLOGY

final result is that the embryo has an ectoderm of smaller cells
on the outer side, an endoderm of larger cells on the inner side,
and a mesoderm between the outer and the inner layer. These
three layers of cells remain distinct, and are destined for dif-
ferent purposes in the subsequent life of the animal, each one
of them developing into certain organs only. The organs that
are developed from the three layers are as follows : —

The mesoderm. — From the mesoderm develop the muscles,
the bones, the heart, and the blood vessels, the lining of the body
cavity, the outer layer of the alimentary canal, the mesentery
which holds the alimentary canal in position, and the repro-
ductive system.

The endoderm. — From the endoderm develop the alimentary
canal, the glands around the mouth, the lungs, the pancreas,
and the liver. The muscles which form the wall of the alimen-
tary canal are developed from the mesoderm, but the lining of
the digestive tract, with all its glands, which secrete the digestive
juices, is formed from the endoderm.

The ectoderm. — The ectoderm gives rise to the skin, includ-
ing the epidermis and the dermis. It also grows inward to
line the mouth and the extreme posterior end of the alimentary
canal. The ectoderm also gives rise to the nervous system,
with all of its parts, including the brain, the spinal cord, the
nerves, and all of the sensory organs, like the eyes, the ears,
organs of smell and touch.

It will be seen that the alimentary canal is made of three
parts: the anterior end is formed by the infolding (imagination)
of the ectoderm, the infolded part forming the mouth or buccal
cavity; the posterior end is also formed by an invagination of
the ectoderm, which forms the cloacal chamber; the rest of the
canal is formed from the endoderm. These three parts are
called the foregut (stomodceum) , the midgut (mesenteron) , and
the hindgut (proctodceum) . Similar relations are found in other
vertebrates and also in the lower animals as well.

Layers similar to those described are found in the embryos

DEVELOPMENT OF THE FERTILIZED EGG 285

of nearly all animals. Among some of the very lowest ( Hydra)
only the ectoderm and the endoderm are formed, the mesoderm
being omitted. But in all except the lowest types three layers
are formed early in the embryological history. The method
by which these three layers are formed differs in different ani-
mals. In Figure 15 is shown a method of formation of the
endoderm, differing from that of the frog, by an infolding of a
hollow sphere to form a double sac. But however differently
the layers are formed, the system of organs which are developed
from them is essentially the same. The nervous system is
always developed from the ectoderm, the alimentary canal
from the endoderm, and the blood system and muscles are
developed from the mesoderm.

4. The Formation of the Body.— While the germ layers have
been forming, the embryo has been elongating (Fig. 132 E\ and
the endoderm forms itself into a hollow tube within the body,
which acquires an opening, first at one extremity and then at
the other; Fig. G. This tube becomes the alimentary canal,
and the two openings are the mouth and the anal, or cloacal
opening. Between this inner tube and the outer wall of the
body lies a cavity, more or less filled with the mesoderm, but
in it eventually appears the body cavity or ccelum, which be-
comes a more distinct cavity as the animal grows. Early in
the development, when the animal has assumed the form shown
at E, openings in the side of the neck break through from the
alimentary canal to the exterior. There are at first two of these,
shown at E, brc, but later others appear. These are known
as branchial openings, and become passages through which
water taken in at the mouth may be passed to the exterior.
They represent the gill slits present in fishes, and are to
have a similar function a little later, when the frog hatches
from the egg and lives in the water. While these changes are
going on there is formed a long, thickened rod of ectoderm in
the middle line of the back, extending from one end of the ani-
mal to the other, which is the beginning of the nervous system;

286 BIOLOGY

Fig. G, n. The result is the formation of a little animal such
as is shown in Figure H, in which the relation to the adult
structure can be clearly seen, although at this stage the em-
bryo only slightly resembles the adult frog. The development
that has taken place up to this point has occupied a period of
several days from the time when the egg was fertilized, the
exact length of time depending to a large extent upon the
temperature, the different stages being more rapidly passed
through if the eggs are kept warm than when they are kept cool.

Various other systems of organs begin to appear at this stage
or a little later. From the ectoderm along the middle line in
the back, develops a rod of nervous matter, and around the
front end of this, outgrowths appear, which become the eyes,
ears, and other sense organs. The nervous mass itself differenti-
ates into the brain and spinal cord; Fig. H. The endodermal
tube also develops outgrowths which in time become the lungs,
liver, and pancreas. One part of the mesoderm forms itself
into a gelatinous rod running lengthwise in the back of the
embryo, just beneath the nervous system. This is the noto-
cord, nc; it represents the beginning of the spinal column,
and in time the vertebrce grow around it. Another part of the
mesoderm develops into the heart, ht, and blood vessels; while
that part of it which lines the body wall becomes the muscles,
and that which is next to the intestine develops into the pm-
toneum<smd mesentery. From the mesoderm, too, the kidneys
and sexual glands arise, with their ducts, s.

These changes take place quite rapidly, although they are
not completed for many days. When they are finished the
whole series of the organs of the frog is present, though yet
incompletely developed. Meantime the animal has hatched
from the egg, and forces its way out of the jelly in which it
has been embedded and assumes an independent life.

5. Metamorphosis. — The further development of the frog
comprises a number of different stages, shown in Figure 133,
the important features of which are as follows: The animal

DEVELOPMENT OF THE FERTILIZED EGG 287

FIG. 133. — THE METAMORPHOSIS OF THE FROG

A, the embryo within the egg; C, at the time of hatching. At about the stage I, t!
animal leaves the water and lives a part of the time in the air.

288 BIOLOGY

elongates, and a slight constriction appears behind the anterior
end resembling a neck. The front portion is, however, not
the head alone, but the head and body fused together, while the
back portion soon grows out into an elongated tail. From the
side of the two branchial openings feather-like external gills
or branchiae develop, which, projecting laterally from the head,
serve as respiratory organs; Fig. D. The free larva is now known
as a tadpole, and from this time it is obliged to depend upon
itself. Its digestive organs have become developed enough to
perform their functions, and the larva begins to feed upon vege-
table food, eating the delicate green plants that are found grow-
ing on the bottom of the pool where the larva attaches itself.
The rapidity with which the animal goes through the subsequent
changes is dependent chiefly upon the amount of food it obtains,
and the temperature; but it soon begins to pass through the
stages represented in Figures C to G. The front end of the body,
which is the head and body fused together, increases in size
and becomes rounded, while the tail elongates and becomes
flatter, serving as a swimming fin. The external gills disappear;
but the gill slits remain, the animal still breathing by the use
of internal gills, not visible from the outside. The size of the
tadpole varies with the different species of the frog; in some of
the ordinary frogs it may become two or even three inches in
length, while in other species it is not more than half an inch.

The next change is the appearance of a pair of small pro-
tuberances, or buds, on the posterior end of the body on either
side; Fig. 133 F. These grow rapidly in length and develop
into the hind legs. A similar pair of buds appears at the an-
terior end of the body, a little behind the gill slits, which later
grow into the fore legs or arms. As these legs and arms grow,
the whole shape of the body changes; the eyes appear on the
sides of the head; the mouth, which is at first a round sucking
slit, elongates into a large slit surrounded by the jaws; the
head assumes more of its final form; the shape of the body
changes from the rounded tadpole to a more elongated structure.

DEVELOPMENT OF THE FERTILIZED EGG 289

The tail also shortens until it disappears. It does not drop off,
but is gradually absorbed into the blood vessels and carried
to the rest of the body, where it is used as nourishment for the
other parts of the body. These changes are not abrupt but take
place gradually as the animal assumes the adult form; Fig.
133, F to K.

By the time the form shown in Figure J is reached, the gill
slits have entirely closed, the skin growing over them; and from
this time on the animal takes air into its mouth and forces it
into its lungs in the ordinary fashion of the adult frog. It
changes, therefore, from a water-breathing to an air-breathing
animal. But even when it is an adult, the animal never quite
loses its power of respiring by means of water, for the skin
of the adult frog is always kept moist, and contains abundant
blood vessels by means of which oxygen can be absorbed from
the water, and carbon dioxid excreted. Not until the gill slits
have closed and the lungs have become functional is the frog
able to leave the water and live in the air. By this time its
legs have become well grown and are strong enough to enable
it to move more or less vigorously on the land, so that the
tadpole may leave the water and assume its adult habits.

Other Types of Metamorphosis. — Such a series of changes
from the embryo to the adult is known as metamorphosis.
Many other animals besides the frog have a metamorphosis.
One of the best-known examples is the metamorphosis of a
butterfly, which hatches as a caterpillar, lives a considerable
part of its life in this stage, and then passes into a pupa iriside
of a cocoon. Here it remains dormant for a considerable time
and eventually emerges in the form of a winged butterfly, the
imago. Many other types of metamorphosis are found among
animals, for it is quite common for them to pass through a
series of stages in their development, each stage being different
from the other, and each different from the adult. Not all
animals, however, have a metamorphosis, many passing by a
very direct course to the adult stage. In the ordinary chick,

290 BIOLOGY

for example, the embryo pursues the most direct course pos-
sible for building itself from the simple egg to the adult, and
the chick, when it hatches from the egg, is practically adult
in form, although not in size. In such cases we call the history
a direct development, in contrast to an indirect development
or a metamorphosis.

Embryology a Repetition of Past History. — It will be seen
from the development of the frog that at one period it resembles
a fish in a number of points. It lives in the water, has a flat
swimming tail, possesses branchial slits, and carries on respira-
tion by means of gills. The study of geology has shown that
in the history of the world fishes preceded frogs, and it is thus
seen that in its embryology a frog shows a tendency to repeat
the past history of animals. Such a repetition is found, not
only in the frog but in many other animals, for it is a funda-
mental biological law that embryology repeats past history.
In technical terms this is expressed by the statement that on-
togeny is a repetition of phylogeny, ontogeny (Gr. on = being +
-geneid) being the individual's embryological history, and phy-
logeny (Gr. phylon = tribe + -geneia = producing) the history of
the race, during the geological ages. This parallel has been one
of the strong arguments which have convinced scientists that our
present forms have been derived by ordinary methods of de-
scent, through the process of reproduction, from the earlier in-
habitants of the world; or, in other words, that the history of
the organic world has been one of evolution and not one of spe-
cial- creation of each species independently, as was formerly be-
lieved. While a few years ago this law of repetition was thought
to be more strictly adhered to than careful study has proved to
be the case, the general fact that embryology tends to repeat
past history remains as one of the interesting and significant
laws of nature. It is sometimes called the biogenetic law.

Oviparous and Viviparous Animals. — Many animals (for ex-
ample, the frog) extrude their eggs into the water as soon as they
are mature and take no further care of them. In some cases,

DEVELOPMENT OF THE FERTILIZED EGG 291

as in birds, snakes, etc., the eggs, after being laid, are still cared
for by the parents, and may be incubated by the parents to
keep them warm during their development. All animals that
thus lay eggs are called oviparous (Lat. ovum = egg -f- parere
= to bear). A few of the higher animals, like the mammals,
retain the egg for some time within the body of the mother.
The sperms from the male in these animals are carried into
the oviduct at copulation by the penis, and the eggs are fertil-
ized while they are still within the oviduct. After the egg is
fertilized it attaches itself to the part of the oviduct called the
uterus, and here undergoes development. The developing
embryo, called the foetus, is nourished through the maternal
blood vessels, and grows to a considerable size while still re-
tained in the uterus and attached to it by a membrane called
the placenta. Eventually, when it has become mature, it is
detached from the uterus and expelled to the exterior at birth.
The young are well developed at birth, and such animals are
spoken of as viviparous (Lat. vivus = alive + parere = to bear).

CHAPTER XV

THE SOURCE AND NATURE OF VITAL ENERGY
MATTER AND ENERGY

PHYSICAL science teaches that the universe consists of twc*
great factors, matter and energy.

MATTER

By matter is meant the substance of the objects found in
nature, such as earth, stones, etc. One of the fundamental laws
of physics is that, while matter may be changed from one form
to another, it can neither be created nor destroyed. The amount
of matter in the universe at the present time is thus exactly
the same as it has been in all previous ages.

ENERGY

By energy is meant the force or power that exists in nature.
Energy is the power of doing work, and may best be explained
by illustrations.

Active Energy. — A cannon ball flying through the air is said
to possess energy. It is flying with such force and momentum
that it requires great resistance to stop it; and if the ball could
be received upon properly devised machinery, its motion might
be made to turn wheels or do any other kind of work. The
revolving flywheel of an engine also possesses energy of the
same type, its motion and its great momentum enabling it, if
connected with machines, to move them and make them do
work. In the same way, any form of motion is energy. In
another type of energy the motion is not so evident. Heat,
liberated from burning coal, is energy, since, when it is properly-
applied to an engine, it may be made to do work. In this case
the heat may be applied to water, which it vaporizes into steam,
and this eventually may produce motion in an engine; but it

292

•THE SOURCE AND NATURE OF VITAL ENERGY 293

is fundamentally the power in the heat that goes into the engine
and finally exhibits itself in the motion. In the same way, the
electric current, flashing along the electric wire, is energy,
since this also, if received by a proper machine, can be made
to set machinery in motion and thus accomplish work. Each
of these four examples of force clearly comes under the defi-
nition given, since they all show the power of doing work.
They also have another common characteristic: they all rep-
resent motion. The cannon ball and the flywheel are evi-
dently in motion, and the physicist has shown that heat
and electricity .are also forms of motion. Each of these four
examples, then, represents energy in action. An indefinite
number of other examples of this same type could be given,
for all forms of light, hoat, motion, chemical action, and elec-
tricity are examples of energy, and, in one form or another, all
represent energy in motion. This general type of energy in mo-
tion is active energy or kinetic energy (Gr. kinetos = moving).

Passive or Potential Energy. — Energy is not always active
but,, under some circumstances, it assumes a dormant form,
which we sped: of as potential energy. By the term "poten-
tial" is meant that the energy, though not at the moment
active, may at any time be converted into active energy. Fcr
example, a heavy stone, poised on the roof of a house, is at
rest, exhibiting no active energy; but it has potential energy,
in virtue of the fact that it is raised some distance above the
ground. The moment it is dislodged it begins to move, falling
to the ground by the law of gravitation, and as it falls it de-
velops the energy of motion. No energy is put into the stone
by simply dislodging it from its position on the roof; hence it
follows that the stone contained the energy when it rested
upon the roof, only the energy was in a dormant or potential
form. When it was dislodged from its position the potential
energy began to be active, and when the stone reached the
earth it became quiet again, its energy having apparently
disappeared.

294 BIOLOGY

A different type of potential energy is illustrated by a
bit of ordinary coal. The coal that is put into a furnace
contains, stored within itself, a large amount of energy in a
dormant form. That it contains the energy is perfectly evi-
dent from the fact that we need only put it under proper con-
ditions, by kindling it, and the energy will be liberated from
the coal in the form of heat, which may be converted into
^notion by an engine. We can get no motion out of the steam
engine unless we put the energy into the furnace in the form
of coal, wood, or other fuel. Evidently fuels may be looked
upon as containing a store of dormant energy. These types of
passive energy, which exhibit no action, but which are capable
of being brought into activity when placed in the right condi-
tions, are spoken of as potential energy or energy of position.

THE CONSERVATION OF ENERGY

Energy can neither be created nor destroyed. Just as we
cannot destroy nor create matter, so we cannot destroy nor
create energy, the amount of energy present in our universe
to-day being the same as it has been in all previous time. This
statement does not seem quite so self-evident as the statement
that matter cannot be created or destroyed, for many ex-
amples occur that, at first sight, seem to be instances of the
destruction of energy. A stone which has been dislodged from
its position upon the roof falls rapidly to the ground and de-
velops energy in falling, but on reaching the ground it stops
suddenly and its energy seems to have disappeared. When a
cannon ball strikes a ledge of rock it suddenly stops. Any
examples of the stopping of motion would seem to be illus-
trations of the destruction of energy.

A careful examination, however, shows that in these cases
there is in reality, no destruction of energy, but simply the con-
version of one form of energy into another. In the case of the
stone lodged on the roof, it is evident that at one time a cer-
tain amount of energy must have been used to lift this stone

THE SOURCE AND NATURE OF VITAL ENERGY 295

into its position, and when the stone fell it only redeveloped
.the energy that was originally required to lift it to its position.
The amount of energy required to lift the stone to its position
is exactly the same as that which is developed by the stone
when it falls to the ground, and the lifting of the stone and
its falling illustrates the conversion of active into potential
energy and reconversion of potential energy into an equivalent
amount of active energy. It would seem, however, that when
the stone reaches the ground the energy disappears. But if
we examine the fallen stone carefully, and the earth under-
neath it, we find that both have been warmed. The moment
that the motion of the stone ceased, heat appeared. Heat is
a form of energy, and thus, when the stone comes to rest on
the ground, the motion of the stone is converted into that
form of energy which is called heat. This heat is soon dissi-
pated from the stone and from the earth, for they presently
resume their former temperature. The heat has simply gone
off into the air; it is not destroyed but has simply distributed
itself, and slightly raised the temperature of the air. Nowhere
in this series of changes has there been any loss of energy, but
simply the conversion of one form into another. Some 5000
years ago the Egyptians lifted a large number of stones and
placed them one on top of another so as to make the pyramids,
exerting a large amount of energy; the energy used in placing
the stones in position was stored away in the pyramids in the
form of potential energy and is there still. If at any time the
pyramids should topple over and the stones fall to the ground,
there would be redeveloped an amount of motion exactly equal
to the amount used to lift them in position. Thus energy
may be stored away and remain in a potential form for ages;
but at any future time the energy originally stored away may
reappear in the form of active energy.

The energy present in a dormant form in coal requires a
little more explanation. Chemists have shown that the small-
est particles of matter which we can see are themselves made of

296 BIOLOGY

much smaller particles called atoms, which are quite invisible
even with the highest-power microscope. They also tell us
that these atoms are united in groups, which are called mole-
cules, each consisting of a number of atoms. Just as it re-
quires the expenditure of energy to lift stones into position
to form a monument, it also requires energy to lift atoms
into position to form a molecule; and if the molecule is
broken down, the energy is liberated according to the same
principle concerned in liberating it when a monument falls.
If, therefore, we look upon the particle of coal as a series of
molecules, each built up of many atoms, it follows that if these
tiny molecules are broken down, so that their atoms will assume
a simpler form, the energy imprisoned in them, in a dormant
state, will be released. Coal is thus made of immense numbers
of complex molecules, each of which has been built by the
expenditure of energy, and the coal contains, in a potential
form, energy which may be released by breaking up the coal.
The molecule is broken down when the coal is burned and its
energy appears in the form of heat, which may then be applied
to the moving of an engine. This of course raises the question
as to how the energy was stored away in the coal, — a question
to which we will refer later.

THE TRANSFORMATION OF ENERGY

Any type of energy may be converted into any other type.
When we lift a stone to the roof of the house we convert energy
of motion into energy of position, and when the stone falls,
energy of position is converted again into energy of motion.
When it is halfway to the ground, it has a certain amount of
energy of motion, because it is moving; but it also has a cer-
tain amount of energy of position, because it is considerably
above the surface of the earth. The more closely it approaches
the earth, however, the more its energy of position is converted
into energy of motion, and the moment it strikes the ground,
all of its energy of motion is converted into heat. The potential

THE SOURCE AND NATURE OF VITAL ENERGY 297

energy in the coal in the furnace is converted into heat ; the heat
is converted by the engine into motion; the motion of the fly-
wheel, by being attached to a dynamo, may be converted into
electricity; the electricity, passing over the wires, may run
into an electric lamp, where it is converted into light, or it
may go into an electric stove to be converted into heat. The
motion of water over a waterfall may easily be converted into
the motion of a wheel by the means of a water-wheel, this into
electricity, and this in turn into light, heat, motion, or any other
form of energy that we wish to obtain.

Some of the types of transformation of energy are more easy to
bring about than others. It is much easier to convert motion
into heat than to convert heat into motion. Any form of mo-
tion is sure to take the form of heat eventually, whether we
are turning a grindstone or putting a brake on a railroad train,
or whether a cannon ball is stopped by a stone cliff. Heat,
indeed, seems to be the type which all forms of energy have a
tendency to assume in the end; it is then radiated into the atmos-
phere and into space, where it is beyond the reach of this earth
and is called radiant heat. It is true that we have some devices
by which heat may be reconverted into motion, but always with
considerable loss as radiant heat. We put into our steam en-
gines five times as much stored energy in the form of coal as
we receive in return in the form of motion, not because the
energy is destroyed, but because four-fifths of the energy of
the coal is wasted in heating the machinery and the air, and
then passes away as radiant heat, only a small part being con-
verted into motion.

Definition of a Machine. — A machine is any device which
converts one form of energy into another. The locomotive is
a machine for converting heat into motion; the electric bulb
is a machine for converting electricity into light; the motor
converts electricity into motion. Even the gas burner is a
machine for converting the chemical energy of the gas into
light. A clock is a machine which converts the potential

298 BIOLOGY

energy in its coiled spring into the motion of its pendulum
and hands; a sailboat is a machine for converting the energy
of the wind into the motion of the boat. So one might illus-
trate indefinitely. In no case is there any creation of energy
by the machine, simply the conversion of one form into an-
other. Not only is there no creation of energy, but there is
an actual loss of available energy, inasmuch as heat always
develops, and after energy has assumed the form of heat, as
we have just seen, it is difficult to get it back into another
form. While there is no actual destruction of energy when it
is converted into heat, there is, in every form of machinery
with which we are acquainted, a loss of available energy. Some-
times this loss is very great. For example, in an ordinary
electric lamp about 95% of the electrical energy that is put
into the bulb is lost; only 5% of it appears as light. The effi-
ciency of a machine is indicated by the percentage of the energy
supplied which we can get back in the form that we desire.
Machines differ much in their efficiency in this respect. It is
quite easy to get very efficient machines for converting motion
into heat, but very difficult to get an efficient machine for con-
verting heat into motion. The most efficient machines that
we have for this latter purpose are gas engines, some of
which give back 25% or 30% of the energy put into them.
Most engines give a far smaller proportion than this. Many
steam engines give back as motion not more than 5% to 10%
of the energy furnished. This matter of efficiency is one of
interest as we come to study the power of living organisms to
convert one type of energy into another.

THE LIVING ORGANISM AS A MACHINE

From the definition above given it is very easy to see that
the living organism, either animal or plant, is a machine, since
it is a mechanism which transforms one type of energy into
another. This may best be understood by considering first
the life of plants and then that of animals.

THE SOURCE AND NATURE OF VITAL ENERGY 299

THE LIFE OF A PLANT

Sunlight furnishes the earth with practically all its energy.
There have been many attempts to make efficient sun engines,
which will utilize the rays of the sun to serve directly as a
source of energy sufficient to run engines. Sun engines have
been made, but as yet they are cumbersome, unwieldy, and im-
practical. But it seems that the time must come, after the ex-
haustion of the coal supply, when sun engines will be a necessity.
A plant growing on the surface of the earth is a perfectly efficient
sun engine, devised by nature to utilize the rays of the sun
and then to transfer the energy thus received to the rest of
the living world. The life of the ordinary green plants consists
of two features: (1) the utilization of the sun's rays and the
storing away of these rays in a form of potential energy;
(2) the liberation of this energy and its subsequent use by
the plant. These two processes will be considered in turn.

Energy Stored by Plants. — All green plants have the power
of absorbing the sun's rays and, by the means of energy thus
obtained, of building up chemical compounds of great complex-
ity which will contain the energy thus absorbed, stored away in
a potential form. Their method of accomplishing this is in part
as follows: In Chapter VI we have learned that plants have
the power of manufacturing starch out of carbon dioxid and
water. This process involves the manufacture of complex
molecules (C6Hi0O5) out of simple ones (H2O and CO2), and
hence requires the expenditure of energy. Since it can take
place only in sunlight, it becomes evident that (1) the sun's
rays are the source of energy used, that (2) the starch manu-
factured will contain in a potential form the energy used in
building it, and that (3) this energy may be liberated in an
active form if the starch molecule is broken down.

Stored Energy Utilized by Plants. — The energy stored in
the starch is the primary source of energy for nearly all the
activities on the earth, except water power. The plant uses it
for two distinct purposes: L While plants do not in their

300 BIOLOGY

ordinary life exhibit a great amount of active energy, they do
develop a little heat and a little motion, and they are constantly
lifting quantities of water from the soil to the tops of the
branches. All this requires energy, which is obtained by break-
ing down some of the starch and utilizing the energy thus lib-
erated. 2. Plants are always at work building other materials
besides starch. Proteids, woods, and fats are manufactured by
combining, within the living cells, the various materials ab-
sorbed by the roots (nitrates, etc.), with the starches made in
the leaves. The chemical processes by which these new organic
compounds are built are not yet understood, but one feature
is significant. Just as starches are more complex than the
water and carbon dioxid out of which they are made, so the
proteids are far more complex than the starches, nitrates, etc.,
out of which they are made. Since it requires energy to build
the complex molecule starch out of the simpler carbon dioxid
and water, so too it requires energy to build proteids out of
the starches and nitrates. For this purpose the plants do not
use the sun's rays directly, but they use the energy they have
stored in the starch. In other words, in making proteids, a cer-
tain quantity of starch or sugar is broken down into a condition
of carbon dioxid and water, and as a result of this destruction
the stored energy in the sugar molecule is liberated. This
energy is liberated within the living cells, and under such
conditions the protoplasm can make use of it for building the
complex proteids out of the simpler materials. This general
process is called metastasis.

Thus it is seen that the plant protoplasm uses the starches
for a double purpose. Part of them are reduced to the condi-
tion of carbon dioxid and water in order to liberate the energy
needed by the plant. Part of them are combined with other
ingredients to enter into the combination of proteids, etc. By
this latter process there is thus (1) an accumulation of proteids
and other substances in the plant body, (2) a destruction of
sugar or starchr (3) an elimination of carbon dioxid and water,

THE SOURCE AND NATURE OF VITAL ENERGY 301

arising from the destruction of that portion of the starch which
was utilized as a source of energy for the constructive processes.
The carbon dioxid and water are waste products and are liber-
ated at once by the process of excretion.

Thus it will be seen that there are two processes going on
in a plant body. One — photosynthesis — is a constructive
process by which the sun's energy is stored; the other — metas-
tasis — is a destructive process by which the energy is liber-
ated. The former process is going on in green leaves and only
in sunlight; the latter takes place in all of the living parts of
the plant, whether in sunlight or in darkness, at all times when
the plant is carrying on its life processes. By the former
process starch is being made; by the latter the plant manu-
factures a host of materials which are stored away in its body
in the form of proteids, wood, fat, cellulose, or other substances.

Plants Produce an Excess of Organic Material. — In all gre^n
plants, photosynthesis is much in excess of the metastasis, and
green plants are constantly manufacturing a quantity of starch
and other organic products, far more than they need for their
own use.

The materials thus produced serve not only as a reserve
for their own future use but also for most other forms of ac-
tivity on the earth. All fuel which is used by our numerous
engines, whether wood, coal, oil, or gas, can be traced back to
plant life, and represents, therefore, the sun's energy stored
by photosynthesis. The food of all animals also comes from
plants.

THE LIFE OF AN ANIMAL

Stored Energy Utilized by Animals. — The only source of
energy available for animals and colorless plants is that stored
up by green plants, and rendered available when liberated by
the destruction of the compounds that hold it. The general
result of animal life is a destructive one, with its resulting
liberation of potential energy. Animal protoplasm is, however,
able to carry on some constructive work. It can make fats

302 BIOLOGY

out of starches, can convert one proteid into another, and can
make new living protoplasm if fed with lifeless proteids; all of
these are constructive processes. Whatever energy is needed
for this work must be obtained by breaking down part of the
food, so that the result is a reduction of the total amount of
organic materials. In their constructive work, animals are not
only unable to make starches and sugars, but they are unable
to make proteids. Since they require these as materials out
of which to manufacture muscles, nerves, glands, etc., it fol-
lows that they are dependent upon plants, not only for starches
but also for proteids, which the plants manufacture and which
the animals utilize.

From this outline of the transformation of energy it is evi-
dent that living organisms, both animals and plants, are in
a strict sense machines. That living beings possess special
powers shown by no other kind of mechanism, and therefore
belong in a category by themselves, is very evident; but so far
as concerns the problem of energy they are machines. Vital
energy is only the energy of sunlight transformed into various
types within the mechanism of the living machine. Since
coal is simply an accumulation of the remains of plant life of
past ages, we now see the source of its energy. It contains
the stored sunlight of the past.

CHAPTER XVI
THE MECHANICS OF THE LIVING MACHINE

IN the general comparison, of the living body with a machine,
a number of significant conclusions are reached when we carry
this comparison out in detail.

Are the Income and Outgo Equivalent? — Can all of the en-
ergy shown by the living organism be accounted for by the energy
furnished by the food, and, conversely, can all of the energy fur-
nished in the food be accounted for in the form of energy exhib-
ited in the living organism?

If the law of the conservation of energy is correct, the
answers to these questions must be in the affirmative. To get
an experimental answer is not easy, but it has been done, as
follows: A large box has been constructed in which is placed
an animal, or sometimes a human being, and then the box is
sealed. By means of ingenious apparatus the person inside of
the box is furnished with the necessary air to carry on his
respiration, and is given plenty of food and water; he remains
in this box for a varying length of time. The apparatus is
designed, not only to determine the exact amount of water
and food that the individual consumes, but also the amount of
oxygen he takes from the air, the carbon dioxid he breathes
into the air, together with all the moisture that is eliminated
from the body, and all other excretions. Moreover, the amount
of energy furnished him in his food is measured, and the amount
of heat liberated from his body is determined with accuracy,
as well as the amount of work that he does.

If the doctrine of conservation of energy holds concerning
the animal body, as it does concerning other machines, it ought
to be found by such an experiment that the amount of energy
exhibited by the individual is identical with that furnished
in his food, and that the amount of excretions is exactly equiv-

303

304 BIOLOGY

alent to the amount of food consumed in his body during
this given time. The difficulties of carrying on such an experi-
ment have been great, but they have been surmounted satis-
factorily, and the results are always the same. There is an
exact equivalence between the income and the outgo of a liv-
ing animal, both as to force and matter. The amount of
excretion from the individual is exactly equal to the amount
of food consumed; and the amount of energy developed is
the exact equivalent of the energy contained in the. food that
he uses during the same experiment. The general conclusion
is that the income and the outgo of an animal balance, and that
the living machine, like other machines, simply transforms one
form of energy into another without creating or destroying it.
In this statement no account is made of the energy of the
action of the nervous system, which does not show itself in
such experiments, the probable reason being that the record-
ing apparatus is too coarse to show an amount of energy so
slight as that exhibited by the nervous svstem.

DETAILS OF THE ACTION OF THE MACHINE

In the running of an ordinary machine, like a steam engine,
we understand fairly well the details of its action. We can
understand how the forces of chemical affinity break up the
chemical compounds in coal; how the heat thus liberated
vaporizes the water; how the water under pressure acts on the
piston in the cylinder, and how this produces the revolution
of the flywheel. It is true that we do not understand the
forces of chemical affinity by which coal burns, but, apart from
this, there is nothing mysterious in the fact that the engine
converts the stored-up energy contained in the coal into the
motion of the flywheel. Is a similar intelligible explanation
possible of the activities that go on in the living organism?
In other words, do chemical and physical forces suffice to ex-
plain the activity of the living machine, just as they do the
activity of the non-living machine?

MECHANICS OF THE LIVING MACHINE 305

To follow out this question 'in detail would take more space
than could be devoted to it here. A few of the more important
functions of life may be considered, and will serve to show
how modern biological science endeavors to explain life phe-
nomena in terms of chemical and physical forces. In this
discussion we shall confine our attention wholly to the life of
animals. The life of plants is far simpler than that of animals,
and if it can be shown that the animal organism works in a
mechanical fashion, we may safely assume that the same prin-
ciple will hold for the vegetable kingdom. In following out
this thought we will consider in succession several of the im-
portant functions of animal life.

Digestion. — Digestion is simply a chemical change in the
nature of the food, and involves nothing mysterious, nor any
special forces. The foods when taken into the body are mostly
insoluble. In order to pass through the walls of the intestine,
they must first be dissolved in the liquids of ^the digestive
tract, and before they are dissolved they must be changed into
a soluble condition. The changes which make them soluble
are not peculiar to the living body, since they will take place
equally well in a chemist's laboratory. One of the most impor-
tant steps in digestion is the change of starch into sugar; and
starch, by proper chemical methods, can be changed into sugar
just as readily in the test tube of a laboratory as in the digestive
organs of an animal. The digestion of starch has nothing mys-
terious in it, and is only an instance of the application of the well-
known chemical forces. The same thing is true of all the other
changes in the food which we call digestion. They are all
chemical changes, resulting from the laws of chemical affinity.
The only feature concerning the process that is not intelligible
in terms of chemical law is the nature of the digestive juices.
The digestive juices contain substances that have the power
to bring about chemical changes. If we mix starch and water
together they will not combine to make sugar, but will remain
a mixture of starch and water. If, however, to this mixture

306 BIOLOGY

we add a little of the secretion of the pancreas, the starch and
the water will chemically combine to produce sugar, a new
compound. The pancreas produces a substance which is called
amylopsin, which has the power of causing a chemical union
of the starch with the water. This substance we call an enzyme.
It is not alive nor does it need any living environment for its
action. If we separate a little of it from the pancreatic juice
and put it in a test tube with water and starch, it will cause
the union of the water and the starch exactly as it does in the
digestive tract. Now we do not know exactly the nature of this
enzyme, nor just how it brings this union about; therefore the
vital process of digestion is not entirely understood at present.
We do know, however, that digestion itself is only a chemical
change, and that the same chemical union of the starch with
the water can be brought about without the presence of this
enzyme. The fact that we do not exactly understand how
the pancreatic juice acts in this case is no stranger than the
fact that we do not understand exactly how a spark causes a
bit of gunpowder to explode. We do not doubt that the ex-
plosion of the powder is the result of chemical and physical
forces, and there is no more reason to doubt that the combina-
tion of the starch with the water, under the influence of amy-
lopsin, is also the result of chemical and physical forces.

The same principle holds in regard to the digestion of all
other foods in the digestive tract of animals. Each of the di-
gestive juices contains special enzymes, each food is acted
upon by enzymes, and in all cases the food undergoes a chemi-
cal change. Apart from the fact that they are brought about
by these enzymes, there is little or nothing to distinguish be-
tween chemical changes taking place in the body and similar
changes taking place outside of the body. Digestion, in other
words, is a chemical process and controlled by chemical laws.

The Absorption of Food. — The digested food passes through
the intestine, being forced along by the muscular action of
the intestinal wall. As it passes through the intestine it is

THE MECHANICS OF THE LIVING MACHINE

307

gradually absorbed, soaking through into the blood vessels
that lie within the walls. This process of food absorption in-
volves another set of forces, which are, at least to a considerable
extent, either chemical or physical. The primary force con-
cerned is what physicists call osmosis or dialysis, a
force which has no special connection with life. If
a membrane separates two liquids of different con-
sistency (Fig. 134), a force is exerted on the liquids
that causes each to pass through the membrane in
an opposite direction, until the constitution of the
liquids on the two sides of the membrane is the
same. The force that drives these liquids through
the membrane is a powerful one, since it is exerted
against a high pressure. In Figure 134 a mem-
branous bladder is attached to the lower end of a
glass tube. If a solution of sugar is placed inside
of this bladder and pure water outside of it, the
sugar and the water will both pass through the
membrane in opposite directions. Under these cir-
cumstances, however, more water passes from the
outside into the bladder than passes from the blad-
der outward. The result is that the bladder be-
comes more and more filled with liquid, and enough
pressure is produced in the bladder to force the
water up the tube, in which it may rise to quite a
height. This force is known as osmosis, and it is
always exerted whenever two solutions of unequal
concentration are separated from each other by a
membrane. Some substances, like the white of an
egg, are not capable of passing through a membrane,
and we refer to them by the term colloidal or non-
dialyzable. Other substances, like salt and sugar, will readily
pass through membranes, and we speak of them as crystalline
or dialyzable.

Osmosis is the fundamental force concerned in the absorption

FIG. 134.—
A DIAGRAM

ILLUSTRAT-
ING THE
FORCE OF
OSMOSIS

308

BIOLOGY

of the food from the alimentary canal. Undigested foods are
not, as a rule, capable of osmosis. Digestion changes them
into a condition in which they are soluble and capable of os-
mosis. After complete digestion the foods in the alimentary
canal have been converted into a dialyzable liquid. More-
over, the structure of the intestine is such as to make osmosis
a natural process. This can be under-
stood from Figure 135, which illustrates
a diagrammatic cross section of the in-
testinal wall. In such a figure the food
occupies the space, in. The walls of the
intestine are thrown into little papillae
called villi, each of which is covered by a
membrane, m; on the other side of this
membrane, at bv, there are blood vessels
containing the blood, which is a liquid
of very different nature from the intes-
tinal contents. Thus it is seen that we
have a membrane separating two liquids
of different consistency, the blood on
the one side and the digestive food on
the other. Under these circumstances,
the force of osmosis will develop and the
material in the solution will begin at
once to pass through the membrane
from one side to the other. Thus the primary factor in the
absorption of food from the intestines is that of osmosis.

The physical force of osmosis is not, however, the only factor
concerned in the absorption of food. If it were, there would
be an equivalent passage of liquid from the blood into the
intestine, as well as from the intestine into the blood. Such an
equivalent passage from the intestine does not seem to take
place, proving that the forces concerned in the absorption of food
are not confined to the process of osmosis. Moreover, a careful
study of the absorptive process shows that it is much more

FIG. 135. —DIAGRAM
SHOWING THE RE-
LATION OF PARTS IN
THE INTESTINE FOR

THE ABSORPTION OF
FOOD

bv, the blood vessels in the
intestinal wall;

in, the intestinal cavity
occupied by the digested
food;

m, the membrane of the
epithelial cell through which
the food dialyzes into the
blood vessels.

THE MECHANICS OF THE LIVING MACHINE 309

complex than has been considered. As the food is being passed
through the intestinal walls it is changed further in its chemi-
cal nature, and by the time it has reached the blood it is in
a different chemical state from that in which it left the intes-
tines.

While, therefore, osmosis is the fundamental factor concerned
in the absorption of food, we are obliged to admit that it is
not the only factor concerned, and that there are some phases
of the food absorption that we do not yet understand. At
the present time we may speak of this unknown factor as the
vital factor of food absorption. By this term " vital factor" we
simply mean the undiscovered forces concerned. No biologist
doubts that the further study of the digestive process will dis-
close the nature of these vital forces, just as a previous study
has explained the early phases of food absorption. In other
words, the general belief of biologists to-day is that here the
term " vital" is only a means of concealing our ignorance of
facts which are yet to be discovered. We have no reason for
believing that there are any peculiar forces concerned in the
absorption of food. Modern biology thus explains the ab-
sorption of food by the application of the same chemical and
physical forces that are found elsewhere in nature.

Circulation. — The next function in animal life is the circu-
lation of the blood, which carries the absorbed food to the
various parts of the body where it is needed. The mechanism
of the circulatory system is very simple and is based upon
mechanical principles. The circulating blood is contained in
a series of tubes, the blood vessels, extending to every part of
the body. At the center of this series of vessels there is a
pump, the heart, which keeps the blood moving. The heart
is like a pump, with valves opening in one direction only.
Its structure is such that the expansion and contraction of
its walls will open and close the valves, and cause the blood
to flow in one direction. By examination of Figure 136, which
represents diagrammatically the structure of the human heart,

310

BIOLOGY

it will readily be seen how the valves work to prevent the
backward passage of the blood, and to force it onward when the
walls of the heart contract. The blood forced from the heart
is received in elastic blood vessels, the arteries, which branch
and grow smaller as they pass from the heart, and finally break
up into extremely minute and even microscopic vessels. After

FIG. 136. — DIAGRAM OF ONE SIDE OF THE HEART, SHOWING
THE MECHANISM OF THE VALVES

A, in the state of relaxation; B, at the time of contraction. In A the open valves admit
the flow of blood from the veins into the ventricles. In B the valve connecting with the
auricle is closed and the contraction of the heart forces the blood up through the semiluuar
valve, as is shown by the arrows. Upon relaxation of the ventricle, the semilunar valve
closes, and prevents the flow of the blood back into the ventricle, while the auriculo-
ventricular valve opens and allows blood to enter from the vein.

a, auricle;

avv, auriculo-ventricular valve;

sh, semilunar valve;
v, ventricle.

passing these capillaries, the vessels are again united into
larger tubes which, by combining with each other, form the
large veins that flow back to the heart. The whole action of
this system is mechanical; and we can arrange a series of
elastic rubber tubes with a central beating force-pump, in a
manner to imitate the chief functions of the circulation. Into
the details of this matter we need not go; for our purpose it
is sufficient to understand that the circulation of the blood is
a mechanical phenomenon which can easily be imitated by

THE MECHANICS OF THE LIVING MACHINE

311

-rc

machinery devised on the same general structure as the heart
and blood vessels.

It is evident, however, that one phase in the circulation
requires further explanation. The force that drives the blood
is the contraction of the walls of the heart. Unless we ex-
plain the beating of the heart, we have not explained cir-
culation. The explanation of this phenomenon belongs to
the study of muscles, for the walls of the heart are nothing
more than a chamber made up of a series of muscles. The
beat of the heart is, therefore, no more mysterious than
the contraction of other muscles, The contraction of the
muscles, it is true, we do
not yet fully understand,
but we do know that mus-
cles constitute a machine
which by physical laws
transforms the energy
stored in the foods into
motion.

Not only is the distribu-
tion of the blood to be
explained by mechanical
principles, but the method
by which the blood sup-
plies the tissues with their
nourishment is fairly simple.
The blood first absorbs nour-
ishment from the alimen-
tary canal and is then car-
ried into the active tissues wc' w^ite corpuscles;

rc, red corpuscles.

of the body — for example,

to the muscles — where again it is placed in a position in which
osmotic pressure will be exerted. The blood passes through the
muscles in thin-walled capillaries, on the outside of which is a
liquid called the lymph (Fig. 137), and thus there is a membrane

FIG. 137. — DIAGRAM OF A FEW CAPIL-
LARIES FILLED WITH BLOOD CORPUS-
CLES AND SURROUNDED BY LYMPH.
THE ARROWHEADS SHOW THE DIALY-
SES FROM THE LYMPH INTO THE
TISSUES, AND FROM THE TISSUES
BACK INTO THE BLOOD

312

BIOLOGY

separating two liquids, i. e., the capillary walls separating the
blood and the lymph. Under these conditions osmosis will
take place, and thus the same general force which was con-
cerned in the passage of the materials from the intestine into
the blood, will cause the passage of the same materials from
the blood vessels into the lymph in the tissues. This lymph
lies in direct contact with the living cells, and these living cells
can now take from the lymph the food material that they
need. This latter function, by which the living cells take the
material that they need, is not explained by any known force,
so we speak of it as due to what we still call vital force.

Respiration. — The absorption of oxygen by the blood in the
lungs of a frog or the gills (branchiae) of a fish, and the elimi-
nation of the carbon dioxid, are also processes which are ex-
plainable by simple chemical laws. The blood contains certain
substances which have a chemical affinity for oxygen, and
others which have a chemical affinity for carbon dioxid. The
red coloring matter, hemoglobin, has a chemical affinity for

oxygen, and will absorb the gas
whenever it is in contact with it,
provided the pressure of the oxy-
gen is sufficient. But this union
is a peculiar one. If the atmos-
phere contains oxygen under
high pressure, the haemoglobin
will unite with the oxygen, but if
the oxygen pressure is low the
haemoglobin will let go of the oxy-
gen. As a result, whenever blood
passes through the lungs, where
there is a large quantity of air and
where oxygen is under high pres-
sure, the haemoglobin combines

with oxygen; Fig. 138. The blood is then carried around the
body, and when it reaches the active tissues, like the muscles,

vein

^artery

FIG. 138. — AN AIR SAC OF
THE LUNGS

Showing the blood vessels distributed
in the wall in position to absorb oxygen
from the cavity of the sac and excrete
carbon dioxid into it.

THE MECHANICS OF THE LIVING MACHINE 313

the glands, or the brain, it finds a condition where there is prac-
tically no free oxygen. Here, since the oxygen pressure becomes
reduced, the haemoglobin at once lets go its hold upon the
oxygen which it has seized in the lungs. The oxygen then
passes off rapidly into the tissues and the blood is carried back
again to the lungs to get a fresh supply. There is a similar
relation between carbon dioxid and the blood; when the pressure
of carbon dioxid is high the blood will absorb it, and when the
pressure is low, the blood will let go its hold upon the carbon
dioxid it has absorbed. In the active tissues and cells, carbon
dioxid is present in considerable quantity, as the result of the
activity of the tissues. When the blood flows through these
tissues, it therefore absorbs carbon dioxid, and then goes back
to the lungs loaded with this gas. In the lungs, however, it
comes in contact with the air, in which the carbon dioxid is
present in very small quantities only. Under these circum-
stances the blood can no longer hold the carbon dioxid. This
gas passes into the lungs and is exhaled in the next breath.
These two processes are purely chemical; they will take place
just as well in a laboratory as in the lungs, and are quite
independent of any vital factors.

Up to this point in the study of the activity of the living
body, there is no special difficulty in reaching the following
conclusions: (1) So far as relates to the general problem of
the transformation of energy, the body neither creates nor
destroys energy, but simply transforms one kind into another.
(2) So far as concerns the functions now considered, the
laws of chemistry and physics furnish for them an adequate
explanation.

It is necessary, however, to question further a function of
life in which the mechanical relation is less obvious. The
nervous system controls all the operations of the body as an
engineer controls an engine. Is it possible that this phase of
living activity can be included within the conception of the
body as a living machine?

314 BIOLOGY

The Nervous Functions. — The primary question is whether
there is any correlation between nervous force and other types
of energy. For this purpose it will be convenient to separate
the phenomena of simple nervous transmission from those
that we speak of as mental phenomena. The former are sim-
pler and offer the greater hope of solution.

Nerve impulse. — If we are to find any correlation between
nervous force and physical energy, it must be done by find-
ing some way of measuring nervous energy and comparing
it with physical energy. There has been devised as yet no satis-
factory way of measuring the nervous impulse directly. In the
experiment of keeping an individual in a large box where all of
the energy exhibited by his body can be carefully and accurately
measured, the attempt has been made to get some indication of
the energy involved in nervous phenomena. But the results
have been quite negative. When in these boxes an individual
simply arises from his chair, the measuring device of the ap-
paratus is accurate enough to show distinct indication of the
expenditure of energy in this very simple motion. But when
this person is allowed to remain seated, not performing any
bodily action, but working hard with his brain, as for example
in writing a difficult examination, there seems to be exhibited
no extra energy, so far as can be determined by the measure-
ment recorded with this apparatus. In spite of all attempts
that have been made, it has hitherto been impossible to get any
indication that the use of the nervous system involves the ex-
penditure of energy. This is probably due to the fact that
the amount of energy thus involved is altogether too small to be
recorded in the coarse apparatus which has been devised for use
in these experiments.

That there is some correlation between nervous force and
physical energy is fairly well proved by experiments along
various lines. The impulse that passes along nerves may be
excited by a variety of forms of ordinary energy. Any mechani-
cal shock, a little heat, or an electrical shock will develop a

THE MECHANICS OF THE LIVING MACHINE 315

nervous impulse. Now, if forms of physical energy applied to
a nerve are capable of giving rise to a nerve stimulus, the
inference is certainly a legitimate one that the nerve is simply
a bit of machinery which converts one kind of energy into
another, i. e., converts physical energy into nervous energy.
If this be the case, of course it is necessary for us to regard
nervous force as one of the correlated forms of energy.

Other facts point in the same direction. Not only can the
nerve stimulus be developed by an electric shock, but the
strength of the stimulus is, within certain limits, proportional
to the strength of the shock producing it. Conversely, we also
find that a nerve stimulus produces electrical energy. In an
ordinary nerve, even when it is not active, there are slight
electric currents that can be detected by very delicate appa-
ratus. If the nerve is stimulated, these electric currents are
immediately affected in such a way that they may be increased
or decreased in intensity. These variations in intensity are
sufficient to be visible by delicate apparatus, and by using a
galvanometer we can actually measure the passage of an im-
pulse passing along a nerve like a wave, and can approximately
determine the shape of the wave.

Since the nervous impulse can be started by some other
form of energy, and since in turn it can modify ordinary forms
of energy, we cannot avoid the conclusion that the nervous
impulse is a special form of energy developed within the nerves.
It is possibly a form of wave motion, peculiar to the nerve
substance, but correlated with and developed by other types
of energy. This of course would make the nerve fiber a simple
bit of machinery.

If this conclusion is correct, it will follow that whenever a
nerve impulse passes over a nerve a certain portion of the food
supply in the nerve must be broken to pieces to liberate energy,
and this would also be accompanied by the elimination of
carbon dioxid and heat. But although careful experiments
have been made, it is as yet impossible to detect any rise in

316 BIOLOGY

temperature when a nerve impulse passes over a nerve. This
is not, however, an objection to the general theory, since the
nerve is such a small machine that it would be doubtful whether
our tests are delicate enough to recognize any rise in tempera-
ture even if such a rise occurred. The total energy of the
nervous impulse is too small to be detected by our rough
instruments for measuring heat.

All evidence goes to show that the nervous impulse is a
form of motion, and hence is correlated with other forms of
physical energy. The nerve is a very delicate machine and its
total amount of energy is very small. A tiny watch is more
delicate than a water-wheel, and its actions are more closely
dependent upon the accuracy of its adjustment. The water-
wheel may be made very coarsely and still be useful, while the
watch must be fashioned with extreme care and nicety. Yet
the water-wheel transforms vastly more energy than the watch ;
it may drive the machinery of the whole factory, while the
watch can no more than move itself. But who can doubt that
the watch as well as the water-wheel is governed by the law
of the correlation of forces? So the nerve machine of the living
body is delicately adjusted, easily put out of order, and its
actions involve only a small amount of energy; but it is prob-
ably just as truly subject to the law of the conservation of
energy as are the more massive muscles.

Sensations. — Up to a certain point, sensations can also be
related to the general problem of the conservation of energy.
The frog has a piece of apparatus, which we call the ear, capable
of being affected by the vibrating waves of the air. It is made
of parts so delicately adjusted that the air waves set them in
motion, and this motion starts a nervous stimulus which travels
along the auditory nerve to the brain. Whenever air waves
strike the frog's ear, they will excite in his auditory nerve
impulses which will travel from the ear to the brain. The ear
is simply a delicately poised apparatus, so adjusted that when
it is stimulated by vibrating air it is discharged like a bit of

THE MECHANICS OF THE LIVING MACHINE 317

gunpowder, and a nervous impulse is produced. In all of this
we are plainly dealing with nothing more than the transforma-
tion of one type of energy into another. In the same way the
optic nerve has at its end, in the eye, a bit of mechanism that
is easily excited by the light waves, and when such waves strike
the eye there will be started in the optic nerve a series of
impulses which pass towards the brain. Thus each sensory
nerve has at its end a bit of machinery designed for trans-
forming certain kinds of external force into nervous impulses.

The second phase of the sensation is, on the other hand,
not explainable by any mechanical principle. When the im-
pulse started in the ear reaches the brain, it is converted into
what we call a sensation, i. e., a consciousness, a perception, a
distinct feeling. In our attempt to trace external forces we can
follow the stimulus to the brain, but there we must stop. We
have no idea how a nervous impulse is converted into sensation.
By no means of thinking can we conceive of the correlation
of the sensation itself with any form of physical energy. It
is true that the mental sensation is excited by the nervous
impulse, and true also that in the development of the individual
the mental powers develop parallel with the growth of the
nerves and brain. Moreover, certain visible changes occur in
the brain cells when they are excited into mental activity.
All of these facts point to a close association between the mental
side of sensation and the physical structure of the machine.
But they do not prove any correlation between them. The
unlikeness between the mental and physical phenomena is so
absolute that we must hesitate about drawing any connection
between them. It is impossible to conceive of the mental side
of sensation as a form of wave motion.

Mental functions. — If we go farther and try to consider the
other phenomena associated with the nervous system — the more
distinctive mental processes — we have absolutely no ground for
comparison. We cannot imagine thought measured by units;
and until we conceive of some such measurement we can get

318 BIOLOGY

no meaning from any attempt to find correlation between the
true mental processes and physical energy.

Reproduction. — The process of reproduction would seem to
be one which cannot possibly be explained as the result of
chemical and physical forces. Nowhere else in nature do we
find this property, and in this respect living organisms cannot
be compared to any other machine. Nevertheless, in its sim-
plest form reproduction also permits a partial explanation.
When a unicellular organism, like the Amoeba (Fig. 19), feeds
and grows, it increases in size. The increase in size is due to
the transformation of the chemical material of its food into a
material like that of the animal, and as these new materials
accumulate, the bulk of the animal becomes greater. As the
animal increases in bulk, it needs a larger supply of oxygen to
keep up its life processes, since all life processes require the
expenditure of oxygen, and the amount of oxygen needed is
dependent on the bulk of the animal that is to be supplied.
Now it is a principle of mathematics that the bulk of a solid
object increases as the cube of its dimensions, whereas its sur-
face increases only as its square. Since this Amoeba is obliged
to absorb all of its oxygen through the surface of its body, it
follows that the surface adapted for absorbing of oxygen in-
creases only as the square of its diameter, while its need for
oxygen increases as the cube. It is evident from this that in
time the surface will no longer be sufficient to absorb enough
oxygen for its increasing size. When this time comes the ani-
mal must either stop growing or devise some way of increasing
its absorptive surface. What happens is that the bit of living
jelly simply breaks in two. The result is that once more the
absorbing surface is large enough to accommodate a larger
bulk, and the animal again begins to grow. This explanation
of reproduction shows how the process may have been due
to overgrowth. Since all kinds of reproduction are forms of
division, it follows that if we can explain the simplest division
upon the basis of physical and chemical forces, we have at

THE MECHANICS OF THE LIVING MACHINE 319

least reached an intelligible understanding of the process.
The more complicated phases of reproduction are, of course,
not explained by this simple process, not even the division of
a cell which we have seen to be very complicated; but if we
can explain this strange phenomenon even in its simplest form,
we have done much toward explaining the functions of repro-
duction in accordance with the principle of chemical and phys-
ical forces.

VITAL FORCE OR VITALITY

With all of the explanation given, we cannot believe that
we have reached a solution of life. There is clearly something
lacking, for we still have to ask the question why it is that all
of these chemical and physical forces play together in such
harmony within the living organism. Nowhere in nature can
the physical forces automatically carry on such functions except
in living organisms. It is quite possible to compare the animal
body to a locomotive at rest. But a locomotive at rest, even
if its boilers are filled with steam under high pressure, ,will
never exhibit any activity without an engineer to control the
forces that are contained in the machine. The living organism
has no outside engineer. What is there in the living organism
that corresponds to the engineer starting and directing the
machinery? To this question we have no answer. Some bi-
ologists claim that there is no more need of an engineer for a
living organism than for a clock, these scientists assuming
that the complexity of the machine gives it automatic activity.
Others would believe that in a living being there is something
that is absent in other machines, to which they would give the
distinct name of vitality. There are certain functions of this
machine, like sensation, thought, etc., that do not seem to be
explainable by chemical and physical laws, and one class of
biologists would group these functions together under the
general term of vitality. Others would claim that vitality
has no real meaning, but is only a name given to a combination
of functions possessed by certain machines. The question

320 BIOLOGY

whether there is anything like vital force has not yet been
solved, and it is by no means certain that it ever will be.
If it were possible for scientists to manufacture a cell exhibit-
ing the properties of life, the great problem of biology would
be settled. This has never been done, and we must leave the
question of the meaning of vitality without an answer. It
cannot be insisted upon too strongly that, while we may
compare the living organism with a machine, it is unlike any
other machine. The living machine consists of a number of
small independent units called cells, each one of which has
its own independent power of growth and reproduction. The
whole combination, too, has functions possessed by no other
machine.

Complex and Simple Living Machines. — An animal as high
in the scale as the frog is evidently an extremely complicated
machine. Not only is it made up of a large number of parts,
each with a different function, but each of these parts is made
up of a number of tissues, each having a different relation to
the organ in general; and furthermore, each of these tissues
is made up of hundreds, thousands, and perhaps millions of
living units, called cells. It seems plausible to think that, if
we could get rid of the complexity seen in the frog, we might
approach nearer to primitive life. In other words, if we can
get at the simplest unit of life we might be able to understand
many mysterious phenomena, since we should thus approach
life in its simplest form. For this purpose biologists have turned
especial attention to the life of the individual cell, since this is
the simplest known unit manifesting life. It is clear, however,
from the study of cells in Chapter II, that the mysteries of
life phenomena are not solved by reducing them to the opera-
tions going on inside of the single cell. Although some cells
are simpler than the one shown in Figure 9, still it represents
practically the simplest form of machinery with which we are
acquainted that is capable of carrying on the functions of life.
But such a cell itself is a complex machine, and if we study in

THE MECHANICS OF THE LIVING MACHINE 321

it the processes of life, it becomes evident that the functions
of this single machine are as mysterious, although not so com-
plex, as are the functions of the whole body of the frog. In
other words, getting rid of the complex machinery of such a
highly built organism as the frog does not help us at all towards
the [solution of the' problems of biology; for it is no easier
to understand the processes of life going on in the single cell
than it is to understand the processes of life going on in the
multicellular animal. While the study of single cells and their
functions has enabled us to understand the processes 'of life
in many respects much better than before, it has not solved
the problem of what life is, nor made it any easier to get
rid of the idea that living organisms show certain powers
not possessed by machines, — powers so mysterious that we
must acknowledge our inability to explain them, and must,
for the present at least, include them under the general term
of vitality.

The recognition that the cell is such a complex mechanism
has recently led to the attempt to analyze it into smaller and
simpler units. Whether any success will follow this attempt
it is too early to predict.

For these reasons it is useful still to retain the term "vital
force"; not meaning by this to imply that there is any special
force in living things, uncorrelated to forces of nature, but
simply indicating our present lack of knowledge. By vitality
we refer to the guiding principles which regulate the play of
chemical and physical forces in this living machine, and which
determine the processes of reproduction, which lie at the foun-
dation of that side of living organisms and their functions which
we call mental. We certainly have not yet explained all the
factors connected with life processes, and we can therefore
most satisfactorily comprehend them under the term "vitality."
With this understanding, it is perfectly legitimate to retain th*
term "vital force" for those phases of life processes which are
not included in any mechanical conception of life.

322 BIOLOGY

SUMMARY

1. All physical energy exerted by the living organism is
distinctly correlated with other forms of energy, the energy of
plants coming from sunlight, and that of animals coming from
the energy stored by plants in their foods. To this extent,
therefore, a living organism is a machine. 2. Nearly all life
functions are explainable by chemical and physical laws. This is
certainly true of such functions as digestion, assimilation, circu-
lation, excretion, respiration, etc. 3. Some of the functions of
the living animal are not yet explained by chemical or physical
forces. This is true of the absorption of the food from the
intestines, and the power which the living cells have of taking
from the lymph the particular form of food that they need.
We may gather these factors for the present under the term
"vital forces" of the living organism. After we have learned
thoroughly to understand them and their method of action,
we may find these processes are also to be included under the
general laws of physics and chemistry. There is really no good
reason for questioning that the living organism is a mechanism,
simply because there are some functions which are at present
unintelligible. 4. In the mental power of the living organism
appear functions which are not found in any machine. The
functions of mind, sensation, and thought are so absolutely
unique, and so different from any other type of energy, that
no one has ever conceived the possibility of correlating them
with physical energy. 5. Only the living machine has the
power of reproducing itself. It is true that some forms of the
process of reproduction may be explained simply as a result
of growth, and growth as due to the chemical forces that are
at play within the living organism. But it nevertheless remains
true that no other mechanism in nature has the power of divid-
ing itself into two parts, each of which develops into an indi-
vidual like the first. Taking all these things into considera-
tion, it is evident that, so far as physical forces are concerned,
the living organism is a machine, and, like other mechanisms,

THE MECHANICS OF THE LIVING MACHINE 323

transforms one type of energy into another. But the living
organism possesses additional powers, some of which may be
explained some day, while others, like thought and reproduc-
tion, appear to be insoluble and place the living organism
in a category by itself. If the living organism is a machine,
it is also more than a machine, and cannot be compared with
any other mechanism in nature.

WHAT IS LIFE?

It may be instructive to ask whether we can define life.
Although many attempts have been made to give the defini-
tion of life, all that can be done is to describe some of its char-
acteristics. The primary characteristic of living things is a
constant activity, and if we mean anything by the term "life,"
it must be the guiding force that controls these activities.
Our understanding of the word "life" is certainly obscure;
but, so far as it means anything, it refers to the engineer that
controls the engine, the machinist that directs the activity of
the machine, the force that guides the activities of the animals
or plants. What this guiding force is we do not know. Some
have called it "vital force," and have believed it to be a special
force in nature. Others insist that there is no special force in
living things, any more than there is in a clock or a watch.
Whether there is any force in nature that can properly be called
vitality is not yet settled, but it is certain that the phenomenon
which we call life is manifested only in those machines which
we call animals and plants, and which come from no source
except that of previously existing animals and plants. We have
no evidence that this force can be created in any way except
from life which previously existed. The life force is capable
of indefinite growth and expansion, since a fraction of life
force, in the form of any single animal, may produce hundreds
of thousands of offspring, each of which has the same amount
of life force as the original ancestor had. But this life force,
although capable of expansion and growth, has, so far as we

324 BIOLOGY

know, no method of origin except from previously existing life.
We must look at life as a unique manifestation of force, stand-
ing by itself. This is perfectly consistent with the recognition
of the fact that the animal body is a machine, acting in accord-
ance with the principles of conservation of energy, and that
a living organism simply transforms one type of energy into
another. This view is also equally consistent with the sug-
gestion that there is a special force, which we call life, directing
the activity of these machines. At all events, for the present
we can go no farther in the discussion of the question than this.
Life is the directive agent which controls the activity of the
living machine, and death means the disappearance of this
controlling agent; though what is meant by its disappearance
we cannot say, any more than we can tell what caused its
appearance in the machine in the first place. The question of
the real significance of life and death is still unanswered by
science.

CHAPTER XVII

THE ORIGIN AND DEVELOPMENT OF ORGANISMS:
HEREDITY AND VARIATION

THE ORIGIN OF THE LIVING MACHINE NOT EXPLAINED

EVEN if it were possible to explain perfectly the working of
the organic machine by mechanical principles, this would not
explain life. As we have noticed in Chapter I, living organisms
come into existence to-day only as the result of reproduction
from previously existing organisms. Granting that animals
and plants have the power of reproduction, we have still to
ask how these complicated machines came into existence.
One of the most revolutionary eras of thought has arisen in
the last fifty years as the result of the attempt of biologists
to explain how the innumerable animals and plants have been
brought to their present condition of existence.

Of the primal origin of life we have no knowledge, and it
must be admitted we have little hope of ever gaining any.
Nor have we much idea of the first living things that appeared
in the world. Probably they were of the lowest type, possibly
even simpler than unicellular forms. One thing seems certain :
the first living things must have been endowed with the prop-
erties of growth and reproduction; for without these powers
they would not have been alive. We know of nothing simpler
than cells possessing these powers, and we cannot therefore con-
ceive the beginning of life as anything simpler than a bit of
reproducing protoplasm.

THE FORCES WHICH HAVE PRODUCED ORGANISMS

It has been the aim of biology to show how the endless
series of complicated animals and plants, now found in the
world, have been produced from the simplest forms of life.

325

326 BIOLOGY

Living organisms possess three properties, by the interaction
of which the present world has been formed. These are re-
production, heredity, and variability. That these three factors
are necessarily concerned is evident. Without reproduction
there could not have been produced the successive generations
which have followed each other; unless the successive genera-
tions had, by heredity, reproduced the characters of preceding
generations, there would have been no connection between one
type and another; and lastly, if the successive generations had
not shown variability, organisms would have remained in a sta-
tionary condition, without any opportunity for change. That
these forces have been sufficient to account for the develop-
ment of the organisms inhabiting the world, i. e., to explain
the origin of the living machines, is not so evident. To show
how the result has been brought about has been the endeavor
of biological discussion for the last half-century. The property
of reproduction we have already considered. The considera-
tion of heredity and variation remains.

Heredity. — The general rule in reproduction is that the off-
spring grow into individuals like their parents, the repetition
of the parent being spoken of under the name of heredity.
Heredity must not be looked upon as any special force or law,
but merely as a word expressive of the fact that one generation
repeats itself in the next. It is evident that this process of
repetition cannot be exact, since most animals have not one
but two parents, and an individual that has a father and a
mother cannot be exactly like both of them if they are in the
slightest degree unlike. Since no two animals are exactly alike,
the natural conclusion would be that the offspring would be a
compromise between its two parents. Successive generations
are thus not identical, but constantly show differences from
their parents. Heredity means, then, that successive genera-
tions resemble their parents as closely as is compatible with
the fact that the individual has two parents, and cannot be
like both.

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 327

Variation. — The offspring of any animal is never exactly like
either of its parents. Sometimes it is a compromise between
them; sometimes, for certain reasons that we do not under-
stand, it is quite different from either. The reasons why any
peculiarity may reappear in successive generations, are probably
partly due to processes connected with the reproductive func-
tions, but they are also partly due to the effect of the environ-
ment in which the animal lives, upon the structure, the nature,
and the life of the organism. Whatever be their cause, the
points in which animals and plants differ from each other, or
from their ancestral types, are known under the general name
of variations.

The life of an individual which is produced by sexual repro-
duction may be said to begin at the moment when the sperm
fuses with the egg, as shown in Figure 121. Previous to this,
there were only the sex cells produced by two parents; but
from this point there is a new individual resulting from the
union. Variations which appear in an animal or a plant may
be caused by influences acting either before or after the union
of the sex cells. If the variation is caused by influences acting
before this union, we speak of it as a congenital variation (Lat.
con = together + genitus = produced) . If, however, the varia-
tion is developed in the animal after the fusion of the sex cells,
and thus produced by influences acting directly on the new
individual, we speak of it as an acquired variation. Although
this distinction between acquired and congenital variations may
be merely a matter of a short time, nevertheless the facts
show that there is a very great distinction between character-
istics produced before and after this period. Variations which
are produced by influences acting before the fusion of the
sex cells (congenital variations) are practically certain to be
handed to the subsequent generations by heredity. Variations
which arise subsequently, and affect the new individual only
(acquired variations), are practically certain not to be handed
on to the following generations by heredity.

328 BIOLOGY

CONFORMITY TO TYPE

Nothing is more marvelous, and at the same time more
evident, than the fact that the individuals of generation after
generation resemble each other so closely. Not only in general
features, such as the .structure of the body, the presence of
the proper number of legs, arms, etc., does the child resemble
the parent, but in an infinite number of details, — in the color
of the eyes, the color of the hair, and even in many obscure
traits. The child may inherit from its parents the tendency
to become bald-headed at a certain age, or at a certain time in
life to put on a large amount of fat, etc. Through an endless
series of details, the child has a tendency to repeat its parents'
characteristics.

Since scientists have begun to study life phenomena, they
have always puzzled over these marvelous facts, and have
advanced many speculations and theories to explain the simi-
larity of the offspring to its parents. Some of these theories
have been ingenious, some have been plausible, but all have
been imaginative. For the last century, particularly, this sub-
ject has been a matter of speculation; but until about 1884
none of the various speculations had sufficient plausibility to
receive any general acceptance.

In 1884 there appeared a little essay by August Weismann
entitled "On Heredity" which advanced a new suggestion for
the explanation of heredity. In some of its phases this new
theory had been antedated by the writings of Brooks in America
and Galton in England. Nevertheless, it did not appear in a
clear-cut form until Weismann's essay came out in 1884, and
the theory has been almost universally known as Weismann's
theory of heredity. From the time of its appearance, the ex-
planation commanded wide acceptance and extended discussion.
As year by year has passed, the theory has been more and more
substantiated by facts, until at the present time it has practi-
cally universal acceptance. While it cannot be claimed that
we have a complete explanation of heredity, it is beyond ques-

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 329

tion that our present understanding gives us an intelligent
grasp of the law of conformity to type. Future experiments
and discussions may modify our present ideas in many details;
but it is practically certain that the fundamental law, in ac-
cordance with which successive generations tend to resemble
one another, is now so well understood that it is not likely to
be changed greatly by subsequent investigation. The prin-
ciple underlying this conception of Weismann's is spoken of
as that of the continuity of germ plasm. A brief resume of
the theory is as follows : —

Reproduction by Simple Division. — It is not difficult to
understand that when an animal multiplies by simple division,
the offspring will be similar to each other. When, for example,
an Amoeba divides, it would be almost impossible to see how
the two parts should be otherwise than alike, since they are
each half of the same individual. So, too, when yeast multi-
plies by budding, it is not difficult to understand that the buds
which grow from the side of the older cell will be like the old
cell. If a cell is thus capable of dividing, it would be very
difficult to see how the bud could in any degree be unlike its
parent, except as it may be changed by future conditions.
So, too, with those multicellular animals and plants that mul-
tiply by the process of budding, the conformity to type is nat-
ural rather than marvelous. When Hydra (Fig. 69) pro-
duces a small bud on its side, which grows to the size of the
original and then breaks away, it is not difficult to see why
the bud should be like the parent, for it would be difficult to
understand how it could be otherwise. So, too, when a branch
of a tree is broken off, takes root, and grows into a new tree
when placed in the ground, it would be difficult to understand
why the new individual should be different from the parent
tree, since it is really a part of it. In all of these cases the
conformity to type is natural and presents no special puzzle,
beyond the fact that animals and plants have the power
of dividing and reproducing at all. Conformity to type in

330 BIOLOGY

animals that multiply by simple division, or by budding, ex-
plains itself.

Reproduction by Eggs or by Spores. — When we come to
the reproduction of the multicellular animals and plants by
eggs and spores, the problem, however, becomes more difficult.
The egg or the spore is a single cell, and from this single cell
develops the many-celled adult. When this cell divides into
many cells, which become differentiated and form themselves
into new individuals, why should these adults be repetitions
of the parent? There can be only one answer. This single cell,
whether an egg or a spore, must contain in itself, in some form
or other, features representing the whole of the animal from
which it came. We may place two eggs in an artificial incu-
bator and hatch them by artificial heat under identical condi-
tions; one of them becomes a duck and the other a chick. It
is absolutely impossible to avoid the conclusion that in one of
the original eggs there were present potentially the characters
which would produce the duck, and in the other the charac-
ters which would produce the chick. This of course indicates
a complexity in the egg far beyond the possibility of our imag-
ination. But we are logically forced to the conclusion that the
facts are as stated. An egg or a spore undoubtedly contains
potentially all of the characters of the animal into which it
develops. \ '

Germ Plasm. — For convenience in discussion it is agreed to
call this substance, which is present in the egg and contains
the hereditary characters, by the name of germ pksm. We
have seen, in Chapter XII, reasons for believing that this mate-
rial is chiefly, if not wholly, confined to the part of the egg that
we have called the chromatin. We have also learned that the
chromatin is capable of growing and dividing and has the power
of self-perpetuation. Using the term "germ plasm" for this
material that possesses the power of determining the develop-
ment of a new individual, it follows that the germ plasm has
the power of growth. In other words, this germ substance,

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 331

when properly nourished, continues to increase in bulk and
may grow indefinitely, becoming more and more abundant,
but not essentially changing its character. If We admit this
power of the germ plasm, the problem of the conformity to
type obtains a ready explanation; for some of this germ plasm
is simply handed on from one generation to the next, constantly
growing in bulk, but not changing its character. At any stage
in the development of the race, there is present in each indi-
vidual a certain amount of germ plasm, containing in itselt
the general race characteristics. The way that this is brought
about is believed to be as follows : —

In each egg produced by an animal or a plant, and in each
sperm produced by the male, there is a small quantity of this

FIG. 139. — DIAGRAM TO ILLUSTRATE HEREDITY, SHOWING
TWO GENERATIONS OF HYDRA

gp, germ plasm;
ov, an ovum;

sp, somaplasm;
s, sperm.

The diagram shows how the germ plasm in the egg, ov, divides: one part, sp, develops
into the next generation, while the other part, the germ plasm, gp, becomes stored in its
reproductive bodies, ov1 and s. In 6, the germ plasm from an egg is combining with the germ
plasm from the sperm, s1, in sexual fertilization.

germ plasm; Fig. 139 gp. It is the presence of this germ plasm
that makes it possible for the egg to develop into a new individ-
ual like the parent. An early step in the development of this egg
toward the adult consists in the division of this germ substance
into two parts. The two are essentially alike and both contain
the same characteristics; but each has a different purpose.
One of them remains exactly as it is at the start, increasing

332 BIOLOGY

by growth but not changing in nature. Thus this substance
remains as germ plasm, gp. The other bit of the original germ
olasm, however, soon begins to develop into a new individual,
and in distinction from the other germ plasm is called soma-
plasm (Gr. soma = body + plasma = substance), sp. With
the development of the egg the dormant power, which this
somaplasm possesses, begins to show itself in an active form.
As a result there appears a new individual; the second gen-
eration (Fig. 139, 5) arises from this somaplasm. The second
generation, in other words, unfolds the characters which lie
dormant in the bit of germ plasm from which it was derived.
As this individual develops, the other part of the original egg,
which remains as modified germ plasm, finds lodgment within
the body of the new individual, and thus, when the somaplasm
has developed into an adult, that adult contains, stored away
somewhere in its body, a bit of this dormant germ plasm of
the original egg. Since this germ plasm has not changed its
nature, but has only increased in amount, its nature is of course
exactly the same as that of the parent germ plasm.

When later the second generation produces eggs, some of
this germ plasm, which has been stored away in the body of
the second generation, passes into each of the eggs; Fig. 139, 4.
If we admit that the germ plasm has certain dormant qualities,
capable of developing into an adult, it will of course follow
that all of the individuals produced from bits of this germ plasm
will be alike. It is thus inevitable that the third generation
should be exactly like the second, since both the third and
the second generations have developed from two different
parts of the same germ plasm. As long as the process con-
tinues, it is evident that successive generations will be alike.
Part of the germ plasm at each reproduction is handed on
unchanged to the next generation; it is retained by that gen-
eration through its life, and then handed on again to the next
generation. Successive generations thus carry a continuous germ
plasm. The race is the result of the continuous germ pla^m;

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 333

the individual is simply the unfolding of a bit of it, the soma-
plasm, which is set aside to develop into an individual for the
purpose of carrying for future generations the germ plasm which
is to continue the race. Heredity is thus due to the continuity
of the germ plasm from generation to generation.

It is seen that, in accordance with this theory, heredity is
simply a name given to a process of handing on from age to
age a bit of marvelous material, the germ plasm, small bits of
which have the property of developing into individuals. As
long as this germ plasm is handed on unchanged, it will pro-
duce a succession of generations identical with each other, and
there will be a conformity of type. It will be seen thus that
the child does not actually inherit anything from its parents;
the child and parent are alike because both develop characters
that are present in the continuous germ plasm.

Variations in Germ Plasm are Inherited. — It is evident that
any modification of the germ plasm must permanently affect
the race. If at any period the germ plasm should be changed
so as to produce in it a new character, the new character will
inevitably appear, not only in the next generation, but in the
following generations. Characters which appear in the germ
plasm at once become, therefore, race characters, handed on with
certainty, unless something subsequently causes their disappear-
ance from the germ plasm.

Variation in the Individual not Inherited. — It is equally evi-
dent that any variation occurring in the body, but not in the
germ plasm, will have a very different effect upon the race.
The individual is only a trustee of the germ plasm which is
stored away somewhere in his body. Among animals, the germ
substance is largely stored away in the ovaries and sperm
glands; among plants it may be more distributed, but here
also it is probably located in certain parts of the plant. If
we admit that the individual has nothing to do with this germ
substance except handing it on to the subsequent generations,
it is evident that no special change which affects the individual

334 BIOLOGY

himself will be transmitted to the subsequent generations. If
an individual should sustain the loss of an arm, it would affect
his own life, but would have no influence upon the germ sub-
stance which he has received from the egg, and which he is
simply holding hi trust for the next generation: his offspring
will not be one-armed. So with any peculiarity developed
during life, as the result of life habits or as the direct result
of environment. Characters which are impressed simply upon
the individual himself will have no opportunity of being
transmitted to subsequent generations.

Congenital and Acquired Characters. — Thus it will be seen
that variations are of two distinct types: (1) variations which
appear in the germ plasm and which therefore affect subsequent
generations; (2) variations which appear hi the body of the
individual and which are not in the germ plasm, and hence
cannot affect subsequent generations. These two types of
variations have been recognized for a long time, but they
were never sharply distinguished until Weismann's conception
of heredity brought them out hi such clear contrast. Char-
acters which result from modification of the germ plasm, and
hence are inevitably transmitted by heredity, to-day are com-
monly called congenital characters. Congenital variations are
fixed in the germ plasm and are therefore inevitably trans-
mitted by the process of heredity.- On the other hand, char-
acters which are developed as the direct result of the environ-
ment, such as loss of limbs, or changes resulting from food habits,
climate, etc., are commonly known by the name of acquired
characters. The term is not a good one, for all characters are
acquired at some time; but this name has been used hi the
discussion of the last quarter of a century, for such variations.
From what has just been stated, it is evident that acquired
characters, if they do not become a part of germ substance,
will not be repeated in subsequent generations. Acquired
characters, therefore, which an individual animal or plant
develops as the result of the conditions of the environment in

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 335

which he lives, would affect his body during life, but would not
be expected to affect his progeny; and acquired characters
should not be transmitted by heredity. This conclusion, quite
at variance with the beliefs of twenty-five years ago, has been
subjected to long and exhaustive study, as a result of which
the belief in the inheritance of acquired characters has gradually
disappeared. The conclusion has been vigorously disputed, and,
since the advancement of Weismann's theory of heredity, the
most active search has been made for proof or disproof of the
idea that acquired characters can be inherited. While many
apparent instances of such inheritance are easily found, they all
prove illusive when carefully studied, and biologists have prac-
tically agreed that there is no good evidence that acquired char-
acters can be transmitted to subsequent generations. While the
possibility of the inheritance of acquired characters cannot yet
be positively denied, it is quite generally believed to-day that
it does not occur. This conclusion has far-reaching results, for
it entirely changes our conception of the relation of parent to
offspring.

Heredity and the Union of Sex Cells. — We are in a position
now to appreciate a little more fully the significance of the
factor of the union of sex cells in sexual reproduction. Thus
far heredity has been spoken of as associated with eggs only.
A succession of similar types in successive generations can be
explained as due to a division and transmission of a continuous
germ plasm. But the result of such a process would seem to
produce a series of like individuals, without any variation in
successive generations. Successive generations are, however,
not alike. Indeed, the development of animals and plants is
dependent upon the fact that successive generations show more
or less divergence from the original type. It is here that we
see one reason for sexual reproduction.

In Chapter XII we have noticed that the reallj- significant
feature of the union of the egg and the sperm lies in the fact
that each of these reproductive cells throws away part of its

336 BIOLOGY

chromatin in order to make room for a similar amount brought
to it by the other of the two uniting sex cells. If, as we have
seen reason for believing, the chromatin contains the germ
plasm, this process has a most natural interpretation. The
maturation of the egg and the union of sex cells bring about
a new individual in which the germ plasm is a mixture from
two individuals (amphimixis) (Gr. amphi = together + mixis
= a mixing) . The result is that the germ plasm of the subse-
quent generations will be different from that which was pres-
ent in either of the parents of the last generation, since it
will be a mixture of the two, and, if the parents are in any
degree unlike, the mixture of their germ plasms will not be
exactly like that of either. It would be impossible for any
such complex things as two bits of chromatin to be mixed
twice without producing differences in the mixtures. In other
words, the following generation will show variations from the
last. Since, however, this mixed germ plasm will be handed
on to form the germ plasm of the next, and all following genera-
tions, it will follow that the variations which thus appear will
be handed on indefinitely by the. process of heredity, and such
new characters as appear from the mixture of two germ plasms
will remain fixed in the race. With the next reproductive gen-
eration this mixed germ plasm will again be combined with
another mixture from another individual, and still further
variations will appear. Successive generations will thus tend
constantly to be more or less unlike their parents. Sex union
of eggs and sperms, therefore, appears to be a device to bring
about variation and divergence from type.

If this conclusion is correct, we should expect those organ-
isms which multiply by sex union to show a greater amount
of variation than those which multiply by the asexual process
of simple division; and this appears to be a fact. If a horti-
culturist wishes to preserve unchanged a type of plant which
he has found, he contrives to multiply the plant by the asex-
ual method of budding, grafting, or cuttings. As long as this

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 337

method is continued the plants remain essentially constant.
If, however, he wishes to obtain new types, he adopts the
method of planting the seeds. Seeds, as we have seen, come
from the sex process of reproduction, and the offspring which
come from seeds show a far greater tendency to variability
than those which come from buds by the asexual process. It
is the general conclusion from observation that variations are
more common among organisms multiplying by the sexual
process. With this understanding, one purpose and function
of the union of the sex cells becomes intelligible.

DIVERGENCE FROM TYPE

The term "divergence from type" is the exact opposite of
the term "conformity to type." It is no less evident that
animals and plants tend to diverge from the race type than it
is that they conform to type. The reconciliation of these two
contradictory facts is that though, in general, successive gen-
erations conform to the type of the race, the individuals show
more or less variation from each other, and, moreover, the
whole race is slowly changing, so that the type itself in time
undergoes modifications.

Individual Variations. — An infinite number of slight differ-
ences are found between individuals of the same species. This
fact is clear to everyone who is at all familiar with animals.
In the human race, it is well recognized that no two individuals
are exactly alike, and the same thing is equally true among all
species of animals and plants. The different individuals of
the same species differ in size, color, habits, and in an infinite
number of minor points, like the length of legs, the length of
hair, the size and shape of leaves, flowers, etc. Indeed, there
is no part of an animal or plant that does not show more or
less of such variation. It is so evident that it needs no further
discussion. While the different individuals conform to the
type of their species in general character, in numerous details
they differ from each other in almost endless fashion.

338 BIOLOGY

Race Variations. — In addition to individual variations, the
whole species may show a tendency to diverge from its original
form. Races are either slowly or rapidly changing from their
previous condition, so that if the members of any race living
to-day are carefully compared with those living in a previous
period of the world's history, it will be found that the whole
race has undergone a general change which has affected all
members. Such race variation commonly occurs by what
is known as divergence. By this is meant that the descendants
of one type have, by this race variation, diverged in several
directions, more or less different from each other. This is
explained by the assumption that the descendants from any
animal remain neither exactly like their ancestors nor like each
other, and that different lines of descent depart from the origi-
nal type in different directions. Examples of this are numerous,
but for illustrative purposes two well-known instances of such
divergence will be briefly mentioned.

Breeds of pigeons. — For some centuries, breeders of pigeons
have been very much interested in improving different strains
of these birds, and pigeon fanciers have been careful to breed
together individuals showing characters that appeal to their
fancy. The result has been that the pigeons have undergone
many profound changes from their original type. The original
pigeon, from which all of our domestic pigeons came, is fairly
well known to be essentially the same as the rock pigeon of
India, a bird gray-blue in color, with bars on its breast and a
tendency to perch on rocks, but never on trees. Historical and
scientific evidence shows that all the numerous strains of
pigeons with which our pigeon fanciers are familiar to-day
have been derived from this bird. The tumblers, the fan-
tails, the pouters, and hosts of others, have all been descended
from this primitive ancestral form. The differences between
these varieties are very numerous, including variations in
color, length of bill, size, wings, tails, and many other points,

The differences between the breeds of pigeons, which have

THE ORIGIN AND DEVELOPMENT OF ORGANISMS 339

thus been produced artificially, are greater than differences
found among many of the wild birds that are regarded as
belonging to distinct species. In the case of the pigeons, it is
known from historical evidence that these different strains have
all come from a common type by methods of breeding.

The dogs. — Another example, perhaps even better known, is
that of the breeds of dogs. Dogs have been domesticated for
a period almost as long as man has been civilized. At the
present time the variety of dogs is very great, ranging in size
from the great Newfoundland to the tiny poodle, and varying
in color, type of hair, disposition, and almost every other
respect. We can hardly conceive of two animals being much
more unlike than the tiny lap-dog and the massive bloodhound
or mastiff, and it is hardly possible to believe that these ani-
mals have all come from the same type. But the most careful
study of the characters and history of the breeds of dogs has
led to the unquestioned conclusion that all forms of domes-
tic dogs with which we are familiar belong to one species of
animal, and all came from the same type far back in history.
Some varieties of dogs, like the dingo of Australia, belong possi-
bly to a different species; but all of our common forms belong to
one species and have been derived from the same fundamental
stock. Here, as in the case of pigeons, the breeds have been the
result of a long series of unconscious breeding experiments.
Different families of human beings have had a liking for certain
types of dogs and have kept by them such individuals as pleased
their fancy. These have been bred together and their masters
have selected from the pups those which most pleased them.
This process has gone on, similar individuals being bred with
each other over and over again, until the whole race has become
slowly changed. Different types of dogs were selected for differ-
ent purposes. The shepherd took a fancy to a different type of
animal from that which was most desirable as a house dog. By
selecting the dogs who could drive sheep, or the big dogs, or the
fierce dogs, or the little dogs, etc., and breeding together those

310 BIOLOGY

nearest alike, there have been produced the different types which
we have in our world to-day. Recognizing, however, that all
of these types of dogs belong to the same species and must have
come from a single common type, the strains of dog illustrate
excellently well what is meant by race divergence.

Both of these examples have been chosen from domestic
animals. There is no reason for doubting that the same facts
may occur in nature and that under proper conditions in nature
there may be a series of race variations similar to those found
in domestication. Perhaps divergence in nature is not quite
so rapid or so extreme as it is when controlled by the fancy
of the breeder, but the same general facts hold true. In nature
as well as under domestication, races are undergoing a constant
series of changes, sometimes slow, and sometimes rapid.

Race variations must be variations of the germ plasm.
Individual variations, as we have seen, will affect the body of
the individual but will not affect the germ substance. From
this it follows that individual, acquired variations will not be
transmitted by heredity and will therefore have no lasting
effect on the race. On the other hand, if the race is to undergo
a change, as we have just seen that it does, this must be due
to modifications in the continuous germ substance. Hence it
follows that the only variations that can continue in the race
and can be carried on for successive generations, are those that
affect the germ material itself. Race variations are therefore
necessarily germ variations.

The Divergence from Centers. — A little thought will show
that the result of divergence of the descendants of any type
in different directions will, in the end, produce extremely wide
diversity among animals and plants. If the descendants of
any animals diverge in two directions, and then later their
descendants again diverge from each other, and if this process
goes on indefinitely,, it becomes evident that in the course of time
the descendants of the original type will become widely unlike
«ach other, and will show great variation from primitive forms.

THE ORIGIN AXD DEVELOPMENT OF ORGANISMS 341

Such has been the history of animals and plants. So far as we
can learn of that history, it has always been one of divergence
from common centers, the process being repeated over and over
again in successive ages, until finally there has resulted the
great diversity of organisms that people the world of to-day.

At the beginning of life in the world there was, apparently,
no difference between animals and plants. We have already
seen that some organisms so closely resemble both groups that
we cannot say whether they are animals or plants. Possibly
some such organisms were the first to inhabit .the world. As
progress continued, however, the descendants of these origi-
nal forms of life diverged from each other along two great
lines, one of which acquired the habit of living upon the
other. The original form of life must have been capable of
utilizing the mineral ingredients hi nature, possibly like the
green plants of to-day. Whether the original organisms were
capable of carrying on photosynthesis we do not know, but
hi some way they must have been able to utilize minerals.
However that may be, their descendants diverged into two
groups, one group acquiring the green coloring matter and the
power of utilizing carbon dioxid and sunlight and, by means
of chlorophyll, building up starch, thus giving rise to plants.
The second group of descendants, losing this power of utilizing
minerals, and acquiring the power of feeding upon the mate-
rials which were manufactured by the first group, developed
into the kingdom of animals.

After plants and animals were thus separated and each had
developed for a time along its own line, some of the plants lost
their chlorophyll and acquired the habit of depending upon
other plants for food, thus becoming the Fungi. As the history
of the world progressed, each of the two great types thus
started continued to repeat the history of divergence. Age
after age the descendants continued to separate into different
lines, until the modern world was finally produced, with its
endless series of different forms, all having been derived from
common centers by descent with divergence.

CHAPTER XVIII

THE ORIGIN OF THE LIVING MACHINE; ADAPTATION;
THE FORCES OF ORGANIC EVOLUTION

ADAPTATION

Meaning of Adaptation. — One of the most striking facts of
the organic world, resulting from heredity and variation, is the
adaptation of animals and plants to their environment. By
this term is meant that the parts of each animal and plant are
so particularly fitted to the conditions of its life that it seems as
if they were intelligently fashioned with this end in view.

A few illustrations will make the matter a little clearer. The
tree, with its roots extending under ground, with its branches
growing into the air and bearing the broadly expanded leaf
surface for the purpose of absorbing air, is
evidently exactly adapted for its life in the
soil and in the air. The roots are mechani-
cally built so that they can push their way
through the soil; the stems are rigid enough
to support the heavy branches, and the
leaves are broad and thin and of exactly the
proper shape to absorb the largest amount of
air. The wing of a bird is an example of
adaptation; for its structure, its shape, the
lightness of its bones, its ability to expand its
feathers, the delicate manner in which the
parts of the feathers are attached to each
other, are all admirably adapted to an organ whose function
is to support the bird in the air. The bird's feet are a beauti-
ful instance of adaptation, since wading birds, swimming birds,
and scratching birds have feet plainly adapted to their peculiar

342

FIG. 140. — THE

PENGUIN, A BIRD
ADAPTED FOR LIFE
IN THE WATER

ORIGIN OF THE LIVING MACHINE: ADAPTATION 343

habits of life; Figs. 140 to 143. The white fur of the polar
bear is an adaptation to its life habits in the north on the ice
sheets; for not only does the heavy hair serve as a warm covering

FIG. 141. — THE FOOT OF A BIRD ADAPTED FOR WADING IN MUD

to protect the animal from the cold, but its color at a distance
is hardly to be distinguished from the white ice, and thus pro-
tects the bear from observation. The marvelous tongue of the

FIG. 142. — THE FOOT OF A BIRD FIG. 143. — THE FOOT OF A BIRD

ADAPTED FOR SCRATCHING ADAPTED FOR SWIMMING

butterfly is adapted for sucking the honey from flowers. The
honey in the flower is at the bottom of the long corolla, and
unless the butterfly had this long tongue to insert within the

344 BIOLOGY

corolla and thus reach the honey, it would not be able to utilize
this food. Each butterfly is provided with a tongue sufficiently
long to obtain the honey from the particular kind of flower
upon which it feeds. The marvelous structure of the human
hand, with its wonderful mobility, its delicate sensations, its
great power of muscle movement, is clearly adapted for use
as an organ of prehension, and one might believe, as has been
vigorously argued, that it was especially made by an intelli-
gent designer for the conditions of life in which man lives.

The principle of adaptation is found everywhere in nature, all
animals and plants being more or less adapted to their conditions
of life. Indeed, perhaps the most characteristic feature of or-
ganisms is that they are adapted to their environment, instead
of being purely haphazard in their shape and structure. In-
animate objects, like stones, have no special relation to their
environment, and having been produced by blind forces, are
not particularly adapted to any purpose. In contrast to this,
all animals and all plants show structure and functions which
fit them for their environment. We may almost regard this
feature of adaptation as the most universal and striking char-
acteristic of life.

Origin of Adaptation. — How came organisms to be thus
adapted to their environment? The explanation of adaptation
which was for a long time regarded as satisfactory, was that
each animal was made by an intelligent Creator, and exactly
fitted to the environment in which it was placed. This sug-
gestion was satisfactory so long as it was believed that each
species was an independent creation. Since, however, the idea
of special creation has been replaced by the belief that our
present species have been derived from older types by descent,
the problem of adaptation to their environment must be given
a different solution. If animals have diverged from common
centers, it follows that types now inhabiting different localities
must have originally come from the same place, and if they
were originally adapted to one locality, they could not be

ORIGIN OF THE LIVING MACHINE: ADAPTATION 345

especially adapted to the conditions of new localities. Hence
their adaptation to a new environment must have been ac-
quired during their growth, and not by an original special cre-
ation. The question of how the adaptation was produced,
therefore, comes up with redoubled force.

More careful study, however, shows that animals are not
always exactly adapted to their environment. The old idea
that each organism is especially fitted for its environment is not
borne out by facts. Of course living animals are always in a
measure adapted to the conditions in which they live, for if they
were not they would long since have been exterminated. Indeed,
the history of animals shows many instances where poorly
adapted animals have been crushed out of existence, leaving
alive only those adapted to their environment. On the other
hand, many instances are known where organisms living in
one part of the world to-day are not particularly adapted
to their habitat, but are really better adapted to other parts
£>f the world if they could only get into new regions. It not
infrequently happens that organisms from one country get
carried by accident to another, and find the new country far
better adapted to their life than their original home. For
example, when the European hare was carried to Australia, it
found conditions far better adapted to it than those of its original
home in Europe, and it multiplied with prodigious rapidity, be-
coming far more abundant in Australia than ever it was in
Europe. The English sparrow, when introduced from England,
finding America better adapted to its life than England, multi-
plied very rapidly, and spread over the country. Our fields in
the eastern states are filled with the so-called white daisy (Leu-
canthemum) . This is a European species which, when introduced
into this country, found conditions better adapted to its needs
than in its original home and became far more abundant here
than in its original home. These three illustrations show that
although animals certainly must be adapted to the conditions
in which they live or be exterminated, they are not particularly

346 BIOLOGY

made for those localities, since in many cases they are better
fitted for other localities than their own homes. The idea that
organisms were especially designed by creation to fit the con-
ditions in which they live is thus disproved.

Adaptation the Result of Growth. — The history of organisms
shows that adaptation to environment has not come suddenly,
but has been the result of slow development, brought about by
race divergence and evolution.

Adaptation in the life of the individual. — When the individual
starts its existence it is simply a fertilized egg. It is a cell, and
is not especially adapted to any particular condition of life.
In its development the cell divides into many cells, and these
cells assume different shapes and relations. As the organism
grows, the adaptation to the environment makes its appearance.
In plants, the roots soon assume a form which adapts them to
the soil, while the leaves become fitted for the air; in animals,
some cells adapt themselves to functions of digestion, others
to the functions of motion, etc. In other words, in the life of
the individual, adaptation is a matter of slow growth and comes
step by step as the egg is gradually molding itself, into the form
of the adult. Concealed in this fertilized egg are marvelous
powers which cause the egg to develop into an adult, and the
powers that cause the development of the egg cause also the
adaptation of the different parts to the conditions of life.

Adaptation in the race. — There is no doubt that a similar
history of growth has brought about the adaptation of the race
to environment. Probably the earliest type of the plant was a
single cell, adapted to life in the water but not in the soil. As
the ages passed on and plants reached the land, an adaptation
to this new environment slowly developed. The structures
which we find in animals and plants to-day, which adapt them
to their environment, were not of sudden origin in any case,
but were the result of a gradual change of the older forms into
newer types, more closely adapted to the new conditions of life.

As an example of such adaptation, may be mentioned the

ORIGIN OF THE LIVING MACHINE: ADAPTATION 347

development of the spinal column of the vertebrates- during
the geological ages, which is disclosed by the fossils in the
rocks. When the vertebrates first appeared, apparently they
had no bones, but in their backs was a rather stiff rod which
gave them rigidity, this being represented by the rod in the
embryo which we have already learned to speak of as the
notochord (see page 286) . Following along through the various
strata of rocks, which represent a progressive development of
vertebrates, we find that this rod in time became broken up
into short sections, a condition which adapted its possessor
very much better to an active life in the water. The short
sections, which became the vertebrae, enabled a lateral flex-
ing motion of the body which could not be brought about so
readily if there were only a stiff supporting rod in the back.
This broken series of bones, forming the vertebral column,
thus adapted the animal to its rapid motion in the water.
Later, when the vertebrates emerged from the water and as-
sumed a life on the land, the type of vertebras adapted to life
in the water was no longer fitted for the condition in which
the animal now lived. The vertebrae were still retained, but
they acquired new connections with each other, a greater solidity
and a greater rigidity, so that the spinal column could now
support the body in the air. Further development of the land
animals into the birds was characterized by a further change in
the form of the vertebrae, which adapted the animal to life in
the air, and, moreover, the vertebrae were changed in another
fashion in the mammals which lived on the land. In all of these
series of changes, from the original unbroken rod of the back in
paleozoic times, to the complicated spinal column of the mam-
mal, we see a successive series of adaptations. The study of fos-
sils has made it possible to trace this series of changes in detail,
and our paleontologists have quite accurately pictured for us
the succession of changes that has produced this long series of
race variations, bringing about an adaptation of the race, first
to one condition of life and then to another, and finally ending

348 BIOLOGY

in the excellently adapted internal skeleton which the higher
vertebrates possess to-day. All of this can be followed out in
the study of fossils, and it represents only one of the many
series of evolutionary changes which have occurred in the history
of animals, adapting the race little by little to new conditions,
or better adapting them to older ones.

Forces Producing Race Adaptation. — While biology has not
yet reached a point where it considers itself capable of explaining-
all of the marvelous phenomena of adaptation, some of the laws
that have been concerned in the production of the phenomena
are fairly well understood. A primary one seems to be the law
of natural selection, first exploited by Charles Darwin. This
law and its action will be considered on a later page.

THE THEORY OF EVOLUTION

The divergence of animals and plants from common centers
to produce the diversified world of to-day has been generally
known under the phrase, the theory of evolution, or the theory
of organic descent. The term "e volution" has a very much
wider application than that which has just been given to it,
since in its philosophical import it involves much more than the
problem of the origin of species of animals and plants. The
general theory of evolution includes the conception of the orderly
development of the whole universe, by a system of natural law
and force, and assumes that the origin of the world from the
original nebulous mass has been, from the beginning, due to
the unfolding of natural law. With the philosophical aspects
of the theory we are not here concerned; but the phase of the
theory that concerns the origin of modern animals and plants
is one of the fundamental factors of modern biological thought.
Indeed, it may be stated that modern biology did not have any
real existence until, under the influence of the writings of Charles
Darwin, the conception of the origin of species from common
types began to be studied.

The idea which has been expressed above, that the adaptation

ORIGIN OF THE LIVING MACHINE: ADAPTATION 349

of organisms to their environment has been a matter of growth,
is the result of the thought of the last half-century. Previous
to the middle of the last century it had been assumed that or-
ganisms transmitted their characters so accurately to their off-
spring that they had continued from the beginning unchanged,
and that species were immutable. The immutability of species
(Lat. im = not + mutabilis = changing) had been assumed as
the foundation stone of biological science, and all conceptions
of nature had been based upon the idea that organisms breed
strictly according to their type, without change, other than
slight fluctuations back and forth from a center, and without
permanent modification. The conception which we have as-
sumed above — that not only are all organisms constantly
undergoing individual variations, but that races are going
through a gradual series of permanent changes, resulting in the
appearance of new forms with successive ages — was quite
revolutionary in thought. The belief that species were not
immutable, but were constantly being transformed into new
species by the ordinary processes of descent, changed the whole
aspect of our attitude toward nature. During the fifty years
after this conception was presented to the world for discussion,
it was subjected to most hostile criticism and most bitter dis-
pute. The objections have now, however, mainly disappeared,
and it has become to-day one of the accepted doctrines of science
that species are constantly undergoing changes, and that our
present species have descended from older ones and will in turn
develop into others. To understand and appreciate this modern
conception, it is necessary to survey briefly the development of
the idea and the fundamental facts that lie underneath it. In
this review we will make reference only to that phase of the
great theory of evolution that has to do with the origin of modern
species, or to organic evolution, as it is commonly termed.

Early Views. — We can trace a beginning of the idea of evo-
lution back to the scientists and philosophers before Christ.
Aristotle, nearly four centuries before Christ, recognized in a

350 BIOLOGY

vague way the idea of a gradual succession of higher and higher
forms of existence; and several other early philosophers specu-
lated concerning the origin of living things upon the earth accord-
ing to general processes of development. But these earlier
ideas were soon lost sight of and it was not until the seventeenth
century that any more modern ideas of the development of
animals from each other were advanced. During all of these
centuries, and indeed until about the middle of the nineteenth
century, so far as the subject was thought of at all, the view
generally accepted was that each different kind of animal and
plant was an independent creation. This view crystallized into
the special creation theory in the writings of John Ray in 1725,
and became the generally accepted view of all scientists. Dur-
ing the seventeenth and eighteenth centuries, however, several
philosophers expressed, in their writings, ideas approximating
the belief that living things do not remain forever constant, but
are ever going through the series of changes that we have al-
ready described as race divergence. Among those whose writ-
ings tended in this direction may be mentioned Kant, Goethe,
Leibnitz, Erasmus Darwin, and others. With the beginning of
the nineteenth century these conceptions began to take a more
definite shape.

Lamarck. — Lamarck was a French naturalist, living in about
the beginning of the nineteenth century, and was well versed in
botany and zoology. He formulated a clearly defined doctrine
of descent, and was the first of the modern scientists who had
any conception of the theory of evolution. Lamarck believed
that the fossils found in the rocks were the ancestors of
animals living to-day, and that the organisms of the present
world have been derived by descent from those that lived in
previous years. The changes that had taken place in their
structure he believed to have been slow and gradual, but contin-
uous, and produced by a variety of causes which he specified,
and which have received the name of Lamarckian factors. The
chief of these causes were the following: —

ORIGIN OF THE LIVING MACHINE: ADAPTATION 351

1. The direct effect of the environment acting upon animals and
plants, modifying them, generation after generation.

2. New physical needs, necessitating new conditions of life;
these new conditions producing changes in the animals them-
selves.

3. Use and disuse. — It is a well-known fact that the use of
any organ causes it to increase, and the failure to use it causes
it to decrease in size and in efficiency. Lamarck supposed that
the arms of birds became wings through continued use in this
direction, and that the hind legs of snakes were lost because
they were not used. This has been the most universally recog-
nized of the Lamarckian factors.

4. The transmission of these acquired characters to posterity.
Lamarck assumed, as everyone else assumed in his day, that
any characteristics possessed by an animal or a plant might be
transferred to its offspring. Hence any of the changes produced
by the environment, by new physical needs, or by use and disuse,
would be transmitted to the offspring, and, therefore, the next
generation would have the body modified by the habit and
environment in which the first generation lived. This would
result in a constant modification of organisms, producing evolu-
tion.

There were certain other factors in Lamarck's conception
which, though really part of the original theory, are not com-
monly included under the term of Lamarckian factors. One
of these was cross breeding, i. e., breeding together of individuals
of different varieties, or perhaps even of different species, the
result being an offspring different from either parent. A second
was isolation, a suggestion that certain individuals became
separated from the rest, and they and their offspring, being
obliged to breed together, produced types in an isolated locality,
which developed along lines different from those taken by other
members of the same species in other parts of the world.

Although these Lamarckian factors are several in number,
it will be seen that there is one common phase. In all it is

352 BIOLOGY

assumed that diversities produced in individuals as the result
of the action of the environment, or of their own habits, i. e.,
acquired variations, are transmitted to subsequent generations,
and serve as the basis of the changes which produce race varia-
tions and evolution. Our study of heredity has shown that such
variations, according to our present knowledge, are almost
certainly not transmitted to subsequent generations. It is
evident that the very foundation of the Lamarckian theory can-
not stand, if the modern conception of heredity is accepted.

Lamarck's views were not accepted in his day. This was
partly because the great French naturalist, Cuvier, one of the
greatest naturalists that ever lived, opposed them strongly;
and partly because the scientific world was not at that time
ready to accept any such natural explanation of the origin of
organisms as that suggested by Lamarck. They were, therefore,
practically forgotten for a period of fifty years, during which
time the idea that organisms had appeared by the process of
descent had practically no followers, special creation of each
species to fit its environment being the generally accepted
view. A new era of thought was inaugurated in the middle of
the nineteenth century by Chalmers, Spencer, and especially
in 1859, by Charles Darwin.

Charles Darwin. — Charles Darwin was the grandson of
Erasmus Darwin, already mentioned. In 1859 he published a
book, the result of twenty years' work, entitled "The Origin
of Species," which produced a revolution in thought, not only
in science but also in philosophy. Darwin accepted the idea of
the origin of modern organisms from earlier ones by a process
of direct descent, recognizing that divergence of type from com-
mon centers has been the law of historical development of ani-
mals and plants. To this extent, therefore, Darwin followed
Lamarck and the early speculators concerning the origin of
animals. Darwin's method of explaining this descent was
totally different from that of Lamarck, and much more in ac-
cordance with facts that could be demonstrated. According to

ORIGIN OF THE LIVING MACHINE: ADAPTATION 353

Darwin, the method by which new forms were produced was
by the law of natural selection. Very briefly stated, that law
is as follows : —

1. Overproduction. — All animals and plants tend to multiply
more rapidly than it is possible for them to continue to exist.
More offspring are produced by even the slowest breeding ani-
mals and plants than can possibly find sustenance in the world.

2. Struggle for existence. — As the result of overproduction,
the individuals that are born are engaged in a constant struggle
with each other for the opportunity to live. This struggle is
sometimes an active and sometimes a passive one; and sometimes
it is a struggle with each other for food. It is a struggle in which
only the victors remain alive, the vanquished being exterminated
without living long enough to leave offspring.

3. Variation, or diversity. — All animals and plants show a
large amount of diversity among themselves, and, as a result,
some must be better fitted for the struggle for life than others.

4. Natural selection, or the survival of the fittest. — It is a logical
result of the struggle for existence that only those individuals
best fitted for the struggle will be the ones, in the long run, to
win in the contest. Hence the "fittest" in the long run will
survive, while those less fitted to exist will be exterminated
in the merciless struggle for existence.

5. Heredity. — By the law of heredity, individuals transmit
to their offspring their own characters. Hence if one individual
survives the struggle for existence by virtue of some special
characteristic, it will transmit this characteristic to its offspring.
The offspring will inherit it, and in the course of a few genera-
tions the only individuals left alive will be those that have
developed the favorable characteristic in question, while those
that did not develop it will be exterminated by the law of
natural selection.

As the result of these five factors working together, Darwin
supposed that there would be a constant accumulation of favor-
able characters, each generation being to a slight extent an

354 BIOLOGY

advance over the last. The struggle for existence and the sur-
vival of the fittest are repeated generation after generation, and
in each successive generation the only members to survive will
be those with qualities that make them better able to contend
in the struggle for existence than their rivals. Hence every
individual character which gives its possessor any slight advan-
tage over its rival will be sufficient to enable its possessors to
survive the struggle for existence, by bringing about the exter-
mination of the less fortunate individuals that did not have the
favorable character in question. This character will be trans-
mitted to subsequent generations, when the struggle will be
repeated again, and once more the best characters of the
next generation will be selected. As this goes on without
cessation age after age, there will be a constant accumula-
tion of favorable characters, and thus the race will in general
constantly advance.

Natural Selection and Adaptation. — This law of natural
selection is especially well fitted to explain the marvelous adap-
tations of organisms to their environment. Since the different
members of any species of animals or plants are not alike, it
will follow that at any period in the history of a race, some indi-
viduals will be more closely adapted to their environment than
others. Since there is always an overproduction of individuals,
so that many more are born than can live, it will follow that the
individuals best adapted to their environment will be the ones
that will survive, while those less adapted to the conditions of
life will be the ones to be exterminated in the struggle for exist-
ence. Hence it will follow that at the close of any generation
the individuals left alive will be those that have the most favor-
able adaptation to environment. These will necessarily be the
parents of the following generations, and, by the law of heredity,
the next generation will inherit the characteristics of these
parents and will be, on the average, a little better adapted to the
environment than the last generation. If this process is repeated
generation after generation, it will follow that each generation

ORIGIN OF THE LIVING MACHINE: ADAPTATION 355

will be slightly better adapted than the last. By an accumula-
tion of the improvements which thus appear accidentally, there
will be developed, as the generations pass, a closer and closer
adaptation to conditions. The final result is a better adaptation
to conditions, and a gradual change of type and production of
new species.

Acquired and Congenital Characters Affecting Natural
Selection. — In the form stated above, and as at first conceived
by Darwin, the characters which are chosen by natural selection,
and upon which the advance of the race is based, might be either
acquired characters, such as those upon which Lamarck based
his theory, or they might be congenital characters, which are in
the germ plasm and essentially due to variation in the heredi-
tary substance. Darwin did not sharply separate these two
types of variation, although he recognized them both. Dar-
win thought that the advancement of type was produced prima-
rily by the natural selection of such characters as were born with
the individual, i. e., congenital characters. He also believed
that, to a certain extent, acquired characters, which were pro-
duced in the animal either by the direct effect of the environ-
ment or by use or disuse, could be transmitted and might thus
affect posterity and have an influence in changing the type.
Darwin did not believe, as did Lamarck, that these acquired
characters were the primary factors in producing divergence of
type, but thought they might be secondary ones, the primary
factor being the selection of most favorable congenital varia-
tions.

Weismann. — The discussion of Darwin's theories continued
vigorously for a quarter of a century, until his views of descent
were quite generally accepted, although with various opinions
as to the efficiency of his law of natural selection. In 1884
appeared the essay L* Weismann "On Heredity," which put a
totally new aspect on the whole problem. His theory of hered-
ity, already described, was so simple, and so readily obtained
confirmation by direct observation, that it soon acquired almost

356 BIOLOGY

universal acceptance. With the acceptance of Weismann's
theory, it was no longer possible to look upon acquired
characters as transmitted to posterity. As a result, the
Lamarckian factors were of necessity thrown overboard, since
they all involved the inheritance of acquired characters. It was
no longer possible to believe that the direct effect of the environ-
ment upon the individual, or the effect of the disuse of organs,
could have any influence upon posterity; and as rapidly as
Weismann's theory of heredity received acceptance the so-called
Lamarckian factors were discarded, until to-day they are not
generally regarded as factors in producing race variation. The
adherents of Weismann have thought that the only possible
factor left to produce evolution was the natural selection of the
congenital variation. Congenital variations, since they are due
to variations in the germ plasm, will be transmitted; and the
natural selection of these congenital variations will remain as
the great factor in the development of type. Indeed, the fol-
lowers of Weismann took this extreme view and held, and still
hold, that the only factor which has produced race evolution
has been the natural selection of those characters which start
as variations in the germ substance. But the dispute between
the followers of Lamarck's older views and Weismann's new
views has never yet been positively settled. Some naturalists
accept Weismann's views in toto; others have not regarded them
as sufficiently well demonstrated; while quite a number of prom-
inent biologists, including Spencer, Packard, Cope, and others,
have held to a modern form of Lamarck's views, believing that
in some way, and under some circumstances, acquired characters
might have influence upon the offspring and therefore might
direct the line of race divergence. The question has not been
definitely settled; but at the present time the balance of evidence
seems to be against believing that acquired characters are trans-
mitted, and therefore against the retention of any of the so-called
Lamarckian factors, that are based upon the direct action of the
environment upon the individual.

ORIGIN OF THE LIVING MACHINE: ADAPTATION 357

The Mutation Theory. — One of the essential factors of the
Darwinian theory was that the change of species was produced
by the selection of minute diversities, such as the slight differ-
ences found among animals and plants of the same species. It
was argued by Darwin that in the struggle for existence, when
the majority must be exterminated that the few may live, even
the slightest differences in structure, shape, body, color, or habits
would be sufficient to determine the question of life or death.
If these slight differences could accumulate, generation after
generation, they would in time become great; and thus, accord-
ing to Darwin, the great differences between type were produced
by the accumulation and heaping up of minute variations. To
many of the more recent students of this subject it has not
seemed plausible that such minute differences could accomplish
all that Darwin claimed for them. Many objections to Darwin's
ideas on this line have been expressed, and have finally found
voice in a more recent conception of the conditions which have
produced the evolution of the living world. This new idea is the
mutation theory (Lat. mutare = to change), and is commonly
associated with the Dutch naturalist, DeVries, although a num-
ber of others have shared in its origin and development. DeVries
based his views upon observations made in a field of primroses,
where he kept thousands of individuals under observation.
As the result of these observations, he came to the conclusion
that new types of plants are appearing constantly in nature;
but that they do not arise, as Darwin had supposed, by the accu-
mulation of little changes one generation after another, but
suddenly, and, as a rule, in single steps. In his field of primroses,
growing side by side, he found several distinct types, abso-
lutely different from each other and with no intermediate
steps between them. They came, not as the result of the ac-
cumulation of little steps, but suddenly, in a single generation.
Moreover, by isolating and experimenting with them, he found
that the new characters, which had thus appeared, bred true,
i. e.y remained fixed m the race.

BIOLOGY

lack

From this series of observations, extended in other directions
by many other observers, has been developed the theory of
mutation. This theory is, in essence, that new characters do
not, as a rule, appear simply as slight diversities found between
different individuals of the same species, but as characters of
considerable extent at a single birth. New features of the race
are thus sudden in their origin instead of gradual, as had been
supposed by Darwin and also by Lamarck. According to this
theory there are two types of variation among organisms: 1.
Individual variations, spoken of above as the diversities which

are shown between
different individuals,
and which come and
go in a haphazard
fashion, having no
part to play in the
change of the race.
These may be ac-
quired characters; at
all events they are
not impressed upon
the germ plasm. 2.
Mutations, which
probably start with the germ plasm; Fig. 144. These varia-
tions may be large or small, but whenever they appear they
are at once fixed in the race. Inasmuch as they are part of
the germ substance, they will be handed on to the next gener-
ation and remain, therefore, as a permanent inheritance of the
race. According to the mutation theory, these sudden large
changes have brought about the race divergence. The theory
of mutation, therefore, abandons Darwin's idea of the accumu-
lation of the minute diversities, and replaces it with the idea
that the steps in evolution may be larger and may be taken sud-
denly. It is, of course, evident that this new conception of muta-
tion is perfectly consistent with Weismann's view of heredity.

Red

BY THE

FIG. 144. — MUTATIONS SHOWN
BEETLE LEPTINOTARSA

A and C are mutants from the original form B. The
actual differences are greater than appears in these fig-
ures because of great differences in color.
(Tower.)

ORIGIN OF THE LIVING MACHINE: ADAPTATION 359

Mendel's Law. — Accompanying the development of the the-
ory of mutation, there has been brought prominently to view a
somewhat new view of the laws of heredity, perfectly consistent
with Weismann's theory, but explaining its method of action.
Darwin in his discussion assumed that the offspring of two
parents, since it could not be like both, would, in general, be
halfway between the two. Even the slightest familiarity with
the laws of heredity is enough to show that organisms inherit
from both parents, and it has generally been assumed that they
inherit, or may inherit, equally from both. It is, however,
manifestly untrue that the offspring is always midway between
its father and mother, inheriting equally characters from each.
The laws of heredity are much more complex than this, for it
frequently appears that an organism inherits mostly from one
parent, the characteristics of the other hardly- reappearing in
the offspring. An attempt to bring some of these facts into a
general law has resulted in what is called Mendel's law of
heredity. Mendel published the result of his work originally
in 1866, but it attracted no special attention for nearly forty
years, when it was revived by modern students in 1900. Since
that time it has been subjected to extensive experiment, and
has produced results of very great practical value in controlling
and directing breeding experiments with animals and plants.
Mendel's law is somewhat complex and difficult to understand,
but the essential features of it -are as follows : —

Unit characters. — It is an assumption of Mendel's law that,
in many cases at least, different characters of animals are unit
characters. By this term is meant that those characteristics
are handed to the offspring as single units, which are inherited
by the offspring in toto or not inherited at all. They cannot be
halved or reduced in total characteristics. In other words, if
the offspring inherits one of these unit characters, it inherits it in
full. Even though the offspring should come from two parents,
one of whom possessed the character in question, while the
other did not, the offspring would either inherit it as a whole

360 BIOLOGY

or not at all. For example, two varieties of peas are known, one
of which has short pods and the other long pods. If they are
crossed the offspring are either short-podded or long-podded,
but not midway between the two. Very many other characters
have been tested out experimentally and found in the same way
to be inherited as unit characters.

Dominant and recessive characters. — Mendel's law further
points out that some of these unit characters are much more
likely to reappear in the offspring than others. It frequently
happens that of two opposite characters, one is much more likely
to appear in the next generation than the other. Those that
are most likely to reappear are called, in this terminology,
dominant (Lat. dominari = to rule), while other characters that
are more likely to disappear in the first generation, are called
recessive (Lat. recessus = receding). These recessive characters,
even though they do not appear in the first generation of off-
spring, are not necessarily lost. The offspring may contain
within its body the germs of these characters, but they may
remain dormant, not appearing at all in the first generation.
In subsequent generations these recessive characters may re-
appear; thus recessive characters, which are present in one gen-
eration, may disappear in a following generation, to reappear
subsequently in the later generations.

Law of inheritance. — The specially valuable contribution of
Mendelism is the formulation of a law in accordance with which
these dominant and recessive characters reappear in subsequent
generations. That law is briefly as follows : When we cross with
each other two individuals, one of which has a dominant char-
acter, while the other has its opposite as a recessive character,
all of the offspring in the first generation show the dominant
characteristics. But although showing only the dominant
characters they actually contain a mixture of dominant and
recessive characters. This is shown by the fact that if these
individuals now are bred together, in the next generation, which
we will call the second generation, only three-fourths of the off-

ORIGIN OF THE LIVING MACHINE: ADAPTATION 361

spring will show the dominant character, while one-fourth
will show the recessive character. If now the individuals show-
ing this recessive character are bred with each other, all their
offspring will show the recessive character, the dominant char-
acter having totally disappeared from them, never to occur
again in any subsequent generation. This race is then a pure
recessive type, from which all of the dominant characteristics
have been eliminated. All of the other three-fourths of the
second generation show the dominant character only. But
tests, similar to the above, prove that only one of these is
purely dominant. The other two-fourths, although in them
the dominant character only is evident, are really mixed, con-
taining both dominant and recessive characters. This is shown
by the fact that if they are crossed, three-fourths of their off-
spring will again show the dominant character and one-fourth
will show the recessive character. This process may then be re-
peated indefinitely.

An illustration may make this clearer. Among mice the color
gray is dominant, while the color white is recessive. If white
and gray mice are bred together, the first generation of offspring
will be all gray. If these gray animals are now bred together,
in the second generation three-fourths of the offspring will be
gray but one fourth will be white. If these white animals are
bred together, their offspring will all be white and will continue
to breed white offspring indefinitely, no gray mice ever subse-
quently appearing in their progeny. They constitute a pure
white race. If the other three-fourths, which are gray, are bred
together, one of the three-fourths will continue indefinitely to
produce gray offspring, no white ones appearing. In these the
white characteristic has been eliminated entirely, and they form
a pure gray race. But the other two-fourths, when bred to-
gether, prove to contain both white and gray characters, and
among their offspring one-fourth will show the white fur and the
other three-fourths the gray fur. If again tested in the same
way, the white animals will be found to produce pure races of

362 BIOLOGY

white with no mixture of gray fur; one of the other fourths will
be found to be pure gray races with no mixture of white, and
the other two-fourths will again prove to be a mixed race con-
taining both white and gray characters. This process may
then go on indefinitely.

The further details of this law are too complicated to be fol-
lowed out in this place, but from the law it is possible to calcu-
late approximately how many of the offspring at each generation
will show recessive, and how many dominant characters. This
law has been of great value in directing breeding experiments,
and breeders who are trying to produce new varieties of animals
and plants find the law extremely useful in controlling their ex-
I HT iments toward definite ends. Mendel's law has thus shown that
the inheritance by the offspring of the characters of the parents
is not a pure matter of chance, but is controlled by definite
laws. While we do not yet fully understand these laws, the fact
that some of them have been discovered gives promise that we
may, in time, be able to control the process of inheritance far
more accurately than hitherto.

It is not believed by those who have worked on Mendel's
law that all characteristics of organisms are thus unit charac-
ters and are transmitted in toto or not at all. Some characters
appear to blend, as for example the cross between the white
race and the negro, the offspring of such crossing being neither
white nor black but mulattoes, a mixture midway between the
parents. Hence the color of the human skin is probably not
like the white and gray color in mice, a character transmitted
by the law of Mendel. This law of Mendel has, however, been
a great contribution to science in showing that large numbers
of characters or organisms are unit characters, and are trans-
mitted according to definite laws that may be clearly formu-
lated.

We may say, in concluding the general subject, that modern
biological science recognizes the principle that race divergence
has been the law of life, and that the evolution of modern types

ORIGIN OF THE LIVING MACHINE: ADAPTATION 363

from earlier ones by descent has been the method by which the
present world was produced. Further, the laws formulated by
Darwin, DeVries, and Mendel, together with Weismann's theory
of heredity, all fit together to explain the method of this evolu-
tion. New variations have appeared suddenly, at least in many
cases, as germ variations (mutations) , and then have been trans-
mitted to the offspring as unit characters by Mendel's law, some
of the offspring receiving the new characters, while others do not;
but if inherited they are inherited as unit characters. Next,
the law of natural selection comes in and selects those indi-
viduals which have received useful mutations. Selection then
"fixes them" in the race by eliminating individuals with char-
acters less useful than those possessed by the survivors. As a
result of all these factors working together the race advances.

CHAPTER XIX
CLASSIFICATION AND DISTRIBUTION

EVEN the slightest familiarity with organisms will disclose
striking similarities between some forms and great differences
between others. The frog is clearly quite like the lizard and
much like other vertebrates, but very unlike the earthworm.
These points of likeness are the basis upon which organisms are
classified.

mt tar

FIG. 145. — THE SKELETON OP A RABBIT

c, carpals;
/, fibula ;
fe, femur;
h, humerus;

il, ilium;
is, ischium;
me, metacarpals;
mt, metatarsals;

pu, pubis;
tar, tarsala;
r, radius;
sc, scapula;

at, sternum;
t, tibia;
u, ulna.

Homology. — The likeness between organisms is of two general
types. The first is likeness in structure, which is called homology
(Gr. homos = like + logos = ratio) . It is frequently found that

364

CLASSIFICATION AND DISTRIBUTION

365

animals which appear quite unlike are really built upon the same
plan of structure, differing only in the manner that the plan is
carried out. For example, the frog possesses a spinal column
made of vertebrae, and two pairs of legs attached to the body by
girdles, each containing a certain number of bones. The rabbit
(Fig. 145) has a skeleton based upon the same type. It also
possesses a spinal column made of vertebra, with two pairs of
appendages attached by girdles to the axis of the body; and each
appendage is made up of bones which can be compared, bone
by bone, with those in the appendages of the frog. If Figure 145
is compared with Figure 88, this similarity can be seen and fol-
lowed out in very close detail, nearly all of the bones of the frog
being represented in the skeleton of the rabbit. This similarity
is found in spite of the fact that the two animals are so unlike
in general appearance and in habits. One lives in the water and
uses its legs for swimming and hopping; the other lives on the

land, using its legs for

support. But although

.£, ,.«.

built for different pur-
poses, the skeleton of
these two animals is
evidently based upon
the same plan. Figure
146 shows another ex-
ample of similarity in
structure representing
the hand of man, C,
and the corresponding
fore foot of four other
animals. Although the
hand of man is used for a totally different purpose from that
of the fore legs of the horse, the ox, or any of the other animals
represented, it is evident that they are built upon the same plan
of structure. In each there are a radius and ulna, and a series of
wrist and finger bones. There are differences, it is true: while

The skeleton of the hand of man, C; and the fore feet
of a horse, A; a rhinoceros, B; a pig, D; and an ox,E.
r, radius ; u, ulna. The other bones are not named but
may be easily compared.

366

BIOLOGY

man has five fingers represented, the others have lost some of
these fingers, and one of them, the horse, A, has left but a single

finger with the rudiments of
two others. That these other
fingers have been lost has been
proved by the study of the
paleontology of these animals;
for by tracing back their his-
tory through fossils it is found
that the ancestors of the horse
had at first five fingers, with a
type of hand similar to that of
man; later they had three and
finally only one finger. This
similarity in the structure be-
tween the frog and the higher
animals is shown in other parts
of the body besides the bone.
Figure 147 represents a section
through the body of a cat, giv-
ing a diagrammatic represen-
tation of the relation of the
organs in the upper part of
the body. This can be com-
pared directly with the anat-
omy of the same region of the
body of the frog, and while
there are many differences in
detail, the general structure is
evidently the same. The spi-
nal cord with the brain is found
on the dorsal surface of both
animals; the mouth, nostrils,
larynx, lungs, and oesophagus are, in essential features, iden-
tical. Thus it is evident from these comparisons that the

FlG. 147. — A MEDIAN VERTICAL
SECTION OF A CAT, SHOWING
DIAGRAMMATICALLY THE RELA-
TIONS OF THE ORGANS

cb, cerebellum ;
cr, cerebrum ;
hy, hyoid;
Ix, larynx;
oe, oesophagus;

sp, spinal cord;
st, sternum;
th, thyroid gland :
tr, trachea.

CLASSIFICATION AND DISTRIBUTION

367

frog, the cat, and the rabbit are built upon the same general
plan, which is carried out in different ways in different cases.
In other words, we have
in these animals a like-
ness of structure quite
independent of differen-
ces in the general pur-
poses for which the vari-
ous parts of the body are
used. A comparison of
Figure 97 with Figure
148, representing the
eyes, respectively, of the
frog and of man, will
show that this similarity
is carried out in the de-
tails of structure, even
of the smaller parts. Al-
though differing in some
minor points, it will be
easy to trace in the eye
of the frog the same parts that are present in the human eye.
It is perfectly clear that these two organs are based upon the
same plan and are identically planned structures.

Such similarities in structure are not by any means confined
to animals with a bony skeleton, but may be found among all
groups of animals. Figure 149 represents a worm, which, by
comparison with the figures of the earthworm in Chapter VIII,
shows a similar structure in spite of differences in detail. The
earthworm bears at first sight little resemblance to the worm
shown in Figure 149, the latter having external tentacles and
gills, neither of which is found in the earthworm. But it will
be seen that both are made up of a series of similar segments,
and that in general shape they are the same. If their internal
anatomy is compared, both are found to have a similar alimen-

FlG. 148. A VERTICAL SECTION OF

THE HUMAN EYE

0, aqueous humor; m, muscle;
ch, choroid; r, retina;
co, cornea; sc, sclerotic;

1, crystalline lens; si, suspensory ligament;

v, vitreous humor.

368

BIOLOGY

tary canal, similar circulatory, nervous, and excretory systems,
and all other parts of their anatomy are essentially alike. Such

a likeness in struc-
ture is sometimes
found in quite unex-
pected places. One
would hardly expect,
for example, that the
arm of man, the fore
leg of a horse, the
wing of a bird, the
fore leg of a frog
and the fin of a fish
would be identical
structures, since they
FIG. 149.— A SEGMENTED WORM RELATED TO viuysomuchinshape

THE EARTHWORM, BUT HAVING TENTACLES and function; but

AND GILLS they are all found to

be homologous.

Analogy. — A second type of likeness is. similarity in function,
irrespective of structure. It not infrequently happens that dif-
ferent animals develop organs of similar functions but of
totally different structure. In this case they are said to be
analogous (Gr. ana = according to + logos = ratio) but not
homologous. For example, the butterfly and the bird have
both developed wings for flying, and their wings are hence analo-
gous. They are of similar shape and are used mucb in the same
way; but the wing of the bird is made of bones, muscles, nerves,
and feathers, while the wing of the butterfly has none of these
parts, being simply an outgrowth of the skin containing air
tubes. It is not homologous with the bird's wing, in spite of
similarity in shape and function. The wing of the bird is, how-
ever, both analogous and homologous with the wing of the bat,
since both are used for similar purposes and both are made of
similar bones and muscles, nerves and blood vessels. As another

CLASSIFICATION AND DISTRIBUTION 369

example of analogous organs, may be mentioned the teeth in
the mouth of vertebrates and the peculiar teeth found inside
the stomach of the lobster. These organs are both used for
grinding food; but they are not homologous organs, since their
structure is so different. The teeth are bony organs arising
from the bones of the skull, which are themselves developed from
the mesoderm of the embryo; the teeth of the lobster are of
horny texture, and are developed from the ectoderm of the em-
bryo which is folded inward to line the stomach. Numerous
other examples of analogous organs might be given, for it
frequently happens that different animals use for the same pur-
pose organs that have quite a different origin and structure.

Explanation of Homology and Analogy. — Analogous organs
sometimes show much similarity, as in the shape of the wings
of the bird and butterfly, and sometimes very little. When
they do show a likeness it is explained by the fact that similar
necessities of life have forced the development of similar struc-
ture. For example, both the vertebrates and the lobster are
obliged to masticate their food, and both have consequently
developed hard cutting and grinding surfaces for the purpose.
There is, therefore, some similarity in the form of the organs;
but there is no necessity for similarity in structure, and in the
two cases different parts of the body have been utilized for the
purpose.

The likeness between homologous organs, however, requires
a very different explanation, because here we find a similarity
in structure in spite of differences in function. We cannot explain
the similarity in structure by any similarity of conditions.
Although the wing of the bird and the arm of man are adapted
to wholly different functions and have developed different shapes
and motions, they are, in spite of this difference, formed upon the
same plan, with an identical structure. The explanation must
be something more fundamental than mere similarity in use.
Naturalists to-day account for likeness in homologous organs by
the theory of descent, saying that two animals with homologous

370 BIOLOGY

organs owe their likeness to the fact that they have descended
from a common ancestor possessing such an organ. The bird,
the dog, and the monkey show homology in the wing, fore leg,
and arm, because they have descended from a common ancestor,
whose fore appendage possessed a certain series of bones and
muscles, and, therefore, all its descendants have, by inheritance,
retained these same bones and muscles. The differences between
the members in question have been brought about by the fact
that they were used for different purposes, and thus were slowly
modified in shape, although they still retained a fundamental
likeness in structure.

CLASSIFICATION (TAXONOMY)

Individuals. — As we look upon nature to-day, we find only
individual organisms, each isolated from all others, and allied
only with its parents. But the most superficial examination
shows that some individuals have resemblances to each other,
while others are very unlike; and it is evident that organisms
can be arranged in groups showing more or less likeness to one
another. Such a grouping is called classification. The general
plan of such classification into groups is as follows: —

Species. — When we find a large number of individuals re-
sembling one another so closely as to be practically identical,
we speak of them as belonging to a single species. For example,
the common dandelion, which is widely distributed over the
world, is made up of countless numbers of individuals; but they
are essentially alike, in root, in stem, in leaf, in flower. We
therefore speak of them all as constituting a single species, So
too is the horse a distinct species, and the ass another. To give
a definition of just what is meant by species is impossible, since
no one knows just what is meant, and the word perhaps does
not always have the same meaning. That the individuals of
a species are not always exactly alike is evident from facts
already mentioned concerning the great variations among dif-
ferent pigeons and dogs. Such great variations as those pre-

CLASSIFICATION AND DISTRIBUTION 371

viously mentioned among dogs are very exceptional, for as a
rule the members of the same species are closely alike.

Just what biologists mean by species, and just what line they
would draw to separate two species from each other, cannot be
stated. It is quite impossible to say how unlike two animals
must be to constitute two species, since sometimes, as with
pigeons, members of the same species may be very unlike,
while in other cases, as with sparrows, animals belonging to
different species are very closely similar. It has been quite
common to regard all animals that can breed together and pro-
duce fertile offspring, as belonging to the same species. But
this is not an accurate definition of the term, for there are many
animals, so different from each other that they certainly deserve
to be ranked as different species, but which can breed together.
Nor can we get any idea as to the meaning of the term "species"
by studying the number of similar individuals. Some species are
composed of an immense number of individuals, as in the case of
the dandelion; while other species comprise very few animals,
sometimes only one or two having been found. Sometimes, too,
the organisms belonging to the same species show a number of
sub-groups, and the biologist calls them sub-species, or varieties.
All of these facts show that no naturalist can at the present time
exactly define the term "species," or state definitely how species
may be separated from each other. When we recognize that
new types are constantly arising from old ones by the process
of divergence, it will be seen that we could not always expect
to draw sharp lines separating the new and the old types that
have arisen from a common center. But although naturalists
are not able to define the term accurately, or separate the species
strictly from each other, species are always recognized and form
the starting point for classification.

Genera. — A little study shows at once that some species have
a much greater resemblance to each other than they do to others.
For example, naturalists recognize the domestic cat as constitut-
ing one species, and the wild cat as a second. But it is quite

372 BIOLOGY

clear that the wild cat and domestic cat show greater resemblances
to each other than they do to tigers, dogs, or wolves. Moreover,
it is evident to anyone in the slightest degree familiar with ani-
mals, that lions, tigers, leopards, wild cats, and domestic cats,
although unlike each other, and recognized by naturalists as
belonging to different species, have many points of resemblance
to each other. They have the same general stealthy habits,
the same kind of toes and feet, and they are much more closely
allied to each other than any one of them is allied to the
dog or the wolf. Naturalists, therefore, group all of these
species together under one group which they call a genus (pi.
genera).

In naming any species, two names are commonly used, the
first of which is the name of the genus, the second that of the
species. For example, Felis is the name given to the whole genus
of cats. Felis domestica is the domestic cat; Felis leo is the
lion; Felis bengalis, the bengal tiger; Felis canadensis, the Ca-
nadian lynx, etc. So, too, Viola is the genus name of all the
violets; Viola blanda, of the white violet; Viola cucullata, of
the common blue violet, etc. If the species should happen to
have more than one variety or sub-species, a third name may
sometimes be added to indicate the particular variety of the
species. As a rule, however, two names only are used.

Families. — Extending observation a little farther, it becomes
evident that many genera show close resemblances which mark
them off distinctly from other animals. As a result, naturalists
group genera together into a larger group, which they call a
family. A family sometimes may contain only a single genus;
it may contain two or three or a large number of genera.

Orders. — In the same way, families are grouped together to
form larger groups, which are called orders. For example, the
various cats already considered have certain points in common
with the dogs, wolves, bears, seals, and walruses. In all of these
cases the teeth are especially adapted for cutting flesh, and the
animals are flesh eaters. There are very many genera among

CLASSIFICATION AND DISTRIBUTION 373

them, and a number of different families; but all agree in the
living upon flesh, and all show certain points of likeness in the
structure of the feet and the skeleton, which place them in a
group by themselves, distinct from animals that live upon
vegetable foods. All of these flesh-eating animals are, therefore,
grouped together into an order called the Carnivora.

Classes. — In a similar way, different orders can be arranged
in still larger groups. For example, although there are many
points of difference between the carnivorous cat, the herbivorous
buffalo, the gnawing rabbit, the flying bat, and the gigantic
marine whale, still they all agree in one fundamental character.
In all of these orders the females have mammary glands and
nourish their young by means of milk, a characteristic which
is totally lacking in fishes, reptiles, and birds. It is evident,
therefore, that all of these milk-producing animals may prop-
erly be classed together under one head. Such a group we then
know as a class; in this particular case we name them the
Mammalia.

Phyla. — Extending our observations still farther, we find
that all of the animals mentioned, together with fishes, reptiles,
amphibia, and birds-, resemble each other in having bones, which
none of the rest of the animal kingdom possesses. The insects,
clams, etc., never have bones, but have other characteristics of
their own. It is evident, therefore, that all animals possessing
bones may be grouped together as distinct from other types.
This produces a group that we know as a phylum or sub-
kingdom. In this particular case we name the phylum the
Vertebrata.

Kingdoms. — Now if we sweep our glance over the whole
organic world, we find that it is divided into two groups, the
animals and the plants. These large groups we call the animal
kingdom and the vegetable kingdom.

Thus it is seen that the whole organic world is divided into
kingdoms, phyla, classes, orders, families, genera, and species.
Occasionally we recognize intermediate groups; for instance,

374 BIOLOGY

between the family and the genera there are sometimes recog-
nized what we call sub-families, between the classes and the
orders we find sub-classes, etc.

THE SIGNIFICANCE OF CLASSIFICATION

Why should there be a classification? — As soon as we
recognize the principle of divergence from type it becomes evi-
dent that the classification of animals has a meaning. < 'l:i>sifi-
cation means history, and if we could get a perfect classification
we should have pictured the history of organisms. The first
step in the development of the organisms of the world was the
divergence of animals and plants from one another, thus form-
ing the two kingdoms of plants and animals. Then the process
was repeated in each kingdom, where there appeared a still
further divergence, a number of different lines of descent start-
ing from common centers, giving rise to the various sub-king-
doms. Again each of these broke up into other lines of descent,
and the smaller groups thus made their appearance. Thus
types continued breaking up and branching out in various
directions, giving rise to a classification which may be compared
to a tree, the trunk being the original type of organisms, the vari-
ous large branches representing the first lines of divergence from
the original stock, while the numerous subordinate branches
represent the successive types that appeared, by the same gen-
eral law. The minute twigs at the end of the branches are the
species of to-day, and they are aH connected by this line of
descent with the original trunk.

The classification of animals is the attempt to reconstruct
this treelike arrangement of organisms according to their histori-
cal relationship. The members of the same species are supposed
to have had a common ancestor in a fairly recent period; the
different species of the same genus had a common ancestor a
little farther back in history; the different genera of the same
family had a still earlier common ancestor; the families of the
same order had their connecting point farther back still, and so

CLASSIFICATION AND DISTRIBUTION 375

on through the whole series, until we get back to the common
starting point, or the common center from which all animals and
plants diverge. Classification is thus an expression of history.
The following is an outline of the classification of animals and
plants. The classification accepted by science is ever under-
going changes, as a more complete knowledge of relations is
obtained, and the classification accepted to-day is different in
many respects from that adopted a generation ago. In turn,
the classification used to-day will doubtless be modified by
future study, until it becomes practically perfect. But even
though we recognize that it is not yet perfect, it is quite necessary
to have such a classification in order to understand the living
world. It must not be inferred that our present classification
represents an accurate history of organisms. The classification
that biologists are aiming at is a genetic one, i. e., one that repre-
sents actual relationships, and to a considerable extent the classi-
fication outlined below does represent such relationships. But
the difficulties of determining the actual history of organisms
have been so great as to seem in some respects almost insur-
mountable. The classification of organisms given to-day rep-
resents, therefore, only an attempt to express genetic relation-
ships, and is recognized as being only in part successful.

AN OUTLINE OF THE CLASSIFICATION OF THE LIVING WORLD
THE PLANT KINGDOM:

Phylum I. THALLOPHYTA: plants without distinction of root,

stem, or branch.
Sub-phylum 1. Algae: thallophytes possessing chlorophyll:

including unicellular forms, pond weeds, seaweeds, etc.
Class I. Diatomacece: the diatoms (Fig. 68 A).
Class II. Cyanophycece: the blue-green algae (Fig. 68 C).
Class III. Chlorophycece: the green algae (Fig. 30).
Class IV. Phceophycece: the brown algae.
Class V. Rhodophycece: the red algae.

376 BIOLOGY

Sub-phylum 2. Fungi: thallophytes without chlorophyll : bac-
teria, yeasts, molds, etc.

Class I. Schizomycetes: the bacteria (Fig. 33).

Class II. Saccharomycetes: the yeasts (Fig. 32).

Class III. Phycomycetes: the alga-like fungi (Fig. 42).

Class IV. Ascomycetes: the sac-fungi.

Class V. Basidiomycetes: the basidio-fungi (Fig. 115).

Phylum II. BRYOPHYTA: the moss-like plants.

Class I. Hepaticce: the liverworts.
Class II. Mustinece: the mosses.

Phylum III. PTERIDOPHYTA: the ferns and their allies.

Class I. Filicales: the true ferns (Fig. 124).
Class II. Equisetales: the horse-tails.
Class III. Lycopodiales: the club mosses.

Phylum IV. SPERM ATOPHYTA: the seed-bearing plants.

Sub-phylum 1. Gymnospermae : the cone-bearing plants,

pines, hemlocks, etc.
Sub-phylum 2. Angiospermse : flowering plants.

Class I. Monocotyledons: endogenous plants.

Class II. Dicotyledons: exogenous plants.

THE ANIMAL KINGDOM:
Division I. PROTOZOA: unicellular animals.

Class I. Rhizopoda: animals with naked protoplasm and
pseudopodia (Fig. 19).

Class II. Infusoria: animals with cilia, flagella, or ten-
tacles, and usually a mouth (Fig. 21).

Class III. Sporozoa: parasitic animals, producing spores
and having a metamorphosis (Fig. 25).

CLASSIFICATION AND DISTRIBUTION 377

Division II. METAZOA: many-celled animals with a differ-
entiation of cells.

Phylum I. PORIFERA: animals with no distinct mouth, but
many incurrent openings : the sponges.

Class I. Calcarea: with a skeleton of calcareous spicules.
Class II. Non-calcarea: with a skeleton of silicious or
horny spicules, or none.

Phylum II. CCELENTERATA: animals with a mouth, but
without an anus and with no body cavity.

Class I. Hydrozoa (Fig. 69).

Class II. Syphozoa: sea-nettles.

Class III. Actinozoa: corals, anemones.

Class IV. Ctenophora.

Phylum III. ECHINODERMATA: radiate animals, with com-
plete alimentary canal and a body cavity.

Class I. Asterioidea: starfishes.

Class II. Ophiuroidea: brittle stars.

Class III. Echinoidea: sea-urchins.

Class IV. Crinoidea: sea-lilies.

Class V. Holothuroidea: sea-cucumbers.

Phylum IV. PLATYHELMINTHES : flat, unsegmented

worms.

Class I. Cestoda: the tapeworms.

Class II. Trematoda: the flukes.

Class III. Turbellaria: the planarians.

Phylum V. NEMATHELMINTHES : round, unsegmented
worms: round worms, threadworms.

Phylum VI. MOLLUSCOIDEA.

Class I. Polyzoa: sea-mats, corallines.
Class II. Brachiopoda: lamp-shells.

378 BIOLOGY

Phylum VII. ANNULATA: the segmented worms.
Class I. Chcetopoda: bristle-footed worms.

Sub-class A, Polychceta; with many bristles (Fig. 149).

Sub-class B, Oligochceta ; with few bristles (Fig. 74).
Class II. Hirudinea: leaches.
Class III. Archiannelida.
Class IV. Gephyrea.
Class V. Choetognatha.

Phylum VIII. MOLLUSCA.

Class I. Pelecypoda or Lamellibranchia: bivalves, clams

oysters, mussels.

Class II. Gasteropoda: univalves, snails.
Class III. Amphineura: many-valved: chiton.
Class IV. Cephalopoda: with long arms: squids, cuttle

fishes.

Phylum IX. ARTHROPODA: with jointed feet.

Class I. Crustacea: crabs, lobsters, barnacles.

Class II. Onychophora.

Class III. Myriopoda: millipedes, centipedes.

Class IV. Hexapoda: insects.

Class V. Arachnida: spiders, scorpions, etc.

Phylum X. CHORD ATA: animals with a notochord.

Sub-phylum, Atriozoa : body cavity opening to the exterior.

Class I. Urochorda: tunicates or sea-squirts.

Class II. Cephalochorda: amphioxus.
Sub-phylum, Vertebrate: animals with a vertebral column.

Class. I. Pisces: fishes.

Class II. Amphibia: frogs, toads, salamanders.

Class III. Reptilia: lizards, snakes, turtles, alligators.

Class IV. Aves: birds.

Class V. Mammalia: mammals.

CLASSIFICATION AND DISTRIBUTION 379

DISTRIBUTION OF ANIMALS IN SPACE AND TIME

We have already seen that while organisms are always
adapted to the locality in which they live, they are frequently
even better fitted for other localities, and their presence in
any part of the world must be due to other factors besides
fitness. The distribution of organisms on the earth's surface
is controlled by three fairly well-known laws:—

1 . The members of a species usually occupy a continuous terri-
tory. We do not find some members of a species in one locality
and others in a distant region, without finding them also in inter-
mediate territory. There are some exceptions to this law, but
in the vast majority of instances each species occupies a continu-
ous territory around a center of origin. The territory occupied
will depend upon many factors of climate, for of course the
habitat must be properly fitted to furnish the organism with
food, water, and a proper temperature.

2. All animals and plants can multiply with a rapidity suf-
ficient to give them, in a comparatively short time, enough off-
spring to cover the face of the earth. The rate of multiplication
of different organisms varies very greatly. The codfish may pro-
duce 8,000,000 eggs per year, while the elephant produces only
a single offspring in two years, and usually not so frequently
as that. Among the lower animals and plants, the rate of
reproduction is sometimes even greater than the higher num-
ber given above. But even the slow rate of the elephant is
sufficient, if the multiplication were unchecked, to enable the
species to fill the world in a few years. The numerous offspring
are always endeavoring to find room for themselves, and food
to eat. For this purpose they distribute themselves as widely
as possible.

3. All organisms distribute themselves from the centers, where
their reproduction is rapid. All organisms, even those that
seem stationary, have some method of dispersing themselves
over the earth. The means of dispersal are chiefly the follow-
ing: 1. By independent migration. This is true of almost all

380 BIOLOGY

animals, but it is not true of plants, which, as a rule, have no
independent power of motion. 2. By winds. Many plants
produce seeds or spores which can be blown for long distances
by the wind, until they land in a favorable locality, where
they can develop into new plants. This dispersal by the wind
is not so common among animals, although some of the lighter
animals which fly, like the insects and bats, may be blown
for long distances by the wind. 3. By water currents. The
ocean currents and fresh-water streams carry many animals
and plants long distances. The Gulf Stream carries living
organisms across the Atlantic Ocean, and a river flowing through
a country may distribute seeds for hundreds, and even thou-
sands, of miles. 4. Incidental means. There are various inci-
dental methods by which seeds or eggs, or even living animals,
may be distributed. Wood-boring insects may be carried on
drifting logs; seeds may be carried in particles of mud clinging
to the feet of flying birds; living animals may be carried for
long distances on floating ice; ships carry living animals and
plants all over the world; migrating animals not infrequently
distribute seeds of plants as they move about from place to
place, and they may even carry living eggs and some living
animals in the same way.

By some of these means all organisms have an efficient
method of distribution, and tend to scatter themselves in all
directions from the centers, where they are produced in large
numbers. Although the dispersal may be slow, in the end
even the most slowly migrating animal or plant might be
distributed over the face of the earth. All organisms tend
to disperse themselves until further migration is checked.
The factors which check their migration are spoken of as
barriers.

Barriers. — The ocean. — Bodies of salt water are effectual
barriers against the distribution of land animals. Flying ani-
mals cross small bodies of salt water, and animals and plants
that are blown by winds may be distributed over the ocean

CLASSIFICATION AND DISTRIBUTION 381

for considerable distances; but for most land animals the ocean
is an effectual barrier.

The land. — For marine animals, the land proves to be an
effective barrier. Although the conditions are essentially the
same on both sides of the isthmus of Panama, the animals on
the two sides of the isthmus are different, the narrow land
barrier being sufficient to prevent animals from crossing from
sea to sea. Land is also a fairly effectual barrier in preventing
the water animals of one river system from passing to another.
The inhabitants of the river may distribute themselves over
a wide territory, but they are usually unable to pass from one
watershed to another, except as they may be carried by inci-
dental means.

Mountains. — The high mountain ranges are perhaps the
most effectual barriers of all. Practically no animal or plant
is able to cross over the higher mountain ranges. Hence it
sometimes happens that the animals and plants upon the two
slopes of high mountains may be quite different, even though
the climatic conditions on the two sides are essentially the same.

Climate. — Each animal and plant is able to live only in cer-
tain conditions of climate. Hence the climate of a territory is
a determining factor in regulating its inhabitants. In their
distribution, animals and plants are frequently completely
checked when they reach territories in which the climate is
unadapted to them. This may be the result of several different
factors.

1. Water. — The absence of water is a most effectual barrier
to the distribution of either animals or plants. Deserts are
uninhabited by any form of life, since no protoplasm can exist
without water. Although most forms of life need a moist
climate, some prefer one that is moderately dry and cannot
live in moist territories. Deserts and semi-deserts will, there-
fore, be barriers for the greater number of animals and plants,
while moist climates will be effectual barriers for the type of
organism which prefers a semi-dry climate.

382 BIOLOGY

2. Food. — Animals and plants are limited to territories which
furnish the food on which they subsist. A territory that fails
to produce sufficient food for any given type of animal will
prove an effectual barrier.

3. Temperature. — Forms of life adapted to a warm climate
cannot live in a cold climate, and vice versa. The temperature
of a territory is, therefore, a highly important factor in deter-
mining its inhabitants. Most animals living in cold regions will
not pass over the equator, and those adapted to the warm
equatorial climate cannot distribute themselves over the colder
regions.

Enemies. — Every animal and plant has its special enemies.
These enemies are sometimes in the form of parasites; they
may be larger animals and plants, or other organisms that are
contending for the same food. The mutual rivalries of organ-
isms make one of the most complex problems of biology, and one
that presents an endless puzzle. The introduction of any new
animals into an old territory may produce unexpected changes
in the life of the animals and plants, the newly arriving organ-
isms seizing the available food, or destroying the life of other
animals and plants, and giving rise to modifications in the
fauna and flora, which can never be anticipated or predicted.
The complexity of these relations is indicated in a famous
example given by Darwin. The clover crop is dependent upon
the bumblebees, which distribute its pollen and produce proper
fertilization; the number of bumblebees is dependent upon the
number of field mice who eat them; the field mice in turn are
eaten by the cats; so that in this roundabout way the number
of cats in a territory regulates the clover crop.

Change of type under new conditions. — The distribution of any
particular species of animal or plant is modified by another
factor of a different nature. When an animal migrates into a
new territory, and comes under totally different conditions as
to food, climate, and enemies, it is very apt to begin to change.
These variations from the original type may, in the new terri-

CLASSIFICATION AND DISTRIBUTION 383

tory, prove of special advantage rather than of disadvantage,
and will be preserved, while the original type may be destroyed.
In the new locality, the species often assumes a form quite
unlike the original type, and becomes so differentiated that the
descendants can hardly be recognized as belonging to the
original species. This peculiar feature is especially noticeable
on some of the oceanic islands. Such islands may be hundreds
of miles from the mainland and only occasionally visited by
accidental stragglers; but they develop peculiar types of ani-
mals and plants distinctly their own, although originally com-
ing from the mainland. So different do they sometimes be-
come that they can hardly be recognized as close allies of the
mainland types. Although this change of type in new localities
is especially noticeable on oceanic islands, it undoubtedly
occurs on the continental areas as well. When a species
migrates into a new territory, and is placed under new condi-
tions of food and climate, and is in rivalry with new enemies,
modifications of the original type are sure to develop, and in
the end the form adopted is more or less different from that
of the original immigrant, which may be limited to its original
home.

DISTRIBUTION OF ORGANISMS IN TIME: PALEONTOLOGY

Geology discloses the fact that the earth's crust is made up
of a series of rocks which have been deposited during the long
ages of the past; and by the study of these successive layers
of rock we can learn various facts concerning the history of
the world during the time when the different strata were de-
posited. In many of these rocks we find remains of living or-
ganisms, called fossils, which comprised the life of the world
at various periods in its earlier history. The study of these
different fossil remains is known as paleontology (Gr. palaios
= ancient + on = being -f- logos = speech), and gives us an
outline history of organisms in the past.

Paleontological history at best is very incomplete, since it

384 BIOLOGY

is only under special conditions that the body of an animal
or plant becomes imbedded in the rocks and preserved in the
form of a fossil. Incomplete as it is, paleontology has shown
us many illuminating facts concerning the earlier life of the
world. It has shown that life has been in existence on the
earth for many millions of years, although we have no means
of determining, even approximately, how many. It has taught
that during this long series of ages there has been a constant
succession of living things, one type after another making its
appearance and giving place to other types. The animals and
plants living to-day represent only the last step in this long
series, nearly all of the species existing at the present time
being of recent origin, some having been in existence only a
few thousands, or perhaps even a few hundreds of years, al-
though some of our present forms may extend back for hundreds
of thousands of years in the past. The immediate predecessors
of our present species were organisms much like them, and from
them the present forms have doubtless been descended; and
preceding these were others, still more remote in time and more
unlike the present ones in structure, representing still earlier
ancestral forms.

The general history of any series of types has been approxi-
mately as follows: Appearing in a certain part of the world,
a group of animals has dispersed itself more or less over the
face of the earth, becoming numerous in species and giving
rise to a variety of subordinate types. The development has
commonly gone on until a climax has been reached, after which
the particular type has perhaps remained constant for a time,
but eventually declined toward its final extermination. As
it disappeared, its place was taken by some other type, better
adapted to the new conditions of the changing world. So the
progress has gone on age after age, type after type appearing,
developing, culminating, and then declining and disappearing.

One general law in this long progress is manifest. During
the whole sories of ages there has been a general progress of

CLASSIFICATION AND DISTRIBUTION 385

type from lower to higher forms. The first organisms, appear-
ing in the oldest rocks, were simple forms of low structure,
while the highest forms of organisms appeared in the most
recent ages. While the progress has not been uniformly con-
stant, the general trend has always been upward. The inverte-
brates, which contain the lower animals, appeared and cul-
minated first, while the vertebrates appeared later. Among
the vertebrates the fishes appeared in the earlier rocks, the
amphibia came next, reptiles and birds followed, and finally
the highest group, the mammals, appeared last, with man at
the extreme end of the series. It is true that in this long suc-
cession of ages, some forms of organisms have degenerated,
becoming simpler and finally disappearing, while others have
remained constant for immensely long periods of time without
any apparent change. But the general tendency of the whole
history has been one of progress from a low form to a higher,
from the simple to the complex; and the living world to-day
represents the culmination of a long period of progress from the
earliest times. This progress, as disclosed by the fossils buried
in rocks, is, in a very general way, parallel to the progress of
the individual animal as it develops from the egg, through the
series of changes which we have learned to call embryology.
The parallel between embryology and paleontological history
has been one of the striking discoveries of biological study,
and has been one of the great factors in the disclosure of the
unity of the living world during these long ages. All the facts
to-day assure us that there have been uniform laws and forces
extending through the whole series of living organisms, from
the earliest geological ages to the present, and from PROTOZOA
to MAN.

GLOSSARY-INDEX

In this index all defined words are printed in black-faced type; words to which only
page reference is given are in roman type. In addition to the words used in the text,
definitions are given for some of the more common biological terms. These may be recog-
nized by their lack of page references.

abdomen. — The ventral part of the body below the ribs, 175.

abdominal vein. — A vein in the frog, passing over the abdominal wall in
the middle line, 191.

abducens (Lat. ab = from -f- ducere = to lead). — A cerebral nerve supplying
the eye muscles, 194.

abiogenesis (Gr. a = without + bios = life -f- genesis = birth) . — See spon-
taneous generation.

aboral (Lat. ab = away from + os = mouth) . — Opposite to or away from
the mouth.

absorption of food, 205, 306.

accretion. — Growth by addition of layers on the outside, 4.

accompanying cells, 106.

acellular. — See syncytium.

acetabulum, 182.

achromatin (Gr. a = without + chroma = color). — The part of a cell that
does not absorb coloring matter, 33.

acquired characters. — Characters first developed in the body rather than
in the germ plasm, 327; inheritance of, 333, 334, 355.

activity, 2, 3.

adaptation, meaning of, 342; origin of, 344, 354.

adipose (Lat. adeps = fat). — Fatty tissue.

adnate (Lat. adnatus = grown to). — United to.

adrenals. — Ductless glands lying on the kidneys.

aerial (Gr. aer = air). — Pertaining to the ah*.

aerial hyphae. — Branches of a mycelium growing upward into the ah* and
developing spores, 98.

aerobic. — Growing only in the presence of oxygen.

afferent fibers (Lat. ad = to + ferre = to bear). — Fibers carrying im-
pulses toward the brain, 172, 212.

albumen. — A proteid, illustrated by the white of an egg, 8, 22.

alcohol, 79.

alimentary canal. — The digestive tract, 157, 185.

alimentation (Lat. alimentum = food). — The process of food-getting, 138.

alveolus. — Expanded sacs at the ends of ducts, as in the glands or lungs.
See page 312.

387

388 BIOLOGY

alternation of generations, in animals, 277, in plants, 269, 273.

amitosis (Gr. a = without + mitos = thread). — Cell division without
karyokinesis, 89.

Amoeba, description of, 52.

amoeboid (Gr. amoeba -\- -oid). — Resembling Amoeba, especially as regards
movements by the protrusion of pseudopodia, 218.

amorphous (Gr. a = without -+- morphe = form). — Without regular
shape.

amphiaster (Gr. amphi = around 4- aster = star). — The double star formed
in karyokinesis, 86.

amphibious (Gr. amphi = around + bios = life). — Capable of living either
in the air or in the water.

amphimixis (Gr. amphi = around + mixis = a mixing). — A name applied
to the mixture of germ substance in sexual union, 336.

amylopsin. — A ferment found in the pancreatic juice, that converts starch
into sugar, 306.

amylolytic. — Capable of converting starch into sugar.

anabolism. — The building up of chemical substances from simpler ones,
139, 225, 299.

anaerobic (Gr. an = without -f aerobic). — Growing only in the absence
of oxygen.

anaesthetics (Gr. an = not -\-aisthesis = feeling). — Drugs that dc.-n<>v
consciousness.

analogy (Gr. ana = according to -f logos = a ratio). — Likeness in func-
tion, 368.

analysis (Gr. ana = up + luein = to loose). — The reduction of a com-
pound to its component parts, 234.

anaphase. — The third stage of karyokinesis, 87,.

anatomy (Gr. ana = up -f temnein = to cut). — The study of the grosser
structure of organisms, 19.

anchylosis (Gr. angkylos = crooked). — The growing together of bones,

animalculae. — Microscopic organisms commonly found in water, 52.

animal functions. — -Those distinctive of animals, i. e., nervous and muscu-
lar, 217.

animals and plants, differences between, 217, 224.

annuals. — Plants which live a single season only.

Annulata (Gr. anulus = a ring). — A group of animals with the body
divided into rings, 157, 378.

Anopheles, 71, 72.

antennae. — Elongated appendages with sensory functions occurring on the
head of certain animals.

anterior. — Pertaining to the head, 155.

GLOSSARY-INDEX 389

anterior root. — The anterior branch of a spinal nerve, carrying impulses

from the center, 194.
anther (Gr. anthos = a flower) . — The sac on the end of a stamen bearing

pollen, 119.
antheridia (Gr. antheros = flowery). — The organs in certain plants that

produce the sperms, 271.

anthropoid (Gr. anthropos = man). — Resembling man.
anus. — The posterior opening of the digestive tract; the vent, 157.
aorta. — The large main artery which carries blood to the lower part of the

body, 188.

aperture. — An opening,
apetalous (Gr. a = without -+- petalon = a leaf). — Flowers without petals,

119.

appendages. — • Elongated projections from organisms, with special func-
tions; like legs, tentacles, etc., 175.
appendix vermiformis. — A small, blind sac attached to the end of the large

intestines.
APPERT, 14.
aqueous humor. — The transparent liquid between the cornea and the lens

of the eye, 197.
arachnoid membrane. — The membrane covering the brain, between the

dura mater and the pia mater.
arborizations (Lat. arbor = a tree). — The fine branches in which many

nerve fibers sometimes terminate, chiefly at their central ends,

170.
archegonia (Gr. arche = first + gonos = race). — The organs in certain

plants that produce small eggs, 271.

arteries. — Blood vessels carrying blood from the heart, 190.
articular (Lat. articulus = a joint). — Pertaining to the joints, 177.
ascent of sap. — • The flow of liquids from the roots to the leaves, 126.
Ascomycetes, 99.

ascospores. — Spores produced in asci, 79, 99.
ascus (Lat. ascos = a sac). — A sac holding a definite number of spores,

79, 99.

asexual reproduction, 238, 243, 262; distribution of, 265.
asexual stage (Gr. a= without + sexual). — The stage in a metamorphosis

in which reproduction is by an asexual method.
Aspergillus, 97, 102.

asphyxia (Gr. a = without + sphyzein = to throb). — Suffocation.
assimilation (Lat. assimulare = to make like). — The power of converting

nourishment into the substance of the body, possessed by all living

things, 44, 62.
astragalus, 182.

390 BIOLOGY

atrophy (Gr. a = without + trephein = to nourish). — To decrease in size
as the result of disuse, or from other causes.

auditory. — Pertaining to hearing, 104.

auricles (Lat. auris = ear). — The chambers in the heart that receive
venous blood, 188.

automatic activity. — Actions started by the organisms and not brought
about by any external stimulus: spontaneity, 3.

autophytes (Gr. autos = self + phyton = plant). — Plants which subsist
upon minerals and gases, which they utilize through the agency of sun-
light, 226.

available energy, 298.

avidity for water, 127.

axial. — Pertaining to the axis.

axial skeleton. — The skull and spinal column, with the ribs and sternum,
177.

axil. — The angle above the attachment of a leaf.

axis cylinder. — See axon.

axon. — The process from a neuron extending outward and becoming the
axis cylinder of a nerve fiber, 170.

bacillus. — A motile, rod-shaped bacterium.

bacteria. — A group of extremely minute plants, the simplest und sm;ilh>st
known organisms, 26, 80, 232, 235.

ball-and-socket joints, 185.

barriers. — Factors that check the distribution of organisms, 380.

basioccipital, 180.

bast. — The fibers of the phloem.

bees, parthenogenesis in, 246.

bell animalcule. — Same as Vorticella.

biennials (Lat. bi- = twice -f annus = year). — Plants that live two years,
growing the first year and fruiting the second.

bilaterally symmetrical. — Having the two sides strictly counterparts of
each other, 155.

bile.— The secretion of the liver; also called gall, 186.

biogenetic law (Gr. bios = life + genesis = creation). — The law that em-
bryology tends to repeat past history, 290.

biological sciences, classification of, 18.

bladder, 199.

blade of leaf, 114.

blastula. — A hollow-sphere stage of the developing egg.

blend. — To mix, as when the offspring shows characters midway between
those of its parents, 362.

blights, 232.

GLOSSARY-INDEX 391

blood, 191.

blood vessels, 158.

body cavity. — The cavity between the intestine and the body wall; also

called the coelom, 157.
body wall. — The muscular walls which lie outside the body cavity; the

outer wall of the body, 143, 157, 166.
bone, 27, 176.

brachial (Lat. brachium = the arm). — Pertaining to the arm, 190.
bract. — A leaf in the axil of which a flower is developed,
brain. — The enlarged front end of the nervous system in vertebrates, 192:

sometimes applied to the cerebral ganglia of invertebrates, 162.
branchiae.— Gills, 288.
branchial openings. — Openings in the neck, through which water may pass

for respiration, 285.
bread raising, 80.
breeding season. — The season of the year during which reproduction

occurs, 214.

bronchi. — The larger branches of the trachea leading to the lungs,
buccal. — Pertaining to the mouth, 185, 284.
budding. — Reproduction by the formation of buds which may become

detached; gemmation, 78, 146, 239.
bulbus arteriosus. — The large arterial trunk arising from the ventricle of

the frog, before it breaks up into branches, 188; also called truncus

arteriosus.

bundle. — A cluster of elongated cells, 104.
butterfly, 72, 289.
calcaneum, 182.

calciferous (Lat. calx = lime + ferre = to bear). — Lime-producing, 169.
callosities (Lat. callum = a thick skin). — Thickenings of the skin,
calyx. — The outer row of leaves of a flower, usually green, 119.
cambium (Lat. cambire = to exchange). — The layer of active, growing cells

inside the bark and outside the woody layers in exogenous plants, 105,

108.

cane sugar, 9.

capillaries (Lat. capillus = a hair). — The microscopic blood vessels be-
tween the ends of the arteries and the beginning of the veins, 190, 207.
capillarity, 127.

carbohydrates. — Starches, sugars, and celluloses, 9, 10.
Carchesium, 91.

cardiac (Gr. cardia = heart). — Pertaining to the heart,
carnivorous (Lat. caro (carnis) = flesh + vorare = to eat). — Feeding on

flesh, 224.

392 BIOLOGY

carotids. — The two large arteries on the sides of the neck carrying blood

to the head, 190.
carpals, 182.
carpels (Gr. carpos = fruit). — The separate parts (leaves) of which a pistil

is composed, 120.
cartilage. — A hard material, softer than bone, which forms part of the

skeleton, 27, 35.

cartilage bones. — Bones first formed as cartilage, 181.
casein. — A proteid present in milk and constituting the curd, 8.
castor bean, 103.
cat, 366.

caudal. — Pertaining to the tail,
cell. — One of the simple units of which living things are composed, 20,

37.

cell doctrine, 38.

cell sap. — A clear liquid inside of plant cells, 30.
cellulose. — A material related to starch and forming the cell wall of many

plant cells; the basis of paper, cotton, etc., 35.
cell wall. — The covering on the outside of the cell, not present in all cells.

29, 34.

central nervous system, 162, 192.
centrosome (Gr. centron = center + soma = body). — A small body lying

near the nucleus in animal cells and apparently the center of active

forces, 29, 34.
centrosphere, 34.

centrum. — The large central disk of bone in a vertebra, 177.
cephalic (Gr. kephale = head). — Pertaining to the head,
cerebellum. — The larger of the two divisions of the hind-brain, 193.
cerebral ganglia. — The large ganglia in the head of an animal, usually two

in number, 162.

cerebral hemispheres. — The anterior and largest part of the brain of ver-
tebrates, 193.
cerebrospinal axis. — The central nervous system of vertebrates, composed

of brain and spinal cord, 192.
chemical composition of living things, 5, 7, 42.
chemical compounds. — Substances made of two or more chemical elements

joined in chemical union.

chemotropism (Eng. chemesa = chemical 4- Gr. trope — a turning). — Reac-
tion to chemical stimuli, 57.

chiasma. — The crossing of the optic nerves in the brain.
Chilomonas, 73.
chitin. — A horny material, like that of which insects' wings are made.

GLOSSARY-INDEX 393

chlorogogen cells.— The cells which fill the typhlosole and cover the intes-
tine in the earthworm, 169.

chlorophyll (Gr. chloros = green + phyllon = leaf). — The green coloring
material in plants which enables them to carry on photosynthesis, 93,
117, 131, 218.

chlorophyll bodies, 37.

chloroplasts (Gr. chloros = green -f- plastos = molded). — Cells which pro-
duce chlorophyll, 117.

Chordata (Gr. chorde = a string). — Animals possessing a notochord, includ-
ing all Vertebrata, 378.

choroid (Gr. chorion = skin). — The pigment-holding layer of the eyeball,
inside the sclerotic coat, 196.

choroid coat, of the eye, 196.

choroid plexus. — A membrane full of blood vessels covering certain cavities
in the brain, 193.

chromatin (Gr. chroma = color). — The material in the nucleus holding the
characteristic features of cell life and concerned in inheritance, 33, 259.

chromatophore (Gr. chroma = color + pherein = to bear). — A pigment-
bearing cell.

ohromidial units, 50.

chromogenic. — Pigment-producing.

chromosomes (Gr. chroma = color + soma = body). — The threads of
chromatin formed preliminary to cell division; the number is constant
in each species of organism, 85.

chyle. — The completely digested food in the intestine.

cilia (Lat. cilium = an eyelash). — Vibratile processes of protoplasm from
the free surface of cells, 60, 218.

circulation, 138, 206, 309; of earthworm, 158; of frog, 187, 205; of Hydra,
146.

Cladonia, 229.

class. — A group of closely related orders, 373.

classification of organisms, 370, 375; significance of, 374.

clavicle, 182.

cleavage. — The division of the egg into cells, 281.

clitellum (Lat. clitellce = a saddle). — A band of swollen segments in the
earthworm, between the 28th and 35th segments, 157.

cloacal aperture (Lat. cloaca = sewer). — The common opening of the intes-
tine and the urogenital organs, 175, 186.

cloacal chamber, 284.

cnidoblast (Gr. cnide = thistle + blastos = a sprout). — The ectodermal cells
in Ccelenterata which produce the nematocysts.

cnidocil, 144.

394 BIOLOGY

coagulate. — To change into a curd-like mass, 22.

coccus. — A spherical bacterium.

cocoon. — The t >ationary stage in the life of a butterfly, 72; the egg case

of an earthworm, 165.
Coelenterata (Gr. kottos = hollow + enteron = intestine) . — Animals with

pnly a single cavity and no body cavity, including Hydra and its allies,

377.
coeliac axis. — The arterial trunk from the dorsal aorta, supplying the

viscera, 190.

coelom (Gr. koilos = hollow). — Same as body cavity, 157.
Coelomata (Gr. koilos = hollow) . — Animals with a body cavity including

all animals above Ccelenterata.

coelomic fluid.— The fluid filling the body cavity, 158, 160.
coenocyte (Gr. koinos = common + cytos — a cell). — A protoplasmic

mass containing several nuclei, but without division into cells; same as

syncytium.
cold-blooded. — A term applied to animals whose blood is of essentially the

same temperature as the surrounding medium,
colloidal.— A term applied to substances which will not dialyze through

membranes, 307.
colony. — A group of connected individuals, usually arising from one by

asexual budding, 73, 92.
columella. — A rod connecting the tympanic membrane with the inner ear

in the fiog, 197.
commensalism (Lat. cum = together + mensa = table). — An association

of two organisms in which neither is benefited nor injured, 230.
commissures (Lat. committere = to join). — Nerve cords connecting ganglia,

162.

communal. — Living in communities.

compound pistil. — A pistil made of several fused carpels, 120.
conductility. — The power possessed by protoplasm of transferring impulses

from one end to the other, 43.
condyies. — The smooth protuberances by which the skull is attached to

the first vertebra, 181.

conformity to type. — The appearance of like individuals in successive gen-
erations, 328.
congenital characters (Lat. con = with + genitus = born). — Characters

that are fixed in the germ plasm, 327, 334.
conidia (Gr conis = dust). — Spores produced by constriction on the ends

of threads rather than in a sporangium, 98.
conjugation (Lat. com = together + jugare = to join). — The union of

two similar cells in reproduction, 65, 94, 247, 262.

GLOSSARY-INDEX 395

connective tissue. — Material, usually fibrous, which connects the various

parts of an animal,
consciousness, 5, 213.

conservation of energy, 294; applied to organisms, 303.
constructive processes, 139, 225, 299.
contagious. — Having the character of passing readily from person to

person.

continuity of germ plasm, 329.
contractile vacuole, 55, 62.
contractility, 54.

convolutions. — Folds of the surface of the brain,
coordination (Lat. con- = with + ordinare = to arrange). — •The orderly

control of the various functions so that they act in harmony; the control

of many muscles so that they act toward a definite end, 140, 162, 211;

in plants, 137.
copulation. — The union of the sexes for the transferring of sperms to the

eggs, 165, 215.
copulatory organs. — Organs used for bringing the sex cells together,

256.

coracoid, 182.
cork, 38.
cornea (Lat. corneus = horny). — The front, transparent covering of the

eye, 196.

corolla. — The second row of leaves in a flower, usually colored, 119.
coronary arteries. — Arteries supplying the heart,
corpuscles. — Any small bodies, but chiefly applied to floating cells in the

blood, 192, 205.

correlation of forces. — Same as transformation of energy,
cortex (Lat. cortex = bark). — The layers of cells inside the epidermis and

outside the cambium of a young stem; the outer layers of any organ,

as the cerebral cortex, 104, 112.
cotyledons. — The leaves of a plant in the seed, 123.
cranial nerves. — The nerves arising from the brain, 194.
cranium (Gr. cranion = skull). — That part of the skull that holds the brain,

180.
crop. — An expanded chamber of the digestive tract for storing hastily

swallowed food, 158.
cross fertilization. — Fertilization of eggs from one individual, with sperms

from another, 165, 267.
crus, 182.
cryptogams (Gr. cryptos = concealed + gamos = marriage).— Plants which

do not produce flowers, 103.

396 BIOLOGY

crystalline. — Applied to substances that will dialyze through membranes,
307

crystalline lens. — The lens in the eyeball which focuses light on the retina,
197.

Culex, 72.

cutaneous (Lat. cutis — skin). — Pertaining to the skin, 191, 209.

cuticle (Lat. cutis = skin). — A thin, structureless membrane forming on
the outside of the epidermis, 62, 166.

cyclical changes. — Changes which pass through a cycle but eventually
return to the starting point, 5.

cyst. — A hard shell which is sometimes secreted around organisms in a dor-
mant condition; any sac with a wall, developing abnormally in the
body, 59, 74, 241.

cytoblastema, 38.

cytoplasm (Gr. cytos = cell + plasma = substance). — The liquid part of
the protoplasm outside the nucleus, 32, 49.

dandelion, 370.

DARWIN, 352.

death, 3, 153.

decay. — Decomposition changes produced by bacteria in the presence of
air; more complete than putrefaction, 81.

deciduous. — A term applied to plants that shed their leaves in the fall;
also to mammals that shed the placenta at birth.

decomposition. — The chemical destruction of molecules. In biology the
disintegration of organic substances, usually produced by bacteria or
allied organisms.

degeneration, 233.

dehiscence (Lat. dehiscere = to' open). — The opening of an organ to dis-
charge its contents, 123.

dendrites (Gr. dendron = tree). — The branching processes arising from
neurons, 170.

denitrification. — The reduction of nitrates to nitrites or simpler compounds.

dentine. — The inner, softer part of the teeth.

depressant. — Having the power of reducing activity.

dermis. — The inner layer of the skin, 176.

descent theory. — The theory that all organisms are genetically inter-
connected: evolution, 348.

dessication (Lat. dessicare = to dry up). — Drying, 57.

destructive processes, 139, 300.

deutoplasm (Gr. deuteros = second + plasma = substance). — The food
yolk in the egg, 249.

DE VRIES, 357.

GLOSSARY-INDEX 397

dextrose. — A form of sugar found in fruits; glucose, 9.

dialysis. — See osmosis.

diaphragm. — A muscular membrane separating the chest from the abdomen.

diastatic. — Capable of turning starch into sugar.

diastole (Gr. diastole = an expansion). — The period in each beat when the
heart is relaxed, 188.

Diatoms, 136, 219.

dichotomous (Gr. dicha = in two + temnein = to cut). — Branching by
regular division into pah's.

differentiate. — To become unlike; usually applied to parts originally similar
but which acquire different structure and function, 95, 283.

digestion. — • A series of changes in the chemical and physical nature of the
food which renders it capable of absorption, 204, 305.

digestive cells, 145.

digestive juices. — • The secretions which render the food capable of absorp-
tion, 55, 62, 204.

digits. — Fingers and toes.

digitigrade. — Walking on the tips of the fingers and toes.

dimorphism (Gr. di- = twice -f- morphe = form). — -Showing two distinct
forms.

dioecious (Gr. di- = twice + oikos = house) . — Having the sexes in different
plants.

diphtheria, 231, 232.

direct development. — • Development without a metamorphosis, 290.

disease germs, 82.

disintegrate. — To break to pieces, 4.

dispersal. — The power of organisms to distribute themselves from centers,
379.

dissepiment. — See septum.

distal. — Farthest from the main body.

distribution, in space, 379; in time, 383.

divergence.—- The appearance of two or more lines of descent from a com-
mon center, 337, 339.

diversities. — The slight differences found among individuals of the same
species.

diverticulum. — Any sac-like outgrowth.

dogs, origin of, 339.

dominant characters (Lat. dominari = to rule). — Those which appear most
prominently in the first generation after the crossing of races, 360.

dorsal. — Pertaining to the back, 155.

drones. — Male bees.

Drosera, 223.

398 BIOLOGY

ductless glands. — Gland-like structures without ducts, pouring their secre-
tions into the blood.

ducts. — The large spiral or otherwise marked cells in the fibrovascular
bundles; vessels, 106. In animals the tubes that carry the secretions
of glands to the exterior, 105.

duodenum. — The first loop of the intestine below the stomach, 186.

dura mater (Lat. durus = hard + mater = mother). — A tough membrane
on the outside of the brain, 194.

ears, 197.

earthworm, 155; physiology of, 216.

ecdysis. — The shedding of the skin.

ecology (Gr. oikos = house + logos = discourse). — The study of the mu-
tual relations of animals to each other and to their environment,
20.

ectoderm (Gr. ectos = outside + derma = skin). — The outer layer of cells
of animals, 141, 283.

ectoparasites. — Parasites living on the outer surface of their host, 230.

ectoplasm. — The outer layer of protoplasm in Protozoa, 54, 61.

efferent nerve fibers (Lat. ex = from -}-ferre = to bear). — Fibers carrying
impulses away from the brain, 172, 212.

egg. — Same as ovum, 267.

egg sac, 163.

electropism (Gr. electron = amber + trope = a turning). — The power of re-
acting to electricity, 58.

elements. — The ultimate varieties into which substances can be chemically
analyzed, 5.

embiyo (Gr. embryon = an embryo). — The young organism in the early
stages of development, 19.

embryology. — The study of the development of the egg into an adult, 19;
of the frog, 280.

embryo sac. — A name formerly given to the macrospore of a flowering
plant, 122, 273.

emulsion. — Finely divided droplets of one liquid (usually oil) floating in
another liquid, 23.

enamel. — The hard, outer covering of the teeth.

encyst. — To inclose in a cyst, 74, 241.

endoderm (Gr. endon = within + derma = skin). — The inner layer of cells
of animals, lining the digestive tract, 143, 145, 283.

endodermis. — A layer of cells within the cortex and next to the wood in
the roots of plants, 113.

endogenous stem (Gr. endon = within + genes = a producing). — Stems
in which the fibrovascular bundles are irregularly arranged, with no
cambium, wood ring, or bark, 112.

GLOSSARY-INDEX 399

endoparasites. — Parasites living within the body of their host, 231.
endoplasm (Gr. endon = within + plasma = substance). — The inner

layers of protoplasm in the Protozoan cell, 54, 62.
end organs. — Special organs at the ends of the nerves, 211; peripheral and

central end organs are recognized,
enemies, relation of animals to, 382.
energy, 292; stored by plants, 299.
English sparrow, 345.
enteron. — The alimentary canal, 158.
entire. — Of a leaf margin, without indentations,
environment. — The surroundings which influence organisms, 351.
enzymes. — Substances secreted by organisms and having powers of fer-
mentation; unorganized ferments, 306.

epiblast. — A name applied "to the ectodermal layer of the embryo,
epidermis (Gr. epi = upon -f- derma = skin). — The outer layers of cells

of any organism, 104, 115, 167, 176.
epiglottis (Gr. epi = upon -f glottis = glottis) . — An elastic lid covering

the glottis, which prevents food from passing into the windpipe,
epiphysis. — -Same as pineal gland, 193.
epithelio-muscle cells. — Cells of the ectoderm of Hydra, with contractile

fibers extending from their base, 143.

epithelium (Gr. epi = upon + thele = nipple) . — Cell layers covering sur-
faces or lining canals or cavities, 169.
equatorial plate. — The flattened mass of chromosomes formed between

two centrosomes, 86.
erythrocytes (Gr. erythros = red + cytos). — • The red corpuscles of the

blood, 192, 205.
Eudorina, 263.
Euglena, 75, 76, 217.
Eustachian tubes. — The tubes leading from the throat to the middle ear,

186, 197.
eversion (Lat. e = out -f vertere = to turn). — The process of turning a

part inside out.
evolution. — The theory that traces the origin of the present world from the

past as the result of the unfolding of natural law, 348.
excreta, 139.
excretions. — Waste products of metabolism eliminated by glands, 56, 139,

210.
excretory system.— 56, 62, 139; of earthworm, 161; of frog, 199; of Hydra,

151; of plants, 225.
exoccipital bones, 180.

400 BIOLOGY

exogenous stems (Gr. exo — without + genes = a producing) . — Stems
with a cambium layer separating a bark from a wood ring, 109, 112.

extensors. — The muscles that straighten the appendages at the joints, 211.

eye, 196; of human being and of frog compared, 367.

eyespot. — A colored spot found in unicellular organisms, sensitive to light,
76.

facial nerve. — The nerve supplying the side of the head with sensations,
194.

Fallopian tubes. — • In mammals the part of the oviduct extending from the
ovary to the uterus.

family. — A group of similar genera, 372.

fat. — One of the three chief food substances; a hydrocarbon made up of
a fatty acid and glycerine, 9, 133.

fatty acid. — One of the materials into which fat may be decomposed, 10.

fauna. — The total animal life of any region.

Felis, 372.

females. — Individuals producing eggs, 251.

female pronucleus. — The matured egg nucleus before union with the
sperm, 254.

female spores, 122.

ferment. — Chemical substances that produce fermentation; enzymes, 306.

fermentation, 79.

fern, life history of, 269.

fertilization. — The union of the egg nuclei and the sperm nuclei, 122,
249, 251, 257, 263. In botany the term is frequently erroneously
applied to the transference of pollen to the pistil, 277.

fibers. — The individual elements of muscles and nerves, 170.

fibrillae. — The minute filaments of which a muscle fiber is composed.

fibrin. — A proteid obtained from clotted blood.

fibrovascular bundles (Lat. fibra = fiber -f- E. vascular). — Bundles of long
cells of various shapes, extending lengthwise and strengthening the
stems of the higher plants, 104.

fibula, 182.

filament. — The, thread-like stem to a stamen, 119; any thread-like organ.

fission (Lat. findere = to split). — Division into two equal halves, 58, 63.

flagella (Lat. flagellum = a whip). — Rather long, lashing processes of pro-
toplasm, one to six to each cell, 73, 218.

flexors. — • Muscles that bend the joints, 211.

flexure. — A bending.

Flora. — The total vegetation of any territory.

flowers, 118.

foam theory of protoplasm, 31.

GLOSSARY-INDEX 401

foetus. — The embryo while within the uterus of the mother, 291.

follicle. — The pocket in which a hair is produced.

foods of plants, 126.

food vacuoles. — Clear spaces in Protozoa representing the remains of

digested food,
foramen magnum (Lat. foramen = opening + magnum = great). — The

opening into the skull through which the spinal cord enters, 180.
forebrain. — The front part of the brain, consisting of cerebrum and thala-

mencephalon, 193.

fore-gut. — The front part of the alimentary canal; the stomodceum.
fossils. — • The remains of animals or plants found in the rocks, 383.
fourth ventricle, 193.
free-living, 227.

frond,— The leaf of a fern, 269.
frontal bone, 180.
fundamental cells. — The cells which make up the bulk of the stem of

young plants; they are roughly spherical in shape and never elongated,

104.

Fungi, significance in nature, 134, 234.
fusion nucleus. — The nucleus formed from the fusion of two nuclei into

one, as in fertilization or conjugation, 65, 241, 257.
gall bladder. — The sac which temporarily stores the bile, 187.
gamete (Gr. gamete = husband or wife). — One of the uniting cells in sex

union; usually male or female, but sometimes not showing any sex

differentiation, 262, 267.
gametophyte (Gr. gamete = husband or wife + phyton = plant) . — • The

stage in the life cycle of a plant that produces sex organs, 272, 275.
ganglion. — A group of aggregated neuron bodies, 162.
gastric glands, 204.
gastric juice. — -The digestive secretion produced in the walls of the stomach,

204.

gastrovascular cavity. — The cavity in the body of Ccelenterata, 141.
gastrula (Gr. gaster = a stomach). — An early stage in the embryology of

animals. See page 285.

gemmae — Special buds formed for reproduction, gemmules, 243.
gemmation. — The same as budding, 239.
gemmules. — Special buds which break away from the parent and become

new individuals; same as gemmae,
generation. — The whole life history of an organism, from any stage to the

same stage again, 67.

genital (Lat. genere = to produce) .— Pertaining to reproduction,
genus (pi. genera). — A group of similar species, 371.

402 BIOLOGY

geotropism (Gr. ge = earth -f trope = a turning). — The power possessed
by many plants of growing toward or away from the earth.

germinal. — Pertaining to reproduction.

germ layers. — The three layers formed in the developing embryo, ^83.

germ plasm. — The substance which bears the hereditary traits and is con-
tinuous from generation to generation, 330.

gills. — Thin, expanded organs, bathed in water for respiratory purposes,
288.

gill slits. — See branchial openings.

girdling, 111, 129.

gizzard. — A muscular chamber of the digestive tract where food is ground,
158.

glands. — Groups of cells which produce secretions, 167, 176, 204.

glenoid cavity, 182.

glomerulus. — See Malpighian body.

glossopharyngeal (Gr. glossa = tongue -f pharynx). — A nerve from the
brain supplying the tongue and throat, 194.

glottis. — The opening of the trachea or larynx into the mouth, 186.

glucose. — A sugar from fruits, or artificially made from starch, containing
maltose and dextrin, 9.

gluten. — A proteid from cereals, 8.

glycerine. — One of the decomposition products of fat, 10.

gonads. — Glands producing eggs or sperms, 251.

Gonium, 240.

grafting. — Inserting a part of one animal or plant into another in such a
way that the inserted part retains its life and grows.

granular. — Filled with granules or minute solid particles.

granular theory of protoplasm, 31.

gregarious. — Congregating.

growth, 4.

guard cells, 116.

gullet. — The oesophagus, 158.

gustatory. — Pertaining to taste.

gyncecium. — Same as pistil.

haemal (Gr. haima = blood). — Pertaining to the blood.

haemoglobin (Gr. haima = blood + Lat. globus = globe). — A red proteid
which colors the blood red, 158, 192, 209.

hair follicle. — The tiny pocket, within which each hair grows.

hairs, 35, 117.

hallux. — The great toe.

hand, 365.

hare, 34£.

GLOSSARY-INDEX 403

Haversian canals. — The canals in bone in which the blood vessels run.

heart, 187, 206, 309; of earthworm, 159.

heliotropism (Gr. helios = the sun + trope = a turning). — The power

possessed by plants of turning toward the sun.
helotism (Gr. helot = a slave) . — An association of organisms in which one

enslaves the other, 228.

hepatic (Gr. hepar = liver). — Pertaining to the liver,
hepatic vein. — The vein from the liver, 190.
herbivorous (Lat. herba = grass + vorare = to eat). — Feeding upon grass,

herbs or other plants,
heredity. — The appearance in the offspring of characters of the parent,

326; nucleus in, 48, 259; Weismann's theory of, 329.

hermaphrodites. — Individuals possessing both male and female reproduc-
tive glands, 251.

heterocercal. — Applied to a tail-fin with one lobe longer than the other,
heterosporous (Gr. heteros = other -f spore). — Producing more than one

kind of spore; i. e., macrospores and microspores, 274.
hibernation (Lat. hibernare = to winter). — The death-like sleep which some

animals show in winter, 214.

high organisms. — Organisms with complex structure, 96.
hind-brain. — The posterior part of the brain, consisting of cerebellum and

medulla, 193.
hind-gut. — The hind part of the intestine, the cloacal chamber; also called

the proctodoeum.
hinge joint, 185.
histology (Gr. histos = a web -f logos = discourse). — The study of the

microscopical anatomy of organisms, 19; of earthworm, 166.
holophyte (Gr. holos = whole + phyton = a plant). — Having the food

habits of plants, i. e., capable of utilizing sunlight and assimilating

CO2, 221.
holozoic (Gr. holos = whole -f zoon = animal). — Having the food habits

of animals, i. e., nourished wholly on organic foods, 221.
homocercal. — Applied to a tail-fin with both lobes equal,
homologous (Gr. homos = like + logos = ratio). — Similar in structure,

364.
homosporous (Gr. homos = like + spore). — Producing only one kind of

spore, 274.
HOOKER, 38.
Horse, foot of, 365.
host. A name applied to an animal or plant upon which another is living

as a parasite, 227,
humerus, 182.

404 BIOLOGY

humor. — A name applied to the transparent liquids in the eye, 197.

HUXLEY. 40.

hybrids. — Organisms resulting from the crossing of different species, 268.

Hydatina, 57, 247.

Hydra, description of, 140.

hydrocarbons, 9.

hydroids. — Animals closely related to Hydra, 148, 277.

hydrophyte (Gr. hydor = water + phyton = plant). — Plants living in

water or in a very wet habitat,
hyoid. — A V-shaped arch of bone under the jaw and surrounding the larynx,

181.
hyomandibular. — A chain of bones attaching the lower jaw to the skull

hi the frog.

hypha. — One of the filaments of a mycelium,
hypoblast. — Applied to the endodermal layer in the embryo,
hypophysis (Gr. hypo = under + phuein = nature). — See pituitary body,

193.

ileum. — A name given to the intestine below the duodeum.
ilium, 182.

imago. — The adult stage of an insect with a metamorphosis, 289.
imbibition (Lat. imbibire = to imbibe). — The action of absorbing water,

shown by many organic substances,
immutability of species (Lat. in = not + mutabilis = changing). — The

theory that species remain constant, 349.
imperfect flowers. — Flowers in which either stamens or pistils are lacking,

120, 121.
impregnation (Lat. impregnare = to make pregnant). — See fertilization,

257.
inbreeding. — Breeding from a male and female of the same parentage, like

brother and sister, 268.
income, of an animal, 219; of a plant, 220.
incubation (Lat. incubare = to he on). — To keep warm.
Indirect development, 290.
individual, 67.

individual variations, 337, 358.
indusium (Lat. induere = to put on). — A covering over the sporangia in

the sorus of a fern, 269.
inferior vena cava. — The large venous trunk bringing the blood from the

lower parts of the body to the heart: same as posterior vena cava, 190.
infundibulum. — Any funnel-shaped or dilated organ, 193.
infusion. — A preparation made by steeping a substance like hay in warm

water.

GLOSSARY-INDEX 405

inner ear, 198.

inorganic, 26.

insertion. — The attachment of a muscle farthest from the center of the

body, 184.

intercellular (Lat. inter = between + cellular). — Lying between the cells,
intercellular digestion, 145.
internodes. — Spaces between the nodes,
interstitial cells (Lat. inter = between + sistere = to set). — Cells in

Hydra lying between the cnidoblasts and the muscle cells, 143.
intestine. — The digestive tract from stomach to cloacal chamber, 186.
intracellular (Lat. intra = within + cellular). — Lying within the cells,
intracellular digestion, 146.
intussusception (Lat. intus = inside -f- suscipere = to take up). — The

process of growth by taking material inside the body and incorporating

it into the body substance, 5.
invagination (Lat. in = within + vagina = a sheath). — The act of turning

inward, as when the finger of a glove is pushed into the palm,
invertebrata. — A name given to all animals below vertebrata.
invertion. — The splitting of a molecule of cane sugar into two molecules

of grape sugar, a molecule of water being added in the process, 9.
iris. — An opaque curtain, containing pigment, covering the front of the

eyeball, 197.
irritability. — The power of reacting under the influence of stimuli, 43, 57,

63, 219.
ischia, 182.
isolation. — The separation of two individuals from the rest of the species

so that they will breed together, 351.
jellyfish. — See medusa.
joints. — Places where bones or other hard movable parts come together,

176, 184.

karyokinesis (Gr. karyon = nucleus + kinesis = movement). — The proc-
ess of cell division accompanied by a peculiar, complicated nuclear

division; mitosis, 85.
karyoplasm (Gr. karyon = nucleus + plasma = substance). — The liquid

protoplasm inside the nucleus; nucleoplasm, 32, 49.
katabolism, 139.

kidneys. — Glands in vertebrates secreting urea, 199.
kinetic energy (Gr. kinetos = moving). — Energy in motion; active energy,

293.

kingdoms. — The two divisions of organisms, animals and plants, 373.
lachrymal (Lat. lachryma = a tear). — Pertaining to tears,
lacteals. — Lymph vessels, carrying absorbed fat from the intestine, 192.

406 BIOLOGY

lacunae. — Spaces among the tissues in which lymph collects, 192, 208.

L..MARCK, 350.

Lamarckian factors. — Forces in evolution first suggested by Lamarck,

351.

lamella. — A thin plate or layer,
larva. — A free-living stage in the development of an animal, unlike the adult,

e. g., a tadpole, 286, 289.
larval history. — The stages in the life history of an animal after hatching

from the egg and before adult form is reached,
larynx. — The enlargement of the air passages containing the vocal cords,

191, 209.

leaf, structure of, 114.

legumen. — A proteid derived from legumes, 8.
legumes. — Plants belonging to the Leguminosae family, like beans, peas,

clover, alfalfa, vetches, locusts, etc.
Leucanthemum, 345.
leucocytes (Gr. leukos = white -f- cytos = cell). — The white corpuscles of

the blood, 192, 205.
lichens. — The grayish green mosses which grow on rocks or trees, etc.; an

association of a fungus and a green plant, 229.
life cycle. — See generation; of nature, 234.
life force, 4, 323.

ligaments. — Bands of connective tissue connecting bones, 184.
lingual (Lat. lingua = tongue). — Pertaining to the tongue, 190.
linin. — The delicate fibers extending through the karyoplasm and forming

a network, 32.

littoral. — Pertaining to the shore.

liver. — A large gland opening into the intestine at the pylorus, 186.
lophotrichic (Gr. lophos = a crest + thrix = hair). — With a tuft of fla-

gella, 81.

low organisms. — Organisms with a simple structure, 96.
lumen. — A cavity in a tube or sac.
lungs, 191, 209.

lymph. — The liquid part of the blood after it has passed out of the capilla-
ries into the tissues, 176, 192, 208.
lymph glands. — Glandular swellings on the lymph vessels, which belong to

the ductless glands; lymph nodes, 192.
lymph hearts. — Four pulsating sacs in the frog, that force lymph into the

veins, 192, 208.

lymph spaces. — Spaces in tissues in which lymph collects, 176.
lymph vessels. — Tubes carrying lymph from the lacunae to the veins, 192,

208.

GLOSSARY-INDEX 407

machine. — Any mechanism designed to convert one kind of energy into

another, 297.
macronucleus (Gr. macros = large). — The larger of the two nuclei in cells

having two, 62.
macrospore (Gr. macros — large).— The large spores ;n certain plants,

which develop into female gametophytes, 122, 273.
macula lutea (Lat. macula = spot + luteus = yellow). — A small spot on

the retina, with most acute vision,
malaria, 69, 232.

males. — Individuals producing sperms, 251, 257.
male pronucleus. — The sperm after entering the egg and before it unites

with the egg nucleus, 257.
male spores, 122.
Malpighian bodies. — Minute rounded bodies in the kidneys filled with a

knot of blood vessels; glomeruli.
Mammalia (Lat. mamma = breast). — Animals, the females of which have

milk glands, 373.

mammary glands. — Glands secreting milk, 373.
mandible. — The jaw bone, 180; also the jaw-like teeth of animals like

insects and Crustacea.

mantle. — A fold of skin more or less enveloping the body of an animal,
marrow. — The soft material filling the cavities of bones,
maturation (Lat. maturare = to make ripe). — The final changes by which

an egg becomes prepared for fertilization, 253.
maxilla. — A bone forming the upper jaw, 180; also mouthparts of insects

or Crustacea,
mechanical theory. — The theory that life phenomena are manifestation?

of chemical and mechanical forces only, 41.
medulla oblongata. — The posterior part of the brain, 193.
medullary rays (Lat. medulla = marrow) . — Bundles of cells extending from

the center to the outer parts of a stem, 111.
Medusae. — The sexual, free-swimming stage of certain hydroids and other

Ccelenterata; jelly fishes, 144, 278.
megaspore. — Same as macrospore.

membrane bones. — Bones formed first as membranes, 181.
membranella. — A band of fused cilia found in some of the unicellular

animals, 61.

Mendelism. — A law of heredity first advanced by Mendel, 359.
mental functions, 317.

mesenteron (Gr. mesos = middle + enteron = intestine) . — The mid-gut, 284.
mesentery (Gr. mesos = middle + enteron = intestine). — A fold of the

peritoneum which slings the intestine in position, 187.

408 BIOLOGY

mesoblast. — The mesoderm of the developing embryo.

mesoderm (Gr. mesos = middle + derma = skin). — The middle layer in

a developing embryo, 283.

mesogloea (Gr. mesos = middle + gloios = glue). — The middle, non-cellu-
lar layer of Hydra and allied animals, 143.
mesophyll cells (Gr. mesos = middle + phyllon = leaf). — The irregular,

loosely packed, chlorophyll cells in the middle of a leaf, 117.
xnesophytes (Gr. mesos = middle -f phyton = plant). — Plants living in a

moderately moist habitat.
metabolism (Gr. meta = beyond + ballein = to throw) . — A name given

to the series of chemical changes going on in organisms, 138, 210.
metacarpals, 182.
metameres (Gr. meta = beyond -j- meros = a part). — Segments of animals

like the earthworm, 155.
metamorphosis (Gr. meta = beyond + morphe = form). — A life history in

which an organism passes through several unlike stages, more or less

independent, 72, 289; of frog, 280, 286.
metaphase. — The second step in karyokinesis, 87.
Metaphyta (Gr. meta = beyond + phyton = plant). — Plants made of

many cells, 223.
metastasis (Gr. meta = beyond + histanai = to place.) — The process of

using foods, 132, 135, 300.
metatarsals, 182.
Metazoa (Gr. meta = beyond + zoon = animal). — Animals made of many

cells, 223.

mice, breeding of, 361.
micronucleus (Gr. mikros = small + nucleus). — The small nucleus in a

cell containing two nuclei, 62.

microorganism. — Any organism of microscopic size,
microsomata (Gr. mikros = small + soma = body). — Extremely minute

bodies in the protoplasm which frequently show motion, 32.
microspores (Gr. mikros = small + spore). — The small spores in a plant,

which develop into male gametophytes, 122, 273.
mid-brain. — The middle part of the brain, consisting of the optic lobes

(called corpora quadrigemina in man), 193.
migration.— The act of changing one's dwelling place from one locality to

another, 379.
mimicry. — Resemblances which some organisms show to other objects, for

protective purposes.

mitosis (Gr. miios = a thread). — See karyokinesis.
mitral valve. — The valve between the left auricle and vent-ride,
molds, 96, 99, 235.

GLOSSARY-INDEX 409

molecule. — The smallest particle of a chemical compound which can exist
without the compound being chemically destroyed.

Monocystis, 241.

monoecious (Gr. monos = one -j- oikos = house). — With both sexes in the
same individual; applied to plants, 251.

monogamous (Gr. monos = one + gamos = marriage). — The sexual asso-
ciation of one male with one female.

monotrichic (Gr. monos = one + thrix = hah1) . — With a single flagellum,
81.

morphology (Gr. morphe = form + logos = discourse) . — The study of the
structure of organisms in all relations, 19.

morula (Lat. morum = a mulberry). — The stage in the egg development
after the egg has become a sphere of cells.

motion, in plants, 136, 218; in the earthworm, 167; in the frog, 211.

motor cells. — The neurons which send impulses over their axons to the
muscles to produce motion, 172, 213.

motor ocularis. — The third cerebral nerve supplying the eye muscles, 194.

Mucor, 97, 247.

mucous membrane. — The lining of the alimentary canal, 187.

mucous. — Applied to glands secreting mucus.

mucus. — A thick, viscid secretion from the mucous membrane.

multicellular organisms (Lat. multus = many + cellular). — Organisms
made of many cells which show a differentiation among themselves,
90, 95.

muscles, 219.

mutations (Lat. mutare = to change). — Sudden departures from the race
character which have a tendency to remain fixed, 358.

mutation theory (Lat. mutare = to change).— The theory of evolution that
assumes that progress has taken place by mutations rather than by
individual diversities, 357.

mutualism. — An associating of organisms for mutual benefit, 228.

mycelium (Gr. mykes = fungus + helos = a nail). — The thread-like fila-
ments of which fungi are composed, 96.

myosin (Gr. raws = muscle). — A proteid in lean meat, 8.

nares. — • See nostrils.

nasal bones, 180.

natural selection. — The law by which the best fitted organisms survive.
353.

NEEDHAM, 13.

nematocysts (Gr. nema = a thread + cystis = sac). — Special cells in Cce-
lenterata which have a coiled poison thread capable of extrusion; net-
tling cells, 143.

410 BIOLOGY

nephridia (Gr. nephros = kidney). — The organs of the earthworm which
excrete nitrogenous waste, 161.

nerve fibers. — The separate fibers of which a nerve is composed, 170.

nerve impulse, 314.

nerves, 163, 194.

nervous system of earthworm, 162; of frog, 192; of Hydra, 146.

netted-veined leaves. — Those with veins branching into a network, 114.

nettle-hairs, 117.

nettling cells. — See nematocysts.

neural arch (Gr. neuron = a nerve). — The arch of bones on top of the ver-
tebrae, inclosing the neural foramen, 177.

neuroglia (Gr. neuron = a nerve + gloios = glue). — The connective frame-
work of the nervous system.

neurons (Gr. neuron = a nerve). — The nerve cells which are the units of
the nervous system, 169, 195.

nictitating membrane (Lat. nictare = to wink). — A semitransparent, inner
eyelid in the frog and some other animals, 175.

nidamental glands (Lat. nidus = a nest). — Glands connected with th<>
oviduct, that secrete the covering of eggs, 201.

Nitetta, 29.

nitrification. — The production of nitrates in the soil from simpler nitrogen
compounds.

nodes. — The places on a stem where branches arise.

nostrils. — Openings into the nasal cavities; nares, 175, 186.

notochord (Gr. notos = the back -f chorde = a string). — A rod in the back
of vertebrate embryos that develops into the spinal column, 286.

nucleolus. — A small body in the nucleus of a cell, with unknown functions,
32.

nucleoplasm. — See karyoplasm.

nucleus. — The vital center of a cell, containing chromatin and controlling
constructive metabolism, 29, 32, 45.

occipital bones. — The skull bones which surround the foramen magnum,
180.

occipital condyles. — The rounded protuberances by which the skull articu-
lates with the first vertebra, 181.

oesophagus. — The tube from the throat to the stomach or crop, or from
the mouth into the body, 60, 158.

olfactory lobes. — Two small lobes of the brain in front of the cerebrum, 193.

olfactory nerve. — The first of the cerebral nerves, supplying the olfactory
sacs, 194, 196, 198.

olfactory sacs. — Minute sacs in the nasal cavities; the seat of the sense of
196, 198.

GLOSSARY-INDEX 411

omosternum (Gr. omos = shoulder + sternon = the chest). — A bit of
cartilage forming the front of the sternum in the frog, 182.

ontogeny (Gr. ont = being + -geneia = a producing). — Development from
the egg, 19, 290.

oocyte (Gr. oon = egg + cytos). — An egg before maturation, 252.

oogenesis (Gr. oon = an egg + genesis = creation). — The development of
the egg in the ovary, 252.

oogonium (Gr. oon = an egg + gonos = offspring). — A sac in some plants
within which are produced one or two eggs.

operculum. — A lid-like cover.

optic lobes. — The section of the brain in front of the cerebellum; the mid-
brain, 193.

optic nerve. — The second cerebral nerve, supplying the eye, 194, 196.

optimum temperature, 132.

oral. — Pertaining to the mouth, 60.

order. — A group of similar families, 372.

organ. — Any part of an animal or plant adapted for a specific function;
usually made of a combination of several different tissues, 26, 95.

organic evolution, 349.

organic substances.— Substances originally derived from organisms, 26;
in chemistry, any compounds of carbon.

organism. — A living being provided with organs; hence any living being,
26.

organisms as machines, 298.

origin. — The attachment of a muscle nearest the center of the body, 184.

Oscittaria, 136, 219.

osmosis. — The force that causes some substances to diffuse through mem-
branes which are moistened on both sides; dialysis, 127, 307.

ossification. — Turning to bone.

osteoblast (Gr. osteon = a bone + blastos = a sprout). — A bone-forming
cell.

otic. — Pertaining to the ear, 180.

otocyst (Gr. ous = the ear + cystis = a sac). — A sac which in many
invertebrates is supposed to have hearing functions.

otoliths (Gr. ous = the ear -f lithos = a stone). — Calcareous bodies found
in the otocysts in some animals.

outgo, of an animal, 220; of a plant, 220.

ova. — The female reproductive cells, 249, 267.

ovary. — In animals the glands producing eggs, 151, 163, 200, 249; in flowers
the lower part of the pistil containing the ovules and seeds, 120.

overproduction, 353.

oviducts. — Ducts for carrying eggs to the exterior, 163, 200, 249.

412 BIOLOGY

oviparous animals.— Animals that lay eggs, 290.

ovules. — The small bodies in the pistil that contain the macrospores and
grow into the seed, 121.

oxidation. — Union with oxygen, as in ordinary combustion, 55, 138.

palatine bones, 180.

paleontology (Gr. palaios = ancient + out = being + logos'). — The study
of the distribution of organisms in the past ages by means of fossils,
383.

palisade cells. — A layer of regular, chlorophyll cells, just beneath the upper
epidermis in most leaves, 117.

pallium.— See mantle.

pancreas. — A digestive gland opening into the intestine just below the
stomach, 187, 204.

pancreatic fluid or juice. — The secretion of the pancreas, 204, 306.

Pandorina, 73, 90, 263.

papilla. — A small finger-like projection.

parallel-veined leaves. — Those with veins running from base to tip, or
from midrib to margin, in a roughly parallel course, 114.

Paramecium, description of, 59.

parasite. — An organism that lives upon and feeds upon a living host, 227.

parasitism, effect of, 231.

parasphenoid bones, 180.

parenchyma (Gr. para = beside + enchein = to pour in). — Short, square-
ended cells in plants, 106.

parietal bones, 180.

parotids. — • Salivary glands in front of the ear in some animals.

parthenogenesis (Gr. parthenos = virgin + genesis = a creation). — Re-
production by eggs which do not require fertilization, 246, 262, 265.

PASTEUR, 14.

PASTEUR'S solution, 83.

patheticus. — The fourth cerebral nerve, supplying the eye muscles, 194.

pathogenic (Gr. pathos = disease + -genie). — Disease producing, 82.

pectoral. — Pertaining to the chest.

pedal (Gr. pous = a foot). — Pertaining to the feet.

peduncle. — The stalk supporting a flower, 118.

pelagic (Lat. pelagus = the sea). — Pertaining to the open ocean.

pelvis. — The girdle of bones attaching the legs to the spinal column, 182.

Penicillium, 96.

penis. — The male copulatory organ, 291.

pepsin. — A ferment in the gastric juice.

peptone. — A soluble form of proteid.

peptonize. — To convert ordinary proteids into peptones.

GLOSSARY-INDEX 413

Peranema, 75, 217.

perennials (Lat. per = through + annus = year).— Plants living year after
year.

perfect flowers. — Those with both stamens and pistils, 121.

perianth (Gr. peri = around + anthos = a flower). — A name given to the
calyx and corolla combined, 119.

pericardium (Gr. peri = around + cardia = the heart). — A sac surround-
ing the heart, 187.

perichondrium (Gr. peri = around + chondros = cartilage). — Fibrous ma-
terial surrounding cartilage.

peripheral system, 163.

periosteum (Gr. peri = around + osteon = bone). — Fibrous material sur-
rounding bone.

peristalsis (Gr. peri = around + stellein = to place). — The writhing mo-
tions of the intestine, 205.

peritoneum (Gr. peri = around -f teinein = to stretch). — The membrane
lining the abdominal cavity, 167, 171, 187.

peritrichic (Gr. peri = around + thrix = hah-). — With flagella distributed
over the body, 81.

perivisceral fluid (Gr. peri = around + Lat. viscera). — See ccelomic fluid.

pes. — The foot.

petals. — Leaves which form the corolla, 119.

petiole. — The stem of a leaf, 114.

phagocytes (Gr. phagein = to eat + cytos = a sac). — Leucocytes with the
power of absorbing solid objects, 205.

phalanges. — -The bones of the fingers and toes, 182.

phanerogams (Gr. phaneros = visible + gamos = marriage). — Plants which
produce flowers, 103.

pharynx. — The throat cavity, 158.

phloem (Gr. phloios = inner bark). — The bark, 105.

photosynthesis (Gr. phos = light + synthesis — composition). — The func-
tion of starch making, possessed by green plants only, 129, 135, 218.

phototropism (Gr. phos = light + trope = a turning). — Reaction to light,
58.

phyla. — The largest subdivisions of animals and plants, 373.

phytogeny (Gr. phylon = tribe + -geneia = a producing). — The past history
of organisms, 290.

physiology (Gr. physis = nature + -logia). — The study of the functions
of the different animals and plants, 19.

pia mater (Lat. piits = delicate + mater = mother). — A delicate mem-
brane surrounding the brain and cord, inside the dura mater, 194.

pigeons, 338,

414 BIOLOGY

pigment cells (Lat. pingere = to paint). — Cells which contain coloring

matter, 176.
pineal gland. — A small body lying on top of the brain; also called the pineal

eye; same as epiphysis, 193.

pistil. — The central row of leaves (carpels) of a flower, bearing female re-
productive organs; also called the gyncecium, 120.

pith. — The central mass of cells in a stem, made of fundamental cells, 104.
pituitary body. — A small body on the under side of the brain; the hypophy-
sis, 193.

placenta. — The membrane by which the embryo is attached to the uterus
in mammals, 291; in plants, the line of attachment of seeds in the
ovary,
plankton (Gr. plankton = wandering). — The living organisms which float

in water, largely microscopic.

plantigrade. — Walking on the palms of the hands or the soles of the feet,
plasma. — The liquid portion of circulating blood, 191, 205.
plasmodium (Gr. plasma = substance). — A jelly-like mass.
Plasmodium malarice, 69, 239.
plastids. — Miscellaneous bodies within a cell, 37.
platelets. — Minute bodies in the blood of vertebrates, 192.
pleura (Gr. pleura = a rib). — Membranes surrounding the lungs. .

Pleurococcus, 77, 239.

plexus (Lat. pleclare = to weave). — A network of nerves, 194.
pneumogastric (Gr. pneumon = lung + gaster = stomach). — A large, cere-
bral nerve extending down the neck and supplying the heart, lungs, and
stomach, 194.
Podocoryne, 277.
poisons. — Substances which, taken into the body, produce injurious effects,

43.

polar cells. — Small cells extruded from the egg during its maturation, 254.
pollen. — The male spores produced by a flower, 119.
pollen tube. — An outgrowth from a pollen grain which pushes through the

style of a flower to fertilize the egg in the ovary, 122, 275.
poll ex. — The thumb.

pollination. — The transfer of the pollen to the stigma, 277.
polygamous (Gr. polus = many + gamos = marriage). — The sexual asso-
ciation of one male with several females,
polymorphism (Gr. polus = many + morphe = form). — The property of

having two or more forms of the same animal, 149.
portal circulation. — The circulation of blood from the intestine through the

liver; it has two capillary systems, 190.
portal vein. — The vein carrying blood from the intestine to the liver, 190.

GLOSSARY-INDEX 415

posterior end, 155.

posterior root. — The branch of the spinal nerve entering on its posterior

side and carrying impulses toward the brain, 195.
posterior vena cava.— Same as inferior vena cava, 190.
potential energy, 293.
precoracoid, 182.

predatory. — Living by preying upon other animals,
premaxillary bones, 180.
proboscis. — An elongated portion of the head of an animal, with special

functions.

process. — • Any small projection.
procoelous (Gr. pro = before -f coilos = hollow). — Applied to vertebrae

which are concave in front only,
proctodaeum, 284.

pronation. — The position of the fore arm with the palm downward,
pronuclei. — • The two nuclei, male and female, which are in the matured

egg, ready to unite with each other, 254, 257.
prophase.— The preliminary stage in karyokinesis, 85.
prostomium (Gr. pro = before + stoma = mouth). — The sensitive lobe

projecting over the mouth in the earthworm, 156.
protective resemblances. — Resemblances to objects, either animate or

inanimate, for the purpose of protection; mimicry.
proteids. — Highly complex compounds of carbon, oxygen, hydrogen, and

nitrogen and some other elements; the basis of living tissues and a

necessary part of animal foods, 7, 133.
prothallium (Gr. pro = before + thallos = a branch). — The small, sexual

stage of the life history of a fern, 271.
protomitomic theory, 50.
Protophyta (Gr. protos = first + phyton = plant). — The unicellular plants,

222.

protoplasm (Gr. protos = first + plasma = substance) . — The living sub-
stance of organisms, 29, 30, 40, 48.
Protozoa (Gr. protos = first + zoon = animal). — The unicellular animals,

92, 222.

proximal. — Nearest to the body.

pseudonavicellae (Gr. pseudes = false + Lat. navicella = a boat).— Spin-
dle-shaped spores formed by some Sporozoa as the result of the union

of cells.
pseudopodia (Gr. pseudes = false -f pous = foot).— Temporary lobes of

protoplasm used in locomotion, 52.

psychology (Gr. psyche = the soul + -logia). — The study of mind, 20.
pterygoid bones, 180.

416 BIOLOGY

ptyalin. — The enzyme in saliva which converts starch into sugar.

pubis, 182.

puffballs, 245.

pulmonary arteries (Gr. pleumon = a lung). — Bloodvessels carrying blood

to the lungs, 191.

pulmonary circulation. — The circulation through the lungs, 191.
pulmonary veins. — The blood vessels carrying blood from the lungs to the

heart, 191.

pupa. — • A stationary, inactive stage between a larva and an adult, 289.
pupil. — An opening in the center of the iris allowing light to enter the eye,

197.
putrefaction. — Decomposition of organic products, taking place without

the presence of much oxygen, 81.

pylorus. — The opening of the stomach into the intestine, 186.
quadrate bones, 180.
quadrato-jugal bones, 181.
rabbit, skeleton of, 364.
race variations. — Variations by which the race is gradually or suddenly

modified, 338.

racemose. — Arranged somewhat like a cluster of grapes,
radiant heat. — Heat which is given off from a hot body into space, 297.
radius, 182.
ramus. — A branch.

reaction. — A response to an external stimulus, 43.
recapitulation theory. — See repetition.
receptacle. — In botany, the end of the flower peduncle on which the floral

leaves are borne, 118.
recessive characters (Lat. recessus = receding). — Characters which fail to

appear in a first generation, but may appear in later generations,

360.

rectum. — The enlarged, posterior end of the intestine, 186.
REDI, 12.
reflex action. — An action produced by a stimulus passing to the central

nervous system and there giving rise to stimuli which pass outward to

the muscles, but without volition, 212.

regeneration. — The redevelopment of parts that have been lost, 150.
reintegrate. — To recombine compounds that have been disintegrated,
renal. — Pertaining to the kidneys,
renal portal vein. — A vein from the legs of the frog that breaks up into

capillaries in the kidney, 191.
repetition, law of. — The law that the development of animals repeats their

past history, 290.

GLOSSARY-INDEX 417

reproduction, 5, 45, 140, 238, 318, rate of.

reproductive cells, 267.

reproductive organs, 163, 199.

reproductive system of Amoeba, 58; of bacteria, 81; of earthworm, 163;

of Eudorina, 264; of frog, 214; of Hydra, 146, 151; of malarial Plasmo-

dium, 71; of Monocystis, 241; of Pandorina, 74; of Paramecium, 63; of

Penidllium, 98; of Ulothrix, 93; of yeast, 78.

respiration. — The exchange of gases between organisms and their environ-
ment, 56, 138, 160, 225; explained, 209, 312.
reticular theory of protoplasm, 31.
reticulum. — A network, 32.
retina. — The sensitive part of the eye, 196, 197.
rhizoids. — • Delicate hairs attaching some plants, like mosses, to the soil,

270.

Ritinus communis, 103.
rigor mortis (Lat. rigor = stiff + mors = 'death). — The stiffening that

occurs after death,
rivalries of organisms, 382.
root cap. — A protective covering of hard cells over the tips of growing

roots, 113.
root hairs. — Delicate, single-celled absorption hairs, on the tips of roots

of plants, 113.
root pressure. — The pressure of sap in roots that forces sap up the stem,

127.

root structure, 112.

rudimentary organs. — Organs only imperfectly developed,
rusts, 232.
Saccharomyces, 78.
saccule, 198.

sacrum. — The fused vertebrae between the hip bones,
salivary glands. — Glands secreting saliva, 204.
saprophytes (Gr. sapros = rotten + phyton = a plant). — Plants which live

upon the dead bodies of other organisms, 227.

sarcode. — A name first given to the living contents of animal cells, 40.
scapula, 182.

SCHLEIDEN, 38.

SCHULTZE, 40.
SCHWANX, 38.
sciatic plexus. — The network formed by the several spinal nerves which

combine to form the sciatic nerve, 194.
sclerenchyma (Gr. sderos = hard +enchyma = infusion). — Plant cells wiih

thick, hard walls.

418 BIOLOGY

sclerotic coat (Gr. sderos = hard). — The outer covering of the eyeball, 196:

sea nettles. — See jelly fishes.

sebaceous glands (Lat. sebum = fat). — Oil glands in the skin.

secreting cells. — Cells which extract material from the blood and secrete

special substances, 145, 161.
secretions. — Materials eliminated by the glands and used by the body

for some special purpose, e. g., gastric juice,
seed. — A young plant surrounded by a shell and lying dormant; developed

in higher plants only, for the purpose of distribution, 122.
seedling. — The young plant in a seed, or just sprouting from a seed, 123.
segmentation. — A term describing the division of the earthworm into

segments, 155; the division of the egg into many cells in development

(deavage), 280.

segment. — The name applied to the rings of which a body like the earth-
worm is composed; melameres, 155.
segregation (Lat. segregare = to separate). — The grouping together of

individuals which show resemblances,
semicircular canals. — Canals in the inner ear, associated with the sense

of equilibrium, 198.
semilunar valves. — Valves at the beginning of the pulmonary arteries and

the aorta, 310.

seminal receptacles. — Sacs of the earthworm for holding the sperms re-
ceived at copulation, 165.
seminal vesicles. — Sacs in the earthworm for holding sperms before they

are ejected during copulation, 164, 200.
sensation. — A conscious feeling, produced in the brain as the result of

impulses reaching it from the various sense organs, 212, 316.
sensations in plants, 137.
sense organs. — Organs at the outer ends of the nerves which are excited

by external stimulation, 167, 172, 195, 212.
sensitiveness. — Same as irritability, 219.
sensitive plants. — Plants which respond quickly to touch by closing their

leaves, 137.

sepals. — The leaves which form the calyx, 119.

septa. — Partitions separating chambers, especially in the earthworm, 157.
serous. — Applied to glands secreting a thin, watery liquid,
serous membranes. — Membranes lining the body cavity and thorax,
serum. — The liquid part of the blood after the clot has separated,
setae. — Minute bristles serving to aid the earthworm in locomotion, 167.
sexual reproduction. — Reproduction by union of eggs and sperms, 71.

238, 240, 262; distribution of, 266. In earthworm, 163; in frog, 214;

in Hydra, 151; in plants; origin of, 263; purpose of, 335.

GLOSSARY-INDEX 419

sexual stage. — The stage in a metamorphosis in which sexual organs are
produced.

shell, of an egg, 250.

shoulder girdle, 181.

sieve cells. — • Large vessels in plants, with perforated partitions separating
them from each other, 106.

sinus. — Any irregular space or dilated blood vessel.

skeleton, 139, 176.

skin, 35, 176.

skull, 180.

sleep of plants, 137.

smell. — See olfactory organs.

sociology (Lat. sotius = a companion + Gr. -logia). — The study of the
relations of organisms in forming societies, 20.

somaplasm (Gr. soma = body + plasma = substance) . — The bit of the
germ substance in the egg that is set aside in the developing egg to give
rise to the new individual, 332.

somatic (Gr. soma = body). — Pertaining to the body.

sorus (plural, son). — A cluster of sporangia in the leaf of a fern, 269.

SPALANZANI, 13.

special creation theory, 350.

specialization. — Adaptation to some special function.

species. — The name given to a group of organisms essentially alike, 370.

sperms. — The male cells in sexual reproduction, 250, 255, 267.

spermaphytes. — Seed-bearing plants, phanerogams, 376.

spermaries. — The glands that produce the sperms, 164, 199, 250.

spermatocyte (Gr. sperma = seed + cytos = cell). — A cell in the spermary
that is to break up to form sperms, 254.

spermatogenesis (Gr. sperma = seed + genesis — creation) . — The devel-
opment of the sperms, 254.

spermatozoids. — A name sometimes given to the motile sperm-cells of
plants, 271.

spinal cord. — The part of the central nervous system of vertebrates extend-
ing through the spinal column, 193.

spinal nerves, 194.

spindle, 86.

spiracles. — Openings of gill chambers, as in tadpoles; also breathing pores
of insects.

spiral cells. — Cells of a fibrovascular bundle with their inner wall thickened
to form a spiral thread, 106.

spireme (Gr. spirema = a coil) . — A name applied to the chromatiu when
it forms a thread, prior to division, 85.

420 BIOLOGY

Spirogyra, 30.

splanchnic. — Pertaining to the viscera.

spleen. — A good-sized organ lying among the folds of the intestine,

187.

spontaneity. — Power of producing movements from internal causes, 2.
spontaneous generation. — The theory that life can arise in some other

way than from previously existing life; abiogenesis, 10.
sporangium (Gr. spora = a seed + angeion = a receptacle). — A sac within

which spores are produced, 100, 270.
spores (Gr. spora = a seed). — Single-celled reproductive bodies, capable

of growing into a new plant without fertilization, 16, 59, 79, 81, 239,

267.
sporoblast (Gr. spora = seed -f- blaslos = a germ). — A sac in which

sporozoites are produced, 242.
sporophyte (Gr. spora = seed + phyton = a plant). — The stage, in a

plant with alternation of generations, that produces spores, 272, 274.
Sporozoa, 241.
sporozoites (Gr. spora = seed + zoon = animal). — Spores that result from

the division of fused gametes, 241.
squamosal bone, 180.

stamens. — The modified leaves of a flower that produce pollen, 1 19.
starch. — A carbohydrate with the general formula CeHioO.s, or some

multiple of this, 8, 129, 134.
stereome cells (Gr. stereos = a solid). — Cells in the bark with very thick

walls, 106.
stereotropism (Gr. stereos = a solid + trope = a turning). — Reaction to

solid objects, 53.

sterility. — Unfertility, or inability to produce offspring, or hybrids, 268.
sterilize. — To treat an object so as to destroy all living things in it, 14,

17.

sternum, 182.
stigma. — The roughened surface on the end of a style, for the reception of

pollen, 120.
stimulus. — Any force applied to an organism which will produce a reaction,

43.

stinging cells, 143.
stipules, 114.
stomach, 186.
stomata (Gr. stoma = mouth). — Openings through the epidermis of plants

through which gas enters and moisture evaporates, 116.
stomodaeum, 284.
strawberry plant, reproduction of, 244.

CtLOSSARY-lNDEX 421

streaming of protoplasm. — The circulating motion of protoplasm within
a cell, 32.

struggle for existence, 353.

style.— The projection on top of an ovary in a flower, 120.

subspecies. — A subdivision of a species; sometimes called a variety, 371.

sugar. — A carbohydrate with the general formula C6Hi2OG (monosac-
charide) or Ci2H22On (disaccharide), 8, 132, 134.

summer eggs. — Eggs produced in the summer which develop at once, 247.

sundew, 224.

sunlight, 131.

supination. — Position of the forearm when the palm of the hand is upper-
most.

support, 139.

supraoccipital bones, 180.

survival of the fittest. — Same as natural selection, 353.

suspensorium, 180.

suture. — A jagged union between two bones, as in the skull.

swarm spores. — Spores with flagella enabling them to swim; zoospores.
74.

symbiosis (Gr. sun = together + bios = life) . — The living together of
two organisms in close relations, which may be advantageous or dis-
advantageous to each, 228.

sympathetic system. — Two chains of nerve ganglia and nerves lying in the
body cavity, parallel to the spinal cord, 195.

symphysis. — A union of two bones in the median line of the body.

syncytium (Gr. sun = together -f- cytos = cell). — A mass of living pro-
toplasm with many nuclei but no cell boundaries; acellular, 89, 99.

synovial glands. — Glands which secrete lubricating fluid into joints, 184.

synthesis (Gr. sun = together + tithenai = to place). — The building of a
compound out of simpler parts, 234.

systematic zoology. — The study of organisms which gives attention to
classification and naming of species.

systemic circulation. — That part of the circulation which includes the
vessels that supply all the body except the lungs, 191.

systole (Gr. systole = contraction). — The period of contraction of the heart,
188.

tactile. — Pertaining to touch.

tadpole, 288.

tarsals, 182.

taste, 198.

taxonomy (Gr. taxis = arrangement + nemein = to arrange). — The study
of the classification of organisms, 19.

422 BIOLOGY

telophase. — The last stage of karyokinesis, 87.

tendons. — Bands of connective tissue binding muscles to bones, 185.

tentacles. — • Appendages from an animal, usually motile, and serving as

sensory and prehensile organs, 141.
testis. — See spermary.
thalamencephalon. — The small section of the brain behind the cerebrum,

with the pineal gland on top of it, 193.
thallophyte (Gr. thallos = a shoot + phyton = a plant). — A plant that

does not show differentiation into root, stem, and leaf, 37.5.
thallus (Gr. thallos = a shoot).— A flat leaf or branch,
thermotropism (Gr. thermos = heat -f- trope = a turning) . — Reaction to

temperature, 57.
thigmotropism (Gr. thigma = touch + trope = a turning) . — Reaction to

mechanical stimulation, 57.
thoracic duct. — The large lymph duct in mammals, carrying lymph from

the lower parts of the body to the veins in the neck, 209.
thymus. — A ductless gland in the neck, especially prominent in the young,
thyroid gland. — A ductless gland in the neck below the larynx,
tibia, 182.

tissue (Lat. texere = to weave). — A collection of similar cells in a multi-
cellular animal, 27, 95.
tongue, 185.

tonsils. — Two ductless glands in the back part of the mouth. '
touch, 198.

touch corpuscles. — End organs of touch,
trachea (Gr. trachea = windpipe). — The air passage from the lungs; the

windpipe, 209.
tracheids. — The thick-walled wood cells with tapering ends, found in fibro-

vascular bundles, 105.
transformation of energy, 296.

transpiration. — The evaporation of water through the leaves,
transverse processes (Lat. trans = across + vertere = to turn). — Lateral

projections from the vertebrae, 177.
Trichina, 231.
trichocysts, 61.

tricuspid valve. — The valve between the right auricle and ventricle,
trigeminal. — The fifth cerebral nerve, supplying the sides of the head with

sensations, 194.

trypsin. — A ferment found in pancreatic juice, digesting proteids.
tubercles. — Knob-like growths, usually indications of disease,
tuberculosis, 231, 232.
turgor. — The pressure of liquids within the cells of plants.

GLOSSARY-INDEX 42?

tympanic membrane (Gr. tympanum = a drum). — The membrane cover-
ing the ear cavity and serving to collect the air waves, 175, 197.

tympanum (Gr. tympanum = a drum). — The cavity of the middle ear,
197.

TYNDALL, 14.

typhlosole (Gr. typhlos = blind + solen = a tube). — A cylindrical rod ex-
tending through the intestine of the earthworm, 158.

typhoid fever, 232.

ulna, 182.

Ulothrix, 93, 136.

unicellular organisms. — Organisms made of single cells, or of colonies of
similar cells, each of which can carry on all the functions of life, 52,
90.

unit characters. — Characters that are inherited as units, 359.

urea. — -An excretion from animals containing the nitrogen waste
(CH4N2O), 210.

ureter. — The duct carrying urine from the kidney to the bladder, 199.

urethra. — The duct carrying the urine from the bladder to the exterior,
199.

urogenital organs (Gr. ouron = urine + Lat. gemtalis = genital). — The ex-
cretory and sexual organs, which, in vertebrates, are united, 201.

urostyle (Gr. oura = tail -f- stylos = a pillar). — The single bone in the frog
which represents the tail, 177.

use and disuse, LAMARCK'S theory of, 351.

uterus. — • A chamber in the oviduct where the eggs are stored, or in mam-
mals where the embryo develops, 200, 291.

vacuoles (Lat. vacuum = a cavity).— Spaces inside the body of cells,
usually filled with a clear liquid, 37.

vagus. — • A branch of the pneumogastric nerve extending to the heart.

valves. — Membranous folds in the vessels or in the heart, which allow
liquid to flow only in one direction, 188.

variability. — The quality of showing variations.

variations. — • Slight differences between animals of the same species, 261,
327.

varieties. — See subspecies.

vasa deferentia (Lat. vasa = vessel + deferens = carrying down). — The
ducts that carry sperms from the spermary to the exterior, 164,
250.

vasa efferentia (Lat. vasa = vessel + efferens = carrying to). — The ducts
carrying sperms from the sperm aries to the kidney in the frog.

vegetative functions. — Those possessed by vegetables as well as animals,
Associated with alimentation and reproduction, 217.

424 BIOLOGY

veins. — Blood vessels worrying blood to the heart, 190; the fibrovascular

bundles in a leaf, 114.
vent. — See anus.
ventral side, 155.
ventral cord. — The nerve cord on the ventral side of the body cavity of

the earthworm and some other animals, 163, 171.
ventricle (Lat. venter = stomach). — The lower chamber of the heart that

forces blood into the arteries, 188.
venus sinus. — A large blood vessel on the dorsal side of the heart of a frog

into which the venus blood collects before passing into the right auricle,

188.

vertebrae. — The separate bones of the backbone or spinal column, 177.
Vertebrata. — Animals which possess a backbone or its equivalent, 176,

373.

vesicle. — A sac.

vessel (Lat. vasa = a vessel). — Any hollow tube or cavity, 105.
vestigial organs. — Functionless remains of organs, formerly larger and

functional,
villi. — Projections on the inside of the intestine which serve to absorb

food, 308.
viola, 372.

viscera. — The organs of the abdominal cavity, 175.
vital energy, or vitality, 41, 309, 319.

vitalistic theories. — The theories that regard life as a distinct force, 323.
vitelline membrane (Lat. vitellus = yolk). — A cell wall of an ovum, 249.
vitreous humor (Lat. vitrum = glass). — The transparent liquid back of

the lens and filling the eyeball, 197.
viviparous. — Producing young alive, 290.
vocal cords. — The membranes in the larynx whose vibration produces the

voice, 209.
vomers, 180.
VON MOHL, 40.
Vorticella, 91.

warm-blooded. — With blood that always maintains an equable tempera-
ture.

WEISMANN, 328, 355.
wilts, 232.
winter eggs. — Eggs of certain animals, designed to live over winter, usually

requiring fertilization, in distinction from summer eggs which do not,

247.

Wolffian body. — The primitive kidney found in the vertebrate embryo,
wood. — Same as xylem.

GLOSSARY-INDEX 425

worker bees. — Female bees whose sexual organs do not mature, and who

perform the work of the colony,
xerophytes (Gr. xeros = dry -f phyton = a plant). — Plants inhabiting

very dry regions,
xylem (Gr. xylon = wood). — The layers of hard woody cells inside the

cambium layer in exogenous stems; the wood, 105.
yeast, 78, 235.
yolk. — The food material deposited in an egg for the nourishment of the

developing embryo, 250.
zooid (Gr. zoon = an animal) . — A more or less independent member of a

compound organism, like a hydroid, 148.
zobspore (Gr. zoon = an animal + spora) . — Spores which swim by the

agency of motile flagella or cilia, 93, 136.
Zoothamnium, 92.

zygapophyses. — Articular processes in vertebrae, 177.
zygospore (Gr. zygon = a yoke -f spora) . — A spore formed by the union

of two gametes, 94, 262.
zygote (Gr. zygon = a yoke). — A cell formed from the union of two other?

in sexual reproduction; a zygospore, 94, 262.
zymogenic. — Giving rise to fermentations.

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