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Received June 27* 1939 
Accession No. 50458 
Given by Amer loan Book Co* 
Place, New York City 

*,t*flo book OP pamphlet is to be removed ttom the Iiab- 
oratopy tuithout the pepmission of the Trustees. 


The Story of Living Things 


Lecturer in Methods of Science Teacliimj 

Department of Education 

Claremont Colleges 


Professor of Biology, Brown Unirersity 


Assistant Professor of Biology, Wesleyan Unirersity 








Copyright, 1937, by 

All rights reserved 

W. P. 2 


This hook is gratefully dedicated to our wives, to whom 
much of the credit and none of the blame is due. 


Here are a few chips left over from the authors' workshop. 

First of all we do not pretend to ha\'e presented herein the last 
word in a field already overcrowded by worthy ri\'als. The "last 
word" has an undesirable mortuary connotation quite out of keeping 
in a book about living things. 

The authors have been teaching biology for a total of ninety-four 
academic years, in addition to over sixty seasons of strenuous service 
in summer field work with classes at marine and fresh-water labora- 
tories, and they can truthfully and enthusiastically say that they 
have enjoyed this experience. 

If what they would pass on to other students of biology appears 
from the table of contents to bear the familiar marks of old stuff, the 
reason is that it represents, in their minds at least, what remains 
after many years of trial and elimination at the hands of an army of 
different teachers and scholars. The fact that much material that has 
been worked over before it was retained does not necessarily prevent, 
it is hoped, some degree of freshness in its presentation. Any text- 
book, the authors hold, should be somewhat like a dish of uncracked 
nuts, accompanied by a good substantial nutcracker. It is desirable 
that the reader should have some of the fun of wielding the nutcracker, 
for no pedagogical cellophane can preserve nuts already shelled in an 
entirely fresh and satisfactory condition for a very long time. 

An inevitable handicap that the textbook method of presentation 
of any subject is bound to suffer, is the fact that between the covers 
of a book the whole banquet is set upon the table at once in a more or 
less complete array. It is the part of the instructor to break up the 
feast into courses and to serve them in digestible portions. Perhaps 
the method of suspense employed in magazine serials woidd furnish a 
better way of arriving at the desired end than presenting the matter 
all at once in l^ook form, since sufficient time shoukl always be pro- 
vided between the planting and harvesting of intellectual ideas to 
allow for unforced sprouting and growth. 

In the use of any textbook it is well to remember that the pages 
may be turned backward as well as forward, and that it is no crime 
either to skip or to reread. 

Every studious and effective reader, moreover, is wary about 


accepting witliout question whatever he may come across in print, 
for even textbooks are often known to l)o incomplete and liable to 

Again, if the art of reading between the hnes has not been culti- 
vated, it does not greatly avail simply to scan the printed hnes 
themselves. Every opening that induces the reader to seek further 
should be gratefully prized. 

Goethe once said: "Wer nicht mit der Bewunderung anfangt, 
werdet nie in das innere Heiligthum eindringen." Wonder is truly 
the mother of wisdom, for once the capacity for wonder slips away, 
one is prone to become blase, imcomfortably sophisticated, and 
intellectually slothful. 

With this explanation of the way it is hoped that this book will 
be used, the authors unite in cordially inviting the reader to join 
them in exploring the following pages. 


The authors wish to make grateful acknowledgment to all who 
have aided them in the preparation of a college textbook in biology. 

In particular, mention should be made of the members of the Biology 
Department of Wesleyan University who so willingly collaborated in 
trying out the ecological approach to a study of general biology for 
several years prior to publication of this book. Grateful acknowledg- 
ment is also made to them for innumerable suggestions as well as for 
their willingness to include certain successful features of the course in 
the text. Their help and advice has frequently been sought and 
willingly given. 

Special mention should also be made of the tireless effort and 
willing help of Wanda S. Hunter and Alice Hall Walter, both of whom 
read the manuscript and proof and contributed much to whatever 
success this book may attain. 

It is impossible here to enumerate all who have aided in the 
production of this book, but the following names must be men- 
tioned : Dr. E. C. Schneider, Shanklin Biological Laboratory, 
Wesleyan University, for reading the entire manuscript ; Dr. Francis 
R. Hunter, Rhode Island State College, for reading the entire proof ; 
Dr. Aurel O. Foster, Gorgas Memorial Laboratory, Panama, for 
reading section XII ; Dr. Hurbert B. Goodrich, Shanklin Biological 
Laboratory, Wesleyan University, for reading sections XIX-XXIII ; 
Dr. Frederick L. Hisaw, Biological Laboratories, Harvard University, 
for reading section XVIII ; Dr. John A. AIcGeoch, Psychological 
Laboratory, Wesleyan University, and Dr. Bernard C. Ewer, Depart- 
ment of Psychology, Pomona College, for reading section XVH ; 
Dr. Philip A. Munz, Department of Biology, Pomona College, for 
reading the botanical portions of the book ; Messrs. Emil Kotcher 
and Wilson C. Grant for aid in preparing the index. 

Acknowledgment is also made to organizations and individuals 
without whose co-operation it would have been impossible to secure 
many of the instructive and attractive illustrations. 






I. The Stage Settin"g (Ecology) 

II. The Biological Conquest of the Would . . . 20 

III. The Interdependence of Living Things — The Web of 

Life 44 

IV. Roll Call (33 


V. Life and Protoplasm 
VL Cells and Tissues 




VII. Beginnings: The Large Group of the Smallest Organlsms 

VHI. The Development of Sexuality in Plants 

IX. Division of Labor in Coelenterates 

X. Being a Worm 

XI. The Popular Insect Plan . 

NIL The Art of Parasitism 

XIII. Advantages of Being a Vertebrate 





XIV. The Role of Green Plants 

XV. The Metabolic Machinery of Animals 

XVI. Support, Motion, and Sensation 

XVII. The Display of Energy 

XVIII. Chemical Regulators 



XIX. Reproduction and Life Cycles 
XX. The Great Relay Race 







XXI. Time Spent (Palaeontology) 473 

XXII. The Epic of Evolution 483 

XXIII. That Animal, Man (Anthropology) .... 530 


XXIV. Man's Conquest of Nature ...... 567 

XXV. Conservation and Its Meaning 589 

XXVI. Man's Fight for Survival 608 

XXVII. The Next Million Years 637 

Index 645 




Preview. Ecology of a typical region • How to study ocolog>' • IMaiit 
and animal associations • Basic environments : water as a factor ; tempera- 
ture ; light as a factor ; chemical factors ; gravity as a factor ; substratum ; 
molar agencies ; biotic factors • Life in the water • Life in the air • Life on 
land • Suggested readings. 


"My heart is fixed firm and stable in the belief that ultimately the sun- 
shine and the summer, the flowers and the azure sky, shall Ijecome, as it 
were, interwoven into man's existence. He shall take from all their beauty 
and enjoy their glory." — Richard Jefferies : The Life of the Fields. 

There is a lure in knowing something intimate about i)lant and 
animal neighbors, their habits and the places where thoy live. A 
trout fisherman finds almost as keen enjoyment in watching a king- 
fisher make its catch as in having a trout take his own fly. Flic 
banks, meadow\s, and woods along a trout stream are aliv'e with 
interesting plants and animals. Even a slight acquaintance with 
what may be expected along the path makes a hike througii the 
forest and field immensely more worth while. An early morning 
walk, if one knows a few permanent bird residents and can recognize 
a migrant here and there, takes on an absorbing interest for tlie 
observer. Such trips in the open are eventful experiences, the joy 
of which is not easily forgotten. One may see the beauty of living 
things, and enjoy the songs of birds and the gay colors of insects, or 
get a thrill out of the sight of the first violet or bluebird, as he drinks 
in the sweet odors of the flowery meadow. From the standpoint of 
the more observant, another side than passive enjoyment of nature 
is to be found. It is discovered in asking and trying to answer the 
how and why of life aroimd us. 

Charles Elton has called the science of ecology "scientific natural 
history." This deals with the occurrences and behavior of organ- 
isms in a given habitat or home. Anyone who feels a genuine response 
to the call from the natural environment surrounding him cammf 



fail to find an interest in this approach to natural history. Why, for 
example, do eertaiii kinds of animals live in the swift water of trout 
streams, while different ones are associated with plants in a quiet 
pond? Why are the types of life found along the seashore so unlike 
those around the edge of an inland lake? Why do forest trees grow 
tall in the dense woodland, more spreading in the open, and stunted 
near the tops of mountains? These and hundreds of like questions 
can be answered truthfully with the background afforded by the 
science of ecology. 

Ecology of a Typical Region ^ 

New England scenery is characterized by rounded granite hills, 
often heavily wooded with second or even third growth. In the 
hollows surrounded by these hills nestle little lakes, bodies of water 
varying in area from a few hundred square feet of surface to many 
scores or even square miles in extent. 

A survey of the inhabitants of one of these smaller lakes, chosen as a 
typical example, reveals relatively few fish and fewer plants in the 
open water. Nearer shore are found unmistakable zoning of plants 
and animals, depending on whether the shore is rocky, sandy, or 
muddy. In sheltered bays having a bottom of soft mud are found 
numbers of pond lilies and other aquatic plants, which give shelter 
to pickerel, bass, and smaller fish, as well as a vast array of small 
crustaceans, insect larvae, and microscopic plants and animals. 

Part of the lake shore is a sandy beach, at one end of which a slug- 
gish stream, after meandering through a meadow, empties into the 
lake. This constitutes quite a typical environment and will yield 
abundant material if searched carefully. 

The edge of the lake bordering on the beach contains relatively 
few plants and animals. It is exposed to the wind and consequently 
to wavelets which cause more or less movement of the loose sand, thus 
giving slight protection to living things. We find here almost no 


Downing, Our Living World, Longmans, Green, 1924. 

Johnson and Snook, Seashore Animals of the Pacific Coast, Maemillan, 1927. 

Lntz, Field Book of Insects, Putnam, 1921. 

Mann and Hastings, Out of Doors, Holt, 19.32. 

Morgan, Field Book of Ponds and Streams, Putnam, 19.30. 

Needhani and Needham, Guide to the Study of Fresh Water Biology, 3rd ed., Comstock Publ. Co., 

Weaver and Clement, Plant Ecology, McGraw-Hill, 1929. 


./. N lOinkin. Jr. 

A slow-ilowing stream presents a habitat for characteristic plants ami animals 
adapted to this type of environment. Head i)afjes ."5-1. 

plants and only occasional bass, pickerel, or minnows. A few 
dragonfly nymphs live under the small stones in shallow water, while 
ninnerous snails (Campeloma) are foimd buried in the sand or crawl- 
ing along the bottom.^ It is possible to collect a few specimens of 
plankton, which consists of minute free-swimming or floating organ- 
isms, but, on the whole, it is a relatively inhospitable environment 
inhabited by comparatively few organisms. 

Within a few yards of this beach the stream flows gently over a 
shallow sandbar, flanked by cattails and rushes. Here are nimierous 
representatives of several groups of plants : in the water a \'ariety 
of algae, Spirogyra (pond scum), streaming filaments of Ocdogonium, 
Oscillatoria, and Cladophora, and iimiunerable unicellular organisms, 
such as desmids and diatoms. Water cress, water plantain, water 
smart-weed, and burr-weed grow along the banks, while in sheltered 
bays the surface of the water may be covered with duckwec'd or per- 
haps yellow and white water lilies. H(>re and there in boggy i)I;ic(>s 
are dense masses of cattails, yellow flowering rushes, and numerons 
sedges, while on the banks are fotmd grasses of se\'eral species. 

'It is expected that the student will make free use of IV, ' Roll Call." for cencral idcntiBcation 
and of the books of reference noted for more intimate and exact classification. 


buttercups, Jack-in-the-pulpit, bog arrow-grass, and a few shrubs 
such as button-bush and willow. The vegetation shows a zonal 
arrangement of, first, submerged or floating water plants, then emer- 
gent forms, growing in the water and along the banks, while other 
plants such as grasses and shrubs are found at a little distance from 
the water. This zonal distribution is characteristic of shore associa- 
tions of plants and animals. 

In the slow-flowing stream live two species of sunfish, two or three 
species of pickerel, bass, three species of frogs, bullfrogs, green frogs, 
and pickerel frogs with their tadpoles, also an occasional painted 
turtle and water snake. Of birds, the redwing blackbirds are numer- 
ous, with occasional kingfishers, and more rarely a great blue heron. 
Although no mammals are in sight, a telltale mound of sticks shows 
that muskrats live there. Of the smaller organisms, the nymphs and 
larvae of the dragonfly and Mayfly are the most abundant. The 
water swarms with two species of water bugs and diving beetles, while 
beetle larvae and the larvae of mosquitoes are numerous. Many 
crustaceans, tiny amphipods and isopods, may be seen swimming or 
feeding on the aquatic plants. The snails, Physa and Lymnaea, are 
very abundant, while a few aquatic worms, Tuhifex, may be found in 
the mud. Colonies of bryozoans may also be found, incrusting the 
stems of water plants, as well as an occasional mass of fresh-water 

These two regions, the lake shore and the stream, although only a 
few yards apart, present tremendous differences in populations. 
Why these differences? At first sight, one might say it was due 
entirely to abundance of food, but this is only begging the question. 
Evidently many factors are at work. The fauna and flora of other 
localities visited would show even greater changes. Across the 
meadow and up into the nearby woods each locality would be found 
to be inhabited by groups of living plants and animals differing in 
many respects from those in neighboring localities. In each of these 
localities there would be certain dominant organisms better fitted 
than any others to live there. These become permanent species in 
that locality. 

How to Study Ecology 

To understand much al)out ecology, one must be able to do much 
more than simply study a book. The place to study the stage setting 
is the stage. The place to learn about the relation of living things 


to their cnviroiuncnt is the luihitat. Kltoii ' in his iiiteivstiiijr intro- 
duction to ecology cU^scribes the attack on a ceilain ccolofrical prob- 
lem in these words : 

"Suppose one is studying the factors limiting the distribution of animals 
living in an estuary. One would need to know amongst other things what 
the tides were (but not the theories as to how and why they occur in a par- 
ticular way) ; the chemical composition of the water and how to estimate 
the chloride content (but not tlie reasons why silver nitrate precipitates 
sodium chloride) ; how the rainfall at different times of the year affected 
the muddiness of the water; something about the physiology of sulphur 
bacteria which prevent animals from living in certain parts of the estuary ; 
the names of common plants growing in salt-marshes ; sometliing about the 
periodicity of droughts (but not the reasons for their occurrence). One 
would also have to learn how to talk politely to a fisherman or to the man 
who catches prawns, how to stalk a bird witli field-glasses, and possibly how 
to drive a car or sail a boat. Knowing all these things, and a great deal 
more, the main part of one's work would still be the observation and coUeo 
tion of animals with a view to finding out their distribution and habits." 

This gives us our approach. Our own interests, our reading, and 
the time involved must largely determine the extent to whicii we 
solve the ecological problems 
of our own environment. 

Plant and Animal 

In making an ecological 
study of living communities 
we notice that one kind of 
plant or one kind of animal 
is never found li^-ing entirely 
alone. Plants, for example, 
are associated together by 
lack or abundance of water ; 
those living under abundant 
water conditions being called 
hydrophytes ; those associated 
in a condition of moderate 

Water lilies, catta 


1 From Elton, Charles. Animal Ecology, p. 35. By permission of The Macmillan Company. 



Typical xerophytic plants of the desert areas. 


water supply, mesophytes ; and those which associate in desert condi- 
tions, xewphytes. Animals which live in the water are said to be aquatic, 
those on land terrestrial, while those that live both on land and in 
water are called amphibious. Animals and plants associated in still 
water are quite different from those in running water, while different 
types of plants and animals are found close to shore, in deep water, in 
rapid water, on rocky shores or on sandy shores, in salt or in fresh 
water, and in tidal pools or on the sand. Everywhere we find dif- 
ferent associations of plants and animals. Many explanations are 
given, but no one explains everything. One investigator, Merriman, 
emphasizes temperature as an all-important factor ; Walker gives 
atmospheric pressure ; Heilprin, food ; and Shelford, in recent experi- 
ments, indicates that the conditions under which an animal breeds 
may greatly influence its distribution. He experimented with tiger 
beetles, using different soils such as clay, clay and humus, humus, 
humus and sand, and pure sand. The beetles lay their eggs only in 
moist soil, therefore this factor was constant with all the soils. In 
this experiment the soils were also placed at a level and on slants. 


Eighty per cent of all the eggs were laid in steep elay, and <)S per 
cent in sloping soil. Thus ho concludes that the egg-laying hahits 
of these beetles determine tiieir habi- 
tat, for if they could not get the kind 
of soil and the slope needed, they 
would not breed. In this case the 
fluctuation and distribution of a spe- 
cies would be dependent upon a single 
factor. This may be true in the dis- 
tribution of a great many plants and 

Basic Environments 

There are three states of matter, 
gas, liquid, and solid. These are evi- 
dent in the land, the water, and the 
air in which living things are found. 
Life is only found in conditions where 
it is at least partially fitted or adapted 
to live. These conditions, called factors of the en\iroinnent. are air 
or its contained gases; water or moisture; temperature; light; 
chemical constituents in soil, water, or foods; gravity; the presence 
of a substratum on which the organism rests, such as soil, moving 
objects in the water, or the sea bottom ; molar agencies, such as 
wind, water currents, or any moving force in the environment ; and 
finally, biotic factors which come through the interaction of other 
organisms in the same environment. 

A birch forest is composed of 
typical me.sophyte.s. 

Water as a Factor 

Water is absolutely essential to life, from 40 to 95 per cent of all 
living things being formed of this substance. It is generally true that 
no growth or life process of either plants or animals can take place 
without water. An example of this relationship of moisture to life is 
shown in the story of the British Mu.seum snail related by Mr. Baird.' 

" On the 25th of March 1846 two specimens of Helix desertorum, colloc-ted 
by Charles Lamb, Esq., in Egypt some time previously, were fixed ui>on 
tablets and placed in the collection among the other ^h)llusca of the .Musmnn. 
There they remained fast gummed to the tal)let. About the loth of .Marcli 
1850, having occasion to examine some shells in the same, Mr. Il'iird 

1 Ann. Mag. Nat. Hisl. (2) vi. (1850). p. 68. 
H. W. H, — 2 




WiiylU J'itrcc 

These photographs were taken from the same spot on the Mohave desert floor. 
The upper was made at the end of the rainy season, the lower about two months 
later. What one factor causes this difference.!^ 

noticed a recently formed epiphragm over the mouth of one of these snails. 
On removing the snails from the tablet and placing them in tepid water, one 
of them came out of its shell, and the next day ate some cabbage leaf. A 
month or two afterwards it began repairing the lip of its shell, which was 
broken when it was first affixed to the tablet." 


The uses to which water is put by an organism are nianit'old. It 
is necessary as a solvent for foods within the body. In HvIiik tissues 
it becomes a medium of exchange between different parts of tlio body, 
while in higher animals it carries off body heat, thus helping in tiic 
regulation of their temperature. In air it causes humidity. In soil 
it carries the raw food materials of green plants. In many alkali 
lakes, such as Great Salt Lake, fish life is practically absent and the 
numbers of insects and crustaceans inhabiting such water are greatly 
reduced because of the high mineral content of the water. On the 
other hand certain crustaceans, such as the brine shrimps, are only 
found in water containing a high concentration of salts. Acid lakes 
and streams contain only certain types of fish, and according to in- 
vestigation by Jewell ^ are lacking in snails, possibly because of the 
absence of lime from which snails build their shells. 


Differences in climate (which after all are largely differences in 
temperature and water supply) are accompanied by changes in the 
appearance and kinds, of plants and animals. The life processes of 
organisms proceed between certain maximum and minimum limits 
of temperature. Somewhere between these is an optimum temi^era- 
ture at which the life processes function most normally. In i)lants 
optimum temperatures vary greatly for different species, and are 
largely instrumental in determining what plants will grow in a gi\-cn 
locality. For example, apple-raising regions must have a mean 
summer temperature of not more than 70° F. The optimum of 
most tropical plants ranges over 90° F., while alpine species require 
a temperature slightly above freezing. The temperature of plants 
changes rapidly, depending on the amount of external heat they re- 
ceive. This has an important bearing on horticulture. Lemons on 
the trees, for example, freeze at a temperature of 28° F., and oranges 
at 26° F. They are often kept from freezing by means of heaters. 
Plant injuries caused by freezing are due to the rapid withdrawal of 
water from the soft parts, therefore plants with a high water con- 
tent are more easily injured. This accounts for the freezing of the 
young tips of trees. Seeds which have a small water conteut are 
capable of withstanding very low temi)eratures. 

In animals, as in plants, the lif(> processes proceed best at oi)timuiii 
temperatures which differ with the species. Mast p rotozo a divide 

1 Jewell, •• The Fishes of an Acid Lake." Tran.. Amer. M \ol. XLIII, 1924. pp. 77-84. 





*"■ ■ ■>^---''^'>j*S3B||)iiN^:'- 


nil wl^^^^^^^H 


(,?) ir. L. Macchtlin 
During the freezing weather in January, 1937, in California, citrus groves 
which were adequately protected by heaters lost relatively little fruit, while many 
unprotected groves suffered a complete loss of fruit as well as some trees. 

much more rapidly at warm than at cold temperatures, and this is true 
of the reproduction of many animals. Many tropical animals may 
withstand cold temperatures, but will not propagate at those tem- 
peratures. H. B. Ward ^ has made observations on the sockeye 
salmon which indicate that these fish in swimming up rivers to spawn 
always take the river of slightly cooler temperature, a difference of 
1° F. being sufficient to divert the fish. Seasonal cycles of activity 
are largely influenced by temperature, this being particularly true of 
reproductive activity, which plays a part in the migrations of birds, 
the rapid multiplication of plankton and other forms. Some animals 
respond to a cold temperature by going into a resting state or hiberna- 
tion, while others go into a dormant condition because of unfavorable 
conditions of heat and dryness. This latter state, aestivation, is often 
seen in regions having marked periods of alternating rain and drought. 

I Ward, H. B. " Some Responses of Sockeye Salmon to Environmental Influences during Fresh- 
water Migration." Ann. and Mag. of Nat. Hist., Vol. VI, pp. 18-36. 



Animals are said to be warm-blooded or cold-blooded. The foriiK r 
term means that they have a constant body temi)erature {honwio- 
thermal), while the latter means that the body temperature varies 
with the external temperature {poikilothcnnal) . Frogs can often 
be frozen stiff and, when thawed out gradually, will live. This is 
true of many animals and is an undoubted adaptation which enables 
them to withstand great cold. Homoiothermal animals, however, 
are more or less independent of the external temperature because 
their internal body heat remains at a constant temperature regard- 
less of outside fluctuations. 

Animals are divided into two groups depending on whether they 
can easily stand changes in external temperature, some being 
restricted to a relatively narrow range of temperature changes {steno- 
thermaV), while others have not only the ability to withstand a large 
range of temperature, but also may become acclimated to new tem- 
perature ranges if they are changed gradually from one environment 
to another {eur y thermal) . A classic series of experiments by Dallinger 
with protozoans showed that he could change their li\'ing conditions 
from 15.6° to 70° C. without having the animals die. It is this ability 
that gives us the plant and animal populations in some hot springs. 

Light as a Factor 

Light is a form of radiant energy. Passed througli a prism it is 
broken up into the primary colors of the spectrum, each of which has 

Left : A nasturtium plant exposed to ordinary lif^hl sin.e ^.vnuuMum. 
Right: Same plant exposed to onr-sideddlunnnal.on for .SIX hours. 

I^ettuce. a long-day plant. 

Salvia, a short-day plant. These series of plants were grown experimentally 
at the Boyce Thompson Institute for Plant Research, Yonkers, N. Y. 




a different wave length. In addition there is the non-visible radiant 
energy of the ultra-spectrum. These different wave lengths ha\c 
various effects on plants and animals. Chlorophyll, the gnnMi color- 
ing matter of plants, which depends on the presence of light, absorbs 
light waves only from the red and blue bands of the spectrum. 
Whereas most of the radiant energy absorbed by a plant changes 
to heat, a very small part of it, estimated at not more than 0.5 per 
cent to 3 per cent, is used by the chlorophyll in the process of starch 
making. As in the case of temperature, optimum light is necessary 
for the best work of plants, some preferring shade and others living 
at their best in bright sunlight. 

Light causes movements in leaves and stems as well as changes 
in the size and shape of these organs. Plants respond to light, tlie 
leaves being placed so as to get the most light possible. The amount 
of light largely determines the shape of the entire plant, trees in a thick 
forest having a very different shape from similar trees in the open. 

The length of daylight has an effect on plants. Some plants, like 
the radish, spinach, and clover, require a long day to produce flowers 
and fruit, while fall flowers, such as cosmos, dahlia, and ragweed, 
require a short day in order to form flowers and fruit. It has been 
shown experimentally that for each species there appears to be a 
most favorable length of day for flowering, fruiting, tuber formation, 
and other food-storing activities. This discovery is of great value to 




30 V 




Diaptomus Lake Eaton 

Holopfdium 8<Q Sirnon Pond 

Cladocera LaVe Madeleine 


13 Noon 2 



6 8 10 13 Noon 

.V. Y. State Conscrralion Drpt. 

Curves showing the variation in nmnbers over a period of 21 hours of scvcriil 
species of plankton organisms from three Adirondack lakes. W hiil factors might 
be expected to influence their dislribiilion? 



Animals definitely respond by movement to the stimulus of light, 
but unlike green plants, some respond positively and others nega- 
tively. The unicellular Ameba is killed by too much light. Earth- 
worms and some other animals are definitely repelled by light. The 
moth, on the other hand, is attracted to light. Although of great 
importance, light may be injurious to some forms, for bacteria and 
some animals are killed by long exposure to it. The dangers from 
certain wave lengths of light are seen in a bad case of sunburn. 

Light influences animals in other ways. Light stimulus coming 
through the eyes of flounder is said to give rise to changes in the pig- 
ment of the skin. Thus the surface of the skin takes on the general 
color and markings of its background. Some animals in caves lack 
pigment, and there seems to be a general relationship between light 
and pigment in the skin. There is a day and night rhythm in the lives 
of many animals. Land snails feed at night, while activities of most 
birds are confined to the daytime. Bees go to flowers during day- 
light. Migrations of plankton are influenced by light, many crusta- 
ceans coming to the surface only at night and going deep down into 
the water during the daytime. 

A dry alkali lake. 

Life is practically absent in such areas. 
why this is so.'> 

II riiilil I'll rcc 

Can you explain 


Fishing boats at the mouth of the Klamath River in northern Cahfornia. 
Salmon run in on the outgoing tide apparently in response to the fresh water 
coming out through the narrow mouth of the river. 

Chemical Factors 

Under this heading are inckided all of the chemical factors in the 
environment of living things. Such are soil, rocks, and the various 
salts and chemical substances found in food and water. Experi- 
mental evidence shows that certain mineral substances are needed 
for plant growth, and that these minerals are found in the composi- 
tion of living matter. 

Alkali soils form a great problem of agriculture. In sixteen west- 
ern states this is the greatest problem outside of the water supply. 
In thirteen irrigated states there is enough alkali present to be harm- 
ful to crops. Alkalies are chiefly harmful because their presence 
causes the soil water to become permeated with these salts, thus 
hindering absorption of water by the plant. 

Acidity of the soil is another problem for the agriculturist. It is 
produced by a number of factors, such as the removal of calciinn from 
the soil, or the production of acids by certain bacteria or from decom- 
position. Acid affects the plant growth by checking the multii)lica- 
tion of useful bacteria and keeps earthworms and other useful animalN 
out of the soil. However, some species of plants demand aciti soils. 
Mountain laurel, rhododendron, blueberries, and cranberries are 
examples, as are sphagnum mosses found in certain bogs. 



The distribution of fishes and other organisms in water depends 
largely on whether these waters are neutral, acid, or alkaline. Brook 
trout, for example, are usually found in acid and neutral waters, 
while sunfish, bass, perch, and certain other fish are typically asso- 
ciated with alkaline waters. 

Carbon dioxide in the atmosphere is another factor which deter- 
mines plant distribution, three parts to 10,000 being necessary if 
plants are to make starch. Oxygen is essential for living things. 
Certain so-called anaerobic bacteria and a few animals appear to be 
able to live without oxygen. Some insect larvae, worms, and molluscs 
live a part of the year in deep lakes where little or no free oxygen is 
present, due to decomposition of the algae. Certainly one factor in 
the distribution of aquatic animals appears to be the oxygen content 
of the water. 

Gravity as a Factor 

The pull w^e call gravity brings about differences in pressure both of 
air and of water. Plants and animals must adjust themselves to this 
factor. In a general way gravity determines the size of organisms. 
Insects and birds which move about swiftly in the air must be small, 
otherwise gravity would bring them down. Gravity is important in 

the growth and orienta- 
tion of plants. It is a 
stimulus for the direc- 
tion taken by the plant 
body, apparently caus- 
ing the root to grow 
downward and the stem 
to grow upward, while 
horizontal branches are 
neutral to the pull of 
gravity. This same 
force acts upon sessile or 
rooted animals, such as 
hydroids and sponges. 
Adaptations to offset 
the force of gravity are seen in the air spaces of floating plants, oil 
drops in eggs, spines and long hairs on the surfaces of aquatic plants 
and animals, and the air spaces in bones and other tissues of birds, 
and in the construction of feathers. 

Successive positions, from photographs, showing 
effect of gravity on a green plant {Impatiens glandii- 
ligera). — After Pfeffer. 


' 2^ 

'• r^' 





Cypress trees have become adapted to live in swampy lands by developing 
buttressed bases of the trunks and erect growths (knees) from the roots. Tliese 
enable the tree to get sufficient air. 


Anything in which a plant grows or on which an animal comes to 
rest is known as substratum. Types of soil differ from cold, dense, 
clayey soils, which though they hold water do not readily give it up 
to humus that is well aerated, has a high nitrogen content, hokls 
water, and gives it up readily. The distribution of plants depends 
to a considerable extent on the kind of soil found in a given locality. 
For example, mosses and ferns grow in moist soil, while cacti are 
found in sandy desert soils. Varying soil temperatures are brought 
about by the kind of soil, whether coarse or fine ; by the pre.'^ence 
of a blanket of living things over it ; by its color (dark soils absorb 
heat more readily than light-colored soils) ; and by the water it will 
hold (wet soils are cooler than dry). Great variations occur in the 
air content of soils and this again determines the plants and animals 
found in a given area. Water-soaked soil, for examj^le, contains 
practically no air and does not ordinarily have a large jjiant or 
animal population. In some cases a plant adapts itself to water- 
soaked soil, as seen in th(> bald cypress. 



Animals also differ with different types of soil. This is particularly 
true of the bottoms of lakes or streams. A different fauna is found 
on the rocky stream bed from the soft mud of the pool below. Mud 
contains more food, but it is also more difficult for organisms living 
in it to carry on respiration. Soil is also the home of such burrowing 
animals as nematode worms, earthworms, ants, beetles, digger wasps, 
and the larvae of various insects. 

Molar Agencies 

Such are any moving agencies. Running water and winds erode ; 
ice moves soil and rocks. Tides cause great differences in aggrega- 
tions of plant and animal life, animals living between tides having 
different problems to face from those below the tidal flow. Moving 
air has a definite effect on vegetation, as is often seen in the wind- 
blown trees on mountainsides or plains. Moving air acts upon seeds, 
tumbleweeds, spores, and fruits, thus spreading plants over vast 

American Museum uf Xatural History 

Tidal shores, along the New England coast, show wide variations in habitat. 
The flora and fauna of the intertidal zone differs greatly from that of the regions 
above and below the tidal flow. 



\y ritjlil /'it re, 

The ell'ect of differences in environiiient upon the same s[)e(ies of tree (Piiins 
ponderosa). Here molar agencies are largely responsible for Ihi; changed ai)pear- 
ance of the tree. 

areas, but it also fells much timber, breaks off branches, and destroys 
crops. Winds may either help or hinder in the migration of insects. 
The cotton boll weevil travels north more rapidly in the years when 
more wind is recorded. Winds blow birds and insects out to sea, 
thus destroying them, or they may land them in a new location where 
they may multiply rapidly. Currents of air as well as water currents 
distribute plants and animals. Many animal forms react to wind 
and water currents. Fish head upstream, an adaptation favorable to 
food-getting. The swiftness of the current not only determines tiic 
distribution of fishes, but also of other forms, such as caddis fly 
larvae and "water pennies." 

Biotic Factors 

These are factors arising from the presence of other li^•ing organ- 
isms. One is concerned when studying ecology not only with the 
environment of living things but also with how li\ing things react 
on others in their immediate environment. There is competition 
not only between plants and animals, but between plants of the 


same and of different species for a place under tlie sun ; literallj'- under 
the sun, for competition is caused by the limited amount of light that 
will fall in a given area. Young plants often die because of the shad- 
ing by the parent plant, and larger plants preempt areas of soil which 
give little or no space for young growth. Feeding by animals, such 

Effect of sheep grazing upon trees. Thousands of young trees are destroyed 

every year in this way. 

as rabbits or sheep, may change the entire flora of a region, while 
parasitic organisms injure a vast number of the hosts on which they 
live. There are also marked cases of partnership between organisms, 
bacteria in the soil giving and taking from both plants and animals, 
and helping to create a cycle of food substances which pass through 
the bodies of both animals and plants. The feeding of animals is their 
biggest business in life, and the presence of a food supply determines 
very largely the presence of animals in a given locality. It is said 
that the oak tree serves as food for over 500 species of insects, the 
apple for 400, clover and corn, over 200 each. Thus man with his 
tilling of the soil, destruction of the forest, and domestication of 
plants and animals has changed the fauna and flora of the land. 

Having discussed the effects of the factors of the environment on 
living (jrganisms, let us now see how the interaction of these factors 
affects life in the situations that living things are forced to meet. 
Animals and plants must be adapted to live either in the water, in 
air, or on land. The pages that follow show some of these adaptations. 



Life in the Water 

Plants are adapted for lite in water hy a mucli reduced root system, 
by leaves which either float, are ribbonlike, or are finely divided with 
air passages and air spaces. The latter spaces help buoy up the plant 
and also allow for an accumulation of oxyu-en and carbon (hoxidc 
Green coloring matter is abundant, such plants being better fitted 
for vegetative propagation than reproduction by flowers and fruits, 
as is shown by their numerous horizontal and thickened stems. In 
general, aquatic plants are restricted to relatively shallow water, 
many species being found floating near the surface. 

Animals, usually locomotor and having definite adaptations for 
movement in the water, have a much wider ^•ertical range. The 
bodies of most fishes are more or less streamlined, and protected by 
mucus which covers the backward-pointing scales, their fins being 
placed where they offer the least possible resistance to the medium. 
In some animals, the limbs are transformed into flii)pers, while in 
lower types, such as protozoa, threads of living matter, cilia, are 
used as whiplike organs of locomotion. Since the oxygen content 
of water is only about 1 per cent as against over 20 per cent in air, 
we find special adaptations 

for taking in oxygen. These 
structures are usually in the 
form of gills, delicate struc- 
tures which will be discussed 
more fully later. 

The water forms an ideal 
medium for vast numbers of 
small, free-swimming, or float- 
ing organisms, the plankton. 
Oceans and lakes swarm with 
them. Every small pool has 
its plankton, and even rapidly 
flowing waters will disclose 
some of these tiny organisms. 
In certain tested regions in 
the Atlantic, plants form about 56 per cent and animals 44 jx-r cent 
of the total plankton. The flora consists mostly of diatoms, bac- 
teria, and many forms of algae, while the fauna includes numerous 
dinoflagellates and other one-celled animals, eggs of fish, molluscs, 

Diatoms have various forms aFid may !><• 
colonial as well as unicellular. riicrc arc 
probably I. "),()()(» species known. 



numerous crustaceans mostly copepods, jellyfish, and the larvae of 
many crustaceans, molluscs, and fish. Some of the plankton, such 
as small crustaceans, tunicates, medusae, small fishes, and larger 
algae, may be visible to the naked eye, but most of it is microscopic. 



























1 M 








1 M 








1 M 










iV. y. Siaie Conseruation Dept. 

Comparison of the distribution of nannoplankton (minute forms that will 
pass through the meshes of a plankton net) at the surface and bottom of four 
Adirondack lakes. 

The larger pelagic organisms mostly found in the ocean, such as fish, 
squid, whales, turtles, and seals, are called collectively nekton. 

Currents, wind action, the shapes of bays and coasts, migrations 
of various animals, all cause differences in the horizontal distribution 
of plankton. Sometimes given forms, as Cladocera, will multiply very 
rapidly, even coloring the water in a large area. The vertical dis- 
tribution is much more regular with reference to plants, since algae 
and other green plants depend upon sunlight. Plants get very little 
light at a depth of 100 meters. At 75 meters' depth, only half as many 
plants are found as at 50 meters, and careful investigation in various 
areas shows that most of the plant plankton lives within a few feet 
of the surface. On the other hand, animals exist at great depths. 
Beebe reports jellyfish, shrimps, and other plankton at a depth of 
over 1000 feet and the tunicate, Salpa, as well as fishes, at his greatest 
depth of 3028 feet. Dredgings from the "Challenger" and other 
expeditions reveal many living organisms, particularly protozoans, 
in the abysmal depths. 

Till': sr\(;|.: si:rnN(; o., 

Towins with a phnikton (a llun-mcshcd uv\ cf l,oltii,K ,-l„tl,) 
near the surlacc of tlu> ocean on an early summer day would yield 
a very different distribution of organisms from those collected "on ;, 
fall or winter day. There is a seasonal variation in distrihutioii. 
The eggs and larvae of animals ar(> abundant in the spring and early 
summer, while great numbers of algae appear then which are Hot 
found later. This rhythm of plant life is believed to be correlated 
with a turn-over of the available phosphates and nitrates in the water. 
In the winter, the (^ooler top layer of water sinks and pushes up the 
water rich in the salts necessary for plant growth from underneath, 
so that with the coming of warmer weather tlu^ life cycle goes on and 
a seasonal rhythm of algae appears. This turnover of plant and 
animal life is very great. The fishing industry on the Grand Banks 
and in the North Sea is largely due to the occurrence of this great 
seasonal rhythm of plankton. 

There is also a considerable variation in the numbers of plankton 
near the surface of the water during the day and night. Many crus- 
taceans, for example, come to the surface at night and go down in 
the daytime, while green algae are usually nearer the surface during 
the day. 

In oceans and lakes, there is a more or less distinct zoning of living 
forms, depending on the depth of water, the type of shore, or the kind 
of bottom. A very different fauna and flora exist on a rocky coast 
from that along a sandy beach. The forms of botii plants and ani- 
mals are different in salt and fresh water areas. 

Life in the Air 

Here life is more circumscribed. There are no true air plants unless 
they be the so-called epiphytes of the tropical rain forest, some algae, 
such as the Pleurococcus found on the bark of trees, or the lichens, 
which encrust rocks and tree trunks. The reproducti\'e bodies of 
plants, such as spores, seeds, and fruits, are furnished with adai)ta- 
tions which enable them to pass long distances through the air, thus 
allowing new areas to be populated. In animals where locomotion 
is possible various special adaptations exist. Flying animals hiivv 
their wings placed wdiere they will not onlv cause the liody to mo\e 
forward, but also assist in balancing it. Instead of one ])ropeller 
placed astern, as in fish, flying animals have two paired i)roi)ellers 
placed forward at a greater breadth of beam. The body is not onl.\- 
streamlined, but in higher forms special adaptations exist for protec- 
H. w. H. — 3 



Epiphytes in a semitropical forest. Note the aerial roots for securiiif? moisture 

from the air. 

tion against low temperatures and moisture. Oiled skin and feathers 
of birds are examples. Bones are hollow and large air spaces are 
found between muscles. In insects a special aerating system exists, 
since in these heavier-than-air machines a very rapid oxidation of 
fuel material must take place if the organism is to be efficient in 
the medium. 

Life on the Land 

Adaptations in plants for life on the land are seen in the widely 
branching root systems, the woody stiffened stems, the leaves placed 
in positions where light may reach them, and in the various adaptive 
movements which enable green plants to get a share of the much 
needed light. In tropical rain forests, this relation to light is seen in 
a vertical zoning where sun plants form long twining stems, making 
their way up the tall trimks of trees to an upper zone where light is 
available, while in the lower areas are found shade-loving plants which 
prefer less sunlight. In animals, where movement is much more 
evident, there are special adaptations in the form of legs, which 
support the body off the ground and allow of various types of loco- 
motion such as climbing, crawling, walking, running, and leaping. 


Various other types of movement are found as, for example, tlir 
waves of muscular contraction in the foot of the slug ; the crawling 
of earthworms where tiny setae are used as levers; the erawlinfr 
of the snake with its definite use of scales as "ground grippers" ; the 
adaptations for leaping in the grasshopper and the frog; adaptations 
for climbing, such as the sucking disks on tiie toes of tree frogs {Ilyln) 
and of some lizards, or the arrangement of the toes in climbing birds. 
These and scores of other adaptations for obtaining food, for brcatii- 
ing, and for protection may be recalled. 


Borradaile, L. A., The Animal and Its Environment, Oxford University Press, 

London, 1923. 

A general book on the natural history of animals. 
Elton, C., Animal Ecology, The IVIacmillan Co., 1927. Chs. I, II, III, I\', \'. 

A fascinating book, written in a charming style. Accurate and authentic. 
Jordan, D. S., and Kellogg, V. L., Animal Life, D. Appleton tV: Co., 1900. 

Contains some valuable chapters fundamental to an understanding of 

Needham, J. C, and Lloyd, J. T., The Life of Inland Waters, Charles C. 

Thomas, 1930. Chs. Ill and V. 

Interesting aquatic natural history. 
Pearse, A. S., Animal Ecology, McGraw-Hill Book Co., 192G. Chs. II and III. 

Rather technical. 
Shelford, V. E., Animcil Communities in Temperate America, University of 

Chicago Press, 1913. 

A pioneer work, but still reliable and usable. 
Shelford, V. E., Laboratory and Field Ecology, The Williams & Wilkins 

Co., 1929. 

Very usable for field work. 
Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co., 

1929. Chs. IX, X, XI, XII, XIII, XV. 

Authentic and well written. It should be of great value in the field. 



Preview. A comparison of two forests • The why of distribution ; 
barriers ; successions and their causes ; overpopulation and its results • 
The shifting world of organisms • Ways of locomotion • Adaptability to 
new conditions • Human interference • Life zones • Life Realms • Sug- 
gested readings. 


The science of Ecology, or the distribution of animals and plants in 
a given habitat, was considered in the preceding section. Chorology 
attempts to determine the laws governing the distribution of animals 
and plants over the surface of the earth. 

So long as man accepted the naive assum]3tion that the earth was 
originally populated by means of isolated creative acts, there was 
no point in attempting to explain the distribution of living things. 
They had all been put arbitrarily in the places where they occurred, 
and that was all there was to it. With the rise of the belief which 
culminated in Darwin's famous theory, that dissimilar species have 
arisen by modification from other species, and that all organisms are 
related, more or less distantly, to one another, the interpretation of 
plant and animal distribution became a very interesting and challeng- 
ing field for study. 

How about the varied populations of living things in arctic, tem- 
perate, and torrid climates ; the absence of animals and plants from 
areas quite suited to their existence? Why is it that tapirs are 
found only in South America and the East Indies, while certain 
fishes, such as the pickerel, occur only in North America and north- 
em Europe ? Equally difficult aspects of distribution cropped out, 
notably in the Australian fauna and flora, which differ so greatly 
from that of the rest of the world, while most perplexing of all, 
probably, the habit of migration that makes certain animals, such 
as birds, seals, salmon, and eels, change residence regularly from one 
region to another. A gradual suspici(jn that two environments quite 
similar in general appearance might nevertheless be populated by 
species of plants and animals different from each other gave the clue 



to a scientific differoiitiution of specific distribution, AVWor///, Irom 
.aionoral distribution, Choroloc/!/. 

A Comparison of Two Forests 

Two writers, Victor E. Shclford, the well-known ecologist. and 
William Beebe, ornithologist and naturalist, have given two widely 
different pictures, one, an accurate description of a hard-wood forest 
in Illinois, and the other, a survey of life in a British Guiana jungle 

A typical beech-maple forest, such as Dr. Shelford describes, can 
be found anywhere in the vicinity of Chicago. A.ssociated with the 
two dominant trees are ash, elm, walnut, linden, and a wealth of 
smaller trees and shrubs forming a lower layer under the higher trees. 
Wild cherry, sassafras, and dogwood are abundant, and in some of 
the more northern forests, azalea and rhododendron form an inter- 
mediate growth. The floor of the forest is covered with herbs and 
flowering plants, large and small, which change with the season. In 
spring, trilliums, violets, wild geraniums, anemones, phlox, and scores 
of other plants are in bloom, succeeded in the fall by asters and other 
composites, in areas having ample light. A relatively large number 
of plants having spiny or hooked fruits occur, which aid in their 
accidental distribution by wandering animals. A few large mammals, 
deer, fox, and hares, are found occa.sionally, though are rarely seen. 
The woodchuck is perhaps the numerous of the mammals, and 
the red, gray, and fox squirrels are not uncommon. Of birds the 
crested flycatcher, wood pewee, blue jay, scarlet tanager, wood 
thrush, and red-eyed vireo nest in the lower trees, while the oven-bird 
conceals its curious architecture on the ground. The wood frog, 
red-backed salamander, and Pickering's tree frog are found, although 
not always in evidence, and insects abound, especially those that live 
on trees, such as borers of various sorts, beetles, millipeds, spiders, 
and in.sect larvae. Inhabiting the lower layer of the forest are snails, 
centipedes, sowbugs, and earthworms. This represents, \\\\]\ \aria- 
tions, a typical association of life in a northern deciduous forest. 

At first sight the jungle forest does not appear to be very difTerent 
from the northern forest. Both contain large and small trees, the 
larger ones in the jungle, such as mora and greatheart. towering to a 
height of two hundred feet or more, but here the likeness stops. 
There is an almost complete absence of large horizontal branches in 
the tropical forest, the trunks of trees shooting straight up for si.xty 



}\'iUiam Beebe American Museum of Xatural History 

CouTtestj U. S. Forest Serrice 

A comparison of two widely separated forests. The right-hand photograph is a 
typical northern mesophyte beech-maple association, the left-hand photograph a 
tropical rain forest of British Guiana. Note the superficial likenesses and dif- 

or seventy feet without a branch, festooned with long cHmbing hanas, 
which in this way work from the forest floor into the upper zones. 
Four general horizontal regions, or zones of life, are distinguishable, 
namely, the forest floor, the lower jungle up to about twenty feet, the 
mid-jungle up to seventy feet, and the tree-tops, towering a hundred 
and fifty or two himdred feet high. Life at first seems almost absent 
in the jungle to the casual observer, but if one stops, and simply 
looks, the jungle wakes up and life appears everyu^here. The forest 
floor is covered with the accumulated debris of ages, fallen trees in 
different stages of decay, fungi, mosses, and lichens, with a generous 
covering of brown leaves, for here the leaves fall all the year around, 
instead of only in the autumn season as in northern regions. The 
ground area is occupied by occasional deer, paca, and tapirs, with 
agoutis and armadillos found more frequently. Partridge and the 
strange tropical tinamou are seen here and there, as well as jungle 
mice and rats, salamanders, frogs, a few snakes, innumerable scorpions, 
beetles, grubs, worms, and rarely, the unique and interesting Peripatus. 
In the low jungle are found manikins of several species, ant-birds, 
with trumpeters and jungle-wrens, while at night opossums climb 


about through the underbrush. During the daytime tiie wonderful 
morphous butterflies, brilhant spots of blue, add a touch of col.,!- to 
the picture. 

The mid-jungle contains the most life. Here iimumeraljlc birds, 
curassows, guans, pigeons, barbets, jacamars, trogons, and smaller 
feathered species abound, in company witli ant-eaters, sloths, squir- 
rels, bats, coatis, and small monkeys such as marmosets. 

The upper jungle of the tree-tops is the difficult region to 
know. Red howlers and be.som monkeys move about in the tree- 
tops, and occasional glimpses may be had of toucans, macaws, and 
great flocks of parakeets and parrots that live ther(\ Fierce ants 
prevent tree-climbing, and the relatively great height and mass of 
foliage make living things not easily acce.ssible to observers in this 
upper layer of the tropical rain forest. 

These two forests, the northern maple-birch and the jungle, by their 
entirely dissimilar populations illustrate contrasts that might be 
found in many parts of the world. Sometimes conditions in widely 
separated areas may be almost similar, with diverse populations 
inhabiting them, and again, localities close at hand may show remark- 
able diversities in their living inhabitants. When regions far apart 
have similar populations, which does not commonly happen, the 
biologist is faced by a puzzling problem. 

The Why of Distribution 

Jordan and Kellogg ^ give three laws to account for the distril)u- 
tion of organisms which they state as follows : E\ery species is found 
everywhere that conditions are suitable for it unless (I) it was unable 
to reach there in the first place, or (2) having reached there it was 
unable to stay because it could not adapt itself to the new condi- 
tions, or (3) having entered the new^ environment it became modified 
into another species. It is not only the normal habitat that deter- 
mines the presence of a given plant or animal, but its accessibility 
from the place of origin. 

Although every species originated historicaUy from some i)receding 
species at some definite place, its present distribution results from 
the working of two opposing factors, expansion and repression. The 
factors of expansion will be mentioned later, lliose of n^pression 
are, first, inadequate means of dispersal because slow-moving animals 

1. Jordan, D. S.. and KellofiK. V. L., Animal Life. Appletou, 190(). 



are necessarily limited in their distribution. A second means of 
repression lies in the poor adaptability of organisms to new localities 
which they have invaded. A round peg will not fit in a square hole, 
nor a square peg in a round hole, but if the peg consists of a plastic 
material it will adapt itself. The normal habitat for a species is the 
place where the organism is most nearly in physiological equilibrium, 
the geographic range being determined by the fluctuation of a factor, 
or factors, which are necessary for the life of a species. 


Each species widens its range of distribution as far as possible and 
tries to overcome obstacles which nature has put in its way. These 
obstacles may be chemico-physical, geographical, or biological bar- 

In general chemico-physical barriers are climatic in nature, such 
as unfavorable conditions of moisture, soil, or temperature. Soil 
deficiencies, salinity, the presence or absence of light, or character 
of the surrounding medium might also be mentioned. These climatic 


Why might such a mountain barrier restrict the distribution of certain plants 

and animals .3 



barriers may be in vertical zones, extending from tlie ocean level to 
mountain tops, as well as horizontal, spreading out north and south 
from the equator in zones of latitude. 

Map showing ancient and modern ranges of the elephants and their ancestors. 
The shaded area shows the former habitat of the maniniolh and mastodon, 
ant^estor of the modern elephant. A land connection probal)Iy existed I)etween 
Asia and North America. Note the restricted range of the present-day elephants 
indicated by heavy shading. How can this be accounted for.^ 

Sometimes natural barriers occur, such as high mountain ranges 
with eternal snow, deserts with unfavorable conditions of moisture, 
or in the case of water-distributed animals such as fishes, high water- 
falls may prevent them from moving up a stream beyond a cciiain 
point. The barrier for one organism, however, might l)e a highway 
for another. A desert would be an impassable barrier- to a squirn^l 
but not to a camel. 

Geographical barriers have not always been fixed. Geological 
history reveals the fact that some land surfaces were once occujiied by 
water and what is now water may have been land. The presence of 
fo.ssil sea.shells in the Panama Canal area indicates that the Isthmus 
was formerly submerged, and there is evidence that as late as Eocene 
times there was a land connection Bering Straits. As bar- 
riers have changed so has the resulting distribution of organisms. 
Distribution often indicates the geography of the i)ast. .Mnnbers 
of the same genus may differ widely in certain isolated localities, as, 
for example, the tapirs found in tropical America and the Malay 


Peninsula with its adjacent islands. In early geological times mem- 
bers of this genus were widespread and abundant, whereas now, due 
to the disappearance of former land connections, there are but two 
widely isolated species in existence. 

The distribution of animals is bound up in their food supply. 
Hence carnivorous animals are restricted to areas wiiere the animals 
on which they prey live. Often a biological barrier is created by the 
presence of animals which are parasitic on a given form. The tsetse 
fly, Glossina, which frequents the river bottoms and shores of lakes 
in certain parts of Africa, prevents the ranging of other than native 
cattle in these areas because of the fact that they transmit a blood 
parasite fatal to such animals. Man himself is most active in both 
creating and breaking down barriers. He introduces new animals 
and plants either purposely or by chance into areas where they thrive 
and replace other species, or by building dams, irrigating, deforesta- 
tion, or accidentally burning over areas, he destroys one kind of life 
perhaps never to replace it with another. 

Successions and Their Causes 

Succession means that in a given area organisms succeed one an- 
other because of changes in the environment, migration taking place 
so that they may reach conditions favorable to their development. 
An example of plant succession may be seen in almost any pond that 
is gradually drying up. In deep water there are a few submerged 
aquatic plants ; in water from 6 to 8 feet deep floating plants such as 
pond lilies are found ; in shallow water from 1 to 4 feet deep, cat- 
tails and reeds are abundant ; while at the edge we find a meadow of 
sedges and some bushy plants. As the pond becomes drier, these 
plants slowly push outward until eventually it may be completely 
filled with plants which build up soil, making first a swamp and 
eventually a meadow, while around the edge of the former pond will 
now be a forest of trees and bushes. In the tropical oceans different 
corals succeed each other, growing on the skeletons of other species, 
thus building their way into shallow and warmer water, or along the 
ocean shore colonial diatoms may occur, to be followed by hydroids and 
seaweeds, the latter becoming a dominant climax formation, a group of 
species that are better fitted to survive in that habitat than any others. 

Erosion, which carries away the original inhabitants, or a deposit 
of new soil by running water, wind, or other agencies, gives oppor- 
tunity for the establishment of new life in a region thus devastated. 

THE BI()LO(;iC,\L CONQUEST ()\- nil] \\(,|u,|) 


The question of how long seeds will survive, uiidci- whal condilions 
they will germinate, and how fast they will grow is of g,vat inipor- 
tance in the repopulation of areas after soil erosion oi- fire. Beale 
reports an experiment where 
ten out of twenty-two species 
of seeds sprouted after hav- 
ing been buried in open bot- 
tles in moist sand at a depth 
of three feet for over forty 
years. After a coniferous for- 
est has been devastated by 
fire, an entirely new series of 
plants spring up in the area ; 
first herbs, such as fireweed 
or wild mustard ; then trees 
or bushes, the seeds of which 
may be brought by birds, as 
raspberry, blackberry, or wild 
cherry ; later a stage of trees 
having wind-blown or bird- 
carried seeds, such as aspen, 
cottonwoods, or birches. Still 
later the forest may become 
repeopled by its original in- 
habitants, which becomes the 

Conditions of wind, mois- 
ture, sunlight, and weather, the sum total of which constitutes climate, 
play a most important part in succession. If drought destroys life in 
a given region, an entirely new group of plants may come to occujiy 
that area, bringing with them a new group of animals. Migrations 
of animals may be brought about by changing seasons. 

The biotic conditions governing successions are many. Man, 
through clearing forests, throwing wastes into ri\ers, or introducing 
new plants or animals which may compete with existing species, often 
completely upsets the balance of life and causes succe.'^sioiis. Indus- 
trial pollution may completely depopulate streams of fish life, bac- 
terial growth replacing the original plants and animals. Sometimes 
new organisms add so many competing mouths to feed in a gix'cn terri- 
tory that it becomes necessary for some to break away if any are to li\('. 

Wriijlu I'itrct 

A lypiciil undt'CKruwtti succession after a 
I'orcsl (ire. 


Overpopulation and Its Results 

One of the I'aclors in dcteniiiiiiug tlie .spread and distribution of 
organisms is overpopulation. An annual plant, for example, pro- 
ducing only two seeds a year, which is far below the actual number, 
and always developing these into mature plants, in only twenty-one 
years would have 1,048,576 descendants. A pair of common house- 
flies which usually produces eggs six times a year, each batch con- 
taining 150 to 200 eggs, with the young flies beginning in turn to 
lay eggs in about fourteen days after hatching and repeating the life 
cycle, might, it is calculated, beginning to breed in April, if all the 
eggs were hatched and no individuals died, give rise to 191,010,000,- 
000,000,000,000 descendants by the end of August. However, each 
species, year in and year out, tends to remain about stationary in 
number. Indeed, many species are actually disappearing. The 
reasons for this check of potential populations are found in lack of 
adequate food supply, lack of favorable breeding conditions, and in 
the fact that many animals and plants become food for others. 

The Shifting World of Organisms 

There is no doubt that desire for food furnishes the greatest urge 
to locomotion and exploration in animals. Dr. Crothers once said 
in one of his essays that the "haps and mis-haps of the hungry make 
up natural history." Indirectly there is the same necessity for food 
on the part of plants, but here the urge is expressed not so much in 
locomotion as in a struggle for position with reference to light, which 
is essential to every green plant in the manufacture of its own food. 

Changing environmental conditions may force the movements of 
organisms and produce faunal and floral repopulations. For example, 
it is known that drifting coconuts frequently float long distances and 
grow into trees upon some distant shore. A recent cataclysm of 
nature has given us an opportunity to see the repopulation of a 
devastated area taking place. In 1883, the volcanic island of Kra- 
katao was literally blown to pieces by a series of terrific explosions 
that destroyed every living thing on the island. Less than three 
years after the volcano became quiescent, a Dutch botanist visiting 
the island found the ash which covered its surface completely car- 
peted with a layer of bacteria, diatoms, and primitive blue-green algae. 
Here and there ferns were found, along with several kinds of mosses. 
There were even a few flowering plants, but no trees or shrubs. In 


that short time, the naked land had been partially ropoijulatcd with 
these several low forms of life, by spores or seeds blown througli tho 
air, or floated in water from the nearest islands, wliich wore aboiil 
fifteen miles away. Twenty-three years after the explosion, I'rofessor 
Ernst visited Krakatao and reijorted a forest of cocoinit pahns and 
figs growing near the shore line, a luxuriant jungle in the interior, and 
considerable animal life, represented by species that could either fly 
or drift to the island on floating wood. Ernst estimated that within 
another fifty years this island would differ in no respect from its 
neighbors, a prediction, however, wliich seems doomed to failure of 
confirmation because the volcano has again gone on a rampage. 

Variations in temperature, brought about by the changing seasons, 
are a factor in the movements of animals. This is particularly true 
in the case of the annual migrations of such animals as crabs, lobsters, 
and squid, which go into deep water in winter, returning to shallow 
shore-water in spring. Movements apparently dependent to some 
extent upon temperature occur in the case of many marine fishes, and 
birds, certain butterflies, and bats, that go north and south according 
to the season. Many animals mo\T up and down mountain slojjes 
probably for the same reason. 

Sometimes other factors than scarcity of food, em'ironmental 
changes, or seasonal differences cause migration. I^emmings. for 
instance, small rodents living in the mountainous districts of Scan- 
dinavia, at intervals of from five to twenty years suddenly mo\'e 
forth in vast numbers, with no apparent Pied Pi])er of Hamelin to 
lead them, but always in the same general direction, swimming rivers 
and lakes, overcoming all sorts of obstacles, and eventually ending 
the mysterious trek in the ocean. Although they feed on the way 
and consume enormous amounts of food material, the search for food 
is not sufficient to explain their fatal pilgrimages. 

The relation of different degrees of salinity to th(> breeding habits 
of food-fishes jjrobably influences their distribution also, by det(>r- 
mining the character of organisms in their feeding grounds. Tetters- 
son found that herring only enter the Baltic when the .salinity g<'ts 
to a certain degree, whereas Galtsoff found that in America tli(> mi- 
gration of mackerel is due not so much to salinity as to temperature. 
Thus, different factors appear to influence different species in deter- 
mining their movements. 

Birds, of their ability to fly, are better aide to seek out :i 
favorable place for abode than most animals. Many tlifferent reasons 


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have been given to account for the long-distance migrations of ducks, 
geese, the Arctic tern, golden plover, and other remarkable feathered 
travelers. Food cannot be the deciding factor, jjecause many birds 
leave for the south while food is still abundant. Neither can tempera- 
ture be the only cause, because a 
majority of migrating birds go 
south when the weather is still 
warm, while robins and other 
lairds often stay behind and win- 
ter successfully in cold climates. 
Humidity, atmospheric pressure, 
winds, have all been considered 
as playing a part in migration, 
but it is more likely that some- 
thing within the bird rather than 
any external environmental factor 
is the impelling cause for this 
impressive phenomenon. For in- 
stance, among the hormones pro- 
duced by the ductless glands, are 
sex hormones which may stimu- 
late the bird to the extraordinary 
activity that results in long mi- 
gratory flights. How to account 
for the direction and exactness of 
these migratory flights is another 
matter, even more difficult to 

Changing climatic conditions 
probably influence plants more 
directly than animals, because the 
latter are more capable of move- 
ment, and, consequently, better able to escape from unfavorable 
surroundings. Nevertheless, living things make up a world of 
shifting organisms, always on the move. 

Ways of Locomotion 

Much of the delight that the naturalist experiences comes from 
observing and interpreting the ways and devices by which the move- 
ments of organisms are brought about. 


The annual migration routes of the 
Arctic tern. It covers about 22,000 
miles in its yearly round trip from its 
winter range in the Antarctic to the 
summer breeding range in the Arctic. 
Note the different routes taken going 
and coming. 



II riijhl I'll rci 

The Russian thistle {Salsolu) introduced into this country in 1!571. Today it 
covers the entire country. What adaptations have enabled this pesi to do tliis? 

In the world of attached animals, like sea-anemones and corals, that 
apparently are doomed to remain in one place, the free-swimming 
larvae seize the opportunity to break away from the maternal apron- 
strings before settling down for life, just as stationary plants by means 
of spores, seeds, and chmbing or trailing vegetative parts are enabled 
to shift about and occupy new territory. Seeds of orchids and certain 
spores of fungi, mosses, and ferns, for example, are light as dust and 
may be wafted hundreds of miles in the air before settling down to 
germinate on some distant soil. Seeds of dandelions and other plants, 
such as milkweed, willow, and cottonwood, have feathery paracluite- 
like structures, which support them in the air for some time, e\-en in a 
wind blowing only two miles an hour. Insects, ballooning spiders, and 
birds make use of air currents, sometimes being carried long distances, 
particularly by heavy winds. Whole plants, like the Russian lliistl(\ 
and the "resurrection plants" of desert regions, may dry uj) and 
break loose from their anchoring roots, and roll along the ground or 
ride the breeze scattering their seeds, thus taking root in newly invatknl 

H. w. H. — 4 



Estimate ok Seeds Produced by a Single LARtiB WE>;n 

Dandelion . 
Cockle-bur . 
Oxeye daisy 
Prickly lettuce 
Beggar's ticks 







Crabgrass . 
Russian thistle 
Purslane (large) 
Tumble mustard 







Some fruits, like those of violets and the witch-hazel, explode, send- 
ing their seeds to a distance. Even gravity may sometimes be re- 
sponsible for spreading plants by means of soil-slides, while animals 
in such accidentally disturbed soil may be carried considerable dis- 
tances to a new situation. 

Birds inadvertently scatter fruits and seeds by first swallowing 
and then depositing them elsewhere with their droppings. As a 
result cherry bushes and poison-ivy vines may often be seen growing 
along fences where birds have roosted. 

Adaptability to New Conditions 

The fact that some organisms do not invariably adapt themselves 
to new localities which they have invaded is a great deterrent to 
their permanent spread. Successful invaders that gain a new foothold 
as pioneers, and retain it as settlers, are conspicuous enough to be 
discovered and remembered, but unsuccessful ones, reaching the 
Promised Land but unable to establish themselves there, escape atten- 
tion. Indian corn, for example, seems unable to reproduce and main- 
tain itself if allowed to run wild. The yellow-fever mosquito has a 
certain dead-line, north of which it cannot successfully continue to live. 

Just as in economic life, so in communities of plants and animals, 
undesirable individuals frequently appear, bumming their way into 
places where they are not wanted. Weecis are notorious plant- 
hoboes that are pre-eminently successful on their own part, but are 
unwanted by man, and reckoned as outlaws with a bad reputation, 
because they rob other plants which man favors, of food, moisture, and 
sunlight. Having great natural vitality, they are successful because 
they usually grow even in unfavorable conditions which would kill 
competing plants, and produce enormous numbers of seed. Their 
persistence and varied means of seed dispersal are easily realized by 
anyone who has tried to pick "beggar's ticks," and "sticktights," 
and burrs from his clothes after a ramble in the autumn woods. 


Human Interference 

Man is often the unwitting of sliifts, .sometimes with serious 
results, of animal and ])lant ixjpulations. Tlie Russian thistle, 
already mentioned, was introdueed into South Dakota in 1S74 with 
flax-seed from Europe. By 1888, it was reported as a troublesome 
weed in both the Dakotas. By 1898, it had covered all the area east 
of the Rocky Mountains from the Gulf of Saskatchewan, and today 
ranges over the whole country. 

There are many curious cases of the accidental transport by human 
agency of animals and plants to regions far from their point of origin. 
Recently a tropical boa landed in Middletown. Connecticut, with a 
bunch of bananas. Tropical tarantulas, too, are known to be carried 
over long distances in the shipment of this fruit. Such instances as 
these, however, usually have no lasting effect on the general spread 
of organisms, yet they emphasize the fact that unanticipated develop- 
ments in distribution are quite jiossible from very insignificant and 
unsuspected beginnings. Man's interferences with the distribution 
of organisms have by no means always been unfortunate or disastrous. 
In many instances his rearrangements of plant and animal popula- 
tions have been eminently successful. The planting of various 
species of trout in new streams has proved to be a wise move, \\hile 
the introduction of reindeer into Alaska and Labrador is of incal- 
culable benefit to both man and beast. The list of cases where man 
has lifted the lid of Pandora's box and set free plants and animals 
for weal or woe into new localities could be extended indefinitely. 

Life Zones 

Reference has already been made to a zonal distribution of i)hints 
and animals in a pond. A similar condition is easily seen in climbing 
any high mountain. Life zones are often rather sharply marked, but 
usually show transitional areas between them. A region which has 
been carefully studied and which shows this zonal distribution in a 
marked way is the San Francisco mountain region in north Arizona. 
Here, a mountain nearly 13,000 feet in height rises out of a desert 
plain. This mountain shows successively two tyj^es of desert zone, 
a lower and upper, each with its own desert fauna and flora, cacti, 
sagebrush, a few birds, mice, lizards, and snakes. Then a r(>gion at 
between 6000 and 7000 feet of pinon pines and red cedars, inhabited 
by more birds and a small number of mammals. Between 7000 and 



Zonal distribution of flora on a moun- 
tain peak rising from a desert area. 
How would you account for these differ- 
ent life zones ? 

8200 feet there are forests of Douglas and balsam fir, with such mam- 
mals as meadow mice, chipmunks, deer, lynx, and puma. Higher still 
between 8200 and 9500 feet, is a typical Canadian vegetation, timber 

pine, Douglas and balsam fir, 
and aspens, while the wood- 
chuck, porcupine, rabbit, mar- 
ten, fox, wolf, and other northern 
forms are found. From 9500 to 
11,500 feet we find a fauna and 
flora almost like that of northern 
Canada and called Hudsonian. 
Stunted spruce and pine exist 
up to the timber line with a 
few typical mountain mammals 
such as the marmot, and pika 
or mountain hare. Above this 
area lies the rocky Alpine zone, 
snow-clad for half the year even 
in this warm, sunny climate. 
Lichens on the rocks and a few 
stunted herbs are the only plant life visible, while a limited number 
of insects and an occasional mammal from the Hudsonian zone are 
the only signs of animal life. 

The facts that the chorologist has discovered concerning life zones 
have been put to practical use by the Biological Survey of the United 
States Department of Agriculture. A life zone map has been pre- 
pared so that the settler going into a new region will know at once 
the kind of plants and animals best adapted to live there. In addi- 
tion, information is available about the character of the soil, the 
rainfall, temperature range, and the particular cereals, fruits, and 
vegetables that can be grown in the region. 

Life Realms 

Different parts of the world, each with its several life zones, have 
been separated into life regions, or realms. If we plot the distribu- 
tion of a given family of animals or plants, we often find that species 
within the group have a wide distribution, in some instances covering 
more than a single continent. Australia has long been set aside as 
a distinct realm because its peculiar fauna and flora differ from those 
in other parts of the earth and so is called the Australian Realm. 



"L,_ Hblarctic p,. 

3 / 



Auslralia-ri ^:;» 

Map showing life realms. 

Similarly there are the South American, or Xcotroi)i('al, Etliioi)iaii, 
Oriental, and Holarctic realms, the latter comprising most of the 
land surface of the Tropic of Cancer. Each of these regions has 
animals and plants peculiar to itself, although resemblances are often 
found between inhabitants in different realms. 


Beebe, C. W., Hartley, G., Howes, P. G., Tropical Wild Life in Britif^h 

Guiana, New York Zool. See, 1917. Ch. VI. 

Contains an interesting description of a tropical rain-forest. 
Borradaile, L. A., The Animal and Its Environment, O.xford University Press. 

London, 1923. Chs. VII, VIII, X, XI, XIII. 

Excellent for general reading. 
Elton, C, Animal Ecology, The Macmillan Co., 1927. Chs. Ill, V, X. 

Fascinating reading. 
Jordan, D. S., Kellogg, V. L., and Heath, H., Animals, D. Applcton ct Co., 

1909. Chs. VII, XVI. 

Old but reliable. 
Pearse, A. S., A7iimal Ecology, McGraw-Hill Rook Co., 192G. Ch. IV. 

Rather a book of reference than a reading book. 
Roule, L., Fishes, Their Journeys and Migrations, W. ^^■. Norton & Co., 1933. 

All of this book makes interesting reading. 
Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 192S. Cli. III. 

Interesting and reliable. 
Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co., 

1929. Chs. IV, V, VII, XVIII. 

Very' scientific and yet interesting. 



Preview. Relations between members of the same species; care of 
eggs by parents; care of young • Relations of mutual aid • Animal can- 
nibalism • Relations of competition • Relation of members of different 
species ■ Adaptations for food-getting in animals • Scavengers • Food- 
getting in plants ; carnivorous plants • Symbiosis • Commensalism • Par- 
asitism • The chemical relationship of plants and animals • Life habits of 
bacteria • Relation of bacteria to free nitrogen • Rotation of crops • The 
relations between insects and flowers • Suggested readings. 


Those who have been fortunate enough to be in California or Flor- 
ida when the oranges are in bloom will never forget their odor ; nor 
will they, when examining the grove, fail to notice the large number 
of bees vi-siting the flowers. The bees are after nectar and pollen, 
yet without these winged agents, the crop of oranges for the follow- 
ing year would probably be small. This interrelationship between 
insects and flowers was noticed by Charles Darwin, who pointed out 
that the size of the clover crop in England depended upon the num- 
ber of cats in a given region. His friend Huxley, who knew better 
than Darwin how to popularize science, immediately went him one 
better and added that the size of the clover crop depended upon the 
number of old maids. When asked to explain, he gave this logical se- 
quence of events. Old maids keep cats ; cats prey upon field mice ; 
mice provide nesting places for bumblebees ; bumblebees pollinate 
clover, upon which pollination the next year's crop depends. So he 
had a perfectly logical chain of events. Throughout nature there is 
this give and take between different organisms which we call the web of 
life. When man interrupts or displaces a link in the chain of interre- 
lationships, the web is broken and the whole fauna or flora of a region 
may be changed, as in the case of the Englishman who took a bit of 
water cress to Australia, planting some in a nearby stream to remind 
him of home. This foreign plant, having no enemies and finding 
conditions favorable for its growth, literally overran the waterways 
until today the rivers of Australia are choked with water cress. Look- 



ing over the world of plants and animals an unescajDabie dcixMidenro 
of one form of life upon another is found in the food relationship 
by which green plants supply animals with food and in the shelter 
relationship, by which animals find safety in the protection given 
by plants. Reducing this search for food and shelter to its ultimate, 
we find that all animals are dependent upon green plants. 

But does the green plant get anything from the animal ? At first 
sight it would seem as though it were all give and no take. As we 
study the situation more closely, however, we find that food-making 
is dependent upon certain raw materials, some of which, such as 
nitrogenous wastes, can only be supplied from the dead bodies of 
organisms or their excreta. Moreover, another important raw 
material, carbon dioxide, used by green plants in starch-making, is 
given off as a respiratory by-product by animals, and in this same 
process oxygen is released. 

All of these facts suggest certain problems. Why, for example, 
when some animals produce enormous numbers of eggs and others 
only a few, do not the former outnumber the latter? Of what 
significance is the mutual aid so frequently observed in nature? 
What is symbiosis and why is it significant? What is the \'alue of 
pollination by insects as compared with pollination by other means? 
What part do bacteria play in the fives of plants and animals? 
What is the reason for parasitism ? Can the oft-repeated statement 
that green plants make food for the world be proved ? A start on the 
answers to some of these questions will be made in the pages that follow. 

Relations between Members of the Same Species 

Many examples of helpful relationships can be .seen between ani- 
mals of the same species, especially in the care of young. Although 
in low forms, such as sponges, coelenterat(>s. echinoderms, and a good 
many fishes, large numbers of eggs are laid and given little or no 
parental care, the production by the male of immense numbers of 
sperm cells in the vicinity of the eggs insures chance fertilization and 
continuity of the species. For example, Norman ' reports that a cod 
w^hich weighed 21^ pounds produced over 6,650,000 eggs. At tiie 
time of egg laying each male of the above .species throws billions of 
sperm cells into the water near the eggs. Higher in the animal scale 
we find greater provision for care of the young correlatcnl with a re- 
duction in the number of eggs laid. Many insects lay their eggs on 

1 Norman, J. R., A History of Fishes. Stokes, 1931. 



Bruinu II 

Ichneumon fly {Ophion macnirum) 
laying eggs in the cocoon of a Cecropia 

plants which will become food for the larvae or caterpillars. Others 
lay their eggs either in the ground where they are protected, or in 

dead bodies of animals on which 
the larvae may feed, as in the case 
of certain beetles, or in a ball of 
dung, as in the case of the dung 
beetle. Certain ichneumon flies 
bore deep into tree trunks in order 
to lay their eggs in the larvae of 
wood-boring insects. Some w^asps 
paralyze caterpillars or spiders, 
laying eggs in the still living victim 
so that when the eggs hatch the 
young larvae will have food. In 
many animals, food is provided in 
the yolk of the egg, the eggs of 
fish and birds being examples. 
Spiders and earthworms form 
cocoons, which in the case of the 
earthworm are usually filled with a nutritive fluid on which the young 
feed after they are hatched, while in the cocoon of the spider the 
young feed upon each other, the strongest of the group surviving. 

Care of Eggs by Parents 

Some of us as youngsters have angled for sunfish and will always 
remember the thrill that came when a brightly colored male dashed 
at the bait dangled over the hollowed nest containing eggs which he 
was guarding. From the simple nest of sunfish and salmon through 
the more complicated nests of the stickleback or lake catfish we come 
to the more elaborate nesting habits of birds. Some birds, as terns, 
sandpipers, or gulls, simply make shallow holes in the sand, as does 
the sand ostrich. Grebes and rails make nests of floating decaying 
vegetation. Nuthatches and woodpeckers make nests in holes in 
trees where the young are protected. At the top of the ladder are 
more elaborate nests such as those of the oriole and oven-bird of our 
latitude or the tailor bird and weaver bird of the tropics. 

Care of Young 

Sir Arthur Newsholme has said that the most dangerous work in 
the world is that of being a baby. If the young of plants and ani- 



A. 1'. .sV((/i CtinserrniUin iJcpl 

Stickleback and nest. Of what advantage would this be to the species? 

mals survive this dangerous stage, their chances of growing to adults 
are very considerable. Although parental care is not associated 
with plants, nevertheless in low forms of plant lif(> locomotor stages 
occur, called zoospores or swarm spores, by means of which the plants 
gain footholds in new areas. Many devices have already been men- 
tioned by means of which seeds are scattered far from the parent 
plant. In higher plants, hard shells, spiny coverings, or inedible pulp 
protect seeds within the mature fruit, thus giving greater ojjpor- 
tunity for the scattering and germination of seeds. 

Adaptations for the protection of young are more evident among 
animals. In crustaceans, the larvae of which form the chief food 
for great numbers of fish, there are not a few protective adaptations. 
In some instances crustaceans have brood pouches in which the young 
are kept, or, as in the case of crayfish and lobster, the developing 
eggs are cemented to the abdominal appendages of the mother and 
carried around by her. The male bullhead .swims arountl with and 
broods over his young, while the male sea horse has a brood pouch in 
which the young are held. In some worms and crustaceans, the eggs 
may be retained in the burrow of the parent, or they may be held 
in the mantle cavity or a space similar to it, as in the fresh-water 
mussels, barnacles, and tunicates. Some spiders, notably the wolf 
spiders, carry the egg cocoon about with them and when the yoimg 
are hatched, they are carried on the backs and legs of the female 



Huijh Spiricer 

A spider with its egg cocoon. 

until large enough to care for themselves. The male of the so-called 

midwife toad (Alytes) carries the eggs entangled around the legs. 

The male Surinam toad places the eggs on the back of the female, 

where each sinks into a tiny pouch as it develops. 

Animals that lay eggs which hatch outside of the mother's body 

are said to be oviparous. A modified form of this procedure is seen 

in some nematodes, arthropods, 
fish, amphibia, and reptiles. Here 
the eggs remain in the oviduct or 
uterus of the mother until they 
are almost ready to hatch, the 
body of the mother acting as an 
incubator. Such forms are said 
to be ovoviparous. Most of the 
mammals which retain the eggs in 
the body until the young are born 
are said to be viviparous. Here 
the young are held as embryos 
within the body of the mother 

and nourished by means of an organ called the placenta. The young 

of mammals are suckled at the breasts of the mother until they 

are able to eat solid food. 

Relations of Mutual Aid 

A certain amount of protection is afforded plants from their habit 
of living in communities. Examples are the aggregations of cacti in 
our western deserts or the acacia and "thorn bush" communities of 
Australia. The animal world, too, shows many examples of protec- 
tion among gregarious forms. The schooling of fishes not only is a 
defense for the group from larger fish, but it also enables small fish, 
working concertedly, to prey on organisms much larger than them- 
selves. The driver ants in Africa, traveling in great swarms, often 
overcome and devour animals hundreds of times larger than them- 
selves. Wolves hunt in packs, several of them rushing together to 
bring down their larger prey. Deer and other herbivorous animals 
move in herds for mutual protection. 

Another relation of mutual aid results from the development of 
division of labor among certain animals. Although social division 
of labor is well seen in the human species, there are many examples 
in the insect world, particularly among the social bees and wasps, 



such as tho division of the colony into castes thai include nuih-s 
(drones), fertile females ((lueens), and infertile females (workers). 
Castes are even more mmierous among ants, there being winged and 
wingless females, intermediates between females and workers, soldiers, 
several groups of workers, and winged and wingless males. Not all of 
these forms, however, are found in any one species. By means of such 
division of labor, life in the colony goes on at a very efficient level. 

Animal Cannibalism 

Most of us have had the experience of having some pet destroy 
her young when they were in danger, or of having laboratory-bred 
rats or mice eat their newborn young. This is probably a perverted 
instinct, but nevertheless animal cannibalism is .seen rather fre- 
quently. The destruction of a wounded member of a pack of wolves 
when hunting is usual. The female spider usually kills the male 
after fertilization of the eggs, this habit being common to some 
other forms. Similarly the eggs may be destroyed by the male, 
as in the case of the mole cricket and centipede, w^hich eat the eggs 
shortly after they are laid, the mothers resorting to numerous pro- 
tective devices in order to thwart the cannibalistic fathers. Many 
fish eat the eggs of their own species. Even the domestic hen at 
times will eat her own eggs. 

Relations of Competition 

Evidences of competition in the plant world are numerous. Be- 
cause of their sessile habit, older plants may overshadow and crowd 
out the young ones, or one group of plants may prevent the growth 
of other plants in the vicinity. Weeds and plants in general pro- 
duce enormous quantities of seed, which are kept from germinat- 
ing by the rapid growth of the older plants. Many grasses and some 
shrubs grow rapidly by means of underground shoots, in this way 
securing territory which might be used by other plants. Thus plants 
with favorable adaptations may completely pre-empt new territory for 
themselves at the expense of others able to use the environment. 

In animals, competition between individuals of a species is almost 
universal. Males fight each other for the possession of females, or 
sometimes just for the sake of fighting. There is a contituial struggle 
for food, for water, and for a place to live. I>arger animals, as we 
have seen, prey on smaller ones and in general those best fitted to 
compete in the battle of \Uo, survive. 



Wright Pierce 

This desert weed, rabbit brush (Chrysothamnus nauseosus) has pre-empted newly 
cleared areas along the border of the Mohave Desert. How would you account 
for its rapid spread ? 

Relation of Members of Dififerent Species 

No one who has carefully watched the life that goes on in a grove 
or forest can escape seeing there the enactment of a drama that repre- 
sents the larger picture of relationships between living things the 
world over. Insects are flying through the air, crawling along the 
ground, or burrowing into decaying logs and the ground. Spiders 
and ground beetles may occasionally be observed making off with a 
victim, while here and there birds such as woodpeckers, flycatchers, 
and warblers may be seen feeding on adult insects or their larvae, 
while a hawk may be watching to pounce upon some one of the 
insect-eating birds. If we were able to make a prolonged study of 
the area we would find that squirrels, rabbits, and wood mice are food 
for larger flesh-eating animals or carnivores, such as foxes. In such 
an area we might also find a series of herbivorous animals ranging 
from plant lice (aphids) living on the leaves of trees to occasional deer 
which browse on the leaves of the same plants. 


plants rrxike 
the fooct fbr- 
tha- worloC 


It will be noted in the illustrations given that animals almost 
mvanably feed upon others smaller than themselves. The same 
relationship is seen in lakes or oceans where microscopic plants and 
animals (plankton) form the food of other larger organisms, especially 
fish. These living things form 
definite "food chains" in 
which larger animals feed on 
smaller and smaller ones until 
ultimately the lowest forms 
subsist on tiny green plants 
or bacteria. For example, in 
a small pond we may find 
billions of diatoms, unicellular 
algae, and protozoa and feed- 
ing on them millions of small 
crustaceans. With them are 
thousands of insect larvae, 
hundreds of small fish, and a 
few large fish, such as bass, 
pickerel, or perch, which are 
dependent upon all the other 
forms of life. In this case a 
few large animals are depend- 
ent for food upon the development of myriads of smaller organisms, 
the basis of this food being very simple plants. Take away any link 
in the food chain and life in the pond becomes disorganized, with the 
ensuing death of many of the inhabitants. 

Since smaller animals reproduce more rapidly than larger ones, 
the food supply for those "on the top of the heap" remains fairly 
constant. It should be borne in mind, however, that the larger 
animals require a range of sufficient size to support them. 

Adaptations for Food-getting in Animals 

Protozoans, if ameboid, engulf their food, but in other members of 
this group, food passes into the cell through a definite opening or 
through the plasma membrane. Sponges and many molluscs pick 
up microscopic food as it comes to them in water currents. Some 
molluscs bore holes through the hard shells of bivalves, in that way 
securing the soft parts of the animal for food. Insects have biting, 
chewing, or sucking mouthparts, each type being fitted to utilize a 

° Gat gi-ass 

\\ hy will a break in the food chain often 
cause disorganization of life in that locality ? 


Wright Pierce 

Adaptations of beaks of birds for 
food -getting. 

different kind of food. Carnivorous 
mammals have sharp teeth fitted for 
tearing and holding prey ; herbivorous 
mammals have flat, corrugated teeth ; 
rodents, gnawing or chisel-like teeth ; 
while snakes, which swallow their prey 
whole, have pointed, needlelike teeth 
to hold their food securely. More 
striking adaptations for food-getting 
are found in birds whose beaks and feet 
both give clues to their food habits. 
The flesh-eating birds have hooked 
beaks and curved claws ; aquatic 
birds have feet shaped like paddles 
and scooplike bills for straining out 
small organisms from the water ; wad- 
ing birds display a remarkable variety 
of highly specialized beaks and feet ; 
and the smaller land birds show 
equally interesting adaptations for se- 
curing food. Bizarre adaptations for 
procuring food characterize the giraffe, 
with its long neck that enables it to 
reach up to feed on branches of trees 
fifteen feet from the ground, the ant- 
eater, with its sticky tongue, and the 
walrus, which digs bivalves with its 


Some forms of life are not only om- 
nivorous in their diet, but are actually 
scavengers, living on dead organic ma- 
terials. The bacteria,^ smallest of all 
plants, feed upon or destroy millions 
of tons of organic wastes which other- 
wise would make life on earth impossi- 
ble. Think of a world without decay. 
Land and water would soon become 

' See pages 165-166. 


roverod witli the dead bodi(>s of plants and animals. TUr i)acteriu of 
decay are very numerous in rich, damp soils containing large amounts 
of organic material. They decompose organic materials, changing 
them to compounds that can be absorbed by plants to be used ii, 
building protoplasm. Without decay life would be impossible. f„r 
green plants would otherwise be unable to get the raw food materials 
to make food and living matter. 

In general all plants, both colorless and green, may be said to play 
a part in ridding the earth of organic wastes. The fungi, or colorless 
plants, get their nourishment from the dead bodies of plants and 
animals, while the green plants take organic wastes from the soil 
to be used in the manufacture of foods. 

Many animals also take part in scavenging. Some of the food of the 
protozoa is made up of decaying unicellular material and the bacteria 
which cause its decay. Certain forms, especially insects, feed upon and 
lay their eggs in decaying flesh, while myriads of insects and their 
larvae help to break down decaying wood in a forest. These are 
only a few instances of this important function. 

Food-getting in Plants 

Although green plants make foods and use raw food materials ' from 
their environment to do this, there are some that destroy foods. 
Fungi, such as bacteria, molds, smuts, and rusts, ruin billions of dollars' 
worth of food plants and plant jiroducts each year. This is seen in 
damage to crops, fruits, stored foods, and animals used as food by 

Carnivorous Plants 

A curious exception to ordinary green plant nutrition exists in 
carnivorous plants, which also illustrates a different interrelationshij) 
between plants and animals. Carnivorous plants add to their nitro- 
gen requirement in several ways. The fresh-water aquatic plants 
known as bladderworts {Utricidaria) catch water fleius and other 
small crustaceans in hving bladderlike traps. Just what lure urges 
the crustaceans to destruction is hard to say, but the fact that they 
are caught in numbers is verified by their decomposed remains found 
in the bladders. Other animal-eating forms are the various pitch(>r 
plants (Sarracema sp.), some of which are found in our northern 
swamps. Insects are apparently lured to the urn-shaped leave^' 

> See pages 253-262. 



by a trail of sweet nectar secreted just outside the mouth of the 
pitcher. Once inside, a shppery surface and incurving hairs prevent 
egress, and the insect is soon digested by the enzymes in the fluid 
contained inside the pitcher. Still another leaf modification with a 
similar function is seen in the sundew (Drosera sp.). Here the leaves 
are covered on one surface by sticky glandular hairs, which close 

The leaf of a bladder wort {Uiricu- 
laria vulgaris). Many of its numerous 
divisions bear bladders (6), especially 
near the place of attachment to the 
main leaf axis (a). Note the aper- 
tures of the bladders (p) into which 
small aquatic animals may crawl or 

The modified leaf of a sundew {Drosera 
rotund if olia) showing the conspicuous 
glandular hairs (g) covering the upper 
surface, the hairs at the right having 
caught an insect. Note that the hairs 
are tipped by a drop of secreted liquid 
{d), which attracts insects to the leaf and 
also entangles them. — After Kerner. 

over the insect, hold it fast, and ultimately digest it and absorb its 
juices. In the Venus's-flytrap (Dionaea sp.), another carnivorous 
plant found in some parts of this country, the leaves have two sensi- 
tive lobes provided with marginal hairs. If an insect lights on a 
leaf, the two lobes close over it and the insect is trapped. After its 
prey is digested, the lobes of the leaf open up and the plant is ready 
for action again. 


The process of living together for mutual advantage is called 
symbiosis. Plants may join forces as may animals, or in some 
instances, plants with animals. Lichens, for example, illustrate this 


W I III III I'll rci 

An encnistiiif^ licht-n. Why docs lliis pl;inl suc- 
ceed in such an unfavorable cm ironiucul i' 

mutual partnership in 

an interesting way. A 

lichen is composed of 

two kinds of plants, a 

green alga and a fungus, 

one of which at least 

may live alone. The 

two plants form a part- 
nership for life, the alga 

making the food and 

nourishing the fungus, 

while the latter gives the 

alga raw food materials, 

protects it, and keeps 

it from dying when the 

humidity of the air is low. 

Other examples are bac- 
teria and the mycelial 

filaments of fungi {my- 

corhiza) which live sym- 

biotically on the roots of certain plants, taking food from the plants, 

but giving them nitrogen in a usable form in return. 

A common example of symbiosis between plants 
and animals is the green Hydra {Chlorohydra viri- 
dissima), which holds in its body wall a unicellular 
alga known as Zoochlorella. These plants contain 
chlorophyll, using the sun to make food. In this 
partnership, the algae get carbon dioxide and ni- 
trogenous wastes from the animal, to which, in 
turn, they give food and the oxygen set free in the 
process of starch-making. There are numerous 
examples of this kind of symbiosis in tlio animal 
world, as is seen in many of the protozoa, sjionges, 
A root' tip of the coelenterates, flatworms, molluscs, and sea urchins. 

European beech xhe symbiotic relationship of animals to each 

{Fagus sylvalica), ^^^ -^ ^j^^^^.^^ y ^j^^ ^j, protozoans iixing in 

showing ectotrophic '^ „ . i •. * 

mycorhiza, the fun- the digestive tracts of termites or white ants. 

gal hyphae forming These Httlc animals act as digestive cells for the 
:nS:the?tLtLt termites, making it possible for them to use 
-After Frank. wood fibers on which they live. In return th(> 

H. W. H. — 5 



a shark- 


protozoans receive food and are protected by their hosts. A some- 
what similar situation prevails in the large intestine of man, where 
certain types of useful bacteria are found. These forms help keep 
down putrefying bacteria, receiving in return a home, food, and 
a favorable temperature in which to live. Certain species of ants 
protect and feed aphids, in turn feeding upon the sweet fluid secreted 
by the aphid. 


Some associations are not obviously to the advantage of either 
organism, the two feeding together as messmates. Animals like 

the small crabs that live 
in the water canals of 
certain sponges, or the 
tiny fishes that live in 
the lower part of the 
body of a "trepang," a 
sea cucumber, are ex- 
amples. The young of 
some species of rudder- 
fish (Stromateidae) ac- 
The shark sucker (Remora brachypiera, Lowe) company large jellyfish, 
showing sucking disk and its method of attach- geekina; shelter under 
ment to the shark. The Remora gets free trans- ,, ■ +• • j. 4. i 

portation and makes sudden forays after food as their stmgmg tentacles 
well as sharing the "left-overs" of the shark's food, when chased by larger 
But it seems doubtful if the shark gains anything ggj^ while another fish 
from the association. . ^ . 

{Nomeus) lives in con- 
stant association with the beautiful coelenterate known as the 
Portuguese man-of-war. 


Not all life is give and take. Some plants and animals live at the 
expense of others, giving nothing and taking all. These are known 
as parasites, the organism which entertains them being called the 
host. From the lowest to the highest forms in the plant and animal 
kingdom there are few which are not attacked by parasites at some 
stage of their existence. 

Parasitism implies plenty of food, shelter, and a relatively protected 
life for the parasite, but it also usually spells degradation in structure 
and loss of activity. It may mean only inconvenience, but more 
Ukely a shorter and disturbed life for the host, especially if the parasite 



causes disease. In some instances, the complicated life history is so 
bound up with more than one host that if one of the hosts is absent 
a hnk in the chain of life is broken, the life cycle cannot be completed' 
and the parasite dies. The black-stem grain rust, which ref,uires 


recC spore 
blov/n to 
anotber^ stem 

recC or 
rixst on 
•wheat stem 

barlserrv rtcst 
spore in?ectintf 
ths cells of -^heat 
stem ira 


mfscts stem 



?«""«. red rust syjrecuil^ , 
from stem to stem/ 
cCixriijg' Sixmme'P 

blaclc or 
^vinter rust 
lives on 

straw thrcuflh , 
winter- * " 

infection form', 
barberry rust 
a cluster cup' 

The life history of black stem grain rust. 


r, ■!, black spora 

inject ing^ 
bocfics, sporicCa 

a spoT~id.ium 

infects the 

Cells of a 

barberry leaf 

Explain how this rust may he 

both the barberry plant and the wheat to complete its life history ; 
the pine tree blister, which lives on the currant or gooseberry at one 
stage of its life history, and on the pine at another ; and the parasite 
causing malaria, which requires both the anopheline mosquito and the 
blood of man to complete its cycle, are examples. 

The Chemical Relationship of Plants and Animals 

The study of plant and animal ecology may be said to be analogous 
to the study of human economics. Social conditions among men, 
animals, and plants are all determined by the environmental factors 
present, but chiefly by the availability and abundance of food. The 
world's food supply in the long run depends upon the chemical ele- 
ments making up the environment and energy derived from the sun. 



Plants and animals are made out of the same chemical elements. 
Burn some beans or a piece of beefsteak, a piece of wood or a bit of 
living bone, an entire green plant or a dead mouse, and the chemist 
would tell us that the same chemical elements are present in animals 
and plants ; that certain of these elements passed off in the smoke, 
others into the air as colorless gases, leaving still others as a 
whitish ash. All living things are composed mainly of carbon, 
oxygen, hydrogen, nitrogen, with about twelve other chemical ele- 
ments found in very minute quantities. These elements are all 
present in the immediate environment of plants and animals, air, 
water, and soil. 

How they get from the basic environment into living things can 
be briefly stated. Carbon, which is contained in all organic foods 
and in this condition is taken into the animal body, can only be 
absorbed in the form of carbon dioxide by food-making green 
plants. This gas, which is present in the atmosphere to the average 
amount of about 0.03 per cent, gets there as a result of oxidative 
processes taking place in plants and animals, as well as by the com- 
bustion of organic substances. Factories and volcanoes alike form 
their quota of carbon dioxide to diffuse out into the atmosphere. 
The cycle of the passage of carbon from plants to animals and from 
animals back to plants is shown in the accompanying figure. 

Hydrogen, another component part 
of living things, cannot be used in 
its pure state by either plants or 
animals. In water (H2O) , it becomes 
an important part of the food of 
animals, and as water vapor it is 
used in starch-making by green 

Oxygen is freely available to both 

plants and animals. As a gas, making 

up over 20 per cent of the air, capable 

of being dissolved in water for aquatic 

plants and animals, it is used by all 

living things in respiration. Green 

plants add this gas to the air during the process of starch-making. 

Nitrogen is one of the most important elements found in living 

things. Making up 79 per cent of the air, it is not usable in the 

form of a gas except by the nitrogen-fixing bacteria. 

The carbon and oxygen cycles in a 
balanced aquarium. Trace the pas- 
sage of an atom of carbon from a 
green plant back to the plant. 



The other mmeral components of living matter, of wliich sulpliur 
phosphorus, calcium, potassium, and iron are among tlie most impor- 
tant, are all found either in water, soil, or both. How the plant makes 
use of them and turns them over for the use of animals is an interesting 
story to be told later. But enough has been said to show that foods 
made by the green plants form the supply on which all animals live. 

carbohydrates^,, .^^^^ carbon^^ /'0>^yg«?'v 

proteins^ X -^-ureot /^ >\ n'til«ts 

slltsZI^ AnimaU (Gr^enPlantC r^f"' 

>.^ter- \^ y-sctlts V y^-^^'■-1^<.^ 

The food relationships between green plants and animals. 

Life Habits of Bacteria 

In this web we call life, bacteria play a most important part. Since 
bacteria contain no chlorophyll, they are unable to make carbo- 
hydrate food, and must obtain their foods from decaying organic 
matter. In order to absorb such food it must be made soluble so 
that it will pass into their bodies. This they do by digesting food 
substances by means of enzymes ^ which they secrete. Bacteria 
that grow or thrive in the presence of oxygen are called aerobic, while 
those which live without free oxygen are called anaerobic. The latter 
need oxygen, like other living things, obtaining it by breaking down 
the foods on which they live, and utilizing oxygen freed in this process. 

Relation of Bacteria to Free Nitrogen 

It has been known since the time of the Romans that the growth of 
clover, peas, beans, and other legumes causes soil to become more 
favorable for the growth of other plants, but the reason for this was 
not discovered until modern times. On the roots of the plants 
mentioned are found little nodules, or tubercles, in each of which 
are millions of nitrogen-fixing bacteria {Rhizobium leguminosarum) , 
that take nitrogen gas from the air between the soil particles and 
build it into nitrites which arc tlu>n converted by otluM- bacteria 
(Nitrobacter) into nitrates. In this form it can be used by plants. 
Nitrogen-fixing bacteria live in a symbiotic relationship with the 
plants on which they form tubercles, their hosts pro\iding them with 
organic food. 

J See pages 127-128. 




Bacteria also act upon ammonia formed from plant and animal 
wastes, one kind (Nitrosomonas) producing nitrites, or nitrate salts, 
and others (Nitrobader) converting the nitrites into the more stable 
nitrates. Thus all of the compounds of nitrogen are used over and 
over, first by plants, then as food by animals, eventually returning to 
the soil, or in part being released as free nitrogen. This process is 

called the nitrogen cycle. 
Although free nitrogen is 
fixed for use by means of 
electrical discharges dur- 
ing thunderstorms, by 
man-made machines, by 
ultraviolet light (which 
is estimated to return 
100,000,000 tons a year 
to the earth's surface), 
and from other sources, 
yet these means give an 
almost negligible amount 
of usable nitrogen to the 
soil, compared with what 
is used in crop produc- 
tion, especially since so much nitrogen is lost from the soil in various 
ways. The nitrogen-fixing bacteria supply the deficiency, thus form- 
ing one of the most important inter-relationships between plants and 
animals because of their direct relationship to the production of the 
food of the world. 

Rotation of Crops 

Plants that are hosts for the nitrogen-fixing bacteria are raised 
early in the season, then plowed under and a second crop of a differ- 
ent kind is planted. The latter grows quickly and luxuriantly be- 
cause of the nitrates left in the soil by the bacteria which lived with 
the first crop. For this reason, clover is often grown on land used 
later for corn, or cowpeas will be followed by a crop of potatoes. 
On well-managed farms, different crops are planted in succession 
in a given field in different years so that one crop may replace some 
of the elements taken from the soil by the previous crop. This is 
known as rotation of crops. ^ 

' Crop rotation is not only a process to conserve the fertility of the soil, but also a sanitary meas- 
ure to prevent infection of the soil. 

The nitrogen cycle. What additions could 
made to this diagram? 




Wriilhl I'Urcc 

Tiger Swallow-tail (Papilio turnns) on rose. 

The Relations between Insects and Flowers 

One of the most interesting symbiotic relationships is that which 

exists between msects and flowers. Flowering plants produce .seed.s 

and fruits, and from 

these come new gen- 
erations of plants, 

but if it were not for 

the visits of insects, 

many plants would 

not produce seeds. 

Insects visit flowers 

in order to obtain 

nectar, a sugary sub- 
stance formed by the 

nectar glands, and 

pollen. The glands 

which produce the 

nectar are usually so 

placed that an insect 

has to push its way past the stamens and pistil of the flower in order 

to reach the desired food. In 
doing this, pollen grains may 
adhere to the hairy covering of 
the insect and be transferred to 
the sticky surface of the upper 
end of the pistil (stigma). Inside 
the pollen grains are the male re- 
productive cells (sperms), while 
in the ovary of the pistil are 
held the female reproductive cells 
(eggs). In order to ha\e develop- 
ment of a new plant, it is essen- 
tial for a sperm cell to unite with 
an egg cell. Pollen grains on the 
stigma are stimulated to send out 
hairlike tubes, wiiich j)enetrate 
the stalk (style) of the j)istil and 
eventually reach the ovary. The 
pollen tube carries one or more 





. .-finicropyle-' 

A longitudinal section of the repro- 
ductive organs of a flower showing the 
penetration of a pollen tube through 
the opening in the pistil called the 
micropyle, and the growth of the pollen 
tube to the ovule. 


sperm cells, which are thus enabled to unite, each with a single egg 
cell, in the ovule of the pistil. The union of the sperm nucleus with 
its egg nucleus is called fertilization. As a result of this process the 
fertilized egg develops into an embryo or young plant which is held 
in the seed. When favorable conditions arise, this embryo m.ay 
develop into a plant. 

Bees are the chief pollinizing agents, although butterflies, moths, 
flies, and a few other insects perform this service as well. Hum- 
mingbirds often pollinate tubular flowers, while other small birds, 
snails, and even bats are agents in the pollination of certain forms. 
Man and animals may accidentally pollinate flowers in brushing past 
them through the fields. The value of cross-pollination is obvious 
and is an example of the close weaving of life in which man, animals, 
and plants are all inescapably entangled. 


Borradaile, L. A., The Animal and Its Environment, Oxford University Press, 

London, 1923. Chs. IV, V, XIV. 

Excellent for reference. 
Elton, C, Animal Ecology, The Macmillan Co., 1935. Chs. V, VI, VIII. 

Particularly valuable on the animal community and the relationship of 

animals to a food supply. 
Needham, J. C, and Lloyd, J. T., Life of Inland Waters, 2nd ed., Charles C. 

Thomas, 1930. Ch. V. 

Interrelationships among fresh-water organisms. 
Pearse, A. S., Animal Ecologij, McGraw-Hill Book Co., 1926. Chs. VIII, X. 

A wealth of material on interrelationships. 
Rau, Phil., Jungle Bees and Was-ps of Barro Colorado Island, privately printed, 

Kirkwood, St. Louis, 1933. 

An ecological study of a tropical environment. 
Wallace, A. R., The Geographical Distribution of Animals, 1876. Books I 

and II. Ch. IV, especially. 

This book forms the basis for most of the modern work in distribution. 

All of Part III, Books I and II, is extremely interesting. 
Weaver, J. E., and Clements, F. E., Plant Ecology, McGraw-Hill Book Co., 

1935. Ch. XVI. 

An interesting chapter on relations between plants and animals, with 

especial emphasis on insect pollination. 
Wells, H. G., Huxley, J. S., and Wells, C. P., The Science of Life, Doubleday, 

Doran & Co., 1931. Book 6, Chs. IV and V. 

A fascinating book for general reading. 



Preview. Earh^ contributions to classification • Binomial nomencla- 
ture ■ Law of priority • What is a species? • A classification of plants and 
animals • Classification of the plant kingdom • Classification of the animal 
kingdom ■ Glossary of terms occurring in the Roll Call. 


It is hoped that this section will be freely used by the student. 
It is not expected that the classification of plants and animals will be 
learned by rote, but rather used for reference from time to time as 
new forms are seen. By this means the diagnostic characteristics 
of different phyla and classes will gradually be learned as needed, and 
the relationship of one group to another become more apparent. 

In order to enjoy hikes or longer trips, the student should be able 
to recognize the larger groups of the plant and animal kingdoms. 
Fortunately there are museums, botanical gardens, and zoological 
parks to which one may refer, all the more intelligently of course if he 
has himself first discovered living animals and plants. 

Identifying plants and animals correctly becomes more of a plea.s- 
ure than a task, if the principles of scientific, as well as common, 
nomenclature are understood. Both scientific and common names 
will be encountered. The former are written in the dead, unchanging 
Latin language, and are of more universal usefulness, since the latter 
are frequently misleading and confu.sing, as more than one common 
name may be applied in different countries, or in different parts of the 
same country, to a single plant or animal. For example, the common 
"chain pickerel" is listed under the scientific name of Esox, indicating 
the larger or generic group to which the fish belongs, and niger, which 
is its specific name, but it has at least twenty-two connnon names in 
different parts of this country. Here are a few of them: black 
pickerel, pike, common eastern pickerel, duck-bill pickerel, green p'lkv, 
little pickerel, and lake pickerel. The terms pike, pickerel, aii.l lake 
pickerel are also quite commonly used in some parts of the country 
to designate another fish, the great northern pike, Esox lucius. In 
still other localities "pike" refers to an entirely different group, the 
pike-perches, belonging to the genus Stizostedion. This examph^ will 



serve to indicate the necessity for the use of Latin scientific names in 
classification. There may be other members of the genus Esox, but 
there is only one niger, although varieties of the same are possible in 
different environments. The terms of genus and species were intro- 
duced to the scientific world in the middle of the 18th century by 
Carl von Linn^ (1707-1777), of Sweden. 

The study of classification is called Taxonomy and is subdivided 
into zoological taxonomy, or Systematic Zoology, and botanical taxon- 
omy, or Systematic Botany. 

Early Contributions to Classification 

In order to secure an idea of the development of taxonomy it is 
necessary to go back several hundred years to some of the earlier 
biologists and glance at a few of the contributions of these students. 
Obviously such an excursion can hope to touch upon only a few of 
the more important workers. Logically, one should go all the way 
back to Aristotle's time, but lack of space forbids such an interesting 
excursion. Consequently we must confine ourselves to the immediate 
forerunners of Linne, or Linnaeus as he came to be called, who intro- 
duced the concept of binomial nomenclature and with it a more ade- 
quate idea of genus and species. 

In 1576, Matthias de TObel published an important work on plants. 
This was an attempt to arrange plants according to their structure. 
He took the shape of the leaf as the basis for this classification, and it 
led him to put such things as ferns in the same group with trees because 
the fronds of the fern bore a superficial resemblance to the needles 
of the hemlock. Another botanist was the Swiss, Kasper Bauhin 
(1560-1624), who described in order 6000 species of plants, beginning 
with the ones he considered most primitive. He approached the 
concept of genus and species, because he grouped together plants 
which resembled one another externally. 

John Ray (1627-1705) deserves recognition along with Linnaeus 
as the founder of the science of systematic biology. This enthusiast 
published a catalogue of British plants in 1670 and later works (1703) 
in which he introduced and explained the groups of Monocotyledons 
and Dicotyledons. He also made less extensive contributions to the 
classification of animals. Some of these he published with his good 
friend Willughby (1635-1672). Ray gave evidence in his work that 
he realized the fundamental differences between genus and species ; 
furthermore, he had the keenness to group together both related plants 


and animals. Ray also advanced the idea that fossils are extinct 

Linnaeus was born in 1707, the son of a Swedish clergyman. Ho 
would have been destined to become a cobbler had it not been for the 
influence of a physician who recognizcnl the lad's abilities. To make a 
long story short, he finally secured his medical degree, aided in no 
small amount by the contributions of his fiancee, and eventually 
became a professor of natural history at Upsala. It seems that Lin- 
naeus had a passion for natural history and for classifying everything 
which came to hand. He initiated several changes in the study of 
systematic biology, many of which are still in use today. 

Binomial Nomenclature 

The most important contributions of Linnaeus center about (1) brief, 
clear, and concise diagnoses ; (2) sharper divisions between groups ; 
and (3) a definite, clear-cut system of scientific terminology, known 
as hinomial nomenclature. These innovations appeared in the 1753 
edition of Species Plantarum and the 1758, or tenth, edition of his great 
work, the Systema Naturae. The tenth edition of this latter work is 
taken as the starting point of zoological nomenclature. Linnaeus 
divided the plant and animal kingdoms into Classes, Orders, Genera, 
and Species. This was a great step over the use of popular common 
descriptive terms, as you can now appreciate if you refer back to the 
example of the pickerel. However, a big mistake made by Linnaeus 
was his concept of fixity of species. 

In 1898 the International Congress of Zoology appointed an inter- 
national commission which drew up a set of rules ajjplying to the 
divisions of the animal kingdom. Thus classification today is really 
an expansion of the Linnaean system which now includes in the case 
of the animal kingdom, for example, the following : 

Animal Kingdom — is made up of 

Phyla — each of which is composed of 
Classes — in turn made up of 
Orders — then 

Families — and finally 
Genera — and 

In the plant kingdom a comparable arrangement is utilized, beginning 
with Divisions (= phyla). 


Law of Priority 

In describing species it sometimes happened that more than one 
person described the same form, giving it different names. In such 
cases the name assigned by the one who first described it is used, the 
second being considered a synonym. This is the reason for writing 
the describer's name and date of pubHcation after the specific name. 
Ordinarily the date and frequently the describer's name is omitted. 
Thus the true daisy is properly Bellis perennis, Linn. 1758, or the 
English sparrow, Passer domesticus, Linn. 

What Is a Species? 

We have taken a glimpse at the contributions of some of the con- 
temporaries and near contemporaries of Linnaeus and have gained a 
sHght concept of the problems these early workers faced in defining 
and describing a species. Biological scientists of today are still 
working on this problem. The principle involved is readily under- 
stood when we look at a sheep, a cat, and a dog. One can easily sepa- 
rate them from each other, various cats being put in one group and 
diverse dogs in another. All domestic cats, whether they be the alley 
variety or pet Persians, and all dogs, whether they be a "dog in the 
manger" or "man's best friend," fall into well-marked and easily 
separable groups, known as species. To continue further, one finds 
in looking over representative mammals that many other species such 
as the jaguars, ocelots, jaguarundis, and cougars, all have certain char- 
acteristics in common with our domestic cats. These characteristics 
are size, build, shape of head, nature of claws, teeth, and fur. The 
zoological systematist, therefore, places them in one larger group or 
genus which is called Felis, a relationship expressed below. 

Kingdom. Animal . Plarzb 

'Phylum - Onovdcdxx . Arthropod a. 'Koiltx£r<i.a.. eh=>. 
Gla55 - "rlocmiTacxIioD , Pisces .Pept/ilia, Aves.cstc-. 
Order"- Carpi vonx . 'R'ocCe-aticc.Chiroptera,ete- 
Kimil/- PeUcLcce^ . <Zo.n idoe ."LCr^icCoca.etc. 

Genzxs . " feli^ . Lumhricus, eto. 



Tndi vidical . lorn , ,Dick .Harry, etc. 


However, species have otlicr characteristics besides oxtornal or 
morphological similarities. They breed true, that is, cats produce 
cats, and dogs produce dogs. Usually diflferent species cannot be 
crossed. There are exceptions, for sometimes one species crossed with 
another may yield a sterile hybrid. Thus a horse crossed with an ass 
produces a mule. But on the whole the preceding statement holds 

Two criteria have been used in classifying organisms, first, struc- 
tural differences, or appearance, which really means comparative mor- 
phology, checked physiologically and genetically by the cross-breeding 
of species, and second, the approach through a study of the early 
development and the life cycle, emhnjology, and the distribution 
of the organism, ecology. The latter leads to a consideration of 
varieties, subspecies, and races, which through mtergradations often 
complicate the problem of determining species. 

Such a study may be made either more complicated or facilitated 
according to whether a so-called natural classification or artificial 
classification is utilized. Thus bats and birds might be artificially 
classified together, simply because they both fly, just as whales and 
fishes are placed together by the ignorant, because both inhabit 
the water. A careful study of the anatomy and development of these 
animals would indicate that if one is trying to show relationships, 
which is what a classification should do, bats and whales would both 
have to be put in the mammalian group. 

Determination of the type of symmetry present is useful in clas- 
sification. Some organisms possess a universal symmetry, as the 
protozoan Volvox. In such cases the organism is divided into equal 
halves by any plane that passes through the center. Starfish and 
hydra, on the other hand, are well-known examples of radial sytn- 
metry. In such forms there is a single axis, as may be seen in a 
cylinder, and a number of planes through such an axis would di\-id(^ 
the organism into symmetrical halves. Most of the more highly 
developed forms possess bilateral symmetry, which is characterized 
by similar halves on either side of a main axis. Other secondarv' 
planes occur in bilaterally symmetrical animals, resulting in anterior- 
posterior, and in dorso-ventral differentiation. Sometimes segmenta- 
tion, or metamerism, is apparent, as in the case of the earthworm and 
many of the Arthropods. 

If one attempts a classification that is based primarily upon struc- 
ture, it is necessary to differentiate between homology and analogy. 


The former refers to similarity of structure and the latter to similarity 
of function. Thus the f orelimbs of a bat, bird, cat, and turtle are ail 
homologous, while the wings of a bat or a bird are analogous to the 
wings of a butterfly, but they are not homologous since they differ in 

A Classification of Plants and Animals 

As stated earlier, the appended scheme of classification is simply a 
tool to be used by the student. Remember that a scheme of classi- 
fication is not only the "who's who" of the plant and animal world 
but it shows relationships as well, indicating what we know at present 
in this field. Classification involves a knowledge of the occurrence, 
distribution, development, and structure of the form studied, and 
so is much more than simply applying a scientific name to an ani- 
mal or plant. The use of scientific names cannot readily be avoided, 
as will be realized from a study of these pages. If one really desires 
to excel in biological work, he must set out cheerfully and with 
determination to acquire an understanding of the use of these tools as 
an indispensable aid to a comprehension of the interrelationship of 
organisms to one another. 

In the first place it is hoped that diagrams which accompany this 
classification are detailed enough to give the student some concept of 
the more common or important kinds of representative organisms 
occurring in each group. It is unfortunately impossible in these 
drawings to represent the different animals according to scale. The 
student hardly needs, however, to be reminded that whales and 
protozoans should be interpreted as decidedly different in size. In 
most cases, the classification will be carried only as far as the class, 
although in a few groups, as with the Arthropoda and Tetrapoda, 
it is necessary to go to the orders. In some instances attempts have 
been made to simplify the classification in order to avoid unnecessary 
scientific terminology. It should be added that the classification 
here presented is only one of many that may be encountered in various 
books, differing in details but agreeing in essential particulars. 

It is impossible to designate readily all of the characteristics which 
are utilized in the separation of the larger plant and animal groups. 
However, it is of importance to know (1) whether we are dealing with 
a one- or many-celled form (uni- or multi-cellular) ; (2) the number 
of germ layers present in the organism, diplohlastic — • two (ecto- 
derm and endoderm) ; triplohlastic — three (ectoderm, endoderm, and 



mesoderm) ; (3) the nature of the body — usually divisible into tubes 
within tubes or sacs ; (4) the symmetry — radial or bilateral ; (5) the 
nature of the appendages — if present, whether jointed or non-jointed, 
paired or unpaired ; (6) whether the organism is segmented or non- 
segmented ; (7) which organ systems or organs are present, in what 
form they occur and how they function ; (8) type of skeleton, ab- 
sent, exo- or endoskeleton ; (9) the presence or absence of a noto- 
chord ; (10) the presence or absence of special organs ; (11) the type 
of tissues present, as bark, phloem, muscular, or circulatory. 

Inasmuch as some of these and other terms appearing in the scheme 
of classification are new, a short glossary is included. This is designed 
to elucidate terms used in the appended classification. Other words 
are defined as they are first used in the text and may be found by 
reference to the index. 


(mainly after Sinnott) 

All members of the plant kingdom are characteristically ses.sile ; typically 
possess chlorophyll; usually take food in inorganic form; cell walls of cellulose 
or hydrocarbon. 

DIVISION I — THALLOPHYTA — Thallus Plants (algae,' fungi, bacteria). 
Chabacteristics : Small, often minute, little differentiated plants some- 
times possessing chlorophyll; se.x organs, when present, typically one 
celled ; spore-bearing organs are single celled ; 80,000 species. 

Subdivision A — Algae — Composed mostly of blue-green, green, brown, or red 

Characteristics : Chlorophyll frequently associated with other pigments ; 
manufactures own food. 

Class I — Cyanophyceae — Blue-green algae {Gloeocapsa, Nostoc, Oscilla- 
tor ia). 
Characteristics : Simplest and lowliest of green plants ; body consists of a 
single cell with nucleus ; sap cavity and chloroplasts absent ; often tend- 
ing to adhere in colonies; usually in threadlike rows ( filaments); cyto- 
plasm homogeneous, pigment evenly dispersed, or a colored outer and a 
colorless inner zone may be distinguishable ; blue-green color probably due 
to chlorophyll mixed with blue pigment (phycocyanin) . 

Class II - Chlorophyceae - Green algae (Cartena, Ulva, Ulothrix, Oedogo- 
nium, Vaucheria, Spirogyra). 

1 Genera in boldface type indicates that the form is illustrated in this unit. 



Class I 


blua-ghsen olgcus 




1. d ^-^^ — ^v. 

LaTY^inarig Txjccus wiopfccryx 

Class 12" 


brov/n algae 

Class 3Zr 









Staphylo Coitus 

Olcxss "SC 



ovass I. 

Class is: 

ol^-like fitnga. 

astreptococcTxs 4.E»cu:ilIuS 5.Spirillurai.$cipi'o1egmgt^^i:ggi3^ 

Class IT 





2. Cbmatr ic"ha 
l.Kemitrichia s.Trichompboro 

Class HE. 
slime "Yucn^i 

, 2, 

-1. ., 

Class "C. 

gac ■{ ^UT7^i 


smuts ^"puff balls 


smutts ondrtxstls 


Chakacteristics : Chlorophyll associated with carotin and xantlKjphyll; 
marine or fresh water organisms, or inhabitants of moist hind; nucleus 
and one or more chloroplasts present; starch synthesizeil in pi/rentmls; 
plant composed of single cells, colony, filament, or plate of cells; most 
species produce motile i-eproductive cells (zoospores) ; botli equal {iso-) and 
different sized (hetero-) gametes present. 

Class III — Charophyceae — Stonewarts {Chara and Nitella). 

Characteristics: Vegetative body consisting of long, jointed stems with 
whorls of short branches arising at joints {nodes) ; asexual spores absent ; 
more complicated antheridia and oogonia than found in Thallophytes borne 
along branches. 

Class IV — Phaeophyceae — Brown algae, kelps, rockweeds, sargassum {Lami- 
naria, Fucus, Ulopteryx). 
Characteristics: Multicellulate; exclusively marine ; brown color (due to 
one or more brown pigments associated with chlorophyll) ; normally found 
in intertidal zone. 

Class V — Diatomaceae — Diatoms (Meridion, Diatoma, Denticula Fragillaria). 
Characteristics : Large group of unicellular algae ; related in color to 
brown algae ; common as plankton organisms in both fresh and salt water ; 
siliceous walls. 

Class VI — Rhodophyceae — Red algae {Nemalion, Polysiphonia, Phyllophora, 
Corallopsis) . 
Characteristics : Mostly marine ; characteristically reddish in color ; 
branched, vegetative body filamentous and delicate; grow entirely sub- 
mersed; cell wall often thick, gelatinous; color due to pigment, phyco- 
erythrin ; no motile cells ; sexual reproduction highly specialized. 

Subdivision B — Fungi — Fungi, bacteria, and molds. 

Characteristics : Chlorophyll lacking; exist as parasites or saprophj-tes. 

Class I — Schizomycetes — Bacteria (Diplococcus, Staphylococcus, Streptococ- 
cus, Bacillus, Bacterium, Spirillum). 
Characteristics : Unicellular plants, usually without pigment, dividing in 
one, two, or three planes; apparently structureless, but probably con- 
taining a diffuse nucleus. 

Class II — Saccharomycetes — Yeasts (Saccharoimjces). 

Characteristics: Sometimes regarded as reduced Ascomycetes; single 
cells with definite nucleus ; cytoplasm and sap cavity ; buds a.sexua!ly ; 
under unfavorable conditions forms four spores, in a modified ascus. 
Class III — Myxomycetes — Slime fungi, slime molds (Hemitrichia, Coma- 
tricha, Trichamphora) . 
Characteristics: Border-Hne plants; spores borne by fruiting bodies, 
germinating into small, naked mass of protoplasm without a wall ; indi- 
vidual cells fuse, forming a Plasmodium. 
Class IV — Phycomycetes — Algalike fungi, molds, and bliglits {Saprolegnia, 

H. w. h. — 6 




1 L vei^-NVortS 


, iverwor ts , mosses 

r^^^-Sporxs capsule 

CD 6'phcc^nixxn 
peoct moss 



acomrrion moss 

arche^nium Qnt"hei4diuTn 
of corartiorL moss 

Olo-SS IC 


Characteristics : Resemble algae ; plant body consists of filaments {hyphae) 
which are not divided into cells by cross walls ; multinucleate. 

Class V — Ascomycetes — Sac fungi {Morchella, Exoascus, Microsphaera). 
Characteristics: Includes over 20,000 species, mostly saprophytes or 
parasites; body consists of branching mycdium throughout substratum 
and a definite fruiting body at surface ; produce spore sacs {asci) contain- 
ing eight spores (ascospores) ; group of asci embedded in sterile hyphae 
may or may not be surrounded by protective envelope. 

Class VI — Basidiomycetes — Basidia fungi, smuts and rusts, wheat rust 
(Puccinia), puff balls. 
Characteristics: Large and varied group; specialized reproductive struc- 
ture (basidium) is swollen terminal cell of hypha, in mushrooms the ija- 
sidium usually bears four basidios pores, each carried on a delicate stalk 
{sterigma) ; sexual reproduction rare; lichens — composite plants in which 
algal cells are entangled in mycelium. Usually regarded as a parasitism 
of algal member rather than an example of symbiosis. 

DIVISION II — BRYOPHYTA — Liverworts and Mosses. 

Characteristics : Alternation of generations in which sexual (gametophyfic) 
stage dominates; asexual (sporophyiic) stage typically parasitic upon 
the gametophyte ; archegonium and nmlticcUulate antheridium pre.sent ; 
gametophyte contains x number of chromosomes while the 2 x number 
occurs in the sporophyte; careful study of archegonium reveals typical 
flask shape, with sterile cells (neck and venter) surrounding the egg and 
associated cells ; antheridium more or less stalked and consisting of layer 
of jacket cells surrounding cuboidal sperm mother cells. 

Class I — Hepaticae — Liverworts (Marchantia, Riccia). 

Characteristics: Intermediate between green algae and higher plants; 
thaUus flattened and attached to soil by rhizoids; growth by repeated 
division of single large apical cell. 

Class II — Musci — Mosses (Sphagnum, Polytrichum, Catherinia). 

Characteristics: In every habitat except salt water; very common in 
alpine and arctic regions; gametophyte erect, consisting of stalk with 
spirally arranged leaves ; attachment by rhizoids. 



Subdivision A 
pi"! mi live, 
vasculctr plants 

Devonian plant ^ 

RsilophyCalcs ^-"^ 
-.i <'..^ .(2)p5notum 



club mosses 

sporopVr/te y,- 





vascular' plants 


ferns . seed plants, 






^''\ cells ' 




>>^ cell i 





dicotyledon ii20i20Ccit/kfGn 


oclVc, iTioLple . elm , ccc . 




Chakacteristics : Fibro- vascular system for transportati..,i „f raw mate- 
rials up and food down; separation of specialized cl.l..njphyll-lK.ari..K 
tissue ; adaptation to absorption of water from soil. 

Subdivision A — Primitive Vascular Plants — {Psilotum and Tmesipteris, Rhynia). 
Characteristics : Fossil primitive vascular plants giving rise in tliree lines 
to Lycopsida, Sphenopsida, and Pteropsida. 

Subdivision B — Lycopsida — Club mosses, ground pines (Lycopodium, Selagi- 

Characteristics: Stem clothed with small, numerous, spirally arranged 
leaves; sporangia borne on upper surface of spowphyll; latter usually 
grouped into terminal cones. 

Subdivision C — Sphenopsida — Horsetails (Equisetum). 

Characteristics : Hollow, typically jointed stems, bearing small leaves at 
joints (nodes); stems ribbed; diaphragms often across stem at nodes; 
sporangia borne in groups on stalked shield-shaped structures forming 
terminal cones ; ribs opposite fibro-vascular bundles which are associated 
with small air-filled canal; abundant in Paleozoic age; now only about 
35 species. 

Subdivision D — Pteropsida — Ferns and seed-bearing plants. 

Characteristics: Typically large leaves; sporophytic generation domi- 
nates ; sporangia relatively large. 

Class I — Filicineae — Ferns. 

Characteristics : Small, herbaceous plants with typical pinnately com- 
pound leaves (fronds) ; stem relatively weak and inconspicuous ; roots 
numerous but do not form an extensive system; small sporangia borne 
on lower surface of leaf in groups usually protected by membrane (iiulu- 
simu) ; spore germinates, forming small, thin gametophyte (prothallus), 
which in turn bears antheridial and archegonial structures. About 15,000 
species, some of which reach a height of 30 feet. From forms like the 
ferns evolved the higher vascular plants whic-h dominate the earth's 
surface today. 

Class II — Gymnosperil\e — Evergreens, pines, hemlocks, spruces, junipers. 
Characteristics: Seeds freely exposed to air; usually nondeciduous 
types; megaspore retained within megasporangium where it germinates 
producing female gametophyte; integument, a new structure, a 
sporangium and embryo sac; reduced male gametophyte transferred 
directly to vicinity of female ; male obtains access to female gametophyte 
by new structure (pollen tube); young sporophyte develops in contact 
with and at expense of parental sporophyte; gametophyte with haploid 
(x) number of chromosomes entirely parasitic upon sporophyte. Mem- 
bers of this group are phylogenetically ancient; only about 450 living 

Class III — Angiospermae — Deciduous trees and plants. Dicotyledons, oak, 
maple, beech ; Monocotyledons, corn. 



Class I 



C3) "'-J 

Clctss IE 


one cellecC animals 

c'C®) \^ 



^^-^ ctsexuctl \^^9© 
Cycle in. / 





Class JSL 



(2) ^^ 



5tyionych i cc 

Class IST 




Characteristics: Seeds enclosed by a case (ovary), so that pollen Rrain 
does not reach the ovule but rests on surface of carpel ; closure to form 
case probably arose by folding together of edges of megasporophyll {carpd) ; 
pollen received on special organ (stigma) at tip of ovary. Members of 
this group probably were derived from gymnosperm stock ; now number 
135,000 species and are subdivided into dicotyledons and monocotyledons 
which may be separated by the following ciiaracteristics : 



Number of cotyledons 
of embryo . . . 



Vascular bundles . . 

arrange to form vas-cylinder 
enclosing pith 



open venation, veinlets end- 
ing freely in margin, which 
is often toothed or lobed 

closed venation (i.e. parallel) 
margin therefore entire 


in sets of four or five 

in sets of three 


(mainly after Hegner) 

All members of the animal kingdom are characteristically free-moving organ- 
isms; generally capable of assimilating organic foods; rarely possessing chloro- 
phyll ; cell membranes composed of protoplasm or proteins. 

PHYLUM I — PROTOZOA — One-celled animals. 

Characteristics : Single cells or colonies of loosely aggregated unspecial- 
ized cells ; rarely differentiated into germ cells ; 8500 species. 

Class I — Sarcodixa — Naked protozoa (Ameba,^ Arcella, Radiolaria). 
Characteristics : Locomotion by means of pseudopodia. 

Class II — Mastigophora — Flagellate protozoa (Euglena, Trypanosoma). 
Characteristics : Locomotion by means of flagella. 

Class III — Sporozoa — Parasitic protozoa (Lankesieria, Myxosporidia, Plas- 
Characteristics : Xo organs of locomotion in adults ; endo-parasites repnn 
ducing by schizogony and spore formation. 

Class IV — Infusoria — Ciliate protozoa (Vorticella, Stentor, Stylorjychia, 
Characteristics : Locomotion by means of cilia. 

1 See footnote at beginning of classification of Plant Kingiioni. 



Class T 


KexactiY^ell ioCa 








fresh- wcxter-Spon^e 




Characteristics : Usually considered as diploblastic animals ; body con- 
sists of a perforated (inhalent pores) cylinder, leading to central canal 
opening to outside through exhalent pore; [peculiar flagellate, collared 
cells (choanocyfes) typically present; body structure frequently compli- 
cated by budding ; 2500 species. 

Class I — Calcarea — (Grantia). 

Characteristics : Small marine sponges possessing one-, two-, or four-rayed 
calcareous spicules. 

Class II — Hexactinellida — Deep-sea sponges (Euplectella). 
Characteristics : Sponges with six-rayed siliceous spicules. 

Class III — Desmospongia — Finger sponge, bath sponge (Chalina, Spongilla, 
Euspongia) . 
Characteristics: Diverse groups of sponges possessing spicules of silicon, 
not six-rayed, with spongin, or a combination of spicules and spongin. 



Olass I 




Portuguese mar?- of -^/ar 




Class IE 


Secc anerrzor^e 

Astra n^ioc 

Class HE 



PHYLUM III — COELENTERATA — Jellyfishes and corals. 

Characteristics: Mostly marine; radially syinniotrioal ; diploblastic- ani- 
mals with a noncellular layer of niesoglea lying between; po.ssL's.sing 
tentacles, armed with nematocysts; body composed of a single gastro- 
vascular cavity ; 4500 species. 

Class I — Hydrozoa — Fresh-water polyps, jellyfishes, and a few stony corals 
{Hydra, Obelia, Physalia). 
Characteristics : Mostly marine ; usually hydroid and jellyfish forms 
occur in the same life cycle; the jellyfish (medusae) po.ssess a shelflike 
velum extending inward from the margin toward the mouth (manubrium) ; 
a few species like Hydra possess no medusoid stage; the stony coral, 
Millepora, represents a colony with a coral-like skeleton of calcium car- 

Cl.\ss II — Scyphozoa — (Amelia). 

Characteristics: Entirely marine, with the medusoid stage dominating; 
produced from subordinate polyp by terminal budding (strobilalion) ; 
velum usually absent; lobate, typically eight-notched. 

Class III — Anthozoa — Sea-anemones, sea-pens, and stony corals (Metridium, 
Pennatula, Astrangia, Sagartia). 
Characteristics : Entirely marine with medusoid stage suppres.sed ; organ- 
isms characterized by an introverted ectodermal mouth (sto7nodaeum) anti 
vertical radiating mesenteries extending inward from the body wall; one, 
two, or more rarely three cihated gullet grooves (siphonoglijphs) carry 
a stream of oxygenated water to interior. Corals produce islands and 
reefs; in addition they sometimes protect a shore from wave action. 





ytonna iphorroc 

Comb i<2-^Vy "'^i^^'S^^w^'i^pM 


Venas* gxindLie 

ROLL C^LL 8:{ 

PHYLUM IV — CTEXOPHORA — Sea-walnuts {Cestus, Hormiphora, Mnemi- 
opsis) . 
Characteristics : Eight radially arranged rows of comhjlike plates typi- 
cally present; fundamentally bilaterally syninictrical; with a distinct 
mesodermal layer (therefore triploblastic); no nematocysts : 100 species. 



Class I 





'Planoria ^' Microsbmum 

Clccss IT 




frog lun^ fluke/' 



Cb J^s bicerC'Lcs 



:••• Cv55 

ovary- ^telloricx. 
2(aO proglottv 

Class HI 



brocccC tapeworm 

of mctn_ 


liver fluke 
of -man. 



lo«^. naPVB J 

ovary - 





Class ISL 



Characteristics : Dorso-ventrally flattened, soft bodies, bilaterally sym- 
metrical, animals lacking true segmentation and blood vascular system; 
no anus; excretory system of flame-cell type; only Class I free-living, all 
others parasitic ; 4600 species. 

Class I — Turbellaria — Free-living flatworms {Planaria, Microstomum, 

Characteristics: Typically free-living, possessing a ciliated ectoderm; 
some ectodermal cells secrete mucus, or produce rodlike bodies (rhnMite.s) ; 
classification into orders depends upon nature of intestine. 

Cl.\ss II — Trematoda — Flukes (Polystoma, Pneumonoeces, Clonorchis). 

Characteristics: Parasitic flatworms with non-ciliated ectoderm in the 
adult, possessing one or more suckers; highly specialized for parasitic 
existence; many are internal parasites having complicated life cycle, 
occupying as many as four hosts during development ; digestive system 

Class III — Cestoda — Tapeworms {Taenia, Diphyllobothrium). 

Characteristics : Members of this group are completely parasitic, living 
as adults in the alimentary canal of vertebrates; digestive tract absent; 
body typically divided into a chain of segments (proglottids), except for 
Cestodaria, budded from neck, gradually increasing in diameter towards 
posterior end; the head (scolex) typically bearing organs of adhesion in 
the form of hooks and suckers. 

Class IV — Nemertinea — Nemertines (Micrura, Cephalothrix, Cerebralidus). 
Characteristics : Members of this gnnij) because of uncertain systematic 
position not always placed with the flatworms ; characteristically found in 
moist earth or fresh water, most forms being marine; characterized by 
possessing alimentary canal with mouth and anus, definite blood-va.scular 
system, and a long proboscis enclosed in a proboscis sheath. 



CAccss I 


^^^ , ■ ' ^" (2) (5) ^ 

Trichi^Gllo: spiralis Trichuris ovis 'NecatDr onnericaTiiK 

■pork TDundvorra NK-'J^ip '•v^orm yiooy<:\i/or-m 




ClotSS IE 


Leptorty-nchoicLes £hecatus 

Clots s HE 



Characteristics: Bilaterally symmetrical ; cylindrical, unsegmented, long 
and slender worms ; usually a distinct alimentary canal with mouth and 
anus ; primitive body cavity present ; papillae or spines at anterior tip of 

Class I — Nematoda — Threadworms (TrichineUa, Trichuris, Necator, Oxyuris). 
Characteristics : Members of this group art; l)oth free-living and parasitic 
on plants and animals; mouth usually terminal and alimentary canal 
composes a relatively straight tube with anal opening near posterior end 
of body ; body cavity not lined by epithelium but bounded directly by 
muscles of the body ; four thickenings of the ectoderm, one dorsal, one 
ventral, and two lateral, produce ridges containing excretory canals and 
nervous system ; sexes separate. 

Class II — Gordiacea — Hairworms {Gordius, Paragordius). 

Characteristics : Long, slender, and hairlike ; free-living adults in water ; 
larvae usually parasitize aquatic insect larvae (often Mayflies) ; asually 
reach a second host, as beetle or grasshopper, in which development con- 
tinues ; escape to water made by breaking through body wall ; no lateral 
lines present; body cavity hned by distinct peritoneal epithelium derived 
from mesoderm ; eggs discharged into body cavity instead of to outside. 

Class III — Acanthocephala — Spiny-headed worms (Leptorhynchoides, Neo- 
echinorhynchus, Macracanthorhynchus) . 
Characteristics: Protrusible proboscis armed with hooks; alimentary 
canal absent; reproductive sy.stem complex; entirely parasitic, larval 
stage in Arthropods. 

h. w. h. — 7 





^iKSisa. animalcule 

Clccss I 



OlccSS 31 




PHYLUM VII — TROCHELMINTHES — Rotifers, Gastrotricha. 

Characteristics : Small, frequently microscopic, identifiable by cilia around 
the mouth region ; about 1300 species. 

Class I — Rotifera — Wheel animalcules (Philodina, Notommata, Trocho- 
Characteristics : Mostly free-living, inhabiting fresh water ; distinct nerv- 
ous system ; universally characterized by presence of jaws inside pharynx 
(mastax) ; usually a foot. 

Class II — Gastrotricha — (Chaetonotus). 

Char.acteristics : Microscopic organisms reaching maximum length of 
about 0.5 mm. ; animal divided into indistinct head, neck, and body ; oral 
bristles on side of head ; often a forked tail containing cement glands ; 
locomotion by ciliary bands or by long bristles. 



Class I 

fresh -water hr/oysan 


moss aniYnals ana. lamp svjetis 

ejdsrnol viev 

(1) lyTageWania 

Class IE 




Class IL 




PHYLUM VIII — MOLLUSCOIDEA — Moss animals and lamp-shells. 

Characteristics : Unsegmented, sessile, typicull}' marine, bilaterally S3'm- 
metrical animals possessing a ridge (lophophore) bearing ciliated tentacles 
which surrounds the mouth ; 5700 species, including fossils. 

Class I — Bryozoa — Moss animals (Electro, Pectinatella, Iletniseptella, Bugula, 
Characteristics: Colonial, sessile, free-living animals; mostly marine; 
lophophore usually horseshoe-shaped ; alimentary canal L'-shaped ; divi- 
sion into subclasses depends upon whether anus opens within or without 

Class II — Brachiopoda — Lamp-shells (Magellania). 

Characteristics : Marine organisms possessing characteristic lophophore ; 
body covered by calcareous, dorso-ventrally arranged bivalve shell, usu- 
ally attached by a stalk (peduncle). 

Class III — Phoronidea — (Phoronis). 

Characteristics : Small, marine, sedentary animals living in tubes ; unseg- 
mented adults are hermaphroditic, possessing a body cavity as well as 
characteristic horseshoe-shaped lophophore; two excretory organs and a 
vascular system. 



Class T 



cMass "K 

Nsreis Chaetoptert£5 

c\ccro..^»/'or-m. tube vorm. (^3) 


segmentecC "^orms 

•■medicinal leech 

Class HL 



Garthvorm 1 


arrow vorm 

Class sr 





PHYLUM I X — ANNELIDA — Segmented worms. 

Characteristics : Segmented animals bearing distinct head, digestive tube, 
coelom, and sometimes nonjointed appendages; frequently supplied with 
chitinous bristles (setae) ; 6500 species. 

Class I — Archiannelida — (Polygordius). 

Characteristics : Marine worms lacking setae or parapodia ; trochophore 
larvae present. 

Class II — Chaetopoda — Clam worms, tube worms, earthworms (Nereis, 
Glycera, Chaetopierus, Lumbricus). 
Characteristics : Members of this class marine, terrestrial, or fresh water ; 
paired setae characteristically arranged in integumentary pits or upon 
parapodia ; further subdivision based upon number of setae present : 
Oligochaeta, a few ; Polychaeta, many. 

Class III — Hirudinea — Leeches (Hirudo, Glossiphonia). 

Characteristics : Hermaphroditic, dorso-ventrally flattened annelids with 
32 body segments, two suckers, one surrounding mouth, the other the 
posterior end ; setae and parapodia absent ; growth of mesenchyniatous 
cells reduces coelom. 

Class IV — Gephyrea — Sipunculid worms (Phascolosoma). 

Characteristics : Non-segmented when adult, without setae or parapodia ; 
characterized by a large coelom and trochophore larvae. 

Class V — Chaetognatha — Arrow worms (Sagitta). 

Characteristics: Small, transparent, marine invertebrates with well- 
developed body cavity, alimentary canal, nervous system, two eyes; 
lobes on sides of mouth armed with bristles which aid in capturing food. 



Class I 


Clccss X 

brittle -star- 


starfishes, etc 



K'#^- Her) 


sect urcHin 


sea: - cucuiTzber 

sccncC dCollocr 

Class is: 


Class IE 



Class ^ 



PHYLUM X — ECHINODERMATA — Starfishes, sea-urchins, sea-curumbors. 

Characteristics : Adults radially symmetrical (pentamerous) ; marine ; 

tube-feet, water vascular system, distinct alimentary canal, large body 

cavity usually present ; frequently a spiny skeleton of calcareous plates ; 

larvae bilaterally symmetrical ; 4800 species. 

Class I — Asteroidea — Starfishes (Asterias, Mediaster). 

Characteristics : Typically five rays or arms not marked off from central 
disk ; each ray possessing ventral ambulacral groove through which numer- 
ous tube-feet extend ; gastric pouches and hepatic caeca extend into rays ; 
blunt spines and pedicellariae present; respiration by dermal branchiae. 

Class II — Ophiuroidea — Brittle-stars (Ophiopholis, Ophiothrix, Ophioglypha, 
Characteristics: Typically pentamerous with arms sharply marked off 
from disk ; no ambulacral groove ; hepatic caeca and anal opening lacking. 

Class III — Echinoidea — Sea-urchins, sand-dollars, spatangoids (Arbacia, 
Strong ylocentrotus, Echinarachnius, Spatangus, Moira). 
Characteristics : Typically pentamerous without arms or free rays ; test 
of calcareous plates bears movable spines; i)ediceilariae usually three- 
jawed ; mouth with five conspicuous teeth constituting part of Aristotle's 

Class IV — Holothuroidea — Sea-cucumbers {Holothuria. Thyone, Leptosy- 
Characteristics : Long, ovoid, soft-bodied cchinoderms ; tentacles about 
mouth; body wall muscular ; skeleton greatly reduced. 

Class V — Crinoidea — Sea-lilies or feather-stars {Antcdon, Halhromelra, Co- 
rnadinia, Pentacrinus). 
Characteristics : Usually five branched arms, possessing featherlike divi- 
sions (pinnules) ; aboral pole sometimes possessing cirri but more gener- 
ally a stalk for temporary or permanent attachment ; a few modern types, 
most forms known as fossils. 



Class I 


Class I 

Class HE 


1 5cb r2och itoio. 

Iccnd. snccil 





tootlri snocil 


clams , Snccils, etc 


Class 3Sr 


ra^or-shell Cicom 



Class ^ 



PHYLUM XI — MOLLUSCA — Snails, clams, and oysters. 

Characteristics : Unsegmented, bilatorally synunotiical, triijloblastic ani- 
mals bearing a shell, muscular foot, and mantle; four main pairs of nerv- 
ous ganglia ; 70,000 species. 

Class I — Amphineura — Chitons (Chaetopleura, Ischnochiton) . 

Characteristics: Bilaterally symmetrical; shell typically composed of 
eight transverse calcareous plates with many pairs of gill filaments. 

Class II — Gastropoda — Snails, slugs, whelks {Umax, Physa, Helix, Lymnaea, 
Campelotna, Busy con). 
Characteristics : Asymmetrical animals with well-developed head ; spi- 
rally-coiled shell. 

Class III — Scaphopoda — Elephant's-tusk shells (Dentalium, Siphonodenta- 
Characteristics: Both shell and mantle tubular; protrusible foot ; rudi- 
mentary head. 

Class IV — Pelecypoda — Clams, mussels, oysters, and scallops {Ensis, Ano- 
donta, Venus, Teredo, Ostrea, Pecten). 
Characteristics: Usually bivalved shells with two-lobed mantle; no 
head ; body laterally compressed ; bilaterally symmetrical. 

Class V — Cephalopoda — Squids, cuttlefishes, octopus, nautilus (Loligo, 
Polypus, Dosidieus). 
Characteristics: Bilaterally symmetrical; with foot divided into siphon 
and arms provided with suckers; well-developed nervous system con- 
centrated in head; mouth possesses strong jaws. 





ClctSS I 


Olci-ss -jn 


PecLiculus /^TXi-jh 

Class IS" 


"Po-pilio . 



PHYLUM XII - ARTHROPODA - Lobsters, crabs, spider., millir>odes 
insects. ' * ' 

Characteristics: External evidence of segmentation, body at least beine 
divisible into a well-defined head, thorax, and abdomen; jointed append- 
ages ; chitinous exo-skeleton ; nervous system of ladder f vpo witl, tondcnry 
toward concentration in head region; main longitudinal blood vessel 
with heart dorsal to alimentary canal; coelom reduced; body cavity 
filled with blood (hemocele) ; 640,000 species. 

Class I — Crustacea — Crayfish, crabs, water fleas, barnacles, sowbugs (Cam- 
barus, Callinectes, Gammarus, Asellus, Trior thrus). 
Characteristics: Mostly aquatic; usually bearing gills; with two pairs 
of antennae (feelers) ; chitinous exo-skeleton ; body divided into head, 
thorax, and abdomen ; head and thorax sometimes fused {cephalolhorax) ; 
further subdivision depending largely upon characteristics of carapace. 

Class II — Onychophora — Annelidlike arthropods (Peripatus). 

Characteristics: Tropical, primitive, wormlike tyi)os j)resumably inter- 
mediate between the segmented worms and the arthropods; excretory 
system of annelid type (nephridial) ; respiratory organ resembles tracheae 
of insect group ; external appendages ringed, suggesting segmentation of 

Class III — Myriapoda — Centipedes and millipedes {Scolopendra, Spirobolus). 
Characteristics : Body relatively long and definitely nietamcric ; one 
pair of antennae ; appendages segmented ; legs similar ; respiratory sys- 
tem of tracheal type ; in millipedes there are two pairs of legs per somit«, 
in centipedes one. 

Class IV — Insecta — Insects, as butterflies, grasshoppers, beetles, bees. 

Characteristics: L^sually possess wings; one pair of antennae; tracheal 
respiratory system ; segmented legs. 
Order 1 — Thysanura — Bristletails, Silverfish (Lepistna, Campodea, Thermobia). 
Characteristics : Wingless arthropods ; primitive ; probably derived from 
wingless ancestors ; 11 abdominal segments; chewing mouth parts; usu- 
ally two or three long, threadlike, segmented caudal appendages; less 
than 20 species in the United States ; no metamorphosis. 
Order 2 — Collembola — Springtails (Archorules). 

Characteristics : Primitive wingless insects with chewing or sucking mouth 
parts; four segmented antennae; usually no tracheae; six abdominal 
segments; a springing organ (furcida) present on ventral side of fourth 
abdominal segment in most species ; no metamorphosis. 
Order 3 — Orthoptera — Grasshoppers, cockroaches, walking sticks {Melanoplus, 
Periplaneta, Diapheromera). 
Characteristics: Members of this order are characterized by two pairs 
of wings (sometimes greatly reduced) ; the fore wings usually thickened. 
sometimes leathery ; hind wings folded fanlike beneath fore wings ; biting 
mouth parts ; gradual or simple metamorphosis. 


Order 4 — Isoptera — Termites or white ants {Reticulitermes) . 

Characteristics : Four similar wings lying flat on back when at rest ; 
workers are wingless; chewing mouth parts; abdomen joined directly to 
thorax ; gradual or simple metamorphosis. 

Orders — Neuroptera — Dobson flies, alder flies, lacewings, ant-lions {Corydalis, 
Chrysopa, Myrmeleon). 
Characteristics : Four membranous wings with many veins ; chewing 
mouth parts ; larvae carnivorous ; tracheal gills usually present on aquatic 
larvae; the larvae of the horned Corydalis known as hellgrammites are 
used by fishermen as bait ; complete metamorphosis. 

Order 6 — Ephemerida — Mayflies (Ephemera). 

Characteristics : Mouth parts of adult vestigial ; two pairs of membra- 
nous, more or less triangular, wings ; fore wings larger than hind wings ; 
caudal filaments and cerci very long; aquatic larvae breathe by tracheal 
gills, usually located on either side of abdomen ; adult's span of life short ; 
mouth parts poorly developed, probably making organism incapable of 
taking food; nymph remains one to three years in water; adults moult 
within 24 hours after acquiring wings, therefore called sub-imagos ; gradual 
or simple metamorphosis. 

Order 7 — Odonata — Dragonflies and damsel flies (Macromia, Agrion). 

Characteristics : Chewing mouth parts ; two pairs of membranous veined 
wings; characteristic joint (nodus) on anterior margin of each wing; eyes 
large, compound ; nymphs are aquatic ; gradual or simple metamorphosis. 
When at rest dragonflies hold their wings horizontally and at right angles 
to body, while damsel flies maintain theirs vei-tically. 

Order 8 — Plecoptera — Stone flies (Allocapnia, Taeniopteryx). 

Characteristics : Chewing mouth parts often poorly developed in adults ; 
two pairs of wings; hind wings usually larger and folded beneath fore 
wings ; nymphs aquatic, bearing filamentous tracheal gills ; usually be- 
neath stones in flowing water; gradual or simple metamorphosis. The 
salmon fly, Taeniopteryx pacifica, is a dangerous pest in the State of 
Washington because it destroys buds. 

Order 9 — Corrodentia — Book- and bark-lice (Trodes). 

Characteristics : Either wingless, or two pairs of membranous wings char- 
acterized by a few prominent veins; fore wings larger than hind wings; 
when at rest held over body like sides of a roof; chewing mouth parts; 
gradual metamorphosis. Book-lice often eat paper and bindings of old 

Order 10 — Mallophaga — Chewing lice or bird-lice (Menopon, Trichodectes). 

Characteristics : Chewing mouth parts ; wings absent ; eyes degenerate ; 
metamorphosis gradual or wanting. Members of this group are ecto- 
parasitic upon hair and scales of birds and mammals. 

Order 11 — Embiidina — Emhiids {Emhia). 

Characteristics : Chewing mouth parts ; wingless or possessing two pairs 
of delicate membranous wings with few veins ; cerci present on two seg- 
ments ; males usually winged, females wingless ; gradual metamorphosis. 
These organisms live under stones, etc., in tunnels formed of silk produced 
in tarsal glands. 


Order 12— Thysanoptera — Thrips (Thnps, Franklinella, Crypiolhnps). 

Characteristics: Piercing mouth parts; either wingless or with two pairn 
of long, narrow membranous wings, practically veinless; large, free pro- 
thorax; feet clawless but possessing small protrusible membranous sacs 
for clinging; manj^ parthenogenotic ; gradual metamorphosis. 

Order 13 — Anoplura — Sucking lice {Pediculus, I'hthirins). 

Characteristics: Wingless ectoparasitic lice with piercing and sucking 
mouth parts; eyes poorly developed or absent; parasitic on bodies of 
mammals ; gradual metamorphosis. At least two species, the head louse 
and crab louse, occur on man. 

Order 14 — Hemiptera — True bugs {Artocorixa, Lethocercus). 

Characteristics : Either wingless, or with two })airs of wings ; in such cases 
fore wings are thickened at base ; mouth parts adapted for piercing and 
sucking; gradual or simple metamorphosis. Members of this group con- 
tain many interesting and sometimes economically important forms. The 
water-boatmen (Corixidae) have long, flat, fringed metathoracic legs which 
are adapted for swimming. These peculiar forms carry a film of air about 
body when under w^ater. The leaf bugs (Xeridae) are frequently numer- 
ous and injurious to plants. Bedbugs (Cimicidae) have been accused of 
transmitting various diseases. The cabbage bug does damage to garden 

Order 15 — Homoptera — Cicadas, aphids, leaf-hoppers, and scales {Euscclis, 
Empoasca, Rhopalosiphum) . 
Characteristics : Mouth parts adapted for piercing and sucking ; two pairs 
of wings of uniform thickness held over back like sides of a roof. The cica- 
das (Cicadidae) are better known as the "seventeen-year locust." Plant- 
lice (Aphididae) are mostly small green insects that suck juices from 
plants and have a gradual metamorphosis. 

Order 16 — Dermaptera — Earwigs (Anisolabis, Labia). 

Characteristics : Either wingless, or possessing one or two pairs of wings ; 
in such cases fore wings are small and leathery, meeting in straight line 
along back; chewing mouth parts ; gradual metamorphosis. Earwigs are 
nocturnal and feed principally upon vegetation. 

Order 17 — Coleoptera — Beetles and weevils {Hydrous, Dytiscits, Photinus, 
Characteristics : Either wingless or with two pairs of wings, fore wings 
being hard and sheathlike {elytra); hind wings membranous and are 
folded two ways under elytra; large movable prothorax; chewing mouth 
parts; complete metamorphosis. Many forms are found in this group. 
as the tiger beetles, fireflies, click beetles, whirligig, ladybird, and leaf 

Order 18 — Strepsiptera — Stylopeds {Xenos). 

Characteristics: Mouth parts reduced or wanting; nutrition by absorp- 
tion; males possessing club-shaped fore wings and large membranous 
hind wings ; females wingless and legless ; life cycle complex ; para.sitic on 
bees, wasps, and homopterous bugs. 


Order 19 — Mecoptera — Scorpion-flies {Panorpa, Bittacus). 

Characteristics : Members of this group are wingless or characterized by 
two pairs of long membranous wings containing many veins; head pro- 
longed into beak; antennae long and slender; mouth parts adapted for 
chewing ; males with olasping-organ on caudal extremity resembling sting 
of a scorpion ; metamorphosis complete. 

Order 20 — Trichoptera — Caddis flies {Phryganea, Molanna). 

Characteristics : Adults with vestigial mouth parts ; two pairs of mem- 
branous wings obscurely colored by long silky hairs and narrow scales; 
antennae long and slender; metamorphosis complete; larvae and pupae 
aquatic, constructing portable cases of sand grains or vegetable debris 
fastened together with silk from modified salivary glands. 

Order 21 — Lepidoptera — Butterflies and moths {Tinea, Alsophila, Papilio). 

Characteristics : Wingless, or with two pairs of membranous wings cov- 
ered with overlapping scales; sucking mouth parts coiled beneath head 
consist of two maxillae fastened to form a tube; metamorphosis com- 
plete ; larvae known as caterpillars ; many species known. 

Order 22 — Diptera — Flies and mosquitoes (Tipula, Culex, Prosimulium, Musca, 
Characteristics : One pair of membranous fore wings on mesothorax, or 
wingless ; knobbed threads (halteres) on metathorax represents hind wings ; 
mouth adapted for piercing and sucking, forming proboscis ; larvae known 
as maggots ; complete metamorphosis. 

Order 23 — Siphonaptera — Fleas (Ctenocephalus, Pulex). 

Characteristics : Wingless insects with laterally compressed body ; head 
small ; no compound eyes ; mouth adapted for piercing and sucking, legs 
for leaping; metamorphosis complete; ectoparasites of mammals and 
more rarely birds. 

Order 24 — Hymenoptera — Saw flies, ichneumon flies, ants, wasps, and bees 
(Cladius, Ophion, Formica, Vespa, Apis). 
Characteristics : Wingless or with two pairs of membranous wings ; fore 
wings usually larger ; venation reduced ; wings held together on each side 
by hooks (hamuli); mouth parts adapted for chewing or sucking; first 
abdominal segment fused with thorax ; complete metamorphosis. 

Class V — Arachnoidea — Spiders, scorpions, ticks, mites, and king crabs 
{Caddo, Lycosa, Phalangium, Buthus, Argas, Sarcoptes, Limulus). 
Characteristics : No antennae nor true jaws ; two of six pairs of jointed 
appendages modified for mouth parts; respiration by lung-books or 
tracheae; first pair of appendages usually contain poison glands, second 
pair used as jaws ; terminal portions as sensory organs ; body usually 
divided into anterior cephalothorax and posterior abdomen ; former bears 
four pairs of legs for locomotion. 







Annelida (Annulata) 

Troehelmi n thes 






Cl asses 







.Si'EciEM DkhciiiiiEU 


Man, cat, horse, bat, whale 
liirds, fowls 

Turtles, snakes, lizards, alli- 
I'rofis, toads, salamanders 

Tunicates, Balanoglossus, etc. 
Total Chordata 


Crayfish, crabs, water fleas, 

barnacles, sowliugs 
Centipedes, millipedes, etc. 
All true insects 
Spiders, scorpions, ticks, 

mites, and king crabs 

Total .\rthropoda 

Snails, slugs, clams, oysters 

Starfish, sand dollar, sea- 

Earthworm, leeches 

Bryozoa, Ijrachiopods 

P'latworms, flukes, tapeworms 

Roundworms, Trichinclla, 

Rotifers, wheel animalcules 

Jelly-fishes, coral animals. 


Ameba, Paramecium, 

Euglena, malarial organ- 
isms, trypanosomes 

Grand total 



















' Modified from Metf-alf and Flint, Destmrtirp and Useful Insects. By pprmis.xjon r)f the McOrnw- 
Hill Book Company, publishers. The discrepancies between this table and the tt'Xt illusirato the 
pragmatic nature of taxonomy. 

H. W. H. — 8 



Sub -phylum I 

half - CL ' cViorcC 

Sub • pliylum H 


chor*<jC - in- t<xil 



sute-pViylvcm, uc 


cViorcC-in- Vj«acC 


(1^ ri'^^i 

Arophioxus ' 

ammals '*\/it*h a -notocVjorcC 


(c) Superclass tetrapoda 

(ii fossil Ostracoderm 


(2 ) 

^etronwjjon. . lamprey 


(1) ^ 





sub-phylura IS" 


'->vit/Vi "toccc-Vctoones 



PHYLUM XIII — CHORD ATA — Animals with notochord. 

Characteristics: All possess a dorsal supporting rod or notochord and 
pharyngeal gill clefts at some stage in life cycle; tubular nerve cord 
dorsal to digestive tract ; 36,000 species. 

Sub-Phylum I — Hemichordata — (Balanoglossus). 

Characteristics : Wormlike marine oiganisms of doubtful relationship that 
burrow in sand and resemble the larval echinoderms in development ; head- 
end with proboscis and collar; with or without a notochord. 

Sub-Phylum II — Urochordata — Tunicates and ascidians. 

Characteristics : Marine organisms with saclike covering {tunic) ; larvae 
resemble tadpoles, possessing notochord in tail; gill slits and endostyle 
present in pharynx. 

Sub- Phylum III — Cephalochordata — Lancelots (Amphioxus). 

Characteristics : Segmented primitive chordates, burrowing in sand ; lat- 
erally compressed ; notochord extending from anterior tip to tail. 

Sub-Phylum IV — Vertebrata (or Craniata) — Vertebrates. 

Characteristics : Animals with definite head, sense organs, closed circula- 
tory system, and axial notochord at some period in life cycle; skull and 
vertebral column present either in cartilaginous or bony stage. 

Super-Class A — Agnatha — Fossil, armored Ostracoderms, lampreys and hag- 
fishes (Cyclostomata). Primitive fishlike forms (Pterichthys, Petromy- 
Characteristics: Animals w'ithout jaws; sucking mouth and primitive 
brain present. 

Super-Class B — Pisces — True fishes. 

Characteristics : Organisms with true jaws ; typically scaled ; charac- 
teristically aquatic; appendages developed into fins; two-chambered 



Clocss I 


Class IE 


spook fislT. 

(B) Superclass PISCES 





Class 12: 



class "JT 


l\OLL CALL jy^ 

Class I — Elasmobranchii — Gristle-fishes {Sgualus, Raia). 

Characteristics: Cold-blooded fishlike vertebrates witli jaws; charac- 
terized by a cartilaginous skeleton, i)(>rsistent notochord and placoid scales; 
upper jaw suspended to ci-aniuin indirectly by means of ligaments and 
cartilages (hyostylic). 

Class II — Holocephali -^ Elephant-fishes (Chimaera). 

Characteristics : Immovable upper jaw fused with cranium (autostyiic) 
resembling higher forms; gill slits covered by flap (operculum); tail 

Class III — Ganoidei — Enamel-scaled fishes (Acipenser, Lepisosteus, Polyp- 
Characteristics : More or less armored fish ; remnants of group dominant 
in Devonian seas; degenerating spiral valve in intestine associated with 
presence of pyloric caeca; scales usually rhomboidal, fitting together 
rather than overlapping; dorsal fin usually close to caudal fin. 

Class IV — Dipnoi — Lung-fishes (Neoceratodus, Lepidosiren, Protopterus). 

Characteristics: Semitropical fishes, passing dry season by aestivating in 
slimy cocoon ; during period of active life use gills, and while aestivating 
breathe air, the modified swim bladder acting as a lung; cycloid scales; 
auricle of heart partially divided. 

Class V — Teleostei — Bony fishes (Ctenolabriis, Perca, Gadus, Microptcrtis). 
Characteristics : Bony fishes, breathing primarily by gills ; well-develoiM-d 
operculate bones, cycloid or ctenoid scales ; tail homocercal. These fishes 
constitute about 90 per cent of all known varieties. 




OrcCe^r- (i) 

OrdiQr (2") 

jg^css amphibia 



"blincC" ^vo^mUke 

(C)5icperclass TE 




spottccC incvt 


Order (3) 

UrodLela . . . ., 
gtnpbibitt wttn taits 

OncCer- (4-) 


taillC96 amphibia 


Super-Class C — Tetrapoda — Four-footed vertebrates. 

Characteristics : Well-defined limbs witli hands and feet typically con- 
structed on plan of five digits; stapes or coluniolla present in ear; Rirdles 
adapted to bear weight on land; body divisible into neck and trunk, tail 

Class I — Amphibia — Frogs and salamander.'?. 

Characteristics: Cold-blooded, naked vertebrates undergoing a meta- 
morphosis ; usually with five-fingered limbs (pentadactylous) ; young 
u.sually aquatic, breathing by gills; adults using lungs and skin, u-suallj' 
air breathers. 
Order 1 — Stegoccphalia — Extinct fossil amphibians (Erynps, Loxomma). 

Ch.^racteristics : Fossil forms resembling amphibia, flourishing in car- 
boniferous age ; probably earliest four-footed air breathers. 
Order 2 — Apoda — Legless amphibia (Herpeles, Siphonops, Caecilia). 

Characteristics : Small, tropical, wormlike, often blind amphibia, burrow- 
ing in ground. 
Order 3 — Urodela — Salamanders {Desmognathus, Necturus, Cryptobranchus, 
Characteristics: Tadpole-like tail retained throughout life; some never 
emerge from water; a few retain external gills in adult stage. 
Order 4 — Anura — Frogs and toads {Rana, Bufo, Ilyla). 

Characteristics : Tailless upon completing their metamorphosis ; capable 
of singing ; characterized by the possession of movable eyelids. 



OrcCeJr, 1 , ^. 

RViy n cho ctepnou loc 

'tJhe. old, t-imer-^'' 


OrcL©3~ 2 • 


Crocodilea . aUigators 


Soft iheileat turtle 

(C)5LqDercla55 TETRAPODA 

"i5o>: turtle^ 

OrcCer -3 


turtles, tortoises 

^ 'osoxers 
fish -like reptile^ 

sub ordter Soci^ria 



./(2) -^^.^^.ffla^ (4) 

'' sub order Serper^teS Hiriosaurs 
SriccKe.S ^lant reptiles 

Ordei^ 4r 

snokss , li3ards 

Orders 5-8 fossil rejjtiles 
Ichthyosouria ,Plesio5auria 


Class II — Reptilia — Turtles, snakes, alligators, and lizards. 

Characteristics: Cold-blooded; usually covered with scales and fre- 
quently bony plates ; air breathers. 

Order 1 — Rhynchocephalia — "The old-timers," Sphenodon. 

Characteristics : Biconcave vertebrae often containing remnants of noto- 
chord; quadrate bone immovable; parietal eye present. This group i.« 
represented by one genus of lizards, Sphenodon, found only in New Zea- 

Order 2 — Crocodilia — Crocodiles and alligators {Crocodiliis, Alligator). 

Characteristics : Anterior appendages bearing five digits, jiosterior four 
with trace of fifth ; longitudinal slit constitutes cloacal opening; vertebrae 

Order 3 — Chelonia — Turtles and tortoises (Amyda, Eretmochelye, Terrapene, 
Testudo, Chelonia). 
Characteristics : Body surrounded by bony case forming a carapace and 
plastron; toothless jaws ; immovable quadrate bone; appendages typi- 
cally with five digits. 

Order 4 — Squamata — Snakes and lizards (Phrynosoma, Heloderma, Tham- 
Characteristics: Usually with horny epidermal scales or plates; movable 
quadrate bone; vertebrae usually procoelous; ril)s with single heads. 
This order is usually subdivided into two sub-orders: lizards (Sauria) ; 
and snakes (Serpentes). 

Orders 5-8 — /)z/;o.s<7;/m — Fossil reptiles (Ichthyosaurs, Plesiosaurs, Pterodac- 
tyls, Dinosaurs). 

In these groups belong such forms as the fishlike reptiles (Ichthyosaurs) ; 
the long-necked reptiles (Plesiosaurs) ; the flying reptiles (Pterodactyls) ; 
and the giant reptiles (Dinosaurs). 



Subcbss A - Arcbaeomithes 
r| fossil reptile-like "bircCs 




Kestserornithifbrmes ^, 
^5sil toothe^blrasl 

« Ca5i:arii|brTOes Kivi^^ 

^Caseovarie^ ^ Ciconiifomes 

C)Icbtbxor^^i7orm<25 ^ /« stork -like bircfs 

Grui formes 
rails at2ct coots 


glover, 5nipi« ,^115 

11^ ^ . ^ 

m.^s??^ Cuculi formes 

stratHorjiformes |f^^ M-^WxV Talcomfl^nes 
Afncan oitrich jV ]j col^'mbilbrmcs falcon-like-binis 
(-r) <?4rV;;^V-- l-oons and Grcbas /^ 

i)moTnitbi(t>rmes ^"^^W ^^^'^S^l ^^^^ -"^ssi^*^ 
Moots ^-r-'^'^/ j'^^'^,.// Coraciiformas 

extinct :^^/ <^^f< — ^'^ 

(8) (^^^ ^ Gcclhformss 

=^ elep hant birds aibatrossa.s .petrels 

RheifoT^m©© . 
American ostrioh 

Subclass D -Neorrzitbes 


percViing- birds 


Class III — Aves — Birds. 

Characteristics: Typically featliered and toothless; \varm-l)Ioo(lo(l. 

Subclass A — Archaeornithes — Fossil birds (Archaeopteryx). 

Characteristics: Ancient re[)tilelike fossil birds; only three specimens of 
a single genus (Archaeopteryx) are known. 

Subclass B — Neornithes — Recent birds. 

Characteristics: Mostly composed of birds which are represented by 
living forms; 21 orders. 
Order 1 — Hesperornithiformes — (Hesperornis). 

Characteristics : Fossil, toothed birds from America ; teetli set in a groove. 
Order 2 — Ichthyornithiformes — {I chthyornis) . 

Characteristics : Fossil, toothed birds from America, whose teeth are set 
in sockets. 
Order 3 — Struthioniformes — Ostriches (Stridhio). 

Characteristics : Naked head, neck, and legs ; flightless, terrestrial forms ; 
feet with two toes; no keel on breastbone {sternum). 
Order 4 — Rheiformes — Rheas (Rhea). 

Characteristics : Distinguished from preceding order by a partially feath- 
ered head and neck; flightless terrestrial birds, with three-toed feet; 
feathers without aftershaft. 
Order 5 — Casuariiformes — Cassowaries and emus (Dromalus). 

Characteristics: Terrestrial, flightless birds, possessing small wings; 
feathers with large aftershaft. 
Order 6 — Crypturiformes — Tinamous (Rhynchotus) . 

Characteristics : Flying, terrestrial birds, with a short tail ; no pygostyle. 
Order 7 — Dinornithiformes — Moas (Palapteryx). 

Characteristics : Recently extinct, flightless, terrestrial birds, with large 
hind limbs ; wing bones absent. 
Order 8 — Aepyornithiformes — Elephant birds (Aepyornis). 

Characteristics: Extinct terrestrial flightless birds with large hind limbs; 
small sternum and wings ; large eggs. 
Order 9 — Apterygiformes — Kiwis (Apteryx). 

Characteristics: Small flightless terrestrial birds; hairlike feathers with- 
out aftershaft. 
Order 10 — — Penguins (Eudyptes). 

Characteristics: Marine antarctic birds, incapable of flight, with small 
scalelike feathers; wings modified as paddles for swimming. 
Order 11 — Colymbiformes — Loons and grebes (Gavia, Podiceps). 

Characteristics : Aquatic birds with feet far back with webbed or lobed toes. 
Order 12 — Procellariiformes — Albatrosses and petrels (Diomedea, Hydrobates). 
Characteristics : Marine birds with great powers of flight ; webbed toes ; 
bill sheath of several pieces. 
Order 13 — Ciconiiformes — Storks, birds, pelicans, cormorants, snake-birds, 
herons, ibises, and flamingos (Phalacrocorax, Ardea, Phoetncoptcru.f). 
Characteristics: Long-legged aquatic marsh birds with feet adapted for 

114 NATUllAL lilSTORY 

Order 14 — Anseriformes — Swans, geese, and ducks (Mergus, Anas, Cygnus). 
Characteristics : Aquatic birds whose beak is covered by soft sensitive 
membrane edged with horny lamellae. 
Order 15 — Falconiformes — Falcons, vultures, eagles, hawks, and secretary-birds 
{Cathartes, Gymnogyps, Sagittarius, Falco). 
Characteristics : Carnivorous birds with curved, hooked beak ; feet 
adapted for perching and provided with sharp, strong claws. 
Order 16 — GalUformes — Tui'keys, fowls, quails, and pheasants ; also the 
hoactzin (Meleagris, Colinus, Bonasa). 
Characteristics : Arboreal or terrestrial birds ; feet adapted for perching. 
Order 17 — — Rails and cranes {Rallus, Gallinula, Fulica). 

Characteristics : Mostly marsh birds. 
Order 18 — — Plovers, snipes, gulls, terns, auks, and pigeons 
{Jacana, Larus, Rhynchops). 
Characteristics : Marine, arboreal, or terrestrial forms. 
Order 19 — Cuculiformes — Cuckoos and parrots {Conuropsis, Coccyzus). 

Characteristics : Arboreal birds, first and fourth toes directed backwards ; 
the latter may be reversible. 
Order 20 — Coraciiformes — Kingfishers, owls, hummingbirds, swifts, and wood- 
peckers (Streptoceryle, Antrostomus) . 
Characteristics : Tree-inhabiting forms with short legs. 
Order 21 — Passeriformes — Perching birds (Passer, Sayornis, Tyrannus). 

Characteristics : More than half of all known birds belong in this order. 
In America representatives of 25 families are found. A few of these are 
the flycatchers, larks, thrushes, thrashers, wrens, warblers, swallows, 
shrikes, nuthatches, crows, orioles, finches, and creepers. 


Class IV — Mammalia — Mammals. 

Characteristics: Members of this class are readily distiiiKuisl)0(l by a 
covering of hair at some time in their existence; the females pos.seKs 
mammary glands which secret(> milk for nourishment of young. 

Subclass A — Prototheria — Monotremes {Echidna and Ornithorhynchus). 
Characteristics: Egg-laying mammals; in case of Ecliidim the egg is 
placed in a temporary pouch and incubated until hatched. 

Subclass B — Metatheria — Marsupials {Didelphys, Petrogale, Macropus). 
Characteristics : Carry young in marsupium or pouch ; allantoic placenta 
typically absent. 

Subclass C — Eutheria — Viviparous mammals. 

Characteristics : Bring forth their young alive ; young never carried in 
pouch ; nourished before birth by placenta. 
section a — unguiculata — Clawed mammals. 
Order 1 — Insectivora — Insect-eaters, moles, and European hedgehogs. 

Characteristics : Small terrestrial clawed mammals with typically planti- 
grade feet ; molar teeth enameled, rooted, and tuberculate. 
Order 2 — Dermaptera — Flying lemurs. 

Characteristics: Members of this group resemble the insectivores in the 
structure of the skull and the canine teeth ; only two genera are known, 
which inhabit the forests of Malaysia and the Philippines. 
Order S — Chiroptera — Insectivorous bats, fruit-bats, and blood-sucking vam- 
Characteristics : Mammals with claws whose fore limbs are modified for 
Order 4 — Carnivora — Flesh-eating mammals, hyenas, raccoons, dogs, cats, 
weasels, bears, sea-lions, seals, and walruses. « 

Characteristics : Carnivorous mammals with claws and large projecting 
canine teeth; incisors small; premolars adapted for flesh-cutting. 
Order 5 — Rodentia — Gnawing animals, hares, rats, mice, squirrels, beavers, 
porcupines, guinea pigs. 
Characteristics : Members of this group are usually separated into two 
suborders depending upon the possession of one or two pairs of incisors 
in upper jaw. 
Order 6 — Edentata — So-called toothless mammals, three-toed sloth, armadillo, 
and pangolin. 
Characteristics: Clawed mammals; teeth entirely absent or mi.-^sing 
from anterior part of jaw ; teeth usually without enamel ; tongue often 
long and protractile. 
section b — primates. 

Order 7 — Primates — M&mmaAs with nails; tarsiers, lemurs, monkeys, apes, 
Characteristics: Toe or thumb usually is opposable to other digits; 
dentition rather primitive ; eye orbits directed forward ; posture usujilly 




®gig-layin^ mamnrzals 




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clawea mammals 


viviparous mamrcKxis 


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OrcCejT 8 


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SECTION c — UNGULATA — Hoofed mammals. 

Order 8 — Artiodactyla — Even-toed ungulates; hippopotamus, camel, deer, 
moose, domestic cattle, giraflfe. 
Characteristics; An even number of digits, axis of symmetry passing 
between digits three and four. 
Order 9 — Perissodactyla — Odd-toed ungulates, horse, zebra, tapir, rhinoceros. 
Characteristics : An uneven number of digits, axis of symmetry passing 
through digit three 
Order 10 — Proboscidea — Elephants. 

Characteristics : Ungulates characterized by long, prehensile proboscis ; 
incisors developed to form tusks ; broad molars. 
Order 11 — Sirenia — Sea-cows, dugong, manatee. 

Characteristics: Aquatic ungulate-type Eutheria; fore limbs finlike ; hind 
limbs absent ; tail with horizontal fin. 
Order 12 — Hyracoidea — Hyrax and coneys. 

Characteristics : Small rodent-like mammals with reduced tail and short 
ears ; four digits on fore limbs ; three digits on hind limbs. 
section d — cetacea — Whales and dolphins. 

Characteristics : Aquatic mammals ; probably derived from the Ungui- 
culata or Ungulata. 
Order 13 — Odontoceti — Toothed whales, dolphin, porpoise, grampus. 

Characteristics : Cetacea with teeth (at least on lower jaw) ; no whale- 
Order 14 — Mystacoceii — Whalebone whales, fin whale, right whale. 

Characteristics: Cetacea without teeth in adult; mouth provided witli 
plates of whalebone. 



Abdomen — the posterior region of the body, behind the thorax of an insect ; the 
region of the body below the chest in man. 

Aestivating — passing the summer in a torpor. 

Aftershaft — an accessory plume arising from the posterior side of the shaft of 
the feathers of many birds. 

Alimentary canal — food tube of animal, beginning with mouth and ending with 

Allantoic — pertaining to a respiratory sac which in early fetal life grows out from 

the hind-gut of an embryo. 

Ambulacral groove — groove in which tube-feet are located. 

Antennae — paired appendages, which are sensory in function, on the head of an 

insect or crustacean. 
Antheridium — organ or receptacle in which male sex cells of ferns are produced. 
Anus — posterior opening of alimentary canal. 

Appendage — an organ or part attached to a body, as a leg, arm, fin, or tail. 
Arboreal — pertaining to forms frequenting trees. 

Archegonium — a female organ in which the young plant begins development. 
Aristotle's lantern — masticating apparatus of sea-urchin. 
Ascomycetes — sac fungi. 

Ascospore — one of a set of spores contained in a special sac or ascus. 
Ascus — a membranous spore sac of fungi. 
Asexual — having no sex. 

Axial — pertaining to the fundamental central line of a structure. 
Basidiospore — a spore formed on a basidium. 

Basidium — the spore-producing organ of certain of the higher fungi. 
Bilaterally symmetrical — having two symmetrical sides about an axis. 
Bill sheath — protective covering for bill. 
Bivalve — consisting of two shells or valves. 
Body cavity — space in which the viscera lie. 
Branchiae — gills. 
Calcareous — containing lime or calcium, chalky. 

Canine tooth — a pointed tooth situated between an incisor and a bicuspid or 
premolar tooth. 

Carapace — a bony or chitinous case covering an animal's back, as in the crayfish. 

Carotin — yellow pigment of plants ; associated with chlorophyll and xantho- 

Carpel — a pistil, or one of the members composing a compound pistil or seed- 

Cartilaginous — gristly substance forming part of the skeleton. 

Caudal — of, or pertaining to, the tail. 

Cerci — bristlelike structures. 

Chitin — a carbohydrate derivative forming the skeletal substance in arthropods. 

Chlorophyll — green coloring matter found in plants and some animals. 

Chloroplasts — small bodies of protoplasm containing chlorophyll. 

Chromosome — a deeply staining body in the nucleus of a cell, supposed to carry 

the determiners of hereditary characters. 
Cirri — slender extensions found on bodies and appendages of many forms, 

which are used for various functions. 


Clasping organ — specialized holdfast structure of certain males used in conula- 

tion. ' 

Coelenteron — internal cavity of a coelenterate, which servos as a diKcstivc 
tract as well as body cavity. 

Coelom — body cavity. 

Columella — rodhke bone of middle ear of anura formed from hyomandihular 


Compound eye — made up of several simple eyes. 

Cotyledon — embryonic leaf, in a seed. 

Ctenoid scales — scales with a comblike or serrate margin. 

Cuticle — an outer layer of the skin. 

Cycloid scales — scales with evenly curved free border. 

Cytoplasm — the living substance of the cell outside of the nucleus and inside 
the cell membrane. 

Deciduous — falling off at maturity. 

Dentition — number, arrangement, and kind of teeth. 

Diaphragm — (Bot.) a septum or membranous layer. 

Dicotyledon — a plant that bears seeds having two cotyledons. 

Digits — terminal divisions of limb in any vertebrate above fishes. 

Diploblastic — having two distinct germ layers. 

Direct development — no metamorphosis, i.e., the young when hatched closely 
resemble adult except for size. 

Dorsal — pertaining to the back or top side of (as of a leaf). 

Dorsoventral — pertaining to structures which extend from dorsal to ventral 

Ectoderm — the outer embryonic layer in a multicellular animal. 

Ectoparasite — a parasite that lives on the exterior of an organism. 

Elytra — the anterior wings of beetles, hard and caselikc. 

Embryo sac — the megaspore in plants. 

Endoparasite — a parasite which lives within the body of its host. 

Endostyle — ciliated groove whose cells secrete mucus. Foimd in urochordate,s 

and cephalochordates. 
Epithelium — cellular tissue covering a free surface or lining a tube or cavity. 
Excretory — pertaining to organs of elimination. 
Exo-skeleton — an outside skeleton such as the shell of a lobster. 
Fibrovascular bundles — collections of tubular cells, supported by woody cells, 

which conduct fluids in plants. 
Filamentous — composed of long, threadlike structures. 
Flagella — threadlike projections of cells, which are used for locomotion. 
Flame cell — the terminal cells of l)ranches of excretory system in flatwornis, 

with cavity continuous with lumen of duct, and containing a ciliuni or l)uncli 

of cilia, the motions of which give a flickering appearance similar to that of 

a flame. 
Foot — thick muscular locomotor organ of molluscs. 
Furcula — a forked process or structure. 
Gamete — a mature sex cell. 
Gametophyte — a stage in the life history of a moss or fern in which sex cells 

are produced. 
Ganglion — a group of nerve cells situated outside of the brain or spinal .•oluinn. 
Gastric — pertaining to or in region of stomach. 
H. w. H. — 9 


Gastrovascular — serving both digestive and circulatory purposes. 

Germ cell — sex cell. 

Gill cleft — a branchial opening formed on the side of the pharynx. 

Gill filaments — the soft filamentous structures on the respiratory organs (gills) 
of aquatic animals. 

Gland — an organ which secretes material to be used in, or excreted from, the 

Halteres — a pair of small capitate bodies representing rudimentary wings in 

flies, used as balancers. 

Haploid — having the number of chromosomes characteristic of mature germ- 
cells for the organism in question. 

Hemocoele — an expanded portion of the blood system which takes the place of 
a true coelom. 

Hepatic caecum — blind pouch or diverticulum of or in region of liver. 

Hermaphroditic — pertaining to an organism with both male and female repro- 
ductive organs. 

Heterocercal — having vertebral column terminating in upper lobe of fin, which 
is usually larger than the lower. 

Homocercal — with equal or nearly equal lobes, and axis ending near middle of 

Hypha — one of the filaments composing the mycelium of a fungus. 

Incisors — front chisel-like teeth of either jaw. 

Incubate — ■ to keep warm and under other favorable conditions for hatching. 

Indirect development — undergoing metamorphosis, i.e., showing a decided change 
in form and appearance from time of hatching until maturity. 

Integument — a covering or protective layer; skin. 

Keel — ridgelike process. 

Lamellae — thin platelike structures. 

Larvae — young stages in the development of some forms of animals, which be- 
come self-sustaining but which do not have the characteristics marking adults. 

Lateral line — longitudinal line at each side of body of certain aquatic animals 
marking position of sensory cells. 

Laterally compressed — narrow from side to side. 

Ligament — a band of connective tissue binding one bone to another. 

Lobate — divided into lobes. 

Lophophore — ridge bearing tentacles. 

Lung-books — respiratory organs formed like a purse with numerous compart- 
ments or a book with edges of leaves exposed. 

Mammary glands — milk-secreting glands. 

Mantle — the soft outer fold of skin in molluscs which secretes the outer shell. 

Medusoid — like a jellyfish or medusa. 

Megasporangium — a macrospore-producing sporangium in plants. 

Megaspore — larger spore of heterosporous plants, regarded as female; embryo- 
sac cell of seed plant. 

Membranous — resembling or consisting of a membrane ; pliable and semi- 

Mesenchymatous — pertaining to mass of tissue intermediate between ectoderm 

and endoderm, derived from mesoderm. 
Mesenteries — peritoneal folds serving as a bridge for blood vessels and for 

holding organs to body wall. 
Mesoderm — the middle layer of tissue in a young animal embryo. 


Mesoglea — an intermediate non-cellular layer in sponges and coeleiiterate.s. 

Metamorphosis — change in form or structure of an animal in its devclotimerif 

from embryo to adult. 
Gradual or simple metamorphosis — young resemble adults at hatcliing cxccDt 

for absence of wmgs or color, shape, and structure of some appendages ' 
Complete metamorphosis — young differ from adults in appearance habitat 

etc., and undergo several changes in form such as larvae, pupae, and adult. ' 
Metathorax — posterior segment of insect's thorax. 
Molars — grinding teeth. 

Monocotyledon — a plant that bears seeds having but one cotyledon. 
Mother cell — primary cell before division occurs. 

Mycelium — the threadlike body of a mold, or other fungus ; made up of indi- 
vidual threads called hyphae. 

Nematocyst — a stinging cell. 

Nervure — one of riblike structures which support membranous wing of insect. 

Notochord — a rod of cells forming the supporting axis of lower chorda tes ; found 
in early stages of development in all vertebrates. 

Nucleus — the center of activity in the living cell. 

Nymph — larva of aquatic forms which undergo gradual or simple metamor- 

Oogonia — female reproductive organs in certain Thallophytes ; the mother egg 

Operculum — a lid or cover. 

Oral — pertaining to the mouth ; side on which mouth lies. 
Ovary — (Bot.) the base of a pistil, containing the ovules. 
(Zool.) — the egg-containing organ. 

Ovule — egglike cell of a plant. 

Papillae — any small nipplelike projections or parts. 

Parapodia — paired appendages used in locomotion, attached to body segments 

of some marine worms. 
Parasites — animals or plants which live at the expense of other organisms. 
Parietal eye — rudimentary eye ari-sing as an evagination on the median dorsiil 

surface of the brain. 
Parthenogenesis — reproduction without fertilization by a male element. 
Pedicellariae — minute pincerlike structures studding the surface of some of the 

Pentamerous — made up of five parts. 

Peritoneum — membrane which lines the abdominal walls and invests the con- 
tained viscera. 
Phylogenesis — history of evolution of species. 
Placenta — organ through which the mammalian embryo is nourished by the 

Placoid scales — embedded scales and dermal teeth of elasmobranchs. 
Plantigrade — walking with sole of foot touching the ground. 
Plasmodium — a single mass of living material which contains many nuclei. 
Plastid — small bodies of .specialized protoplasm lying in cytoplasm of some 

cells — especially plants and certain protozoans. 
Plastron — ventral bony shield of tortoises and turtles. 

Poison gland — gland which secretes poison, used for protection or food-getting. 
Pollen tube — a tubular process developed from pollen grains after attachnu-nt 

to stigma. 


Polyp — a separate zooid of a colonial animal. 
Prehensile — adapted for holding. 

Premolars — bicuspid teeth between canine and molar teeth. 
Proboscis — any of various tubular prolongations of the head of animals ; mus- 
cular protrusible part of the alimentary canal in certain worms. 
Procoelus — with concave anterior face. 

Prothallus — a small, thin, gametophytic mass of tissue developed from spores 
of ferns. 

Prothorax — anterior thoracic segment of arthropods. 

Protractile — capable of being thrust out. 

Pseudopodia — protrusions of protoplasm (false feet) serving for locomotion and 
prehension in protozoa. 

Pygostyle — an upturned compressed bone at end of vertebral column of birds, 
formed by fusion of caudal vertebrae. 

Pyloric caecum — a blind diverticulum or pouch in the pyloric region. 

Pylorus — the aperture between the stomach and the small intestine. 

Pyrenoid — a colorless plastid of lower plants, a center of starch formation. 

Quadrate bone — the bone with which the lower jaw articulates with the cranium 
in some forms. 

Radially symmetrical — having similar parts arranged on either side of a central 


Reproduction — the process by which organisms produce off .spring. In asexual 
reproduction a new organism is formed by the separation of a cell or cells 
from a single parent; in sexual reproduction two cells from two plants or 
two animals of different sexes join together to form a new individual. 

Rhizoids — rootlike organs. 

Rhomboidal — shaped more or less like an equilateral parallelogram, having its 
angles oblique. 

Rodent — animal with a habit of gnawing or nibbling. 

Sap cavity — a vacuole, filled with water and dissolved substances in mature, 
live plant cells. 

Saprophytes — organisms which live on dead and decaying organic matter. 

Scale — a flat, small, platelike external structure, dermal or epidermal. 

Schizogony — repeated division of the nucleus without immediate cell division. 

Sedentary — not free-living; animals attached by a base to some substratum. 

Segmentation — the division or splitting into segments or portions ; cleavage of 
an ovum. 

Sessile — stationary or attached, opposite of free-living or motile. 

Setae — bristlelike structures. 

Siphon — a tube through which water may pass into and out from the mantle 

cavity of a mollusc. 
Sperm — male sex-cell. 

Spicules — siliceous or calcareous secreted skeletal structures of sponges. 
Spiral valve — a spiral infolding of intestinal wall. 
Spongin — material of skeletal fibers of certain sponges. 
Sporangium — a sac containing spores. 
Spore — a type of reproductive cell, usually asexual, with a protective covering 

enabling it to survive unfavorable environmental conditions. 
Sporophyll — a sporangium-bearing leaf of ferns. 
Sporophjrte — spore-bearing stage in the life cycle of a plant. 
Stapes — stirrup-shaped innermost bone of middle ear of mammals. 


Sterigma — a slender filament arising from basidium, giving rise to spores by 

Stigma — the part of a pistil which receives the pollen grains. 

Tarsal — pertaining to the tarsus or last region of the leg of an insect ; the ankle 
bones of vertebrates. 

Tentacles — flexible organs at the oral region of an animal, used for feeling, 
grasping, etc. 

Thallus — a simple plant body not differentiated into root, stem, and leaf. 

Thorax — the part of the body between the head and abdomen. 

Tracheae — respiratory tubes of insects. 

Tracheal gills — small winglike respiratory outgrowths from the abdomen of 
aquatic larvae of insects. 

Triploblastic — with three primary germinal layers. 

Trochophore — free-living pelagic, ciliated larval stage. 

Tube-feet — organs of locomotion of echinoderms. 

Tuberculate — resembling or having root-swellings or nodules. 

Ungulates — hoofed animals. 

Vascular — consisting of or containing vessels adapted for transmission or cir- 
culation of fluid. •*; 

Vein — branched vessel which carries blood to heart; rib or nervure of insect 

Velum — a membranous partition likened to a veil or curtain. 

Ventral — pertaining to the belly surface or under side. 

Vertebrae — bones of the vertebral column (backbone). 

Vestigial — small and imperfectly developed ; rudimentary. 

Whorl — (Bot.) circle of flowers or parts of a flower arising from one point. 
{Zool.) spiral turn of univalve shell. 

Xanthophyll — a yellow pigment invariably associated with chlorophyll in higher 

Zoospore — a motile spore of either plants or animals. 




Preview. What is being alive? • Metabolism • Some signs of mani- 
festation of life • The production and use of enzymes associated with living 
things ■ Structure of protoplasm • Protoplasm and the cell • Chemical or- 
ganization of living matter • Protoplasm a complex mixture • Protoplasm 
a colloidal mixture • Diffusion ■ Osmosis and its significance to living cells • 
Suggested readings. 


Being alive is something that we all know a little about. Liveliness 
is associated with those of one group who are "up and coming," 
those who are active, both mentall}^ and physically. If living things 
are thought about a little more closely, certain things are attributed 
to them : they move, feed, grow, are sensitive, and they reproduce their 
kind. The scientist goes a step further and compares the living thing.s 
with those which do not possess this mysterious something we call 
life. He says life is a manifestation of forces, like a flame, or elec- 
tricity. He goes beyond superficial observation and asks himself 
a good many questions about the make-up and action of the living 
things which fill his environment. Some of the problems with which 
one is confronted are relatively simple and may be solved with a little 
close observation, even without the aid of a microscope, but other 
problems are speculative and may never be answered in full. 

If the problems were to be assembled with a view to attempting 
their solution, some of the more important might be the following : 
What is being alive? What differentiates living stuff from non- 
living? What is known about the ultimate composition of the living 
stuff ? Is it different for animals and for plants ? And what common 
characteristics can be found for i)lants and animals? 

It is obvious that our problems resolve themselves into two gr()Ui)s. 
those which are more or less speculative and those which depend on 
the knowledge provided by the physicist,, and biologist. 
The newer knowledge of chemistry and physics and the of the 
refinements of the compound microscope have made possible mudi 
that was undreamed of a i'o^^■ decades ago. It is only 260 years since 



the first simple microscopes of the Dutchman, Antony van Leeuwen- 
hoek, but enormous advances have been made in this period. The 
most rapid advances in chemistry, physics, and biology have come 
in the last two or three decades, but we are still far from the solution 
of the great riddle of the universe — What is life, and from whence 
did it come ? 

What Is Being Alive? 

The chemist or the biologist weighs the food an organism eats and 
thus finds out that much of the energy locked up in food is trans- 
formed within the body of the organism, ultimately to be released 
in another form, either in heat production or in work of some kind. 
Not only do living things release energy but they also grow and are 
able to repair parts that are wasted or lost. Think of the athlete, 
hale and hearty, winning points for his team ; losing weight in a 
football game, making it up after the game at the training table, or 
imagine the same athlete recovering from a severe illness, or with 
his leg in a cast after an accident. One may feel fairly sure that he 
will soon be well again. The living stuff of which he is made will 
not only use the food to release energy for his normal processes, but 
will also rebuild the expended body material and rid itself of such 
wastes as result from the process. Put in another way, this living 
stuff of which an athlete is composed has the ability to take in food, 
to use this food for the release of the energy stored up in it, or, under 
certain conditions, to make some of the food over into living ma- 
terial. Living things thus have the capacity for growth, for waste 
and repair, and, like man-made machines, have the abihty to use food 
fuel, and to release energy from it. 


The sum total of all the processes involved in the business of being 
alive is called metabolism. This series of processes is twofold : first, 
constructive metabolism, or anabolism, in which the food material 
becomes a part of the living organism, the energy being held there in 
a potential form ; and second, by destructive metabolism or katabo- 
lism, in which the body material is broken down to release energy, 
and in which, as a result, there is the production of work and a 
passing off of waste products. This text is concerned, by and large, 
with the various phases of metabolism which will be considered in 
greater detail later. 


Some Signs of Manifestation of Life 
One sign of life is the release of cnersy, which is a rosult of respir-i- 
tion. It occurs in all living things, ho it a tree, a frog, or a man 
when oxygen is taken into the body, whore it combines with oxidiz- 
able materials to release energy. The by-products are carbon dioxide 
and water, which are given off by the organism. 

Living things are sensitive to and respond to various stimuli in 
their environment. The plant in the window, the earthworm in 
the ground, the fish in the water, and the bird in the tree are all 
sensitive to and respond in different ways to the stimulus of light. 
Temperature, chemical substances, gravity, electricity, radiations, 
and mechanical factors, all are stimuli which affect living things in 
different ways. It is this characteristic of li\ing things that we call 
irritability or sensitivity. 

One direct outcome of the ability of living things to respond to 
stimuli in their environment is their adaptiveness. Thus li\ing 
organisms ha\e the capacity to adjust themseh-es to changes in con- 
ditions. Some plants throw out new roots, or suckers, or trailing 
stems, by means of which they can get a foothold in slightly different 
environments from those in which they are accustomed to live. 
Certain low forms of plants have even become adapted to li\e in the 
hot springs as those in the Yellowstone National Park, in a habitat 
many degrees warmer than that of their near relatives found in 
adjoining pools. Fishes, and certain small crustaceans, may similarly 
adapt themselves to life in water containing a high concentration of 
salts. This power of adaptation is a quality of the organism as a 
whole, and results in adjustment between the external environment 
and the internal body material. 

The Production and Use of Enzymes by Living Things 

In recent years a good deal of work has been done by physiologists 
to see how the cell is able to perform the cycl(\ in which material 
is taken into the organism as food or is made into food as in the case 
of green plants. Food is changed into a j^oluble form so that it may 
pass through the delicate living membrane of every coll. Meantime 
each cell is using oxygen, which also has to be taken in through the 
cell membrane, while wastes are given off by the same road. Physi- 
ologists seem agreed that those living processes, called digestion, 
absorption, respiration, and excretion, are made possible by the 



presence of substances called enzymes, which act as catalyzing agents, 
thus hastening by their presence the performance of such functions. 
(See pages 279-280.) Enzymes are manufactured in certain cells 
and it is believed that every cell, even an egg cell, contains enzymes 
which are capable of digesting food substances, as well as those which 
aid in oxidation within the cell. 

In plants, enzymes seem to be made in almost any cell that is active 
and these enzymes usually have a reversible action. For example, 
certain insoluble foods may be broken down or hydrolyzed in the 
cells of the leaf, so that they are soluble, then they will pass in that 
condition to the stem, the roots, or the fruit, where a reverse action 
takes place and the food is stored in an insoluble condition. In 
animals, the hydrolyzing enzymes which make digestion possible are 
usually formed by groups of cells forming glands. 

Structure of Protoplasm 

This living material, known as protoplasm, has been called by the 
biologist Huxley "the physical basis of life." It is this stuff that is 

always present in things 
that are ahve. In our 
present state of knowl- 
edge we may liken it to 
the albumen or white 
of egg, a nearly colorless 
and translucent sub- 
stance, like soft jelly. 
This substance seen un- 
der the compound microscope has many granules floating in it. 
It is more or less elastic, although in some cases it will flow like 
a dense liquid. Seen under a high magnification it may be almost 
homogeneous in structure or may appear foamy or spongelike, or 
even fibrillar in appearance. A study of living cells shows that it 
is obviously quite different in structure at different times and in 
different animals and plants. 





Three states of protoplasm. 2 and 3 have much 
higher magnification than 1. 

Protoplasm and the Cell 

Although cells were first described in 1665 by Robert Hooke, it 
was not until the nineteenth century that the cell theory came into 
the spotlight. The knowledge that all organisms, plant and animal, 
are composed of fimdamentally identical protoplasmic units, or cells, 



forms ono of the most important corner stones in th.. roun.JMli.,,, of 

Wiiile plant and animal colls possess some rather striking cJifT.T- 
ences m organization, they ar(> fnndamentally similar. Prac-tic-iUy 
every cell that is microscopically visible possesses several difTerent 
kmds of structures located within its borders. Some of these struc- 
tures are alive, some lifeless. In the first group 
may be placed the -plastiih of plant cells, the 
mitochondria or chrondrio somes, some of which 
probably give rise to plastids, fibers of various 
kinds, the Golgi bodies and the centrosomes, the 
latter of importance in animal cell division. In 
the second group may be placed such inclusions 
as yolk, or other food substances, fatty droplets, 
granules of pigment or of secretions (as in gland 
cells), and crystals of various kinds, such as 
calcium oxalate in plant cells. To this list may 
also be added vacuoles, which in plant cells often 
occupy the major space within the cell membrane. 
All of these structures are confined to the cyto- 
plastn or part of the protoplasm outside the 
nucleus. In Elodea, the cells present a green t^. 
appearance, due to the presence of many tmy pi.yii cell of a K-af; 
ovoid bodies, the chloroplasts, which are plastids <"• ihl()ro[)last ; n, nu- 
containing chlorophyll. Careful obser^•ation of 
a single cell shows that the chloroplasts move 
slowly down one side of the cell, across one end, and uj) the other side, 
keeping rather close to the outer edge of the cell during the process. 
This is due to the movement of the cytoplasm. In the cells of the 
hairlike stamen of Tradescantia, the movement of the cytopla.'^m is 
also evident. Here it can be seen actively streaming in currents 
within the cell, carrying along within it tiny crystals of inorganic 
origin, as well as colorless plastids and granules. The latter term is 
usually applied to inert materials, such as granules of stored food in the 
form of starch grains (in plants), fat or yolk granules, or pigment 
granules which frequently occur scattered througiiout |)r()toi)lasm. 
Between the strands of (ytoplasm are spaces or vaeuolrs filled 
with a watery fluid, called cell sap. In young jilant cells, the 
vacuoles are small and the cytoplasm occupies the greater part 
of the cell, but in mature plant cells the cytoplasm is found 

{'k'us ; r 
u\ cell wal 

\ a<n(il(' 



Xell membrane 





Diagram of a typical animal cell. 

to the outer part of the cell, while the vacuoles form large sap cavities 
within the cell. Although Golgi bodies appear much less stable and 
more changeable in form than plastids, they are found in many kinds 
of plant and animal cells. Fibrils of various kinds, such as those 

seen in a muscle cell, are frequently 
found. In plants the cell wall, a 
delicate but rigid, secreted cellulose 
covering, is lined with a delicate 
living membrane which separates 
the living stuff from the cell wall. 
At one point can be found a slightly 
denser jellylike part of the proto- 
plasm called the nucleus. Both 
the vacuoles and the nucleus are 
separated from the cytoplasm by 
delicate membranes. In many but 
not all nuclei, dense, dark-staining 
nucleoli appear. While their func- 
tion is not clearly understood, they 
generally break up and disappear 
during cell division. The nucleus proper is a vital, definite part of 
every living cell and is of great importance in cell division which 
must take place if a many-celled organism is to grow in size, for 
growth takes place by an increase in the number of cells, not in the 
size of the cells. The nucleus is filled with nuclear sap, in which 
is found a network of linin fibers. On these fibers are scattered 
numerous granules of chromatin. This material, which as we will 
see later forms chromosomes, is of the greatest importance, as through 
it plants and animals are able to pass on to successive generations 
their inheritable qualities. 

Chemical Organization of Living Matter 

A dozen or more of the ninety-odd elements recognized by the 
chemist are found in living protoplasm, — carbon, hydrogen, oxy- 
gen, and nitrogen comprising the greatest bulk. These elements also 
form the basis of our so-called organic foodstuffs, which are called 
proteins, carbohydrates, and fats. The two latter groups of sub- 
stances are made up of carbon, hydrogen, and oxygen, while the pro- 
teins have the element nitrogen added to their constitution, along 
with sulphur, phosphorus and sometimes iron. In a simple carbo- 


hydrate, such as ghicose, for example, the chemist writes a fornuihi 
representing a molecule of the substance. In such a simple nujlecule 
the atoms of hydrogen and oxygen are usually united in the same 
proportion as in water and the empirical formula is written CcHi-Oa. 
This water proportion (H,.0) is maintained in other more complex 
carbohydrates such as starch, but here the chemist writes an x after 
the empirical formula (CeHioOa)^. This means that the molecular 
formula is not exactly known but in the case of starch the x should 
probably be about 200, which makes the molecule very much larger 
than that of the simple sugar. The simple sugars with their small 
molecules are ea.sily soluble in water, while the complex molecule of 
the starch is not .so soluble. In fatty substances, oxygen is present in 
a much smaller proportion than in the carbohydrates. An example 
might be oleic acid, one of the components of butter fat, (Ci8H,3402). 
Proteins have still more complex molecules. In the first place tliey 
are built up of simpler substances, called amino acids, and in some 
cases other radicals are added to them. For example, in the cell 
nuclei the protein is combined with nucleic acid, which has the aston- 
ishingly complex formula C38H49029Xi5P4, which really means very 
little except to the student of chemistry. 

Protoplasm a Complex Mixture 

Living stuff, having the same elements as the complex foodstuffs 
for a ba.sis, is even more intricate. No chemical compounds in nature 
are quite as complicated in composition, for protoplasm not only 
is made up of the foodstuffs but it also consists largely of water. 
One estimate by weight gives 80 per cent water, 15 per cent proteins, 
3 per cent fats, 1 per cent carbohydrates and other organic substances, 
and 1 per cent inorganic salts. It has been determined that carbon, 
nitrogen, hydrogen, oxygen, and phosphorus are alwaj's present in 
protoplasm and are called the primary elements. Magnesium, pota.^- 
sium, iron, and sulphur appear equally nece.s.sary for life. Sodium 
and chlorine are always found in animal but only infrequently in jilant 
tissues, and calcium appears necessary for life in the higher forms. 
Other elements, bromine, fluorine, iodine, silicon, boron, manganese, 
and even copper, zinc, and aluminum, are found in some organisms. 

While some of these elements are solids and others gases, none of 
them, except oxygen, typically occurs to any marked extent free 
in the organism. Nor are th(>y found free in the foods or waste 
products, but rather as various kinds of chemical compounds which 



may be further subdivided into inorganic and organic compounds. 
The former comprise most of the non-Hving compounds such as soil 
and rocks and their decomposition products. However, in proto- 
plasm, inorganic compounds are usually present as water, salts, or 
gases. Water is important not only because it comprises 70-98 per 
cent of protoplasm by weight, but also because it dissolves so many 
different substances. Furthermore, water is an important factor in 
promoting the dissociation of many salts into their constituent ions. 
The inorganic salts which occur in marine organi.sms, for example, 
are usually those commonly found in sea water. Some, such as ni- 
trates and nitrites, occur chiefly in plants, while compounds con- 
taining sodium and chlorides are characteristic of animal tissues. 
Only three gases are found in varying amounts in the living cell, — 
free oxygen, carbon dioxide, and ammonia. 

Protoplasm a Colloidal Mixture 

Matter exists in three states, gaseous, liquid, and solid. Frequently 
it passes from one state to another, as when ice melts under the in- 
fluence of heat, turn- 
ing to steam as the 
water boils away. 
That protoplasm at 
different times and 
under different con- 
ditions varies in ap- 
pearance is probably 
due to the fact that 
A B c it is a colloid and as 

Colloidal constitutions. The continuous phase in a such can change from 
being fluid; in a jell (C) solid; while in the "sol " or liouid tO 

sol (A 

intermediate phase (B) the solid forms a net through 

which the fluid is continuous. 

a "gel," or solid state 
and then, under cer- 
tain conditions, back again. The scientist examines protoplasm 
under the ultramicroscope and finds tiny dancing particles which 
are invisible under the ordinary illumination of the microscopic field 
(Brownian movement). This condition is known as a dispersion, 
the dispersed particles being carried in the dispersion medium, 
in this case water. A fog composed of tiny droplets of water is 
an example of dispersion in nature. If the particles in a disper- 
sion are small, the substance is called a crystalloid, when large it 



is called a colloid. Xow these terms are not api)lie(l t(j fixed sub- 
stances but to states of matt(M-. Gelatin passes from a li{|uid to ii 
solid state on being heated or cooled. A study of the diap;ram shows 
how this might be possible. In the left-hand diagram the solid i)ar- 
ticles are floating freely in the fluid of the medium ; in the middle 
diagram the solid portion is becoming a loose mesh; while in the 
right-hand diagram the mesh has become a solid mass, including the 
liquid within it . The protoplasm within the cells of plants and animals 
probably behaves in a similar manner, under some conditions assum- 
ing the "sol," and at others the "gel" state. Remembering that 
protoplasm is not a single protein substance, but rather a mi.xture 
of proteins, fats, carbohydrates, and sometimes even other sub- 
stances, it is clear why there are many slightly different protoplasms 
depending on the part of the animal or plant examined. This fact 
may help us to see why the living matter of a muscle, the blood, or 
the brain differs visibly in structure. For one thing, the water con- 
tent differs greatly. Living bone is said to be 25 per cent water, 
muscles about 75 per cent, the jellyfish almost 99 per cent, and some 
fruits as high as 98 per cent water. 


We have spoken of the work of the enzymes in making food sub- 
stances soluble. Let us now see why .'^olul)ility is necessary for the 
life processes of cells. The physical phe- 
nomenon of diffusion is easily demonstrated 
by the slow spread of red ink when a droj) 
is put into a glass of water. Brownian 
movement of dancing particles visible under 
the high power of the microscope is a mani- 
festation of molecular kinetic energy caused 
by the water molecules bombarding these 
particles. It is a similar movement of 
molecules that occurs when diffusion takes 
place. Molecules of any substance are 
always in motion. If this substance is 
soluble (the solute) in another substance 
(the solvent), there is always a tendency 
for these molecules to move from the 
place of their greatest concentration to i^laces where they are not 
so highly concentrated, until an equilibrium is reached and there 

Lonfjit udiiial sect ion 
through a tumbler of watt-r 
containing soluble crystal. 
sliowiiif; by arrows tlic direc- 
tion of (lillusion. ami by 
(lotted circles the lines of 
equal concentration. 



are just as many molecules of the solute in one part of the solvent 
as in another. In the case of the diffusion of red ink in water, 
the eosin (which is the coloring material used) was more concen- 
trated in the drop than in the water, so the molecules of eosin began 
moving away from this place of high concentration until they were 
equally dispersed throughout the water. As a general rule we may 
say that, if other conditions are equal, the diffusion rate between 
two points is proportional to the differences in concentration of the 
substances at these two points. One thing which affects the diffusion 
rate is the nature of the medium, w^hether it be a gel, emulsion, or 
some sort of semisolid (porous). Gelatin, for example, which is a 
gel, offers no effective resistance to the diffusion of molecules of a 
crystalloid nature through its meshes, but, upon the other hand, 
this network may serve to block effectively the passage of colloidal 

Suppose a membrane were stretched crosswise in a jar where 
diffusion was taking place. Could the molecules of the diffusing 
substance pass through the membrane? This depends on whether 
the membrane is permeable to the diffusing substance. In some 
membranes the ultramicroscopic "pores" are believed to be quite 
large, thus letting through molecules of larger sizes, while in other 
membranes the "pores" through which substances can diffuse are 

very small. Other substances 
penetrate in proportion to their 
lipoid solubility. Thus some 
membranes allow certain sub- 
stances to pass through, while 
they keep out others. Such mem- 
branes are said to be selectively 
'permeable. An ordinary parch- 
ment membrane will allow the 



" . • - •'•. ^«.-- ■',■ ■' i'ly. l,v-i .'•:~-r 





e p "'"-"-^f 


Diagram of an imaginary section . i •. t-> - -u 

through the cell wall and protoplast to eosm to pass through it. But the 

show a, outer water ; iv, cell wall ; c, ecto- cell membrane does not act in the 

plast or cytoplasmic membrane next ^^^^ manner, as it is a vldsma 
to the cell wall; p, general cytoplasm; i i j.- i 

/. tonoplast or inner cytoplasmic mem- membrane, and selectively per- 

brane next to the water, thus forming meable. 

a continuous pathway which carries ^j^^ plasma membranes sur- 
solutes irom (a) to (?)) ; i\ vacuole. ,. ,. . , ,. , 

rounding living cells are believed 

to be colloidal in nature, made of a combination of fatty and protein 

substances. Careful experiments have demonstrated them to be 



selectively permeable. Most living cells allow oxygen and carhon 
dioxide to pass freely through their mcmijranes, while diss(jlved 
sugars and digested proteins in the form of amino acids 
through more slowly. Water of course passes through, acting as a 
vehicle for other substances. Such membranes are impernicablc to 
certain salts and not to others. The permeability of living cells 
to dissolved substances differs with the cell, and naturally with tlu; 
organism. Salt- and fresh-water fishes are examples of types, the 
cells of whose gills exhibit different permeabilities. Dead cell mem- 
branes are usually permeable to crystalloid solutes, while living 
cell membranes permit but few salts to enter. In general, cells are 
not permeable to colloids, because of the large size of the particles 
constituting the colloid. 

Osmosis and Its Significance to Living Cells 

We have already seen that if a membrane is sc^lectiveiy permeable, 
then some substances, such as water or certain solutes, will pass 
through readily, but other 
solutes may not. because 
their molecules are too large 
to pound their way through 
the ultramicroscopic "pores" 
of the membrane. The 
process by which substances 
diffuse through membranes 
is known as osmosis. It is 
of the greatest importance 
to living cells, as it is by this 
means that dissolved gases, 
such as oxygen, and dis- 
solved food substances get 
into the cell, as well as the 
process by which waste ma- 

- sugar 


lJ» selectively 




^ • •wat«r'mo\eculti =^ °'TS^^ 


=1. "WCCt&V 



Diagram to explain osmotic pressure. Sii>:ar 
solution is of equal density in ea<h tube. 
Explain rise of solution in left hand tube. 

terials pass out. Perhaps a further word of oxplana .on ,s u, orcler. 
Other things being equal, if two soh.tions «' '''«""": ™"''™'7,;" 
are separated by a permeable membrane, the chffus.on ^v,ll ^fll , 
in the direetion of the greater to the lesser ™';'-"- "' ; V^ 
it a sugar solution be .separated by a pern,eabIo men,b,a,.o 
another more dilute sugar solution, diffusion -l'/^' ;;■'," 
the more coneentrated. If, however, we separate «at,. from .. 

H. W. H. — 10 



sugar solution by a selectively permeable membrane, the water mole- 
cules tend to pass through the membrane (since it is permeable to 
water) from the water, to the sugar solution where the water is in 
less concentration. Actually it is a question of the water molecules 
of the solvent reaching an equilibrium. 

Osmotic pressure, in living cells, is one of the factors that accounts 
for the rise of water in roots and up the stems of plants. Its effects 
can easily be demonstrated experimentally in the laboratory by plac- 
ing, for example, living cells of Spirogyra in a 10 per cent solution of 
salt and water. The water from within the cell (where it is in greater 
concentration) passes out through the cell membrane to enter the salt 
solution (where water is in less concentration than in the cell). The 
result is that the cell body shrinks away from the cell w^all and the 
shrunken cell is said to be plasmolyzed. A solution which contains 

a greater number of mole- 
cules of the substance in 
solution (solute) per vol- 
ume than the interior of 
the cell is said to be hy- 
perosmotic; if it has less 
concentration than the in- 
terior of the cell it is 
hyposmotic; and if it has 
the same number of solute 
molecules per unit volume 
as the interior of the cell 
the solution is isosmotic to 
the cell. 

When a cell is placed in 
a hyposmotic solution it 
will tend to swell up, be- 
cause water is diffusing 
more rapidly inward, and so, unless the cell is surrounded by 
heavy walls as in the case of plants, the cell will tend to burst. 
When this happens it is called cytolysis. This may be demon- 
strated when human red blood corpuscles are placed in distilled 
water. It is evident, therefore, that osmotic pressure differs greatly 
in the cells of different organisms, possibly depending on whether 
they live in fresh or salt water and the consequent concentration of 
salts present. As a matter of fact, fresh- water organisms live in a 

pelliole — 

.cytoplasm. . 



Plasmolysis in a plant and an animal cell. 
Note how the cytoplasm has shrunk away from 
the wall in the case of the plant cell and the 
pellicle in the case of Paramecium. Why has 
this occurred ? 


hyposmotic solution. In plants, the cell walls prevent the cells from 
swelling up, while in animals there are special ways of ridding the 
body cells of excess water. 


Calkins, G. N., Biology of the Protozoa, Lea and Febiger, 192(). Cli. L 

A full and scientific approach to the cell. 
Plunkett, C. R., Outlines of Modern Biology, Henry Holt & Co., 1930. Chs. 

I and IV. 

A chemical and physical approach to the study of protoplasm. 
Singer, C. J., The Story of Limng Things, Harper & Bros., 193 L Chs. IV 

and IX. 

An interesting history of biology. Chapters IV and IX deal with the 

historical approach to the cell theory. 
Wilson, E. B., The Cell in Development and Heredity, The Macmillan Co., 


A classic authority on the ceU. 



Preview. Living things composed of cells • Plant and animal cells 
differ in size, shape, and structure • Why cells divide • How plant cells 
divide • How animal cells divide • Tissues • The tissues in plants; the 
meristematic tissues ; the protective tissues ; the fundamental tissues ; the 
conducting tissues ; the tissues in animals ; the epithelial tissues ; the sup- 
porting tissues ; the muscular tissues ; circulatory tissue ; the nervous tissues ; 
reproductive tissues • Why are living organisms so called? • Suggested 


One characteristic of living things is that they are organized into 
tiny units of Uving matter which have been called, rather inaptly, 
"cells," because an Englishman, Robert Hooke, as early as 1665, 
described the construction of cork which he saw under a lens as 
"little boxes or cells distinguished from one another." He cut 
cross sections with a penknife and saw that they were "all cellular or 
porous in the manner of a honeycomb, but not so regular." What 
Hooke saw was the woody walls enclosing spaces which in younger 
plants would be filled with living matter. 

From a comparison with the simplest organisms, it is evident that 
the more complex forms are built up of cells, and that, although each 
cell can function as an organic whole, far more efficient results are 
obtained when groups of cells organized into tissues do the work. The 
consideration of groups of cells, according to their structure and func- 
tion, constitutes in itself a major chapter in biological study, called 
Histology. The study of individual cells, which make up the sub- 
ject matter of Cytology, is absolutely indispensable to a proper 
understanding of the organism as a whole. 

The problems for reading and further study are so numerous that 
we might spend the major part of our available time in discussing 
them. Why and how do cells divide? What are the differences 
between plant and animal cells? What are the reasons for having 
tissues and organs? How did many-celled organisms come into 
existence, and why? The pages which follow will enable the student 
to make at least a start on some of these interesting questions. 



Living Things Composed of Cells 

A very small proportion of living plants on the earth :uc uniccliui-ir 
but accordnig to Hegncr, tiio number of species of protozoa or single- 
celled ammals must be nearly, if not quite, as great as all the other 
species of animals put together. He bases his estimate upon the fa<-t 
that practically every kind of animal has its own species of parasitic 
protozoa living upon or within it. Nevertheless the mctazoa, as the 
many-celled animals are called, make up most of the living animals 
that we know about on earth today, just as the many-celled plants 
make up the visible and familiar plant life. 

Just how the many-celled forms of life evolved from the unicellular 
forms is a matter of conjecture. Two theories of origin in animals 
have arisen, one of which, the colonial theory, postulates many-celled 
organisms evolving as colonies of cells, which hold together after fi.ssion 
to form plants or animals, instead of separating into individual isf>- 
lated cells. As these cell masses evohcd, they became more and 
more complex, different systems of organs appearing in more highly 
organized forms. In the animal series shown on ])age 146, this 
theory seems to be pretty well substantiated. But another theory, 
the organismal theory, considers the living thing as a whole, being 
divided into units of structure in the many-celled organism. Accord- 
ing to such a theory unicellular organisms would become first much 
chfferentiated within their own bodies, as is .seen in many of the 
protozoa. These theories need not concern us further at present. 
Both have many facts to support them, substantiatetl by the devel- 
opment and structure of various types of organisms. 

Plant and Animal Cells Differ in Size, Shape, and Structure 

An examination of the figure on page 140, will sliow that cells 
are far from uniform in size and shape. They differ in size from the 
smallest bacteria which can just be distinguished with an ultra-micro- 
scope that magnifies 3000 diameters, to cells that can be seen with the 
naked eye. The egg-cell of the chick, for example, includes the con- 
spicuous yolk, while certain cells in the human spinal cord, altluiu^h 
microscopic in size, may have prolongations reaching down irito the 
muscles of the fingers or toes. Cells are not of n(>cessity lar-jcr in 
large animals or plants, some of our largest cells being found li\ing 
isolated and alone. But under normal conditions a cell of a given 
size and shape always reproduces the same kind of cell as itself. 



idL "bouiillus/ 

Anthro^ bacillus^ 

As to shapes, their name is legion. A typical cell might be thought 
of as a spherical or ovoid body, but we find them cubical, flat, thread- 
like, spindle-shaped, columnar, or irregular in outline. They are often 

modified by being com- 
pressed by other cells, 
but frequently if given 
opportunity will resume 
their original form when 
released from pressure. 

Structural differences 
exist between plant and 
animal cells, the chief of 
which is the cellulose wall, 
characteristic of plants, 
which gives such cells 
the rigidity and yet the 
flexibility found in woody 
stems. Other physiolog- 
ical differences will be 
discussed in the following 

red. corynxsc^s^ 
.of f nog. 


r<Ed corp 
of )inoa7 



Spex-m ofTnarj. 

livei- cell 

Comparative size of cells. The anthrax bacillus 
shown is among the largest of the bacteria, while 
the human liver cell is not large as cells go. 
(After Wells, Huxley, & Wells.) 

Why Cells Divide 

Every cell has its limits 
of size and when that 
size is reached, if food is 
sufficient and conditions 
favorable, it will divide. In both plant and animal cells, the mech- 
anism and the. end results reached by cell division are similar, in 
that the chromatin from within the nucleus is redistributed so that 
the daughter cells have approximately the same amount of chromatin 
and eventually the same size as the parent cell from which they 
came. Cell division is a universal phenomenon and seems to be a 
part of the normal life of cells. Theories advanced to account for 
cell division are (1) colloidal changes in the protoplasm of which 
they are composed, (2) electrical changes within the cell, (3) oxidative 
changes within the cell, and (4) changes in surface tension. The 
latter can be experimentally proven by treating unfertilized eggs with 
certain chemicals which cause a change in surface tension and initiate 
subsequent cell division. 

How Plant Cells Divide 


Both plant and animal cells are said to divide by a proc-ss c,f n-ll 
division called mitosis. In plants, the resting cell has a nuclcM.. wl,i,-l. 
contains a network of linin fibers, on the strands of which are f,.,n„l 
irregular chromatin granules. When the cell is activated to divide 
these granules assume the form of a thickened, irregularly coiled thread' 
called a spireme. This thread splits lengthwise into two thr(.-„is 
which remam so close together that for some time they appear as 
one, finally splitting crosswise into a number of chromo.sonus that 

cell ^ 

prophets e. metapWe 

anaphase telophase 

Mitosis in plant cells. Read the text and ('\|)laiii the (li;i;:r;iiii 



are constant in number in all cells of a given species. While this proc- 
ess is going on there has appeared in the cytoplasm on oj)j")osite sides 
of the nucleus two caplike masses of delicate fibers, which later will 
give rise to the so-called spindle fibers. Now the iiuchar membrane 
disappears and the fibers grow into the center of the nucleus, where 
some become attached to the chromosomes while others join with 
fibers from the opposite side or pole. This series of changes is 
known as the prophase. These two cone-shapetl of fibers 
form the spindle, while the split chromo.somes arrange themselves 


in a plane in the middle, or equator, of the spindle, this being 
known as the meta phase. Next the half or split chromosomes 
appear to be pulled apart by the spindle fibers so that an equal 
number move toward each pole, where they come to rest. These 
changes are called the anaphase. 

Here the spindle fibers which extended from one pole to the 
other begin to thicken at the equator. The swellings grow larger, 
fuse, and spread out to form a delicate plate, which eventually extends 
clear across the mother cell. This cell plate is in the nature of a 
plasma membrane which splits into two, forming the new cell wall 
between the two new cells. The fibers of the spindle now disappear 
and cell division is completed. Meantime the recently split chromo- 
somes lose their identity and again take on the netlike appearance as 
in the original resting cell. The last series of changes comprises 
the telophase. 

How Animal Cells Divide 

The resting animal cell undergoes a similar process in division. 
However, in the animal cell a new structure is found, called the 
centrosphere, which is a small body lying in the cytoplasm near the 
nucleus. A central granule, called the centrosome or centriole, is 
found within this centrosphere. The centriole usually divides to 
form two of these granules at the beginning of mitosis. The initial 
stages of cell division, collectively called the prophase, occur when the 
particles of chromatin scattered throughout the nucleus take the form 
of the spireme or tangled thread. This thread thickens and shortens 
and then breaks up into the individual chromosomes. The number 
of chromosomes for the body cells of the individual of a species is always 
constant. Among plants, for example, in the pea there are always 
14, in the onion 16, and in the lily 24 ; while examples taken at 
random among animals show 4 for certain roundworms, 8 for the fruit 
fly, Drosophila, of which you will hear more later, 32 in one of the 
common earthworms, 200 in one of the crayfishes, 24 Mn a common 
locust, 24 in one of the frogs, and 48 in man. 

During the formation of the spireme the threads of the future spindle 
are growing out from radiations, called asters, which appear around the 
centrioles. (See figure on page 143.) As the process continues the two 

' This is not quite exact, for it has been found that in some animals at the time when the chromo- 
somes are reduced in number in the process of maturation (see page 429) , there is an even number 
in the female sex cells but an odd number in the male sex cells or vice versa. 


centrioles move farther apart, the spindle fibers elongate, the n.icl.-ir 
membrane disappears, some spindle fibers api)ear to attach to tlic 
chromosomes, and gradually the longitudinally-split chromosomes 
collect at the equator of the spindle. The next step in mitosis, known 
as the metaphase, is the arrangement of the chromosc^mes. with each 
split body on opposite sides of the equator of the spindle. Then the 
two sets of chromosomes begin to move toward the opposite poles of 
the spindle, the fibers which are attached to them getting shorter tunl 
shorter. At this time comes the first external appearance of cell 

- (tentrosome 
^ chromatin 


resting cell 

-spindle thread 


linVm - pi'ophase 

spireme sViortens 
anoL thickens 

and of prophase 
nuclecLr membi*ane 



end of anaphase 

nuclear membranes 

ctoir^ter Cells 

restivxT stage. 

Mitosis in animal cells. Compare this diagram with that on page i 11. 

division, a slight constriction appearing in the cell body. The 
constriction in the cell becomes more evident and, as the i)rocess 
continues, the chromosomes become grouped so as to form the new- 
nuclei of the two daughter cells. These progressive changes are 
collectively known as the anaphase. In the final stage, or Idophasc, 
the two sets of chromosomes gradually lo.-;e their individuality and 
become Httle masses of chromatin grouped on linin fibers in the new 
nuclei around which a nuclear membrane is formed. .Meantime the 
constriction in the cell has gone far enougii to form two daughter cells, 
the new separating partition appearing along the line nf the (-(luator 


of the spindle. The centriole in many daughter cells divides imme- 
diately into two, although in some cells it remains as a single body 
until a new mitosis begins. 


Cells form aggregates called tissues, examples of which may be seen 
in the woody cells making up the greater part of the stem of a plant ; 
the elongated cells in this same stem which form the conducting 
tissue; the flat protective cells covering the outside of the leaf, called 
collectively the epidermis; and the large columnar cells filled with 
green chloroplasts that form the parenchyma layer directly under the 
epidermal cells. In our own body, we find numerous examples of famil- 
iar tissues set apart for doing some particular work, such as the epithe- 
lial, or protective, tissues ; the connective tissues, which serve to bind 
the various groups of cells together ; the muscular tissues, of several 
kinds ; the supporting tissue cells, which help to build the bones ; 
glandular tissues ; the nervous tissues of several kinds ; and the blood, 
which, though fluid, yet contains cells, and is classed as a circulating 

The Tissues in Plants 

It is a difficult matter to make a classification of tissues that will 
fit all plants and yet be simple enough to use at this stage of our 
biological knowledge. But the following will give us a general survey 
which can later be expanded by the student of botany. 

The Meristematic Tissues. These cells in general are small, 
thin walled, and rich in protoplasm. They are found in the rapidly 
growing parts of plants, the buds, the tips of the roots, and in growing 
layers. They represent the primitive and embryonic tissues. 

The Protective Tissues. Such are the epidermal cells covering 
leaves. These are often waterproofed with a waxy material called 
cutin. Such layers are found on the outside of the stem, root, and 
even the fruit, forming a protective covering. In the stem and the 
root, the epidermis is often replaced by a layer of corky cells, while on 
leaves, stems, and flowers the epidermal cells frequently develop 
hairs or scales, which sometimes secrete sticky substances. 

The Fundamental Tissues. These groups of cells form the great 
mass of plant tissue, such as the soft green parts of the leaf, the pith 
or cortex of plant stems, the soft parts of flowers and fruits. These 
cells differ greatly in size and shape in different parts of the plant, 



but in general they are alive and act as storage cells. Some of the 
parenchyma cells, called collectively collnichynw, become f liickcncd 
at the corners, as seen in a cross .section, and serve as strengthcnii.n 

'menStQWatiC P'^'^^'^.X"'-^ colknch^roa sderenchv^xt 
tissues "tancCameatial tissues 


■ Sedition 


"plant- -, I", y, ... 

... i:icxir^ :jcylem M '.phloem 

prouective t-issues <:tondxxctin<^ tissues 

Types of plant tissue cells. 

units in the outer part of the stem. The walls of the other funda- 
mental tissue cells become much thickened and are called sclcrcrichyma 
cells, which may become fibrous, helping to .support the stem, while 
others form stone cells making up the covering of nuts antl other 
hardened parts. 

The Conducting Tissues. In the liigher plants, woody bundles 
of elongated cells act as tubes for the conduction of water and food 
substances. The water-conducting tissues are collectivelj' known 
as the xylem and consist largely of supporting dead cells (trachcids) 
impregnated with a strengthening substance called lignin, and long 
tubular cells (vessels) which have lost their cross walls. Scattered 
amongst them are various other types of cells, including jiarenchynia. 
The tissues which conduct food materials down the stem from the 
leaves, where food is made, are known collectively as the phlonn. 
The characteristic conducting cells of the j^hloem are known as sifir 
tubes, which have perforations in the end or .side walls known as the 
sieve plates. Long threads of cytoplasm through these holes. 



connecting cell with cell and making a pathway for the food sub- 
stances. Small companion cells are attached to the sieve tubes. 
The phloem is also provided with parenchyma and fibrous cells, 
which give strength to the tubular bundle. 

The Tissues in Animals 

Although the histologist makes a much more detailed classification 
of tissues, a convenient grouping for animals is the following : 

The Epithelial Tissues. Not only do these cells form the outer 
layer of the animal body, but they also are responsible for the forma- 
tion of such protective body structures as the calcareous shells of 

reticular odiposs 



^ , fibrous Z,^ ..^ -- ^ ° ^' w^- -' .^^^l? 

stratifM ^.,^.:.:-:,^,:mm\ Striated: 





vv ;.:>: -v: :. ;;,U^ loom. 

Yiyoline Cartilage 



© <!orpuscles 




Types of animal tissue cells. Into how many groups may they be classified ? 

clams and oysters, the chitinous covering of the insect, or the outer 
covering of the crayfish. These tissues line all body surfaces as 
well as the digestive tract and other inpocketings of the outer body 
covering. They are of the utmost importance because they also 
form the glands of the body, structures which secrete, for example. 


digestive enzymes or the waste products of metabolism, such as 
perspiration. They also form a largo portion of many of the sense 
organs of the body. In shape, the cells of epithelial tissues as they 
lie side by side may be fiat, cuboidal, columnar, or even ovoid. 

The Supporting Tissues. These tissues .serve to bind together or 
support the various parts of the body. They include bone, cartilage, 
and connective tissue, and they differ from other ti.ssues in that it is 
the material formed by the cells, rather than the cells themselves, that 
is of functional importance. In bone or cartilage, for example, the 
supporting portion or matrix is produced by the cytoplasm of cells 
and surrounds it. Fat cells are connective ti.ssue cells in which the 
body of the cell becomes a storehouse for a drop of fat, the living part 
of the cell being much reduced. Pigment cells are branched irregular 
structures of a somewhat similar nature. Most characteristic of 
true connective tissues are the white non-elastic fibers that make a 
network in certain parts of the body, or form the glistening cords or 
tendons \\\nQh. connect bones with muscle, or ligaments, which connect 
bones with bones. Other forms of connecti\e ti.ssue that might be 
mentioned are the areolar, which forms an elastic padding underneath 
the skin; and the yellow elastic fibers found in the air tubes of the 
lungs and the walls of arteries. 

The Muscular Tissues. Motion of certain cells is produced by 
ameboid movement, or by the lashing of tiny threads of protoplasm, 
that is, flagella or cilia. But in higher animals movement is brought 
about by the muscle cells in which the propert}' of contractility is 
greatly developed. In higher animals, muscles are groups of highly 
specialized cells bound together by connective tissues. There are 
three kinds of muscle cells, namely, smooth, striated, and cardiac. 
Smooth muscle cells are long with an outer contractile fibrillar layer 
surrounding a central area of semifluid protoplasm containing a 
nucleus. In vertebrate animals, smooth muscle is found i^articularly 
in the walls of the blood vessels and the walls of the digesti\-e tract. 
Striated muscle fibers in higher animals are groups of cells slu)wing n(» 
cell boundaries and held together by connective ti.ssue. They s1k)w 
curious cross striations and on the whole in man are under control (»f 
" the will," hence are called voluntary muscles. A third type o( 
muscle, the cardiac, is striated, but involuntary in action, making 
up the tireless muscles of the heart. 

Circulatory Tissue. Although the blood, lymph, and other 
fluids that serve to transport foods and wastes in the body are <-on- 


stantly in motion, we must classify them as tissues, for they contain 
living cells or corpuscles of various kinds, carried about in a fluid 
matrix or plasma. These tissues are of the utmost importance to 
animals, as it is only by means of them that the living cells of the body 
receive nourishment and oxygen, and get rid of their wastes. 

The Nervous Tissues. Even in its simplest form we have seen 
that protoplasm is sensitive and responds to stimuli. In higher 
animals this sensitivity and conductivity of sensations is taken over 
by the nervous tissues. The unit of structure is the neuron, or nerve 
cell. The elongated fibers from these cells are bound together into 
nerves or conducting ])athways for nerve impulses. All parts of the 
vertebrate body, with the exception of the cartilages and epidermal 
derivatives, are supplied with nervous tissue, which may be said to 
be the master tissue of the body. 

Reproductive Tissues. These cells which, as one author puts 
it, are "within the body though perhaps not of the body," form tis- 
sues, eggs and sperms, that have to do with the futures of all animals. 

Why Are Living Organisms So Called? 

In the preceding pages, we have referred to living things as organ- 
isms. The anatomist calls collections of tissues, which do specific 
kinds of work, organs. The hand is an example of an organ which is a 
collection of tissues. Muscles are attached to the hones by means of 
tendons and bones are joined together by ligaments. The skm, 
composed of several different kinds of tissue cells, is supplied with 
blood and nervous tissues, while the whole organ is interlaced through 
and through with other connective tissues. Living things are made up 
of organs, and we call them organisms. The living world about us, 
plant and animal, is a collection of organisms, some very simple, 
others aggregates of simple cells, still others formed of untold billions 
of differentiated cells, grouped into tissues forming an organism, 
such as an insect, a fish, a tree, or a man. Yet all these different and 
complex entities basically are made of the living stuff called proto- 
plasm. In animals, this grouping of organs which are united in the 
performance of some general function gives us a number of organ- 
systems. There is, for example, the integumentary system, or outer 
body covering ; the supporting system, which forms the body frame ; 
the systems which have to do with the nutrition of the body, the 
digestive, respiratory, circulatory, and excretory systems ; the nervous 
system, which controls the activity of the body ; and the reproduc- 


live system, which has to do witli the contiiiuaiice of Hfc It is on 
the structural development of these systems, ({('velojx'd to ;i greater 
or lesser extent in all of the many-celled animals, that the various 
groups of the metazoa are classified. 


Dahlgren, U., and Kepner, W. A., Textbook of the Principles of Animal 
Histology, The Macmillan Co., 1908. Chs. I, II. and \'. 

Holmau, K. M., and Robbins, W. W., Elements of Jiotany, 2iul cd., John 
Wiley & Sons, Inc., 1928. Ch. III. 

Maximow, A. A., Textbook of Histology, edited by W. Bloom, W. B. Saunders 
Co., 1930. 
Rather technical. Chapters I and II useful. 

Stohr, Philip, A Textbook of Histology, .')th ctl. (arranged by J. L. Bremer), 
P. Blakiston's Son & Co., 193(3. 
Chapters I and II make excellent reading. 

Wilson, E. B., The Cell in Development and Heredity, 3rd cd., The Mac- 
millan Co., 1925. 

The most authoritative text on the cell. Rather advanced, but with 
excellent figures. Chapters I and II especially us(>ful. 





Preview. Some forms found in a drop of fresh water: Ainoba. an 
animal cell ; Euglena ; Paramecium ; Diatoms : Desmids ; Bacteria ■ Func- 
tional differences between plant and animal cell • Suggested readings. 


Over two hundred and sixty years ago, when the Dutchman, Antony 
van Leeuwenhoek, examined what he called "little animals" under 
his homemade microscopes, he made the first real exploration of a 
drop of water ever attempted. His microscopes were simple affairs, 
consisting of a single lens. They had no tube or mirror such as our 
microscopes of today have. When objects were examined they had 
to be brought into position and focus through the use of rather coarse 

Besides being the first person actually to see the capillary circulation 
of the blood (a thing that Harvey knew must be so, but which he wa.s 
unable to prove), van Leeuwenhoek made numerous other llhJ^sio- 
logical and anatomical observations which gave him the title of 
"founder of histology." One thinks of him most often as the first 
man who saw protozoa, unicellular plants, and own bacteria in 
standing water. 

Let us read his own description and judge for ourseh-es a.s to what 
he saw. The following extract is taken from a letter written on 
October 9, 1676, to Henry Oldenburg, First Secretary of the Koyal 
Society of London. It describes the finding of "little animals" in a 
drop of rain water. 

"Of the first sort that I discovered in the said water, I saw, after divers 
observations, that the bodies consisted of 5, 6, 7, or 8 ver>' clear globules, 
but without being able to discern any membrane or skin that held these 
globules together, or in which they were inclosed. When these aninuilcules 
bestirred 'emselves, they sometimes stuck out two little hnrns, which were 
continually moved, after tlie fashion of a horse's ears. The i)art l)etween 

H. W. H. — 11 131 


these little horns was flat, their body else being roundish, save only that it 
ran somewhat to a point at the hind end ; at which pointed end it had a 
tail, near four times as long as the whole body, and looking as thick, when 
viewed through my microscope, as a spider's web. At the end of this tail 
there was a pellet, of the bigness of one of the globules of the body ; and 
this tail I could not perceive to be used by them for their movements in 
very clear water. . . . 

"I also discovered a second sort of animalcules, whose figure was an oval ; 
and I imagined that their head was placed at the pointed end. These were 
a little bit bigger than the animalcules first mentioned. Their belly is flat, 
provided with divers incredibly thin little feet, or little legs, which were 
moved very nimbly, and which I was able to discover only after sundry 
great efforts, and wherewith they brought off incredibly quick motions. 
The upper part of their body was round, and furnished inside with 8, 10, or 
12 globules : otherwise these animalcules were very clear. These little ani- 
mals would change their body into a perfect round, but mostly when they 
came to lie high and dry. Their body was also very yielding : for if they so 
much as brushed against a tiny filament, their body bent in, which bend also 
presently sprang out again ; just as if you stuck your finger into a bladder 
full of water, and then, on removing the finger, the inpitting went away." 

His description of the cause of movement in his little creatures is 
amusing, yet it shows that he saw cilia plainly and estimated their 
size quite clearly. 

"But many of the things we imagine, and the natural objects that we 
inquire into, are very insignificant; and especially so, when we see those 
little living animals whose paws we can distinguish, and estimate that they 
are more than ten thousand times thinner than a hair of our beard ; but I 
see, besides these, other living animalcules which are yet more than ten 
thousand times than a hair of our beard ; but I see, besides, these other 
living animalcules which are yet more than a hundred times less, and on 
which I can make out no paws, though from their structure and the motion 
of their body I am persuaded that they too are furnished with paws withal : 
and if their paws be proportioned to their body, like those of the bigger 
creatures, upon which I can see the paws, then, taking their measure at but 
a hundred times less, it follows that a million of their paws together make 
up but the thickness of a hair of my beard ; while these paws, besides their 
organs for motion, must also be furnished with vessels whereby nourishment 
must pass through them." ' 

Van Leeuwenhoek was made a member of the Royal Society for his 
clear reports of what he saw and at his death he had sent the Society a 

1 Dobell, C, Antony van Leeuwenhoek and his "Little Animah," pp. 118 and 180, Harcourt, 
Brace and Co. By permission of the publishers. 


case containing 26 of his microscopes, a gift which was later lost ( )„,. 
of the few remaniing of the 419 lenses put up at auction after van 
Leeuwenhoek's death was recently examined by an expert who 
reported that the biconcave lens that he inspected "was very good 
indeed" and proved that its maker had attained "a very high degree 
of proficiency in grinding extremely small glasses." 

With the modern microscope of the college laboratory, infinitely 
better work can be done than with this old pioneer. The best of \an 
Leeuwenhoek's lenses are said to have magnified not more than 270 
diameters, while the " high dry " power of the average modern micro- 
scope gives a magnification of about 440 diameters, so that the college 
freshman today has a far better physical equipment than did this 
famous Dutchman. He also has much more. In the years that have 
intervened between the time of van Leeuwenhoek and the present, 
patient observations of minute forms of life ha\-c been made by 
hundreds of scientists whose results may be found in these pages and 
in other books suggested for collateral reading. With this intro- 
duction the student might begin the study of simple organisms in 
some such way as Antony van Leeuwenhoek did, by examining a 
drop of pond water. 

Some Forms Found in a Drop of Fresh Water 

The pages that follow^ will serve to give us a slight acquaintance 
with some of the simplest plant and animal forms that are likely to be 
met in the examination of a drop of pond water or water from a 
laboratory aquarium. Li addition to the unicellular organisms, 
scores of other higher forms are likely to be seen. Countless protozoa, 
including the many tiny species of monads, dart across the field of the 
microscope ; others many times larger, with their highly specialized 
cell parts, as Euplotcs or Stylonychia, may be found browsing on tiny 
plants. Frequently one also encounters threads of the filamentous 
algae, Zygneyna or Sjpirogyra, while debris, consisting of tiny bits of 
wood, sand grains, and the glasslikc cases of diatoms and desmids. 
may abound. 

Many tiny crustaceans, water fleas, and cojx'pods are usually 
present, and in addition one finds the easily recognizable rotifers, 
with their whirling wheels of cilia, their prominent grinding organ 
or mastax, and their slender toelike posterior foot by means of which 
they often become attached to .solid objects. Sometimes a small 
roundworm may be found working its way through the dt^bris. while 


hyaline cap.. 


ptemagsl.m o _, 

many types of insect larvae and pupae may also be seen. This brief 
list includes only a few of the many new acquaintances to be found 
in a drop of water. 

AmebOj an Animal Cell 

Ameba is the classic representative of a single-celled animal which 
illustrates the action of living protoplasm. Found in ooze taken 
from the bottom of small ponds or sluggish streams, it is seen to be 

an irregular and almost 
transparent cell. When 
in motion the protoplasm 
of its body apparently 
flows out into newly 
formed bulging projec- 
tions of the body called 
pseudopodia (Gk. pseu- 
dos, false; pous, foot). 
The cell body consists of 
two substances, an inner, 
more fluid, granular por- 
tion, the endoplasm and 
a more viscous area, the 
ectoplasm, on the outside. 
The whole Ameba is 
surrounded by a deli- 
-foodvacuole cate plasma membrane. 
When the animal moves, 
the protoplasm appears 
to flow into the pseudo- 
podia. According to S. 
O. Mast of the Johns 
Hopkins University, 
when an Ameba is mov- 
ing in a given direction the endoplasm sol pushes out in a pseudopo- 
dium and becomes changed to a gel, the "gel" at the other end of the 
cell becoming a "sol" that moves into the cefl body. This illustrates 
a characteristic of protoplasm mentioned earlier. 

This cell, like others of its kind, has a nucleus containing chromatin. 
Certain vacuoles are present, some of which are filled with a watery 
fluid, others hold food in different states of digestion, while a single 


li.L.-fooct vacuole. 


Ameba proteus. The direction of progress of 
the cell is shown by arrows. What happens to 
the protoplasm in the extreme anterior end during 
movement. (After Mast.) 


vacuole, called the contractile vacuole, rhythmically collects and expels 
fluid. The function of the contractile vacuole may he to eliminate 
wastes from the cell, or it may have a hydrostatic function, that is, 
it may control the amount of water contained in the ccjl. Food 
particles are actually ingested or taken into the cell by the proto- 
plasm which flows around the food, engulfs it, and then surrounds it 
with digestive fluids in a food vacuole. 

A recent series of observations by Mast and Hanliart ' indicate 
that the Ameba selects certain kinds of food, ])referring, for instance, 
Chilomonas to Monas, although both are flagellates of about the 
same size, form, and activity. It was further sliown that Monas 
was not digested in the food vacuoles, while Chilajnonas was, and 
also, some organisms, such as mold spores, certain algae, and other 
flagellates, might be eaten but were not digested. 

The process of constructive and destructive metabolism may take 
place in a single cell. Indigestible waste materials are pa.s.sed out 
any^vhere from the surface 
of the cell body, while 
respiration takes place by 
means of an osmotic ex- 
change of the gases, oxy- 
gen and carbon dioxide, 
through the cell mem- 

As a result of the taking 
of food, the cell gradually 
increases in size and then 
divides by a process known 
as binary fission. Accord- 
ing to a recent study by 
Chalkley and Daniel - the 
division of the nucleus 
shows the typical stages 
of mitotic division, the 

entire process lasting, under normal temperature conditions, about 
half an hour. During the process the Ameba is quiescent and the 

late prophorse. 

mid- anaphase 

Gccriy anaphase 


Mitotic division in the nucleus of Vruflia. i, After 
GliiilklcN ;uni Daniel. ' 

iMast, S. 0., and Hanhart, W. L., " Feedins, Digestion, 
(Leidy)." Phusiol. -Zool.. Vol. 8, lO.'?."). Pp. 2,5,5-272. 

and the 
Pp. 592-619. 

and StarviiliiiM in .■inwrbii pmlrw 

Iv)." Ph„.no}. 'Zool.. Vol 8. \9^r,. Pp. 2,5,5-272. ,i„„,vll 

Chalkley, H. W.. and Daniel. G. E.. "The Relation between the lorn, of he I. v .« r 1 
the Nuclear of Division in Amoeba protcu.. (Leidy). Phmol. looU \o\. rt. !..,«•«. 


pspiidopodia are relatively small. After the nucleus divides, the cell 
body separates into two equal parts, each of which grows into a 
full-sized individual. 


Although Ameba is usually looked upon as the simplest of all animal 
cells, there is another group of organisms containing equally simple 

forms, making up a large 


stigma .^^<a. 

I^lagsllcu- granule 


basal granule 

vacuoles -•'- 

nuole-us — 

Central bcx^ 


striation$ — 


Euglena viridis. Read your text and give 
the functions of each of the structures shown. 
Note that the drawing makes the cell appear 
flat whereas in cross section it is oval. What 
evidence of holophytic nutrition is seen in 
this diagram ? (After Hegner.) 

proportion of the microscopic 
plankton of the ocean and 
bodies of fresh water. This 
grotip, w^hich comprises one 
of the classes of the Phylum 
Protozoa, includes the Masti- . 
gophora, or flagellates, cells 
that move by means of one or 
more long, whiplash threads 
of protoplasm. Certain of 
the Mastigophora bear a close 
relationship to plants, and the 
organism Euglena, selected as 
a representative of the group, 
is often claimed as a plant cell 
by botanists. Euglena may 
be found in shallow and some- 
times temporary freshwater 
ponds, where it often grows 
with such rapidity as to give 
a dull greenish color to the 
water. When unfavorable 
conditions set in, the organism 
settles to the bottom, becomes 
covered with a resistant coat 

or cyst, and is only recalled 
to active life by a recurrence of favorable environmental conditions. 
Some species of Euglena have conspicuous spiral markings on the 
surface of the body, which is roughly ovoid, with a depression at the 
anterior end, called the gullet. A single flagellum has its origin near 
the base of the gullet, in the form of a long axial filament anchored 
in the protoplasm, that gives the filament free movement. By 


means of a rotary movement of the Hagelluni, the cell is pulled forward 
on a spiral course, which is caused partly by the way the flaRellum 
moves and partly by the irregular shape of the cell. At the same 
time a current of water is swept into the gullet, bearing with it par- 
ticles of potential food. The niembranous covering of the bod\- 
allows the shape of the cell to change, often moving by what is known 
as euglenoid motion, that is, by a wave of contraction over tlie whole 
body, thus causing a slow movement like that characteristic of Ameha. 

Although some species of Euglena appear to ingest the food i)ar- 
ticles that are swept into the gullet, the ordinary nutrition is tiie same 
as that of a green plant. The imuM- protoplasm of the cell is filletl 
with chloroplasts (chromatophores) by means of which the raw mate- 
rials, water, carbon dioxide, and mineral salts, are synthesized into 
food, thus storing the energy of sunlight. Different species of Euglena 
are sensitive to different degrees of sunlight and are found to turn 
towards a source of light, the anterior part of the cell, which contains 
a red "eyespot," being most sensitive to the light stimulus. When 
they are exposed to strong sunlight, they change their direction, 
coming to rest in an area of moderate or "optimum light." Respira- 
tion is carried on as in any other unicellular form by exchange of 
gases through the membrane covering the body. During the period 
when starch is being made in the sunlight enough oxygen is released 
within the cell body to supply its needs. Excretion of waste products 
appears to be taken care of by a number of very small contractile 
vacuoles, that collect fluids from the cell, eliminating them period- 
ically into a small reservoir which empties into the gullet. The 
individual cell in some respects acts hke a plant, and in others like an 
animal. It is a borderline representative, and as such must be 
regarded as a very primitive organism. 

Reproduction takes place as in other sim])le forms by fission, the 
free-swimming cell splitting lengthwise*. The si)Iit begins at the 
anterior end, the two new cells finally having the same structures as 
are found in the parent cell. In some species of Euglena that encyst, 
the cell divides by fission during the quiescent period, so that two 
or more cells are eventually released from the cyst. In some instances 
as many as 32 have been released from a single cyst. 


Although protozoa are single cells, some representatives ..f the 
phylum are much more highly specialized than tiie snnple Ameha. o. 


Euglena. These living cells may often be seen with the naked eye as 
whitish specks, moving slowly near the surface of a laboratory hay 
infusion that has been standing for some time. There are several 
different species commonly found, some larger than others, although 

in a drop of infusion 
much variation in size 
within the same species 
may be found. The 
class, Infusoria, con- 
tains a large number of 
forms, one of which, 
longitudinal fiber Paramecium, or the 
— -trichocvst "slipper animalcule," is 



^basal i^TamAs. 

1-^- - thread of attachment 

Diagram showing structure of the pelHcle in Para- 
nicciurri nnillimicronucleata. Under high power 
of the microscope the peUicle is seen to form minute 
hexagonal areas, from the center of each of which a 
cilium protrudes. The cilia arise from basal gran- 
ules (microsomes) which are located on strands of 
protoplasm (longitudinal fibers). Where do the 
t richocysts lie with reference to the cilia .•* Where 
are the openings through which the trichocysts are 
discharged? (After Lund.) 

very common. It has a 
somewhat flat, elliptical 
body with the anterior 
thinner end more blunt 
and the broader poste- 
rior end more pointed. 
The cell body of Para- 
mecium is almost trans- 
parent and is made up 
of an outer, non-granu- 
lar layer, the ecto'plasm, 
and an inner semifluid, granular layer, the endoplasm. The ectoplasm 
is covered with a delicate, elastic, but lifeless covering called the 
pellicle. Under it is the living cell membrane and through the pellicle 
project numerous threads of protoplasm, the cilia, which are distrib- 
uted over the surface of the body in regular rows. The cilia are 
quite uniform in size except at the posterior end of the cell, where 
they are a little longer. It is by means of a lashing movement of 
these cilia that locomotion takes place. Embedded in the clear ecto- 
plasm are also fotmd nimierous defensive structures, called trichocysts. 
Under certain conditions, delicate filaments or threads are discharged 
from them which serve as organs of offense and defense. It is be- 
lieved that they may contain minute quantities of poison which 
paralyzes other protozoa. 

On one side a depression, the oral groove, runs diagonally from the 
anterior end of the body to about the middle. This oral groove ends 
in a gullet, which in turn leads to the interior of the cell. The 



diagonally beating cilia which cover the body cause the rotation of 
the Paramecium on the longitudinal axis. Since the cilia in the oral 
groove are longer and capable of more vigorous motion, th(> b<,<lv 
tends to swerve toward the left. As the water passes down the <,ral 
groove towards the gullet, the waving undulatinq mnnhranv f<,r,ne,l 
of ciha fused together, guides particles of potential food down the 
gullet by means of its wavelike motion. At the inner f..od 
vacuoles are formed within the body. The food vacuoles an<l other 
granular inclu.sions shift about in a definite course within the cndo- 
plasm of the cell. Gradually the food particles within a given vacuole 
are digested by means of 
enzymes formed in the endo- 
plasm and released into the 
food vacuole. The digested 
food material is absorbed 
into the protoplasm, there 
to build up living matter or 
to be used later in the release 
of energy. Food wastes are 
passed out of the cell through 
the anal spot. Excretion of 
wastes may also take place 
through the cell-membrane 
by diffusion, or through two 
contractile vacuoles, one at 
each end of the cell, which 
consist of a central cavity 
with canals radiating out 
from it into the endoplasm. 
Many experiments have 
been made to test the sensi- 
tiveness of Paramecium to 
various stimuli. As in other 
living cells responsive to 
stimuli, factors of the envi- 
ronment have a distinct in- 
fluence upon its movements. 
Paramecium swims in a spi- 
ral course partly as a result of its shape and the arrangement and 
diagonal beating of the cilia, and partly on account of the anteriorly 

anLericfT end. 

i pellicle- 



— ectoplccSm. 

--oral droavQ, 
- -moLcth 

;- gullet.. 


arxxl ^pot 

Contractile ,, 

^_^ -.tricHocy^t 

.;\^,,<o^..„ cilicx. 

'"'^<:'.. posterior ond. 

Internal structure of Pdrdnirritirn attula- 
lurn. The cilia cover the cut ire surface of 
the cell and are somewhat Ioiiltit .iI I In" jmis- 
terior em]. 


pointed groove which turns the cell to the left as it progresses through 
the water. When moving into an unfavorable environment or hitting 
against a solid object, Paramecium reverses the direction of its ciliary 
lashings, backs away, and goes forward again in a slightly different 
course, repeating the performance until the obstacle is eventually 
avoided. Other reactions take place with reference to light, gravity, 
heat, dissolved chemicals, electricity, and water currents, all of which, 
whether positive or negative, are co-ordinated by means of a so-called 
neuromotor mechanism within the cell that enables it to adjust itself 
to its environment. Under careful methods of staining a number of 
very minute fibrils may be found in the cell which arise in a central 

I I 

Binary division of Paramecium caiidaiiim. Note the position and structure of 
micro- and niacronucleus in I. Follow these structures to the formation of the 
daughter cells (IV). Do both micro- and macronuclei divide by mitosis? What 
other changes take place in the cells .3 (After Hegner.) 

body near the nucleus and radiate out to the bases of the cilia. This 
apparatus apparently aids in co-ordinating the action of different 
parts of the cell. 

Occupying a central area in the cell are two denser bodies, the 
larger, knowni as the macronucleus, has to do with the metabolic 
activities of the cell, while the smaller, or micronucleus, contains the 
chromatic material which is associated with heredity. 

In a hay infusion Paramecia may be found dividing by simple 
fission. In this process both macro- and micronucleus elongate, and 
then divide. ' A new gullet buds off from the original one, two new 
contractile vacuoles appear, and the cell, which has been constricting 
in the middle, pulls apart to form two new cells. This process may 
continue for a good many generations where food is plentiful and 
conditions of life favorable. Woodruff has kept one culture of Par- 



amecia in his laboratory at Yalo University lor thirty yoars and (hir- 
ing that period over twelve thousand generations vv.t,. I,,-,,! I.y fissi,,,, 
It has been observed in these cultures, however, (haf after 4() or inor.- 
divisions have occurred, a i)rocess called cndomixis takes plac.-. in 
which the old active niacronucleus is replaced by a new one made 


Endomixis in Paramecium aurelia. The normal condition of Parainociiiin is 
shown in I showing niacronucleus and two niicronuclei. Follow throufrli the 
series pictured. What happens to the niacronucleus? How many iniiTonuclci 
are formed? What, happens next? Note in IV that only one daughler cell is 
shown. How does this cell obtain the normal number of niicronuclei? Where 
does the new niacronucleus come from? This rhythm of cell actixity seems to 
occur with considerable regularity every 10 to .^0 generations and it gi\ es the new 
macronucleus chromatin from the reserve sujiply held in the micronucleiis. This 
process does not appear in all ciliates and is not beliexed to be necessary for 
normal growth. (After Hegner.) 

from chromatin of the reserve micronucleus. This process is similar 
in many respects to conjugation, except that no foreign chromatin 
is added. 

Under normal conditions, another process known as amphimixis or 
conjugation takes place somewhat resembling the sexual procc^^ses of 
higher animals. Two cells come to lie with their gullet surfaces next 
to each other and a bridge of protoplasm forms between them, \\hile 
this is going on the micronucleus in each cell moves away from the 


macronucleiis, elongates and divides twice in rapid succession. Three 
of the micronuclei thus formed in each cell disappear, but the fourth 
one divides again. In this last division two irregular masses of chro- 
matin are formed. This process has been likened to a similar division 



Conjugation in Paramecium caudafiim. Shortly after the conjugating pair 
come together with their ventral surfaces opposed (I) a protoplasmic bridge is 
formed, the macronucleus breaks down (II) and each micronudeus divides a 
second time (III). What happens to three of the four micronuclei? Compare 
this stage with the figure on page 429 (maturation). Next the micronuclei 
remaining in the cell divide into two, the smaller (migratory) micronucleus passing 
over by the protoplasmic bridge into the opposite cell, there to unite with the 
larger (stationary) nucleus (VI). Trace the subsequent divisions of the fused 
micronucleus (VII, IX). How do we get back to the original cell condition? 
(X-XIV). (After Hegner.) 

that takes place in the eggs of animals, at the period known as matu- 
ration, when the sex cells are losing part of their chromatic material 
in preparation for fertilization of the egg by the sperm cell. The 
smaller mass is thought to correspond to a sperm cell of the many- 
celled animals, while the larger one corresponds to the egg cell. In 
any event, each of the smaller micronuclei migrates reciprocally over 



the protoplasmic bridge, and unites with the larger niicroiiuclcus of 
the cell left behind. The two conjugating cells now separate, and the 
newly fused nucleus, composed of a male and female microinu'lcus, 
is left in each cell. Then a series of divisions of this nucleus takes 
place until eight nuclei are formed, four of which become macro- and 
four micronuclei. Three of the micronuclei next disintegrate, leaving 
the cell with four macro- and one niicionucleus. The latter divides 
again and with it the cell, so that two cells result, each witli a inicro- 
and two macronuclei. A second division leaves the daughter cells 
each with a single macro- and micronucleus, which, thus rejuvenateil. 
start off on a series of several hundred cell divisions until another 
period of old age comes on, when conjugation or endomixis is repetited. 


These beautiful microscopic plants, sometimes called "jewels of the 
plant world," are among the most numerous of the one-celled plants. 
Over 2000 species have been identified and named. They form one 
of the most abundant components of plankton in 
both fresh and salt water, and are also found in 
damp earth and on moist rocks, where they may 
occur singly or massed together in groujis. Certain 
species stick together because of a gelatinous ma- 
terial which they secrete. Some diatoms move 
with a slow gliding motion when they are in con- 
tact with solid objects, although lacking visil)le 
organs of locomotion. They secrete a glasslike 
shell exquisitely marked by tiny ridges and rows 
of extremely minute holes. 

Diatoms have been, and still are, among the 
most abundant of li\ing organisms. So abundant 
were they in past ages that large deposits of their 
shells exist in the form of diatomaceous earth. 
In California, there are deposits of diatomaceous 
earth lying hundreds of feet thick over an area of 
many square miles, while the floor of the ocean is 
covered with ooze made up of skeletons of diatoms, 
which after death sink to the bottom of the water. 
This diatomaceous material is used as a basis for i^ohshnig lu.wders 
in the manufacture of bacteriological filters, and of certain kinds o\ 
porcelains and glass. 

The (lialoiii \(i- 
vinild («) Niilxt-sidf, 
{h^ ;:inilc side. sIkiw- 
inf.' IIk' rcliilion of 
tlic\;iK('s. ThiMiii- 
clfiis iiiid the two 
rililmiilikc chlorn- 
pliislsitrc iiol sliowii. 
(After Plil/«T.i 


One of the most common diatoms found in pond water is Navicula. 
In this form the cell wall consists of two valves, one of which fits into 
the other. The part that fits over the inner valve is called the girdle. 
The cell appears quite different in structure when seen from the valve 
side or the girdle edge. In the latter view, a bridgelike mass of pro- 
toplasm containing a nucleus appears, while in a valve view a line 
running down the center, called the raphe, is seen, that shows three 
tiny spots, one in the middle and one at each end. A mucilaginous 
material exudes through a series of pores which form the base of the 
raphe. Navicula has two chloroplasts, colored yellowish-brown by a 
pigment called carotin. These can be seen best when the cell is 
viewed from the flat side. At the time of cell division, the chloro- 
plasts first increase in size, pushing the two valves apart so that they 
barely touch. Then the nucleus, chloroplasts, and cytoplasm of the 
cell divide, an inner valve forming for each cell. Each of the new 
cells thus formed is much smaller than the parent cell. 


Another one-celled form common in fresh water is the bright 
green desmid, Closterium. Like diatoms, desmids are of various 
shapes and sizes. They are beautiful symmetrical structures with 

large, bright green chloro- 
plasts, which may be lobed, 
starshaped, or platelike. The 
cell wall is thin and transpar- 
ent, the granular protoplasm 
within being obscured by 
chlorophyll, but the nucleus, 
in the center of the cell, may 
be easily recognized. 

Desmids divide by a simple 
transverse splitting, forming 
two cells, each new desmid 
consisting of half of an old 
cell from which an entire cell 
is formed. In addition, a process of conjugation takes place, in 
which two cells come together, each sending out a protoplasmic 
protuberance that forms a connecting canal. The contents of the 
two cells meet in this tube, fuse, and form a single cell which grows a 
thick wall, whereupon it remains as a dormant spore or zygote until 


Two cells undergoing conjuga- 
. tion. 


conditions are favorable for germination. When the zyjrotr .l,„..s 
germinate, two new individuals come direetly from it. 

Many other forms of algae may be found in fresh and .s.h w.-.i.t. 
Some, like Scenedcsmus, occur in colonies, their end cells being ..ftcn 
provided with characteristic spines. Another colony of gr,.,.,, cells 
Pediastrium, made up of a flat plate of sixteen cells, is also frecpiently 
seen. These are only a few of the many forms of green algae that 
may be found in a drop of water debris tak(Mi from a (iniet poml 


Various kinds of bacteria are common in a drop of i)ond water or 
hay infusion. They are sometimes seen moving through the water, 
but more often are massed together in a scum covering the surface 

""a^o?. O^tfJ /f#^^' 


op oo oo 

«■& S? 

QO QO ^ 

ig^Hfl GO .. ao 

rmcrococcA diplococci staphylococci streptococci 
Forms of l);icleria. 

of the water. Three large groups of bacteria ha\-e been established 
according to their shape, coccus, baccillus, and spirillum. The coccus 
or spherical-shaped bacteria may live singly, as micrococci. Anotlier 
form, the diplococci, divides and remains attaclied s(j as to form 
pairs ; a third, streptococci, reproduces to form chains ; while a fourth. 
staphylococci, forms irregular groups of eight cells or more, resem- 
bling a bunch of grapes; Sarcina divides in three directions to pro- 
duce cubical packets. The rod-shaped bacteria, or bacilli, \i\ry a 
good deal in size and shape, as well as in tiieir ability to form spores, 
some being very short, others many times longer than wide. The 
third type, comprising the spirilla, are cur\e(l or twisted in shape, 
and move through the water rapidly by spiral movement. This 
form can often be seen hi a droj) of pond water or hay infusion. 
BacilH and spirilla move by means o( Jlagdla, protoplasmic threads 


which are difficult to see except under the highest power of the 

The cell wall of a bacterium is usually considered as a selectively 
permeable membrane, very delicate, and secreted by the cytoplasm. 
A gelatinous capsule may be formed by some bacteria, so that groujis 
of them clump together in masses. Although pigments are often 
present, bacteria contain no chlorophyll, and consequently most of 
them are dependent on other organisms for their food. They feed 
both on living and dead organisms, using not only organic foodstuffs, 
such as starches, sugars, and proteins, but even leather or wood. 
Since their food must be liquid in order to be absorbed, they form 
digestive enzymes within the cell which exude to digest the food out- 
side of the cell body. 

In addition to these foods, bacteria need certain mineral salts that 
are found in protoplasm, water, and nitrogen in a usable form. Not 
all bacteria are capable of nitrogen fixing, but many obtain their 
supply of nitrogen for tissue building as green plants do, in the form of 
compounds of ammonia or nitric acid. 

The chromatin material is scattered through the cell, there being 
no distinct nucleus in most bacteria. Bacteria need moisture, a favor- 
able temperature, and food, in order to grow. Under favorable con- 
ditions they multiply with great rapidity by simple fission. Under 
unfavorable conditions, many bacterial cells can contract, lose con- 
siderable water, and form resistant coats, thus making spores, which 
can stand extreme conditions of dryness and temperature. While 
bacteria are usually killed by heating to 100° C, some spores can 
withstand this temperature for long periods. 

Functional Differences between Plant and Animal Cells 

A comparison of the several types of unicellular organisms described 
might seem at first to show hard and fast distinctions between plant 
and animal cells. Although chlorophyll is associated with plants, it is 
sometimes found in borderline animals, while many plants, such as 
the fungi and bacteria, lack chlorophyll. Locomotion is not exclu- 
sively an animal characteristic. Some animal cells, as Vorticella, are 
fixed during a part of their life history, while many unicellular plants 
move freely through the water. Other plants, although fixed for part 
of their lives, produce sex cells that are motile in water. The greatest 
difference exists in methods of nutrition. In the green plant cell, for 
instance, food substances are made inside the cell in the presence of 


sunlight while in animal cells, food is made outside and has to be 
absorbed before it can be used. The method of nutrition used by the 
green plant is called holophytic, and that of the animal cells, holozoic. 
The differences between these two types of nutrition are summed up 
in the table below. 

Animal Cell 

Plant Cell 

No chlorophyll 

Chlorophyll present 

Cannot make organic foods 

Can synthesize organic foods out of law 

food materials 

Only source of energy is organic food 

Source of energy is the sun 

Ingests solid food 

Cannot ingest solid food 

Usually moves about after food, therefore 

Does not ordinarily move about, and uses 

greater destructive metabolism 

sun's energy, therefore greater construc- 

tive metabolism 

Depends on other organisms for food 

Supplies other organisms with food 


Calkins, G. N., Biology of the Protozoa, Lea & Febiger, 1926. Chs. I, III, 

and IV, especially. 
Dobell, C., Antony van Leeuwenhoek and his "Little Animals," Harcourt, Brace 

and Co., 1932. 

The entire book, which contains excellent translations of most of the 

original letters of van Leeuwenhoek, is well worth reading. It is a 

most authentic picture of this interesting Dutchman and his times. 
Giltner, W., Textbook of General Microbiology, P. Blakiston's Son & Co., 1928. 

Ch. III. 
Holman, R. M., and Robbins, W. W., Elements of Botany, 3rd cd., John 

Wiley & Sons, Inc., 193(3. Ch. X. 
Locy, W. A., Biology and Its Makers, Henry Holt & Co., 1908. Ch. V. 

An excellent historical survey. 
Needham, J. G., and Lloyd, J. T., Life of Inland Waters, 2nd ed. Charles C. 

Thomas, 1930. Ch. IV. 

Excellent descriptions and illustrations of the life found in pond water. 
Singer, C. J., The Story of Living Things, Harper & Bros., 1931. Ch. IV. 

An interesting and authentic history of biology. 
Ward, H. B., and Whipple, G. C, Fresh-Water Biology, John Wiley & Sons, 

Inc., 1918. 

This book is invaluable for reference. Chapters VI, IX, and XVII arc 

especially useful. 

H. w. H. 




Preview. The beginnings of sex in the algae • Oedogonium • A repre- 
sentative fungus • Alternation of generations in the plant kingdom • Sug- 
gested readings. 


The one unescapable fact that stands out in the observation of 
plants and animals in the world about us is the remarkable variety 
among living things. They range from tiny forms too small to be seen 
with the unaided eye to huge organisms such as elephants or trees. 

The biologist is not satisfied with random looking. He looks for 
certain things, tries to interpret what he sees, but as Thoreau once 
said, "We must look a long time before we can see." One of the 
striking facts already noted in the Roll Call of forms of life is that 
both plants and animals may be placed in groups having similar 
characters, and that these groups arrange themselves in a series of 
gradually increasing intricacy of structure, which goes hand in hand 
with an ever increasing complexity in functions. Simple plants or 
animals do things simply. Almost any part of the one-celled Ameba 
can do any part of the work of the cell although lacking organs found 
in higher forms. More refined ways of doing things, and a more' 
efficient division of work, come with increasing complexity of organic 
structure. The true investigator is ever alert to find forms that 
illustrate this increasing division of labor, and is always asking why 
and how such things come about. Biologists have picked out certain 
representative forms that clearly suggest certain facts and principles 
that are worth knowing. It is possible, for example, through the 
study of some simple forms of organisms, such as the Thallophytes, 
to discover the beginnings of sexuality in plants. 

The Thallophytes include most of the simplest plants and are 
divided into two great groups, algae and fungi, the latter containing 
no chlorophyll. Wliile there are six classes of algae, four, namely, 
the blue-green, the green, the brown, and the red, are classified 
largely on color. All of the four groups are essentially water- 
loving plants, showing in many ways that they are simple and 
rather primitive organisms. In size they range from tiny uni- 




cellular forms to some of the great brown seaweeds, or kelps of 
the California coast which may be several hundred feet in length. 
Ascending the scale of increasing complexity in structure, we find the 
appearance first of sex cells and later of sex organs evolved to form 
and protect these sex cells. 

By selecting other representatives from the higher plant groups, 
such as mosses, ferns, and flowering plants, we can follow this evolu- 
tion of sex through the entire plant kingdom. The pages that follow 
will at least give us a start on the answer to the question : How and 
where does sex originate in plants and what is its meaning ? 

The Beginnings of Sex in the Algae 

Pleurococcus, or Protococcus as it is sometimes 
called, is one of the simplest of all living plants, 
familiar to most of us as the green "moss" 
usually seen on the north side of trees. Indians 
used it to find their direction through the forest, 
as persons lost in the woods do today. Its 
habitat suggests that the life of the plant has 
direct relation to moisture, temperature, and 
light. It would be injured by the direct rays 
of the sun, because some rays such as those of 
ultraviolet light are injurious to unprotected 

The cell of Pleurococcus is very simple as seen 
under a microscope. It is found single, in twos, 
threes, fours, or flat colonies of several cells 
hanging together. Examination of a single 
cell discloses the presence of a thin wall sur- 
rounding a mass of green protoplasm, the protoplast, which almost 
completely fills the cell. If a drop of iodine solution is placed under 
the coverslip, the detailed structure of the cell becomes more evident. 
The nucleus is completely surrounded by one large, spherical chloro- 
plast. The cell is a complete entity, in spite of the fact that it is 
often attached to other cells. Physiologically it is able to carry on 
all the functions of a living green plant, making food, and digesting 
it as well as absorbing food and water. It grows to a certain size 
and then reproduces by simple fission, part of the mother cell going 
into one daughter cell and part into the other. Theoretically the 

Reproduction in Pleu- 
rococcus. Each cell is 
considered as an indi- 
vidual, although colonies 
(seen above) may be 
formed. The protoplasm 
of the cell body is not 
shown, the single chloro- 
plast being surrounded 
by protoplasm in active 


protoplasm of the pleurococcus is immortal, since it 
passes from cell to cell by means of cell division. 

Spirogyra is one of the multicellular green algae. 
It is a slimy thread, called "pond scum," found near 
the surface of a pond, often buoyed up by bubbles of 
gas which it forms. The filamentous plant body 
consists of several cells joined end to end, each with a 
characteristic spirally-banded chloroplast. 

Examination of a single cell shows a colorless cell 
wall, the cytoplasm of the cell mostly adhering to its 
inner surface. Strands of cytoplasm radiate from a 
central colorless nucleus, which is suspended in a large 
vacuole or sap cavity. The most characteristic fea- 
ture is the large twisted chloroplast, on which are 
scattered many pyrenoids, bodies which contain some 
of the starch manufactured by the chloroplast. In- 
dividuals grow in size by forming, through transverse 
division of the cells, longer or shorter filaments, de- 

A Spirogyra pending upon the environmental conditions. 

cell showing the 
spiral chloro- 
plast containing 
pyrenoids, and 
the nucleus. 

At certain times in the year, the plants form resting 
spores called zygospores. Two adjoining filaments 
come to lie parallel, the cells opposite to each other 
sending out bulging outgrowths which meet to form a 
connecting tube. Meantime, owing to the dissolving of the cell wall 
at the end of the outgrowths, water gets inside of the cells, so that 
they show signs of plasmolysis, rounding up into ovoid masses. 
Curiously, however, the cells of one filament remain stationary, 
while the cell contents from the other filament move over through 

Conjugation of Spirogyra. Explain what happens. (After Coulter.) 



the tube and fuse with th(^ quiescent cells. When this fusion takes 
place, the nuclei unite so that a single resting cell is formed, called the 
zygote, which develops a thick wall, very resistant to drought and cold. 
The zygote is heavy enough to sink to the bottom of the pond when 
the rest of the filament dies, and under favorable conditions will 
germinate, giving rise to a new filament. 

Since these cells from different filaments join or fuse, somewhat 
after the manner of conjugation in Paramecium, we think of them as 
sex cells, or gametes. Although the two cells are of the same size, 
yet one is active and the other passive. In higher plants and animals, 
the active cell is referred to as the male gamete, or spei^m, and the 
non-active cell as 
the female gamete, 
or egg. A compari- 
son of Spirogyra 
with higher forms 
suggests a very sim- 
ple type of sexual 
reproduction, known 
as conjugation. 

In another fila- 
mentous form, Ulo~ 
thrix, certain cells 
are modified to be- 
come free-swimming 
zoospores, provided 
with four cilia which 
may swim about for 
as long as an hour 
before settling 
down. It is obvious 
that such a free- 
swimming cell may plant a new individual at some distance from 
the original filament. Gametes of Vlothrix are also formed as free- 
swimming cells, all alike, having two cilia instead of four. These 
gametes fuse by conjugation and produce a zygote, which, like that 
of Spirogyra, has a thick resistant wall, and is capable of developing 
even after exposure to very unfavorable conditions. 

In the formation of the conjugating gametes of both Vlothrix and 
Spirogyra a significant thing happens to the nuclei of the cells before 

Ulothrix: a. base of filament with holdfast; b, fila- 
ment producing; zoospores or gametes; c. young filament 
developed from zoospore; d. filament discharging zoo- 
spores and gametes ; e. an escaped zoospore ; /, escaped 
and pairing gametes ; g, zygospores ; h, zygospore pro- 
ducing zoospores by reduction division, (a-f/. After 
Coulter; //, after Dodel-Port.) 



they conjugate. By a 
series of divisions, such 
as is shown in the dia- 
gram, the number of 
chromosomes in the nu- 
clei of the zygote, result- 
ing from the union of the 
two gametes, is reduced 
. . to half this number. If 
\ spelling disappear g^j^^g g^^j^ dcvice as this 

' ^ / \ were not used, every time 

sex cells united, the num- 
ber of chromosomes 
would be doubled. How- 
ever, by this so-called 
reduction division during 
the formation of the ga- 
Diagram to show how reduction division takes metes, which OCCUrs in 
place in the zygote of Spirogyra. both plants and animals, 

the number of chromosomes is halved. We speak of the single number 
of chromosomes as haploid and the double number, which comes with 
the union of the two gametes, as diploid. 


first division, 
of ci-ji-TomoSomcS 
from ?T-; to ri , 



In another of the filamentous algae, 
Oedogonium, there is the first appearance 
of two kinds of sex cells. This alga repro- 
duces by zoospores and in addition forms 
two sex organs, structures called anther- 
idia, which produce a number of ciliated 
sper7n cells and oogonia, the latter holding 
a single egg cell. The sperm cells swim 
through the water from the antheridia, 
one uniting with the egg cell, and almost 
immediately a thick wall is formed about 
the fertilized egg. This oospore does not 
produce a new plant directly, but gives 
rise to zoospores, which in turn eventu- 
ally become new plants. 

Life history of Oedogonium. 


Another form of Oedogonium forms antheridia and oogonia on 
separate filaments, the male filament being much smaller than the 
female filament. Thus the filamentous algae illustrate three big 
ideas, namely, division of labor, development of sex, and reduction of 

In the simplest plants all cells tend to do the same work, but in the 
more specialized algae there is a differentiation of work and an 
accompanying differentiation of cells to accomplish it. In the 
development of sex and of structures to take care of the sex cells, as 
found in the forms described, the contribution of the sex cells seems 
to be to provide a greater vigor to the offspring, especially when the sex 
cells come from different individuals. Most important of all is the 
fact that cells which fuse, as in the case of the sex cells, must have 
some way of reducing the number of their chromosomes, else they 
would be doubled each time two sex cells united. This is accom- 
plished by the reduction division referred to above, by which process 
the number of chromosomes, doubled at the time of fertilization, is 
halved. This reduction process occurs in both plants and animals, 
and although in plants it occupies a different place in the life cycle, 
its ultimate effect is the same in both cases. 

A Representative Fungus 

Bread mold, Rhizopus nigricans, one of the most common of the 
fungi, may easily be grown in the laboratory by exposing a moist piece 
of bread to the air for a few moments. Mold spores are so numerous 
everywhere that under ordinary conditions a growth of mold will be 
evident within one or two days, first appearing as a white, fluffy 
growth that rapidly covers the surface of the bread. This is the 
mycelium, which consists of branching tubelike filaments, or hyphae, 
containing many nuclei, but without cross walls. The absence of 
chlorophyll shows the inability of the mold to make its own foods 
and explains why the mycelium sends down into the bread, root- 
like branches called rhizoids, that secrete enzymes, by means of 
which the food substances in the bread are digested. Some of the 
hyphae form long branches called stolons, which run along the sur- 
face of the bread, forming new plants. At points where rhizoids 
are developed, there arise later numbers of erect branches, or spo- 
rangiophores, on the tips of which are developed sporangia, or spore- 
bearing organs. 


Great numbers of tiny spores are produced by division of the dense 
terminal portions of the sporangiophores. As a sporangium becomes 
mature an outer wall is formed and the spores turn black in color. 

When this outer wall 
breaks, the minute spores 
are scattered far and wide 
by air currents. 

Molds also reproduce 
sexually, by means of con- 
jugation. Rhizopus has 
two different strains of 
mycelia, one of which is 
called a plus ( + ) and the 
other a minus ( — ) strain. 
If hyphae of two such 
strains come in contact 
with each other, zygo- 
spores are formed. Short, 
club-shaped branches are 
developed from the hy- 
phae, the dense proto- 
plasmic tips are cut off from the end of each by cell walls, and these 
"cells," each of which contains several nuclei, unite to form a 
zygote. The zygote with the hyphae which develop from it proba- 
bly represents the diploid stage of chromosome in the life cycle, 
the haploid stage being reached when the spores on the sporangium 

The fungi are of even more interest by reason of their method of 
nutrition. They are typically neither holozoic nor holophytic, since 
they live as saprophytes on dead organic materials. This means that 
they must absorb food materials which are supplied to them from 
outside sources after digesting them by means of enzymes, when 
absorption takes place through the plasma membrane of the cell. 

Alternation of Generations in the Plant Kingdom 

The most important difference in the life cycle between the Bryo- 
phytes or Mosses and lower forms, aside from a greater differentiation 
of the plant body, is the alternation of an asexual with that of a sexual 
generation in the hfe cycle. The asexual generation, which produces 
spores, is called the sporophyte, while the sexual generation, which 

Reproduction in bread mold (Rhizopus nigri- 
cans). Read the text and then explain the 


gives rise to gametes of two different sexes, is known as the gameto- 
phyte. The latter generation is the conspicuous green plant that 
manufactures food and serves as host for the sporophyte generation 
which is permanently attached to it. 

The gametophyte of the simple moss, Funaria hygrometrica, is a 
short upright stalk bearing usually three spiral rows of simple leaves, 



Sfertili^ect egg 



dbcmetophone , 




ycrunS gb-metophyt' 

The life cycle of Funaria, a moss. Which stage is more prominent, 
gametophyte or sporophyte? 

each containing numerous chloroplasts. At the lower end, a group of 
small brown rkizoids furnish the means of attachment to the sub- 
stratum. The moss plant is dioecious, having separate sex organs 
on different plants. The male gametophytes are shorter than the 
female gametophytes and bear at the upper tip a cluster of structures 
known as antheridia. Each mature antheridium looks like a tiny 
club with a wall formed of rather large, thin cells, which forms a recep- 
tacle for numerous motile sperm cells. The female gametophyte 
bears at the apex of the short stem, although in the mature plant 


hidden by leaves, a cluster of flask-shaped structures called archegonia, 
at the bottom of each of which is a single rather large egg cell. 

Fertilization of the egg can take place only when the antheridia and 
archegonia are wet from rain or dew. In such an event the sperm 
cells ooze out in a mucilaginous substance secreted from the walls of 
the antheridium and pass in drops of water to the necks of the 
flask-shaped archegonia. Here they are chemically attracted by a 
substance exuded from the inside of the archegonium and swim down 
the tubular neck until one meets the egg cell, when fertilization takes 
place. The gametophytic phase of the moss is the haploid stage of the 
chromosomes, fertilization of the egg restoring the diploid number 
characteristic of the sporophyte. This generation begins with the 
cell division which follows the fertilization of the egg in the archego- 
nium and results in the growth of a tiny stalk, bearing at its upper end 
a capsule, that in the adult sporophyte is filled with asexual spores. 
During the formation of the spores within the capsule, the formative 
tissues produce a number of large, rounded spore mother cells, from 
each of which by nuclear divisions tetrads, or groups of four spores, 
are formed. During this tetrad formation, a reduction division 
takes place so that the spores contain only the haploid number of 

The moss capsule is quite a complex structure with a cap, or oper- 
culum, that covers an urn-shaped affair bearing at its upper end a 
circle of teethlike structures collectively called the peristome. As the 
sporophyte ripens it dries up and the numerous ripe spores are scat- 
tered by the action of the peristome teeth, the latter being very 
hygroscopic, or sensitive to moisture. When the weather is humid or 
wet, the teeth of the peristome curl up and when dry they straighten 
out, thus expelling the spores, which may then be scattered by the 
wind. The germinating spore does not grow directly into a leafy 
plant, but first forms a protonema or algalike filament from which 
upright stalks later arise, while rhizoids grow downwards from it, 
thus forming again the moss plant. This life cycle with its alterna- 
tion of gametophytic and sporophytic stages is characteristic of the 
life cycle of mosses and liverworts, as well as the higher group of the 
ferns (Filicinae). 

In the flowering plants (Angiospermae), one finds an almost com- 
plete suppression of the gametophytic generation, the sex cells or 
gametes being produced in modified leaflike parts of the flower. The 
floral parts — sepals, petals, stamens, and carpels — are thought of 



as leaves which have become metamorphosed from their vegetative 
form and function to hold the sex structures. The stamens and 
pistil (carpel) contain spore-forming tissues which, by means of 
reduction division, produce pollen grains containing microspores 
(sperms), while ovules produce a female gametophyte and its egg. 
The sperm cells are formed in the pollen grains, while the egg cells 

germinating poller 


osUs of 

form pollen grains with— ^ sperm 2,-^ ^; 

tuba nuclau^ 

TRe embr/o sac contains a dividimg nuclaus ^ v '/ bolUn 
eight cxne jlnally formed, tuJo forn-i fusion rnAcleu^ /^tL-ubo 

Development of male and female gametophyte in the flowering plants. Only 
the cells which actually form these structure are shown. The parts of the sporo- 
phyte upon which the gametophyte is parasitic are omitted for the sake of clarity. 
Read the text carefully and then use the diagrams. 

are held within the ovary of the pistil as has been previously stated. 
In the angiosperms or flowering plants the male gametophyte is so 
much reduced that it consists of only three cells, a tube nucleus and 
two generative cells (see figure). Just previous to the formation of 
the pollen grains (male gametophyte) reduction di\ision takes place 
so that its cells contain the haploid number of chromosomes. The 
female gametophyte is also greatly reduced. After reduction divi- 
sion, the megaspore divides (see figure) one nucleus migrating to each 
end of the etnbryo sac (female gametophyte). The nuclei continue 
to divide until eight are formed in two groups at opposite ends of 
the embryo sac. From each group a single nucleus then unites with 
the other to form a fusion nucleus (see figure). At this stage the egg 
nucleus is ready for fertilization by the sperm nucleus. A double ferti- 
lization now takes place, the sperm nucleus fuses with the egg nucleus 
and the second sperm nucleus unites with the fusion nucleus. The 


former gives rise to the young plant, the latter to its food supply, the 
endosperm. The transfer of pollen in flowers of the same species 
may result in the fertilization of the egg and subsequent growth of 

Division I 


algcxe. "^ 

Division I 


Division BE Tr-ccchsopViyta.- vasculcti- T=lcL"ts 

subdivision A*B;vc ^abdVvi/ion D PteropsicCcc 

ferns ^^rnnospe4*ms angiospcrrrjs 

primiuve plants 

Lvcopsida ,Sl*enopJic£a 


Diagram showing relation of sporophyte and gametophyte generations in the 

plant kingdom, 

the plant body (sporophyte generation). The evolution of sporo- 
phytic and gametophytic generations in the plant kingdom is shown 
in the above chart. 


Coulter, J. M., Barnes, C. R., and Cowles, H. C., A Textbook of Botany, 

Vol. I, American Book Co., 1930. 

This text gives an excellent foundation for the understanding of sexu- 
ality in plants. 
Gager, C. S., General Botany, P. Blakiston's Son & Co., 1926. 

A general botany which gives much information on economic questions, 

as well as sex development in simple plants. 
Robbins, W. J., and Rickett, H. W., Botamj, D. Van Nostrand Co., 1929. 

Chs. XV-XXIV. 

Excellent diagrams help in the understanding of the development of sex. 
Sinnott, E. W., Botany, Principles and Problems, 3rd ed., McGraw-Hill Book 

Company, 1935. Chs. XI and XIV-XXIII. 

A thoroughly up-to-date treatment of the subject. 
Wilson, C. L., and Haber, J. N., Plant Life, Henry Holt & Co., 1935. 

An interesting and well-written elementary text. 




Preview. The Hydra, a representative of the phylum Coelcnterata ; the 
ectoderm and its functions ; the endoderm and its functions ; reactions to 
stimuli ; reproduction ; regeneration ■ Hydroids • Suggested readings. 


It has already been shown that unicellular animals may exhibit 
considerable complexity of structure, and that associated with this 
complexity, there is a separation of functions in different parts of the 
cell, but we have not traced this division of labor into the many- 
celled animals or metazoa. The colonial forms, such as Pandorina, 
Eudorina, and Volvox, claimed by both botanists and zoologists, are 
interesting exam]iles of aggregations of many cells showing little 
evidence of organization or division of labor. Even in the colony of 
Volvox, most of the cells have common functions, only the reproduc- 
tive cells being set off from the others. 

The Hydra, a tiny animal little higher in the scale of life, gives every 
evidence in its structure of being a simple organism and not just a 
collection, or colony, of cells. It shows, in a convincing manner, how 
a simple, many-celled organism lives. It answers the question of 
how division of labor might arise among the cells of a simple organism, 
For this reason it is chosen as a type in most courses in biology and 
so has a place in this text. 

The Hydra, a Representative of the Phylum Coelenterata 

Hydras are quite abundant in many ponds or slow-moving streams, 
where they may be collected on the stems and leaves of aquatic plants. 
In an aquarium, they often leave these plants and become attached 
to the glass walls of the aquarium, where they appear as tiny brown 
or green cylinders one-half of an inch or more in length. At the free 
or so-called oral end, a circle of tentacles surrounds a conelike area, 
the hypostome, in which the mouth is found. The opposite, or aboral, 
end forms a disklike structure which is provided with mucous cells 
that aid it in sticking to a surface. Hydras are able to move slowly 
by a looping motion of the body. The green ones, which are much more 



active than the brown ones, frequently change their position if food 
is not abundant. They respond to chemical stimuli of food, to light, 
and to unfavorable temperatures, food being the chief factor in their 
environment. The color of green hydras is due to the presence of 

Hydra is able to change its position both by turning "handsprings" as shown 
in the diagram and also by contraction and expansion of the basal portion of the 

minute green algae, called Zoochlorellae, that live in a symbiotic 
relationship within the endodermal cells. 

The term, Coelenterata, which is the name of the phylum to which 
the common Hydra vulgaris belongs, comes from the Greek words 
koilos, hollow, and enteron, intestine, which may be translated "hav- 
ing an internal digestive cavity," an apt title, since a Hydra is really 
a hollow, double-walled bag. 

The Ectoderm and Its Functions 

The bulk of the outer layer of cells (ectoderm) is made up of large 
epitheUo-muscular cells, having a layer of muscle fibers placed lon- 
gitudinally at their bases, that enable the animal to lengthen or 
shorten its body. A similar layer of fibers on the inner layer of cells 
which run circularly around the body allows it to expand or contract 
in diameter. Between the epithelio-muscular cells and near the inner 
margin of the ectoderm are found numerous smaller interstitial cells 
from which are derived numerous other cells, including the cnido- 
blasts. Nerve cells are likewise scattered throughout the ectoderm, 
forming a nerve Jiet at the base of the epithelial cells. 

Cnidoblasts are most abundant on the tentacles, although they are 
found on all parts of the body exclusive of the basal disk. They hold 
four kinds of stinging capsules, nematocysts, by means of which the 
animal paralyzes living prey that comes in contact with its tentacles. 
The nematocysts are capsules containing a hollow inverted thread 
which under certain conditions can be thrown out, together with a 
poisonous substance, hypnotoxin, that has the power to paralyze any 
other small animal which it touches. The nematocyst reacts to cer- 



tain chemical stimuli that apparently cause a change of osmotic pres- 
sure within the cell, thus forcing out its threadlike portion. After a 
nematocyst is protruded, the cnidoblast dies and is soon replaced by 




cell ^ 


cell -^ 

cell ® 


The Endoderm and Its Functions 

By cutting a section through the body of a Hydra its similarity to a 
two-walled sac is evident. Between the ectoderm and the inner layer 
of cells (endoderni) a thin, structureless layer called the mesoglea 
forms as a secretion 
from the cells of the «^toclerm j e«dod^m 

inner and outer layers. 
Mesoglea forms much 
of the bulk of other 
coelenterates like the 
jellyfishes. The endo- 
derm consists principally 
of large vacuolated cells 
that have flagella at the 
free or inner end, al- 
though they are also 
capable of developing 
pseudopodia at this 
end. Circular contrac- 
tile fibers are developed 
at their basal end. Thus 

they are endothelial-muscular cells. In the third of the body nearest 
the basal end, gland cells develop, which secrete digestive enzymes. 
Nerve and sensory cells are also found in the endoderm. 

For a simple animal, the Hydra seems to have many kinds of cells. 
What is the use of so many ? The answer is found in the way it gets 
food, ingests it, and finally absorbs it into the body cells. By watch- 
ing a hydra in the aquarium it will be seen that its tentacles are con- 
stantly moving as if seeking food. If a tiny bit of raw beef is placed 
within reach, the animal will bend over and carry the meat to the 
mouth, the edges of which soon close around it, forcing it inside. If 
the piece is too large to be taken in, the Hydra actually turns inside 
out in an attempt, usually successful, to put the meat inside the 
gastrovascidar cavity. Once inside the cavity, digestive enzymes from 
the glandular cells act upon the food, gradually breaking it down into 

Sections through the body wall of hydra showing 
the two layers of cells separated by the striated 
lamella secreted by the basal parts of the ecto- 
dermal and endodermal cells. 


smaller and smaller fragments. Digestion appears to be aided by 
the churning movements caused by expansion and contraction of 
the body wall. Ultimately some of the food is reduced to a soluble 
state, and absorbed into the endodermal cells. Meanwhile some of 
the large vacuolated cells put out pseudopodia and engulf some of 
the undigested food particles, finishing the digestive process inside 
their own cell-bodies. Thus Hydra has two types of digestion, one 
intracellular, like that found in all unicellular animals and, there- 
fore, more primitive ; the other, extracellular, that is, taking place in 
the digestive cavity. Most of the food of the Hydra is digested in 
the latter way, the cells lining the cavity absorbing the digested food 
before passing it along to the cells of the ectoderm. According to 
Hegner, part of the absorbed food is in the form of oil globules which 
are passed over to the cells of the ectoderm and stored there for future 
use. Unusable or undigested material is thrown out of the digestive 
cavity by a sudden contraction, there being no other way of eliminat- 
ing such wastes except through the surface of the body, as in lower 
forms. Hydra like other animals uses oxygen to release energy. 
Respiration probably takes place through the surface of the entire 
body, the cells receiving oxygen and giving off carbon dioxide by 
diffusion through the cell membranes. 

Reactions to Stimuli 

Hydra show very definite reactions to certain stimuli, most of 
which have to do with obtaining food. Hungry Hydra are much 
more active than well-fed ones, and respond to various chemical 
stimuli besides reacting to mechanical stimuli, to heat, to light, and 
to electricity, all of which indicates the possession of some sort of 
simple nervous system, since the movements made are more or less 
co-ordinated. If touched lightly on a tentacle with a needle, only 
the tentacle contracts, but with increased stimulation, the other 
tentacles contract, until finally, the whole animal draws down into 
a little ball. Its physiological condition, according to Jennings,^ 
determines whether it " shall creep upward to the surface and toward 
the light, or sink to the bottom ; how it shall react to chemicals and 
to solid objects ; whether it shall remain quiet in a certain position, 
or reverse this position and undertake a laborious tour of exploration." 

The nervous system of Hydra forms a nerve net. It consists of a 
concentration of primitive nerve cells about the base of the hypostome 

1 Jennings, Behavior of the Lower Organisms. Columbia Univ. Press, 1915, p. 231. 



and the foot. This network of cells lies in the ectodermal layer of 
the animal, and receives impulses from sensory cells as well as trans- 
mitting them to the muscle fibrils. The sensory cells of the ectoderm 
.vary in their location ; one type occurs on the tentacles, one on the 
hypostome, and a third on the foot 
(base). Neuro-sensory cells which 
are located in the mid-body area 
.also resemble nerve cells, except that 
they send processes to muscle fibrils 
and so become intermediate between 
those receiving stimulation and those 
making the response. Some nerve 
cells appear in the endodermal layer 
but are not, so far as can be deter- 
mined, connected with the ecto- 
dermal nerve net. 


Probably the most important 
function of the interstitial cells is 
their growth into sex cells. Most 
Hydras are hermaphroditic, that is, 
have both kinds of sex cells present 
in the same individual, but since the 
sperm cells and ova ripen at different 
times, fertilization is accomplished 
by sex cells from different indi- 
viduals. Sperm cells are produced 
by the mitotic division of interstitial 
cells, each of which first produces a 
number of parent male cells, contain- 
ing the somatic number of chromosomes. 

The nerve net in a young hydra 
as seen with an intravitani methylen- 
blue stain. Note the ringlike ar- 
rangement in hypostome and foot. 
What effect might such an arrange- 
ment have on movement? (After 
J. Ilodzi.) 

These cells divide four 
times and in the process a reduction division takes place, leaving the 
sperm cells with just half as many chromosomes as the body cells. 
A somewhat similar process takes place in the formation of the ova. 
One interstitial cell becomes larger than the others, rounds into a 
sphere, and is surrounded by other interstitial cells, which serve as an 
ovary for the growing egg. The latter continues to grow in size, form- 
ing yolk from the surrounding cells. Just before the egg becomes 
mature, the process of maturation takes place (see page 429), dur- 
H. w. H. — 13 


ing which the number of chromosomes is reduced to half the body- 
number. Spermaries and ovaries can be seen in the Hving Hydra as 
little lumps on the ectoderm. The spermaries are always found near 

the free end of the body, the 
ovaries, when present, being 
nearer the base. The egg 
is fertilized while still at- 
tached to the parent and 
develops into an embryo 
surrounded by a protective 
chitinous case, in which 
stage it sinks to the bottom 
of the pond for a resting 
period before emerging as 
an adult. 

Asexual development 
also takes place. A small 
bulging area, formed by 
the interstitial cells, ap- 
pears on the side of the 
body, which more or less 
rapidly grows into a short 
column surrounded by 
tentacles, depending on the 
food supply available for 
the parent Hydra. When 


fully developed the bud 
may separate from the 
parent and lead a separate 
existence. A Hydra fre- 
quently produces more than 
one bud on a single animal. 


— cells 



Longitudinal section through the body of a 
Hydra, showing both sexual and asexual repro- 
ductive structures. 


Although regeneration takes place in other groups of animals it is 
best seen in the phylum, Coelenterata. The primitiveness of Hydra 
is shown by the fact that it can regenerate or replace lost parts by 
growth of the body cells. It may be cut lengthwise or crosswise, or 
even into small pieces, and the fragments will, under favorable con- 
ditions, give rise to complete individuals. 




Hydra vulgaris is a fresh water form, but many more representatives 
of the Coelenterate group are found in salt water, the most famiUar 
being the hydroids found attached to the piles of wharfs and other 
submerged objects. Among the most common hydroids are members 



^9 ^onacC 

medusa /;<^^^»v^^",'^^ 
^^^ (^..fertiTe 

asexual *~-^-Viyctrorhi3a. - y 

Stage / 



Life cycle of Obelia — showinff alternation of generations.' Compare with text 
pages 18.5-186 for explanation of diagram. 

of the genus Obelia. These animals form colonies, in which the indi- 
viduals, called polyps, or zooids, are attached to each other by means 
of hollow stalks, covered with a chitinous, cellophanelike perisarc. 
At the tip of each branch, the covering expands into a cuplike hydro- 
theca, which surrounds the living polyp. As in Hydra, each individual 
polyp of Obelia is hollow and two layered, with a circle of tentacles 
about the raised hypostome, in. which the mouth is located. The 
tentacles are provided with nematocysts that act in the same manner 
as in the Hydra. The food cavity, however, extends down each stalk- 
like branch or individual and is continuous with that of the other 
polyps, thus forming a common gastrovascular cavity in which food 


is digested. There are also cells, as in Hydra, which perform intra- 
cellular digestion. 

Obelia gives rise to another type of polyp than the nutritive individ- 
ual just described. This is the reproductive polyp, or gonangium that 
grows out as a bud, expands into a knoblike central axis known as 
the hlastostylc within a chitinous, closed vase, called the gonotheca. 
On the sides of the blastostyle budlike structures, called medusa buds, 
develop. These break off and swim away as tiny bisexual jellyfish, 
or medusae, representing the sexual stage in the life history. A sperm 
cell from one of these medusae fertilizes an egg from another, which, 
after a developmental period, becomes a free-swimming ciliated larva, 
called a planula. After a short time the planula settles down and 
produces a new asexual colony of Obelia. Other related forms as the 
jellyfish, Aurelia, possess a predominating free-swimming stage, while 
the sessile, non-sexual generation is reduced. 

This life cycle is reminiscent of a similar condition in plants, which 
also have an alternation of generations. During the maturation of the 
sperm and egg cells, reduction division takes place in which the chro- 
mosomes of the sex cells are reduced to half the body number. In 
alternation of generations of plants, all the cells of the gametophytic 
generation are haploid, but as in animals only the mature sex cells are 
haploid, the body cells having the same number of chromosomes as 
the body cells of the sexual generation. The end result accomplished 
in both plants and animals is the same. 


Curtis, W. C., and Guthrie, M. J., Textbook of General Zoology, 2nd ed., 

John Wiley & Sons, Inc., 1933, pp. 278-301. 
Guyer, M. F., Animal Biology, Harper & Bros., 1931, pp. 197-206. 
Hegner, R. W., College Zoology, The Macmillan Co., 1936. Ch. X. 

An authentic description of Hydra and its activities. 



Preview. A typical worm ; external structure of the earthworm {Lum- 
bricus terrestris) ; the digestive tract and its functions ; how blood circulates, 
the blood and its functions ; organs of excretion ; the muscles and their work ; 
reactions to stimuli ; the nervous system and its functions ; the reproductive 
system and reproduction • Regeneration • Suggested readings. 


Passing from the simple two-layered development of the Hydra, in 
which division of labor among the cells is slight, we come to the earth- 
worm, another lowly animal, but one which represents the big idea 
of a typical three-layered, segmented form. 

In Hydra, the egg develops into an adult form having two layers, 
namely, edodertn and endoderm, but in the earthworm, a third 
layer, the mesoderm, appears, w^iich is characteristic of all the higher 
animals. These three germ layers are of great significance in the 
study of animals, for all of the complex tissues of the body are derived 
from them. 

Another reason why the earthworm is chosen for study is because it 
represents a very simple type of segmented or metameric animal of 
which a great variety is found not only among worms but also among 
insects and crustaceans. Judging by the insects, segmented animals 
are the most abundant and successful of all animals, since they out- 
number all other species. The pages that follow will concern them- 
selves chiefly with the "hows and whys" of the activity of the 
common ''night crawler," some of which are: How far has division 
of labor progressed ? What organ systems are well developed ? How 
does co-ordinated movement take place, and how do worms become 
aware of their surroundings ? 

A Typical Worm 
External Structure of the Earthworm (Lumbricus terrestris) 

The body of the earthworm is divided into segmented parts, or 
metameres, which in adult worms may number over one hundred. 
The body tapers bluntly at each end, the anterior end being easily 



distinguished by the rounded mouth which is just ventral to or under 
a small protuberance, the 'prostomium, while the anus, or posterior end 
of the digestive tract, is a tiny slit in the last segment. The posterior 
end is also flattened, and between segments 32 to 37, not counting 
the prostomium enclosing the mouth as the first, there is found a 
swollen region, called the clitellum, important in reproduction. 

The upper or dorsal side may be distinguished by its darker color, 
while the ventral side is slightly flattened and contains four double 
rows of tiny projections called setae, which give the worm a grip on 
the ground when in locomotion. The dorsal side is devoid of any 

The common earthworm, Lunibricus ierresiris. 

Wright Pierce 

Note the swollen area, or clitellum. 

openings except some very minute dorsal pores that communicate 
with the body cavity, or coelom, but the ventral side has several 
paired openings, difficult to find, which lead to the reproductive and 
excretory organs. The surface of the body is covered with a delicate 
iridescent cuticle, secreted by the living epithelial cells of the skin, but 
which is itself dead. Its iridescence is caused by the presence of 
numerous grooves (striae), and its surface is pierced with small holes, 
which are openings for the mucous gland cells of the skin. The coelom 
or body cavity is cut up into small compartments by partition walls, 
or septa, that are absent or incomplete in the extreme anterior region, 
between the 18th and 19th segments, and in the region posterior to 
the reproductive organs. The coelom in the living worm is filled 





also 3,4-.S 

seroinal — 

with fluid which passes from one segment to another through single 
perforations in each of the septa. The fluid contains ameboid cells, 
that probably serve as scavengers, and it acts as blood, bathing and 
nourishing the tissues and carrying away wastes. 

The Digestive Tract and Its Functions 

The food of earthworms, bits of animal or vegetable matter mixed 
with soil, is taken into the mouth by means of suction. A muscular 
pharynx, previously moistened by the fluid poured out from small 
glands in its wall, is able to 
pull the material into the 
esophagus, a thin-walled part 
of the tube which extends from 
the 6th to the 15th segment, 
beside whose walls, between 
segments 10 to 12, there are 
embedded three pairs of whit- 
ish structures, the calciferous 
glands. These glands produce 
a limy secretion supposed to 
neutralize the food materials. 
The esophagus leads into a 
thin-walled crop, occupying 
the 15th and 16th segments, 
which opens into a thick- 
walled, muscular gizzard ex- 
tending over segments 17 and 
18. The latter organ has an 
internal chitinous wall, and is 
probably used to macerate bits 
of undigested food by means 
of muscular contraction. The 
remainder of the food tube, ex- 
tending from the 19th segment 
to the anus, is called the intestine. Its inner surface is increased by a 
fold on the dorsal side (typhlosoJc) , while surrounding it there is a layer 
of yellow-brown tissue cMorogogen cells, which are thought to aid in 
excretion and possibly digestion of food. The wall of the intestine 
contains gland cells that secrete at least three kinds of enzymes, which 
digest starches, fats, and proteins. The digested food is absorbed 






rzerve CorcC 


three ofhen-.-r^-l 
vessels *■ 

The earthworm {Lnmhricns ierrestris) 
opened from dorsal side to show internal 
structure. (After Sedgwick and \\ ilson.) 


through the walls of the intestine, most of it passing into the blood 
and directly into the coelomic fluid, where it may continue to the 
muscular wall outside the coelom. Unusable material, mostly earth, 
is passed off by muscular contraction through the anus, and may often 
be seen on lawns as little piles of "castings." 

How Blood Circulates 

Since in the earthworms there is a very different arrangement than 
in Hydra, where food is directly available to all the cells, we would 
expect to find some means of distributing it to the tissues where it 

may be used. This is accomplished by 
means of a closed system of blood vessels. 
Some idea of the circulation may be derived 
by a study of the accompanying diagrams. 
Five large blood vessels run lengthwise 
through the body, one dorsal vessel, close 
to the food tube, into the walls of which 
it sends two pairs of lateral vessels in each 
segment ; another, the ventral vessel, runs 
just ventral to the digestive tract and also 
sends lateral branches into its wall. There 
are also three others, the paired lateral 
neural vessels and the suhneural vessel, 
which run longitudinally, the latter directly 
under the nerve cord, and two other smaller 
ones lying parallel one on each side and 
above the nerve cord. Five "hearts," so 
called because of their frequent contrac- 
tions, encircle the esophagus in the region 
of the 7th to the 11th segments, connecting 
the dorsal with the ventral vessel. Blood 
passes into the dorsal vessel especially 
from a long typhlosolar vessel which helps 
drain absorbed foods from the intestinal 
walls, flowing forward until it reaches the 
" hearts." Its forward movement is caused 
by slow, regular contractions of the dorsal 
blood vessel. The blood passes posteriorly through the "hearts" 
and then flows into the ventral blood vessel. Here it passes poste- 
riorly, although some of it moves from the hearts toward the 


buccal cavity 
Esuprcicsophatfeol , 
Sub esopho^ol 






- - (trop 

.nsrve ccnet 
■with. IntM-al 
neurai vesssis 


The circulatory system of the 



anterior end of the body. Blood also passes tliroush two intestino- 
integumentary vessels which pass off at the 10th segment to supply 
the walls of the esophagus and the skin, and to nephridia of that 
region. Parietal vessels connect the dorsal and subneural vessels, 

cross Section of typyosolar vessel 


'>_Jat^rccl-y2eu:ral vessel 

V nerve 

The '"hearts" of the earthworm. How do they function in circulation.^ 

that branch from the ventral vessel to supply the body muscle walls 
and nephridia. Blood also passes from the ventral vessel to the body 
walls, and to nephridia, and returns to flow, after passing through 
capillaries, into the lateral neural trunks. In the subneural vessel, 
the blood flows posteriorly and thence up by way of the parietal 
vessels into the dorsal vessel. Both dorsal and ventral vessels supply 
the anterior part of the worm. 

The Blood and Its Functions 

The blood of the earthworm consists of a liquid plasma, carrying 
colorless corpuscles which are flattened spindle-shaped bodies. The 
red color is due to hemoglobin, the same oxygen-carrying substance 
found in the blood of man. But in the earthworm the plasma is 
colored rather than the corpuscles. The exchange of food and 
oxygen, which the blood picks up in the intestine and body walls, 
respectively, occurs in the tiny lymph spaces around the individual 
cells. Respiration takes place through the moist outer membrane 


of the skin, where the oxygen is picked up and combined with the 
hemoglobin, to be later released in the cells of the body where work 
is done. Carbon dioxide and wastes are here taken up by the blood 
and carried back to the skin and to the nephridia or excretory organs. 
One can easily demonstrate the network of tiny capillaries in the skin 
where this exchange takes place. 

Organs of Excretion 

The paired nephridia are essentially coiled tubular organs, made 
up of a ciliated funnel or nephrostome that opens into the coelom, 

a thin ciliated glandular 


like region 

tube, that loops on itself 
about three times, and a 
pore, the ncphridiopore, 
through which the excre- 
tory products pass to the 
exterior. Some excretory 
materials are probably 
taken directly from the 
coelomic fluid by means 
of the currents caused 
by the cilia, while other 
wastes may be taken 
directly from the blood- 
capillaries which cover 
the surface of the glan- 
dular tubules. One characteristic feature of the nephridium is that it 
always passes through the septum separating two segments. 

A nephridium of an earthworm. Trace the 
passage of fluid from the coelom to the exterior of 
the worm. Note the ciliated surface of the neph- 
rostome. What is its function .!^ (After Wolcott.) 

The Muscles and Their Work 

Movement is brought about by muscular contraction. As an 
earthworm crawls, a wave of contraction from the posterior toward 
the anterior appears to move up the body of the worm. A careful 
examination shows that movement is brought about by the contrac- 
tion and relaxation of two opposing groups of muscle fibers and by 
the movement of the rows of setae on the ventral surface. The 
muscles are arranged in two layers just under the skin, an outer 
circular layer running around the body and an inner longitudinal 
layer. When the worm lengthens, the longitudinal muscles relax 



and the circular muscles contract, while a shortening of the worm 
results from a contraction of the longitudinal muscles and a relaxing 
of the circular muscles. Each stiff seta is placed in a little sac, 
from which it extends out beyond the surface of the body. Inside 
the sac, attached to the seta and to the outer body wall, are two pairs 

endocterra ^ 

peritoneum ^^^ 

^Cuticle ectoderm Circtxlar 




/ verztro-l vessel 
lataml vessel 

'wentral rjerve- cord. 
subnsLcral vess-©! 

Cross section through earthworm. Compare this with cross section of Hydra. 
What advances in complexity of structure flo you find:' In the earthworm the 
most noticeable difl'erence is seen in the coelom. which is formed by a sphtting of 
the mesodermal bands in the embryo (seen on page 197). Note that the coeiom 
is completely lined by a delicate membrane, the peritoneum. Notice also the 
longitudinal fold or typhlosole which gives more surface to the inner wall of the 
intestine. What is its function .^ In the diagram, the funnels of the nephridia 
are not shown. Explain why this is so. 

of muscles by means of which the seta can be directed forwards or 
backwards, depending on the direction the worm is traveling. When 
the worm is moving forward, the anterior end is extended, the setae, 
that are pointed backward, are set into the ground, serving as an- 
chors, while the posterior end of the worm is pulled forward by means 
of the contraction of the longitudinal muscles. 


Reactions to Stimuli 

Earthworms live in soil and make burrows which extend from a few 
inches to several feet under ground. They are nocturnal and lie in 
their burrows not far from the surface during the day time, coming 
out at night to forage for food. In winter, they go below the frost 
line, remaining there inactive. In hot and dry weather, they go as 
far down as possible into the earth, while a heavy rain will bring them 
out of their burrows in great numbers. Earthworms react positively 
to mechanical stimuli. A vibration on the earth will send them down 
into their burrows. They are positively attracted to surfaces of solid 
objects, as can be seen if worms are placed on moist blotting paper in 
a covered pan. They will soon be found lying along the edges of the 
pan, where two surfaces are in contact with the body. This response 
to contact apparently keeps them quite constantly in their burrows. 
They react positively to certain chemical substances, like foods, and 
move away from others. A match that has been dipped in ammonia 
and placed near the anterior end of an earthworm will demonstrate 
this reaction. They respond positively to moderate moisture, which 
is needed for respiration through the body covering, and to different 
intensities of light, by withdrawing from bright areas and moving 
toward weak illuminations. Like Hydra, however, reactions to 
stimuli depend largely on the "physiological condition" of the worm, 
that is, upon internal rather than upon external factors. 

The Nervous System and Its Functions 

The earthworm has a simple type of central nervous system con- 
sisting of a ventral nerve cord, with thickenings, called ganglia, in 
each segment, a dorsal "brain" or supraesophageal ganglion, made up 
of two ganglia, and a "ring" of nervous tissue, called the circum- 
esophageal connectives, which extends around the esophagus, connect- 
ing the "brain" with the ventral nerve cord. Lateral nerves, which 
leave the "brain" and cord to end in muscles, skin, and other organs, 
form a peripheral nervous system. The worm does not have visible 
organs of sensation, but the skin, especially at the anterior and 
posterior ends, is dotted with groups of tiny sensory cells. Some of 
these are sensitive to light, and still others probably to odor. Stimuli 
received by these cells are transmitted to the central nervous system 
by means of nerve fibers. Those which lead from the sensory cells 
to the central nervous system are known as afferent fibers, while out- 



going fibers which originate in nerve cells within the cord are known 
as efferent or motor fibers, since they end in muscle cells and stimulate 
them to contract, thus causing motion. The unit over which these 
impulses travel is called a neuron, which is the term given to the nerve 
cell and its prolongations. (See page 340.) In the earthworm sensory 


SerjSory c<=ll5{r<2cepto«) 

■muscle cells 

, ^—Septu.rrL \j 


The nerve cord of the earthworm showing neurons concerned in the reflex 
arc. Explain how adjustment to an unfavorable condition might be affected. 
How might movement in another segment of the worm be co-ordinated with the 
one shown in the diagram.'' (After Curtis and Guthrie.) 

impulses are passed longitudinally, both anteriorly and posteriorly, 
by means of the peripheral nervous system, and these impulses are 
modified by means of adjustor neurons in the central nervous system. 
This accounts for the co-ordination between segments as the worm 
crawls toward a desirable object or suddenly withdraws from a harm- 
ful situation. 

The Reproductive System and Reproduction 

Earthworms have both testes and ovaries in the same animal, 
and are therefore hermaphroditic, but they are not capable of self- 
fertilization. Two pairs of testes lie attached to the anterior walls of 


segments 10 and 11, and are enclosed by the ventral unpouched 
portion of two of the three seminal vesicles. Dorsally the three pairs 
of large pouches of the seminal vesicles in segments 9, 11, and 12 are 
light-colored structures easily seen in a dissection. Immature sperm 
cells are passed from the testes to complete their development in the 
seminal vesicles. Two pairs of vasa efferentia in somites 10 and 11 
fuse to form the paired vas deferens that carry the sperm to the 
exterior through the male openings on segment 15. A pair of tiny 
ovaries are attached to the anterior septum of segment 13, the eggs 

i^..-^ Semirzal rsceptocle 


^^..)... seminal vesicle 


spsrm. duct^ 

Reproductive organs of the earthworm. The seminal vesicles are cut away on 
one side to show the funnels of the sperm ducts. Read your text carefully and 
explain how reproduction takes place. 

passing from this into the oviducts which open to the surface on seg- 
ment 14. Fertilization of the eggs is accomplished by the process of 
copulation in which two worms, placing themselves in opposite 
directions, become "glued" together on their ventral surfaces by 
means of mucus secreted from the glands of the clitellum region. 
While they are thus placed a mutual transfer of sperm cells from the 
seminal vesicles of one worm to the seminal receptacles of the other 
takes place, rhythmic muscular contractions of the body helping to 
force the sperms along. Then the worms separate. Later, when the 
eggs are to be laid, a cocoonlike band of mucus is formed by the clitel-. 
lum, which is forced forward by movements of the worm, and as it 
passes by the oviducal pores, receives the ripe eggs. When it passes 
over the opening of the seminal receptacles on the ventral surface of 







segments 9 and 10, it receives sperm cells from the other worm that 
have been stored there. The girdle is passed down over the anterior 
end of the worm, slipped off, forming a closed case which contains the 
eggs, sperms, and a nutritive fluid. These capsules may be found in 
late spring under stones, 
boards, logs, or in manure 
heaps. After fertilization, 
the egg of the earthworm 
divides first into two, then 
four, then eight cells, and 
so on, continuing until a 
hollow ball of cells, called a 
hlastula, is formed. These 
cells are not all the same 
size, larger cells appearing 
on the lower pole of the 
sphere, which begins to 
flatten and show a depres- 
sion, forming eventually 
a hollow cuplike affair, 
called the gastrula. This 
process known as gastrula- 
tion places the larger cells 
of the lower pole on the in- 
side of the cup where they 
become the endoderm, 
leaving the outer cells of 
the sphere to form the 
ectoderm. Meantime a 
third layer of cells which 
lies between the other two 
layers buds off and be- 
comes the mesoderm. This latter layer gives rise to the musculature, 
blood vessels, and most of the excretory and reproductive tissues ; the 
endoderm forms the food tube and much of the glandular material con- 
nected with it ; the ectoderm gives rise to the epiderms, the nervous 
system and sense organs, and the outer portions of the nephridia, repro- 
ductive ducts, and digestive tracts. The young worms remain in th(^ 
egg case until they are about an inch in length. When first hatched 
they have no clitellum, since this organ appears only in mature worms. 


Stages in development of earthworm. Fig- 
ures II-V. Segmentation of egg and formation 
of blastula. Figures VI-VIII. Sections, show- 
ing formation of mesoderm as a band of cells. 
IX. Late stage of gastrula, showing coelomic 
spaces in mesoderm bands. X. Longitudinal 
section of young worm showing food tube, mouth 
and anus. (After Sedgwick and Wilson.) 



Earthworms, like other members of the lower phyla of the animal 
kingdom, have the ability, under certain conditions, to grow new 
parts. Experiments have been made by Hazen, Morgan, and others 
that show if a sufficient number of segments are present a worm may 
regenerate a new posterior end, or even a new anterior end. Earth- 
worms have even been successfully grafted end to end. 


Curtis, W. C, and Guthrie, M. T., Textbook of General Zoology, 2nd ed., John 

Wiley & Sons, Inc., 1933. 

Excellent chapter on the Annulata, pp. 350 to 375. 
Darwin, Ch., Formation of Vegetable Mould, D. Appleton & Co. 

An easily read classic which ought to be known to every student of 

Hegner, R. W., College Zoology, 4th ed., The Macmillan Co., 1936. 

Chapter XV is a well-written and authentic chapter on the Annulata. 



Preview. The insect body plan; the head and its appendages; the 
thorax and its appendages ; honey manufacture ; digestion ; circulation, 
respiration, and excretion ; the nervous system • Reproduction and life his- 
tory • The life in the hive • Suggested readings. 


It would seem right in a text on biology that a representative of the 
largest and most successful group of animals should be described and 
that more than a passing glance be given to this enormous group, 
which contains far more than half of all living animals. We are 
always meeting insects, because they are so plentiful rather than 
from choice. They annoy us when we are in the woods, they bite 
us when we are lolling on the beach at the seashore, they get into our 
foods and render them unfit for use, or they eat our stored clothes. 
Worse than this, they defoliate trees, and sometimes destroy forests, 
and take their tithe of the nation's food crops. A good many have 
been implicated in the transfer of disease and some have actually 
rendered regions uninhabitable by man. 

Biologists have a good reason for a study of representatives of the 
great phylum, Arthropoda, because the arthropod plan of structure is 
the one employed by the majority of the species of the animal king- 
dom. In its simplest form, it represents an organism made up of 
segments, each body segment bearing a pair of jointed appendages. 
The head always bears at least one pair of jointed antennae or feelers, 
jointed mouth parts, and usually compound eyes. The body is pro- 
tected by an exoskeleton composed of chitin secreted by the cells 
beneath. A digestive tract passes straight through the body and 
there is a nervous system such as we saw in the Annelids, consisting 
of a ventral nerve cord, a dorsal "brain," and a nerve ring about the 
esophagus. Dorsal to the food tube is an elongated heart, there 
being no closed system of blood vessels. Such a simple arthropod 
would be difficult to find for laboratory purposes, so we have to use 
other more specialized forms. 

From the strictly biological point of view there is another reason 
for the study of an insect. It offers an example of a segmented ani- 
H. w. H. — 14 199 


mal that has gone in for specialization in a big way. The insects 
are a subdivision of the Arthropods, animals that have jointed legs 
and jointed bodies, and as such show definite repetition of similar 
parts, or metamerism, a phenomenon previously noted in the Annelids. 
As a group they have become differentiated to such an extent from 
their not so distant relatives that, like the man on the flying tra- 
peze, they ''fly through the air with the greatest of ease." In no 
other group except the birds has this ability been so exploited. In 
addition some forms, such as the bees, ants, and wasps, show an 
astonishingly complex social life. 

As a successful group insects show numerous adaptations, not only 
in structure but in life habits. They are not only active but often 
so inconspicuous as to pass unnoticed by their enemies. Insects are 
characterized by a rapidly growing larval period associated with an 
abundance of food. The protected pupa is characterized by internal 
changes fitting the organism for the active reproductive life of an 
adult. They deserve our careful consideration as a type for study. 

The Insect Body Plan 

Adult insects are readily identified because the body is made up of 
three parts, an anterior head, a mid region or thorax, and a posterior 
region, the abdomen. The body may be further subdivided into 

Wright Pierce 

The large vagrant grasshopper {Schistocerca vaga Scudder) normal size. A typical 
insect. Give all the distinguishing marks of an insect as shown in this photograph. 



segments and has three pairs of jointed thoracic legs. These charac- 
ters distinguish any insect. If you will refer to the "Roll Call" 
you will see that the various orders of insects are distinguished by still 
other characters, such as the presence or absence of different kinds of 
wings, or differences in the structure of the mouth parts, which may be 
modified for various purposes. All insects breathe through tracheal 
tubes and have a body 


upper Up 


rTncuciUotry pcdptxs 





^ TTjcxxilla 



Mouth parts of the locust. 

armor of chitin, a protein 
substance something like 
cow's horn. 

Many zoologists like to 
use a locust or "grass- 
hopper" as a laboratory 
type for study. This is 
because the body parts 
are easy to see and be- 
cause it is a form ha^'ing 
relatively simple mouth 
parts. It is provided 
with two pairs of jaws, a 
forklike pair, the 7?iax- 
illae, and a pair of hard 
toothed jaws, the mandi- 
bles. These parts when 
not in use are covered by two flaps, the upper and lower lips (labrum 
and labiujn). Such mouth parts are found in the bee, although some- 
what modified from the more primitive type seen here. Moreover, 
the locust is a more typical insect because it has three distinct thoracic 
segments, known as the pro-, meso-, and metathorax, and it also has 
a more nearly typical number of abdominal segments, which in most 
insects is ten or eleven. The bee, although not such a typical insect, 
shows so many adaptations, and in addition has so complex a social 
life, that it is selected as a representative of the class Insecta. 

The honey bee {Apis mellifica) forms colonies which include three 
kinds of individuals ; first, workers, bees with undeveloped female sex 
organs, which form by far the largest number in the colony ; second, 
drones, or males ; and third, a queen, or fertile female. An average- 
sized colony of bees may contain from 35,000 to 50,000 workers, 
several hundred drones, and one adult queen. In the following 
description the worker bee is used, unless otherwise specified. 


A study of the accompanying illustration indicates that the bee, 
like other insects, has three body divisions — head, thorax, and abdo- 
men, but instead of the usual three thoracic parts, there are four, 
since one segment from the abdomen becomes fused with the thorax, 

ocellus or- 

Simple^ eye 
Compound eye. 



ccndi othei^ 
mouth ports-- 


Worker bee, lateral view, hairs removed, showing parts of body and appendages 

on left side. (After Snodgrass.) 

leaving only six visible segments in the abdomen. The head bears 
a pair of jointed antennae, or "feelers," large compound eyes, and 
mouth parts much modified from the plan shown by the locust. 
Three pairs of jointed legs and two pairs of membranous wings are 
attached to the thorax, the wings growing out of the meso- and meta- 
thorax. At the posterior end of the abdomen an ovipositor in the 
female is modified in the worker into a sting, which is withdrawn 
inside its sheath within the body when not in use. The body is 
covered with a horny three-layered coat made up of an outer chitiii- 
ous cuticula that covers the entire body except at the joints, where 
it becomes membranous, thus allowing movement ; a middle layer 
of cells called the hypodermis ; and an inner delicate basement mem- 

Protruding from the chitinous covering are many hairs and bristles, 
outgrowths formed by the hypodermis, in which there are several 
kinds of cells, some forming the chitinous coat, others the hairs, and 
still others gland or sensory cells. In some cases the hairs are hollow 
and contain sensory nerve endings. We must picture these animals 
covered with heavy armor, through which sensation is impossible 



except where sensory nerve endings penetrate the armored surface, 
ending in various sense organs such as compound eyes, antennae, 
and sensory hairs. 

.epicCar misl cuticulo. 



structure- of bocCj^ woJl 
(.. hair 


.Cell cf h/podermis 
basement membrane 

msmbrone •/*^^^ 

•chit 11 


g^^hypodermis «o Chitin in fSlcts or at joints 

some, celts form bains 

The body wall and its modifications. The epidermal portion of the body wall 
is composed of a horny substance called chitin, the dermal portion having a some- 
what different chemical nature, like cellulose. In places where movement is 
necessary the chitin is replaced by a flexible membrane. Several types of hairs 
are found, some solid, others hollow, all outgrowths of the exoskeleton. (After 
Snodgrass. ) 

The Head and Its Appendages 

According to the observations of embryologists the head of the 
bee is made up of six segments that are fused together in the adult. 
This statement is based on the well-estabhshed fact that every seg- 
ment in its embryonic condition bears a pair of appendages. Two 
compound eyes, which are very large in the drones, are placed on each 
side of the head, while between them in a triangle on the top and front 
of the head are three simple eyes, or ocelli. Below and between the 
compound eyes are the jointed antennae. The mouth parts consist 
of lahrimi and labium, the latter a complicated structure which con- 
tains the long, flexible Ugula or tongue with a spoonlike labellum used 
by the bee in withdrawing nectar from flowers. Attached to each 
side of the ligula are two jointed labial palps. The base of the labium 
consists of two pieces, the submcntum and mentum. The upper jaws 
or mandibles are on each side of the labrum, while the lower jaws or 
maxillae, with their tiny palps, fit closely and laterally over the men- 
tum. The liquid food, nectar, is first collected by means of the hairs 
on the ligula, the maxillae and labial palps being formed into a tube 
through which the ligula works up and down with a kind of pumping 


motion although the entire labium aids in the process. While feed- 
ing, the flap of the labrum or epipharynx is lowered, making a pas- 
sageway for the nectar to pass into the mouth. Thus the mouth 
parts, which are all present in the locust as separate structures, 

here form a sort of pro- 
boscis, that when not in 
use is folded back under- 
neath the head. 

Bees also feed on solids 
such as pollen and "bee 
sugar," which they mois- 
ten with regurgitated 
honey and saliva before 
swallowing. The mandi- 
bles and maxillae are both 
used in feeding on solids, 
but the chief uses of the 
mandibles are in building 

Bees are well provided 
with sensory structures. 
Experiments by Mclndoo 
and Von Frisch indicate 
that bees can distinguish 
between different-tasting 
substances, for some of 
which they show strong 
preferences. But whether 
they can actually taste or 
whether they distinguish substances by means of a sense of smell 
is difficult to prove. Several experiments have been made that 
prove the presence of a well-developed perception of odor. Among 
the most convincing experiments were those in which Von Frisch 
trained bees to select certain odors, such as oil of orange peel, out 
of 43 other odors. He concludes that not only can bees discover 
feeding places through a sense of smell but they tell other bees of the 
existence of food supplies by means of a "round dance" in which the 
successful bee probably holds the odor of the particular flowers on 
which she has been feeding and disseminates it to the bees that crowd 
around her in the hive. 

Wright Pierce 

Head of worker bee. Anterior view. Com- 
pare this with the accompanying hne drawing 
and identify as many structures as you can. 



Experiments in which the antennae were removed, together with 
evidence from microscopic examinations of the antennae, indicate 
that they hold many of the sense organs which perceive odors. Small 
pits, in which these sensory cells are located, are found on the surface 


-;-simple. eyes 

compcurjct eye 





Simpla eyes 
Compound.- eye^ 





..labial palp 




labial palp 


Ij.... tongue^ (glossal 
i*»-labellu.ra X 

L Head of worker, lateral view, mouth part labeled. H. Head of worker, 
lower view, lower part of proboscis cut away. Compare these mouth parts with 
those of the locust. Which shows the more primitive condition!' 

of the antennae. The queen has about 1600 of these pits on each 
antenna, the workers about 2400, and the drones about 37,800. This 
large number probably makes it possible for the drones to find the 
queen during her nuptial flight, at which time sperm cells are placed 
within her body so as to insure fertilization of the eggs as they are laid. 
The eyes of the bee, as well as those of other insects and crustaceans, 
are compound. This means that they are composed of individual 
units called ommatidia. Each onmiatidium consists of the retinula, 
a group of elongated sensory cells, which encloses a rodlike rhahdom, 
the latter made up of the sensory edges of the retina] cells. At the 
outer edge is a corneal lens, under which is formed a crystalline cone. 
The retinal cells are connected with the optic nerve fibers, the entire 
apparatus being covered with a layer of i)igment cells, so that each 
ommatidium is a unit, and according to experimental evidence, is 


-crystalline lens 

used as a single eye, in conjunction with the several hundred others in 
the compound eye. Such eyes are not very efficient. It is probable 

that they do not have any sharp vision 
for distant objects and not very clear 
vision for near objects. Bees have 
been conditioned to visit boxes of 
different-colored flowers in order to get 
honey, but recent experiments by Lutz 
and others indicate that they are 
guided to flowers by odor rather than 
by color. 

Bees also have a tactile sense which 
comes through tactile hairs on various 

oCistol retir?alar nicclexcs P^rts of the body, these hairs being 

most numerous on the antennae. 

-Cr/stalline cone 

.outer piginent. Cell 

jcorneal pignQenL cell 

.i_retinalcti-- cell 

-Outer pigment/ oall 

The Thorax and Its Appendages 

The entire body of the bee is covered 
with hairs, which indirectly play an 
important part in pollen collection and 
cross pollination, for the bee in rubbing 
against the stamens of a flower gets a 
good deal of pollen on the head and 
back. The thorax is armored and thus 
serves well its purpose as a base for 
the attachment of legs and wings. The 
delicate membranous wings, with their 
ramifying veins and veinlets serving as 
supporting structures, are outgrowths 

appendages. A wing in flight describes 
a figure eight course, its rapid move- 
ments being caused by four pairs of 

The legs have most interesting special adaptations for the several 
trades which the worker bee carries on. It is a typical insect leg, of 
five divisions consisting of a heavy basal coxa, a short piece called 
the trochanter, a long femur which with the adjoining tihia is pro- 
vided with long hairs, and a five-jointed tarsus. The tarsus is 
provided at the tip with a pair of strong claws, between which 


Detail of an ommatidium. 



is found an adliesive organ that enables the animal to hold fast to 
slippery surfaces. 

Each pair of legs bears different structures which are of use in pollen 
gathering and the making of wax. The anterior pair of legs has along 
the anterior margin of the tibia a fringe of short, stiff hairs, eye brushes, 
used for cleaning pollen or other materials from the compound eyes. 



Spina of the 

front ^ 
of vorker 

g.... tibia 


> tarsus 



middle leg' 
of worker 
honey beer- 

„f)dten Ijasket 

hind leg" 
Money bee 


inner .surf a<ie 
of hincC le^ 

These appendages are used for more purposes than locomotion. Find all the 
adaptations shown and give the use of each adaptation to the bee. 

The first joint of the tarsus is provided with long hairs which form a 
pollen brush. This is used to collect pollen grains scattered over the 
hairs of the body. At the base of the first joint of the tarsus is a 
semicircular notch lined with short, stiff bristles, while a flat spur 
projects from the distal end of the tibia. This apparatus is the 
antennae cleaner. To accomplish this function the front leg is ex- 
tended with the notch placed at the base of the antenna, which 
when drawn backward through the notch is effectively cleaned of 

The middle pair of legs is not so highly specialized as the anterior 
pair. There is a large spine near the outer end of the tibia which is 
used as a pick for removing flakes of wax secreted from the wax pockets 
located under the abdomen. The flattened basal segment of the 
tarsus is called the planta. Its hairy surface is used for brushing 
pollen from the body hairs. 

The hind legs are larger and broader tlian the two anterior pairs. 
They carry most of the pollen gathered from flowers to the hive. 
The slightly concave outer surface of the tibia, called the pollen 
basket, is lined by long outward-curving hairs, and may often be seen 


filled with pollen in bees returning to the hive. The inner surface 
of the first tarsal segment, or metatarsus, is covered with rows of stiff 
bristles forming the pollen comb, while the lower edge of the tibia ends 
in a row of spines, called the pecten. The pecten of one leg is scraped 
over the pollen comb of the opposite leg, the pollen thus obtained 
being pushed up into the pollen basket by means of a Uttle projection 
on the upper edge of the metatarsus. 

Honey Manufacture 

Although bees make honey, which is a good energy-releasing food, 
they do not live entirely upon it because of its lack of proteins needed 
for building up the body. Both adults and larvae use pollen mois- 
tened with saliva and honey, which forms "bee bread." Bees suck 
up nectar from flowers, pass it through the esophagus into a thin- 
walled crop, or honey stomach. This organ is an extensile sac which 
when filled holds only a drop or two of fluid so that numerous trips 
to and from the hive are necessary to fill a single cell of honeycomb. 
The gathered nectar remains in the honey stomach until the bee 
returns to the hive, when it is regurgitated and placed in the cells of 
the honeycomb. As honey, it is still too watery, so some of the 
workers, by a rapid vibration of their wings, cause enough water to 
evaporate to bring it to the right consistency. Just before the honey 
is capped in the comb, the worker places a minute amount of formic 
acid from its poison glands in the cell. This aids in the preservation 
of the honey. Bees store somewhat over two pounds of honey a day 
for the average hive. This is in addition to what the adults eat and 
what is fed the young. Honey storage, of course, varies with the 
weather. Bees, like human outdoor laborers, do not work on rainy 

Dr. L. Armbruster of Berlin made some interesting computations 
on the number of visits of bees to flowers necessary to store up about 
two and one half pounds of honey. He found that bees have to visit 
at least 6,000,000 clover heads, as clover honey seems to require the 
most work. Peas, at the bottom of the scale, called for as low as 
80,000 visits from the bees, and other honey-producing plants fell 
within these two limits. Among the most important honey-produc- 
ing plants are white clover, buckwheat, and fruit trees in the East 
and North ; alfalfa, sweet clover, and a few trees, as the tulip tree, in 
the Central West ; the citrus fruits, palmettos, and mangrove in the 
South ; and alfalfa, sages, citrus and other fruit trees in the far West. 



-Salivary glarjcCS 

- - honay Stomach 

Digestion, Circulation, Respiration, and Excretion 

The digestive tract posterior to the crop has to do with the digestion 
of food. The stomach, a large cyhndrical structure, has a valvelike 
arrangement between it and the crop to prevent nectar not used as 
food from going further. It leads into a small intestine, which in 
turn expands to form the 
rectinn at the posterior • 

end of the body. - (^'W'^'^ ■ - pl^^'^yn&al glands 

Attached to the an- x% '^-^A<a#-P°stc«nlbml%iands 

terior end of the intestine 
is a circle of Malpighian 
tubules, about one hun- 
dred in number, named 
after their discoverer, 

Marcello Malpighi, who '^V^^^^^^'^^^-'J^f 
first pictured them in 
his Anatomy of the Silk- 
worm published in 1669. 
The tubules are excre- 
tory in nature, as is 
proven by the fact that 
small crystals of nitrog- 
enous wastes are formed 
in them. 

In the insects and 
crustaceans, there is no 
closed system of blood vessels as was found in the earthworm, but 
in the former there is a well-developed, dorsally placed, tubular 
heart, located in the abdomen and perforated by paired openings, 
or ostia, through which blood enters. Blood is forced out of the 
anterior end into spaces, or sinuses, which in the insects are found 
throughout the body cavity and take the place of blood vessels. 
The heart acts somewhat like a rubber bulb syringe in a pail of 
water, serving, along with the muscular movements of the insect, 
to keep the blood in motion through the blood sinuses. Snod- 
grass ^ shows that there is a rapid and complete circulation of 
blood through the main sinuses, the blood being forced backward 
into the abdomen on the ventral side of the body by the pulsat- 

smdl intestine 

rectal gkncC 

The food tube of worker bee and glands con- 
nected with it. The pharyngeal glands form the 
royal jelly or brood food given to the larvae by the 
workers. The postcerebral glands secrete a fatty 
substance, which is thought to be mixed with wax 
in making honeycomb. The salivary glands are 
true digestive glands. (After Snodgrass.) 

1 Snodgrass, Anatomy and Physiology of the Honey Bee, McGraw-Hill, 1925, pp. 189-190. 



.tracheal tube 

spiracle^ 1— 

ing vibrations of the so-called ventral diaphragm, a sheet of thin 
tissue which is stretched across the ventral part of the abdominal 
cavity, while on the dorsal side it is pumped by the heart toward the 
anterior end of the body. The blood, which bathes all the tissues, 
consists of a plasma, and colorless blood corpuscles or leucocytes. 
The plasma is rich in food substances, but there are no oxygen- 
carrying substances in it, so that insect blood only carries foods and 

Oxygen is brought directly to the tissues by a very efficient type 
of respiratory organ, the so-called tracheae and their branches. Along 
the sides of the thorax and abdomen of insects are found paired 
openings called spiracles. In the worker bee there are ten pairs of 

these openings, three pairs 
in the thorax and seven 
in the abdomen. The 
spiracle is an oval open- 
ing which can be opened 
and closed by means of 
a flat plate attached to 
its rim. Each spiracle 
leads into a tracheal tube, 
the wall of which is 
strengthened by a spiral 
thread of chitin, thus 
keeping the tube filled 
with air. These tubes 
branch again and again 
until they finally end in 
tiny tubules between the 
body cells. Expansion 
and contraction of the 
muscles of the body wall 
force air in and out 
through the tracheae, thus securing circulation of oxygen to all body 
cells. In addition to the tracheae, large air sacs are developed in 
the thorax and abdomen, as are seen in the above diagram. Since 
insects that fly rapidly usually have better developed air sacs than 
those that are sedentary, it is evident that the air sacs must serve 
to "lighten the load" of the body in its flight as a heavier-than-air 

Spiracle 3.. 

spiracle 4....^ 
Spiracle 5... 
Spiracle 6- 
spiracle 7- 

-i|K — anrsac 

-,-tube5 join 
■ dorsal sclcs 

If. .commissure 

A portion of the tracheal system of the worker 
bee. The dorsal trachea and air sacs have been 
removed. Three spiracles are not shown. What 
advantages are there in having this type of re- 
spiratory system? (After Snodgrass.) 



The Nervous System 

The nervous system of the bee is well developed, consisting of a 

series of ganglia, forming a double ventral nerve cord with a dorsal 

cerebral ganglion (brain), 

antennal . 


optic loloe 

to -vin^? 

to leg 

connected by a circum- 
esophageal nerve ring with 
a subesophageal ganglio7i 
directly underneath it. 
Although typically in a 
segmented animal there 
should be one ganglion 
for each segment, we find 
fewer ganglia than seg- 
ments in the adult bee. 
This is because certain of 
the ganglia have fused, 
there being seven in the 
ventral ner^'e cord of the 
bee. From each of these 
nerves efferent fibers ex- 
tend to the muscles while 
afferent fibers from sense 
cells end in the ganglia to 
make up the reflex arc 
previously described (page 
195). Not all co-ordi- 
nation of muscles is controlled by the brain, for a headless bee will 
still walk and experiments have shown that the body ganglia are 
independent centers of control over the appendages. Insects of the 
order Hymenoptera, to which the bee belongs, have the best brain 
development of any of the insects, a fact that seems to be correlated 
with their complex social habits and their keen senses. 

Nervous system of worker bee. Why are the 
ganglia in the thorax so much larger than those 
of the abdomen? Note that the brain is on the 
dorsal side, the esophagus (not shown) passing 
between the two nerves that connect it with the 
first thoracic ganglion. (After Snodgrass.) 

Reproduction and Life History 

Although the workers possess undeveloped ovaries, all the eggs are 
laid by the fertile female or queen. While a worker may live about 
six weeks in summer and never more than a few months, the queen 
lives three or fotir years, or even longer. The ovaries of the queen are 
made up of a number of tubules, in which are eggs in all stages of 


development. The fully developed eggs pass out through the oviduct 
into the vagina, where they are fertilized by sperm cells that were 
placed in a sac called the seminal receptacle by a drone during the nup- 
tial flight of the queen. The drones form the sperm cells in two testes, 
but the sperms are stored in seminal vesicles from which, during mat- 
ing, they are transferred to the seminal receptacles of the queen. 

The queen lays fertilized eggs in honeycomb cells of the worker 
and unfertilized eggs in the larger drone cells. Just how she controls 

the actual fertilization 
cocoon. .W-^ 

of the egg is not known. 
According to Nolan,^ the 
queen produces an 
average of about 900 
eggs a day during the 
season, but may lay as 
many as 2000 a day 
during the period of 
greatest honey making. 
The queen places the 
eggs in the cells by 
means of an ovipositor, 
which in the workers is 
modified into a sting. 
The latter structure is 
made up of two darts, 
closely applied to each other so as to form a tube through which 
poison from a poison sac flows when the darts are forced out of their 
sheath as the bee stings. Two different poisons are produced, one of 
which is formic acid, the other an alkahne substance. Worker bees 
usually die after stinging, as the sting with its attached parts, along 
with some of the intestine, is left in the wound. The queen, which 
also has a sting, uses it only in combat with other queens and does 
not lose her life in its use. 

The life history of the bee is rather brief. Three days after fertiliza- 
tion the egg hatches into a larva which lies in the cell surrounded by 
a plentiful supply of "bee milk," a mixture of digested honey, pollen, 
and saliva. After three days of feeding by the young "nurse" bees, 
the larvae are given more and more undigested food. Drones are 

<^©er2 cell 

Cells of hive of honey bee. Note the stages in 
development of worker. How many kinds of cells 
are shown? (Read page 213.) 

1 Nolan, " Egg-laying Rate of the Queen Bee." Gleanings in Bee Culture, Vol. 52, 1924, pp. 428- 



fed undigested honey and pollen after the fourth day, while young 
queens are fed upon an especially nutritious albuminous "royal 
jelly" until they pupate. During the larval period, the young 
insects grow rapidly, changing their skins or molting several times 
during the process. About the end of the fifth day the larvae are 
given their last food by the attendant bees and the cell is capped with 
wax. Then the larva spins a cocoon, molts for the last time, and 
becomes a pujM. In this stage it begins to assume adult characters 
and, after the next molt, emerges from the cell as an adult. This 
process, in which the insect undergoes certain changes not in line 
with its direct development, is called a metamorphosis. The last 
molt in which the young adult is ready to emerge from the cell takes 
place about 20 days from the time the egg was laid. The young 
adult bee, or imago, chews its way out of the cell, usually emerging 
on the 21st day. The metamorphosis of the drone takes 24 days and 
the queen 16, the greater rapidity of the latter probably being due to 
the more nutritious food received. 

The Life in the Hive 

The activities in a bee hive are numerous and interesting. Besides 
collecting nectar and pollen and making honey, the most important 
work is that of building the wax cells of the comb. Wax is secreted 
by the wax glands on the abdomen and transferred to the mouths of 
the workers, where it is mixed with saliva, kneaded by the mandibles, 
and shaped into the familiar hexagonal cells of the honeycomb. 
Six kinds of cells are made : (a) drone cells, (6) worker cells, (c) queen 
cells, {d) transition cells between worker and drone cells, (<?) attach- 
ment cells which fasten the comb in place, and (/) honey cells. 
Worker bees also bring back propolis or "bee glue," resinous materials 
collected largely from the buds of trees. The propolis is used to fill 
up cracks in the hive and to strengthen the comb. Water is also 
carried to the hive in dry, hot weather. Besides the above activities, 
others must be performed if life in the hive is to go on. The workers 
must have plenty of fresh air, for they do hard work. To this end 
certain of the bees are delegated to the task of vibrating their wings 
rapidly, thus creating currents of air through the hive. Some workers 
rid the hive of excreta, dead bees, or any other substances that 
interfere with its cleanliness. Still other bees guard the entrance of 
the hive against such enemies as bee raoths or yellow-jacket hornets, 
which come to steal honey. 


One other activity is that of swarming. In early summer, the hive 
frequently becomes overcrowded, and when such conditions arise 
several queen cells are built and young queens are raised. When a 
young queen hatches, the old queen gathers together several thousand 
of the workers, who fill their honey stomachs with honey and then set 
out to form a new colony. Sometimes scouts are sent out in advance 
to seek a place for the new hive, which may be in a hollow tree. 
Often the swarm, forming a large ball about the queen, will come to 
rest on the branch of a tree and the beekeeper may then hive it in an 
artificial hive. It is interesting to note that our honey bee {Apis 
meUifica) is an emigrant from Europe and that there are no native 
honey bees in this country. 

This social life with its accompanying division of labor is seen in 
varying degrees all through the order Hymenoptera. Beginning with 
the solitary bees, we find increasing social complexity of life until, in 
the ants, a highly organized group is developed having several different 
kinds of workers, soldiers, and males. If you want fascinating reading 
along this line, look into William Beebe's Jungle Life, or better, into 
Wheeler's masterly volume on Ants. 


Carpenter, G. H., The Biology of Insects, The Macmillan Co., 1928. 

Chapters II, III, IV, V, VII, and IX make interesting reading. 
Fernald, H. T., Applied Entomologij, McGraw-Hill Book Co., 1935. Chs. IV, 

V, and XXXIII. 

A useful book of reference. 
Kellogg, V. L., American Insects, Henry Holt & Co., 1908. 

Still an authentic book of reference. 
Metcalf, C. L., and Flint, W. P., Fundamentals of Insect Life, McGraw-Hill 

Book Co., 1932. Chs. II, IV, and V. 
Plath, 0. E., Bumblebees and Their Ways, The Macmillan Co., 1934. 

A fascinating study of one type of social insect. 
Snodgrass, R. E., Anatomy and Physiology of the Honeybee, McGraw-Hill 

Book Co., 1925. 

Parts of Chapters II, III, and IV are particularly useful, but a student 

can cull much from the entire book. 
WeUs, H. G., Huxley, J. S., Wells, G. P., The Science of Life, Doubleday, 

Doran & Co., 1931. 

Pp. 1147-1182 give one phase of insect life worth reading about. 



Preview. Who qualifies? • Some host-parasite relationships : The host- 
parasite conflict ; effects of a parasitic life ; keeping the cycle going ■ The 
complexity of parasitic relationships : External parasites ; temporary para- 
sites, periodic parasites, pennanent parasites ; internal parasites ; parasites 
requiring one host, parasites requiring two hosts, malaria, parasites requiring 
more than two hosts • Suggested readings. 


According to the definition, a parasite is one that "lives on or 
within, and at the expense of some other organism," and thus might 
include forms from the smallest, such as filtrable viruses and bacteria, 
to some of the largest species. As a matter of fact, parasitism is 
well-nigh universal, for examples are found among nearly all groups 
of plants and animals. 

In many instances there appears to be a remarkable balance 
between the parasite and its host. A dead host is of little use to a 
parasite since it implies a loss of free transportation as well as board 
and lodging. Consequently the existence of a parasite must be a 
compromise, for it must be able to secure enough nourishment to 
maintain and reproduce itself and yet do this either without injuring 
too much the vitality of its host, or actually reducing its own numbers. 
As a result of this rather elaborate compromise parasites have become 
so adapted that they usually destroy only small portions of the host 
tissue which usually can be replaced by regeneration. 

Whenever a parasite reaches a final host, the problem of propaga- 
tion arises. Most parasites produce large numbers of eggs, cysts, or 
spores that are discharged with the waste products of the host. 
Through the medium of food or drink, these reach the next host, which 
is sometimes intermediate or secondary, the parasites thus becoming 
dependent upon the food habits of more than one organism to main- 
tain their cycles. 

Most animal parasites are essentially carnivorous in their feeding 

habits. True carnivores, however, destroy their prey, whereas 

parasites as a rule do not, and while carnivores are much larger than 

their prey, parasites are smaller. Elton says, to summarize, "The 

H. w. H.— 15 21. "S 


difference between the methods of a carnivore and a parasite is simply 
the difference between Hving upon capital and upon income ; between 
the habits of the beaver, which cuts down a whole tree a hundred 
years old, and the bark beetle, which levies a daily toll from the tissues 
of the tree ; between the burglar and the blackmailer. The general 
result is the same, although the methods employed are different." ^ 

Who Qualifies? 

Parasites vary greatly in their types of relationships. Some may 
be classified as either internal or external, according to their location in 
or upon the host. They may be otherwise classified as temporary, or 
free-living during a part of their life cycles ; yermanent, or parasitic 

throughout their life span ; 
and 'periodic, only visit- 
ing their hosts to obtain 
nourishment. Actually 
there are almost as many 
gradations and variations 
in the degree of parasitism 
among animals and plants 
as there are kinds of par- 
asites. Mosquitoes and 
some fleas visit their hosts 
just long enough to sat- 
isfy their appetites. The 
cattle tick, Boophilus an- 
nulatus, never leaves its 
host except when ready to 
lay eggs. Scab mites and 
some lice are permanent, 
living upon the same host 
from one generation to the next, only leaving or being transferred 
by direct contact. In between these extremes occur such well-known 
forms as the hookworm, which has a free-living larval stage, and 
the botflies that pass their larval existence as parasites. 

Among plants, the large and heterogeneous group of bacteria exhibit 
many varieties of parasitism, while higher in the plant scale such forms 
as dodder and broomrape exemplify true parasitism. Other groups 
are partially parasitic during their life cycle. 

■ From Elton, C, Animal Ecology. By permission of The Macmillan Company, 1935. 

Wright Pierce 

Dodder, an example of a plant parasite which 
starts life as a self-respecting plant growing in 



Some Host-Parasite Relationships 

In the event of parasitism, the association is definitely in favor of 
the parasite, since it usually "lives on" the second party concerned, 
the host. Such a relation- 
ship constitutes a fourth 
type of habitat, namely 
parasitic, that is available 
to both plants and ani- 
mals along with the well- 
recognized terrestrial, 
fresh-water, and marine 
habitats. That many or- 
ganisms take advantage 
of this type of existence 
may be clearly proved by 
observing the plants and 
animals of any locality. 

The Host-Parasite 

Theoretically a conflict 
exists between the para- 
site and the host. The 
latter has as its chief 
weapon a lytic or dissolv- 
ing power which is a nor- 
mal physiological reaction. Likewise the weapons that probably 
were first brought by the parasite from its hypothetical free living 
ancestral state must also have been of a lytic, toxic, or otherwise 
destructive nature. In many cases the host seems to have adapted 
itself to bear the burden of parasitism with the least possible 
outlay of energy on its own part, so that eventually there has devel- 
oped a balance between the two organisms, which might be called 
a Jwst-parasitc equilibrium. In order to reach this equilibrium the 
parasite has likewise gradually e^'olved some sort of protectixe device, 
often a capsule which becomes interpolated in the cycle, or an anti- 
enzyme or anticoaguUn to counteract the destructive action of the 
host's secretions, thus necessitating a counter attack upon the part 
of the host. This apparently was made, first, through the o\or- 

W right I'icTCe 

The large masses in this tree represent a true 
plant parasite, the mistletoe. 


development of the host mechanism which elaborates specific pro- 
tective substances called antibodies, and secondly, through "the 
adaptation of certain normally phagocytic cell groups to the intern- 
ment and gradual destruction of the parasite." ^ 

Sometimes the introduction of a parasite has a visible effect upon 
the host. In the case of certain gall insects, such as the Cynipoidea, 
the deposition of an egg by the female in a plant tissue, or the subse- 
quent movement of the larva, furnishes the stimulus which causes 

H^HI^^^^^^^HBf^#i ^ 





iPn ■'■ 


Wright Pierce 

Unopened oak gall beside one which has been opened to show the enclosed 


abnormal proliferation of tissue, resulting in the enclosing of the insect 
larva and the production of a so-called gall. The type of gall pro- 
duced on a given plant appears to be specific, whether it occurs in 
root, leaf, twig, or stem. Usually a gall ceases to grow about the 
time when the enclosed larva finishes feeding. In such instances it 
dries and forms a protective covering inside of which the insect 
pupates, ultimately gnawing its way out. 

Effects of a Parasitic Life 

Parasitism as a biological phenomenon probably has a more far- 
reaching effect upon the structure of the parasite than upon the host. 
In the first place the former no longer has to worry about locomotion 
or the securing of food because these two important functions are 
taken care of by the host. Consequently a gradual simplification 
of the organs of a parasite takes place, until in forms like the tape- 

> Smith, T., Parasitism and Disease, Princeton Univ. Press, 1934, p. 111. 


worm, and the spiny-headed worm, for example, there is no trace 
whatever of a digestive tract in the adult. Such worms, however, 
have access to various digested foods which are ready for absorption 
by the host and it appears certain that these gutless forms must be 
able to absorb and utilize materials from the alimentary canal of 
their benefactor. Other worms, such as the flukes or trematodes, and 
roundworms, possess a well-developed alimentary canal, the secretions 
of which, in some instances at least, cause a liquefaction of the tissue 
in the immediate vicinity of the parasite, thus making it available as 
food for the organism. 

Another problem which parasites have had to solve is that of respira- 
tion. In the case of cellular or blood-inhabiting forms the parasite 
obviously has access to plenty of oxygen, whereas intestinal parasites 
face a difficulty, since the alimentary canal is known to contain little 
oxygen. Many investigators now believe that these worms secure 
their energy from the breakdown of dextrose. This substance results 
from the hydrolysis of more complex carbohydrates and is the form 
in which it is absorbed from the intestine into the blood stream. 
Presumably oxygen is secured during the process of anaerobic fermen- 
tation that results in the splitting of dextrose or glycogen (if the 
carbohydrate has been converted into glycogen during the metabolism 
of the parasite) into fatty acids and carbon dioxide. This type of 
metabolism is characteristic of some bacteria and yeasts. 

One of the most striking effects of the parasitic habit lies in the 
tremendous development of the reproductive capacity of the parasite, 
a process undoubtedly correlated with the numerous hazards which 
must be met if its life cycle is to be completed. The development 
occurs in two ways, — first by the production of enormous numbers 
of eggs, and secondly by the interpolation of asexual stages in the cycle. 
Thus it has been estimated that a single free-swimming, ciliated stage 
{miracidium) of a fluke may be the indirect parent of as many as 
10,000 free-swimming, tailed larvae {cercariae). 

External or ectoparasites also show marked evidence of adaptation to 
their type of existence, as shown by the piercing and sucking mouth 
parts of the parasitically inclined arthropods or the degeneration 
of the mouth parts in the case of the adult botflies, as well as by 
the laterally compressed body of the flea, and the loss of wings in 
lice and bedbugs. Limitation as to the host and as to the location 
on the host shows specialization among this group. These factors 
tend to illustrate stages in the development of ectoparasitism. 


Keeping the Cycle Going 

The chief problem of any species centers about maintaining itself, 
a statement to which there is no exception in the world of parasites. 
Obviously those organisms which have become adapted as ectopara- 
sites are not faced with complicated problems relating to the transfer 
from host to host. By means of simple contact a new host may be 
reached, or if a portion of the life cycle of the parasite is free living, 
it may leave the host to deposit its eggs. Even in cases where the 
eggs are laid among hairs of the host they usually fall to the ground 
to develop. 

A more difficult problem of maintaining the species must be faced 
when internal parasites are involved. Bacteria which are capable of 
producing protective capsules or spores of one sort or another are 
tided over unfavorable periods and so aided in reaching new hosts. 
They are adapted also for rapid reproduction. One worker has esti- 
mated that if the multiplication of bacteria were unchecked one cell 
would be the parent of 281,500,000,000 bacteria in two days. Such a 
mass at the end of the third day would weigh about 148,356,000 

Many parasitic protozoa as well as metazoa are adapted to be 
transferred from one host to the next by means of resistant cysts 
secreted by the organism. Others, like the blood-inhabiting try- 
panosomes or malarial organisms, secure transference by adapting 
themselves to insects which act as wholesale distributors for the 
parasites. Some produce harmful toxins which occasionally kill the 
host. In such instances, however, one may be sure that the host is 
abnormal and the parasites have not become adapted to it. In the 
case of the trypanosomes of man in Africa, antelopes are their natural 
hosts and are quite tolerant to these blood parasites. Since man and 
domestic animals are unnatural hosts, they are consequently much 
more severely affected by them. 

The Complexity of Parasitic Relationships 

The most satisfactory way to secure a general idea of the surpris- 
ingly varied adaptations to a parasitic existence is by a study of 
a few examples. Such a study emphasizes clearly the almost uncanny 
adaptations which have been made by parasites to insure the com- 
pletion of their life cycles. While various types of parasitism clearly 
exist, nevertheless the line that demarks one kind of parasite from 



another may not always be sharply drawn. However, for the sake of 
convenience an attempt will be made to outline briefly a few examples 
of such relationships. 

External Parasites 

External parasites are found throughout the plant and animal king- 
doms. Even among the minute protozoa, ectoparasitic organisms 
occur, such as Cyclochaeta, a parasite on fishes, which may cause an 
appreciable economic 
loss under epidemic 
conditions. The lam- 
prey eel among the 
chordates is a large 
external parasite on 
certain fishes. 

For the sake of con- 
venience, external par- 
asites may be classified 
as to whether they are 
temporary, periodic, or 
permanent. Some 
forms, like the house 
fly, do not really belong 
in any of these cate- 
gories. Yet the house 
fly certainly deserves 
mention, since it serves as a mechanical carrier from one host to 
another for the transfer of numerous bacteria and their spores, as 
well as the cysts and eggs of various other parasites. 

Temporary Parasites. As an example of temporary parasitism 
may be mentioned the parasitic Hymenoptera that lay their eggs 
on the eggs, larvae, or even the adults of other insects. During the 
developmental interlude they remain as true parasites within the 
body of the host until they eventually destroy it, at which time they 
cease their parasitic existence and become free living. The ichneu- 
mon flies, that belong in this group of parasitic Hymenoptera, each 
year attack and destroy great numbers of injurious as well as some 
beneficial insects. Another example of a temporary parasite is the 
ox botfly, the free living adult of which attaches its eggs to hairs on 
the legs of cattle. Upon hatching, the larvae penetrate the hide and 




1^' '^HP 



American Museum of Natural History 

These brook lampreys are close relatives of a 
larger form which frequently attacks fish and remains 
as a temporary external parasite until the host is 
destroyed. What type of mouth is characteristic 
of this group ? 


wander through the underlying tissues of the host until in the spring 
of the year they come to lie beneath the skin, which is soon punctured 
to serve as an air vent. Finally, when the larvae are full grown they 
burrow out, fall to the ground, and there pupate, finally emerging 
as adult free living flies, destined to ruin many million dollars' worth 
of hides annually. 


larva fall? to 
drouncC , pupatss 


becomes j 






hide of 
cattle and ,^^ 

tissue during 
the. vinter 

Life cycle of the ox botfly. 

Periodic Parasites. Other arthropods definitely fall into the 
group of those that are periodically parasitic. Such forms are 
predators, and most of them are blood suckers, in which manner they 
may serve as a link in a chain of parasitism. Thus the female mos- 
quito serves as the carrier for organisms that cause malaria, yellow 
fever, and filariasis. Others like the tick or rat flea may not only 
secure a meal of blood from one host but at the same time be the means 
of transmitting Rocky Mountain spotted fever or bubonic plague to 
some other host. Certain species fall into the realm of parasites by 
their own right, the tick and botfly clearly belonging in this latter 

thp: art of parasitism 


\\ ri(jhi I'iirct 

Longitudinal section showing mistletoe invading 
the tissues of its host. 

Permanent Parasites. Comparatively few organisms belong in 
this category. Some of the flukes with a continuous life cycle like 
the marine Epidella melleni, or the gill fluke, Ancyrocephalus, pass 
their entire cycle upon the 
same host, adding their 
progeny to the same ani- 
mal and so on ad infinitum. 
Similarly, the female head 
louse that cements her 
eggs, or "nits," to human 
hair from which newly 
hatched lice appear within 
six to ten days is another 
example of a permanent 
parasite. The new addi- 
tions to the community of 
head lice must soon feed 
upon the roots of the host's 
hair or else they will die. 

The parasitical mistletoe is practically permanent in habit, since 
it not only taps the life-giving fluids of its host but also lives for 
many years upon the same tree. 

Internal Parasites 

The food cycle plays a vital role in the dispersal of all internal 
animal parasites. It frequently happens that animals which suck 
the juices of plants or the blood of other animals play an important 
part as an intermediate host. It should be borne in mind that when 
carnivores are included in the chain of parasitism, the cycle tends 
toward greater complexity. A few examples will serve to illustrate 
this point. 

Parasites Requiring One Host. The adult hookworm, Necator 
americanus, lives in the small intestine of man, where the adult female 
is attached to the walls of the intestine and produces great numbers 
of eggs which are eliminated from the digestive tract in the early 
developmental stages. Under proper conditions of soil, temperature, 
and moisture, development of the larvae proceeds rapidly, so that 
hatching may take place within 24 hours. The small larval form is 
only about 0.25 mm. in length, but by the end of the third day it has 
nearly doubled in length and soon molts twice, then being in the 


infective stage. Hookworm larvae may enter the body in food or 
drink, but the normal method of entry is by actively boring through 
the skin of the human hand or foot. For this reason the disease is 
called "ground itch" because of the inflamed areas caused by the 

to stomach 

egg passed, 
in fexies 

larva ready'' 
to infect -^ 
human iTost 

Life cycle of the liookworm. 


entrance of the larvae. Once liaving effected entrance into the host, 
the minute worms are passively carried through the blood stream to 
the heart and thence to the lungs, where they migrate out from the 
capillaries of the lungs and work their way through the delicate 
walls of the air sacs into the lung cavities. They next migrate up 


the lung passages over the "saddle" to the esophagus, and there 
are swallowed, reaching the stomach and eventually the intestine. 
Within the next fortnight two more molts occur, after which the 
parasites reach maturity, copulate, produce eggs, and continue the 

The large roundworm, Ascaris lumhricoides, lays eggs which de- 
velop into infective embryos within three weeks under proper con- 
ditions of temperature and moisture. After reaching the digestive 
tract of the host together with food or drink, the newly hatched 
larva burrows through the mucous layer and starts on a "10-day 
tour" following essentially the same itinerary as that of the hook- 

Among the protozoa the Ameba, Endamcha histohjtica, the cause 
of amebic dysentery, is transmitted from one human host to another 
and thence to the outside world, and back again to the human large 
intestine by means of resistant cysts carried in contaminated food 
and drink. 

Parasites Requiring Two Hosts. The dread pork roundworm, 
Trichinclla spiralis, while a permanent parasite having a relatively 
simple life cycle, nevertheless requires two hosts to complete its cycle. 
The encysted larvae occur in a variety of hosts, but are normally 
secured by man through eating insufficiently cooked pork. The 
parasites mature rapidly in the small intestine and reproduce within 
twenty-four to forty-eight hours of their arrival. Each viviparous 
female produces between 10,000 and 15,000 larvae, which are depos- 
ited directly in the lymph or capillaries lining the intestine, and are 
thus circulated by the blood until they reach the voluntary muscles of 
the body. There, these minute roundworms leave the blood stream, 
enter the muscle fibers, where within a month a lemon-shaped cyst is 
deposited about them. Since man is not cannibalistic, the introduc- 
tion of these parasites into his body becomes a blind alley so far as 
completing the life cycle is concerned. Unfortunately, when these 
parasites are once established in the body, there is no way of getting 
rid of them. In due course of time, calcium carbonate is deposited 
about the cyst and eventually the parasite dies, but the obnoxious 
cyst remains to remind the infected person of his injudicious meal 
by frequent muscular pains which may accompany this infection 
for years. The normal hosts of TrichineUa seem to be the rat, 
mouse, and pig. The former are commonly found in numbers about 
slaughter houses and the percentage of their infection is usually 


liigh. A great number of other animals have been experimentally 

Nearly all of the taenioid tapeworms have a rather simple life 
cycle. In the case of the beef and pork tapeworms, for example, the 
infective stage occurs in the flesh of the host in a milky white cyst. 
When this larval tapeworm, or cysticerus, consisting of an inverted 
head or scolex and its outer cyst wall, is ingested by man, the head 

if imppcfparly 
cooked. , pork with 
"^ cysLs In musda. 
f ibens may develop 
vhan eaten 'by 

encysts in pork 
if hog' is "host- 



tacome adults 

in smoll intestine, 

vithin afev dajs- 

■females burro*^ into 

U5C mucosa , depos-it 

over 10,000 larvae^ 

of hog- 

encyst in human 
Yntcscl© if man 
is host 

larvae enter blooeC 
stream , are carried 
to vol untoj'y muscles 

•muscles ^ , . , , 

of xnan which larvae penetrate and then 

The life cycle of Trichinella. 

becomes everted, and then attached to the intestinal wall, where the 
worm starts budding segments or proglottids and soon reaches sexual 
maturity. Proglottids of Taenia saginata, or proglottids together 
with free eggs in the case of T. solium, are passed with the feces and, 
when eaten by the proper intermediate host, develop into cysticerci. 
Cattle, buffalo, giraffes, and llamas may harbor the larval form of the 
beef tapeworm, while the hog, camel, monkey, dog, and man are the 
only known hosts for the pork tapeworm. The chief difference 
between the cycle of these two parasites centers around the possibility 
of auto-infection in the case of the latter. This occurs by ingesting 
the eggs destined for the outside, which hatch in the intestines, 






migrate to the blood stream and so reach various parts of the body, 
there producing cysticerci. As in the case of Trichinella, human 
infection really becomes a blind alley for the parasite. 

Malaria. One of the most economically important parasites is 
the causative organism of malaria, a minute spore-forming protozoan 
of the genus Plasmodium. 
The infective stage, or spo- 
rozoite, reaches the blood 
stream of man in the saliva 
of the mosquito, which is 
poured into the wound im- 
mediately after the victim 
is punctured. This minute 
parasite promptly pen- 
etrates a red corpuscle and 
starts to de\'elop asexually, 
growing until it fills about 
one half of the corpuscle. 
It is now ready to undergo 
the asexual reproductive 
cycle. The chromatin mate- 
rial is gradually separated 
into a number of tiny 
masses, each one of which 
finally becomes surrounded 
by a bit of cytoplasm. 
Growth continues until the 
red corpuscle is filled with e eye e o 

a number of new indi\-iduals called merozoites. Soon the corpuscle 
bursts, liberating these merozoites, each one of which seeks out a 
new corpuscle and begins the asexual cycle all over again. 

This asexual cycle recurs regularly, the intervals depending upon 
the species of parasite infecting the blood stream. Thus in the case 
of tertian malaria, schizogony is completed every twenty-four hours, 
while in the quartan type it takes forty-eight hours to complete it. 
The periodic chills and fever so characteristic of malaria occur at 
the time of the bursting of the red corpuscles with the subsequent 
release of the asexually formed merozoites and the accompanying 
waste matter. Quinine is the most widely used drug to combat the 
infection as it destroys the newly "hatched" merozoites. 



After a number of asexual generations have been produced, special 
larger, sausage-shaped crescents appear within the red corpuscles. 
These are the gametocytes, or sexual forms. If a female mosquito 
sucks blood from a person having mature male and female stages of 

the parasite in the blood, such 
i^a, parasites are taken mto the diges- 

tive tract of the mosquito, where 
union of the male and female 
gametocytes takes place. After 
conjugation the resulting zygote 
forms an ookinete or cyst that 
enters the lining of the stomach 
of the mosquito, in the outer 
walls of which a complicated de- 
velopment then ensues for about 
twelve days, ending with the 
formation of a large number of 
spindle-shaped structures called 
sporozoites. The cyst then bursts 
and the sporozoites migrate to the 
salivary gland of the mosquito. 
After that time, if the female mos- 
quito bites an uninfected human 
host she infects him with the sporo- 
zoites, which enter red blood cells. 
Animals are not the only group 
having complicated parasitic 
cycles. The various smuts, mildews, and rusts are plant parasites 
that annually take their toll throughout the country. Wheat rust is 
probably one of the most destructive of the parasitic fungi. This 
rust has been the most dreaded of plant diseases because it destroys 
the harvest upon which the civilized world is most dependent. Wheat 
rust has long been associated with barberry bushes. As early as 
1760, laws were enacted in New England providing for the destruction 
of barberry bushes near wheat fields, although nothing was actually 
known of the relationship between the barberry and rust until com- 
paratively recent years. It is now known that wheat rust may pass 
part of its life as a parasite on the barberry, whence it migrates to the 
wheat plant and there undergoes a complicated life history. Since 
the nourishment and living matter of the wheat are used as food by 

Diagram of eggs, larva, pupa, and 
adult of Culex (left) and the malarial 
carrying Anopheles (right). How could 
you tell the eggs, larvae, and adults of 
these two genera apart ? 



the parasite, the plant is weakened and Httle or no grain is produced. 
A few of the wheat rusts do not require two hosts but complete their 
life cycle on wheat alone. Such rusts pass the winter by means of 
thick-walled spores which may remain in the stubble or in the ground 
until the young wheat 
plant appears the follow- 
ing year, or the spores are 
carried by the wind from 
other regions. 

Parasites Requiring More 
Than Two Hosts 

Tapeworms show a va- 
riety of adaptations and 
exhibit a unique and deli- 
cate balance that permits 
the completion of their 
various cycles. Roughly 
they may be divided into 
two groups, one in which 
the eggs reach water, sub- 
sequently passing through 
some aquatic organism, 
and a second in which ova 
are scattered in the soil 
and reach the intermedi- 
ate host by means of food 
or drink. In the first 
group are the broad tape- 
worm of man, the bass 
tapeworm, and many 
others, while the second 
includes the various tae- 
nioid worms and their 
relatives. All of these par- 
asites show a remarkable 
degree of specialization. 

N. Y. State Conservaiion Depi. 
The life cycle of the bass tapeworm {P. arnblo- 
plilis). (1) The mature tapeworm occurs in the 
intestines of the large- and small-mouthed hass. 
(2) Contact with water causes the proglottids 
to liberate the eggs which are eaten, (.3) by 
various copepods. \\ hen infected copepods are 
eaten by many species of plankton-feeding fish 
(1) a larval tapeworm (plerocercoid) develops in 
the mesenteries, liver, spleen, or gonads of these 
fish. Heavy infections in the small-mouthed 
bass affect reproduction. The tapeworm reaches 
maturity when fish infected with the larval stage 
are eaten by larger ones. How could this cycle 
be controlled in fish hatcheries!' 

The broad tapeworm of man, Diphyllobothrium latum, was brought 
to this country sometime during the last century by immigrants from 
the shores of the Baltic Sea. The worm matures in 

the digesti\e 


tract of the host, producing a string of as many as 3000 to 4000 seg- 
ments or proglottids, often reaching a length of ten meters. Mature 
proglottids, passed from the host with the feces, must reach water, 
where the eggs are shed. After a developmental period in the water, 
the eggs hatch into free-swimming larvae (coracidia), which to continue 
development must be eaten by a copepod. The parasites penetrate 
the intestinal wall and so reach the body cavity of this host, where 
they develop until the copepod is in turn eaten by a fish, when they 
usually penetrate to the flesh of the host and grow to approximately 
six millimeters in length. Various fishes, such as the northern pike, 
Esox lucius, wall-eyed pike, Stizostedion vitreum, sand pike, S. cana- 
dense griseum, as well as the burbot, Lota maculosa, may all serve 
as second intermediate hosts for this important parasite. Man and 
other carnivores acquire the infection by eating improperly cooked 

The bass tapeworm which matures in large- and small-mouthed 
black bass also requires three hosts — copepods, small fishes which 
carry the larval stage encysted in the viscera, and the final host, or 
adult bass. The life cycle of this parasite illustrates very clearly 
the interdependence of organisms necessary for the completion of the 

Adult yellow grub, enlarged 
from mouth cavity of hejxn« 

•i>;^@ e-Maturtegg ^o^' — "— "^^--tS 

N. Y. State Conservation Dept. 

Diagram of the life cycle of the yellow grub of bass (C. marginatum). (1) The 
adult fluke in buccal cavity. (2-4) Embryo within egg hatches as free living 
miracidium which, upon entering snail, produces a mother sporocyst and two 
generations of rediae (5-8), cercariae (8-9), liberated by the daughter redia, 
penetrate many species of fish (10-11) and mature when eaten by various 
herons (12). 



cycle. The adult tapeworm matures sexually in the spring of the 
year, the mature eggs being shed into shallow water where the fishes 
come inshore to spawn. The eggs of the parasites are soon eaten by 
copepods and the developmental period necessary for the larval 
parasite to reach its second infective stage is closely correlated with 
the time interval between the laying of the bass eggs and the absorp- 
tion of the yolk sac of the bass fry. At the time the young fishes 
begin feeding upon plankton, the copepods in the vicinity of bass 
nests are found to be much more heavily parasitized than at other 
seasons of the year. It is adaptations such as these which enable 
parasites to complete complex life cycles. 

Flukes, or trematodes, probably undergo more complicated cycles 
than any other group of parasites. In considering the complex 
cycle of a trematode one should keep in mind that there are usually 

Diagram explaining the life cycle of endoparasitic trematodes. 

two free-living stages, — the miracidium and the cercaria. The 
variations that may be expected in such a cycle are apparent upon 
inspecting the above diagram. 

The frequent presence of a second intermediate host suggests a 
characteristic of most trematodes. For example, the great blue 

H. W. H. — 16 


heron harbors an adult fluke, Clinostomum marginatum, in its mouth 
cavity. Eggs discharged by the parasite reach the water and soon 
hatch, the miracidia penetrating snails. After several generations 
in the snail, fork-tailed cercariae emerge to penetrate under the 
scales into the flesh and sometimes on the fins of many species 
of fresh-water fish. Here they grow into the typical yellow grubs 
commonly found surrounded by a cyst formed by the connective 
tissue of the host. 

As a result of the above discussion of parasitism it is hoped that 
some concept of the elaborate food chains and interrelationships and 
interdependence characteristic of the various groups of parasites and 
their hosts may be gained. Because these relationships are so com- 
plicated and form so intricately woven a pattern, it becomes prac- 
tically impossible "to predict the precise effects of twitching one 
thread in the fabric." 


Cowdry, E. V., et al., Human Biology and Racial Welfare, P. B. Hoeber, 1930. 

Ch. XVII. 

Popular discussion, resistance, etc., from the bacteriological point of 

Elton, C., Animal Ecology, The Macmillan Co., 1935. Chs. V, VI. 

Excellent readable discussion of parasitism from an ecological view- 
Massee, George, Diseases of Cultivated Plants a>}d Trees, The Macmillan Co., 

1910, pp. 1-23, 59-77. 

A good discussion of parasitic plants. 
Needham, J. G., Frost, S. W., Tothill, B., Leaf-Mining Insects, The Williams 

& WUkins Co., 1928. Ch. I. 

Deals with natural history of group. 
Smith, T., Parasitism and Disease, Princeton University Press, 1934. 

Excellent general, but somewhat technical, discussion of the parasitic 




Preview. Vertebrate cliaracteristics • Skeletons • Invertebrate attempts • 
The vertebrate endoskeleton • Suggested readings. 


How fortunate it is that we are vertebrates, not only vertebrates 
in general but mammalian vertebrates of the royal primate line which 
has blossomed finally into human beings ! 

When one thinks over the myriads of lowly, less endowed animals 
scattered along the devious highways of evolution, who might have 
been our near relatives, it is a real privilege to claim relationship 
with such highly endowed primates as monkeys and apes. With 
the inclusive vertebrate type, to say nothing of the specialized Pri- 
mates, there are certain outstanding structures and qualities which 
we as mankind are thankful to possess. They are so famihar to us, 
however, that we are apt to forget how far our fortunate biological 
heritage is dependent upon them. 

Only a few of these distinctive vertebrate characteristics that give 
us occasion for self-congratulation may be pointed out here. A 
consideration of the Vertebrates as such forms a biological science 
in itself, set forth in a voluminous Hbrary of descriptive and inter- 
pretative books. 

Vertebrate Characteristics 

Even a partial list of the distinctive vertebrate endowments 
would include the following: 1, a highly developed nervous system, 
based upon a hollow dorsally-located nerve cord ; 2, a unique embry- 
onic skeletal axis, called the notochord, which is the foundation for a 
living internal skeleton, adaptable to the changing demands of growth ; 
3, a peculiar kind of blood, that in the higher forms makes the mainte- 
nance of a constant body temperature possible regardless of the sur- 
rounding temperature ; 4, various devices for effectually transporting 
the blood to every part of the body, devices that are as superior to the 
methods employed by non-vertebrates as modern highways and 
means of transportation are better than the conditions encountered 
in the days of the trackless wilderness ; and 5, locomotor organs for 



getting about on land, in water, and in air, far surpassing those 
employed by lower animals. 


Let us consider briefly just one vertebrate feature, namely, the 
living inside skeleton, which gives the name "vertebrate" to the group. 
It is the culmination of an endless array of experiments and adapta- 
tions that have been going on since the beginnings of life on this 
planet, and there is every indication that the end is not yet. The 
skeleton of man, for example, is by no means the final mechanism of 
its kind. There are to be expected in the future other models nearer 
to perfection, though based upon all that has gone before. 

Invertebrate Attempts 

The microscopic protozoa made brave experiments with the idea 
of a skeleton, in their case an armor mostly for protective pur- 
poses and consequently found located on the outside of the animal 
itself. In fact, protection seems to be the primary service of skeletal 
structures in general, although secondarily supplanted largely by the 
function of support and muscle attachment. It still plays an important 
role even in the vertebrates, since the brain and cord, being ex- 
tremely liable to injury, are enclosed within a protective skull and 
enveloping vertebral arches, while the viscera are in part stowed away 
within a bony thoracic basket. 

In the great group of arthropods, that includes both crustaceans 
and insects, the skeleton is plainly a protective external covering 
which, being a lifeless excretion of the skin, does not change in size 
after it has been laid down. As the anima'l grows, the dead inelastic 
skeletal armor thus formed fits more and more tightly over the 
enlarging body until finally it has to crack open in order that the 
animal may emerge and become refitted, after an interval of rapid 
bodily expansion, with a new and larger skeletal garment. This 
complicated process is called molting. To elaborate and then periodi- 
cally to reject all this material is not only a physiologically expensive 
performance but it is also a hazardous one, since while shifting into 
a more commodious suit of armor, the animal may lose a leg or two, 
and is always exposed for some time to enemies while in its defenseless 
shell-less condition. 

Insects, caught in the same evolutionary blind alley with their 
crustacean cousins, have taken an upward step by secreting a much 


thinner chitinous envelope than the more cumbersome "crust" of the 
crustaceans. Instead of molting at repeated intervals throughout 
life, they have hit upon the idea of metamorphosis, whereby they do 
all their molting early during the growing larval stages. Then, as 
adults of established and unchanging size, they live happily ever 
after without being troubled by the inconveniences and perils of 
growth within an unada])tive external encasement. 

Another and paramount objection to a protective exoskcleton is 
the increasing burden of a heavy armor which soon becomes insup- 
portable, necessitating a limit to the size of the body encased within 
it. The largest known representative of the enormous group of the 
insects is probably smaller even than the smallest adult vertebrate. 

The mxoUuscs have gone at the problem of evolving a skeleton in 
another way. Although the skeleton is still on the outside, excreted 
and consequently lifeless, it is never wastefully molted after the 
crustacean fashion. The parsimonious molluscs keep every particle 
of their old dead shells and simply add new layers on the inside, as 
growth demands. The layers, being a little more extensive with each 
addition, form by their edges the familiar "lines of growth" showing 
as parallel ridges on the outside of the shell. This particular experi- 
ment in skeletons, however, has cost the group of molluscs dear, for 
the heavy shell, together with the accompanying policy of passive 
defense, has either impeded the power of locomotion with all attendant 
advantages that would accrue for the evolution of the nervous system, 
or has brought about its complete abandonment. The clams and 
their allies, therefore, have stuck conservatively in the mud and 
lagged behind in the race for life, while other animals without the 
incubus of a molluscan shell have toiled successfully on to higher 
levels of attainment in working out their organic salvation. 

The Vertebrate Endoskeleton 

The vertebrates alone exploit a fundamentally different model in 
skeletal structure. 

An increase in size being necessary for dominance in the struggle for 
existence, an adequate supporting scaffolding for the body is de- 
manded, and as a result the skeletal function of protection now be- 
comes secondary. Levers and muscles to work them to attain 
locomotion, with ample skeletal surface for their attachment, are also in- 
dispensable for animals that are to develop a successful nervous sys- 
tem. The vertebrate skeleton provides for these adaptive advances. 


The fact that the vertebrate skeleton is inside the body makes it 
a changeable living structure which, by reason of its capacity for 
continuous growth, keeps pace with the increasing demands of the 
enlarging organism. With the introduction of such a scheme of 
mechanical support, the ban upon size imposed by a lifeless exoskele- 
ton is lifted, so that during the Age of Mesozoic monsters there were 
dinosaurs and similar beasts, for example, that were able to lift tons 
of flesh into the air upon majestic bony scaffoldings. These prehis- 
toric giants proved impracticably large, however, and vanished 
forever from the face of the earth after recording by means of their 
fossil remains the results of these colossal experiments in the mech- 
anism of living inside skeletons. There still remain today, elephants 
on land and whales in water as living illustrations of how far it is 
possible to go in the matter of size when an adequate living internal 
support is provided. 

The remarkable superiority of the vertebrate endoskeleton over 
all other skeletal devices is evident. It would be possible to go much 
further and to unfold some of the marvelous details of adaptation 
which every separate part of the vertebrate skeleton presents. That 
would call for many pages. It is the task of the comparative anatomist 
to assemble and elucidate the innumerable facts about the vertebrate 
plan of structure, of which those involved in the skeleton are a 
sample, and to point out wherein we are fortunate to be constructed 
as we are. This is a fertile field, inviting intellectual adventure for 
those who have the curiosity to explore it. 


Adams, L. A., An Introduction to the Vertebrates, John Wiley & Sons, Inc., 


A fine text. 
Keith, Sir A., The Engines of the Human Body, J. B. Lippincott Co., 1919. 

Parallels intriguingly worked out for the mechanically minded. 
Neal, H. V., and Rand, H. W. Comparative Anatomy, P. Blakiston's Son 

and Co., 1936. 

Written by two masters of the subject. 
Newman, H. H., Vertebrate Zoology, The Macmillan Co., 1920. 

Just what the title indicates. 
Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 1928. 

Many illustrations. Bibliography. 
Wilder, H. H., History of the Human Body, Henry Holt & Co., 1923. 

Told with literary grace without sacrifice to accuracy. 




Preview. Structure of green plants • The raw food materials used by 
plants • The root and its work • The stem, structure and function • The 
structure of the leaf • How green plants make food ; carbon dioxide as raw 
material ; the role of water ; chlorophyll and light ; relation of artificial 
light to food making ; what goes on in the green leaf in sunlight ; chemistry 
of food making • Enzymes and their work • How food is used by the plant 
body • Respiration • Transpiration • The rise of water in plants • Produc- 
tion of oxygen by plants • Suggested readings. 


It is a trite statement to say that the destiny of man on the earth 
depends upon green plants. All living stuff is made up of the ele- 
ments found in air, in water, and under the earth's surface, yet 
no laboratory technician has ever been able to put this material 
together and make protoplasm. That energy is displayed by plants 
and animals is obvious, but no man has ever been able to energize 
matter and create a living organism. We know that the units of 
structure, the cells, do release energy and that this energy comes, as 
does all other energy, from the oxidation of fuel substances. Such 
fuels used by living things we call foods. Moreover, these foods, 
be they from plant or animal, in the long run depend upon energy 
derived from the sun. The Biblical declaration that ''all flesh is 
grass" is literally true, for without green plants animals would have 
no food. 

We do not think of plants as very dynamic objects compared with 
animals. Nevertheless, if we look at the soil pushed up by growing 
seeds, the pavement broken by the growth of trees, and even the 
hardest rocks split apart by the wedge action of growing stems and 
roots, we realize that plants are very much alive. They respond to 
the various stimuli in the environment, reacting like animals to 
temperature changes, to gravity, to various chemical substances, or 
to the directive force of currents of water. 

Unlike animals, whose metabolism is catabolic, the green plant's 
metabolism is more completely anabolic. It builds up materials to 



a greater extent than it tears them down in the metabolic process. 
Compare the growth of an animal with that of a plant such as a big 
tree. While the animal is more fixed in size and limited in age, the 
tree grows for a longer period of time and grows to a greater size. 
These differences are due to a continuous growth of the meristemic 
tissue already mentioned, and also to the fact that new tissues and 
organs grow continuously from this area of meristem that is found in 
growing buds, stems, and roots. The most important difference 
between green plants and animals lies in the fact that the green plant 
can make use of the sun's energy to manufacture foodstuffs on which 
not only it, but also the animals which eat it, depend. 

In an investigation of a living green plant, two methods of study 
present themselves. We can rather carefully dissect and study each 
system of structures which makes up the living organism and direct 
our attention to the microscopic make-up of each part. In this way 
a fairly complete picture will be had of the organism in its entirety. 
But such a picture will lack vitality. If the plant is a living thing, 
then why not study it from the point of view of function, of what it 
does and how it lives, using only so much reference to the structures 
as will make intelligible the work of the parts of the plant? This is 
the viewpoint adopted for this unit. The plant is to be thought of as 
a living, working organism, performing the same metabolic functions 
as any other organism, but in addition doing a different kind of work 
from that of animals, that of synthesizing organic foodstuffs out of 
chemical raw materials from the air, the soil, and the absorbed water. 
This unit, then, will bring up a number of important points. Among 
them will be such questions as these : What are the adaptations 
which enable the green plant to do its work? Where does the raw 
material from which food is manufactured come from and how does 
the plant get it ? Under what conditions is the work of food manu- 
facture performed? Where is food made and how does it get to the 
cells where work is done? Why is light necessary for green plants 
and why do they bleach in the darkness? Are green plants really 
as important as is here indicated ? These and similar questions will 
be answered in the pages that follow. 

Structure of Green Plants 

It is not easy to give a general description of a green plant. In the 
higher plants it is obvious that there are several well-defined regions 
which are called root, stem, leaves, flowers, and fruits. In these 

Wright Pierce 

(1) Eucalyptus trees, natives of Australia , which have found California so well 
suited to their needs that they are a dominant form there. (2) Rose, shrub showing 
bushy habit. (3) Snapdragon, a common annual. (4) Carrot, a biennial; note 
the food storage in the root. Of what value is this to the plant ? 




e e i 

regions several different methods of growth occur which will be 
described later. Some plants that grow more or less continuously, 
forming a woody body which resists cold and storm, are called trees 
or shrubs. Others die down at the end of the year, although they 
have some wood fiber in the body. These are the herbaceous plants, 
examples of which are peas, beans, and a variety of garden plants and 

roadside weeds. Herbaceous plants 
DiCotyledoiv MoaocotyledoR that produce seeds and die before 

the following winter are called 

A second group of herbaceous 
plants, called biennials, store food 
in the roots or underground portion 
of the stem. After the upper part 
of such a plant is cut down by un- 
favorable weather conditions, the 
following spring the underground 
portions send up a new shoot from 
the subterranean food supply. This 
gives rise to flowers and seeds at 
the close of the second year. Ex- 
amples of biennials are carrots, 
parsnips, and beets. 

A third type of herbaceous plants 
is the perennial, which grows each 
spring from the underground parts 
that remain alive during the winter. 
Many of our common weeds have 
this habit, which makes them 
difficult to eradicate. 

Woody plants, such as trees and 
shrubs, as we have seen in the 
unit on classification, are grouped either as conifers (the softwoods, 
pines, firs, hemlocks, and their relatives) or as deciduous hardwoods. 
The latter are placed with the flowering plants, and may be either 
monocotyledons or dicotyledons. These two groups have differences 
in the structure of leaf, stem, and seed. The monocotyledons usu- 
ally have parallel-veined leaves, like those of grass or lily. Their 
stems have scattered "closed" woody vascular bundles and a single 
cotyledon in the seed. The dicotyledons have netted-veined leaves, 

iS tern 

Differences between monocoty- 
ledons and dicotyledons; c, cotyle- 
don ; e, endosperm ; fb, fibrovascular 
bundles; h, hypocotyl; p, plumule. 


sudh as are seen in the elm, oak, or sassafras; stems with "open" 
vascular bundles which usually appear as a ring of growing tissue; 
and seeds with two cotyledons or seed leaves. These structures will 
be referred to in more detail later. 

The Raw Food Materials Used by Plants 

For a good many centuries after the time of the Greek philosophers 
who first hold this theory, it was thought that green plants absorbed 
food from the soil, but it was not until the time of the Belgian philoso- 
pher van Helmont, who lived in the sixteenth century (1577-1644), 
that it was clear that water played a very important part in the 
growth of a plant. One of van Helmont's experiments consisted of 
placing a willow slip weighing five pounds in a vessel containing two 
hundred pounds of dried soil. For five years he watered the tree with 
distilled water, making careful observations on it until it had grown 
to be a sapling weighing one hundred and sixty-nine pounds and three 
ounces. But when he weighed the soil in which the tree had grown, 
he found it had lost only two ounces. Clearly then, the gain came 
largely from sources other than soil, and he rightly concluded that 
water was largely responsible for the increase in weight. In the first 
half of the eighteenth century, an English clergyman, Stephen Hales, 
worked out the daily water consumption of a plant by ascertaining 
the relation between leaf and stem surface and the quantity of water 
absorbed. He went a step further than van Helmont in suggesting 
that plants take something from the air as well as the soil with which 
to build up their body material. In 1779, Ingen-Housz, a Dutch phy- 
sician, who was a co-worker with the famous surgeon, John Hunter, 
showed that the green part of a plant, when exposed to light, uses the 
free carbon dioxide of the atmosphere, but that it does not have this 
power when kept in darkness. A little later, in 1804, de Saussure, 
by a series of experiments, proved that carbon dioxide and water 
were both used by plants in the sunlight and that as carbon dioxide 
was taken from the atmosphere, about the same amount of oxygen 
was returned to it. He, however, missed the use of the green coloring 
matter of the leaf in its connection with the sun's energy in building 
living matter and food. The real explanation of the function of this 
green substance (chlorophyll) was left for Julius von Sachs, a famous 
botanist of the nineteenth century. He was the first investigator to 
demonstrate the fact that green plants make food for the world. Just 
how they do this is still not fully known, although plant physiologists 


have been experimenting and are still experimenting in the attempt 
to solve the problem. 

With this background, our point of view is to consider the living 
green plant as an organism, faced by the same kinds of problems as a 
living animal, taking a living from its environment, storing up food 
for the inevitable time of food shortage, and eventually forming 
fruits to hold the seeds which are necessary to pass the stream of life 
on to the next generation. Unlike an animal, the green plant takes 
raw food materials from its environment and, under certain favorable 
conditions, synthesizes them into organic foods, a process effected 
by means of a number of adaptive structures, in certain, favorable 
environmental conditions, the chief of which is sunlight. 

By burning the body of a hving plant until nothing but ash remains, 
and then making a careful analysis of this residue, frequently as many 
as thirty chemical elements are found. Twelve are nearly always 
present, eight of which are essential to plant growth. The latter are 
boron, calcium, iron, magnesium, manganese, phosphorus, potassium, 
and sulphur. It will be noticed that this list does not agree exactly 
with the previous list of elements usually found in the protoplasm of 
Hving things (page 131), but the implication is clear. The chemical 
elements found in living matter, as previously noted, are also found 
in rocks or soil, air, and water. The stage is set and it remains for 
the scientist to discover just how these elements, found in the environ- 
ment, can be made into food and living stuff by the green plant. 

A good many experiments have been made with plants to determine 
more exactly the function of these elements. It has been shown 
that if green plants are placed in a nutrient solution containing the 
necessary elements,^ growth will take place. If, however, certain 
elements are subtracted from the solution, the plants will not develop, 
or their growth will be considerably slowed down. Such experiments 
give us our first clue to one important use of the root. It is evidently 
an absorbing organ through which the plant takes in not only water, 
but some of the essential mineral materials necessary for its growth. 

1 A list of the most commonly used nutrient solutions for plant growth are given below. 

Crone's solution : Water, 2.0 1. ; KNO3, 1.0 g. ; FeP04, 0.5 g. ; CaS04, 0.25 g. ; MgSOj, 0.25 g. 

Detmer's solution: Water, 1000 g. ; Ca(N03)2, 1.0 g.; KCl, 0.25 g. ; MgS04, 0.25 g. ; KH2PO4, 
0.25 g. ; FeClj, trace. 

Knop's solution: Water, 1000 g. ; Ca(N03)2, 1.0 g. ; KNO3, 0.25 g. ; KH2PO4, 0.25 g. ; MgS04, 
0.25 g. ; FeP04, trace. 

Pfeffer's solution : Water, 3-7 1. ; Ca(N03)2, 4 g. ; KNO3, 1 g. ; MgS04, 1 g. ; KH2PO4, 1 g. ; KCl, 
0.5 g. ; FeCls, trace. 

Sach's solution : Water, 1000 g. ; KNO3, 1.00 g. ; NaCl, 0.50 g. ; CaS04, 0.50 g. ; MgS04, 0.50 g. ; 
Ca3(P04)2, 0.50 g. ; FeCls, 0.005 g. 



The Root and Its Work 

Recent experiments made by Weaver ^ and others show that plants 
have extremely comphcated root systems. The roots of an old oat 
plant, for example, although extending through only about two cubic 
yards of soil, were found to have a total length of over 450 feet. 
Weaver found that hardy wheat plants sent their rootlets into the 
soil six feet below the surface 

^,CeJ7tml Cylinder- 
-_>v&ocf^ bundle 

-root "hciiT~ 


of the ground. In the bush 
morning-glory, a common 
plant of the mid-western 
plains, the roots may extend 
ten feet into the ground and a 
distance of twenty-five feet 
away from the parent plant. 
The roots of corn extend 
laterally three to four feet 
from the stem and sometimes 
over seven feet into the soil. 
All this is evidence for the 
great importance of the root 
as an absorbing organ. 

Examination of longitudinal 
sections cut from growing 
roots shows that the body of 
a root is made up of a central 
woody cylinder surrounded 
by layers of softer cells, collec- 
tively called the cortex. Over 
the lower end of the root is 
found a collection of cells, 
most of which are dead, ar- 
ranged in the form of a cap 

covering the growing tip. As the root pushes through the soil, the 
outer cells of this root cap are sloughed off, and are rapidly replaced 
by growing cells of meristem that are just under the root cap. The 
root cap proper is evidently a protective adaptation. In the woody 
region of the root are vascular tissues consisting of xijlc77i and phloem. 
These tissues form a series of tubelike structures which together with 

. >■-' J~OOt/ 


Root of a dicotyledon, greatly magnified. 
Find the functions of each part labeled. 
How might soil water get from the outside 
of the plant into the woody bundles.'* 

' Weaver, Root Development of Field Crops, McGraw-Hill Book Co. 



strong supporting woody cells constitute the vascular bundles that 

put the root in connection with the stem and leaves above it. 

If mustard seeds, for example, are germinated in a moist chamber, 

a few days after germination the lower part of the root will be found 
to be covered with a delicate, fuzzy growth. Ex- 
amination of the root at this stage shows an actively 
growing area of meristem, an elongating zone of tissue 
directly back of it, with a zone of maturing tissue 
above, which together make a zone of growth coincid- 
ing more or less directly with an area covered with 
fuzzy structures known as the root hairs. 

Root hairs vary in length according to their posi- 
tion on the root, the longer ones being found at some 
distance from the tip. They are outgrowths of the 
outer layer of epidermis. A single root hair examined 
under the compound microscope is found to be a 
threadlike, almost colorless structure. The delicate 
cellulose wall is lined by the 
protoplasm of the cell, the 
outer layer of which forms a 
selectively permeable mem- 
brane. Inside the root hair 

are found numerous vacuoles filled with cell 

sap. A nucleus is always present and may be 

found in the basal part of the cell, or in the 

hairlike portion itself. The root hairs are 

evidently living epidermal cells. 

An examination of a young plant growing 

in moist soil shows that the root hairs reach 

out between the particles of soil, apparently 

being closely cemented or attached in places 

to them. Each particle of soil is surrounded 

by a delicate film of water, which, with the 

dissolved minerals found in it, is absorbed 

into the root hair by the process of osmosis. 

The wall of the root hair is covered with a 

delicate layer of mucilagelike pecten formed 

by the outer layer of the cell wall and is also 

in contact with the moist protoplasm within 

the cell, which forms a delicate membrane 

Root hairs of 
corn, showing 
their relation to 
the root tip. 

Root hair, showing its 
relation to an epidermal 
cell. How do you account 
for the attachment of the 
soil particles to the surface 
of the root hair ? 


just under the wall. Diffusion takes place following the laws of 
osmosis, according to which water passes through a selectively per- 
meable membrane from a point of its greater to a point of its lesser 
concentration. This means that water passes from the soil into the 
cell sap, which has a higher concentration of solutes than does the 
water. Since the cell sap within the root hair has received a greater 
quantity of water, it in turn becomes a point of higher concentration 
of water than the cells lying next to it interiorly, and consequently, 
the flow continues from these outer cells to the adjoining cells which 
have a higher concentration of solutes. In this manner water is 
passed through the cells of the root i)ito the woody cylinder inside 
the cortex. Once having reached this region it passes up the tubes 
into the stem and on into the leaves as will be shown later. 

The Stem, Structure and Functions 

In thinking of the tree as a li^'ing organism, we are not so much 
concerned with the internal structure of the stem as with the way it 
functions. For many centuries it has been known that water passes 
up through the wood. If a tree is girdled — that is, if a narrow strip 
of bark extending inward as far as the wood is removed — the tree will 
keep its leaves for some time, indicating the upward passage of water 
which keeps them from wilting. If, however, a strip of wood directly 
under the bark is removed, enough of the bark being left intact to 
allow for passage of fluids, the leaves will wilt within a very few mo- 
ments. A cut branch of apple or willow placed in red ink after a few 
hours shows by a red circle, visible in sections cut across the stem, 
that the colored water has passed up through the outer layers of the 
new wood. 

In order to understand better the pathways for the rise of sap in 
the dicotyledon stem, one must study its growth. When seen in 
cross section, the vascular tissues of such stems are arranged in a circle. 
In some herbaceous stems, the woody bundles are separated by a 
parenchyma, but in trees, shrubs, and a good many herbs, the bundles 
are united to form a complete ring around the stem. These vascular 
bundles are open at each side and grow more or less continuously 
from a single row of meristem or embryonic cells which form a layer 
around the stem. This layer is called the cambium, and the growth 
of the wood and bark of our large trees is due to the activity of this 
always youthful layer of cells which, like the cells of embryonic tissue, 
continually divide and multiply to form internally the xylem or wood 



and externally the phloem tissue. In the spring when this tissue is 
very active, it forms a soft layer of cells that allows of the easy sepa- 
ration of bark from wood, a fact well known to any small boy who 
has made a willow whistle. 

It is not necessary to go into the details of stem structure, except 
to note that the cambium layer gives rise each year to new layers of 

Cross section of stem of Ricinus communis, a dicotyledon, showing cambium 
ring. In what area of the diagram does growth take place ? 

tissues, both internally and externally. The inner layers made up 
of secondary xylem are from the annual rings of a tree. In spring 
the growth of the tissue is rapid, while in winter it is very slow indeed 
or stops entirely, thus making the differences in the cross section 
shown in the figure. As the tree ages, changes may be noticed in the 
appearance of the older woody area forming the interior of the trunk. 
This wood becomes darker in color, its chemical composition changes, 
and it loses its ability to conduct water. It is known as the heart- 
wood as distinguished from the outer rings of wood called sap-wood. 



The latter conducts water, while the heart-wood functions merely as 
a supporting tissue. As the tree increases in diameter, the area of 




Section through a dicotyledonous stem. Explain its method of growth. 

heart-wood increases while the sap-wood, although greater in cir- 
cumference, gets proportionately smaller in extent. 

The bark, or area outside the cambium, is made up of several 
different tissues, which have a somewhat different 
arrangement in conifers than in deciduous trees. The 
area known as phloem is formed immediately outside 
the cambium. This area contains many living sieve 
tubes through which elaborated food is carried down 
from the upper part of the plant. The sieve tubes 
in the conifers are more or less regular in arrangement 
while in deciduous trees they are scattered. In both 
stems they are all surrounded by parenchyma. 
Scattered through the bark of deciduous trees are 
masses of tough, stringy schlercnchyma cells of two 
types, phloem fibers — fibrous, elongated cells that 
give strength and elasticity to the trunk — and thick- 
walled, hard stone cells. Outside the latter area is 
formed the corky layer, produced by a layer of growing 
cells known as the cork cambium. Cork cells, which 
have their walls impregnated with an insulating sub- 
stance called suherin, are of great value to the tree 
because they prevent a rapid loss of water from the 
tissues. It is this layer in the Spanish cork oak which 
is of commercial value. In some trees, such as the 
redwoods, the bark forms a coating highly resistant 
to fire. 

H. w. H. — 17 

Above, sieve 
\ (' s s e 1 (of 
phloem) with 
c()mf>anion cell; 
below, sieve 
plate, with 
section of com- 
panion cell. 
(After Stras- 



Wright Pierce 

The characteristic lenticels of the white birch 
{Betiila populi folia). Note the placement of the 

Scattered over the surface of twigs and young tree trunks are 

found many lenticels, openings in the corky layer which become filled 

with loose masses of cells. 
They are found both on 
roots and stems and act 
as pores which allow for 
the exchange of gases be- 
tween the living cells of 
the cortex and the me- 
dium outside. Lenticels 
are often spoken of as 
"breathing jDores" and 
experimental evidence 
seems to make this title 

As the stem or trunk 
of a tree grows larger in 
diameter, there is an in- 
creasing area that uses 

water and foods. Cells cannot grow without food, and food in a 

growing plant cannot be made without water. The structures which 

put the water-conducting tissue of 

the inside of the stem in connection 

with the phloem of its outer part 

are known as vascular rays. They 

may be seen in almost any cross 

section of a tree which has 

produced secondary xylem and 

phloem. Here the cambium has 

rows of irregularly placed cells 

that instead of forming xylem and 

phloem produce ingrowing masses 

of more uniform parenchymatous 

cells making vertically placed 

strings of tissue. These bands 

act as conducting pathways 

for water from the xylem to 

the phloem and also as chan- 
nels for elaborated food from the phloem to the xylem, thus dis- 
tributing these materials to the growing trunk. Experiments by 




\ — •^yiein 


Note the bands of living parenchym- 
atous tissue that grow inward toward 
the pith. 



Aiichtor ^ have shown that food and water are not transferred from 
one side of a tree to the other, but instead that ahnost all of the 
water taken in is used directly above where it is absorbed, while 
food passes down from the leaves on the same side of the tree. There 
is seemingly little cross transfer of food or water in a plant stem. 

Vascular rays must not be confused with the so-called pith rays 
which are formed in herbaceous stems such as Ranunculus or in the 
stem of Clematis where, as the primary wood bundles grow in the pith, 
the pith forms narrow plates between the bundles. These appear as 
the pith rays in a cross section. 

Conditions of growth upon which the passage of food and water 
depend differ in monocotyledons from those in dicotyledons. If a 
stalk of celery or asparagus is placed in red ink over night, the color 
is seen to be located in little fibrous bundles of tissue which are scat- 
tered throughout the stem. If such a stained stem is examined in 
cross section under the microscope, it is found to be made up of pa- 
renchyma or pith which is dotted with little groups of woody cells 
of irregular size and shape. These are the vascular bundles which, 

Transverse section of stem of corn, a monocotyledon, showhiK the " scattered " 
vascular bundles which are cut in cross section. 

■ Auchter, E. C. 
in Woody Plants?' 

" Is There Normally a Cross Transfer of Foods, Water, and Mineral Nutrients 
Univ. Maryland .\gric. Exp. Station, Bull. 251, Sept. 1923. 



instead of being located in a ring as in the dicotyledons, are scattered 
through the pith although more concentrated toward the outer edge of 
the stem. Examination of this outer edge or rind shows that there 
is no true bark, but that this outer area is made up of these same 
woody bundles closely massed together. Under high power, the 
bundles are seen to have outer strengthened walls of wood cells 

enclosing tubelike cells of 
different diameters of 
which the larger have 
pitted surfaces. The area 
containing these tubes is 
the xylem. Other elon- 
gated tubular cells having 
their ends perforated with 
small holes like a sieve, 
form the sieve tubes, 
w^hich are the conducting 
tissues of the phloem. In 
the phloem, the tubes pass 
foods down from the 
leaves, while the xylem 

A cross section through a closed monocotyle- carries water up from the 
donous bundle. Note that the thick-walled roots to the leaves. The 
xylem cells completely enclose the cells of the entire WOody bundle is en- 

^ °^™' closed w^th a tough wall of 

sclerenchyma which gives strength and resiliency to the stem. Since 
this hard tissue binds the entire bundle, it is called a closed bundle. 
Monocotyledonous stems grow, then, through an increase and 
lengthening of closed bundles in the parenchyma of the stem. 

The end result in both monocotyledon and dicotyledon stems is the 
same. The vascular bundles put the root, stem, and leaves in direct 
communication. The root hairs at one end and the cells of the leaf 
at the other end are the opposite terminals of long communicating 
woody tubes. These tubes carry water and solutes up from the soil 
to the cells of the leaf, and, as will be shown presently, carry elaborated 
food materials down from the leaves to various parts of the plant, 
where they may be stored for future consumption or used immediately 
to liberate the energy needed in growth and in destructive metabolic 
changes. The vascular bundles which leave the stem to enter the 
leaves do so by way of the petiole or leaf stalk. As they enter the blade 



of the leaf, they branch into bundles of ever smaller and smaller 
diameter to form the veins of the leaf. In the monocotyledonous 
leaves, these veins run in a more or less parallel direction as seen in 
grass blades or palm leaves. In the case of the dicotyledonous plants 
characteristic irregular and netted veins 
are found, reminding one of the branch- 
ing of the capillaries in the human body. 
These veins are made up structurally 
of tracheids and tracheal vessels, ser\'ing 
as water-conducting tissues ; sieve tubes, 
which carry out food materials from the 
leaf; and supporting tissue, which 
makes up the mechanical framework of 
the veins. Thus the veins act as a sup- 
porting skeleton for the leaf as well as 

The Structure of the Leaf 

The outer covering of the leaf (epi- 
dermis) is composed of a layer of 
irregularly shaped cells, usually rather 
flattened. In some plants, like the 
mullein, these cells are prolonged into 
hairs, or again the layer, as a whole, is 
frequently covered with a waxy cuticle 
which is impermeable to gases and 
water. The under surface of the leaf, 
as seen through the compound micro- 
scope, shows many tiny oval openings, 
which are called stomata. The position 
of the stomata varies in different leaves. 
Some plants, as, for example, water 
lilies, whose leaves float on the surface 
of the water, have them in the upper 
epidermis. Others have them on the 
under .side, and .still others have them 
on both surfaces. The estimated num- 
ber of these openings varies. Mac- 
Dougal estimates that as many as two 
million are on the under surface of an 

Stomata from the loaf of an 
Easter lily (Lilium lonyiflorum) : 
Above, a stoma, as seen in sur- 
face view, showing the two 
kidney-shaped guard cells {g), 
which enclose the stomatal aper- 
ture (s), the more deeply shaded 
portion representing the central 
slit ; note the chloroplasts in the 
guard cells; (b) subsidiary cells. 
Below, a stoma, as seen in cross 
section ; note the guard cell (g) 
next to the subsidiary cell (6) ; the 
outer slit (o) is enclosed between 
the cutinized outer guard-cell 
ridges (r), the enlarged area just 
below being the outer vestibule 
(o') ; below the central slit (s) is 
the inner vestibule (('), which 
here opens directly into the 
cavity (c) underneath the stoma. 



oak leaf of ordinary size, while four or five hundred thousand to a 
leaf is a common estimate. Surrounding the opening of each stoma 
are found two kidney-shaped cells, the guard cells, which can easily 
change their shape under certain conditions. They are of great 
importance in the life of the plant, since they control to a great 
extent the amount of moisture that may be lost from the leaf's sur- 
face. The guard cells are noticeably greener than the epidermal cells, 
the color being due to many tiny green chloroplasts. 

If the leaf is cut in cross section and examined under the microscope, 
it will be found to be made up largely of a tissue known as mesophyll. 

Lying close to the epi- 
dermis are one or two 
layers of elongated cells 
with the long axis placed 
at right angles to the sur- 
face of the leaf. These 
layers of cells are collec- 
tively called the palisade 
layer. Each cell of this 
layer contains numerous 
chloroplasts which are 
found in the protoplasm 
close to the cell wall. It 
has been estimated that a 
square inch of a sunflower 
leaf contains as many as 
thirty million of these 
chloroplasts, which are 
most important structures 
in the plant so far as food 
making is concerned. 
Below the palisade layer 
is a layer of numerous irregular cells containing fewer chloroplasts. 
These cells are known collectively as the spongy parenchyma. Be- 
tween them are found air spaces connected with the exterior of the 
leaf through the stomata. We have already noted that the veins 
form the framework of the leaf and in a cross section are often found 
occupying part of the area of spongy parenchyma. These veins 
connect the vascular tissue of the root and stem with the leaf. The 
petiole, or leaf stalk, is made up largely of vascular and supporting 

Cross section through a leaf; e, upper epider- 
mis, e', lower epidermis, showing stomata (s) ; 
I, intercellular spaces in the spongy parenchyma. 
Note the cross section of the vein (v). Why is 
the palisade layer (p) so placed ? 



woody tissue. At one point on the petiole, usually close to the main 
stalk, a little time before the leaves drop from deciduous trees in the 
fall, a layer of delicate, thin-walled cells is formed which extends 
completely across the petiole. This is called the separation or ab- 
scission layer, and it is at this point that the leaf is cast off. 

How Green Plants Make Food 

The general biologist is concerned not so much with the structure 
of the organism or with detailed minutiae as with the general 
metabolism of an organism as a whole. He wants to know how plants 
and animals act as living things, both alone and in relation to each 
other. We have examined the green plant from the standpoint of 
structure and are ready to consider it as an organic whole, as a living 
organism that releases en- 

leof on live plant 
+ light- 


■— r^ in -^oodi 
■\>^ alcoViol 

positive reaction 
"where sta.r-cVi was 

ergy, respires, feeds, repro- 
duces, and in time dies. 
But we must remember that 
in addition, the green plant 
makes food, and it is this 
process upon which we will 
now focus our attention. 

It is a relatively simple 
matter to prove that sun- 
light is necessary for starch 
making in a leaf. Place a 
healthy green plant in dark- 
ness for a couple of clays. 
Then pin strips of black 
cloth over parts of some of the leaves and expose the plant to bright 
sunlight for a few hours. Later, remove the leaves and boil them to 
soften the tissues, adding alcohol to extract the chlorophyll, and 
finally, place them in a solution of iodine. A blue color will appear in 
those parts of the leaves exposed to sunlight, while the covered areas 
will be colorless. The appearance of the blue color in the presence of 
iodine is the regular test for starch, thus showing clearly that sunlight 
is necessary for starch making. 

Another simple experiment may be performed to show that air is 
also a necessary factor. Place a healthy green plant in darkness for 
two or three days, then carefully smear vaselin(> on th(> ui)i)(>r and 
lower surface of two or three l(?aves, leaving the others uiitoiiclicd. 

Proof that light is necessary for starch forma- 
tion in green leaves. 


Place the plant in full sunlight for a few hours, then remove the 
vaselined and untouched leaves, and treat both in the manner de- 
scribed in the last experiment. The leaves to which no air penetrated 
will be shown to have no starch. 

The need of carbon dioxide in the process of starch making may also 
be demonstrated by a relatively simple experiment. If plants are 
grown under similar conditions in two bell jars, but in one case the 
carbon dioxide in the atmosphere is removed by means of soda lime, 
while the other plant is left in the bell jar containing normal air, the 
latter continues to grow while the one lacking carbon dioxide does 
not increase in size. 

By burning a plant in a hot flame, it can be ultimately reduced to 
mineral ash equaling about 4 to 5 per cent of the entire weight. Ac- 
cording to Raber, from 1 to 55 per cent of the plant is consumed, 
while from 40 to 95 per cent, roughly speaking, consists of water. 
Since a green plant is immobile and since it has no way of obtaining 
material except from the air, water, and the soil that surrounds it, 
it may be safely assumed that if food is found in the plant body, 
it must be made there. That foods are found in plants is common 
knowledge. We eat roots, stems, fruits, and leaves of plants. Grains 
form our staples of food. Roots and various types of fruits form 
part of our dietary, while herbivorous animals live upon grasses and 
fodder crops. This brings us then to the sources of the raw 
materials out of which these elaborated foods must be formed. 

Carbon Dioxide as Raw Material 

Carbon dioxide is not only a product of respiration of animals but of 
plants as well. A man gives off about nine hundred grams of carbon 
dioxide daily into the air. Carbon dioxide also gets into the air from 
the combustion of inflammable materials. Volcanic eruptions and 
other sources of combustion increase the amount, while decaying 
organisms and the oxidation of rocks and soils add a very appreciable 
amount daily to the store. While it is estimated that there are only 
two grams of carbon in each ten liters of air, nevertheless the fact 
that carbon dioxide is universally available in the air and oceans close 
to the surface of the earth shows that it may readily be made use of 
by growing plants. Its need in food manufacture is well illustrated 
by the statement that the world crop of wheat requires annually one 
hundred and fourteen million tons of carbon dioxide in order to pro- 
duce the seventy million tons of carbohydrates which form this crop. 

Wrijjla Pierce 

The role of water. Upper photograph : The Mohave River near Victorville. 
This river rises in the San Bernardino Mountains and loses itself in a desert sink. 
What effect does it have upon the desert .►> 

Lower photograph: An irrigated orange ranch in the desert near Clareniont, 
California. Thousands of acres of trees now grow where desert conditions 
existed before irrigation. 



The Role of Water 

Water as a raw material needs little mention. The soil always con- 
tains more or less water, and the original source of water in its cycle 
through the oceans, the air, the clouds, and rain gives the earth a 
never ending water supply. When mm aids Nature in carrying 
water to dry areas by irrigation the desert literally is made "to blos- 
som as the rose." Certain chemical elements find their way into the 
plant body with this water. If the green plant is to manufacture 
organic food substances, it is evident that the elements carbon, oxy- 
gen, and hydrogen must come from the water and air. Various 
mineral salts, taken in by the root, furnish the necessary amounts of 
calcium, iron, potassium, sodium, and other elements, which leaves 
only nitrogen to be accounted for. Although nitrogen makes up 
approximately four fifths of the atmosphere, it is nevertheless unusable 
in that free form. It is an extremely inert gas and does not unite 
readily in combination with other substances. By means of the proc- 
ess of decay, however, and particularly through the nitrogen-fixing 
bacteria found on the roots of certain types of plants, this highly im- 
portant element is made available to plants. So much for the raw 
materials. Now let us turn to the machinery of food manufacture. 

Chlorophyll and Light 

Common observation shows that there is a relation between light 
and the green color of plants. We are familiar with the bleaching of 
celery stalks, with the curious blanched elongated shoots of a potato 
which sprouts in darkness, and with the fact that young seedlings are 
devoid of chlorophyll until after they have sprouted. Seedlings 
grown with light coming from one side turn to the source of light, while 
plants grown in a dark box having a hole on one side work their way 
toward the light. Obviously light has a very potent effect on the 

Sunlight passed through a prism is broken up into seven primary 
colors ranging from violet to red, but passed through a spectroscope 
shows numerous dark lines traversing different areas in the spectro- 
prism. The most conspicuous are used as landmarks by physicists 
and for convenience have been designated by the letters A to H by 
Fraunhofer, their discoverer. These several wave lengths of light can 
be measured and it has been fovmd that they vary from 0.00076 mm. 
at the red end of the spectrum to 0.00039 mm. at the violet end. 


Rays of greater and shorter length are also found at eaeh end of the 
spectrum forming the ultraviolet and infrared portions. The heat 
of light rays varies, Ijcing greater at the r(>d end of the spectrum. 
Since all life depends upon this I'adiant energy whose source is the 

1 z ^ 4 I n m EA 

When a green leaf is placed in the path of light passing through a {)risni. dark 
strips appear, due to the partial or conipleh^ blocking of the light energy. These 
are shown in the absorption spectra above. .4, chlorophyll of Alliumiirsi- 
mim in alcohol; B, chlorophyll of English ivy {Iledera helix) in alcohol; 
C, chlorophyll of OscUlatoria in alcohol; D, carotin. 1, 2, 3, 4. absorption bands 
of chlorophyll; /, //, III. absorption bands of carotin; EA, end absorption. 
The lettered broken lines mark the position of the principal absorption hnes of the 
solar spectrum (Fraunhofer lines); the numbered solid lines form a scale from 
which wave lengths (X) in nullionths of a millimeter may be found by adding 
a cipher; note the increasing dispersion from left (red) 1o right (violet). 
(After Kohl.) 

sun, the green plant is no exception to this rule. Certain parts of 
the plant, however, are more susceptii)le than other portions to ra- 
diant energy. While the green leaf as a whole needs sunlight, it is 
only chlorophyll in the chloroplasts that is al:)le to utilize it for food 

If a chloroplast is examined under a very high magnification of the 
microscope, it is found to be a mass of living matter somewhat 
denser than the protoplasm surrounding it. In its disk-shaped struc- 
ture the green coloring matter is arranged around the outer part of 
the chloroplast, while the central portion usually contains a clear area 



filled with fluid. Chlorophyll is a very complex protein, apparently 
made up of two substances known as Chlorophyll A, having the chemi- 
cal formula C55H7205N4Mg, and Chlorophyll B, C55H7o06N4Mg. 
It is found to be somewhat like the hemoglobin of the human blood 
except that it has an atom of magnesium instead of iron and the 
property of fluorescence, its color being different in transmitted or 
reflected light. Chlorophyll in solution, when extracted from the 
leaf by means of alcohol, appears green as light passes through it, but 
red when light is reflected from it against a black background. Other 
pigments are closely associated with chlorophyll, a group of yellow 

pigments called carotins, 
which give the yellow 
color to carrots and other 
fruits or vegetables, and 
xmithophylls, pigments 
that help give color to 
leaves in the fall. 

Numerous experiments 
have been made to dis- 
cover how chlorophyll 
does its work. It has 
been found that if light is 
passed through this sub- 
stance and then broken 
up by a prism, that part 
of the light which is 
absorbed by the chlorophyll may be detected by the presence of 
absorption bands in the spectrum where the chlorophyll has taken 
out the light. By this means we learn that the red band of the spec- 
trum is most active while parts of the blue, violet, and indigo regions 
of the spectrum are also absorbed. A classic experiment by Engel- 
mann illustrates this in another w^ay. A filament of an alga was 
placed in a culture of bacteria which were active only in the pres- 
ence of oxygen. The filament was then put in darkness until the 
bacteria had used up all the oxygen present. Then the slide con- 
taining filament and bacteria was placed on a microscope under a 
solar spectrum. In a short time the bacteria were found to mass 
themselves in abundance at the red end of the spectrum and to a lesser 
extent at the blue end, because at these points more oxygen was 
given off by the alga, thus indicating activity in starch formation. 


3 C 




D F 







■'«■- •-•'.■'..•?-'- 

^/A_ _ _ 

: •.•.-'•* .■-*■.■.'.■?... . 


1 ^ 


■■^.■'■'•'.•.■■-- X-' 







•' ''Z^'J^' 


Engelmann's experiment to show the areas in 
the spectrum most favorable for oxygen release 
in a green alga. The dots represent bacteria. 



Relation of Artificial Light to Food Making 

We have already noted that there are great differences in the 
amount of sunhght required by plants. As a matter of fact, very 
strong sunlight may cause harm since it overheats the protoplasm, 
thus endangering the life of the plant. Moreover, it increases the 
rate of transpiration so that water is evaporated too rapidly. Experi- 
mental evidence with growing plants shows also that too much sun- 
light may retard growth. Some plants are shade loving, as may be 

Shade loving plants on a forest floor. Note the leaf arrangement with reference 

to light. 

seen in any field trip to a forest. The differences in illumination are 
correlated with differences in the structure of the leaf, the ])lants 
which are exposed to bright sunlight having a well developed palisade 
layer, while the spongy parenchyma is not so well developed. The 
reverse is true in shade-loving plants. In addition, plants that live 
in the shade are apt to have a very thin epidermis and usually ha\-e 
dull leaf surfaces which do not reflect the light as reatlily. 

Contrary to common belief, it is possible to grow i:)lants without 
sunlight as pro^'ed by recent experiments (Harvey) with a large 
number of different crop plants such as grains, tomatoes, squash, 
peas, potatoes, and others. Plants exposed continuously to the light 



of nitrogen-filled tungsten lamps of from 200 to 1000 watts produced 
both viable fruits and seeds. The bearing of this experiment upon 
growing crops in areas where the days are short and the intensity of 
sunlight not great is readily seen. Lamps have been put on the 
market for use in the home which provide space directly underneath 
the bulb for stimulating plant growth during the winter season. 

What Goes On in the Green Leaf in Sunlight 

When we examine the green leaf to see how it is adapted to use the 
energy of sunlight, several interesting facts are discovered. One is 
that a plant places its leaves so that they get the largest possible 
amount of sunlight, in a given period. Petioles and even stems of 
plants turn with the sun so that a maximum amount of green surface 
is exposed to its rays. Looking at a tree from above as the bird sees 

Diagram to show the cells of the palisade layer of a leaf at two different times 
during the day. Which of the two receives full sunlight ? 

it, leaves are found to be so arranged that there is a minimum amount 
of overshading, the leaves forming a sort of mosaic or pavement on 
which the sunlight falls. Examination of the internal structure of the 
leaf also shows that the palisade layers which contain the greatest 
number of chloroplasts per cell are massed close under the upper part 
of the epidermis. It is this layer of palisade cells wdiere most of the 
work of starch or sugar making takes place. In the cells themselves, 
the green chloroplasts are so placed that a maximum amount of light 
falls upon them. When the sun's rays are slanting during the morning 
and afternoon, light can reach all of them readily, while at the period 
of greatest illumination, when the sun's rays are direct, less light 
reaches them as they lie one above the other. Their position may be 
changed in the protoplasm, their movement being controlled by the 


liviii<i; substance in which they rest. The ciiloroi)hists are the 
structures in the cells which utilize the sun's rays, and it is within 
them that the raw materials, carbon cUoxide and water, are manu- 
factured into sugar. 

Chemistry of Food Making 

The actual processes of sugar and starch formation in (he ](-if are 
not fully known. The end process can easily l)e shown by the 
equation : 

6 COo + 6 HoO = CeHioOe + 6 O2 

(carbon dioxide plus water = plus oxygen) 

but how this glucose actually comes into existence is still problem- 
atical. Many theories have been advanced to account for the con- 
version of raw materials into foods. The one proposed by von 
Baeyer in 1870 is still accepted with modifications. He assumed that 
formaldehyde is formed by the breaking down of carbon dioxide into 
carbon monoxide and oxygen at the same time the water in the leaf 
is broken up into hydrogen and oxygen. The carbon monoxide and 
hydrogen unite to form formaldehyde, which is then built into 
glucose as shown by the following formula : 

CO2 — ^ CO + O 
H2O — >■ \h + O 
CO + H2 — ^ CH2O (formaldehyde) 
6 CH2O — ^ C6H12O5 (sugar) 

One objection to this theory is that carbon monoxide is extremely 
poisonous and is almost never found free in plants, while the product 
formaldehyde is also a poison. Later theories postulate that by 
first reducing carbon dioxide and water to carbonic acid, then to 
formic acid and hydrogen peroxide by the addition of a molecule of 
water, formaldehyde and hydrogen peroxide result, the peroxide being 
finally reduced to water and oxygen : 

C02+H20 = H2C03 (carbonic acid) 
H2C03+H20 = HCOOH (formic acid)+H202 (hydrogen peroxide) 
HCOOH+H2O = CH2O (formaldehyde) +H2O2 
2Ho02 = 2H20+02 

The last step in this process is brought about by an enzyme, known 
as catalase. Plant physiologists believe that although formaldehyde 


is a poison, it is probably changed into sugar so rapidly that at no 
time is there much present in the cells of the leaf. The last part of 
this process, that of changing the formaldehyde to sugar, seems to be 
brought about by the action of the two chlorophylls, A and B. One 
recent writer, Gordon, > has given the following suggestive formula: 

6 C55H70O6N4Mg + 6 H2O = 6 C55H7205N4Mg + 602 
(Chlorophyll B) (Chlorophyll A) 

6 C55H7205N4Mg + 6 CO2 = 6 C,r,H7o06N,Mg + CeHisOs 

(Chlorophyll A) (Chlorophyll B) (sugar) 

To the amateur chemist this means very little, but it suggests the 
double action of the two chlorophylls in the formation of sugar. 
All we really know is that sugar is first formed in the green leaf and 
that later this is changed to starch and stored in that form in various 
parts of the plant. 

Of the manufacture of foods other than sugar very little is known. 
There are tiny droplets of fat in the vacuoles inside the chloroplasts. 
We know that fats can be synthesized out of carbohydrates by 
animals. Therefore, a similar process may take place in plants. 
Fatty tissue is undoubtedly manufactured out of the carbon, oxygen, 
and hydrogen contained in the sugar molecule. Probably a like 
situation exists in the chloroplasts of the leaves, although we do not 
know just how this process takes place. 

Proteins are even more complex than carbohydrates and fats. 
Their molecule contains nitrogen and a number of mineral salts, 
in addition to carbon, oxygen, and hydrogen. Protein foods are 
found not only in leaves, but in most of the storage organs of the plant. 
Apparently proteins can be synthesized out of the sugar plus the 
elements nitrogen, sulphur, and phosphorus, wiiich combine with 
the carbon, oxygen, and hydrogen of the glucose. Proteins are 
probably manufactured in other cells than those containing chloro- 
phyll, wherever .starches, sugar, and the essential salts are found, 
although light does not seem to be a necessary factor in the process. 
Proteins are undoubtedly used in any of the cells of the plant, just as 
they are in animal cells, for the making of protoplasm, since the plant 
is a living organism composed of cell units each of which is doing a 
common work for the plant as a whole. 

1 Gordon, R. B. : " Suggested Equation for the Photo-synthesis, Action." Ohio Journal of 
Science, 29: 131, 1929. 


Enzymes and Their Work 

The changes just described which take place in food making as well 
as in food storage, all belong to a series of oxidative and reducing 
changes that are presided over and brought about by enzyme action, 
another indication of the importance of these omnipresent substances. 

We have already spoken of enzymes and their work, but reference 
to them again may not be amiss at this point. They are found 
practically everyw^here in the living cells of plants and animals, being 
much more numerous than was at first believed. Although their 
nature is not fully known, we do know that they are colloidal sub- 
stances, because they will pass through porcelain filter, but not 
through membranes. We also know that some of them are doubtless 
proteins, and that they are sensitive to light and ultraviolet rays as 
well as to heat, acid, alkali, and substances which are toxic to proto- 
plasm. They are powerful catalyzers, as is shown by the fact that a 
single gram of the enzyme invertase, for example, will quickly hy- 
drolyze one million times its weight of sugar. Enzymes are found in 
all living cells and are specific in action, that is, one enzyme will only 
do a certain type of w^ork. In general, they may be divided into a 
number of groups, depending upon their function, such as the hy- 
drolases, that act in the digestive processes of plants and animals by 
hydrolyzing materials ; the oxidases, which enable cell respiration to 
take place ; the fermentases, as, for example, remiin, that is used in 
cheese making, and the coagulascs, to which pedasc belongs that is 
used commercially in substances sold for use in jelly making; and 
finally, the carboxylases, which cause organic acids to split into carbon 
dioxide and other simpler substances. 

Specific examples of these various plant enzymes include the en- 
zyme, diastase, that causes the digestion of starch. Another enzyme, 
maltase, aids in the digestion of maltose to glucose, a still simpler 
sugar. A similar action takes place by means of the enzyme, ptyalin, 
in our own salivary digestion. Bacteria carry on a slightly different 
type of digestion in which cellulose or wood fiber is broken down and 
used as food. Here another enzyme, cellulase, causes this digestive 
change. Still another enzyme, called lipase, is instrumental in the 
digestion of fats. In fruits and seeds rich in fat, such as the avocado, 
Brazil nut, walnut, almond, or pecan, the fats are broken down into 
fatty acids and glycerine just as in animals where lipase is formed by 
the pancreas. 

H. w. H. — 18 


Protein digestion is l^rought about by a different group of enzymes, 
called proteases. These enzymes are found in abundance in leaves and 
germinating seeds of plants and to a lesser extent in practically all 
plant tissues. In the living plant, the digestive enzymes carry on a 
necessary and important work. If plants make foods in the green 
leaf, and they do, and if they store foods in the root, stem, fruit, and 
seed, then there must be some way to transfer the foods made in the 
leaf in a soluble form to those parts of the organisms where the food 
is finally used. This work of changing insoluble foods to soluble foods 
is obviously performed by enzymes. A still more interesting phenom- 
enon sometimes takes place. Many of these enzymes under certain 
conditions are capable of reversing their actions, that is, of converting 
a soluble substance like sugar into an insoluble one such as starch, 
or of changing proteins to soluble forms so that they can be transported 
through the vascular system of the plant and stored in insoluble form 
in seeds, nuts, and roots. 

The changes from sugar to starch may take place in leaves wherever 
certain plastids known as amyloplasts exist. These bodies have 
the power to form starch in the presence of a series of enzymes which 
first bring about the transformation of simple sugars to more complex 
sugars, and then to an intermediate substance between sugars and 
starches, called dextrin. Dextrin is changed into soluble starch by the 
enzyme, amijlase, and finally the soluble starch is converted into 
insoluble starch by the enzyme, coagulase. Thus we see that the 
work of enzymes is absolutely essential to the life of the plant. Al- 
though plants and animals obtain their foods in different ways, they 
probably assimilate it in much the same manner, for foods serve 
exactly the same purposes in plants and in animals, namely, they are 
oxidized to release energy and they build up living matter. 

How Food Is Used in the Plant Body 

Although, basically, the uses of food are production of energy and 
making of protoplasm, certain substances are produced by plants 
which are not found in animals. For example, the plant cell is charac- 
terized by its cellulose wall which in old cells is strengthened by the 
addition of a complex substance, known as lignin. This forms the 
useful substance we call wood. In addition, other products charac- 
teristic of plant activity should be mentioned : the fatty substances, 
known as cutin and suherin, as well as waxes which give the "bloom" 
to certain fruits ; the essential oils in resins, such as lemon, pepper- 



mint, wintergreen, menthol, eucalyptus, camphor, and the like ; va- 
rious alkaloids ; poisonous substances such as nicotine and strych- 
nine ; acids such as mahc, citric, and tartaric. Plant protoplasm, in 
addition, as we have seen, manufactures many characteristic enzymes 
and produces pigments like the 
chlorophylls and carotins al- 
ready mentioned. The carotin 
present in green grass fed to 
dairy cows gives the deeper 
color so much desired in cream 
and butter. Another interest- 
ing substance found in carotin 
is a precursor of Vitamin A 
which exists in plant bodies as 
a form of carotin and is prob- 
ably transformed by the liver of 
animals into Vitamin A. This 
is another example showing how 
closely the lives of plants and 
animals are interwoven. (See 
pages 277-279.) 


A diagram of the outer portion of a 
cross section of a wheat grain showing the 
various layers of tissues : h, the different 
integuments of the ovary and seed which 
make up the husk ; o, the cells of " tileu- 
rone layer" of the endosperm, which are 
loaded with protein grains : and b, the layer 
of starch-bearing cells. (After Cobb.) 


Respiration is essentially the 
same process in plants as in 
animals. In its simplest terms it is the release of potential energy 
from foods by means of the process of oxidation, whereby oxygen 
is used and carbon dioxide is given ofT. Glucose is perhaps the 
chief fuel of the plant body, although fats also serve this purpose. 
The latter are probal^ly changed to sugar before actually being 
utilized in the respiratory process. 

In order to have respiration take place, there must be an exchange 
of gases through a selectively permeable membrane. This means that 
there will be an exchange of oxygen and carbon dioxide in the cells 
where the oxidative process is taking place. Sin.ce respiration occurs in 
all living cells and since there is a greater volume of carbon dioxide and 
oxygen in parts of plants that are growing rapidly, it is obvious that 
growing roots must have a supi)ly of oxygen. This is a reason for the 
loosening of soil particles around plants in cultivation to allow air to 
have access to the root hairs. The actual oxidative |)rocess is con- 



siderably influenced by external conditions. Low temperatures slow 
up the process as do very high temperatures, there being an optimum 
temperature for each organism at which the rate of respiration goes on 
best. Seeds have survived a temperature of —250° C. Experiments 
with leaves show that the respiratory rate increases rapidly from 
0°-40° C, from which point it falls slowly until the death of the 
organism. The amount of food present in the plant is a second 
factor influencing the rate of respiration, while the rate also varies 
with the amount of protoplasm in the cells. Light usually increases 
the respiratory rate, probably because of a parallel increase in food 
and temperature. It is also found that wave lengths which increase 
photosynthesis also increase the respiratory rate. Finally, the rate 
of respiration is greatly affected by poisons or anesthetics, at first 
being increased, but later slowing down rapidly. In brief, respiration 
in plants, as in animals, is induced by the action of enzymes, and 
results in the release of energy. 


If a healthy potted plant is placed in a dry bell jar and left in the 
sun for a few minutes, drops of water are seen to gather on the inside 
of the jar. By covering the pot with a rubber tissue to exclude the 


vith 5heet 



24 Viours later 

Experiment to show transpiration. Read your text and explain what has 


possibility of the evaporation of water through its surface and return- , 
ing it to the jar under similar conditions, drops of moisture are again 
found after a time on the inner surface of the jar. Obviously, water 
must come out through the leaves or stem of the plant, a fact which 
can be demonstrated by weighing it before placing it in the jar, and 



again after a brief period of exposure to sunliglit, when it will be found 
to have lost weight. This loss ol' water takes place through the sto- 
mata and to some extent through the lenticels of the stem, a loluniom- 
enon closely associated with the process of i)hotosynthesis, for which 
a relatively enormous amount of water is required. The reasons for 
this are that living matter is largely composed of water ; that the pro- 
cess of food making cannot take place in plants unless the interior of 
their leaves are moist ; and, in the third place, because water is one of 
the raw materials used in making sugar. The amount of water given 
off by plants through transpiration is very great. Early in the eight- 
eenth century Stephen Hales (see p. 241) estimated that an average 
crop of cabbages loses from three to four tons of water per day per acre 
in warm weather. An acre of pasture grass is said to give off over 
100 tons of water on a hot, dry day. A medium-sized tree will evapo- 
rate about five to six tons of water on a hot day. One writer, von 
Hohnel, estimated that an acre of large beech trees would transpire 
30,000 barrels of w^ater in one summer. Such figures show that a 
green plant loses water very rapidly during hot, dry days. 

The amount of water lost differs greatly under different conditions. 
If the air is humid, or if the temperature is lowered, or if the tempera- 
ture of the plant becomes low, the rate of transpiration is greatly 

Diagrammatic cross section through a stoma to show movement of guard cells. 
The dotted lines show the closed position. Closure is brought about by the 
guard cells becoming more elongated and flattened, while the outer wall (w) 
remains in place, the ventral wall (/) and dorsal wall (V/) assume the positions (/') 
and id') moving toward the central slit (s) of the opening of the stoma. This 
movement is largely brought about through the change in position of the base 
or hinge {h) {h') of the guard cell. (After Schwendener.) 

reduced. The stomata tend to close under certain conditions, thus 
helping to prevent evaporation. The opening and closing of the 
stomata depend on changes in turgor of the guard cells. The stomata 



open when the guard cells become more than normally turgid, but if 
the turgor of all of the living cells of the leaf is reduced by water loss, 
then the stomata seem to close automatically. 

Light increases the amount of sugar formed in the guard cells 
because of the chloroplasts present, which results in a concentration 
of sugar, thereby causing a change in turgor. When the leaf is not 
illuminated by direct sunlight, or at night, the amount of sugar con- 
centration in the guard cells becomes less, and consequently the 
stomata close. They usually are closed at night but remain open 
from shortly after sunrise until late in the afternoon. Toward the 
middle of the afternoon they begin to close, thus decreasing the 
amount of water lost in the latter part of the day. Plants wilt on 
hot, dry days because they cannot obtain water rapidly enough from 
the soil to make up for the loss through the leaves. Many adapta- 
tions are found in the leaves which help prevent this water loss, such 
as waterproofing of the outer cells, hairs on the leaf surface, the 
absence of leaves, as in the cactus where the minute leaves are early 
replaced by spines, or the actual turning of the leaves in order to 
place a small surface to the sun, as in the compass plant, thus causing 
the rate of evaporation to decrease. 



; ] 














> 1 









W riylu J'icrcc 

Capillary tubes of various sizes. Is there any relation between the size of 
the bore of the tube and the water level in the tube ? Explain. 



The Rise of Water in Plants 

We have spoken of the passage of water from the root up the stem 
into the leaf. Osmotic pressure has been shown to be sufficient to 
start this column of water on its way up the stem, but it is not enough 
to account for the rise of water sometimes hundreds of feet into the 
air in the stems of trees. Several theories have been advanced to 
account for this phenomenon. The most satisfactory of these is the 
theory that such a column of water is held together by the force of 
cohesion. Experimental evidence shows that the cohesive quality of 
water in capillary tubes is very great. The core of water acts as a 
fine, extremely ductile wire. When we realize that a core of water in 
a tube 2^ of an inch in diameter will withstand a pressure of over 
4600 pounds to the square inch, it will be seen 
that such resistance is a factor in the rise of 
water through the very tiny tubes found in 
the vascular bundles of a tree. Another 
factor in the rise of water in a plant or tree is 
the evaporation that takes place through the 
leaves, causing a pull on the cores of water in 
the tubes of the vascular bundles. During 
the daytime this is undoubtedly the chief 
factor in causing the rise of fluids in the 

Production of Oxygen by Plants 

A good many years ago the botanist Sachs 
proved that a green plant placed in the sun- 
light will give off oxygen, an experiment easily 
shown in the laboratory. If an aquatic plant 
such as Elodca is placed under an inverted 
funnel in a bell jar of water, and an inverted 
test tube of water is placed over the mouth 
of the funnel, bubbles of a gas are seen to 
leave the plant and gradually displace the 
water in the test tube. If a sufficient amount of this gas is collected, 
it can be tested with a glowing splint of wood and proved to be 
oxygen. The amount of the gas can be shown to depend approxi- 
mately on the amount of sunlight and consequently the rate of 
photosynthesis. Going back to the formula which shows the making 

How would you pro\e 
that the gas on the test 
tube w as oxygen ? 


of sugar in the leaf, we find oxygen is given off as a by-product. 
The reaction may be expressed by the following formula : 

6 CO2 + 6 H2O + energy from sunlight = CeHi^Oe + 6 O2 


The value of this reaction to mankind is obvious. The by-product 
oxygen, which is poured into the air by green plants, is used by 
animals as well as plants in their respiratory processes. This exchange 
of oxygen and carbon dioxide by plants and animals gives us one of 
the most significant and far-reaching interrelationships seen in the 
organic world. 

Briefly summing up the process of food making in plants we find 
that raw materials pass in the form of water and soil solutes from 
the soil through the root hairs and up the vascular bundles of xylem 
into the leaf, where water is taken into the individual green cells. 
Carbon dioxide reaches the cells from the air through the stomata 
and to a lesser extent probably *in the water stream through the roots. 
In sunlight, the process of photo.synthesis takes place. Elaborated 
foods made in the form of sugars may be changed by enzymes to 
starches and immediately stored in the leaf, or may be passed down 
through the sieve tubes of the phloem to various parts of the plant 
where they may be used or stored. Fats are probably synthesized 
from carbohydrates in the green parts of plants, while proteins seem 
to be formed in the cells irrespective of the presence of chlorophyll. 
Enzymes play a very important role both in the manufacture and 
in the use of food and are essential to respiration and oxidation. The 
digestive processes which go on in the leaf and other cells of the plant 
are also due to enzymes. 

All that has been said in the preceding pages leads to the most 
important plant function, the reproduction of the species. With 
vegetative propagation by means of budding, runners, underground 
stems, tubers, or some of the other asexual means of continuing life, 
plants would not go far. To establish outposts in far-flung dominions 
they must have means of travel. These can only be obtained through 
free moving parts. Such plants are seeds and fruits, which may be 
dispersed by outside agencies far from the parent plant. 

The life of the flowering plant culminates in the production of seeds 
and fruits. As growth progresses and food is accumulated, a time 
comes, sooner or later, when the energies of the plant are directed to 
the rapid production of the reproductive organs. Often this growth 

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is much more rapid than vegetative growth, and almost overnight, 
flowers appear. 

The flower, as has been previously shown, holds the gametophyte 
generation of the plant and produces from fertilized eggs the seeds 
which hold the embryos or future plants. The fruit arises from the 
ovary of the flower, together with the parts that may be attached to it. 



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Wright Pierce 

Adaptations in certain fruits for seed distribution. Can you describe the 
specific adaptation in each case? 

Sometimes the parts are fleshy, forming edible fruits such as apples, 
pears, or plums ; occasionally they form hard coverings such as the 
shefls of nuts, and often they are prolonged into feathery outgrowths 
which aid in the distribution of the fruit and seeds. Enough has been 
said of distribution for us to grasp the significance of such adaptations, 
the ultimate purpose of which is to place the embryo in new areas so 
that when the seed germinates it may develop into a new plant and 
thus complete the life cycle. 


Biisgen, M., and Miinch, G. (translated by Thompson), The Structure and 
Life of Forest Trees, John Wiley & Sons., Inc., 1929, 
Interesting and authentic, 


Ganong, W. F., The Living Plant, Henry Holt & Co., 1913. 

A not too technical account of how plants Hve. 
Holman, R. M., and Robbins, W. W., Textbook of General Botany, 3rd ed., John 

Wiley & Sons, Inc., 1933. 

Excellent chapter on photosynthesis. 
Macdougal, D. T., The Green Leaf, D. Appleton & Co., 1930. 

A fascinating account of the work of the green leaf. Readable and 

Raber, 0. L., Principles of Plant Physiology, The Macmillan Co., 1928. 

A readable, but thoroughly scientific, book of reference. Especially 

valuable are chapters IV, VI, XVI, XIX, XX, XXI, XXII, and XXIV. 
Sinnott, E. W., Botany, Principles and Problems, 3rd ed., McGraw-Hill Book 

Co., 1935. 

Chapters IV, V, VI, VII, and VIII are useful for reference. Note the 

many suggestive questions at ends of chapters. 
Wilson, C. L., and Haber, J. M., Introduction to Plant Life, Henry Holt & 

Co., 1935. 

A general botany with a new point of view. Readable and usable. 



Preview. Section A . Intake devices and how they function • Foods and 
their uses ; energy producers ; non-energy producers ; vitamins ■ The acti- 
vators — enzymes • Digestion in lower animals • Digestion in higher ani- 
mals ; methods of increasing digestive surfaces ; parts of the digestive 
system : The oral cavitj^, the pharynx and esophagus, the stomach, the 
small intestine, the large intestine ; the digestive glands and their enzymes : 
The salivary glands, the gastric glands, the intestinal glands, the pancreas, 
the liver, the secretions of the small intestine ; absorption and the fate of 
absorbed foods • Section B. The how and why of circulation • Why a 
transportation system • Unspecialized transportation systems ■ Open cir- 
culatory systems • Closed circulatory systems : Among invertebrates ; among 
vertebrates • The blood • The lymph • The conduits — arteries, veins, and 
capillaries • The heart • The aortic arches • The course of blood in the 
body ; functions of the blood • Section C. Respiratory devices • Respira- 
tion ; the protein, hemoglobin ; external respiration : Respiratory papillae, 
respiratory pouches or trees, lung-books, the body surface, gills, tracheae, 
lungs, internal respiration ; respiratory system in man • Section D. Ex- 
cretory mechanisms • Excretion ; types of excretory devices : Contractile 
vacuoles, intracellular excretion, other excretory devices ; excretory devices 
of vertebrates — kidneys ; the mammalian excretory system : The liver, 
other devices for waste elimination, the kidneys • Suggested readings. 


The body has often been compared to a machine. This analogy 
probably holds best when speaking of the preparation of food for 
combustion, the actual release of energy, and the resulting work 
done, as well as the disposal of the end products. It is this group of 
processes with which we will here be concerned. All animals are in 
constant competition with one another for food. If herbivorous they 
may be competing amongst themselves directly for plant food ; if 
carnivorous, the competition is more indirect. Food, whether it is 
animal or plant by nature, is being continuously sought to maintain 
that complex series of processes called by some authors the "flame 
of life." An earlier unit describes how plants take raw materials, 
such as water, carbon dioxide, and nitrogenous compounds, and build 
them up into foods which may then be used or stored. The plant 



in order to transport or to utilize this stored material must first 
break it down into simpler soluble compounds so that it may pass to 
the cells of the organism where it is utilized. A somewhat similar 
situation occurs in animals since complex protoplasmic material of 
animal or vegetable nature is taken in by the organism, broken down 
into simpler units, and then utilized or stored in the cells of the 
body during the normal processes of metabolism. This breakdown 
of foods is known as digestion, the intricacies of which make a fas- 
cinating study. 

There are a number of important and interesting problems which 
present themselves at this point. The most important problems 
involved are : What is food and how is it digested ? How is it 
disposed of after absorption? How is energy released? How are 
waste products removed? Briefly, they center around questions 
which we should answer, for it is both interesting and profitable 
to understand something of the human machine. Consequently, 
although other animals are mentioned, the fact should not be lost 
sight of that we have a selfish interest and are anxious to know about 
ourselves. The answers to these stimulating questions will be found 
in the discussions that follow. 



Foods and Their Uses 

Any substance taken into the body that can be utilized for the 
release of energy, for the regulation of body processes, or for the 
building and repair of tissues falls into the category of food. If this 
broad definition of food is adopted, then water, inorganic salts, vita- 
mins, carbohydrates, proteins, and fats should be included. Food 
substances may be further subdivided into those capable of releasing 
the latent or potential energy bound up within the molecule and those 
which, though non-energy producers, are still essential to life. Energy 
which is so essential to the metabolism of an organism is largely 
secured through the breakdown of a complex series of molecules into 
simpler ones. Non-energy producers are equally as essential to the 
well-being of the organism since water and inorganic salts, for ex- 
ample, are necessary for the maintenance of the normal composition 
of tissue. 




Energy Producers 

Carbohydrates, proteins, and fats are the sources from which energy 
within the animal body is derived. Of these, carbohydrates and fats 

are more readily oxidizable than proteins, 
a fact which is taken advantage of by 
the Eskimo, who secures much of his 
energy from oils and fats. The white 
man in the tropics uses carbohydrates 
chiefly for the same reason. 

No two foods contain the same per- 
centages of carbohydrates, proteins, or 
fats. At water analyzed many foods in 
the calorimeter which bears his name. 
Such a bomb calorimeter consists essen- 
tially of an outer insulated chamber sur- 
rounding one containing a known amount 
of water. The inner compartment in 
turn encloses the metallic chamber in 
which a certain amount of oxygen and 
food are placed and burned by means of. 
an electric current. The amount of heat 
generated is transmitted to the water in 
the chamber surrounding the bomb and 
the value of this in terms of calories 
is then determined. It will be recalled 
that a calorie is the amount of heat necessary to raise one gram of 
water one degree centigrade. 

l— ^ater around, bomb 
■varmect by bixcmug' 
of fooct 

Diagram of a bomb calorim 
eleri How does it, work.^ 

Non-Energy Producers 

Three widely diversified groups are represented by water, inorganic 
salts, and vitamins. All serve the common end of keeping the animal 
in a state of well-being, yet each group does so in a very different way. 

Water constitutes a large portion of the animal body which may 
compose even as much as five sixths of the daily intake. Estimates 
vary from 62 to nearly 75 per cent of water by weight in the case of 
the human body. The quantity in the different tissues varies accord- 
ing to the metabolic state of the tissues of the organism. It is well 
established that bone contains only about 22 per cent of water, while 
other organs, as the liver, muscles, kidney, and brain, contain much 


larger amounts. In the case of man the amount of water in tlie adult 
body remains approximately the same under normal conditions, but 
if decreased beyond a certain point intolerable thirst results. On the 
other hand, if the amount of water is increased, the blood pressure is 
raised in the renal capillaries and the excretion of urine is stimulated. 
The consumption of a hberal supply of water is a characteristic 
biological process as it favors the removal or dilution of waste and 
poisonous materials from the body. 

Along with water, the presence of certain chemical elements such as 
sodium, potassium, calcium, magnesium, iodine, iron, chlorine, phos- 
phorus, sulphur, silicon, and fluorine is necessary to maintain the 
various kinds of tissues. Much experimental work has been per- 
formed upon various animals, indicating the importance of a proper 
balance of these elements in the diet. The absolute withdrawal of 
any of these may end in the death of the organism. 

Since these salts form, a part of all tissues and serve a variety of 
functions it is impossible to mention all of them. The important 
part w^hich calcium salts, for example, play in the formation of 
bone is well realized. In this connection it has been said that 
there is enough lime in a human body " to whitewash a small hen- 

Certain parts of foods rich in carbohydrates contain indigestible 
material that serve as roughage and are useful in stimulating the 
muscles of the large intestine. Bran, whole wheat, fresh vegetables, 
and fruit provide some of the best sources of these materials. Other 
examples may be found in the cellulose of plant cells which can be 
used as food by only a few animals. 

Flavorings, stimulants, and condiments, such as pepper, mustard, 
tea, coffee, and cocoa, are not true foods. However, they have a 
real value in making food more appetizing. 


It might seem that an organism could be kept alive, well, and 
healthy upon a balanced diet of the necessary inorganic salts and 
water, together with energy producers and tissue builders, such as 
amino acids, carbohydrates, and proteins. Modern scientific work 
has dispelled this illusion by a series of laboratory experiments and 
by observations of experiments performed in nature. We now know 
that regulating substances, called vitamins, are some of the most 
essential ingredients of all foods. 


These health regulators have been lettered and are known as 
Vitamins A, B, C, D, E, and G. More recent experiments show that 
what was previously believed to be a single vitamin may prove to 
be a mixture of two discrete fractions. These may be referred to as 
A\ A^, and so on, or they may be given a new letter, as, for example, 
H. Thus Vitamin B has become subdivided into B or B^ the anti- 
neuritic vitamin, the absence of which results in a disease known as 
heriheri, and B- or G, the antipellagric vitamin, the lack of w^hich 
produces pellagra. 

The initiation of scientific work in this field is usually credited to 
Eijkman, who in 1897 produced beriberi in fowls by feeding them on 
certain restricted diets. This was really "putting the cart before the 
horse," for through the pioneering contributions of Grijns (1901) it 
was shown that the disease is produced by the absence of some essential 
constituent of the diet. This important conclusion has been corrob- 
orated and extended materially through the efforts of Hopkins in 
England and McCollum, Eddy, Osborne, and Mendel in the United 
States. Research in this field has taken great strides since 1910 and 
is still going on. 

Vitamin A is found in the fatty and oily constituents of such 
foods as butter and cream, egg yolk, liver, carrots, cod-liver oil, 
yellow corn, and leafy vegetables. Experiments have demonstrated 
that this vitamin is a necessary adjunct to growth. Without it rats 
die, but if even such minute amounts as 0.005 mg. of the purified 
vitamin are added to the normal diet, the sick animals are restored 
to general health. 

Scurvy has long been the curse of those embarking upon long sea 
voyages or expeditions where it has been necessary to provide diets 
deficient in fresh meats and vegetables. It has also been known that 
such a disease can be cured by the use of fresh vegetables and fruits. 
As early as 1804, lemon juice was issued regularly to British sailors, 
who became known thereafter as "limeys." It is only within com- 
paratively recent years, however, that this remedy has been known to 
be due to the presence of Vitamin C, the antiscorbutic vitamin. It 
may be secured most conveniently in oranges, lemons, or tomatoes. 
Apparently food can be dried or canned without marked injury to 
the vitamin. Almost as soon as this vitamin is eliminated from the 
diet degenerative changes begin, although some time is necessary 
before the first symptoms appear. This has been designated as the 
depletion period. 


Vitamin D, better known as the antirachitic vitamin, is chiefly con- 
cerned with maintaining an adequate supply of phospliorus and 
calcium in the blood, bones, and teeth. The discovery of this vitamin 
is associated with a study of rickets. Early workers noted that cod- 
Uver oil had a beneficial effect. The cure was attributed to Vitamin 
A until, in 1923, McCollum of the Johns Hopkins University and his 
co-workers showed that the efficacy of cod-liver oil remained even 
after treatment which destroyed Vitamin A, an observation which 
led to the identification of Vitamin D. The best sources of this 
vitamin are cod-liver oil, butter, and egg yolk. 

More recently it has been shown that the precursor or "pro- 
vitamin" of Vitamin D, a substance known as ergosterol, will yield the 
vitamin after irradiation with ultraviolet light. Ultraviolet rays of 
the sun, or X-rays, are likewise a great help in overcoming rickets. 
At the present time four methods are used to increase the amount of 
Vitamin D in the bodj' : (1) irradiation of the skin by exposure to 
sunlight or other sources of ultraviolet light ; (2) the addition of 
cod-liver oil to the diet ; (3) the introduction of irradiated ergosterol 
(viosterol) ; and (4) the use of Vitamin D concentrates in foods. This 
latter method has been most successfully introduced by Zucker, by 
the addition of this concentrate to milk, thus facilitating its adminis- 
tration to young children. 

A survey of the prevalence of rickets shows that this disease is 
much more common than has been supposed, especially in young 
children, a fact strikingly brought out when 83 per cent of a group 
of over 200 children from New Haven, Connecticut, who were exam- 
ined by X-ray, showed mild evidence of rickets. 

Vitamin E is commonly known as the antisterility vitamin. This 
important substance has been shown to be present in greatest quantity 
in lettuce, whole wheat, and, to a somewhat lesser degree, in egg yolk 
and milk. It is fat-soluble and quite resistant to heat. There is 
evidence suggesting that the animal body has the ability to store this 

The Activators — Enzymes 

It will be recalled that the metabolic processes of plants and animals 
include about every type of reaction known to the chemist. It has 
been demonstrated that enzymes not only are essential for diges- 
tion, but also that all chemical changes in the body are mediated by 
enzymes. Glucose may be taken as an example. The decomposition 

H. W. H. — 19 


and oxidation of this simple sugar produces over 100 different sub- 
stances. The living cell yields only a few of these, and then in a 
regular succession. Such remarkable specificity and speed of reaction 
in the living cell is largely due to the action of enzymes which have 
the property of accelerating some particular reaction. As was 
pointed out previously (p. 128), enzymes may be regarded as catalysts 
because they are not expended and primarily serve to speed up a 

While the properties of particular enzymes will be discussed in some 
detail as they are encountered later, certain of their general char- 
acteristics as determined by the biochemist will be briefly mentioned. 
In this connection it is interesting to note that six enzymes have been 
prepared in crystalline form, and all are proteins. While this evidence 
is not conclusive it suggests the probable chemical nature of a con- 
siderable number of these activators. Most enzymes have what the 
chemist calls a reversible reaction and so may be capable of serving 
as a catalyst for both hydrolyses and syntheses. However, it should 
not be forgotten that under some conditions an action may be practi- 
cally irreversible. Such is the case with glucose which, although 
theoretically capable of reacting in several different ways, continues 
to react in one direction because of the presence and concentration 
of a particular enzyme. Nearly all enzymes appear to have an opti- 
mum working temperature of about 40° C. (104° F.). Furthermore, 
enzymes appear to be specialized, at least to the extent of requiring 
a definite acidity or alkalinity of the surrounding medium. One 
classic example is the pepsin of the stomach, which reacts only in an 
acid environment. 

Many enzymes seem to have the common function of splitting com- 
plex molecules into simpler ones, a process usually accomplished 
through the addition of water, or hydrol3^sis. Enzymes acting in 
this manner may be described as hydrolytic, the term being formed 
by adding the suffix lytic to the Greek stem for water. The enzymes 
themselves are designated by adding the ending ase to the name of 
the substance upon which each acts, as, for example, maltase or lipase, 
signifying, respectively, action upon maltose or the lipins (fats). 
Such activators may be spoken of collectively as hydrolases since 
they act through the addition of water. Similarly the catalyzing 
enzymes for oxidations and reductions are spoken of collectively as 
oxidases. A few other enzymes do not fall into either of the above 


Digestion in Lower Animals 

Digestion within the animal kingdom is primarily of two sorts, 
intracellular taking place within the cell and extracellular which is 
carried on outside the boundaries of the cell. Sometimes both types 
of digestion occur in the same organism. The complexity of the 
picture among one-celled forms may be appreciated when it is real- 
ized that within the confines of a single cell are carried on all the 
essential processes characteristic of a many-celled organism. 

Euglcna, for example, shows evidence of being a rather generalized 
physiological type (see page 157). Within the group to which it 
belongs three types of nutrition are found: (1) holophijtic nutrition 
carried on by the aid of chlorophyll ; (2) saprophytic nutrition cor- 
responding to that carried on by the chlorophyll-less molds and 
fungi ; and (3) holozoic nutrition, involving the ingestion of solid 
food particles, a type characteristic of animals. Both Ameba and 
Paramecium are characterized by relatively simple intracellular 
digestion, the potential food reaching the interior of the cell by 
means of a food vacuole, the indigestible particles being egested 
from the cell later. 

In sponges ingestion and digestion principally occur in the collared, 
or choanoflagellate, cells where food vacuoles are formed and wastes 
egested. The nutritive material is then passed from one cell to the 
other and, according to Hegner, may be circulated to a certain extent 
by wandering ameboid cells found in the middle region by a similar 
intracellular action. 

In the coelenterates one first finds evidence of extracellular digestion. 
Here a special layer of cells called the endoderm, which lines the prim- 
itive gastrovascular cavity, is set aside. This cavity appears in Hydra 
as a simple sac lined by cells possessing the ability to send out either 
flagella or pseudopodia. Some of these cells are glandular and 
secrete digestive enzymes which are passed into the gastrovascular 
cavity, making digestion an extracellular process. A certain amount 
of intracellular digestion does take place, however, since some of the 
food particles are surrounded by pseudopodia and so brought within 
the walls of the endodermal cells. 

Most of the parts of the digestive system found in v(^rtebrates are 
represented in the earthworm (see page 189). The digestive system 
of a crayfish will be discussed here as representative of the Arthro- 
poda. Its food consists of such organisms as frogs, tadpoles, small 



fish, insect larvae, snails, and decaying organic matter. The max- 
illipeds and maxillae around the mouth are used to hold the food 
while the mandibles crush it into small pieces that are then passed 
into the esophagus. The large stomach contains a series of chitinous 
ossicles, forming the gastric mill, which grinds the food. When the 

food has been broken up 


and. mou-th- 

Gsophagixs —■ r 

sixbesophodeal- " 



g-a rig lion. * 

conn<activ<e.s ^ 

grinding stomach 


artsry" "" 


vas d&^&rsns^ 


...dorsal artery 




sufficiently, it passes 
through the strainer into 
the pyloric chamber, where 
the digestive glands or 
"liver" empty their secre- 
tions through hepatic ducts. 
These glands secrete en- 
zymes which digest both 
proteins and fats. From 
this chamber the dissolved 
food passes into the in- 
testine where nutritive ma- 
terial is absorbed through 
the intestinal wall. 

Digestion in Higher 


anus— w 

Sagittal section of crayfish showing diges- 
tive system. 

In the vertebrate series 
the parts of the digestive 
systems are analogous and 
even homologous with some 
invertebrate structures. 
All except the lowest and 
parasitic types of inverte- 
brates are characterized by 
an alimentary canal. Dif- 
erences which occur in the digestive tracts of vertebrates are largely 
attributable to the different kinds of food handled by different types 
of systems. Carnivores digest their foods more rapidly than herbi- 
vores and so can get along with a shorter alimentary tract. 

Methods of Increasing Digestive Surfaces 

One of the first problems in digestion is the production of an ade- 
quate absorptive surface. Greater digestive surfaces may be prO' 



cured by increasing the length or diameter of the aUmentary tract, or 
by the formation of pockets, or caeca, of different sizes and shapes. A 
carnivore such as a cat has an ahmentary tract which is only three to 
five times the body length, whereas a cow, being herbivorous, supports 
one over twenty times the length of its body. Man, who is interme- 
diate as well as an "omnivorous beast," has one about ten times longer 

small intestine. 


man °PP^-^^^ rabbit 

appendix dixode«UTn 




Comparison of digestive tracts of a 
carnivore and herbivore. How can the 
differences in the size of the caeca and 
length of the gut be explained P (After 
WeUs, Huxley, and WeUs). 

£..r<3ctcx\ glandi 

Alimentary canal of the dogfish. 
State the function of the vahular 
intestine. What are the principal dif- 
ferences between this digestive tract 
and that of the rabbit P 

than the body. A modification quite characteristic of some groups is 
the caecum, which is noticeably large in rodents. Other types of caeca, 
like the pyloric caeca, sometimes occur near the juncture of the 
stomach and the intestine among fishes, and there should be men- 
tioned here the longitudinal fold, or typhlosole, of the earthworm. 
When such a longitudinal fold is twisted spirally, there results a 
structure known as a spiral valve, v/hich is characteristic of sharks. 



Circular folds or 
plicae circulares in 
the intestine of 
man. These occur 
from the duodenum 
to the anus. 

Other devices such as throwing the surface into transverse ridges 
are quite common, for example, in man they occur in the intestine 
and colon as plicae circulares. 

Parts of the Digestive System 

The Oral Cavity. The various mouth cavities 
of vertebrates are all developed for one fundamental 
purpose, namely, the ingestion of food. The 
mouth cavity is specialized in many different ways 
and is further complicated in air-breathing forms 
by the necessity for completely separating the air- 
intake apparatus from the digestive tract. This 
is accomplished quite readily in water-inhabiting 
species through the use of gill-slits. In land forms, 
however, the external nares (nostrils) and associated 
nasal passages are dorsal and the lungs ventral to 
the opening of the digestive tract. It is neces- 
sary therefore to arrange in the pharyngeal region 
for the crossing of the air passageways over the 
food tube. This separation is facilitated in most 
forms by the presence of a hard bony plate known as the hard palate 
that lines the roof of the mouth. At the posterior end of the hard 
palate is attached a flap of soft tissue, the soft palate, which further 
expedites the separation of respiratory and digestive tracts. 

The oral cavity is lined throughout by a mucous membrarie the cells 
of which secrete mucus that serves as a lubricant facilitating the 
passage of food. This same tissue is found throughout the entire 
surface of the food tube. In various parts of the alimentary canal are 
found openings of various glands which add their digestive ferments 
to the mucus. These glands will be considered in detail under the 
digestive processes of man. 

Usually the surface of the palate, especially the posterior part, 
known as the soft palate and uvula, contains numerous mucus-secreting 
glands called the palatine glands. The secretions of these glands help 
to keep the cavity of the mouth moist. In many animals, especially 
carnivores, there appear a number of washboardlike palatine ridges 
that appear to be an adaptation to enable its owner to secure a surer 
grip upon the unfortimate victim that has been seized in its jaws. 
The large, bulky tongue, which occupies practically the entire floor of 
the buccal cavity, likewise plays an important role in eating. 



Teeth are found in the vertebrate group from fishes up to man. 
While derived from a common embryological source, they have 
developed in many different ways during the course of e\olution to 
serve such various uses as grasping, grinding, or cutting food. In 








ctura mater 

pineal glanct 




Sagittal section of human head. 

many of the lower fishes they are unspecialized and are continu- 
ously being replaced as worn out. Thus the shark always has a 
new set developing behind the old, a device suggestive of an end- 
less chain. 

The garpike has a series of long, pointed, unspeciahzed teeth which 
are used merely as holdfast organs. In such types, teeth are not 
crushing or tearing devices. The amphibia and reptiles show little 
tendency toward specialization, except among the poisonous reptiles 







, -tir^ 






X . V. Sinir CnnxcnaHo)! />ipl. 

Unspecialized teeth of the garpike, Lc/x'sosleus ossciis. 



with their fangs and the toothless jawed turtles that make up for the 
lack of teeth by sharp cutting horny beaks suggestive of the bird's 

The greatest development and specialization of teeth occurs among 
the mammals. According to their shape and function they are divided 

into incisors, or cutting 
chisels, canines, or graspers 
and tearers, premolars, or 
grinders, and molars, or 
crushers. Here we find 
a real relationship between 
the type of teeth and the 
diet of the organism. In 
the carnivores, for exam- 
ple, the anterior grinders 
are so constructed that 
they slide like shears while the canines are specialized for grasp- 
ing animal food, the back molars tending towards degeneracy. In 
herbivorous animals except the rodents the front teeth, especially 
the canines, are reduced while the molars become greatly developed. 
The teeth of man play a definite role in the mechanical preparation 
of food for digestion. Instead of 

Skull of a squirrel, a rodent (left), and a cat, 
carnivore (right). Compare carefully for differ- 
ences in dentition. 

holding the prey, they crush, 
grind, and tear the food so that a 
greater surface may be exposed to 
the action of digestive juices. 
Man like some other organisms 
develops more than one set of 
teeth. The first, or milk teeth, 
are only twenty in number while 
there are thirty-two secondary, 
or permanent teeth. 

Each tooth is divisible into an 
upper gum-protruding crown, a 
lower embedded root, and an in- 
termediate neck. The outer part 
of the crown is protected by the 
hardest substance of the body, 
enamel, that surrounds the bony 
dentine. This in turn protects 








Sagittal section through a tooth, 
are cavities painful.^ 




the pulp cavity where during the Ufe of the tooth nerves and blood 
vessels are housed. Each tooth is held in a socket of the jaw by 
means of another hard tissue, the cement. 

Nearly every vertebrate organism possesses some sort of tongue 
which serves a variety of functions. The lassoing tongue so char- 
acteristic of certain amphibia, for example, is provided with special 
glands secreting glutinous mucus that helps to ensnare insects. In 
lizards the tongue may become extremely long and extensile, it 
also servii^ig to aid jn capturing food, while among some of the 
birds it may even be adapted for impaling insects, as in the case 
of the "horny, spearlike tongue" of the woodpecker. The mam- 
malian tongue is likewise specialized, for in many of the herbivores it 
is definitely muscular and prehensile, being used to grasp tufts of grass 
which are then cut off against the lower incisors, while in dogs and 
cats it is used as a spoon to take 
up liquids. The tongue helps me- 
chanically in swallowing and in 
man it also plays a vital part in 
speech. The tongue of higher 
forms is covered with a variety 
of sensory structures which test 
the various foods before they are 

The Phaeynx and Esophagus. 
This region is both membranous 
and muscular. We may think of 
the pharynx in all air or land verte- 
brates as being an irregular cavity 
supplied with openings. Dorsally 
and anteriorly are two posterior 
nares, or internal nostrils, laterally 
the openings of the Eustachian tubes 
connecting with the middle ear, 
while medianly and ventrally lies 
the opening to the oral cavity. 

Posteriorly there are two openings, one down the esophagus and the 
other, the glottis, leading into the trachea (see fig., page 285). Above 
the soft palate is a mass of lymphoid tissue, known as the adenoids, 
or pharyngeal tonsils, while anteriorly and laterally lie the true, or 
palatine tonsils. 





Stomach" of bird. What are the 
functions of the different parts .^ 



The Stomach. The stomach of vertebrates is likewise subject to 
considerable variation. In the case of grain-eating birds a distended 
esophageal region, the crop, is developed for the storage of food. 
Below this region is the stomach proper, divisible into a glandular 
stomach, which secretes digestive enzymes, and a muscular gizzard, 

or grinding stomach, 
that compensates for 





Stomach of a ruminant. What is the function 
of the valve and what is the significance of "chew- 
ing the cud" ? (After Walter.) 

the absence of teeth. 

A second example of 
an outstandingly differ- 
ent type of stomach ap- 
pears in the compound 
stomach of ruminants 
as, for example, a cow. 
Here there are four 
parts, namely, the ru- 
men, recticulum, psalte- 
rium, and abomasum, 
the first two being 
derivatives of the esoph- 

agus. The more solid food is temporarily stored in the rumen, or 
paunch, as fast as it is ingested, gradually being passed on into the 
reticulum where it is mixed further with digestive juicfes and softened. 
From time to time, ball-like masses of this food are regurgitated from 
the reticulum and thoroughly mixed with saliva by chewing. This 
process is commonly known as "chewing the cud." After a time the 
food is swallowed a second time and if the chewing has sufficiently 
reduced the mass to a small slippery wad, it passes directly into the 
psalterium and thence to the abomasum, where it undergoes gastric 

The human stomach as compared with the compound stomach of 
the ruminants is of a more simple type, although divisible both 
histologically and physiologically into several parts. The esophagus 
enters an expanded cardiac region the entrance of which is guarded 
by a ringlike sphincter muscle. The stomach is always curved to 
some extent, the inner or concave surface being known as the lesser 
curvature and the outer or convex as the greater curvature. The blind, 
rounded part of the stomach lying to the left and usually opposite the 
entrance of the esophagus is called the Jundus, while the region closest 
to the point of entrance of the esophagus is called the cardiac portion ; 



the lower end is known as the pyloric part, the extreme Hmit of which 
is indicated by a groove called the pylorus. The pyloric and fundic 
parts of the stomach differ in the nature of their musculature as well 
as in their physiological activity during digestion. The pyloric part 
is separated from the small intestine by a sphincter muscle, called the 
sphincter pylorus. The shape and position of the stomach may \'ary 
according to the posture and amount of food ingested. Thus, while 
the stomach is supposed to lie in an "obliquely transverse position," 

Cocrdiac region. 



ga-s bubble 






The human stomach (1) as usually depicted, (2) the shape and position of the 
stomach as shown by X-ray, (3; stomach and large intestine showing position of 
food at varying hours after ingestion. (After Howell.) 

it really assumes a J-shape as detected by X-rays. The folded wall 
of the fundus is dotted with thousands of tiny pits, the mouths of 
gastric glands, or little tubes the epithelial lining of wliich secretes 
the gastric juice. (See page 294.) 

As in the case of the remainder of the digestive tract, the stomach 
wall is made up of several layers of tissue. Beginning with the inside 
is the soft, thick, glandular mucosa, usually thrown into folds, or 
rugae, which tend to disappear when the stomach is distended. 
A second layer, the submucosa, composed of loose connective tissue 



lies between the mucous and muscular layers. The latter is made up 
of three layers of involuntary muscles, an inner, poorly developed 
obUque layer over which lies a circular layer that in turn is enclosed 
by an outer layer of longitudinal muscles. The fourth or outermost 

coat is known as the serosa, 




tissoe ^ 


A typical gastric gland. Explain the 
functioning of each part. 

which is continuous with the 
peritoneum and as such covers 
both organs and their associ- 
ated glands. This covering is 
moist and serves not only as 
a protection but also facilitates 
the movement of one portion 
over the other. 

Food in order to reach the 
stomach must be rolled into 
boluses and then swallowed. 
This is a complicated reflex 
movement which apparently 
may be more or less volun- 
tarily initiated as the bolus 
passes into the pharyngeal 
region, past the trap door (epi- 
glottis) which covers the open- 
ing into the larynx and trachea. 

Failure of this flap to close properly results in food "going down the 
wrong way," when the mass is expelled after a paroxysm of choking 
and coughing. 

Liquids and soft foods reach the stomach in about 0.1 second while 
more solid boluses are passed along by a series of slow-moving wavelike 
contractions, called peristalsis. Boluses require about six seconds to 
reach the stomach. The entrance of food into the stomach is prob- 
ably controlled by the cardiac sphincter. Solid food may remain 
in the stomach for several hours. One of the first noteworthy obser- 
vations of this process was made upon Alexis St. Martin, a Canadian 
voyageur who was studied by Beaumont in 1847. The adventurer 
had a permanent opening into his stomach as a result of a gunshot 
wound, which permitted direct observation of processes going on within 
the stomach. These and other studies indicate that the fundus largely 
fimctions as a reservoir which retains the bulk of the food while the 
more muscular pyloric portion churns it, forcing it periodically into 


the first part of the small intestine (duodenum). It is interesting to 
remember that carbohydrates pass out of the stomach soon after 
ingestion, remaining only about one half as long as proteins. Fats 
hkewise remain a long time within 
the stomach even when combined 
with other foodstuffs. 

The Small Intestine. The 
intestine is subdivided into a 
region principally devoted to 
absorption of digested foods, 

namely the small intestine and the (Y 

large intestine which to a lesser M 

extent is devoted to a continua- '^i^^cting' nioUon shoNvn in. 

tion of absorption, and to the ^■'^•^ ■ '^^/'^hmic.Segnrjenting- 

collection of waste products. ''T^overrjenLs. 

The entire small intestine of man, r\ f^^"--^ 

some twenty feet in length and e^ "^^^-^ 

about an inch in diameter, is cLiastalsis is Cannon's nanxe. 
concerned with the digestion and ^°^ '^^'^ perisLxlLic wave xvl^ich 
„. ,. c c 1 1.,- Tnoves olonS U^e intestine, 

absorption of foods and their prccecCad Joy -inhibition 

transfer to the blood stream. It t^. , .„ • , • 

, , ,. , , Diagram to illustrate peristalsis. 

IS also believed that some waste 

materials are actually excreted into the lumen of the gut. These 
functions are accomphshed by a series of adaptations, one of which 
is the extraordinary length of the small intestine, together with 
numerous small circular ridges, -plicae circulares, which serve the 
double function of giving an increased absorptive surface and of 
retarding the rate of passage of foodstuffs. The other but by no 
means the least important adaptation, is the presence of millions of 
small knoblike projections, or villi. These tiny structures according 
to Howell move actively either by lateral lashings or by extension 
and retraction. It is believed that these movements are associated 
with the act of absorption and probably play an important part in 
emptying the lymph sac, or lacteal, lying in the center of each villus. 
By means of the plicae circulares and the villi, the small intestine is 
estimated to have an absorbing surface equal to twice that of the 
surface of man's body. 

The internal structure of the villus is best seen in a longitudinal 
section. The outer wall is composed of a thin layer of epithelial cells 
in which the more complex fats are resynthesized before being 



passed to the ladeals. Beneath this is a mass of connective tissue 
permeated by a network of capillaries that in turn surround the 
central lymph channel (lacteal) into which fat is absorbed. Between 
the villi are found the openings of the intestinal glands which 
are associated with the compound duodenal glands in the production 
of intestinal juice. Aggregations of two types of lymph nodules 
appear, solitary lymph nodes about the size of a pin head and groups 
spoken of as Peyer^s patches. The latter are sometimes the seat of 
local inflammation and ulceration as in typhoid fever. 

The same four coats which were found about the stomach occur 
in the small intestine except that the oblique layer of muscles is 
missing, while the mucous layer is very thick and vascular. 

The Large Intestine. The large intestine of man has somewhat 
the same anatomical structure as the small intestine except that it 
lacks villi and has a greater diameter. It is separable into a shallow 
blind pouch at the juncture of the small and large intestines, and an 
enlarged colon and rectum, terminating with the anus. The entrance 
of material into the large intestine is regulated by the ileo-caecal 
valve, formed by two flaps of mucous membrane, which permits entry 
into it but effectively prevents back flow. At the end of the caecum 

is a A'estigial continuation of 
it, the vermiform appendix, a 
blind pouch usually about three 
inches long. Inflammation of 
this structure usually results 
in a condition recognized as 

The colon of man is divisible 
into four parts known respec- 
tively as the ascending, trans- 
igrr?oJd verse, descending, and sigmoid 
-Oiorv colons. In other mammals, 
the colon may not always be 
rectum separated into these parts 

The caecum, appendix, and colons of although the juncture of the 
man. Why is the appendix so frequently .n^alland large intestines is 
the seat ol bacterial inlections.J ^ 

clearly set off by an ileo-caecal 

valve and a caecum. The anus is guarded by both an external and 
an internal sphincter which keep the orifice closed except during 
defecation. The external sphincter is composed of striated muscle 




and is under the direct control of the will, while the internal sphincter 
is derived from one of the coats of the rectum and consists of un- 
striated or involuntary muscle. 

The process of absorption is thought to be continued to a limited 
extent in the large intestine as its contents are retained for a consider- 
able time. The secretions of this region are alkaline, containing much 
mucus l)ut apparently no enzymes. By the time the contents reach 
the large intestine the water content is considerably reduced through 
absorption. Bacteria, which compose nearly 50 per cent of the human 
feces, carry on putrefactive protein fermentation in the large intestine. 

The Digestive Glands and Their Enzymes 

The chemical processes of dige.stion occur largely through the activ- 
ity of enzymes which are produced in a variety of different glands. 
Practically all vertebrates possess salivary and gastric glands, a liver, 
pancreas, and various intestinal glands. 

v/^^*>,*.^i^^^ SLcblirj^t^cd duct 

submaxillary ^^itblii' 
glancC gl^^c 

Salivary glands in man. What enzyme do these glands secrete ? (After Walter.) 

The Salivary Glands. Saliva, which acts as a lubricant in the 
mouth, is manufactured in the cells of three pairs of glands that 
empty into the mouth by ducts, and which are called, according to 
their position, the parotid (beside the ear), the submaxillary (imder 
the jawbone), and the sublingual (under the tongue). In addition, 
the salivary glands, which are absent in most aquatic forms, secrete 
a digestive enzyme, ptyalin, that acts upon starch in an alkaline 
medium, splitting it partially or entirely into a disaccharide sugar 
known as maltose. Ptyalin is present in all mammals except those 
which are entirely carnivorous. 


The chewing process theoretically inixes food with saliva thoroughly 
but in man the bolus is invariably swallowed before the ptyalin has 
completed its action. Recent studies indicate that salivary digestion 
continues in the stomach for some time until stopped by the hydro- 
chloric acid of the stomach. 

The Gastric Glands. The inner surface of the stomach is 
covered with cells producing mucus, the entire region being dotted 
with thousands of tiny gastric glands secreting gastric juice. Most 
of the lumen of each gland is lined by columnar epithelial cells called 
chief cells, while between the basement membrane and the chief cells of 
the glands lie scattered parietal cells. The chief cells of the neck 
of the gland secrete mucus while those lower down secrete an in- 
activated enzyme or zymogen, called pepsinogen. Oval parietal cells 
secrete hydrochloric acid, which activates the pepsinogen, converting 
it into an active enzyme (pepsin), that, in the presence of this acid, 
breaks down proteins to the intermediate products, peptones and 
proteoses. Gastric juice is slightly acid in its chemical reaction, 
containing about 0.2-0.4 per cent of free hydrochloric acid together 
with another enzyme called rennin. The latter curdles or coagulates 
casein, a protein found in milk, which is the basis of cheese. After 
milk is curdled pepsin is able to act upon it. "Junket" tablets, 
which contain rennin, are used for this purpose in the preparation of 
a dessert which has milk as a basis. 

The stomach is the place where the digestion of proteins is initiated 
and where digestion of carbohydrates may be continued. Some 
investigators believe that emulsified fats such as cream are digested 
by a gastric lipase. However, since saponification and emulsification 
must take place before absorption, and after the fats reach the intes- 
tine, it appears probable that fats undergo no digestive changes in 
the stomach. 

Although little or no absorption takes place in the stomach, under 
certain conditions water, salts, alcohol, and drugs may be absorbed. 
There appears little evidence at present to support the contention that 
sugars and peptones are appreciably absorbed in this organ. 

Food, after being mixed with gastric juice, becomes increasingly 
liquid and is known as chyme, in which state it passes through the 
pylorus. The next step is facilitated by the muscular movements of 
the small intestine, which are primarily of two kinds. The first, 
peristalsis, helps pass the food slowly along the intestine. The second, 
rhythmical contractions or segmentation, may be described as a series of 


local constrictions occurring at points where the food masses lie. 
Such contractions break up the food into a number of segments 
enabling the enzymes to reach all parts. 

The Intestinal Glands. The partly digested food in the small 
intestine comes in contact almost simultaneously with secretions from 
the liver, pancreas, and intestinal glands. 

The Pancreas. As the acid chyme enters the duodenum it 
activates some "prohormone," probably -prosecretin, which is first 
absorbed into the capillaries of the blood vessels and then carried 
throughout the body. Some secretin ultimately reaches the pancreas, 
which is then stimulated to further activity causing the chemical 
secretion of the pancreatic juice. The pancreas is one of the most 
important digestive glands in the human body. It is anatomically 
a rather diffuse structure resembling the salivary glands in form. 
Its duct, joined with the bile duct from the liver, empties into the 
small intestine a short distance below the pylorus near the juncture 
of the duodenum and the ileum. 

The secretions of the pancreas or "stomach sweet bread" contain 
three groups of enzymes, (1) amylopsin, (2) trypsin and some erepsin, 
and (3) lipase, which act respectively upon carbohydrates, proteins, 
and fats. The first, amylopsin, breaks down starches by hydrolysis 
to double sugars, finally yielding the disaccharide maltose, and dextrin. 
Maltose is further broken down by maltase into a monosaccharide, 
glucose (dextrose), which may then be absorbed. 

Second, in order for absorption to take place in proteins they must 
be broken down into their constituent arnino acids by the action of 
at least trypsin and erepsin. Protein material reaches the first por- 
tion of the small intestine, or duodenum, in the acid chyme which is 
generally neutralized somewhat before the proteolytic enzymes do 
their work. 

Third, fats, thus far unchanged in the process of digestion except to 
be melted by the heat of the body, are then emulsified by the bile and 
finally are hydrolyzed in the intestine by the action of lipase into 
glycerol (glycerin), and also one or more fatty acids. These are 
absorbed by the epithelial cells of the villi, resynthesized into more 
complex fats, and passed into the lymph channels, or lacteals. 

Aside from the noteworthy office of "secretor of the pancreatic 
juice," the pancreas has another important function. One might 
say that it is one of the "board of directors" governing the li(>altli 
of the body. When the sugar content of the blood becomes too high 

H. V. H. — 20 


and sugar appears in the urine, diabetes, a disease caused by a 
dearth of insulin in the blood, occurs. Insuhn is a hormone* formed by 
groups of cells collectively called the islands of Langcrhans, which 
function as ductless glands. Since 1921, when Banting, Best, and 
Macleod found that insulin injected into animals showing symptoms 
of diabetes caused a decrease of sugar in blood and urine, this pan- 
creatic hormone has become a veritable lifesaver to man. 

The Liver. The liver is the largest gland in the body, and in man 
is found just below the diaphragm, a little to the right of the mid 
line of the body. It is not primarily a digestive gland, although it 
secretes daily about a quart of bile, which while containing no en- 
zymes may have the power of rendering the lipase of the pancreatic 
fluid more active. Bile when mixed with the pancreatic juice helps 
emulsify liquid fats into minute separate droplets, in this way pre- 
paring them for digestion. Certain substances in the bile aid espe- 
cially in the absorption of fats. Another important function of bile 
is the neutralization (wholly or in part) of the acid chyme when 
it enters the duodenum, thus preparing it for the action of the 
pancreatic juice. Bile also stimulates the peristaltic movements of 
the intestine, thus preventing extreme constipation. It is also thought 
by some to have a slight antiseptic effect in the intestine. Bile seems 
to he mostly a waste product from the blood. Its color is due to 
certain substances wiiich result from the destruction of worn-out red 
corpuscles of the blood. 

Besides these digestive and excretory functions the liver is also 
concerned with the formation of a nitrogenous waste, urea, CO(NH2)2. 
This product is largely ]:)roduced in the liver, whence it is transferred 
to the blood and carried to the kidneys where it is excreted. 

Perhaps the most important function of the liver is the formation 
and storing of an animal starch, or glycogen. The liver is supplied 
with blood from two sources, some from the heart, but a greater 
amount directly from the walls of the stomach and intestine. This 
latter blood supply is very rich in food materials and from it the cells 
of the liver take out sugars in the form of glucose (dextrose), which is 
synthesized into animal starch in the liver. Glycogen is stored in 
the liver until such time as energy is needed. It is then reconverted 
to the monosaccharide form, glucose, and carried by the blood stream 
to the tissues where it is oxidized with an accompanying release of 
energy. A limited amount of glycogen may be found and stored 
in the muscles and it is also thought to be produced from proteins and 


possibly fats as well as carbohydrates. Storage of glycogen in the 
liver has been demonstrated by taking two rabbits, which were fed 
heavily on clover after a period of starvation. After allowing suit- 
able time for digestion and assimilation, one rabbit was killed and 
glycogen was demonstrated in the liver cells, while the other was 
given strenuous exercise before being sacrificed to science. Upon 
examination the second rabbit showed a greatly reduced quantity of 
glycogen in the liver cells. 

The Secretions of the Small Intestine. There can be no 
doubt of the importance of the part played by the pancreas and liver 
in digestion which is supplemented by secretions of the intestinal 
wall, called collectively intestinal juice, or succus entericus, a substance 
containing five important enzymes secreted by small intestinal 
glands of the mucosa (see figure of villus). The first, enterokinase, 
acts as a co-ferment on proteins and was formerly thought to be an 
activator for trypsinogen. Erepsin, while appearing to be the same 
as that appearing in the pancreas, hydrolyzes peptides to amino acids ; 
maltasc, as previously noted, converts maltose into dextrose, while 
lactase hydrolyzes milk sugar into the simple compounds of galactose 
and dextrose, and invertase converts ordinary table sugar into levu- 
lose and dextrose. The three are frequently spoken of col- 
lectively as inverting enzymes. 

It should be remembered that the large intestine produces no 
enzymes, wherefore it is as.sumed that little or no digestion takes 
place there. The bacteria of the large intestine attack any protein 
material which has escaped digestion and break it down by putre- 
factive fermentation. 

Absorption and the Fate of Absorbed Foods 

In animals that possess circulatory systems the diffusible end-prod- 
ucts of foods are passed through the epithelium of the gut into the 
blood stream, or, in the case of fats, through the lymphatics to the 
blood. In higher vertebrates most of the absor})tion takes place in 
the walls of the small intestine. While diffusion and osmosis are im- 
portant factors in the passage of food and water through the walls 
of the intestine, many physiologists agree that the living matter in 
the cells lining the intestine exerts energy which affects the absorption 
of the substances that pass into the blood and lacteals. This is proved 
by the fact that if these cells are injured or poisoned, absorption 
follows the laws of osmosis and diffusion. Ordinarily the cells lining 



the intestine are like tiny chemical laboratories. Carbohydrates in 
the form of monosaccharides, or glucose (dextrose), are absorbed 
through the epithelial cells lining the villi and reach the capillaries of 
the circulatory system. Proteins in the form of amino acids likewise 
reach the blood stream in this way. Glycerin and fatty acids are 
absorbed by the epithelial cells, resynthesized in these minute chemical 
laboratories into more complex fats, and are then passed on to the 
lymph channels, ladeals, of the lymphatic system in the villi. This 

gobWt cell ,cap\lk 

Cells of 




Diagram of intestinal villi and glands. Can you explain the part played by 
the villi in absorbing digested " foods " ? 

fluid or lymph then passes into the other lymphatics, eventually reach- 
ing the blood through the thoracic duct which enters the jugular vein 
in the neck. On the other hand, simple sugars and amino acids pass 
directly into the blood and reach the blood vessels which carry them 
to the liver, where, as we have seen, sugar is taken from the blood 
and stored as glycogen. From the liver the food within the blood is 
carried to the heart and is then pumped to the tissues of the body. 
A large amount of water and some salts are also absorbed through 
the walls of the stomach and intestines. The greatest loss of water, 
however, occurs in the large intestine. 



We have already traced the changes taking place in the absorbed 
sugars, chiefly dextrose, and have shown how they may be taken from 
the blood stream, converted into glycogen, and temporarily stored. 
Some of this sugar is usually available in the circulating blood which 
contains 0.1 to 0.15 per cent of it. The muscles likewise store glyco- 
gen that is used as work is done. Carbon dioxide and water are 
the final products of carbohydrate oxidation. Experimental evidence 
indicates that glycogen may be produced from some of the metabolic 

PTOcass: builds protoplasm; 
. ^lastss. mostly crccttinin 
and. pifTin. booCies 



formecL by 

process : excretion of ^fastes, 

as arao. anct uric acict, 

r'eptilas a.not bircts . 



^ JT 


process: oxidation, 

Gnergsy relsasscC for~ 
msto-Dolisra ■ wastes, 
■woter ancCearpon dioxide 


Summary of metabolic processes. 

products of proteins.^ The production of glycogen from fats still 
lacks conclusive evidence, although there is some indication of indirect 

The proteins which have been absorbed may be utilized in two 
ways : (1) in the rebuilding of broken-down protoplasm ; (2) in the 
supply of energy for work. Consequently, protein substances are 
often differentiated into tissue builders and energy producers. 

Fats ultimately reach the circulating blood from which they are 
taken up and used by the various tissues. Fats may be oxidized 
within the cell to supply energy. In such cases the final products 
are carbon dioxide and water. When excess fat is eaten it is held in 

1 Howell, Textbook on Physiology, 12th ed. Saunders, p. 869. 


reserve in adipose tissues. Sonic animals must build up a large sui)ply 
of fat so that they may draw upon it when their food supply is low. 
This is particularly true of such hibernating animals as the bear that 
emerges in the spring from a period of sleep at a time when its fat 
supply is depleted. Fat storage in man, upon the other hand, is 
entirely unnecessary from a physiological point of view and, due to 
the frequency of meals, is usually quite involuntary. 


Why a Transportation System? 

Within the body of nearly all of the metazoa evidence of a highly 
specialized system of internal transportation is found. The degree of 
development of such a system depends mostly upon the size of the 
organism, the amount of activity it displays, the complexity of its 
internal organization, and whether or not it is a warm blooded animal. 
The size of the body, the speed and frequency with which the animal 
moves are some of the factors that determine how "specialized and 
well trained" the "handy man" about the body, i.e., the circulatory 
system, must be. With specialization comes greater division of 
labor, yet specialized parts such as nerve cells and muscle fibers require 
not only nourishment but also the elimination of waste products 
from their immediate vicinity as well as favorable conditions of 
temperature. The solution of the problem is met in part by more or 
less bathing all cells in lymph which serves for bringing food to the 
cells and for the removal of wastes. In order to secure a continuous 
food supply and to insure the adequate removal of wastes such a 
transportation system is necessary. 

In all but the simplest organisms such a system is composed of 
vessels containing lymph which brings its contents to locations where 
it can eliminate the wastes, take up the energy-releasing oxygen, and 
pick up food for the tissues. Without such a system the organism 
cannot exist. 

Unspecialized Transportation Systems 

Unicellular animals obviously have no need for a circulatory 
system as each individual cell is in a position to excrete its own 
wastes and secure oxygen and food for itself through its own cell 
membrane. Even in slightly more specialized forms, such as the 


coelenterates, tliere is no need for a specialized transportation system 
for circulating digested foodstuffs other than that furnished by the 
ramifications of the gastrovascular system. Since the organism is 
composed of only two layers of cells, each is capable of securing the 
necessary materials forits metabolism either from outside of the body 
or from a neighboring cell lining the cavity. 

However, in the flatworm Planaria, a more highly developed gastro- 
vascidar system appears. In animals of this type the gut ramifies 
between nonspecialized cells composing the parenchymatous tissue 
in which the various organ systems of the body are embedded. As 
the food is digested it is circulated directly throughout the gastro- 
vascular cavity by means of contractions of the body, the food readily 
passing from the branched gut to surrounding tissues of the body 
by diffusion. The waste products reach the gastrovascular cavity 
and by similar muscular contractions are passed to the outside, or 
they may be excreted through the flame cell excretory system (see 
page 320). 

Still further advances in the development of specialized circulatory 
devices occur in types having a body cavity, or coelom. In a number 
of invertebrates the coelom is filled with a lymphlike fluid which may 
contain corpuscles resembling white corpuscles, or leucocytes. This 
may be looked upon as an advance over the gastrovascular type of 
distributing system. And, as we ascend the animal scale and the cir- 
culatory devices tend to become more complex, we note the tendency 
to develop definite tubes in which the circulatory fluids may l)e con- 
fined. These types are usually muscular and contractions of the 
body facilitate the movement of the fluid. In segmented forms like 
the earthworm the coelomic fluid supplements the work of the regular 
circulatory system. 

Open Circulatory Systems 

This type of transportation reaches its peak of development in the 
Crustacea. The lobster or crayfish, both aquatic forms, furnish 
familiar examples, in which the blood serves the three purposes 
of respiration, transportation of foodstuffs, and the elimination of 
wastes. As in all well-developed circulatory systems, there is a 
muscular pumping mechanism, or heart, which, by its contractions 
forces the blood along a group of so-called arteries. in turn 
usually break down into smaller vessels terminating in the tissues. 
The blood bathes the tissues and then finds its way back, usually along 


a system of sinuses, through the gills to the pericardial sinus surround- 
ing the heart. It passes into the heart by means of a series of openings 
called ostia, guarded by one-way valves. 

Insects, a still more highly specialized group, have a very direct 
respiratory system called a tracheal system, which takes over the job 
usually handled by the blood stream, bringing the oxygen directly to 
the tissues through a network of tubules, or tracheae. This has been 
discussed previously in detail (pages 209-210). 

Closed Circulatory Systems 

Among Invertebrates 

Systems of this general type are found in a large and diversified 
group of organisms beginning with the invertebrates and extending 
throughout the vertebrate group. The motive power of such cir- 
culatory devices consists essentially of a central pumping plant or 
heart, from which extends a series of arteries that break down into 
minute capillaries in the tissues and then pass into gradually larger 
vessels known as veins which return the blood to the heart. Some- 
where in the capillary circuit the blood is aerated, giving off carbon 
dioxide and taking in oxygen. The earthworm furnishes an example 
of such a system in the invertebrates. 

Among Vertebrates 

In all of the vertebrates there is a well-developed closed type of 
circulatory system, although the supplementary lymphatic system 
might be construed as a sort of open system. In order to understand 
the work performed by these systems we must turn our attention to 
the various component parts involved and consider their functions. 

The Blood 

Blood is a red fluid which, examined microscopically, is seen to be 
composed of three types of corpuscles, red and white, circulating in a 
liquid plasma, and the much smaller blood platelets. The first con- 
tains hemoglobin, which combines with oxygen in a loose combination 
forming oxyhemoglobin , useful in respiration. The white corpuscles, 
on the other hand, are the scavengers of the body. They are ame- 
boid in shape and are concerned, in part at least, with the defense 
of the body against bacterial invasion. Under certain stimuli great 


numbers of one sort or the other of these blood eells are produced. 
The blood platelets are now generally beheved to play an important 
role in the clotting of blood. 

In the web of a frog's foot the blood may be seen rushing along 
through relatively large vessels which break down into smaller ones 
until reaching the capillaries, through which the corpuscles slide in 
single file at a much slower gait. It is here that oxygen and food 
diffuse by osmosis to the surrounding lymph and so reach the tis- 
sues. Under the microscope the blood appears to be traveling at a 
headlong pace, due to the fact that this instrument magnifies only 
space without reference to time. The pace of the corpuscles quick- 
ens again as they reach the larger venules which, after anastomos- 
ing, ultimately lead to the heart as veiris. Two interesting facts 
might be mentioned here, one dealing with the capillaries and the 
other with blood. Dr. Krogh, a Nobel prize winner from Denmark, 
says that if an average human being was selected and all of his capil- 
laries were opened up and spread out flat, their total area would nearly 
cover that encompassed by an average city block. The other fact 
centers about the numbers of corpuscles present, of which various 
estimates have been made. In normal women and men there should 
be 4,500,000 to 5,000,000 red corpuscles (erythrocytes) per cubic mil- 
limeter of blood, while somewhere between 5000 and 10,000, nor- 
mally about 7500 white corpuscles (leucocytes), is considered an 
average count. Red corpuscles vary in number with altitude, a 
greater number being necessary in high altitudes where less oxygen is 
present in the atmosphere and, consequently, greater numbers are 
needed to transport the amount of air necessary for life. 

The plasma of the blood also contains a great variety of protective 
substances which are known under the general heading of antibodies. 
They are induced by bacteria and other parasites which, acting as 
foreign proteins, stimulate some living body cells to manufacture them 
(see page 626) and turn their protective substances loose into the 
blood stream. 

The Lymph 

Even though capillaries are distributed widely, each is surrounded 
by narrow lymph spaces, that are filled with plasma and white 
corpuscles, the latter being mostly lymphocytes. Lymph is concerned 
with the transportation of food, oxygen, and other substances neces- 
sary for the successful metabolism of the organism. It is lymph which 



comes into contact with the tissues and serves as the go-between for 
the blood and cells. Lymph gradually flows from the lymph spaces 
into lymph capillaries, which in turn unite to form larger and larger 
lymph vessels, interspersed with numerous lymph glands and lymph 

nodes. Finally the lymph vessels unite into a 
large thoracic duct emptying into the jugular 
vein in the neck region. 

The Conduits — Arteries, Veins, and 

Having considered the "stuff" that blood is 
made of, we can now turn to a consideration 
of the vessels through which it passes. The 
chief function of the capillaries centers about 
the exchange of the products of metabolism 
with the lymph. Some of the plasma of the 
blood actually transudes through the walls of 
the capillaries, while certain types of leuco- 
cytes also pass through the walls, which are 
composed of nothing more or less than a 
single-celled layer of epithehal cells, called 

Distinct structural differences exist between 
the capillaries and the arteries and veins of all 
vertebrates. Both arteries and veins are cov- 
ered externally by a rough protective coat of 
connective tissue. Between this and the inner 
endothelial lining lies a layer of elastic mus- 
cular fibers. -In veins, this layer is relatively 
thin, while in the arteries, it is quite well de- 

Principal lymph chan- 
nels of man. Note the 
abundance of lymph ves- 
sels in the region of the 
intestine. What function 
do they serve ? Find the 
thoracic duct emptying 
into the jugular vein. 

veloped, probably being correlated with the 
greater pressure to which arteries are subjected as evidenced by the 
periodic spurting of blood whenever an artery is cut. 

In the veins blood is prevented from flowing back away from the 
heart by a series of cuplike valves that open in the direction of the 
blood-flow toward the heart but which close when the reversed move- 
ment is attempted. They are quite similar to the semilunar valves 
of the heart (page 308). 

Veins collapse when cut while arteries do not. This fact proved 
a stumbling block to the proper interpretation of the anatomy and 






physiology of arteries and veins by the early scientists. William 
Harvey (1578-1657) was the first to understand thoroughly the cir- 
culatory system, but other earlier and 
contemporary workers were not far 
behind him. The great artist, Leo- 
nardo da Vinci (1452-1519), left in 
manuscript numerous drawings and 
notes on the heart and other vessels, 
stating that the aorta "subdivides 
into as many principal branches as 
there are principal parts to be nour- 
ished, branches which continue to 
ramify ad infinitum.'' Vesalius 
(1514-1564) in his famous anatomical 
treatise, Fahrica, first published in 
1543, expressed doubt as to the exist- 
ence of the connecting "pores" be- 
tween the two sides of the heart. 
This was an attack upon one of the 
main features of the teachings of 
Galen, who believed there was "an ebb 
and flow of blood within both veins 
and arteries throughout the system." 
blood and the arteries vitalized blood. 



An artery 

A capillary 

Comparison of the walls of an 
artery, vein, and t-apillary. 

Diagram showing how valves of a vein 
prevent the back flow of blood. 

The former contained crude 
Yet neither Vesalius nor Galen 
-200 A.D.) apparently under- 
stood the circulatory system. 

William Harvey is rightfully 
known as the father of physiology 
for in 1616 he began presenting his 
views on the circulation of the 
blood. His book, however, did not 
appear until 1628. In it we find 
evidence for the thesis that the 
heart is the pump,^ that the arteries 
dilate passively as the heart forces 
the blood into them, that the blood 
goes from the right ventricle 
through the lungs to the left auricle, 

' All stages of this phase of the argument are 
not outlined fully. 



and that the amount and rate of flow of the blood from the heart makes 
it necessary to assume that most of it must return to the heart. This 
latter fact was shown by assuming that the ventricle held only two 
ounces ; then, if the pulse beats 72 times per minute, in an hour it 
would force 72 X 60 X 2, or 8640 ounces, or 540 pounds, into the aorta, 
which is considerably more than the weight of man. The return of 
the blood to the heart is accomplished by veins, thus completing the 
circuit. This summarizes briefly the gist of Harvey's contributions 
on circulation. Small wonder that after so many misleading beliefs 
this master should be acclaimed for his careful thinking and his 
accurate observations upon the action of the heart. His study in- 
volved examinations of about forty species of animals, and ulti- 
mately led to the fundamental concept of the circulation of blood. 

The Heart 

The vertebrate heart is really a pumping station which in its 
simplest form, as found in the fishes, consists of a receiving auricle 
and a pumping ventricle. Back flow is prevented by a series of valves 
placed at strategic points. Ascending the vertebrate scale and leav- 
ing behind water-inhabiting forms, we find the circulatory system 


hlrd. and, 

Evolution of four-chambered heart. Contrast situation in fish and amphibia 

with reptiles, birds, and mammals. 

becoming more complicated and the heart evolving from a two- 
chambered form, typical of fish, to a four-chambered type found in 
birds and mammals. Intermediate stages in this progression appear 
in the amphibia and reptiles. 

The heart of man is a cone-shaped, muscular organ about the size 
of the fist. It is surrounded by a loose membranous bag called the 



pericardium, the inner lining of which covers the heart and secretes 
the pericardial fluid in which the organ lies. The heart of an adult 
mammal may be divided into a right and left side, each having no 
direct internal connection with the other. Each half may likewise 

innomlnatfi- left, 
ar-tery.... subclavian 

; arterx.^ 




vsna casid} 





. left, . 1 
— -vetttncle 

A section through the mammalian heart. Read the text carefully and trace 
the course of blood through the heart. 

be divided transversely into an upper relatively thin-walled auricle 
and a more muscular lower ventricle. The right side contains 
unoxygenated or venous blood, while the left auricle and ventricle 
contain arterial blood saturated with oxygen. 

The right auricle receives the venous blood by two vessels known 
as the superior vena cava, or precava, entering on the anterior surface 
and bringing the blood from the head and neck, and the inferior vena 
cava, or postcava, which empties into the lower portion of the right 
auricle, returning the blood from parts of the body below the dia- 
phragm. The blood passes into the right ventricle through the; 
tricuspid valve which, as the name suggests, is composed of three 
irregularly shaped flaps. The tips of these flaps project into the 


ventricle, where they are attached by tendinous chords, the chordae 
tendineae, to small muscular projections called the papillary -muscles, 
extending from the wall of the ventricle. Back flow is prevented 
upon contraction of the ventricle by the closing of the flaps due to 
pressure, while a reversal of their position is prevented by the chordae 
tendineae and the contraction of the papillary muscles. Thus the 
blood passes from the right ventricle into the pulmonary artery, the 
lower portion of which is guarded against back flow by three lialf- 
moon-shaped cups, called the semilunar valves. The blood has now 
started toward the lungs through the pulmonary artery, which is the 
only artery carrying unoxygenated blood, to the lungs, where car- 
bon dioxide is given off and oxygen taken in by the hemoglobin in 
the red blood corpuscles. It then passes into one of the larger 
pulmonary veins and so reaches the left auricle of the heart. Here 
the process described for the right half of the heart is repeated ex- 
cept that the left auricular- ventricular orifice is guarded by the 
bicuspid valve, w^hile the semilunar valves on this side of the heart lie 
in the aorta which is the outgoing artery carrying the blood about the 

The "beating" of the heart is a more complicated story than can 
be elaborated here. First, as the ventricles relax, blood flows from the 
veins into the auricles and ventricles, then the two auricles contract 
simultaneously, further dilating the two ventricles. This is followed 
by the immediate contraction of the two ventricles. Then follows 
a brief period of relaxation or rest during which the auricles and 
ventricles are being filled again, after which the cycle is repeated. 
This forces the blood from the heart in a series of spurts, accounting 
for the type of bleeding noted when an artery is severed, and for the 
expansion of the elastic arteries as the blood is forced out of the heart 
into them. 

The Aortic Arches 

As the blood goes out through the pulmonary artery it is passing 
through the embryological remains of the aortic arches. Originally 
six in number, these paired aortic arches are of great interest to 
students of evolution since embryological and comparative anatomical 
studies have yielded a very striking picture of the changes in this 
region involved in the shift of vertebrates from water to land. From 
fishes on up to mammals only these functional aortic arches have 
persisted, although six pairs of aortic arches are usually reckoned as 


the fundamental numbcM-. An idea of the changes involved from life 
in the water to life on land may be secured from tli(> figure. 

11 w E "2: "St 

^.^.^ ^ 

duarsoX aorta 

primitive condition 

. . r ^ventro.! aorta. 

carotid j- _ i ► 

dorsal aorta 

^ -^= ^ 

yauriclez^^^^) - . . 

-ventral aorta~ri^^ri;;^:^i 

*^T - t; — '^^\iM?lliTl Ir^l f --^ ^ Oorsol OLorto. 

'\~\^ \'\\'\ '\'\\sA^^Na\x4\\^ \V ^^^^^^ am phibian 

V ventrol oorxa 
carotjdC left systemic arch. 

,^;^'_-'^-^ -i-"^ - - CZ;X!r ''''r^~^-"r;m^ — - ^ dorsql Op rto. 

uAV^^^^^ ^igber am phibia 
^_ ____^ ^ ot^ re ptiles 

'pvuTOonary trunk 

Carotid.) ri^t systemic arcVj , 

* ^ "''' rr"'-'- '\ ^ ^ '- ^ ' '^.' ■ / - - -^■■;-..-^Wx, pv.\trtort(xrir a.-rtti^y 

■ Vi /^^' ^7 — \ V, l AjT l ^^— --•^ cAorsol oiorto. 

-.;..> ,^ , ,., ^ wi^-- -'-^^i-^^^^ bird: 

^^^j;!?'^-^.. ^ S"^e^^ig!l. great a rch of oorta 

7T— '•TY'; — '■ r\\\Vl"\\/9 '"•';' ""V'"'7yf( aors-o-i aorcct 

'--■"■^"-'-:!"....i^ ^.^ '''^ ;;::_-----"'^xJpuiTnonarx tnmk 
Fate of the aortic arches of the vertebrates as seen from the side. (After (iiiyer.) 

The Course of the Blood in the Body 

There are two distinct systems of circidation in the body. The 
pulmonary circulation, noted in connection with the study of the 
heart, takes blood from the right auricle and ventricle to the lungs, 
passing it back to the left auricle. The longer circulation is known 
as the systemic circulation in which the blood leaves the left ventricle 
through the dorsal aorta and through ever-branching arteries pa.sses 
to the muscles, nervous system, kidneys, skin, and other organs of 
the body. It gives food and oxygen to these tissues, receives the 
waste products of oxidation while passing through the microscopic 
capillaries, and returns to the right auricle through veins. 


Some of the blood on its way from the heart passes to the walls 
of the food tube and so on to its glands. From these parts it is sent 
with its load of absorbed food to the liver. Here the portal vein 
that carries the blood breaks up into capillaries around cells of the 
liver, which take out the excess sugar from the blood and store it as 
glycogen. From the liver the blood passes directly to the right auricle. 

Functions of the Blood 

The blood being the circulatory tissue plays a very important part 
in the maintenance of the organism. Most waste products of the 
tissues are carried by the blood from their point of origin to some 
other region of the body which is adapted for their elimination. 
Thus the nitrogenous waste, urea, is carried to the kidneys. Other 
wastes are eliminated through the sweat glands of the skin or the 
lungs. The blood stream is also concerned with the transportation of 
oxygen from the lungs, and nutrient material from the intestines to the 
tissues. In addition, it carries the products of one tissue to another ; 
for example, internal secretions which are produced in glands must 
be transported elsewhere to do their work. Secretin, already referred 
to, will serve as an example of this type of action. 

In addition to the three transportation jobs already mentioned 
the blood also serves to remove various waste products of metabolism 
from the point of their formation to the organs which excrete them, 
i.e., the lungs, skin, intestines, and kidneys. Through its accessi- 
bility to the various organs and glands of the body, the blood may aid 
in maintaining the normal acid-base balance of the tissues as well as 
the water content of the body. 

We know that oxidation generates heat, which means that in 
the human body heat is being constantly released by the working 
cells. It is carried by the blood stream to the outside layers of the 
body and there dissipated in the surrounding environment unless 
special heat-regulating devices are present. Man regulates his body 
temperature very largely by controlling the heat loss through nerve 
impulses causing contraction of the minute blood vessels in the skin. 
The expansion of these blood vessels, resulting from the stimulus of 
the vasomotor center of the medulla oblongata, allows greater radiation 
and consequent loss of heat (see page 351). What is of perhaps still 
greater importance to man in cooling his body is the ability of sweat 
glands to increase their action under proper nervous stimulation 
and to pass out more sweat to be evaporated. Heat is required 


to vaporize the sweat on the body surface, and body heat is lost. 
Conversely, by performing muscular work, heat may be produced in 
greater quantity through the increase of oxidations in the body. 

Clotting is another very important function of the blood. We are 
all familiar with the fact that while blood is fluid when drawn from 
the body it soon becomes viscous and later gelatinous. Finally a 
clot is formed, which may be seen floating in the blood serum. It 
was initiated in part by the dissolution products of the blood 
platelets. In the gelatinous stage, both red and white corpuscles 
are caught in the fibrin network, and as the clot shrinks the red 
cells are held more tightly by needlelike fibers of fibrin. There are 
too many theories of clotting to present here, but when blood is 
exposed to air chemical changes finally transform the soluble fibrino- 
gen, which occurs normally in the blood stream, into insoluble fibrin. 
The blood of a normal person ordinarily clots in about five minutes. 
The blood of a few persons, however, forms clots very slow^ly or 
refuses to clot at all. Such a condition is known as hertiophilia, and 
the person affected as a hemophiliac. 

Finally, the blood plays an important part in health and disease 
both through the distribution of antibodies and the defense mechan- 
ism of the white corpuscles against bacterial invasion. 



Every living organism requires oxygen for its metabolic processes, 
which demands that every cell shall take in oxygen and give off 
wastes, largely carbon dioxide and water. This exchange of free 
oxygen and carbon dioxide is necessary for combustion. In all ver- 
tebrates respiration may be divided into two types, external and inter- 
nal respiration. The former involves the exchange of gases between 
the atmosphere and the blood through some specialized device such 
as gills or lungs, while internal respiration is an interchange between 
the blood and the cells of the body. 

In looking into the story of respiration, one finds the first relevant 
suggestions coming from John Mayo who in 1668 suggested that res- 
piration and combustion were analogous processes. His work was 
antedated by another early worker, Robert Hooke, the same man 
who described the dead cells in cork, and who demonstrated by the 
use of experiments that air is necessary for the maintenance of life 
H. w. H. — 21 


in animals. It was Priestley (1733-1804), however, who discovered 
oxygen and recognized its great importance to all living matter. The 
name of one more important early worker, Lavoisier (1743-1794), 
should remain in our memory as he was the first man to attempt a 
quantitative scientific study of the phenomenon of respiration. It 
was he who first stated ''life is a chemical action" and who realized 
that animal heat was the result of an oxidation process involving 
substances of the body. Both he and LaPlace (1749-1827) carried 
on numerous experiments on respiration and its relation to the pro- 
duction of animal heat. Out of this humble beginning has come all 
the later fascinating studies upon respiration by such workers as 
Liebig, Voit, Rubner, Pettenkofer, Atwater, Rosa, Benedict, and 

The Protein, Hemoglobin 

Before turning attention to the various devices developed to meet 
the problem of respiration -one mechanism that is universally present 
in the vertebrates should be mentioned, namely, the respiratory 
pigment hemoglobin. This is a protein compound found in the red 
corpuscles of vertebrates. It has the ability of combining readily 
with oxygen to form oxyhemoglobin, thus enabling the blood stream 
to carry much more oxygen than it could possibly do by saturating 
the plasma. 

The interchange of oxygen and carbon dioxide may be explained by 
physical laws. It is known that a gas tends to pass in the direction of 
the least pressure. Even when a moist, permeable membrane, or a se- 
lectively permeable membrane, such as the epithelium of the lungs and 
capillaries, is placed between different gases the molecules pass freely 
back and forth. In the event of a difference in pressure between the 
two sides of the membrane, the gases pass through from the region of 
greater pressure to that of the lower pressure until it is equalized. 
Oxygen constitutes nearly 21 per cent of the atmosphere and is pres- 
ent in sufficient amounts to furnish enough pressure to transfer it 
to regions of lower pressure. If we keep in mind the fact that the 
pressure of oxygen outside the body must always be greater than that 
in the blood stream in the lungs, we can readily understand why 
oxygen must pass through the moist permeable membranes and into 
the blood stream, thus giving us the explanation of external respiration. 
On the same basis internal respiration may be explained. The first 
step involves the liberation of oxygen from the blood to the lymph. 


while the next centers around its transfer to the cells of the body. 
An examination of the first stage shows the blood passing through the 
capillaries which are bathed in lymph where the oxygen pressure is 
very low. This condition brings about dissociation of the oxyhemo- 
globin to such a degree that it loses over a third of its oxygen during 
its brief passage through the capillaries. The lymph in turn loses oxy- 
gen to the cells in the same way. While oxygen is being liberated 
carbon dioxide is being returned to the blood stream in exactly the 
same manner, for carbon dioxide is present in greater concentration 
in the cells than in the lymph and in the blood stream respectively. 

External Respiration 

While the phenomenon of respiration is a common one yet it is 
accomplished in manj' different ways. Small, single-celled, or rela- 
tively simple organisms have no need of a complicated respiratory 
system. However, it is well to remember that while the surface of a 
body varies as the square, its volume varies as the cube of its diam- 
eter. This means that as an object increases in size the ratio of its 
surface to its volume becomes smaller. By transferring this thought 
to biological fields we can readily appreciate that as animals increase 
in size respiratory systems become a real necessity. 

A survey of the animal kingdom shows that organisms have met 
this need in a great variety of complex and sometimes rather 
peculiar ways. Four types of respiration are commonly found, 
namely, respiration through the surface of the body, by means of 
gills, tracheae, and lungs. Three other methods are less commonly 
found, namely, by means of respiratory papillae, respiratory trees, and 

Respiratory Papillae. These occur as evaginations from the 
dorsal surface of such forms as the starfishes, where they are known 
as dermal branchiae. They are really outpocketings of the body 

Respiratory Pouches or Trees. These tubular and more or 
less branched pouches occur in such groups as the sea urchins, holo- 
thuroideans, and some starfish. In the first group the pouches are 
outgrowths from the mouth, while in the holothuroidea they are 
outpocketings from the rectal region (see figure, page 314). 

Lung-books. Such structures consist of a series of folds suggest- 
ing the pages of a book. Each "leaf" is filled with blood spaces and 
is exposed on two sides to the air. Respiratory devices of this type 





body vail 

are found in many spiders while a similar structure called a gill- 
book occurs in the horseshoe crab, Limulus. Gill-books may more 

properly be considered as 
"^'^'^ modified gills. 

The Body Surface. 
This type of respiratory 
system is probably the 
most simple. It consists 
of an exchange of gases 
through the surface of 
the body. It is found, 
however, not only in such 
simple one-celled animals 
as the protozoa, which 
have no specialized sys- 
tem for respiration, but 
also in sponges and coe- 
lenterates. Even in the 
parasitic and free-living, 
flat worms and some 
roundworms, respiration 
is of the same type. 
Some of the smaller forms 
of the higher groups may 
also resort to this method 
of gaseous exchange. 

In some of the more 
highly specialized forms 
such as the earthworms, a circulatory system is present although 
respiration still takes place through the cuticle. The blood of the 
earthworm is red and contains hemoglobin which is dissolved in the 
plasma, just the opposite of the situation in the vertebrates where 
hemoglobin occurs in the red blood cells. 

Complete dependence upon integumentary respiration does not 
occur among vertebrates. Probably the closest approach to such a 
situation is in the lungless salamanders (Plethodontidae) and in 
certain other urodeles, such as the hellbender, Cryptohranchus. In 
the former, integumentary respiration is usually supplemented by a 
capillary network in the pharyngeal region and is therefore designated 
as buccopharyngeal respiration. A highly developed system of capil- 




'lungs," or respiratory tree of the sea 
cucumber, a holothurian. 




laries which almost penetrate to the outer surface of the epidermis 
is found in the integument of many amphibians. In some amphibia 
as much as 74 per cent of the carbon dioxide is given off through the 
skin. Such adaptations are possible only where a cool environment 
keeps down the metabolic rate of these forms. 

Gills. Gills are either flattened or feathery, and are external or 
internal in their location. Invariably the blood circulate.s in them and 
is separated from the surrounding water by a thin membranous wall 
through which the dissolved gases are exchanged. Among the 
invertebrates the position of the 
gills varies in accordance with 
the habitat of the animal. In 
such forms as the crayfish for to pericardial 

example, they are in a protected Sirjus 

outer chamber covered by chitin. 
Circulation is accomplished by 
the creation of a water current 
through the action of the swim- 
merets and certain appendages 
about the mouth. In fishes 
and amphibians, water typically 
enters the mouth where it is 
passed to and o^^er the pharyn- 
geal gills and from there through 
slits to the outside. 

Tracheae. These are com- 
posed fundamentally of air- 
carrying tubules, which, by a 
series of anastomoses and rami- 
fications, penetrate to nearly all 
parts of the body. They are 
characteristically found in most 
insects, myriapods, protracheates, and some arachnids. Such a sys- 
tem starts with a series of openings known as spiracles, occurring 
along the outer surface of the thoracic and abdominal segments. 
Leading from the spiracles are air tubes, or tracheae, which show 
great numbers of anastomoses, frequently forming abdominal reser- 
voirs, or air sacs. The tracheae are nothing more or less than a 
series of pipes, for they are lined with chitin and stiffened by a spiral, 
fiberlike thickening:. The finer subdivisions of the tracheae extend 

efferent branchial 

V(3.5S<2^1. .- 

afferent branchial 


from, lateral 
bloocC $inix.$ 

l^jrtion of gills of crayfish. 

protecting branchiostegite. 
blood aerated .3 

Note the 
How is the 


to all inner jjarts of the body where they end blindly making possible 
the delivery of oxygen directly to the cells. Here again external res- 
piration takes place in the spiracular region, while internal respiration 
centers about the diffusion of gases to and from the tracheae and the 
cells. The efficacy of this system is suggested by the rapid and sus- 
tained metabolism common to many of the insects. 

Lungs. This type of respiratory system is found best developed 
among the birds and mammals. The lungs of birds are specialized 
for a high metabolic rate and for making lighter the load which must 
be lifted in flight. Air sacs connected with the lungs are found 
throughout the viscera, and even the bones are filled with air and so 
are very light. The connection between these and the rest of the 
respiratory system has been demonstrated by closing the trachea and 
opening the air sac in an upper wdng bone. The fact that the bird 
continues to breathe demonstrates this connection. 

The mammalian respiratory system is essentially the same regard- 
less of the form studied. The most important part of the lungs are 
the terminal air sacs called alveoli, in which the inspired air contacts 
the many capillaries of the circulatory system found throughout 
the moist mucous membranes. Oxygen and carbon dioxide diffuse 
through the capillary walls surrounding the alveoli and so the 
exchange of gases is effected. 

Internal Respiration 

It has been shown in the case of very simple animals, such as 
Paramecium, that when oxidation of food takes place in the cell 
energy results. In forms which possess complicated circulatory sys- 
tems, external respiration must first take place, after which oxygen 
is transported by the hemoglobin of the blood to the various parts 
of the body where the actual work is to be done. Here real or in- 
ternal respiration takes place, since cell activity depends upon food 
and oxygen. 

As aerated blood passes through the capillaries these are bathed 
in plasma in which the oxygen pressure is low. The oxyhemoglobin, 
a compound of oxygen and hemoglobin, is stable only in an environ- 
ment where oxygen pressure is comparatively high. Therefore the 
hemoglobin delivers itself of the oxygen to the lymph, which in turn 
transfers it to the cells. The pressure of carbon dioxide on the other 
hand is higher in the cells thus facilitating its transfer to the lymph 
and so to the blood stream proper. 



Respiratory System in Man 

Air passes from tjie nostrils through the shthke glottis into the 
windpipe. This tube, called the trachea, the top of which may easily 
be felt as the " Adam's apple " of the throat, is supported by a series of 
cartilaginous rings complete in front but incomplete behind and divid- 
ing into two hronchi. Within the lungs, the bronchi break up into a 
great number of smaller 
tubes, the hronckiolcs, which 
divide somewhat like the 
small branches of a tree 
and arc lined with ciliated 
epithelial cells. The re- 
mainder of the tubes are 
also lined with ciliated cells, 
the cilia of which are con- 
stantly in motion lashing 
with a quick stroke toward 
the outer end of the tube, 
that is, toward the mouth. 
Hence any foreign material 
in the tubes will be ex- 
pelled first by the action 
of the cilia and then by 




.bronchial tubes 

coughing or 

"clearing the 

The respiratory system of man. Note the 
cartilagenoiis rinjjs supporting the ducts. 


The bronchial tubes end, 
as already noted, in very 
minute air sacs called al- 
veoli. Great numbers of these are present, thereby increasing the 
respiratory surface tremendously. These tiny pouches have elastic 
walls into which air is taken when we inspire or take a deep breath. 
Around the walls of the pouches and separated by a xcry thin 
membrane, are numerous capillaries from the pulmonary artery 
which brings the blood from the right ventricle of the heart to the 
lungs. Through the very thin walls of the air sacs a diffusion of gases 
takes place, which results in the blood giving up carbon dioxide and 
taking in oxygen. Consequently the blood becomes a brighter red, 
due to formation of oxyhemoglobin by the combination of oxygen with 
the hemoglobin in the red corpuscles. 





Carbon dioxide . . . . 
Nitrogen and other gases 
Water vapor 

In Outdoor Aik 


In Air Expired 





As shown in the above table, there is a loss of nearly 5 per cent of 
oxygen and a corresponding gain in carbon dioxide and water vapor 
in expired air. 

The lungs are located in a triangular, air-tight sac or thoracic cavity, 
with the sternum or breastbone in front, the ribs on the side, the 
immobile vertebral column at the back, and the convex diaphragm 
below. The ribs, connected to the breastbone in front and the back- 
bone behind, are united to each neighboring rib by a sheet of intercostal 
muscles. Furthermore the articulation of the rib with the vertebral 
column is higher than its connection with the sternum, and the shape 
is such that when the lungs are empty the "convexity of the curve 
points slightly downwards." Inspiration results from the contraction 
of the intercostal and associated muscles which not only pull the 
ribs toward a horizontal position but also force the sternum ventrally. 
The diaphragm., which also assists, is a combination of a membrane 
and muscle and forms a partition between the thoracic and abdominal 
cavities. The concave surface of the diaphragm is towards the pos- 
terior, that is, down. Contraction reduces the concavity so that the 
result is an increase in the capacity of the thoracic cavity. Keeping 
in mind that the chest cavity is air tight, the lungs elastic, and that 
the sole entrance of air is from the trachea, it is not difficult to see 
that when the capacity of the chest cavity is increased by the move- 
ments described above, the lungs naturally expand and inspiration 
takes place. Expiration is produced in part by special muscles, the 
relaxation of the diaphragm and walls of the chest cavity, and the 
elasticity of the lungs themselves. 

The nervous mechanism that controls this process is found in the 
respiratory center of the medulla oblongata (see page 351.) Under 
normal conditions respiration results from the alternate stimulation of 
two sets of fibers in the vagus nerve leading from the lungs to the 
respiratory center. The inspiratory fibers are stimulated at each ex- 
piration by the collapse of the lungs, which results in an increase in 


the rate of inspiratory discharge from the center down the cord to the 
various levels where the relay apparatus or sympathetic system causes 
inspiration. As the inspiration occurs the expiratory fibers of the 
vagus are stimulated by the expansion of the lungs and the inspiration 
is partially inhibited. Experiments clearly indicate that the gases in 
the blood have a direct effect upon the activity of the center since, 
for example, an increase of carbon dioxide in the blood results in an 
increase in the force or rate of the respirations. This however does 
not tell the whole story. Recently accumulated data furnish evidence 
for the belief that the activity of the respiratory center is controlled 
by the hydrogen-ion concentration of the blood passing through it, 
which in turn is affected by the pressure of carbon dioxide in the 



This term is used to cover the separation, collection, and elimi- 
nation of the waste products of metabolism from the body. These 
waste products naturally vary within the organism itself from time to 
time, and show even greater variation between different species of 
animals. Fundamentally such devices center about mechanisms 
which are adapted in different ways for the elimination of one funda- 
mental by-product — nitrogenous wastes. In addition liquids in the 
form of water, dissolved inorganic salts, and gases, as, for example, 
carbon dioxide, are likewise eliminated by excretory devices. Like- 
wise the digestive tract furnishes the avenue through which solid 
wastes may be eliminated, although this latter method should not 
be regarded as true excretion. Furthermore it should be realized 
that, in the vertebrates at least, there is a constant elimination or 
sloughing off of the exposed cells on various epithelial surfaces, as 
well as from the linings of various tubes and ducts which connect 
more or less directly with the outside. This section, however, is 
primarily concerned with the various urinary devices for the disposal 
of liquid wastes. 

In highly specialized forms such as mammals a number of devices are 
adapted in one way or another for the elimination of waste products. 
Before studying these mechanisms in any detail, we shall consider 
briefly the various types of excretory systems found throughout the 
animal kingdom. 


Types of Excretory Devices 

Contractile Vacuoles. Protozoa are usually characterized by 
some sort of contractile vacuole which serves to eliminate such sub- 
stances as carbon dioxide, surplus water, and perhaps some non- 
volatile nitrogenous substances. In addition to contractile vacuoles, 
protozoa may store and later eliminate more solid wastes by the 
formation of granules or crystals within vacuoles in the body. 

Intracellular Excretion. In some of the simplest metazoa a 
so-called intracellular excretion takes place. This involves the inges- 
tion of particles of waste products by certain ameboid cells which 
leave the body and disintegrate, freeing the excretory matter within 
their protoplasm. Associated with this process is the excretion of 
other wastes from the surface of the body, as is characteristic of some 
of the sponges. In addition, certain cells may store waste products 
or there may be localized areas for excretion. 

Other Excretory Devices. In some of the coelenterates the 
first evidence of true excretory organs appears in the form of pores 
connected with the alimentary tract through the canal system {e.g., 
Hydra and Discomedusae). Although other types exist they are 
unimportant for our purposes and may be omitted. 

Among slightly higher forms than sponges and coelenterates the 
waste products are carried to the outside through a complicated 
system of connecting tubules in which are located occasional ciliated 
cells, whose function appears to be to keep the fluids in motion. The 
blind ends of these tubules are capped by minute ciliated cells of the 
protonephridial excretory system called flame cells. These lie in the 
parenchyma and by their movement initiate the flow of liquid and 
soluble waste products which they have secreted through the wall. 
The waving of the tuft of cilia in each cell is responsible for the intro- 
duction of the term flame cell. In some cases it is believed that the 
cells of these convoluted tubules may also reabsorb food material 
from the passing "wastes" as well as contribute excreta to the stream. 

Reaching the higher segmented worms like the earthworm, the 
excretory apparatus is composed of a system of paired nephridia for 
each somite. Such nephridial systems are really a series of separate 
units, each of which is composed of a ciliated funnel, or nephrostome, 
and a duct that passes through the posteriad septum to empty to the 
outside. A portion of the canal is usually glandular or secretory in 
function and serves to discharge waste products into the tubule and 


possibly to reabsorb any nutrient materials which escaped in wastes 
from the fluid in the body cavity (see figure, page 192). 

In the insects still another type of excretory system is composed of 
special tubules called Malpighian tubules. The cavity of each tubule 
is surrounded by large cells covered by a peritoneal lining, emptying 
into the intestinal canal. The free ends of the tubules lie in the body 
cavity, where they are bathed in blood. The waste products pass 
into the Alalpighian tubules from the blood. This interpretation is 
supported by the detection of considerable quantities of nitrogenous 
material in the tubules (see figure, page 210). 

Excretory Devices of Vertebrates — Kidney 

The excretory organs of vertebrates are known as kidneys. While 
several different forms of kidneys are known to exist, they are all 
derived embryologically from paired segmented structures, which in 
many of the lower types may be connected with the body cavity 
by a series of ciliated funnels reminiscent of the earthworm. Along 
with the complex changes of the various systems of organs found 
in the higher forms, especially of the circulatory system, there is a 
much more intimate association of the circulatory and excretory 
systems and a decrease in the importance of the part played by the 
body cavity in the removal of wastes. 

The Mammalian Excretory System 

A typical mammalian excretory system is a complex affair, for it 
involves not only the kidneys and their associated duets, but also 
the bladder and portions of the circulatory system as well. This 
does not tell the entire story, for the lix'er, lungs, skin, and alimentary 
tract also play an important part in the excretion of wastes. 

The Liver. The liver, which was considered in connection with 
the digestive system, also plays a vital role in the elimination of cer- 
tain wastes from the body. Proteins are absorbed from the digestive 
tract in the form of amino acids. Too heavy a protein diet results in 
the absorption of more nitrogen-containing material than can be 
utiUzed by the cells of the body for tissue building. The cells of the 
liver have the ability to split off the nitrogen-containing radical and 
in some instances resynthesize the remaining materials to carbohy- 
drates and even fat. The nitrogen which is thus left behind may 
have been removed as ammonia (NH3) which is quite toxic to the 



body, especially the nerve centers, but the liver also splits off 
hydrogen and unites it with carbon dioxide to produce a relatively 
harmless substance called urea (CO(NH2)2), and water, thus 

2 NH3 + COo 

^0 = C 


+ H0O 

which in turn is removed from the blood stream by the kidneys. 
Other products which are eliminated by the hver include bile, its 
pigments, as well as various salts, neutral fats, cholesterin, and 

Other Devices for Waste Elimination. There are parts of 
other systems that should be mentioned in a consideration of the 
phenomenon of excretion. These are the lungs, skin, and alimentary 
canal. The former, as previously noted, excretes through the alveoli 
most of the carbon dioxide produced in the body of man. This may 
be indicated in tabular form ^ for man as follows : 






Alimentary canal . . 

Carbon dioxide 

Water and soluble salts, re- 
sulting from metabolism of 
proteins, neutralization of 
acids, etc. 

Solids, secretions, etc. 

Heat regulator 

Water, heat 
Carbon dioxide, heat 

Water, carbon dioxide, salts, 

Water, carbon dioxide, salts, 

hair, nails, and dead skin 

The skin serves a variety of purposes, one of the most important 
being regulation of the elimination of small amounts of carbon dioxide. 
When the kidneys are not functioning properly the skin may be 
stimulated to excrete more waste substances. The alimentary canal 
serves to rid the body of nondigested and nondigestible substances 
which, through the processes of digestion, have yielded up their 
content of foods. Furthermore, the alimentary canal actually excretes 
waste products through its walls into the lumen of the canal. 

The Kidneys. We think of these structures as the principal organs 
of excretion, and perhaps rightfully so. Nevertheless ehmination of 
wastes is not the only important function of the kidneys. They help 

'From Kimber, Gray, and Stackpole, A Textbook of Anatomy and Physiology. By permission of 
The Macmillan Company, publishers. 



to keep the ingredients of the plasma of the blood standardized, thus 
regulating the salt content of the blood by altering the ratio of salt to 
water produced in the urine, depending upon the amount taken into 
the body. The normal healthy person eliminates the following 
amounts of waste per day, through the kidneys : 30 grams of urea 
(converted ammonia) ; 15 grams of urea salt ; 10 grams of other 
soluble urea substances. The remainder, 96 per cent by weight, is 
water, making a total of one to one and a half liters that is eliminated. 
A sagittal section through the kidney reveals the expanded upper 
end of the ureter on the median side draining the basinlike pelvis 
of the kidney. The outer portion is a compact region called the 
cortex, while the inner striated portion ending in the irregular margin 
of the pelvis is known as the medullary substance of the kidneys. 


diistal tubule. 

.glomerulus'] sift 

descending limt A i 
ascending limb ,^ f 

Hanles loop 

papillary diccts 

Diagram of the human excretory system. 


. wr-athra 
How do urea, water, and inorganic 

salts reach the pelvis i* 

The inner margin of the medullary substance forms renal pyramids 
the tips of which are projections, or papillae, that lie in closely invest- 
ing cuplike depressions of the pelvis, called calyces. The tip of each 
papilla is dotted with the openings of the collecting ducts, which in 
turn are formed from the union of several renal or uriniferous 


These uriniferous tubules begin in an expansion (Bowman's capsule) 
about a little arterial knot of capillaries, called a glomerulus, which 
together make up the functional unit of the excretory system, known 
as a renal or Malpighian corpuscle. In order to understand the work- 
ings of these million odd excretory units, it is necessary to understand 
the anatomy of the kidney. 

The main trunk line of the arterial system gives off a pair of renal 
arteries that are broken down into many very small afferent vessels 
each of which enters the glomerulus, leaving as a smaller efferent 
vessel that breaks down into a typical capillary network over the 
convoluted surface of the tubule. As the wall of Bowman's capsule 
surrounding the glomerulus is thin, it is believed that water and 
inorganic salts are mechanically filtered out into the cavity by means 
of differences in pressure between the blood vessels and the lumen of 
the tubule. In the second set of capillaries the urea and other specific 
urinary constituents are first transferred by the cells and so secreted 
in the uriniferous tubule. Water and certain salts are reabsorbed 
into the blood stream at this point. 

In any event, the kidneys remove the waste products from the blood 
stream, transferring them to the pelvis of the kidney, and thence 
down the ureters to the bladder. Here the urine is stored until 
finally released to pass to the outside through the urethra. 


Clendenning, L., The Human Body, Alfred A. Knopf, Inc., 1930. Chs. III-VII. 

INIore popularized anatom}^ and phj^siology. 
Haggard, H. H., Devils, Drugs, and Doctors, Harper & Bros., 1929. Ch. VI. 

A popular account of early anatomy and physiology. 
Howell, W. H., Textbook of Physiology, 17th ed., W. B. Saunders Co., 1933. 


A detailed, technical account of physiology. 
Kimber, D. C, Gray, C. E., and Stackpole, C. E., Textbook of Anatomy 

and Physiology, 9th ed.. The Macmillan Co., 1934. Chs. XVII and 


An anatomy and physiology of the human respiratory system. Technical 

but condensed. 
Locy, W. A., The Growth of Biology, Henry Holt & Co., 1925. Ch. X. 

An account of Harvey's contribution to our knowledge of the circula- 
tory system. See also other books by this author, or others on the 

history of biology, 


Metcalf, C. L., and Flint, W. P., Fundamentals of Insect Life, McGraw-Hill 

Book Co., 1932. Chs. Ill and IV. 

A brief account of insect anatomy and physiology. 
Pearse, A. S., and Hall, E. G., Homoiothermism, John Wiley & Sons, Inc., 


An interesting discussion of the origin of warm-blooded vertebrates. 
Plunkett, C. R., Outlines of Modern Biology, Henry Holt & Co., 1930. Ch. V. 

A good physico-chemical account. 
Rogers, C. G., Textbook of Comparative Physiology, McGraw-Hill Book Co., 

1927. Chs. XVI, XVII, XXII. XXIII, and XXVII 

An advanced account of physiological digestive processes from a com- 
parative viewpoint. 
Wells, H. G., Huxley, J. S., and Wells, C. P., The Science of Life, Doubleday, 

Doran & Co., 1934. Ch. II, Sec. 7, Book 1 ; Ch. II, Sees. 4, 5, and 6. 

A readable, popular account. 



Preview. Section A. Skeletal devices • The interdependence of 
parts • The kinds of skeletons : Exoskeletons ; endoskeletons ; the axial 
skeleton ; the appendicular skeleton • Functions of skeletons : Support ; 
protection; movement • Section B. Devices for movement • The "why" 
of motion and locomotion ; protoplasmic extensions ; demio-muscular 
sacs ; water vascular systems • Muscles and muscular systems : Smooth 
or involuntary muscles, skeletal or striated muscles, heart muscle, muscular 
contractions • Section C. Mechanisms of sensation and co-ordination • 
The morphological unit — The neuron • The physiological unit — The 
reflex arc • Types of nervous systems : Neuromotor mechanisms ; co-ordina- 
tion by a network ; co-ordination by a nerve ring ; co-ordination by a linear 
nervous system ; co-ordination by a dorsal tubular nervous system • Pro- 
tective devices for the central nervous system • Anatomy and development 
of the brain : The early development of the central nervous system ; the 
parts of the vertebrate brain : The cerebrum or telencephalon, the 'Twixt- 
brain or diencephalon, the mid-brain or mesencephalon, the cerebellum or 
metencephalon, the medulla oblongata or myelencephalon ■ The cranial 
nerves • The spinal cord ■ The spinal nerves • The autonomic nervous 
system • The sense organs — Receptor devices : taste ; smell ; simple light 
receptors ; compound eyes ; camera eyes ; ears ; cutaneous sense organs • 
Suggested readings. 


It will be seen from the preceding unit that one of the most impor- 
tant essentials for an animal is to carry on successfully its metabolic 
processes. This is equally necessary for plants although they have 
the advantage of being able to secure most of the raw food materials 
they need from their immediate environment. Animals have to 
move to get their food. The necessity for motion involves three 
factors, a mechanism to support the body when seeking food, 
machinery to do the moving, and an apparatus to detect the location 
of food. In order to locate food, a co-ordination of eye and Hmb 
under control of the nervous system is required. The eye receives a 
stimulus the instant that the color or shape of food is noted by the 
receptor devices in the retina. The motions of the arms and legs then 
supplement the desire for food, followed by the act of taking it. In 



this triple process some of the thousands of pressure endings that 
are scattered over the body come into phiy. Many of these, in t\m 
case of man, are conveniently concentrated in the finger tips which 
relay messages to the brain. It is readily seen that the process of 
getting food requires co-operative action of the skeletal, muscular, 
and nervous systems. 

The limb action involving stooping, standing, and reaching calls 
into play different sets of voluntary or skeletal muscles. This empha- 
sizes one of the fundamental principles of the study of muscles 
(myology), namely, that for every muscle group there is an opposing 
set which performs the opposite type of movement. Muscles are 
effective during contraction and not during relaxation. We speak 
of the muscles that extend the arm or leg as extensors and those 
which bend them as flexors. Such muscles are very different from 
the smooth, involuntary muscles in the walls of the intestines. Here 
the food undergoes rhythmic segmentation and is broken up into 
boluses by the intermittent contractions of smooth muscle cells. 
Fortunately, the control of these involuntary muscles is taken off 
the hands of the voluntary or central nervous system. Such routine 
functions are put under the control of the autonomic nervous system, 
which frees the brain of the necessity of "willing" all these things to 
happen and leaves the central nervous system free for "higher evolu- 
tionary adventures" by taking over the "drudgery of living." In 
order to understand these processes, commonly taken as a matter of 
course, we must investigate carefully the "why and how" of loco- 
motion and then try to see how this complicated performance is 


The Interdependence of the Parts 

The material covered in this unit consists of representatives of three 
well-defined and anatomically separable systems, namely, the skeletal, 
muscular, and nervous systems. Although they are frequently con- 
sidered separately for the sake of clearness it should be kept in mind 
that, physiologically, the muscles, skeleton, nerves, and blood supply 
are all intimately interwoven. In the human body, there are numer- 
ous muscles most of which are under voluntary control and as such 
are concerned with posture, with maintaining the relationship of the 
various skeletal parts to one another, or with some sort of movement. 
H. w. H. — 22 


All of these muscles are under the control of the nervous system, while 
energy for their continued movement must be furnished by means of 
absorbed food transported through the circulatory system to every 
part of the body. To visualize this inter-relationship think of the 
sustained movement of an arm or leg which is dependent upon the 
activity of numerous muscles. The action of the muscles is in turn 
controlled by the nerves which conduct messages to the tissues from 
the brain and spinal cord. The entire network of nerves and their 
branches has often been likened to a telephone system with its compli- 
cated series of connections and relay wires. Closely associated with 
the nerves are the arteries and veins, forming the triumvirate so 
often pictured in histological or medical texts. 

The Kinds of Skeletons 

Skeletal support is of common occurrence in the animal kingdom. 
Skeletons may be divided typically into outer coverings, or exo- 
skeletons, and inner supporting devices, or endo skeletons. 


Generally speaking, any creature or organism possessing 07ily an 
exoskeleton belongs to the large group of invertebrate, or non-chordate, 
animals. Such forms may be present in some members of a given 
phylum and not in others. Even in the protozoa, for example, the 
shelled arcellidae occur in the same class with the naked Ameba. 
Other examples within this same group are the foraminifera and 
radiolaria which possess limy or glassy skeletons. This suggests that 
on the whole these types of exoskeleton are not essential for loco- 
motion but are primarily protective devices. That is certainly true of 
the sessile sponges, corals, sea-lilies, and lamp-shells (brachiopoda), 
and would also probably hold for most of the clams, snails, star- 
fishes, and brittle-stars. In the great phylum of the arthropods, the 
exoskeleton is specialized and definitely associated with an equally 
highly adapted muscular system, the two being definitely designed for 
effecting locomotion. Even among the vertebrate chordates an 
exoskeleton as well as an endoskeleton sometimes occurs, as, for 
example, in the turtles. In such forms the vertebral column becomes 
fused to the dorsal shell which is formed by the flattened ribs plus 
dermal costal plates. 



Endoskeletons are characteristic of chordatc animals. An internal 
supporting rod {notochord) is clearlj^ present in the larvae of the 
tunicatcs and in the adult amphioxus, while a well-developed endo- 
skeleton is found in all of the so-called higher forms from fishes to man. 

The skeleton of vertebrates is divided typically into three parts : 
the axial skeleton, which includes the skull, thoracic basket, main 
spinal column, and tail ; the appendicular skeleton, which pertains to 
the appendages ; and the visceral skeleton, which is developed in con- 
nection with the various modifications of the gill region. In adult 
fish, the visceral skeleton forms the cartilaginous or bony bars (gill 
arches). In other vertebrates, the visceral skeleton becomes con- 
verted into various highly modified structures involved in the forma- 
tion of the jaws, the hyoid support of the tongue, the larynx, accessory 
parts of the skull, and even the bones in the middle ear. 

The Axial Skeleton 

Anteriorly, the axial skeleton of vertebrates is specialized into a 
skull, a bony case covering the expanded anterior end of the spinal 
cord, or brain. Incorporated into this skull are specialized protective 
capsules for several of the major sense-organs, namely, the eyes, ears, 
and nose. 

Many bones are fused to form the skull. These are of two sorts, 
either memhranous or cartilaginous. The former are developed 
directly from a connective tissue membrane, while the latter type 
pass through a preliminary cartilaginous stage before becoming 
bone. In primitive vertebrates, the brain is protected by cartilage 
which later in the evolutionary picture becomes ossified. Still 
later, this original cartilaginous cranium is further protected by 
the addition of a group of thin, flat membrane bones, shingled over the 
skull. In higher forms the number of embryonic bones in the skull 
has been reduced. The skull of a dog, for example, contains fewer 
bones than that of a codfish. A study of the earlier stages of develop- 
ment in mammals shows, however, that representatives (or homo- 
logues) of many of the bones present in the cod skull may be found. 
These embryonic elements fuse in later development, making the 
smaller number of skull bones found in the adult. In the skull of a 
reptile, for example, there are four occipital bones surrounding the 
point of exit of the spinal cord from the skull, which in most adult 



mammals are fused into a single occipital hone. Further study of a 
series of forms from fish to man would furnish remarkable evidences 
of homology besides emphasizing the interpretative importance of 
the study of comparative anatomy. 




V L 1 u:m'vm r — ^^<2-noicC 

■^ 1 "^^^is^y/ \ loccriTTial 

Wi\ *^ -^^ incclccr' 

■<-/^ ■mccicjllcc 




Tnasto\di process../ / 
styloid process.../ 

" ' ..-.ethynoid 


— lacrimal 

<- . fe> nxxjcilloL 


Bones of a human skull. (After Walter.) 

The skull bones of man are frequently divided into cranial bones, 
which surround the brain itself, and those which are designated as 
facial hones. 

The remainder of the axial skeleton is composed of the vertebral 
column and its associated bones. In aquatic forms hke the fishes, 
this part of the axial skeleton is comparatively unspecialized, being 
divisible into the rib-bearing vertebrae of the trunk, and those without 
ribs, called caudal vertebrae, which go to make up the tail. With 
the evolution of land animals, protection of the under side of the 
body became essential and therefore a ''thoracic basket" was de- 




veloped, composed of ribs attached to a ventral breastbone (sternum) 
and to the dorsal backbone. Ascending the evolutionary tree farther 
the organism became better adapted to turn the head. A fish or 
frog must not only roll the eyes but also change the entire position 
of the body in order to look behind. Not so with a cat, which may 






L /..scapula. 

-thoracic sternuTn 





fxbula,. , 

1 1 fl / f i// 

In/ 1/ i//j 


Human skeleton. Can you recognize the bones of a disarticulated skeleton.^ 

roll its eyes and is also able to turn its head. This ability to rotate 
the head is due to varying numbers of cervical, or neck, vertebrae. 
Four-footed animals are further characterized by four other sets of 
vertebrae, thoracic (with ribs), lumbar (without ribs), sacral (for the 
attachment of the pelvic girdle), and caudal. 





glenoid -j^ssa 


f ooi\. .■Tneta<;arpal 5 

The Appendicular Skeleton 

A study of any group of land animals shows a fundamental simi- 
larity of limb construction. Even such apparently diverse structures 
as the flippers of a whale or a seal and the wings of a bird are found to 
be identical in fundamental plan. All sorts of land animals typically 
possess shoulder and hip girdles, respectively known as pectoral and 
pelvic girdles. These girdles are attached directly or indirectly to the 
axial skeleton, thus providing rigidity and facilitating movement of 
the appendages. It is significant that the pentadactyl limb of the 

land vertebrates is built 
upon a generalized plan, 
in which each girdle is 
formed of three bones. 
Each front and hind leg 
is likewise composed of 
three major bones. In 
the anterior limb, a single 
humerus articulates with 
two bones, the ulna, a 
process of which forms 
the "funny bone " of the 
elbow, and the radius. 
In the posterior limb the 
corresponding bones are 
the femur, which is typi- 
cally characterized by a 
prominent "ball" at one 
side of the main axis 
fitting into a socket in 
the pelvic girdle ; the 
tibia, or shin-bone ; and 
the smaller fibula. In 
addition to these larger 
bones is the group of 
wrist {carpal) and ankle 
(tarsal) bones, followed by the metacarpal and metatarsal bones, 
depending upon whether they belong to the anterior or posterior 
limb. The bones of the fingers or toes are technically known as 

•pre lirnb 

^ c c V 





ODD tarsals 

f h '^ \ 
I ;Br ■ 

Diagram of the bones of the fore and hind hmbs 
arranged to show their homology. 

'\imd. HtoId 




The feet of animals show many remarkable adaptations. Foot 
posture involves more than fallen arches ; it determines the speed 
at which an animal can travel. 
If the wrist and ankle are raised 
from the ground the result is a 
longer leg capable of a longer 
stride, which means covering 
more ground in the same inter- 
val of time. Anatomists dis- 
tinguish three types of feet : 
plantigrade, the primitive fiat- 
footed type found in man and the 
bear ; digitigradc, characteristic 
of cats or dogs that are literally 
"on their toes " all the time ; and 
the unguligradc, restricted to 
forms which walk on their nails, 
like horses, cows, and camels. 

Functions of Skeletons 

digitigixxcCe -unguligracCe^ 

Types of mammalian feet. State the 
advantages and disadvantages of each 
type of foot. (After Pander and D'Al- 

Skeletal devices usually serve 
one of three functions, namely, 
support, protection, or movement. Examples of each type will be given, 
although it is sometimes difficult to separate these functions. 


It is quite apparent that organisms living in water have much less 
necessity for a supporting framework than land-inhabiting animals. 
This is due to the fact that the body approximates more closely the 
density of the surrounding medium and is consequently buoyed up by 
it. Cuttlefishes and jellyfishes maintain their shape in their natural 
environment but out of water collapse more or less completely. 

In like manner, the bivalve shells of clams and mussels form a 
supporting skeleton, to which is attached the mantle that in turn 
encloses the viscera. Crayfish and lobsters offer still another ex- 
ample of skeletal support, for their movement is largely brought 
about through the interaction of a well-developed exoskeleton and 
inside muscles. 

In land-inhabiting forms, the function of the skeleton as a sup- 
porting device becomes most apparent. It is hard to envisage any 


other form of mechanical supporting mechanism which would permit 
the general physiological setup as we know it in land animals today. 


It is difficult to speak of the skeleton without associating it with the 
idea of protection. Special devices suggestive of protection are 
scattered throughout representatives of most of the phyla. Certain 
types of spicules in the sponges, the calcareous exoskeleton of stony 
corals, and the thickened horny layer of other branching colonial 
coelenterates (hydroids) probably serve for the protection of these 
animals. Skeletal protective devices are also quite obvious in snails, 
starfishes, sea-urchins, arthropods, armored fishes, fossil armored 
reptiles, and turtles. 


Movement is one of the almost universal characteristics of animals. 
Even in the protozoa special locomotor organs such as pseudopodia, 
flagella, and cilia are found. The earthworm uses its setae in crawling. 

The greatest use of the skeleton for movement, however, occurs in 
the arthropods and vertebrates, two highly specialized groups. The 
former have well-developed exoskeletons while the latter are charac- 
terized by an endoskeleton. This means that in the case of insects, 
for example, the muscles are inside the skeleton while in vertebrates 
they are outside. In both groups, however, the skeletal elements 
articulate with one another, usually by means of curved and rounded 
surfaces permitting free movement of one part upon the other. 


The " Why " of Motion and Locomotion 

In the first place, animals must actively seek food and must be 
constantly on the move if they are to keep from starving. In addi- 
tion, many animals, especially the higher vertebrates, give evidence 
of enjoying play, another type of muscular activity. This is more 
apt to be true of the young, but is also characteristic of many 
adults. If an organism is to survive in the struggle for existence, the 
ability to become adapted to different environments by moving from 
one place to another is a third essential. For example, grazing ani- 


mals must be able to go from one feeding area to another. This 
holds good not only from the standpoint of competition for food but 
also from that of avoiding unfavorable climatic conditions, such as 
drought, which destroys those animals that are unable to keep on 
moving to a better feeding ground. Other animals use this same 
ability of movement in flight and so survive by being able to escape 
capture. Lastly, the part played by motion in perpetuating species 
should be mentioned. The strutting and bowing of a male pigeon, 
or the battle between two male deer in the silence of the forest are 
common examples of movement employed in the perpetuation of the 

Protoplasmic Extensions 

The concept of movement is usually associated with the contraction 
of muscles, but muscles do not tell the whole story. Three distinct 
types of locomotor devices — namely, pseudopodia, flagella, and cilia, 
which are so characteristic of the protozoa, have already been 

The cirri of protozoa are probably the most highly specialized of all 
unicellular motile structures as they may be moved in any direction. 
Certain organisms like Stylonychia or Euplotes actually walk or run on 
the tips of their cirri. The action of the cirri is thought by some to 
be controlled by a so-called "neuromotor apparatus" present in these 
"simple" one-celled organisms. 

Der mo-Muscular Sacs 

Many of the soft bodied invertebrates possess locomotor muscles 
concentrated in the outer layers of the body. The earthworm is an 
example of such a type. The body is shortened by the contraction 
of the inner longitudinal muscles and elongated by the action of the 
outer circular set lying immediately beneath the cuticula and hypo- 

Water Vascular Systems 

The echinoderms have exclusi^'e patents on this method of loco- 
motion that functions by means of water pressure in their numerous 
tube-feet. The apparatus opens on the dorsal surface of a starfish, 
for instance, through a sievelike structure, called the m.adreporite. Sea 
water may be added to the so-called amhulacral fluid through the 



madreporite by means of cilia which send it along the stone canal. 
The latter structure leads straight down to the circumoral ring canal. 
Five radial canals branch from this and extend down the five arms 
sending off smaller branches which end in the tube-feet lying along the 
ambulacral grooves. The proximal end of each foot has a muscular 

ws/ s :Q 

Vertical section through an arm of a starfish : b, ampulla ; d. water canal 
opening at madreporite plate, sl\ i, radial water tube; m, mouth; //, tube feet; 
py, digestive gland ; sic, stomach ; a, anus ; v, ring canal ; n, nerve ring. 

bulb, the ampulla, which is capable of contracting, thus forcing the 
ambulacral fluid into the tube-foot. When the sucking disks at the 
free end of the distended tube-feet become attached to an object, the 
muscles of these tubular organs contract, forcing the water back into 
the ampullae, and the animal through its grip is enabled to move 

Muscles and Muscular Systems 

Great differentiation of muscles is invariably related to a well- 
developed skeletal system. In two large diverse groups of animals, 
the arthropods, with a chitinous exoskeleton, and the vertebrates, 
with a calcareous endoskeleton, individual muscles rather than 
muscle layers have been developed. Examples of exoskeletal muscles 
are the colorless, transparent, or yellowish-white muscles typical of 
the insects. Although soft and almost gelatinous in appearance, these 
muscles which are usually striated are very efficient, as may be seen 
in the common house fly whose wings beat over 300 strokes per 
second. Among vertebrates there are found smooth or involuntary 
muscles, skeletal or striated muscles, and heart muscles. 

While the muscles of a frog and those of a man may be homologous, 
that is, comparable embryologically and morphologically, it does 
not necessarily follow that they are analogous, that is, alike in 



the particular function which they perform. The frog's leg, for 

example, is relatively incapable of more than a flexing motion or a 

straight swing of the limb, whereas the human arm responds to flexing, 

rotating, or swinging, 

origin of 

according to the way 
in which it is moved. 
Human musculature is 
much more complex 
than that of a frog be- 
cause it has many more 
diverse functions to per- 

Evidently there is a 
definite relationship 
between the types of 
motion which are possi- 
ble from the standpoint 
of skeletal structure 
and the development 
of muscles that make 
such movements effec- 
tive. Actual movement 
results from the con- 
traction of muscles and 
is stimulated into activ- 



terjoCon cf 
Achilles ' 

lir— .insertion 
f\ of muscle 


Comparison of the arrangement of the muscles 
and supporting skeleton oi an insect's and verte- 
brate's leg. (Former after Berlese.) 

ity by nerves. Since the muscles, nerves, and skeleton are closely 
correlated parts, their degree of usefulness depends to a marked 
degree upon the proper development and functioning of all the 

Smooth or Involuntary Muscles. This tissue is characterized 
by the absence of striations and the presence of a single nucleus in 
each cell or fiber. It is the type of muscle which carries on most of 
the internal movements of the body. The walls of the intestines 
are lined by layers of circular and longitudinal involuntary muscles. 
The muscles in the walls of blood and lymph vessels, the tracheal 
tube, reproductive ducts, the ureters, and tlie skin are also of this 
type. Typically sluggish in contraction, they are the principal kind 
of muscles found in the lower animals. 

Skeletal or Striated Muscles. In this category fall all of the 
muscles which are under the control of the central nervous system and 


which move the boiios of tlie skeleton. There are approximately over 
five hiindnnl sucli mviseles distinguishable in man. They form the 
body wall, thus constituting, through a three-ply arrangement, the 
chief means of keeping the viscera in position. They regulate the 
position of the head and the degree of curvature of the backbone, as 
well as the shape of the thigh and the calf of the leg, and the contour 
of the arm. Since these muscles are responsible for all quick, con- 
sidered movements, as well as simple reflex actions, they must be 
built upon a plan whereby one set of muscles through contraction 
may perform an opposite type of movement from the other, that is, 
work in opposition to each other. 

Individual skeletal muscle fibers may reach something over an inch 
in length, but average only about 5^^ of an inch in diameter. If a 
single fiber of skeletal muscle is examined under the microscope, the 
regular rows of striations become visible. Careful study reveals a 
series of dense strands of protoplasm running the entire length of the 
muscle fiber, between which are spaces filled with a watery proto- 
plasmic material. It is believed that these delicate protoplasmic 
strands are capable of forcible contraction which, by mass action, 
results in the shortening of the entire muscle fiber. Each muscle 
fiber is enclosed in a modified elastic connective tissue membrane 
called sarcolemma, that bears scattered nuclei on its inner surface. 
Practically every muscle fiber cell is stimulated by a nerve ending. 
Groups of these muscle fibers are bound together with connective 
tissue, numbers of these bundles forming the muscle proper, which 
is then spoken of as a biceps, triceps, and so on. 

The ends of a muscle are usually tapered. One end is anchored 
to an immovable portion of the skeleton, and is termed the origin, 
while the opposite end, which is attached to the portion of the skeleton 
to be moved, is termed the insertion. The helly of a fusiform muscle 
is the mid-portion between origin and insertion which swells during 
contraction. The tough sheath of connective tissue surrounding the 
muscle becomes continued as a tendon merging into the periosteum of 
the bone, thus giving a firm attachment. Striated muscles are also 
arranged in flat, fan-shaped masses, or in thin sheets. 

Heart Muscle. This variety of muscle occurs in all of the higher 
animals. Although it has characteristics similar to the muscles 
previously described, cytological and physiological differences place 
it in a category by itself. Notwithstanding the fact that the action 
of the heart is involuntary, the cells composing heart muscle are 


striated and nucleate, resembling skeletal muscles in being capable 
of rapid, powerful contractions, but unlike other muscles by reason 
of their regular automatic contraction and relaxation. 

Muscular Contractions. That muscle contraction is stimu- 
lated by a nerve impulse in the living animal has long been proved, 
but this is as far as we can at present safely go, for in seeking a physico- 
chemical explanation of what actually happens within the cell itself 
we are treading upon dangerous ground. At the present time there 
does not appear to be an accepted theory that accounts completely 
and satisfactorily for muscle contraction. 

Certain things, however, are definitely known. In the first place, 
muscles shorten when they contract. Under the microscope, the light 
and dark bands so readily seen in striated muscle appear to exchange 
places. In reality, the light bands have become dark and the dark 
ones light so that there has been no actual exchange of position but 
only a change in physical make-up. Chemically, muscular action is 
due to a series of complex chemical reactions which imdergo a number 
of complicated changes, yielding in the end specific amounts of lactic 
acid. It is known that the shortening of the muscle fibers occurs 
before and independeyiily of the formation of the acid and therefore it is 
difficult to believe that the two are unrelated. When muscular activ- 
ity is prolonged, or when it is carried out under conditions implying a 
lessened supply of oxygen, there is an accumulation of so-called waste 
products, especially of lactic acid. According to Hill (1923) experi- 
ments on man caused an increase of from 29 to 104 mg. of lactic acid 
per 100 cc. of blood in the case of violent exercise carried on for one 
and a half minutes. This large increase in acid has been interpreted 
as meaning that the supply of oxygen to the contracting muscles was 
inadequate. Even with increased respiration and circulation, lactic 
acid accumulated in the muscles and was given off to the blood, thus 
creating an "oxygen debt" to the muscles. This phenomenon is 
associated with the condition of fatigue and has been studied in ath- 
letes, especially track men, where it was found that an accumulation 
of lactic acid hinders muscular relaxation. In races the intake of 
oxygen is of course determined . by the efficiency of the lungs and 
heart. In long distance running the athlete reaches an equilibrium 
between his oxygen intake and lactic acid production. In short races 
he may breathe but once or not at all and so builds up a large oxygen 
debt. In such cases a state of exhaustion may be reached in a few 





The Morphological Unit — The Neuron 

In order to get at the secret of control of skeletal, muscular, and 
nervous systems, it is necessary to examine the various nervous devices 
foimd throughout the animal kingdom which have been developed as 
co-ordinators. All animals, except perhaps the protozoa, are built 
up of a number of essentially similar cell units. The complexity of 
the adjustment device is directly related to the way these units are 

put together, as well as to the 
actual number of the units 
comprising the nervous system. 
Since the fundamental unit of 
structure of the nervous system 
is the nerve cell (neuron), we will 
do well to examine it further. 
A typical neuron consists of a 
cell body and two kinds of out- 


oP impulse'. 

5— deridrite^ 



.■naksd. a^on 

nucleus of— . 

..medullary 5beatb growths, the many branched 

dendrites which receive impulses. 

indicate^ _ . . 

6reat lendtVi fi^ , ro 

^ ^ Vi..r2odsofl?arJvier 


and the elongated axon, that 
conducts messages away from 
the body of the cell, and ter- 
minates in the end organs. The 
''naked" axon is characteristic 
of the gray matter of the cen- 
tral nervous system. Around 
many of the axons is a thin, 
membranous protective cover- 
ing, called the neurilemma, or 
Schwann^s sheath. This is liv- 
ing tissue as shown by the 
nuclei scattered through it, and 
by the fact that it may be 
regenerated after injury. Neu- 
rons of this latter type are found in most invertebrate nervous 
systems, in some of the prochordates, and in some of the peripheral 
nerves. In parts of the central nervous system of vertebrates the 
neurilemma is replaced by segments of white fatty substance, called 


\j2rmir2a\ branched 

A typical multipolar nerve cell. 



the medullary sheath, while other periplieral nerves possess both a 
medullary sheath and an outer neurilemma. 

The manner in which neurons operate depends upon their 
"hook up." Contact without fusion (synapse) is made between 
the end organ of one neuron and the dendrite of another, resulting 
in continuity from the physiological point of view. 

The Physiological Unit — Reflex Arc 

The physiological unit of the nervous system is a reflex arc. Such 
arcs are made up of two or more neurons and a muscle or gland ele- 
ment. A simple arc consists of a receptor neuron, the dendrites of 



"Sensory neuron. 








Simpk$t form reflex ai 

of a reflex: arc J vith 

]^~ ne, 




associat ior> 
neixrorL ' 

■muscle fibers 

Diagram of reflex arcs. Explain why this is often called the "physiological unit 

of the nervous system." 

which receive the stimulus and transmit it via the axon to the spinal 
cord where a synapse with the dendrites of an effector cell occurs. 
The impulse is then transmitted by means of the hitter's axon to 
the muscle or gland cell. Reflex arcs generally require one or more 
adjuster neurons in the circuit between the receptor and effector 
cells. Such adjuster cells are usually located in the spinal cord or 
in the brain. 

Even so brief a discussion of reflexes cannot be concluded without 
mention of the compound reflex arcs which are formed by a single 
receptor neuron and two or more effectors that may be widely sepa- 
rated in the body, or l)y two or more receptors and a single effector. 
Varying complexities of these latter types are made by the inter- 


polation of adjustor neurons. It is a moot question whether or not 
the simplest type of refiex arc, involving only two neurons, ever occurs 
in vertebrates. Most of the so-called "refiex actions" of man are 
usually not isolated from the rest of the nervous system. 

In lower vertebrates, such reflexes as are concerned with locomotion, 
breathing, swallowing, and escape from danger are automatic spinal 
cord reflexes. When it comes to forms with complicated and highly 
developed nervous systems, such as man, many actions become auto- 
matic, relieving the brain ordinarily of any responsibility concerning 
them. In this category fall such phenomena as breathing, sneezing, 
and shivering. Certain actions, namely jerking of the knee, dodging 
a blow, closing the eyes to keep out foreign particles, are reflexes 
which may be controlled or inhibited by a conscious effort. Still more 
complex reflexes are called into action when playing a musical instru- 
ment, or in walking, talking, swimming, or driving a car. 

Types of Nervous Systems 

One of the fundamental characteristics of protoplasm is irritability. 
In simple types of animals, like Ameba or the sponges, where co-ordi- 
nation between parts is not essential, no specialized nervous system ia 
developed. With the aggregation of cells in higher forms thera 
arises the necessity of correlating the interaction of component parts 
and consequently some sort of definite nervous system has been 
evolved. To be sure, such devices are quite unspecialized compared 
with the complicated nervous apparatus of a vertebrate, but never- 
theless they appear to be reasonably effective. 

Neuromotor Mechanisms 

Ameba, although a very simple type of organism, gives evidence of 
being definitely affected by stimuli. This is shown by the passage of 
stimuli from one point on the surface to the general mass of the body, 
causing the animal to move away from the source of stimulation 
and resulting in the formation of pseudopodia on the opposite side. 
Experiments upon Ameba suggest that stimuli are transmitted in the 
clear outer layer of ectoplasm. 

Probably the highest development of a co-ordinating system among 
the protozoa appears in some of the ciliates. We have already 
discussed movement in Stylonychia and in Euplotes. Considerable 


experimental work has been performed, largely by K(jfoid and his 
students at the University of California, upon co-ordination in the 
latter form. Euplotcs is characterized by a group of anal cirri, while 
the anterior surface possesses an undulating membrane, near one end 
of which lies a co-ordinating center, or motorium. From this, fine 
protoplasmic threads emanate leading to various parts of the ciliate. 
Five of these strands lead to five anal cirri. Cutting these proto- 
plasmic threads causes disruption of the rhythm of their movement, 
thus furnishing experimental evidence of the existence of a neuromotor 
apparatus in certain ciliates. 

Co-ordination by a Network 

In some of the most primitive metazoan forms, such as the sponges 
and the lower coelenterates, there is evidence of a very elementary 
and simple type of co-ordinating mechanism. A form like Hydra, 
which makes a variety of different movements, reacts to various 
stimuli since it feeds, contracts, expands, creeps, and occasionally 
turns cart-wheels. The mechanism which makes such acrobatics 
possible in Hydra has been described as a nerve net, and as such forms 
a part of the sensory-neuro-muscular mechanism, or as it is sometimes 
called, the receptor-effector system. 

Co-ordination by a Nerve Ring 

Only two of all the great phyla of animals, the coelenterates and 
the echinoderms, are apparently radially symmetrical. The nervous 
system of the first of these radially symmetrical groups has just been 
described and it can be seen how unspecialized are its co-ordinating 
devices. Turning to the echinoderms, as examples of the second 
radially symmetrical group, we find that in spite of the fact that 
embryos of these invertebrates are bilaterally symmetrical, the nerv- 
ous system of the adults has developed along the lines of radial sym- 
metry. This type of nervous system is composed of several parts, the 
relative development of which varies in the different classes, the star- 
fish having numerous nerve cells lying among the ectodermal cells. 
Some of these nerve cells may connect with nerves from the fairly 
definite ridges of nerve tissue known as the radial nerve cords nnming 
the length of each arm and uniting to join a nerve ring that encircles 
the mouth. In addition there may be an apical nervous systern that 
H. \y. H. — 23 


innervates the dorsal muscles of the arms. As might be expected, 
the tube-feet in the starfishes (Asteroidea) are supplied with sensory 
organs. It is also interesting to note that at the tip of each arm of a 
starfish there occurs a light-perceiving organ. 

Co-ordination by a Linear Nervous System 

Once the flatworms are reached in the evolutionary series one finds 
the beginning of a linear type of nervous system. In the segmented 
worms, or annelids, the nervous system is composed of two main 
longitudinal, closely associated nerve trunks from which the several 
branches in each somite pass laterally. Each segment of the worm 
usually contains one, or, if the longitudinal cords are widely separated, 
two ganglia arranged in parallel lines. In such cases the ganglia are 
connected by a transverse commissure. This ladderlike type of nerve 
co-ordination reaches its peak in the arthropods, where well-developed 
ganglia occur in most somites. In nearly all types of the higher 
invertebrates there is in the head end a ganglionic mass of nervous 
tissue which has been dignified by the appellation of "a brain," 
whereas it should have been more properly known, because of its 
position, as a supraesophageal ganglion. All nerve cords of similar 
type are ventral in position and lie beneath the gut. In order to reach 
the supraesophageal ganglion, the nerve cord splits at the large 
infra- or subesophagcal ganglion, and passes around the esophagus by 
means of the circumesophageal connectives or loop. 

Reaching the arthropods, the primary change in the central nervous 
system is found to be a greater concentration of ganglia. In the 
larval forms of insects, there is little change from the linear nervous 
system of annelids. In adult insects, however, ganglia are concen- 
trated, and even fused, in the regions of special organs. For instance, 
the "brain" and subesophagcal ganglia are connected with the ocelli, 
antennae, and mouth parts, while thoracic ganglia are associated with 
the wings and other appendages. An autonomic (sympathetic) nerv- 
ous system, which is believed to control the action of the heart, 
digestive system, and spiracle muscles, makes its debut in the 

Co-ordination by a Dorsal Tubular Nervous System 

Among the vertebrates there is a highly developed dorsal, tubular, 
central nervous system with evidence, even in the lower forms, of 


distinct cephalization. The nervous system serves to correlate move- 
ments and to give information of changes in the environment. In- 
numerable fibers extend from an elaborate central controlling device 
to all parts of the body. Such a nerve mechanism may be subdivided 
into several parts. For example, in man there is a central nervous 
system, a pcj-iphcral nervous system, and an autonomic or sympathetic 
nervous system. 

Protective Devices for the Central Nervous System 

As this centralized ner^'ous system is the master which controls all 
voluntary acts and indirectly all parts of the body, it is of primary 
importance to protect so delicate a mechanism from injury. Since 
the situation is essentially the same among the different members of 
the large group of vertebrates, attention will be primarily directed to 
the system as it is found in mammals, and more particularly in man. 

The skull and the A^ertebral column serve as the ''first line of 
defense" for the all-important brain and spinal cord against possible 
attack or injury. However, "secondary defenses" must also be 
present. The inner surface of the skull and the vertebral column, 
therefore, is lined with a tough membrane of fibrous connective tissue, 
called the dura mater. Inside the dura mater the central nervous 
system itself is also covered with a thin, closely investing membrane, 
the pia mater, while between it and the dura mater lies the delicate 
serous membrane known as the arachnoid. These three membranes 
furnish additional protection to the central nervous system, but they 
would be relatively ineffectual without the buffering effect of the cere- 
brospinal fluid which fills the spaces between the arachnoid and pia 
mater. Thus the vertebrate nervous system is insulated, cushioned, 
or, to put it more graphically, furnished with "shock absorbers," that 
enable man and other vertebrates to withstand severe shocks without 

Anatomy and Development of the Brain 
The Early Development of the Central Nervous System 

Amphioxus gives Init slight evidence of an enlargement of the 
cephalic end of the dorsal tubular nerve cord, but in the bony fishes 
there are already five main divisions in the adult brain. These same 
divisions are to be found in every one of the different vertebrate 
classes, and all representative vertebrate brains have a similar embry- 



ological history. In other words, these structures are both homolo- 
gous and in a general way analogous. 

Some of the more important changes in the growth and expansion 
of the nerve cord are as follows. Early in its embryonic develop- 
ment, before the five regions of the brain are developed, the anterior 
portion of the growing nerve cord becomes differentiated into three 
enlargements, designated, beginning anteriorly, as the iore- (prosen- 
cephalon), mid- (mesencephalon), and hind- (rhombencephalon) brains. 





telencephalon / 
1/ '' ^ 

mescncepholoa v tnyelencephcJon. 


Development of the vertebrate brain from a simple encephalon. 

Most of the subsequent development takes place in the fore- and 
hind-brains (page 347). As growth continues, the anterior part of 
the fore-brain divides, grows out into two pouchlike lateral lobes, 
called the cerebral hemispheres (telencephalon), or collectively the 
cerebrum. The jiosterior portion of the primitive fore-brain is now 
designated as the 'twixt-brain (diencephalon) . The mid-brain (mesen- 
cephalon) meanwhile remains imdivided, while the hind-brain becomes 
separated into an anterior dorsal outgrowth, called the cerebellum 
(metencephalon) , and a posterior medulla oblongata (myelencephalon) , 
which is continuous with the cord. 



Before going further with a consideration and (hseussiou of the 
human nervous system, a comparison of brains of (Hfferent verte- 






lobe,' ,, 




brates should be made 
for the sake of clearness. 
Remembering that the 
brain of a fish is not 
folded upon itself as is 
the brain of a mammal, 
it is easy to see that the 
introduction of flexures 
tends towards greater 
compactness. Another 
outstanding develop- 
ment is the increase in 
size of some of the re- 
gions of the brain. In 
lower forms, the domi- 
nating portions of the 
brain from the stand- 
point of mass are the 
optic lobes of the mid- 
brain and the medulla 
oblongata, and, as 
might be inferred, both 
the cerebrum and cere- 
bellum are quite small. 
In the higher mammals, 
however, these organs become two of the most important centers 
in the brain, increase in the size of the cerebrum being in direct pro- 
portion to the intelligence of the animal. 

An examination of a few of the more important landmarks of the 
divisions of the brain in order to secure a general idea of the function- 
ing of each of these parts will furnish a background for the discussion 
of the '' Display of energy." 

The Parts of the Vertebrate Brain 

The Cerebrum or Telencephalon. As the adult condition is 
approached, certain other characteristic structures appear. From 
the anterior portion of the cerebrum grow the paired olfactory lobes. 
In lower xertebrates these may extend into expanded olfactory hulhs. 


Representative vertebrate brains. o.L, olfactory 
lobes. W hat reffions increase noticeably in mass from 
fish to manmials ? How are these changes correlated 
with the shift from water to land:* (After Guyer.) 


from which the olfactory nerves pass directly to the nostrils, thus 
receiving stimuli which are interpreted in the brain as odors. 

The cerebral hemispheres contain cavities known as the first and sec- 
ond ventricles, which are continuous posteriorly with the third ventricle, 
found in the 'twixt-brain or diencephalon. Dorsally and laterally the 
cerebral walls are known as the pallium, that furnishes a foundation 
on which the outer layer, or coriex is developed. In the higher verte- 
brates a connecting bridge of white fibers, called the corpus callosum, 
unites the two cerebral hemispheres. The higher the mammal is in 
the scale of life the more convoluted the cortical surfaces of the hemi- 
spheres become, and the more the cerebrum weighs in proportion to 
the rest of the organ. A rabbit's cerebnun composes slightly more 
than half of the mass of the entire brain, while in man it exceeds four 
fifths of the total weight. 

The sui)erficial cortical layer of the cerebrum forms a mass made 
up of numberless nerve cells interwoven into an intercommunicating 
network. The axons of some of these neurons pass over the bridge 
of the corpus callosum from one side to the other while other axons 
extend downward in great bands as far as the cerebellum and to other 
more posterior centers. It has been demonstrated that this portion 
of the brain is the seat of consciousness and the controlling center of all 
our higher mental life. As the cerebral functions increase, the instinc- 
tive reflexes retire further into the background. Herein lies the chief 
difference between the so-called lower animals and the higher ones. 
The former are chiefly at the mercy of their hereditary limitations and 
their environment, while the latter have risen sufficiently above their 
environmental conditions to begin at least to become "the captains of 
their souls." A series of experimental operations on a dog, in which 
the entire cerebrum was finally removed, illustrates the importance of 
this part of the brain to the higher animals. The dog in question 
apparently became an idiot, unable to associate experiences or to 
learn. It had no ability to differentiate between solid objects in its 
path and patches of sunlight on the floor, which could in no way 
hinder its progress. 

The 'Twixt-Brain or Diencephalon. This region is compara- 
tively inconspicuous, but very essential to the biologist, since the 
ventral floor of the 'twixt-brain gives rise to an outgrowth called the 
infundibulum, which fuses with a dorsal outgrowth from the roof of 
the mouth to form the pituitary gland (hypophysis), the "generalissimo" 
of the ductless glands. Possibly because of its importance it has 



,<— . 



temporal arecc 

Two I'lKurt's illustralinK tiie intercoiniiiunicating pathways of nerve libers in 
the human brain. 

a. Various association fibers of the human brain. ,1, between adjacent areas; 
B. between frontal and occipital areas; (;, D, between frontal and temporal 
areas; E, between occipital and temporal areas. Note the corpus callosum 
which contains larf):e groups of association fibers and connects the cortex of the 
right cerebral hemisphere with that of the left. The caudate nucleus, CN, and 
the thalamus, OT, both contain gray matter. 


b. Scheme of projection fibers connecting the cerebrum and other parts of 
the brain. .1, tracts fnmi frontal lobe to the pons varolii and thence to the 
cerebellum via (!; B, motor (pyramidal) tracts; C, sensory tracts; D and E, 
the visual and auditory tracts, respectively; F, fibers connecting the cerebrum 
and cerebellum ; 6', fibers connecting the cerebellum and the brain stem ; 11, 
fibers between the cerebellum and the cord ; ./, fibers connecting the auditory 
nucleus and the brain stem ; A', crossing over of motor (pyramidal) tracts in the 
brain stem ; V7, fourth ventricle. The numbers refer to the cranial nerves. 
(Both modified from Starr.) 


become exceptionally well protected. In all mammals this little gland 
is lodged in a protective median depression in the sphenoid bone of the 
skull called the "Turk's saddle," or sella turcica. The 'twixt-brain 
also gives rise laterally to outgrowths of the lateral wall which form 
the optic stalks that are essential to the development of the eyes. In 
this region of the brain several problematical structures, of particular 
interest to the comparative anatomist, such as the pineal eye, have 
their origin. The cavity of the 'twixt-brain is called the third ventricle. 

The Mid-Brain or Mesencephalon. This portion of the brain 
has kept many of its primitive embryonic characters, its gray matter 
being still found largely in ganglionic masses. Anatomically, it is a 
small region, the lumen of which, communicating anteriorly with the 
third ventricle, is called the aqueduct. In lower forms like the fishes 
and amphibia, the roof of this cavity is expanded dorsally into two 
rounded protuberances, the optic lobes, or corpora bigemina. The 
optic lobes of reptiles, birds, and mammals become further divided into 
two pairs of centers known as the corpora quadrigemina, from which 
are sent out bands of fibers, the anterior pair being connected with the 
eyes, and the posterior pair with the ears. In forms below the mam- 
mals the mid-brain functions as a co-ordinating center for impulses 
entering through the eye, ear, and certain nerves of the body. In 
the mammals much of this co-ordinating function has been taken over 
by the cerebrum. Upon the latero-ventral surface of the mid-brain 
may be seen a band of fibers, the crura cerebri, forming a highway of 
communication between the cerebrum and the posterior parts of tlie 
central nervous system. Two motor cranial nerves, the oculomotor 
(III) and the trochlear (IV), which supply muscles of the eye, arise 

The Cerebellum or Metencephalon. While the surface of the 
cerebellum is not convoluted in the same manner as the cerebrum, 
nevertheless its surface of gray matter is increased by being thrown 
into numerous furrows. It is composed of two hemispheres connected 
by a bridge, the vermis, and has consequently been likened to a butter- 
fly with the bridge forming the body. The cerebellum lies just pos- 
terior to the cerebrum and dorsal to the mid-brain. When cut in 
sagittal section it is seen to be composed of radiating folds, arranged 
in an outer layer of gray matter and an inner core of white matter. 
Taken as a whole the white matter somewhat resembles a tree and so 
has been called the arbor vitae, or "tree of hfe." 

Ventrally, a swollen band of fibers, called the pons varolii, is 


plainly evident because of the transverse direction of its fiber, crossing 
from one side of the cerebellum to the other. Nerve fibers arising in 
the frontal, parietal, and occipital lobes of the cerebrum reach the 
cerebellar hemispheres by way of the anterior -peduncles in front of the 
pons, the latter bearing some resemblance to a pair of legs supporting 
the body of the cerebellum. There is a second pair of lateral "legs" 
behind the pons, the posterior peduncles, which contain communi- 
cating fibers between the cerebellar lobes and the posterior regions 
of the central nervous system. Thus, a highway of communication 
with the cerebrum is at hand and herein lies a partial explanation of 
man's ability to perform purposive acts as the result of the various 
visual, auditory, and other impressions of the senses. Experimental 
evidence indicates that this portion of the brain is primarily a seat of 
muscular co-ordination. 

If the cerebellum of a dog is removed, the animal is unable to 
co-ordinate its movements at first. Later it learns to walk, but the 
gait is always slow and staggering. In a similar condition, a pigeon 
is unable to fly, but like the dog, may eventually learn to walk again, 
although resembling the proverljial inebriate in its gait. It has often 
been claimed that man would make a better recovery after removal of 
the cerebellum than either the dog or bird since his more highly 
developed cerebrum would compensate the loss. In any case, from 
these experiments the importance of the cerebellar region of the brain 
in our everyday activities is better understood. 

The Medulla Oblongata or Myelencephalon. The brain at 
this point is anatomically little more than an expanded region of the 
si)inal column, but it is the sole means of communication between the 
cerebrum and the body. Its dorsal surface is partially covered by 
the posterior peduncles, and there is also a very thin non-nervous 
roof, the mctatela, which covers the fourth ventricle, or the large cavity 
of the brain of this region. Ventrally, two raised convex columns of 
fibers may be seen, known as the pyramids. 

In the gray matter of the medulla, the controlling centers for many 
of the essential functions of life are found, for example, the reflexes 
concerned with the vasomotor and respiratory functions. Numerous 
other centers that control swallowing, coughing, sucking, sneezing, 
salivary secretions, gastric secretions, heart inhibition, and other 
activities connected with the living body are located in the medulla. 

All of the cranial nerves, except the first four [)airs, arise from this 
region. It is here, too, that the pyramidal tracts of transmitting fibers 



cross from one side of the brain to the other, like the letter "X," 
so that the control of the left side of the body is located in the centers 
of the right side of the brain and vice versa. 

The Cranial Nerves 

There are typically ten pairs of cranial nerves in the lower verte- 
brates and twelve in mammals, arising from different parts of the 
brain. Of these, four pairs are of particular interest. Three pairs 
(I, II, and VIII) are concerned with the innervation of the organs of 
special sense, while the fourth (X) is that great wanderer, the vagus. 

The first, or olfactory nerve (I) receives 
stimuli from the nose and conveys 
them to the brain. The second, or 
optic nerve (II) emerges from the 
lateral floor of the diencephalon, its 
fibers more or less completely crossing 
in the optic chiasma, that lies just 
anterior to the infundibular outgrowth 
of the pituitary body already men- 
tioned. This nerve transfers the im- 
pulses which are interpreted in the 
brain as sight. The third pair of cranial 
nerves associated with a special sense 
is known as the auditory nerve (VIII), 
and has the dual function of hearing 
and equilibration. 

The remainder of the cranial nerves 
will be omitted from further discussion 
except the vagus (X), the ramifications of which are more extensive 
than those of any of the other cranial nerves. The vagus is a 
mixture of motor and sensory elements, the former supplying muscles 
of the pharyngeal and laryngeal region, most of the digestive tract 
and the liver, pancreas, and spleen, the kidneys as well as the heart, 
and certain blood vessels. The sensory fibers are distributed to the 
mucous membranes of the larynx, trachea, lungs, esophagus, stom- 
ach, intestines, and gall bladder. Inhibitory fibers also reach the 
heart and, in addition, this versatile nerve supplies the gastric and 
pancreatic glands with secretory fibers. Much of phylogenetic in- 
terest may be gleaned from a careful comparative study of the 
distribution of this and other cranial nerves from fish to man. 

Diagram showing the oplic 
chiasma in man. Note that the 
crossing is not complete, a con- 
dition probably related to the 
binocular method of vision. 


The Spinal Cord 

The medulla ohlonsata is {'outiiniecl almost im])erceptibly over into 
the spinal cord, which extends in adult man from the foramen magnum 
of the skull posteriorly through the vertebral column for seventeen to 
eighteen inches. The spinal cord is, roughly speaking, the size of the 
Uttle finger, or about 0.4 of an inch in diameter. Two enlargements 
occur in it, one in the region of the shoulder-blades, and the other 
below the small of the back, respectively knowm as the cervical and 
lumbar enlargements. 

The internal structure of the cord is quite characteristic. In cross 
section, the central gray matter somewhat re.sembles the letter "H," 
the position of the gray and white matter being apparently reversed 
from their ])osition in the brain. As a matter of fact, in both cord 
and brain the gray matter is disposed inside close around the cavity 
that extends throughout the whole central nervous system. Outside 
this central gray matter are the transmission fibers which app(>:ir 
white. In the cerebellum and cerebrum of the brain there is super- 
imposed an outer layer of gray matter that constitutes the centers of 
adjustment. This secondary gray layer is .so pronounced in the brain 
that it gives rise to the popular impression of a reversal in the arrange- 
ment of white and gray matter between the cord and brain. The gray 
matter is composed of a ventral, or anterior, and a dorsal, or posterior, 
column, divided into these two parts by the tran.sverse bar of the "H." 
The white matter may also be subdivided into three parts on either 
side, a ventral, lateral, and dorsal funiculus. 

The Spinal Nerves 

The nerves in this group, like the cranials, are paired, there being 
31 pairs in man. Each nerve, moreover, is "mixed," that is, it is 
composed of a dorsal or sensory root containing receptor neurons, 
carrying messages toward the brain, and a ventral or jnotor root bearing 
effector neurons, which carry messages away from the brain to muscles 
and glands. It will be noted that some of the cranial nerves, unlike 
the spinal nerves, have lost this original ability to transmit messages 
both ways and have been reduced to one-way traffic, for example, the 
three pairs of eye-muscle nerves (III, IV, and VI) handle only outgoing 
impulses, wdiile the auditory nerve (VIII) can only transmit .stimuli 
inward toward the brain. From the point in a mixed nerve where the 
incoming and the outgoing roots fuse are typically given off the 



following braticlios : (] ) a dorsal branch; (2) a nioro prominent ventral 
branch, whicli supplies the skin and body nuisculaturc ; (,S) a com- 
municating branch, going to 
the ganglia of the autonomic 
system and thence to the 
viscera ; and (4) a small 
meningeal branch, going back 
to the protective layers of 
the cord. Thus the nerves 
emanating from this point of 
fusion are mixed in character 
while their roots are not. 


cfcrsol raot 


dorsal root 
Ventral root 



; visntroli? 

Components of a spinal nerve. Somatic 
motor fibers are indicated by solid lines; 
\ isceral motor by lonp dashes; somatic sen- 
sory in short dashes ; \ isceral sensory by 
dotted lines. 

The Autonomic Nervous 

The term autonomic nervous 
system embraces all nerves 
and ganglia located outside of 
the spinal cord, which reg- 
ulate the activities of smooth, 
or involuntary, muscle and 
various glands. It should 
also be thought of as an auxiliary, or perhaps more properly a relay 
apparatus to supplement the work of the central nervous system. 

Anatomically the system consists of two ''longitudinally con- 
nected" chains of ganglia lying on either side and just ventral to the 
cerebrospinal cord together with various ganglia scattered throughout 
the viscera and groups of connecting nerves extending to the central 
nervous system. This system is divisible into two parts. The first 
is called the thoracicolumbar jiart, and consists of the double chain of 
ganglia mentioned above together with the connections through the 
spinal nerves. It reaches the blood vessels, heart, digestive tract, and 
many other parts of the body. The second, or parasympathetic part, 
is characterized by having three centers, two cranial centers, one in 
the mid-brain, one in the medulla region, and a posterior center in the 
sacral region. 

Masses of nervous tissue are scattered as ganglia which are located 
in various organs, such as the walls of the digestive tract, where they 
are known as the solar, cardiac, or aortic plexvses. These serve as relay 
centers for impulses coming from the main trimk line of the autonomic 



system, and since each of these centers usually presents a fanlikc 
arrangement of efferent fibers, they serve to increase the number of 
available pathways. 

The autonomic system is full of contradictions, for there appears to 
be an antagonistic action on the part of the thoracicolumbar to 


' sphsno gcJotirg g>T^lacrimal gland 

1 \xxmhar 

i Sacral 

Submaxillary^^^ "osa . palctta. 

■i^-2fe^^i^ subling,ual ^. 

mucous rtiem. 

panoticC glancC 


^vsssds of abcL 

liver and. 

^^ pancrsQ-S 


Smail . 




Serf, or^n 


Diagram of the autonomic nervous system. The parasympathetic part appears 
in solid lines and the thoracicolumbar part in dotted hues. 

impulses from the cranial or sacral parts of the parasympathetic 
system. Thus the cranial part slows the heart while the thoracicolum- 
bar accelerates it. This has often been spoken of as "reciprocal 
innervation, " a principle which plays a very important role in the 
proper functioning of various organs. 



The origin of the autonomic system has been the subject of con- 
siderable speculation. Some investigators believe that it has been 
secondarily derived from the central nervous system probably by the 
migration of cells. Others support the idea that it is in reality a 
primitive ancestral apparatus which is more or less homologous with 
the nervous system of invertebrates. According to this theory the 
autonomic system has become secondarily subservient to the volun- 
tary nervous system of the vertebrates. 

The Sense Organs — Receptor Devices 

The mechanism and functioning of many of the different parts of 
the vertebrate nervous system have been considered in some detail 
for the purpose of showing how the voluntary system controls actions, 
and also how the involuntary system has taken over the burden of 
running the body. It now remains to trace the various devices that 
have been developed to help an animal keep in touch with its environ- 
ment, in other words the sensory receptors, which range from special- 
ized to rather generalized structures and are usually classified as 
organs of taste, smell, sight, hearing, and the tactile sense. 


In the lower vertebrates the sense of taste is quite widely dis- 
tributed. For example, in some of the fishes sensory cells of chemical 

reception are scattered 
^,fV^,'P^2-^-,— ^-^-^ somewhat widely over the 

body surface. In higher 
vertebrates such organs are 
mostly restricted to the sur- 
face of the tongue and are 
known as taste buds. Most 
people labor under the de- 
lusion that they can dis- 
tinguish between a great 
variety of flavors. Actually, 
however, buds are sensitive 
to only four kinds of stim- 
uli, sweet, sour, bitter, and 
salty. The confusion results from the inclusion of interpretations 
of sensations received by the olfactory senses. 





A taste bud. Explain how it functions. 






This is one of tlio more important organs of special sense. Even 
aquatic forms have been shown to possess a fairly keen sense of smell. 
In land forms, the nasal chamber becomes supplied with sensory 
olfactory cells that are quite primi- 
tive, or undifferentiated. The in- 
sects, which in some cases have a keen 
sense of smell, have the olfactory 
organs located on the antennae. 
Loeb performed an experiment that 
clearly demonstrated the acuteness of 
this sense in a butterfly, by suspend- 
ing a female butterfly in a box and 
then opening the window. In less 
than half an hour a male butterfly of 
the same species was nearby. It 
soon reached the window, flew into 
the room, and perched on the box. 
Two other males also came during the afternoon. Their sense 
of smell no doubt was responsible for their discovery of the female. 
Man, whose sense of smell is by no means as keen as that of some 
other animals, can, nevertheless, detect, for example, one part in a 
million of iodoform. 



Olfactory cells. 

Simple Light Receptors 

The reaction of animals to light is one of the most characteristic 
responses found in the animal kingdom. In the simplest organisms it 
has been demonstrated that this reaction may be classified as a positive 
or negative attraction to light. The ability to react to light indicates 
the presence of cells or tissues in the animal which are photosensitive. 
Since, in lowTr forms, the response to light may be detected by the 
manner in which the animal reacts in the presence or absence of light, 
or in avoiding illuminated areas, it appears probable that there is a 
more or less direct connection between the photoreceptor cells and 
the muscles. The responses to light of such animals as the protozoa, 
hydroids, and earthworms apparently fall into this category, and 
has led to their being designated as positively or negatively 
phototropic. Much interesting experimental work has been done 
along these lines. 


Compound Eyes 

The intergradation from the type of photosensitive cells mentioned 
above, to a primitive eye, or eye spot, is a gradual one. One of the 
first steps in the production of a simple eye spot appears to be the 
concentration at a given point of a number of light-sensitive cells con- 
nected with nerves. From such simple beginnings two types of eyes 
have been evolved in the animal kingdom, the compound eye of the 
insects and Crustacea, and the camera eye of certain molluscs and the 
vertebrates. The compound eye is composed of a varying number of 
complete individual eyes called ommatidia. Each ommatidium is di- 
rectly connected with the brain and produces a separate image that, 
joined to others, gives a unified picture. It has been ascertained by 
counting the exposed surfaces, or facets, of the ommatidia that there 
may be present any number from a few dozen up to several thousand. 
Some ants have about fifty, while the swallowtail butterfly has seven- 
teen thousand, and dragonflies still more in each eye. The walls of 
each ommatidium are surrounded with pigment cells that absorb all 
tangential rays, consequently only those rays which penetrate straight 
in through the facet reach the sensory areas located in the retinular, 
or photoreceptive, cells. On account of this restricted intake, each 
ommatidium receives for interpretation only a small portion of the 
rays entering through the cornea. It is believed that there is no 
marked overlapping of images since each image is recorded in a differ- 
ent spot, the end result being a series of small images one next the 
other, which act to produce the completed picture, called an erect 
mosaic (see figure, page 206). 

Camera Eyes 

The camera type of eye in invertebrates reaches its peak in the 
molluscan squid and, among the vertebrates, in the human eye. 
These two types offer a good illustration of analogous structures. A 
study of the development of these two types of eyes shows that the 
position of certain elongated cells of the retina, called the rods and 
cones, are reversed in the two forms, and consequently while their 
function is in general the same, or analogous, their type of structure, 
or homology, is different. The vertebrate eye is almost spherical, 
and fits into a funnel-shaped socket of bone, called the orbit, while 
the stalklike, optic nerve connects the eye directly with the brain. 
Free movement is made possible by means of six small muscles which 



are attached to the outer coat of the eyeball and to the bony wall 
around the eye. 

The wall of the eyeball is made up of three coats. The outer tough 
white coat of connective tissue is called the sclerutic coat. In front, 
where the eye bulges out a little, the outer coat becomes transparent, 
forming the cornea. A second coat, the choroid, is supplied with blood 



Sagittal section of a inaiiimalian eye. 

vessels and cells containing considerable quantities of black pigment. 
The iris, which shows through the cornea as the colored part of the 
eye, is a part of this coat. In the center of the iris there is a small 
circular hole, the -pupil. The iris is under the control of involuntary 
muscles, and may be adjusted to varying amounts of light, the hole 
becoming larger in dim light and smaller in bright light. The inmost 
layer or coat of the eye, called the retina, is double, consisting of an 
outer pigmented and an inner sensory part. This is perhaps the 
most delicate layer in the entire body. Despite the fact that the 
retina is less than ^^ of an inch in thickness, it is composed of several 
layers of cells. The optic nerve, made up of a chain of relaying 
neurons, enters the eye from behind and spreads out over the surface 
of the retina. At its point of entry a cross section of the optic nerve 
shows that the nerve consists only of axons of neurons, and conse- 
quently this "blind spot" is not sensitive to light. The ultimate 
photoreceptors are numerous elongated cells, called rods and cones. 
The function of the rods is a highly specialized sensitivity to light, 
and of the cones the perception of color. In the optical center of the 
H. w. H. — 24 



l)osterior part of the retina lies a region known as tlie yellow spot, or 
macula lutea. The central pitlike portion of the macula lutea, where 
cones predominate, almost to the exclusion of rods, is designated as 
the fovea centralis, since it is here that the keenest vision occurs. 
The retina is thinner at this point and the black pigment of the outer 
layer shows through from behind, making it dark purple in color, due 
to a layer of cells next to the choroid coat. The retina acts as the 

sensitized plate in a camera, for 

outar Surface of retivict 

*- pigment 






(outer , 
I crarjular 


1 ganglionic. 

fibens- of 
optic nerve 

on it are received the impressions 
of light and shade and color which 
are transformed and sent to the 
brain resulting in sensations of 
sight. Like the camera, the eye 
has a lens formed of transparent 
elastic material, a circumstance 
permitting a change of its form 
and, in consequence, a change of 
focus upon the retina. By means 
of this change in form, or accom- 
modation, both near and distant 
objects may be seen. In fishes, 
unlike mammals, accommodation 
is accomplished by shifting the 
position of the lens, as in a camera, 
rather than by changing its shape. 
In front of the lens is a small 
cavity, divided by the iris into 

inner surface of" retina. 
Detail of retina showing rods and cones. 

two chambers that communicate through the pupil, filled with a 
watery fluid, the aqueous humor, while behind it is the main cavity 
of the eye, filled with a transparent, almost jellylike, vitreous humor. 
The lens lies directly behind the iris and is attached to the choroid 
coat by means of delicate ligaments and by pressure of the two 
liquid media. 

In order to function properly, the surface of the eye must be kept 
moist, and various glands are located in the cavernous orbit of the eye 
and along the edges of the eyelids which serve this purpose. The best 
known are the tear or lachrimal glands with their associated ducts that 
open into the nasal chamber. These glands increase their normal 
production of moisture to form visible tears when the surface of the eye 
is irritated by foreign particles or when the emotions gain control. 




The structures making up the compUcated mechanism of iiearing 
primarily serve two purposes, namely equilibration and hearing. Of 
these functions the first is luidoubtedly the more primitive. 

Most invertebrates, whether jellyfish, molluscs, or crayfish, main- 
tain their equilibrium by some sort of otocyst. Roughly described, 
this consists of a sac lined throughout or in part by cell-receptors and 
containing concretions called otoliths. As the animal changes its 
position the otolith shifts due to the forces of gravity and thus stimu- 
lates by contact the different receptor nerve cells, which transmit the 
impulse of pressure to the ^ ^ 


endolymphcctic duct 






brain, where it is interpreted 
so as to enable the animal to 
right itself. 

The ecjuilibratory mecha- 
nism of vertebrates functions 
principally through stimuli 
received from nerve cells 
located in the arnqmllae or 
swollen ends of three semi- 
circular canals, occupying 
roughly the three planes of 
space. The animal is enabled 
to adjust its position wdth 
reference to the stimuli re- 
ceived through the influence 
of gravity. In such cases the 
fluid within the semicircular 
system stimulates differen- 
tially the nerve endings in 
the ampullae. Stimuli reach 
the nerve-receptors in the same manner as they do in the lower 
forms, being carried by branches of the auditory nerve (VIII) in the 
brain. The entire structure is protected by a surrounding mass of 
cartilage which in higher forms becomes ossified. 

As to the function of hearing, it is possible that in the case of 
fishes vibrations are transmitted by the water through the skull to 
the sensory inner ear. However, when air is substituted for water 
as the chief environment some other more sensitive device must be 




The inner ear of a fish showing the essential 
features of this balancing? organ. Where are 
the ampullae!' These, together with areas in 
the alriculus and sacculus, contain patches of 
sensory cells connected with branches of the 
auditory nerve. How are such areas stimu- 
lated:* The lagena produces the cochlea. 



developed. In the land vertebrates amplifying devices are developed 
in the form of a vibrating ear drum or tympanic membrane beneath the 
skin, and a chain of middle ear hones that transmit the vibrations to 
the inner ear where the sensitive receptor-cells are located. Thus 

A cross section of the coiled cochlea which contains the or^an of Corti in which 
the sensitive hair cells are located. The scala media is filled with fluid endolymph 
which is separated from the fluid perilymph of the scala vestibuli by Reissner's 
membrane. Vibrations of the ear drum are transmitted throuKh the middle 
ear bones which cause the vibration of a membrane at one end of the scala 
vestibuli, thus disturbing the perilymph in the scala vestibuli. How are the hair 
cells stimulated ? 

there is gradually developed an elaborate mechanism by which vibra- 
tions are transmitted and amplified through the ear drum and the 
three bones of the middle ear to the spirally coiled portion of the inner 
ear, or cochlea, where the receptor-cells are located. These essential 
cells receive stimuli which are carried by branches of the auditory 
nerve to the brain for interpretation. 

Cutaneous Sense Organs 

There remains for consideration that diverse group of sense organs 
located in the integument. In fishes, the tactile sense consists princi- 
pally of pressure receptors, which are usually concentrated along the 


lateral line. The entire surface of the body of \ertehrates in general 
is practically covered with receptors capable of iiitcrpretinp; touch or 
pressure, temperature, and pain. 

These integumentary receptors, of which there are many modifica- 
tions, are not located with imiform density over the body surface. 
It has been estimated that there are between 3,000,000 and 4,000,000 
pain, 500,000 pressure, 150,000 cold, and 16,000 warm receptors 
located in the human skin. 

An understanding of the sensations and impulses which are received 
from the organs of special sense is the primary means of keeping our- 
selves informed about changes taking place in our immediate sur- 
roundings. From these sensations and impulses are built up definite 
reactions as well as certain convictions or attitudes which enable us to 
secure the maximum (^r minimvmi out of life. 


Clendenning, L., The Human Body, Alfred A. Knopf, Inc., 1930. Pp. 53-70, 


More popular reading. 
Howell, W. H., A Textbook of Physiology, 12th ed., W. B. Saunders Co. 

1933. Chs. I-V. 

A thorough technical account of the j)hysiolog3' of muscle and nerve. 
Rogers, C. G., Textbook of Com-parative Physiology, McGraw-Hill Book Co., 

1927. Chs. XXVI and XXVIII. 

Advanced account from the comparative viewpoint. 
Wells, H. G., Huxley, J. S., Wells, C. P., Science of Life, Doubleday, Doran 

& Co., 1931. Pp. 32-38, 523-524, 697-698, 1200-1226. 

Popular account with emphasis on man. 



Preview. Why living things are responsive • Various kinds of stimuH • 
Tropisms • Nature of responses • Mechanism of response in plants • Mech- 
anisms of response in animals • Tropisms, reflexes, and native behaviors • 
Native behaviors may be modified • Habit formation • Conditioned behav- 
iors • Are behaviors adaptive responses? • When are animals conscious? • 
Emotional responses • What is intelligence? • Intelligence of apes • Intelli- 
gence of man • The measurement of intelligence • Suggested readings. 


The display of energy is characteristic of all living things. We 
may predict quite accurately what forms energy will take in very 
simple plants and animals, since they react variously but consistently 
to factors of the environment, such as light, temperature, and mois- 
ture, by making definite turning movements, growth movements, or 
by other behavior. These expressions of behavior are called tropisms. 

When it comes to answering the question, "Why do we behave 
like human beings?" we are faced with a much more difficult problem, 
for the more complex the organism, the more complicated are its 
behavior patterns. 

Comparing the behavior of plants with that of animals and using 
the same stimuli in each case, we find in general that, correlated with 
the lack of muscles and a nervous system, in plants responses to 
stimuli are slow and usually expressed as growth movements. In 
animals which, except in the lowest forms, have both muscles and a 
nervous apparatus, the reaction to a given stimulus is a response in 
the form of some sort of motion such as swimming, flying, crawling, 
walking, or running. 

Two very definite theories of animal behavior are held. One 
theory recognizes animals as living machines, giving definite and 
unchangeable responses to certain stimuli. In such a mechanistic 
view of life the organism is considered in terms of groups of cells and 
tissues, or of the elements of which it is composed. When the ma- 
chine is very complex its actions are less predictable because the 
same stimulus may cause a different reaction to a different part of 
the machine. Light, for example, would evoke a response only from 



photoreceptive organs, while differences in temperature might affect 
many different groups of tissues or organs in different ways 

Another view, quite opposite to this, is the organismal theory. 
Here the unity of the organism as an interacting whole is stressed. It 
is considered as an individual and not as a collection of cells and tis- 
sues. The study of embryology bears out this idea, for in the develop- 
ment of the egg certain regions of protoplasm, instead of certain cells, 
develop into the future embryo. The egg at an early stage shows 
polarity, a right and left side as well as an anterior and posterior end 
of the future organism, some time before it divides into cells. Profes- 
sor Child of the University of Chicago has developed and tested a 
theory of the unity of the (organism which he calls the axial gradient 
theory that helps in understanding the complex response patterns ob- 
served in the higher forms of life. He considers an animal as having 
definite axes of polarity, or symmetry, the anterior end containing 
the most sensitive recei^•ing structures. Since the brain is the most 
active protoplasmic substance its metabolic rate is higher than that 
of the rest of the organism, while its activity controls other parts of 
the organism. 

This concept of the organism is an aid to a better understanding of 
the complicated reactions and responses that are found in higher 
animals. It is difficult to explain the complex response patterns of 
vertebrates unless they are considered to be organized masses of proto- 
plasm which respond as units to the total pattern of stimuli rather 
than to individual stimuli. Living animals, at least those high in the 
scale of life, respond to total situations rather than to isolated stimuli. 
Such a point of view is taken by the "Gestalt " group of psychologists, 
who use the term insight to describe an organized response at the con- 
scious behavior level. Such a response can be shown to be directed 
toward a goal, the complex movements being organized in relation to 
that goal, the result of which is that the animal is able to solve its life 
problems. According to this theory, a child who is learning to walk 
does not make random "trial and error" movements. The uncer- 
tainty of its first steps is due to a lack of maturity of the muscles and 
of the nervous system, and not to the lack of a goal. This can be seen 
in a comparison of two children of the same age, one of whom is 
allowed to walk early, the other who has been kept off its feet for fear 
of having the legs bowed. The latter will walk almost at once when 
allowed to try the new "stunt." When maturity of muscles and 
nerves is attained it becomes possible for a total behavior pattern to 


appear and walking takes place. The second child has both condi- 
tions present. 

This explanation of the display of energy helps us to understand the 
mental life of higher animals, especially with reference to a directed 
urge toward definite goals of behavior. In the pages that follow an 
attempt will be made to show how conscious life has developed. No 
set theories or beliefs will be imposed on the reader, but a brief presen- 
tation of the facts will be given as we see them. The student can 
then do his own thinking. 

Why Living Things Are Responsive 

Life has been likened by many writers to a flowing river which 
continually moves in one direction. Meeting obstacles, it is diverted 
from its course, moving rapidly over steep declivities and meandering 
slowly in level valleys. We do not think of a river in terms of water 
alone, but also in terms of the rocks in its bed, of its banks of gravel 
or soil, even of the forests in which it takes its source, and of the 
wharves and bridge abutments of the cities through which it passes 
in its course. We know that eddies in the river mark submerged 
rocks, that sharp curves may be caused by areas too hard for the river 
to erode, that ledges may cause waterfalls. It is not possible to think 
of the river without the environment which surrounds it. 

Guided by this comparison, we note the cause of sensitiveness of 
living matter of which an organism is made up in the fact that wher- 
ever factors of the environment impinge upon the organism, changes 
in the latter are sure to take place. These factors, forces, or things 
that cause changes in the life activities of plants or animals are 
called stimuli, and changes in relation between the organism and its 
surroundings, reactions to stimuli. Such responses may be sudden, 
as the involuntary start which comes as a result of some unexpected 
noise or the quick withdrawal of one's hand from a hot object, or 
they may be extremely slow and continuous, as is seen in the gradual 
turning movements of a plant placed in an area of unequal illumina- 
tion. The sum total of all the reactions of an organism to the stimuli 
which impinge upon it constitutes its behavior. 

Various Kinds of Stimuli 

In order to understand what causes behavior, we must analyze the 
various kinds of stimuli which act upon plants and animals, as follows : 


1. Thermal, that is, changes of temperature, as extremes of heat or 

2. Photic, Hght changes both in direction, intensity, and color. 

3. Chemical, changes that occur in the concentration of certain 
substances which may come in contact with the organism. 
Such changes might be the presence of salts, acids, alkalies, or 
other substances in the soil, or various types of chemical sub- 
stances such as are found in the food of animals. 

4. Electric, changes in the direction and strength of electric cur- 
rents. Since the modern concept of matter is interpreted in 
terms of electricity, it must be realized that these changes may 
have a profound effect on living organisms. 

5. Mechanical stimuli, such as changes in osmotic pressure within 
cells, the pull of gravity, changes in pressure of the medium. 
Contact with various objects, and sound waves, are also impor- 
tant. Many animals and plants respond definitely also to cur- 
rents of air or water. 

In unicellular organisms responses are usually more predictable than 
in higher organisms because the latter are complex structures in which 
different parts may be differently affected by the same stimulus. For 
example, gravity may act negatively on the stems of green plants and 
positively on the roots of the same plant. While the stem of a plant 
may be influenced to grow toward light the roots grow away from it. 
These examples might be multiplied many times. 


In 1918 Jacques Loeb, one of the foremost investigators in this 
country, brought out a book entitled, Forced Movements, Tropisms, 
and Animal Conduct. The author took for his thesis the mechanistic 
point of view of life. To him, and to other members of his school, living 
organisms are mechanisms whose activities are directly influenced by 
the stimuli in their environment, the sum total of behavior being the 
direct result of their reactions to various stimuli. In a series of con- 
vincing experiments, Loeb showed that animals are forced to do certain 
things because of a purely mechanical effect brought about by the 
stimuli impinging upon them. If, for example, the common shrimp 
(Palaemonetes) is placed in a trough through which an electric current 
flows, with its head toward the anode pole, the tail at once becomes 
stretched out . If it is placed with its head toward the cathode pole, the 
tail is bent under the body. In the latter case the animal can only 



swim backwards, while in the former case it can only crawl forward. In 
both cases the change in position is caused by the action of the current 
on the flexor and extensor muscles, which in one case are contracted 
and the other case extended, thus causing the animal to assume the po- 
sitions mentioned. Experiments 
such as these give rise to the theory 
of tropisms, which is simply another 
term for a series of responses of an 
organism to the various factors of 
its environment. Tropisms may 
be briefly classified as phototro- 
pisms, or responses to light ; geo- 
tropisms, or responses to gravity ; 
hydrotropisms, or responses to 
water ; chcmotropisms, or responses 
to chemical substances ; thermotro- 
pisms, or responses to temperature 
changes ; galvanotropisms, or re- 
sponses to electricity; thigmotro- 
pisms, or responses to contact ; 
rheotropisms, or responses to water 
currents ; and aneinotropisms, or 
responses to air currents. 

A tropism is a kind of directional 
urge. It represents a condition 
within an organism, resulting from 
the interaction between its struc- 
ture (nervous) and the stimuli of the environment. Loeb explained 
tropisms as specific irritabilities or sensitivities to stimuli at the 
surface of the body, and in terms of body symmetry, since corre- 
sponding parts on two sides of the body would show the same sen- 
sitivities. Noncorresponding parts, according to this theory, would 
show unequal sensitivities, resulting in directive movements. 

Loeb explained his famous example of the reversal of tropisms in a 
caterpillar by showing that the caterpillar moves toward light when 
hungry and is irresponsive to light when satisfied. The result is 
most useful to a caterpillar, because as it leaves its nest when hun- 
gry, it is near the surface of the ground and is drawn by light to the 
tips of the branches where young edible leaves are sprouting, returning 
to the lower branches when nonresponsive to light. 

Position taken by lejjs of shrimp 
when current goes laterally through 
animal, from left to right. (After Loeb 
and Maxwell.) 

Which direction would the animal 
be forced to take in movement .3 


The typical moth is positively phototropic. 'IMiis is an advantage 
in its natural environment because it flies at night and gets its food 
largely from white flowers which are mon^ conspicuous at night. If, 
however, the factor of artificial light is introduced, the moth flies to 
its death. This is not because it "thinks" it sees a white flower, but 
because its eyes, its central nervous system, and its wings are all 
connected as a unit, so that the animal has to turn in flying to the 
flame not once, but again and again. 

Jennings found Paramecium equally responsive to paper, silk, or 
particles of carmine placed in its immediate environment, thus 
showing a purely mechanical response. It took these foreign sub- 
stances into its gullet and the material was passed into the body. 
Such responses are not advantageous. On the other hand a purely 
thigmotropic response may be advantageous to these animals. Para- 
mecia feed on bacteria, which may form raftlike masses. As soon as 
a Paramecium comes in contact with such a mass, its response to this 
stimulus causes it to remain quiet, while it feeds upon the bacteria. 
Its sensitivity to other stimuli at this time is decreased, making it 
seem as if its attention were "fixed upon its meal." 

Nature of Responses * 

The nature of a response to a stimulus depends upon the intensity 
and nature of the stimulus as well as upon the structure of the part 
stimulated. The nature of this response may differ greatly. In 
unicellular organisms the entire cell may move in response to a 
stimulus, though sometimes there is only a turning or the movement 
of cilia on one side. If a simple animal such as Hydra is touched, 
withdrawal of the tentacles touched may occur, or, if the stimulus is 
more intense, the entire body may contract. In plants, responses to 
stimuli may result in movements caused by diff'erences in osmotic 
pressure of the cells, or in turning movements brought about by the 
growth or turgor of certain cells. There may be glandular responses, 
too, such as the production of nectar in flowers, or the flow of saliva, 
or the dry mouth of "stage fright" in man. The newt gives off slime 
when touched, and the gland cells in the skin of a toad exude poison 
when it is roughly handled. 

As a result of response to pressure, gas is secreted into the swim- 
bladder of some fish. Certain areas in jellyfish or in fireflies become 
luminous when touched, while some fishes and other animals, such as 
squid, octopuses, tree frogs, and chameleons, respond to change? of the 



•>i*'':-: ■•■.-■- ■ *■ ■ ■."■■ ,■ 

..." ••■^{;.-'. ■ • » >• », 

•• V;.. ,. v.- ■■•,'■.■■■ ■■■•... '-J,' •>■■■• ■-'•;■' 

Francis B. Sumner 

Dr. Sumner's experiments with flounders show 
a response of the animal to different backgrounds. 
How would you attempt to account for this .•* 

environment by chang- 
ing their color pattern. 
There may even be elec- 
tric responses to stimuli 
as seen in the discharge 
of as much as 300 volts 
from the electric organ of 
the electric eel, a shock 
sufficient to kill a horse. 
In the higher animals 
where well-developed or- 
gans have been evolved, 
an organ is usually at- 
tuned to one kind of 
stimulus and responds 
only to that particular 
stimulus. The eye, for 
example, responds to light 
waves, but to no other 
ether waves, while the 
organ of Corti in the mam- 
malian ear distinguishes 
with accuracy betw^een 
different wave lengths 
which cause sounds. Thus 
the nature of responses 
depends not upon the 
stimulus, but upon the 
kind of cells stimulated. 

Mechanisms of Response in Plants 

It is much easier to show that plants respond to stimuli than to 
explain how they do. Most of the responsive activities of plants do 
not, as one author puts it, result in "discriminating movement" so 
much as in ''discriminating growth." If a growing root is photo- 
graphed every ten or fifteen minutes and these pictures greatly 
magnified are projected as a slow motion motion picture, the root 
seems to act like an intelligent "white worm," pushing aside soil 
particles, avoiding obstacles, and ultimately finding its way to an area 
where water exists. 


In spite of the work of Sir J. C. Bose, the distinguished Indian 
botanist, who used very dehcate instruments to measure tlie irrita- 
biUty of plants, scientists as a group have not accepted his behef that 
t\\o transmission of stimuh in plants is by means of a mechanism 
similar to the nerves of animals. There is no doubt that certain 
parts of the plant stem do conduct stimuli more rapidly than others, 
but it is doubtful whether the conducting strands of protoplasm in the 
sieve tubes of the phloem are actually the areas of special transmission. 
Experiments have been made in which the stimulus of an electric 
current can be cut out by the use of anesthesia, just as in the case of 
the nerves of animals, but since the cells in the area where the stimulus 
is transmitted are much shorter than the neurons in the animal, 
transmission is naturally slower and anesthetics have the same 
effect on living protoplasm in each case. One investigator, Ricca, 
has shown that a stimulated region of a plant secretes a hormone that 
travels to the region of response, causing a reaction to the stimulus. 
Other workers have even shown that if the tip of one plant is grafted 
to another plant from which the tip has been removed, the stimulus 
will be transmitted to the responsive region of the latter plant. A 
number of experiments upon plants indicate that stimuli are trans- 
mitted by means of hormones which are carried in the transpiration 
stream through the vascular bundles. Too little is knawn at the 
present time to say with certainty exactly what effect hormones 
have, but it is quite evident that they do play a part in the trans- 
mission of stimuli. 

One of the most studied responses is geotropism. Roots are 
assumed to respond positively to the pull of gravity while stems are 
considered to be negatively geotropic. Branches and leaves usually 
grow at right angles to the force of gravity while some roots place 
themselves at a definite angle to this force. Gravity has been shown 
to be a stimulus by experiments which either replaced it by some other 
force, or neutralized its effect. For example, plants are placed on a 
slowly revolving disk called a clinostat. If the })lant is revolved 
horizontally on the disk, which rotates parallel to the long axis of the 
plant, the roots and stems will continue to grow in the same direction 
as they did at the beginning of the experiment. Gravity in this case 
acts on all sides of the plant eciually, with the result that there is no 
change in the position of the plant's organs. In the famous experi- 
ment of Thomas Andrew Knight, who worked in the early part of the 
nineteenth century, plants were placed on a rapidly rotating disk in 



which centrifugal force was substituted for gravity. In this experi- 
ment the roots grew outward while the stems grew toward the center 
of the revolving disk, instead of assuming the normal geotropic 

Roots of Vicia faba with tips in glass slippers: at left, a, b, c, three stages 
in the curvature of the same root, to 20 hours; at right, a, b, two stages of the 
same root; h, 18 hours after being placed in position a. (After Czapek.) 

Experiments by Czapek, in which the tips of growing roots were 
placed in glass slippers smaller than those used by Cinderella, show 
that the region sensitive to the pull of gravity, "at least in certain 
plants," is located in the last two millimeters of the root-tip. Recent 
investigators have tried to account for this location of the response. 
In animals, definite organs which "perceive " gravity are found. Such 
are the otocysts of the crustaceans and the balancing organs (semi- 
circular canals) of higher animals. In the crustaceans small but 
relatively heavy particles, known as otoliths, give the animal its sense 


iMY' "hairxs 

part of an 


otolit'bs— .„ ^, >, 

enlarged view 
of otocysts 

Balancing organs of a crustacean. How do they function.^ 



of position in space when they come to rest on the sensory hairs which 
Hue tiie httle pits, or otocysts. A somewhat similar explanation has 
been advanced to account for the 
response to gravity in plants. 
Cells of plants are filled with fluid, 
but they also have in them various 
solid bodies, some of which are 
starch grains, and others tiny crys- 
tals of calcium oxalate, or other 
minerals. It is thought that the 
movement of these bodies within 
the cell may give the stimulus for 
the turning movements attributed 
to gravity. The twining move- 
ment and spiral growth of stems 
also seems to be related to the 
stimulus of gravity, for if such 
plants are placed on a rotating clinostat, the twining movement ceases. 
There are many other kinds of responses, but the mechanism of the 
response is not always clear. Roots travel for long distances toward a 
source of water. A case is cited in California of a eucalyptus tree 
which sent out its roots over 100 feet underneath a boulevard, the 
fine roots ultimately clogging a cement water pipe on the other side 

Perceptive region 
of Roripa amphibia; 
of the granules in 

in the root cap 

with the position 

the cells. (After 







Wrhjhl I'iirct' 

The Sensitive Plant (Mimosa pudica) before and after stitmilation. Time 
required for reaction can be measured in seconds. 



of the boulevard. The Carolina ])oplar has lost its vogue as a tree for 
city planting largely because of this habit of clogging drain })ipes by 
the response of the roots to water. The movements seen in the wilting 
of leaves, or the changes in the position of leaves in bright sunlight 

and in slight illumination, are 
familiar to all. There may 
even be a quite rapid opening 
and closing of flower petals, 
and there are also definite 
noticeable changes in the posi- 
tion of the leaflets of clover, 
alfalfa, oxalis, and other 
plants in the morning and at 
night. The relatively rapid 
responses of the leaves of 
the sensitive plant, Mimosa 
pudica, are all brought about 
by the functioning of struc- 
tures called pulvini, cushion- 
like enlargements of the 
petiole of the leaf at the point 
of its insertion in the stem. 
When the leaflets of the large 
compound leaves of the mi- 
mosa are stimulated by heat, 
pressure, or anesthetics, they 
tend to droop, the stimulus 
from the leaflets being trans- 
mitted at the extraordinarily rapid rate (for plants) of from one to three 
centimeters per second. When the stimulus reaches the pulvinus 
where the cells are large and are rich in water, a change in turgor takes 
place in these cells, with the result that the leaf stalk droops. In some 
plants there is a rapid and temporary fluctuation in growth on opposite 
sides of the leaves. This causes a comparatively rapid turning move- 
ment, but it is evident that these forces are not in themselves sufficient 
to explain all the changes that take place in such plants as the Mimosa. 

Leaf motility in the sensitive plant 
(Mimosa pudica): above, an open leaf; 
l)elow, a leaf whose leaflets (/) have been 
closed by niechanieal impact ; note also that 
the petiole (p) has dropped; s, stipule; 
m, pulvinus. 

Mechanisms of Response in Animals 

The mechanism of the reflex arc has already been described in some 
detail in the discussion of the various types of nervous systems found 



..oral cUia 

...orccl ^roov(2^ 



in animals. It will not be amiss, however, even at the risk of repeti- 
tion, to take up, from the standpoint of function, the effects of some 
of the forms of animal behavior. 

In simple animal cells, such as Ameba, the outer portion of the 
cell is in contact with the stimulus which is transmitted through the 
protoplasm to the in- 
terior of the cell. In 
cells with cilia, continua- 
tions of these structures 
that reach down into the 
protoplasm apparently 
act as organs for recep- 
tion of stimuli. Euglena 
has a pigmented "eye- 
spot" which is definitely 
sensitive to light. In 
some specialized proto- 
zoan cells a motoriuni or 
co-ordinating center is 

In higher forms of 
animals there are defi- 
nite receptors in the form 

of sense cells, organs which act as stimulating centers with nerves 
serving as conductors to the parts that are fitted for resi)onse, the 
so-called effectors. Examples of such effectors are the muscle cells, 
gland cells, and the cells of such organs as the luminous areas of the 
fire-fly, and the electric organ of the electric eel. In the nerve net of 
such animals as Hydra, or the jellyfish, apparently no synapses 
exist between the cells, the nervous system being a tangled net through 
which the nerve impulse flows. In such a nervous apparatus the 
nerve activity is slower than in a type of nervous system found in 
animals like the earthworm. The so-called "ladder nervous system" 
exists in worms and in arthropods generally, and is seen at its highest 
development in the insects, where there is a series of units in which 
the neurons are connected by synapses. Such types of nervous 
systems are more effective because the nerve impulses travel only in 
one direction through a neuron, while in the nerve net they may travel 
in any direction. Receiving neurons in the sense organs are found 
at the surface or in a situation where they may be exposed to stimuli. 

H. w. H. — 25 

.caudal Cirri 

Euplotes, a hypotrichous ciliate. Note the 
thickened cilia or cirri by means of which the 
animal is able to make siuUlen jumping move- 



Connecting neurons tie up these with the effector neurons which stimu- 
late the muscles to contract, or the glands to secrete. The dorsally 

placed vertebrate nervous sys- 

a few cells of -tfic^ 
nerve. neLof 


\adde-r system systsra of odultrnid^ 
of a myriapod ■ *^'" ' 

tern is considered the most 
highly developed type of all. 
Here centralized function is 
found at the anterior end of 
the body in the so-called brain. 
In the animal series all animals 
except those built on the radial 
plan show a very distinct 
centralization of sense organs 
(receptors) at the anterior end. 
The organs of sight, hearing, 
taste, and smell are found in 
a relatively small area on the 
head close to the brain. It is 
easy to see how evolutionary 
development has brought this 
about, since it is the anterior 
end which is constantly ex- 
ploring for the rest of the 
Upon the success of this exploratory ability rests the suc- 

Three types of nervous systems. What 
are the general Hkenesses and diflerences ? 
Which would be called the highest type and 

why i' 


cess of the animal in its struggle for existence. 

Tropisms, Reflexes, and Native Behaviors 

The term reflex action has been given to the response which comes 
from the stimulation of a single reflex arc, a receptor with its neuron 
leading inward to an effector neuron which in turn causes movement 
through the effector muscles or glands. In most if not all cases, how- 
ever, there is more than a single series of neurons engaged in the 
action of the reflex arc. There is always a direct response in the 
reflex. The response is quite predictable and results in movement 
of a relatively small part of the animal's body. A tropism, on the 
other hand, may be considered as a steady response to a continued 
stimulus. As one writer well puts it, the tropism is "a steady under- 
lying bias in behavior brought about by a constant stimulus." The 
tropism affects the organism as a whole, the reflex directly affects 
only a small portion of it. The activities of all animals, but espe- 
cially the lower forms, are a continual series of reflexes and tropisms. 


When reflexes follow one another in an orderly succession involving 
a chain of reflexes, one step of which determines the next, they are 
called native behavior 'patterns. That these are inherited patterns is 
seen in such acts as cocoon-making, egg-laying, or mating behavior, 
which only take place once in the life of the individual. 

There have been two lines of e\olution in behavior patterns, one 
culminating in the insects, the other in nian. These two groups are 
the most successful in the animal kingdom. The insect group 
embraces probably over 625,000 species, while man is but a single 
species. It is estimated that many insects, particularly the ants, 
have undergone no significant structural changes since the Oligocene 
period some thirty million years ago. They are at the summit of 
their development while man is just beginning. Insects mark the 
top notch of these native behavior patterns. Their innate stereo- 
typed functions make them, in the words of one writer, "a bag of 
tricks." Their actions depend upon a series of associations which 
form a sequence or chain of events. These chain-reflexes in many 
cases have formed so complicated a pattern that the ensuing actions 
appear to be intelligent. However, when these actions are carefully 
analyzed, by means of experiments, they exhibit a far different type 
of response. The well-known example given by Fabre will suffice to 
illustrate how such a chain of reflexes works. One of the Sphex wasps 
habitually paralyzes a cricket by stinging it, and then drags it to its nest 
as food for its larvae. After the female w^asp has dragged the paralyzed 
victim to the entrance of the burrow, she leaves it there and goes inside, 
apparently to inspect conditions. In his experiment, Fabre moved the 
cricket a short distance from where it was left and when the wasp came 
out, finding the cricket out of its original position, she seized it again and 
dragged it back to the mouth of the nest, and again went in. Fabre re- 
moved the cricket forty times, and for forty times the wasp repeated its 
actions. As Huxley has so aptly said, all she knew was, "drag cricket 
to the threshold — pop in — pop out —pull cricket in." In this case the 
initial stimulus that started this whole chain of events was the maturing 
of the egg in the body of the wasp, and the breaking of a single link in the 
chain of associations was sufficient to break the sequence of events. Ex- 
amples of these chain reflexes, which have been called instincts for v/ant 
of a better term, are so numerous that volumes have bcnni written about 
them. The many fascinating books of Fabre, the intriguing volume 
on wasps by the Peckhams, the still interesting classic entitled, Ants, 
Bees and Wasps, by Sir John Lubbock, are all worth reading. 


Native Behaviors May Be Modified 

Although native behavior is usually predictable, there is some 
evidence that it may be modified under certain conditions. Howes ^ 
gives such a case. The sphecid wasp places a single paralyzed cicada 
in its burrow after laying an egg in the body of the unfortunate vic- 
tim. The burrow is then sealed with earth, the young wasp feeding 
on the paralyzed insect until the larva pupates. The adult wasp 
carries the cicada, which is larger than itself, by means of two power- 
ful up-turned hooks on each side of its hind legs. Howes removed 
these hooks from the legs of a sphecid wasp and after several hours 
replaced the wasp near the burrow, but close to a cicada which it 
had previously captured and paralyzed. The wasp paid no attention 
to the cicada but flew off, shortly returning with another victim 
which it carried between the first and second pairs of legs. This 
shows a marked modification of its original instinctive behavior. 

The following examples show how in two nearly related species there 
may be differences in behavior. The mud-dauber wasp builds a 
small nest of from eight to ten cells, filling each cell with paralyzed 
insects or spiders which are used as food for the developing young. 
In filling the cell, Howes found that the wasp averaged one spider for 
every seven minutes of time until its tenth visit, when it brought a 
small pellet of mud which it flattened and placed across the opening of 
the cell. This was not enough to close the cell, so the insect flew away 
to get more mud. While it was gone Howes removed the spiders and 
the cell cap. The wasp, upon returning, resealed the cell without 
examination and without depositing spiders or another egg. In the 
case of the paper wasp, a near relative, when Howes replaced an 
unfinished cell with one of papier-mache the wasp immediately tore 
the papier-mache cell down and proceeded to build a proper one. 
This indicates that the chain of native behaviors in some cases may 
never be broken without a complete re-acting of the whole scene, 
while in others modification of behavior which looks like a low-grade 
intelligence is found. 

In considering the insect with its "bag of tricks," all of which can 
be played expertly but which cannot be changed, we must think in 
terms of structure as well as in terms of function. Contrast, for 
example, the strongly built claws, legs, or mouth parts of an insect, 
or a crustacean, with the hands of a man. The former, each of which 

I Howes, P. G., Insect Behavior, Badger, 1919. 



is fitted to perform a very limited number of unchangeable acts, are 
rigid. The latter, on the other hand, are plastic, extremely flexible 
and adaptable, capable in some instances of playing a Chopin noc- 
turne, or in others of fashioning the cunning work of a Cellini. 

Habit Formation 

The patterns of behavior that we call habits are closely allied to 
native behaviors. If animals can make associations, any act which 
comes as a result of a contiguity of stimulation and useful associa- 
tion tends to be repeated. If there are many repetitions the per- 
formance of such an act becomes more and more certain. In other 
words, it becomes a habit. It has been said that our lives are 
bundles of habits. This is particularly true of man, since many of the 
activities learned in early life, such as walking, learning to drive a car, 
riding a bicycle, skating, swimming, writing, typewriting, and hun- 
dreds of other activities common to this machine age, are habitual. 

One object of education is the training of different cerebral areas 
so that they will do their work efficiently. In learning to write one 
exerts a conscious effort in order to make the letters at first. Later, 
the actual forming of letters is done without conscious effort, for by 
training the act has become habitual. 

Conditioned Behaviors 

More than thirty years ago the famous Russian physiologist 
Pavlov began a series of experiments that have changed much of 
our thinking regarding the 
fixity of animal behavior. His 
best known work was done 
with dogs. It is proven that 
when food is offered to a 
dog saliva is secreted. This 
effect is partly psychic and 
partly mechanical, as can be 
seen when one thinks of a 
particularly sour pickle or 
lemon, or chews dry food. 
Pavlov found that the dog's 
saliva, which was normally 

f-^ -f^paroticC 
inUmoa duct t^JJ 'dlcincC 

"^a"0^ lintsrncU) 



- submayrillar)^ 

drop of.^alivtt^ 

Diagram to show Pavlov's experiment. 
Under what conditions would saliva be 
caught in the tube? Explain why he ob- 
tained a conditioned reflex. 

secreted when the dog saw food, could be caused to flow by the 
ringing of a bell, or by the presentation of a plate of a given color. 


But this behavior was only hroufilit about through tlie presentation 
of food many times in succession at the same time or just after the 
ringing of the bell, or the use of the colored plate, thus forming 
an association between food and bell, or food and plate. Eventually 
when food was not presented but the bell rang, or the plate was 
shown, saliva would flow from the parotid gland just as if food was 
present. The reflex established originally with food was changed 
through association of food with bell or plate. Thus Pavlov estab- 
lished a law of the conditioned reflex, which may be stated thus : 
"If a new indifferent external stimulus is many times present along 
with one which has also a definite response, the subsequent presenta- 
tion of the new stimulus causes the reflex to be given." 

Conditioned reflexes have been demonstrated in forms as simple 
as the Ameba, earthworm, crab, snail, octopus, as well as in higher 
animals. It is, however, unlikely that conditioning plays a very 
important part in the lives of lower animals. In the case of fishes, 
reptiles, amphibians, and vertebrates, the "training" which comes 
through the conditioning of behavior may play a minor part. In the 
highest vertebrates, apes and men, conditioning undoubtedly plays a 
very definite part in the learning process. Experiments made in 
Pavlov's laboratory have shown that while a dog may take from 
thirty to one hundred trials before it is conditioned to food, a young 
child may show the same conditioned effect in from ten to twenty-five 

Are Behaviors Adaptive Responses? 

It is easy to show that all responses to stimuli are useful to a plant 
and, therefore, enable it to adapt itself more easily to the environment 
in which it lives. The turning of stems and leaves toward light, the 
"seeking" of roots for water, the twining movements of plants are 
all well-known examples. 

When it comes to animals, there are two views of their response to 
stimuli, one mechanistic, the other adaptive. The first considers the 
organism to be a machine that responds blindly to the various physical 
and chemical stimuli which impinge upon it, regardless of the conse- 
quence to the organism. This is much easier to see in simple animals 
than in more complex ones, because in the latter the behavior of the 
organism is influenced not only by different combinations of stimuli 
but also by the reinforcement or weakening of stimuli. The behavior 
of the organism at a given time will be determined, not by a single 

Tin: DISPLAY OF i:nkr(;y :m 

stimulus but by the aggregate of all the stimuli which impinge upon 
it. The stimulus pattern causes the behavior pattern. The fact 
that organisms behave in a purposeful way and that frequently their 
behaviors are modified or "conditioned," has given rise to the point 
of view that behaviors are adaptive. 

To understand this philosophy it is necessary to go back to the Avork 
of Child. In recent years he has shown that all organisms exhibit a 
definite polarity. Even in a single-celled organism, polarity is shown 
not only in an anterior and posterior end, but also in a physiological 
gradient which extends from the surface to tlie interior. The proto- 
plasm at the surface exhibits the highest rate of metabolism, the 
protoplasm near the center the lowest rate. If the organism is cut 
in two, a new center forms as far away from the surface as possible 
and a new field of metabolism comes into existence. 

The following test of this metabolic gradient was made with flat- 
worms, animals so simple in structure that they lend themselves 
readily to experimentation. After removing the head and tail end of 
a number of worms, the remaining part of the worm was cut into four 
pieces, as many as a hundred worms at a time being used in the 
experiment. After sorting the cut pieces into groups of anterior 
sections, second, third, and fourth sections, it was found that the 
metabolic rate in these groups was constant, the most anterior group 
using the most oxygen and giving off the most carbon dioxide. The 
most posterior group used the least oxygen and gave off the least 
carbon dioxide. There was thus a chemical gradient of physiological 
activity correlated with the nervous differentiation of the organism, 
the latter acting as a physiological unit and not as a cell aggregate. 

Physiological gradients are seen every^vhere. Eggs exhibit polarity, 
the potential energy at one end being much greater than at the other. 
Gland cells are j^olarized so that they always secrete in a certain 
direction, while nerve cells in higher animals invariably conduct 
impulses in only one direction. In embryos, an early polarization 
takes place and, as we have seen, all animals except radially sym- 
metrical ones exhibit polarity. 

The beginnings of behavior in embryonic animals start as mass 
movements of the organism as a whole. This has been found to be 
due to the fact that the central nervous system has not grown out 
into the surface of the body. A new group of behaviorists start with 
the general thesis that behaviors, such as tropisms, are organized 
responses to a total pattern of stimuli, the organism modifying its 



behavior according to the stimulus pattern it receives. This modi- 
fication results in a change or tension on the part of the organism, 
leading it toward a goal. This goal may be food, or some other 
"desirable" situation. Many experiments have been made which 
indicate that modifications of behavior which result in learning take 
place very early in the animal scale. Schaeffer ^ made an interesting 
experiment with Ameba. He put a particle of glass close to an 
Ameba that had been starved for some time, making the glass vibrate 
by means of a rod. The Ameba immediately surrounded the glass, 
forming a food vacuole, but after six minutes expelled the glass. 
Five minutes later the glass was again presented, and again the 
Ameba ingested it, this time expeUing the glass after three and a half 
minutes. In a third presentation the glass was only partially ingested 
and two more trials gave slight food response. All further trials 
showed the Ameba completely indifferent to the glass. This con- 
tinued placing of nonedible material in front of the animal set up a 
tension in the protoplasm which resulted in a modified behavior, 
causing indifference to the vibrating glass. 

In animals which have only a nerve net, modification of behavior is 
also possible. In the famous experiments of Loeb and of Parker bits 

of meat were presented 
to the tentacles of a sea 
anemone, the meat being 
passed by the tentacles 
into the mouth. Then 
the same tentacles were 
fed with bits of filter 
paper which had been 
soaked in beef juice. At 
first the animal made no 
distinction between food 
and filter paper but after 
ten trials learned that 
filter paper was not food and constantly rejected it. Hundreds of 
other examples might be given to show that behaviors are modifiable. 
But if lower animals live in a world of present actions and "have no 
thought for the morrow," then it is doubtful if this conditioning of 
behavior means much in the ultimate solution of their life problems. 


filter papei~ N/ilb meat jui<ie, 
is token , , 

taken to 

popct- is rejecteoC 

A sea anemone will learn to distinguish between 
meat and filter paper flavored by meat juice. 

1 Schaeffer, A. A., "Choice of Food in Amoeba." Journal of Animal Behavior, 1917, Vol. VII, 


When Are Animals Conscious? 

If we accept Loeb's mechanistic point of view, no animals lower than 
man would be considered conscious. As Professor Hodge once said, 
"A house fly is about as intelligent as a shot rolling down the board." 
Once a chain of behavior is set in motion, it continues until the life 
cycle is completed by egg-laying. 

The theory most commonly accepted among psychologists today 
is that when an animal improves its responses through the use of 
experience, then it has some degree of consciousness. Just because 
an earthworm may "learn" to turn to the right instead of the left 
in a T-tube to avoid an electric shock does not mean that it has either 
consciousness or memory in the true sense of those terms. Nor does 
the dog which can be conditioned necessarily have a consciousness of 
the sound of a bell or of color in the sense that man does. In insects, 
for example, where there is a highly developed nervous system of a 
specialized type, the animals live largely in a world of odor. Their 
perception of food, nest, or surroundings is largely dependent upon 
odor. Ants recognize each other and their tribal enemies by odor. 
The male moth recognizes its mate by odor. 

We must be careful not to read our own sensations into the re- 
sponses of simple animals. As Wells, Huxley, and Wells aptly say, 
"The jelly-fish only pulsates. A sea urchin with its nerve-net has no 
sense of wholeness." It is a mistake to assume that lower animals 
live in a world where space and time play a conscious part. The eyes 
of worms, insects, or most molluscs do not "see" in our sense of the 
word. The insect may perceive colors and moving objects, but to 
many animals the world is a world of light and shadow. Three states 
of existence are probably found, — that of mere reception of stimuli ; 
another, in which objects become stimuli ; and in higher animals a 
perception of space and time. The world of recognized cause and 
effect is probably open only to the highest animals, such as apes and 
man. Therefore, consciousness is a very variable term and at most 
does not mean much to the psychologist. 

Emotional Responses 

The emotional responses of higher animals are a type of nervous 
and glandular activity that plays a tremendous part in their lives. 
Feelings, joys and sorrows, fear, anger, worry, or optimism, how 
much they govern the life patterns of the average man ! Biologically 


such activities are closely tied up with hormone activity. Definite 
changes in the body are recognized as associated with certain emotions. 
Sudden fright causes the heart to beat faster, the hair "stands on end," 
the face blanches, and the digestive glands cease their accustomed 
activity. The biologist sees in these physical accompaniments of 
the "feelings," changes that hark back to native behaviors, actions 
that make for self-preservation. Under the stress of unpleasant 
emotions the glandular activities of the digestive tract are reduced, 
so that more blood flows to the muscles, thus allowing greater muscular 
activity. The automatic sympathetic nervous system invokes secre- 
tion from the adrenal glands which in turn tune up the sense organs 
to greater sensitivity and the circulatory and respiratory systems to 
greater activity, with a resulting increase in oxygen and in food to the 
muscles. Emotions are evidently self-preserving activities, but they 
also add and subtract much from the lives of men. The highly 
emotionalized person who has his "ups and downs" may get more out 
of life than his lethargic neighbor, but he also suffers more deeply and 
may make more mistakes in judgment when under emotional stress. 

What Is Intelligence? 

The term intelligence has been much misused, for we are apt to read 
our own point of view into the actions of lower animals. Psychologists 
say that an animal is intelligent when behavior is flexible enough 
to make it profit by experience. Stereotyped functions having a 
pattern handed down by heredity have been shown to be native 
behaviors. Patterns of conduct not inherited, but acquired by 
many repetitions, are habits. The intelligent act shows choice. It 
involves analysis of a situation and the comparison of past experi- 
ences in relation to the present, that is, there must be memory or a 
record of past events. In addition, the intelligent act also involves a 
synthesis with past experiences built up with the aid of memory and 
imagination. Intelligent animals show a certain amount of insight. 
Intelligence involves the solving of problems, in other words, the 
directional mind set toward a goal. 

Intelligence in animals appears to be correlated with a definite 
development of the cortical layer of the cerebrum. Although the size 
and weight of the brain have little to do with intelligent action, the 
size of the cerebrum in relation to the rest of the brain is definitely 
correlated with intelligence. More than this, the number of convo- 
lutions in the surface of the cerebrum, with a consequent increase 


in the number of cortical brain cells, has a decided correlation with 
the degree and kind of intelligence that an animal shows. A comi)ar- 
ison of the brains of normal with those of feeble-minded individuals 
shows that in the latter the number or depth of the cerebral convo- 
lutions is much less than in the former, thus giving an anatomical 
evidence for differences in intelligence in man. 
As one of the authors ^ has said. 

"With an increase of cerebral function the instinctive reflexes take more 
and more to the background, and therein is a great distinction between 
' lower ' animals which are largely at the mercy of their environment and 
heredity, and the ' higher ' animals, which to an increasing degree have risen 
above environmental conditions, and become more and more ' the captains 
of their souls.' The most prized possession of mankind is the ' capacity for 
individuality,' yet even what passes for ' free will ' has its basis in the neurons 
and reflexes built up in the brain, that after all must be regarded as the 
mechanism through which consciousness, memory, imagination, and will are 
affected, rather than as the seat of these manifestations of intellectual life." 

Types of intelligence differ widely in the animal scale. The so- 
called "Gestalt" psychologist would consider modified or conditioned 
behavior as evidence of some degree of intelhgence. Perkins and 
Wheeler have shown that goldfish could be trained to make correct 
responses to light of various intensities even when the absolute in- 
tensities of the lights were changed as well as their positions. Scores 
of similar experiments performed wuth higher animals could be cited 
to show adaptative configurational behaviors. If, however, w'e take 
the criteria given in the above paragraphs it would seem that memory 
and a synthesis of previous action are necessary to the possession of 
true intelligence. The "hold-up" bear of Yellowstone Park appears 
to be intelligent when it lumbers out of the forest and holds up the 
passing autoist for candy. It simply associates the moving cars and 
their contents with sweets. Probably chance started it on its nefar- 
ious career. A dog taught to do certain tricks seems intelligent but 
has simply formed associations between the food given as a reward 
and the act learned. A dog which welcomes its master after a long 
absence probably does not remember or have a deep attachment 
for its master, but simply responds to a blind though increasing urge 
brought about by a stimidus pattern in which associations exist 
between master and food, or some other goal. 

1 From Walter, H. E., Biology of the Vertebrates, p. 631. By permission of Tiie Macmillan 
Company, publishers. 



Intelligence of Apes 

Because of their relationship to man, the higher apes have been the 
source of much fruitful experimentation of late years. Kohler ^ has 
demonstrated that the chimpanzee shows evidence of emotionalized 
response as well as a comparatively high degree of intelligence. 
A chimpanzee shows emotion not only by actions, but also in facial 

Yale LaboTotOTies of Primate Biology 

Chimpanzees are the most emotional as well as intelligent of the apes. 

expression. The ape jumps up and down to show excitement, knocks 
its head on the floor of the cage when unable to solve a difficult 
problem, or looks vacuously into space and smiles when lost in con- 
templation of some object that interests it. Yerkes ^ shows that 
chimpanzees have wide differences in emotional or intelligent conduct. 
One may be gloomy, another happy, one lethargic, another active, 
one dull mentally, and another bright. They may be as tempera- 
mental as some human beings or just as stoical. They also show 
great differences in mentality and like man have their "off" days. 

Kohler describes one series of experiments which show that apes 
have intelligence to solve problems difficult enough to test the inge- 
nuity of a young human child. The ape Koko was the subject. 
In his cage was placed a box and from the top of the cage a banana 
was suspended well out of reach. The ape first tried jumping for the 

1 Kohler, W., The Mentality of Apes, Kegan, London, 1924. 

2 Yerkes, R. M., and Learned, B. W. : Chimpanzee Intelligence and Its Vocal Expression. 
liams and Wilkins Co. 



fruit, but finding this did not work, approached the l)ox and gave it 
a push toward the food, looking meanwhile at the banana. Dr. Kohler 
then made the goal more interesting by adding a piece of an orange. 
After a brief pause, Koko went back to the box, pushed it vigorously 
until it was directly under the fruit, then climbed up on the box and 
got his reward. The same problem was given Chica, another ape. 
This was solved successfully several times until one day her com- 
panion Teserca was resting on the box. While this was happening 
Chica jumped in vain for the fruit, finally giving up in despair though 
not attempting to use the box. Presently Teserca got down from the 
box. At once Chica dragged the box under the fruit, climbed up, and 
got her reward. Evidently the box on which Teserca was resting 
meant to Chica something to "rest on" and not until the box alone 
was seen with the fruit did it mean "something with which to get the 
fruit." This simple problem was made more difficult by raising the 
fruit to a greater height and adding three boxes which had to be piled 
one on the other before the fruit could be reached. Such a problem 
was solved by Sultan, an ape of unusual intelligence. Yerkes ^ made 
a similar experiment with the gorilla Kongo in which three boxes had 
to be stacked in order to reach food. K year after the successful 
solution of this problem, the gorilla was furnished with a similar 
problem, the three boxes being of slightly different size. The problem 
was solved immediately, thus showing evidences of memory. 

The most difficult problem of all was solved by Dr. Kohler's ape 
Sultan. Food was placed just out of reach outside the bars of the 
cage and Sultan was given two sticks, one of which would fit into the 
other. Sultan made a good many useless and rather stupid move- 
ments before he finally "got the idea" with the aid of the experi- 
menter, who had put one finger into the hole of one stick while holding 
the stick close to the animal. Sultan, after playing with the sticks, 
got the two sticks in a straight line and at once pushed the thinner one 
into the opening of the thicker one. Once having made a long pole 
with the two sticks, he immediately drew the banana into the cage 
and was so well pleased with his performance that, without waiting to 
eat the fruit, he proceeded by means of the double stick to pull in 
other pieces of fruit. The second time the experiment was tried 
Sultan almost immediately stuck one stick into the other and got 
the fruit. In a later experiment he was given two similar sticks the 
smaller of which was a little too large to go into the hole of the latter. 

1 Yerkes, R. M., " The Mind of the Gorilla." Comp. Psy. Mon., 1928, Vol. V, No. 2. 


Sultan chewed the smaller stick down into a wedge and then, inserting 
it into the larger hollow stick, proceeded to get the fruit. This is a 
degree of intelligence such as might be seen in primitive cave men who 
chipped stones to make weapons, or hollowed out trees to make canoes. 

Intelligence in Man 

Man, however, is a long step above the ape because he not only 
can do things that the ape can, but in addition, he has memory 
which enables him to make complex abstractions and to think of 
objects and things which are not present. This ability to form com- 
plex abstractions and to use them in thinking are things that an ape 
never could do. As Herrick^ has well said, "The chimpanzee does 
not know the meaning of F- = 2 PX, and he never can find out J' 
In addition, man has a tool which the apes cannot use, and that is 
language. One ape has been taught a very few words, but it is 
doubtful whether these words have any meaning to him. The reader 
of these lines not only can see the printed word, but can understand 
the meaning of the symbols employed and can express it in terms of 
speech. He has traveled a long way further than the apes because 
he can read, write, and speak. 

The Measurement of Intelligence 
Most young people today hear a good deal about "I. Q's." Nu- 
merous tests have been devised which are supposed to measure the 
intelligence of the human being. The experts who prepared the tests 
have established norms, or average scores, for different ages. The 
I. Q., or intelligence quotient, is found by establishing a ratio between 
the mental age (M. A.) and the chronological age (C. A.) of the subject. 
If, for example, a child's chronological age is 9 and he makes a score 
which is that of a child of 10, his I. Q. is found by dividing his mental 
age (M. A.) by his chronological age (C. A.) and multiplying by 100. 
In this case he would have ^ X 100, or an I. Q. of 111. An I. Q. 
of from 90 to 1 10 is about normal. If a person has over 140 I. Q. 
he is considered to be a genius, only about 1 per cent of all persons 
falling in this group. A glance at the chart shows the normal dis- 
tribution of intelligence as foimd by testing large numbers of people. 
While the tests now used are far from perfect, testing factual knowledge 
rather than ability to think, they do indicate in most cases intelligence 
with reference to the subject tested, and so fulfill a practical purpose. 

' Herrick, C. J., Brains of Rats and Man. Univ. of Chicago Press, 1925. 









<n 15?. 










V lor. 



? .■St 






-_ 01 










I.Q 69and below 70-79 80-89 90-99 I00-IO9 ilO-H9 120-129 laoondtip 

Distribution of intelligence in school children. I. Q.'s are shown below graph. 


Caldwell, 0. W., Skinner, 0. E., and Tietz, J. W., Biological Foundations of 

Education, Ginn & Co., 1931. Chs. XIV, XV. 

An elementary luit valuable text. 
Kohler, Wolfgang, The Mentality of Apes, Harcourt, Brace and Co., 1926. 

Fascinating reading. 
Pavlov, I. P., Lectures on Conditioned Reflexes, International Publishing Co., 

N. Y., 1928, 

Authoritative and based on experimental evidence. 
Walter, H. E., Biology of the Vertebrates, The Macmillan Co., 1929. Chs. 


A well-established authority easily read. 
Wells, H. G., Huxley, J. S., Wells, C. P.. The Science of Life, Doubleday, 

Doran & Co., 1934. Book VIII. 

Interesting discussion of consciousness in animals. 
Wheeler, R. H., and Perkins, T. H., Principles of Mental Development, The 

Thomas Y. Crowell Co., 1932. Chs. Ill, V. 

An excellent exposition of gestalt psycholog>\ 
Yerkes, R. M., and Learned, B. W., Chimpanzee Intelligence and Its Vocal 

Expression, Williams and Wilkins Co., 1925. 
Yerkes, R. M., and Yerkes, A., The Great Apes, Yale University Press, 1929. 

Both of Dr. Yerkes' books give the latest experimental work on the emo- 
tional and mental life of the apes. 



Preview. Chemical co-ordination • Regulators of digestive processes • 
Regulators of general metabolism : Adrenals, thyroid, parathyroids, pan- 
creas ■ Growth regulators : Thyroid ; gonads and pituitary ; pineal • Repro- 
ductive organs as regulators • The master gland or "generalissimo," the 
pituitary : the anterior lobe, growth stimulation, gonad stimulation, lacta- 
tion hormone, thyreotropic hormone, adrenotropic hormone, blood sugar 
raising principle, fat metabolism-regulating principle, parathyreotropic 
principle ; the intermediate lobe ; the posterior lobe • Suggested readings. 


Co-ordinating devices are necessary as soon as cells become grouped 
together in large enough masses to isolate the inner ones from external 
stimuli. As the cell mass increases in size, there is a tendency for 
greater division of labor to be developed, and we find organisms evolv- 
ing with special tissues to perform specific functions. These tissues in 
turn are woven into more complex systems that call for a still greater 
division of labor. 

Probably the chief co-ordinating mechanism which keeps the 
organism in touch with its external environment is the nervous 
system. Even the primitive nerve net of the coelenterates serves 
quite adequately in this capacity, while the linear type of nervous 
system with its more highly specialized co-ordinating centers fur- 
nishes a more complex and efficient mechanism in the higher forms. 
There is another equally important, though far less thoroughly under- 
stood mechanism that acts as an "internal co-ordinator," since both 
nervous and chemical correlation is necessary to secure a symmetrical 
development and orderly functioning of the related parts. 

The study of chemical co-ordination is a field literally bristling 
with thousands of unanswered questions and holding promise of 
becoming one of the most productive phases of modern physiological 
and medical research. Within its pages are already told some of 
the most thrilling tales of intellectual adventure one could hope to 
encounter. Only a few of these will be enumerated, but we might 
well seek an answer to such questions as : What are the controlling 
devices of the body for producing and regulating normal growth? 



What starts and governs the changes in voice and body that accom- 
pany the maturing of reproductive systems? Wluit is the explana- 
tion of that last little ounce of strength which enables a sprinter 
to put on the final burst of speed? How can such correlation be 
possible without the existence of a ''master mind" for the l)ody? 
Answers to these questions will be found in the pages that follow. 

Chemical Co-ordination 

Our knowledge of chemical regulation of the body is far from 
complete. Workers in the field of endocrinology, as in other fields, 
are continuously pushing back the frontiers of ignorance, at best a 
slow process. Nevertheless, each year new excitants, or hormones, 
are discovered, their effect noted, and their refinement or synthesis 
accomplished. Rarely there occurs the discovery of the existence of 
a new and hitherto unsuspected gland that produces one of these 
hormones. Thus far we know quite definitely that the thyroids, 
parathyroids, pituitary, gonads, liver, placenta, adrenals, pancreas, 
the mucosa of the stomach and intestine, and possibly the pineal 
and thymus glands, function as ductless or endocrine glands. In 
some instances more evidence is needed, but on the whole a ma- 
jority of scientists are in agreement regarding this list. 

Early zoologists, including such leaders as Johannes Miiller and 
Jakob Henle, failed to attach enough significance to the ductless 
glands. According to Rogers,^ the former stated that "the ductless 
glands are alike in one particular — they either produce a change in 
the blood which circulates through them, or the lymph which they 
elaborate plays a special role in the formation of blood or chyle." 

Probably the first experimental study in endocrinology was made 
by A. A. Berthold of Gottingen in 1849 when he began a study of 
the results following the removal of the testes of fowls. Shortly after 
this Claude Bernard, Addison, and Brown-Sequard made significant 
contributions. The first of these investigators worked on the liver, 
while the other two studied the adrenals. Brown-Sequard actually 
extirpated the adrenal glands and noted that the accomjjanying 
weakness and death could be prevented by transferring blood from a 
normal animal to the one from which the adrenals had been removed. 

From the physiological point of view, endocrine glands may be di- 
vided into five large groups as regulators of : (1) digestion ; (2) general 

' Quoted by permission of the publishers from Rogers, C. G., Textbook of Comparative Phyni- 
oiogy, p. 361. McGraw-Hill Book Company, 1927. 
H. W. H. — 26 


metabolism ; (3) growth ; (4) reproduction ; and (5) the master center 
which serves as the "generahssimo" of all the endocrines. Such an 
arrangement means that a gland which produces more than one 
hormone may have to be considered in more than one category. 

Regulators of Digestive Processes 

Little can be added to the description of the hormone secretin 
except to point out that physiologists are somewhat uncertain as to 
whether the hormone is first produced as an inactive substance called 
prosecretin, or as the active agent known as secretin. The action of 
this internal co-ordinating mechanism may be seen, for example, 
in the secretion of the pancreatic juice which is always poured at 
apparently just the proper time into the small intestine. At first, 
this co-ordination was believed to be due to some sort of undiscovered 
nervous reflex mechanism that was stimulated as the food passed a 
given point. This interpretation was discounted by two promi- 
nent English physiologists, Bayliss and Starling, who in 1902 showed 
that it is the passage of the acidulated food past the pylorus into the 
upper part of the small intestine {duodenum) which stimulates the 
production of a hormone, secretin. This substance is absorbed by 
the blood and carried throughout the body, the portion reaching the 
pancreas furnishing the necessary stimulus to effect the release of its 
digestive enzymes. 

Regulators of General Metabolism 


The paired adrenal gland is composed of an outer cortex and an 
inner medulla, each part having a different embryonic origin and pro- 
ducing a different hormone. In the lower vertebrates the cortex is 
represented by an elongated mass of glandular tissue called the 
interrenal, lying between the kidneys and derived embryologically 
from the lining of the body cavity. The medulla on the other hand 
is at first a separate structure, composed of so-called chromaffin 
cells, which have their origin in the nervous tissue of the autonomic 
nervous system. In the higher vertebrates the interrenal and the 
chromaffin cells become incorporated to form the adrenal gland. 

The outer portion, or the cortex of the adrenals, secretes a hormone 
known as cortin which has been proved to be essential to life. If 




there is a deficiency of this hormone in the human body heart action 
slows down, the skin becomes discolored, and the vital energy is 
overcome by a growing, and usually fatal lassitude, symptoms 
characteristic of Addison's disease. Biologists and the medical pro- 
fession were led to this conclusion as to the effects of adrenal hormones 
through numerous observations and experiments. Swingle, of 
Princeton, recounts how cats 
with extirpated adrenals 
barely survived eight to ten 
days. During this time their 
temperature fell six to seven 
degrees. Yet such animals, 
at the brink of death, were 
saved and restored to ap- 
parent health within seventy 
hours by the subcutaneous 
injection of beef cortin. 

Cortin appears to have an- 
other property, namely, to 
stimulate the development of 
the sex organs. This has been 
shown by a series of experi- 
ments on young male rats, 
in which the injected animals 
showed a much more rapid 
growth of the sex organs than 
the controls. These studies 
suggested that the occasional 
precocious sexual develop- 
ment of young children may be due to an over-enlargement of the 
adrenals through the presence of a tumor either in the cortex of the 
gland or in the pituitary gland which largely regulates general 
endocrine balances. Young girls under similar conditions develop 
masculine characters. More or less complete cases of the reversal 
of secondary sexual characteristics in women are on record, in a few 
of which the removal of tumors involving the adrenals has restored 
a normally characteristic feminine condition. 

The secretion of the medulla or inner portion of the adrenal gland has 
been known to science for some time as adrenin or epinephrine. 
This hormone was first isolated by Takamine in 1901 and has since been 

The location of the ductless glands. 



synthesized. Its effect is very interesting. It is known that small 
amounts of adrenin are being continuously secreted and passed into 
the blood stream to have an effect upon the involuntary muscles 
of the body. In cases of emotional excitement there is an increased 
secretion of adrenin, in consequence of which there results a more 
rapid heartbeat, together with an increase of blood flow and of the 
glucose output from the liver. This in turn brings about greater 
efficiency of the muscles and so increases the capacity for work. If 
this portion of the gland is not operating normally such symptoms 

as muscular fatigue, cold hands 

and feet, sensitiveness to cold, 
mental indecision, and sometimes 
collapse and heart failure ensue. 
Adrenin is efficacious in reliev- 
ing severe bronchial spasms dur- 
ing attacks of asthma and it has 
also been successfully used to 
mitigate the distress caused by 
hives and by hay fever. 

.thyroid cartilage 

pry amid lobe 

-P (xra-thyro i cC 

-left lobe-^of 
thyroid. ^lancC 
-pa.rat by ro i cC 



viewed, from |i'«nt/ 

Diagram showing the location and 
relationship of the thyroid and para- 
thyroid glands. 


Some sort of thyroid gland is 
present in all of the vertebrates. 
In every instance it arises as an 
outgrowth from the pharyngeal 
region and is, therefore, a deriva- 
tive of the digestive tract. In man the thyroid is definitely bilobed 
and in cases of goiter may be considerably enlarged. 

The secretion of the thyroid gland, thyroxin (C11II10O3NI3), was first 
isolated by Kendall in 1914 and later improved isolation methods gave 
Ci5Hii04NI,i (Harrington, 1926). Under normal conditions but little 
of this substance is secreted at a time, in evidence of which is the fact 
that three and one-half tons of fresh thyroid glands are necessary to 
produce 36 grams of crystalline thyroxin. This substance regulates 
the rate of the transformation of energy in the body, thus controlling 
the metabolic rate. Its potency is almost uncanny, as is evidenced 
by the fact that one milligram of thyroxin produces a two per cent 
increase in the total oxidation of a resting adult body. 

One concept of the rate of metabolism in the human body may be 
secured through the basal metabolism test, a device to measure the 



oxygen-carbon dioxide Ixilance, which determines the amount of 
energy required to keep th(> body aHve, maintain its temjierature, 
muscle tone, rate of breathing, and heartbeat. It has been con- 
ckisively shown by a sufficient number of studies that a comparison 
of people who have been placed under similar conditions may be 
made, and it is now pos- 
sible, as a result of these 
tests, to gain a highly 
accurate idea of the meta- 
bolic rate of different 
people, and so to detect 
an over- or under-activity 
of the thyroid gland. 
Both conditions are ab- 
normal, indicating rather 
serious metabolic malad- 

If the thyroid is over- 
active, a person so affected 
usually has a high basal 
metabolic rate. Such a 
person finds his combus- 
tion rate speeded up and 
is a heavy eater, but at 
the same time that he 
burns his food products 
rapidly, he gradually be- 
comes weaker and weaker. Evidence of a high metabolic rate shows 
further in nervousness and irritability. The individual is also char- 
acterized by protruding eyeballs, an increased and more irregular 
heartbeat, as well as a higher temperature, insomnia, and general 
nervousness, which in advanced cases may seriously undermine both 
the mind and health. This general picture of overactivity is typically 
associated with the variety of goit(T known as exophthalmic goiter. 

Another type of goiter, "common goiter," is frequently encoun- 
tered in regions wdiere there is a material lack of iodine in the water and 
soil. In such cases there is an insufficient supply of thyroxin secreted, 
which is sometimes due to a decrease of iodine in the chemical com- 
position of the thyroxin molecule. Nature apparently endeavors to 
compensate for this by increasing the size of the gland with rather 

Xew York AcfuUmy of Medicine 

An example of a goiter. What type is it .3 
What caused it !> 


grotesque results. Fortunately this condition may l)e alleviated in 
the early stages by the addition of iodine to the diet, or in advanced 
cases, by extirpation of a portion of the gland. 

Occasionally an individual gives evidence of an underactivity of 
the thyroid gland. In such a case, the amount of thyroxin produced 
by the gland is actually decreased. Well-defined and characteristic 
symptoms result. Ingested food is not utilized, with the result that 
the excess is soon deposited as fat, and definite obesity becomes visible. 
Certain other symptoms are quite characteristic, as a slowing down of 
the mental and nervous activities, which may result finally in feeble- 
mindedness or imbecility. If the thyroid gland be hereditarily de- 
fective or non-functional a lamentable condition known as cretinism 
develops. In such cases skeletal development is arrested and a stunted 
misshapen individual results since normal growth becomes impossible. 
This condition is alleviated by administering thyroxin. 


The parathyroid glands, likewise outgrowths of the pharyngeal 
region of the body of nearly all vertebrates, vary slightly in number 
and position with the form under consideration. In man, there are 
normally four parathyroid glands having a total weight of not over 
0.4 gram. Nevertheless, they persist during life and are now known to 
play a very important part in maintaining the calcium balance of the 
body, and by means of it, the irritabiUty of the cells. The active 
hormone of the parathyroid glands was demonstrated by Hanson in 
1925 and later isolated by Collip. 

While the exact nature and method of the functioning of these 
glands is not thoroughly understood, it is known that their removal 
is usually fatal. The first effect of the removal of these glands is that 
the calcium salts are reduced and the threshold of stimulation thereby 
lowered ; the peripheral nerves and muscles of the organism become 
more irritable and the various reflexes become extreme. Severe 
muscular contraction, or tetanus, finally results and the death of the 
organism usually ensues unless calcium is added to the blood. This 
may be done by the injection of the hormone or solutions of calcium 


While the pancreas has long been recognized as a gland secreting 
various important digestive enzymes, it was not until 1889 that it 


was shown to have an equally important role as a ductless gland, 
producing hormones. Von Mering and Minkowski showed that its 
extirpation was followed in all cases by the appearance of sugar in 
the urine. Evidence has accumulated indicating that the oval or 
spherical islands of Langerhans, that are embedded in the pancreatic 
tissue, are the source of the hormone now called insulin, which 
regulates the sugar metabolism of the body. 

The story of the long struggle of scientists to demonstrate the 
existence of this hormone is a fascinating one. Banting, Best, and 
Alacleod ^ in 1921 gave the first successful demonstration of the 
isolation of insulin. We now know that the general physical and 
mental condition of people suffering from diabetes can be markedly 
improved through the administration of this hormone. While the 
exact nature of the reaction is not fully understood, it is certain that 
the amount of sugar in the blood stream is reduced sharply after the 
injection of insulin. 

Growth Regulators 


One must think of the thyroid as a gland with a dual function. We 
have already noted the effect which the secretions of this gland have 
upon general metabolism. The second effect is its influence upon 
growth. When the thyroid is removed in young dogs, for example, a 
retardation of growth occurs in a few weeks. These experiments 
substantiate observations made upon children with congenital lack 
of thyroid. 

Gonads and Pituitary 

As will be noted in more detail later, these glands are both associated 
with growth, and play an imj^ortant role in the normal development of 
the individual. 


Although the function of the pineal gland is not clear, it should be 
mentioned at this point. It is a small body which appears in nearly 
all vertebrates as an outgrowth from the roof of the 'twixt-brain 
(diencephalon). The pineal body reaches its greatest development in 
man at about the seventh year. After that age, and particularly 

' Banting, Best, Macleod. Am. Jour. Physiol., 59 : 479. 1922. 


after puberty, it undergoes involution and finally disappears, its 
place being taken by fibrous tissue. While there is some evidence 
that extirpation of the pineal gland accelerates the development of 
the sexual organs of the male, as found in experiments on the guinea 
pig, its functioning is still a moot question. 

While the thymus has long been a subject of controversy, it now 
appears likely to many students that this gland will not prove to 
belong to the endocrine group. 

Reproductive Organs as Regulators 

It has been known for many years that the gonads are structures 
in which are produced the eggs and spermatozoa that are essential 
to reproduction in most forms of life. However, scientists have 
learned within comparatively recent years that the reproductive or- 
gans function also as ductless glands, producing hormones associated 
with the development of those features known as secondary sexual 
characters. One group of these hormones, though partially under the 
control of that "generalissimo" of the endocrines, the pituitary, is 
really responsible for the normal cyclical functioning of the sex 
glands. Besides producing eggs and sperm, the ovaries and testes 
play a vital part in the development of those mental and physical 
characteristics which constitute maleness or femaleness. The exist- 
ence of some regulatory mechanism has been clearly demonstrated 
in various animals by the removal of the sex glands and the subsequent 
failure of certain secondary sexual characteristics to develop. Nu- 
merous examples might be cited. Male deer (Cervidae), for instance, 
are typically adorned with antlers that are annually renewed. A 
young castrated buck fails to grow antlers, thus suggesting that the 
key to this phenomenon lies in the production of some secretion of 
the testes. 

Many other experiments, performed in recent years upon the lower 
vertebrates, tend to support the idea that such secretions are indis- 
pensable to the proper development of many male and female char- 
acteristics. When emasculated male rats or guinea pigs are given 
ovarian transplants, the skeleton and hair soon begin to resemble 
those of a female and before long the mammary glands enlarge to 
functional size. These results suggest that the effect is due to some- 
thing secreted in, or by, certain cells of the transplanted gonad. 

Other experiments indicate that the hormones of one sex dominate 
expression of the other sex. Such a case is that of the "free-martin," 


which is a sterile female calf, born with a normal male twin. Lillie 
discovered in these instances that there was a fusion of the embryonic 
circulations between the twins and that, since the male gonads develop 
before those of the female, the male hormone appeared first in the 
united fetal circulation and not only interfered with the growth of the 
ovary to such an extent as to cause sterility, but even caused a tend- 
ency toward the assumption of secondary male characters. 

Evidence relating to a second type of secretion associated with the 
rhythmical recurrence of ovulation in the female of all vertebrates 
leads to the belief that in mammals at least two ovarian hormones 
occur, — one derived from the follicular cells surrounding the egg 
before it escapes from the ovary, and the other from the mass of cells, 
or corpus lutcum, that fills the follicle after rupture. 

The cells of the follicle secrete a hormone known as ocstrin into the 
follicular fluid. This substance has the dual function of initiating 
some changes in the female and completing other reactions. Ocstrin 
is secreted by the ovaries of all vertebrates which have been studied so 
far. It is a growth-promoting hormone which governs the develop- 
ment of the secondary sexual characters, including the reproductive 
tract of the female, while the corpus luteum, as known at present, is 
really a mammalian gland w^hich has appeared in association with 
lactation and viviparity. The corpus luteum hormone, progestin, 
prepares the uterus for the reception of a fertilized egg, and if one 
does not appear the corpus luteum involutes, the uterus returns to a 
resting condition, and a new cycle is started. Progestin quiets the 
uterus by inhibiting its rhythmic, spontaneous contractions. In the 
strict sense the corpus luteum may be regarded as a gland of preg- 
nancy. Several interesting experiments have been performed on 
various mammals. It is well known that the mating instinct is lost 
when a normal female is spayed (removal of ovaries). Allen and 
Doisy were able to produce characteristic cychcal changes in the genital 
tract of spayed rats and mice by the injection of the hormone from 
the follicular fluid. 

The interstitial cells of the testes evidently yield hormones which 
produce secondary sexual characters in a castrated male. Much 
work still remains to be done on this point. 

The Master Gland or "Generalissimo," the Pituitary 

The pituitary gland, or hypophysis, might well be regarded as the 
commander-in-chief of all the endocrine glands. Embryologically the 


anterior part of the gland arises as a dorsal evagination (Rathke^s 
pocket) from the buccal ectoderm, while the posterior part develops 
as a downward outgrowth {infundihulum) from the portion of the 
brain (diencephalon) lying directly over the mouth. The anterior 
outgrowth in man finally produces the anterior lobe, a small inter- 
mediate lobe, and a thin layer extending to the brain as the pars 
tuberalis, while the posterior portion forms the so-called posterior 
lobe, or pars nervosa. 

The Anterior Lobe 

There appears to be fairly good evidence of the existence of at 
least five and possibly eight hormones produced by this portion of the 
pituitary gland. It is probable, since the histology of the gland 
indicates remarkably little diversity of tissue, that the substances 
produced are very closely related chemically. 

A. Growth Stimulation. If overactivity of tliis portion of the 
gland occurs when young, giants will result. On the other hand a 
similar overactivity arising when adult, results in excessive growth 
of the bones of hands, feet, and face — a condition known as acro- 
megaly. The intraperitoneal injection of fresh anterior pituitary 
extracts resulted in the production of giant rats. Additional evidence 
has been secured through the autopsies of various giants, who showed 
a greatly enlarged pituitary. 

B. Gonad Stimulation. During comparatively recent years it 
has been shown that anterior pituitary implants will produce sexual 
precocity in sexually immature mammals. This operation has been 
performed on all of the more common laboratory mammals, includ- 
ing cats, dogs, and monkeys, and thus far holds for all vertebrates 
studied. In the female such implants stimulate the development of 
both follicles and corpora lutea, which are associated with the growth 
of the female secondary sexual characters. Implants in the male 
stimulate the development of the semeniferous tubules and inter- 
stitial tissue correlated with the growth of male secondary characters. 
These effects, as determined by hormone isolation, are due to two 
hormones secreted by the anterior pituitary, — one which stimulates 
the growth of follicles in the ovaries and tubular growth in the case 
of the male, and the second which produces the formation of the corpus 
luteum and the secretions of the interstitial cells of the testis. 

C. Lactation Hormone. Knowledge of the existence of this 
hormone is comparatively recent. Various workers reported that 


they were able to induce lactation in spayed, virgin rabbits, which 
had developed mammary glands prior to the operation, througii 
the injection of a substance secured from the anterior pituitary. 
Some years later 'prolactin was extracted in an im|)uro form which, 
while not causing development of the mammary gland, nevertheless 
brought about the onset and continuation of the secretory phase. 
Prolactin \^^ls effective after castration. 

D. Thyreotropic Hormone. While many investigators have 
demonstrated a close relationship between the pituitary and the 
thyroid gland, it was not until 1927 that the pituitary gland of the rat 
was removed to show that the thyroid is dependent upon this structure 
for stimulation. In 1933, a purified extract under the name of the 
thyreotropic hormone was prepared. 

E. Adrenotropic Hormone. It was shown in 1930 that if the 
anterior lobe of the pituitary is removed in rats atrophy of the cortex 
of the adrenals follows, although normality may be restored by in- 
jecting pituitary extracts. Later Houssay and his co-workers showed 
that the active agent in such experiments is a product of the an- 
terior lobe, also proving the existence of this adrenotropic principle. 

Most biologists now concede the existence of these five hormones 
from the anterior lobe of the pituitary gland. Evidence is rapidly 
accumulating which supports the idea of the existence of three more, 
F to H. 

F. Blood Sugar-raising Principle. It has been previously 
shown that the removal of the pancreas results in the appearance 
of sugar in the urine, that is, experimental diabetes is produced. 
Overactivity of the pituitary, as acromegaly, for example, is usually 
associated with hyperglycemia (over the normal amount of sugar in 
the blood) and glycosuria (sugar in the urine). Furthermore, a nor- 
mal animal develops the same condition when injected with anterior 
pituitary extracts. Now, when the hypophysis is removed hypo- 
glycemia results and the animal is very sensitive to insulin. Also it 
has been shown that if both the anterior pituitary and the pancreas 
are removed the experimental diabetes resulting from the loss of the 
pancreas is greatly decreased. It is apparent, then, that in the 
absence of the pancreas the anterior pituitary tends not only to 
increase the blood sugar but also to make the animal sensitive to 
insulin. This clearly indicates that there is a balance between these 
two glands. It might be added that extracts of the anterior pitui- 
tary increase the blood sugar in the absence of the pancreas, thy- 


roids, adrenal medulla, and sympathetic system. It appears quite 
conclusive, therefore, that the action of the anterior pituitary hor- 
mone is at least partially direct. 

G. Fat Metabolism-regulating Principle. Several groups of 
experimenters have produced evidence, since 1930, that the anterior 
lobe of the pituitary gland also produces a hormone that regulates 
the metabolism of fats in the body. 

H. Parathyreotropic Principle. While the evidence is not 
irrefutable there are some grounds for believing that the control of the 
parathyroids is made possible by a secretion from the anterior lobe of 
the pituitary gland. 

The Intermediate Lobe 

This portion of the pituitary gland produces a hormone known as 
intermedin, which has been found in all vertebrates so far studied. 
The effects of this hormone may be readily demonstrated in frogs and 
other amphibians. At the present time its function in mammals is 
not known. 

The Posterior Lobe 

The posterior lobe of the pituitary gland consists of contributions by 
the pars nervosa and the pars intermedia. It is possible, therefore, 
that its products may contain secretions from both sources. Two 
fractions have been isolated from the posterior lobe, called respectively 
pitressin and pitocin. However, much work on these hormones still 
remains to be done before the various effects noted on the cardio- 
vascular, respiratory, uterine, renal organs, and the smooth muscle- 
tissue of the intestine and mammary glands are proved to be due to 
one or to several discrete fractions. Several characteristic reactions, 
however, might be noted. First there is the pressor effect which is char- 
acterized by an increased blood pressure and a decreased heart-rate. 
Injections of the posterior lobe cause an increased secretion of urine 
and also bring about a contraction of the plain muscle of the uterus. 
This latter action has been made use of by the medical profession to 
stimulate the contractions of the uterus at childbirth. If an animal 
is lactating, injections of the posterior lobe will bring on an increased 
flow of milk. 

From this brief account may be gathered some idea of the way in 
which this small endocrine gland functions as the commander-in-chief 
of the metabolism of the body. Although much work remains to be 


done in this connection, it is nevertheless apparent that the pituitary 
gland exercises an interlocking directorate over the remaining lesser 
lights of the endocrine constellation. 


Clenaenning, L., The Human Body, Alfred A. Knopf, Inc., 1930. Ch. IX. 
Interestingly written account of the problem of co-ordination. 

Cobb, I. G., The Organs of Internal Secretion, 4th ed., Wm. Wood and Co., 

Fairly technical discussion of functioning and non-functioning of endo- 
crine glands. 

Rogers, C. G., Textbook of Comparative Physiology, McGraw-Hill Book Co., 
1927. Ch. XXV. 

Discussion of the endocrines and their work from a comparative view- 




Preview. Where did life come from ? : Refutation of spontaneous genera- 
tion ; other theories of the origin of hfe ; Hfe produces Ufe • Regeneration • 
Asexual types of reproduction : Budding ; fission • Sexual reproduction in 
the invertebrates : jjrotozoa ; (^tlier invertebrates ; hermaphroditism • Par- 
thenogenesis • Paedogenesis • Alternation of generations • Sexual reproduc- 
tion and development in the vertebrates : Germ cells versus soma cells ; 
fertilization, results of fertilization ; early cleavage and variations caused 
by yolk ; blastulation ; gastrulation ; mesoderm formation ; early differ- 
entiation of the embr\'o • Tissue fonnation • Protective devices for the em- 
bryo : Egg shells ; the yolk sac ; amnion and chorion ; allantois ; placenta • 
Elaboration of germ cells or gametogenesis : Formation of sperm — spermato- 
genesis ; formation of ova — oogenesis • The new embryology : Genes ; 
environment ; natural potencies ; organizers • Suggested readings. 


It was obvious to the early philosophers that the earth preceded the 
living things upon it and they advanced the interesting idea that 
living things arose spontaneously from their surroundings. The 
Bible alludes to this belief when Samson propounded his riddle, 
"Out of the eater came forth meat and out of the strong came forth" Samson saw flies coming out of the decaying body of a 
lion, took the flies for bees, which he believed were arising spontane- 
ously from the lion's body, hence the riddle. The story of the long 
struggle to disprove spontaneous generation, ending with the conclu- 
sive demonstrations of Louis Pasteur, makes one of the fascinating 
bits of reading in the field of biology. 

With the disproval of the existence of spontaneous generation and 
the perfection of the microscope, great interest was evidenced in the 
many different ways in which plants and animals reproduced. Today 
the student of embryology sees the apparently many diverse ways of 
reproducing the species reduced to a few essentially similar funda- 
mental patterns. 

Likewise the exactness with which the chromatin is segregated and 
divided within the developing germ cell is a never-ending source of 



wonder to the biologist. Another interesting study centers about 
the development of the various protective devices that surround the 
embryo and keep it from injury until it is hatched or born. The 
infinite care with which these devices have been developed is a credit 
to the ingenuity of Mother Nature. 

In this unit the student will find the answer to questions arising 
in his mind concerning the nature of these reproductive devices. 

Where Did Life Come From? 

Greek and Roman literature is full of references to the possible 
origin of life and to the probability that it arose spontaneously. A few 
brave souls dared to doubt this almost universally accepted concept. 
However, even as late as the 17th century Alexander Ross writes, 

"So may we doubt whether in cheese and timber worms are generated, 
or if beetles and wasps in cow-dung, or if butterflies, locusts, shellfish, 
snails, eels, and such life be procreated of putrefied matter, which is to 
receive the form of that creature to which it is by formative power disposed. 
To question this is to question reason, sense, and experience. If he doubts 
this, let him go to Egypt, and there he will find the fields swarming with 
mice begot of the mud of Nylus, to the great calamity of the inhabitants." 

Refutation of Spontaneous Generation 

Belief in spontaneous generation was first shaken by the Italian 
physician Redi, who noticed that flies were attracted to decaying 
meat. In an experiment he put sterilized meat into several jars, 
covered one lot with parchment, another lot with a fine netting, and 
the third he left open. Fly maggots were found later in the meat 
in the open jars, fly eggs on the netting, and no maggots in the parch- 
ment-covered jars. This experiment should have exploded the belief 
that maggots arose spontaneously from rotting meat. However, 
the belief kept constantly recurring because it was very difficult to 
prevent the invasion of food materials by bacteria, even after the 
substances and vessels containing them were apparently sterilized. 
The Abbe Needham, seventy years after the Redi demonstration, 
experimented with living germs, and because of the errors arising from 
improper sterilization found living germs in flasks of nutritive fluid 
that had first been heated and then were sealed with a resinous 
cement. A little later the Itahan, Spallanzani (1729-1799), placed 
nutrient fluids, such as meat and vegetable juices, in glass flasks, the 
necks of which were sealed in a flame ; then he placed the flasks in 


boiling water for three quarters of an hour. The contents of the 
flasks remained unchanged. Spallanzani then op(!ned the flasks and 
after a short period they were found to be full of living organisms. 
Needham objected to the experiments on the ground that the boiling 
had killed the "vegetative force" of the infusion. However, the 
idea of spontaneous generation was not finally disproved until the 
time of Pasteur and T3mdall, who proved that living germs may 
be carried about by dust in the air and that only when air con- 
taining dust particles can be excluded from substances it is certain 
that bacteria will not grow in them. 

Other Theories of the Origin of Life 

The theory of the simultaneous creation of life and this planet does 
not agree with such theories as the scientists offer to account for the 
origin of the eartli. Whether we accept the nebular hypothesis of 
LaPlace, the later planetesimal hypothesis of Chamberlin, or the still 
later theories of Green or Shapley, we are confronted in all of them by 
the formation of our jjlanet from material far too hot to sustain life. 
As Jeans says, ''The physical condition under which Hfe is feasible is 
only a tiny fraction of tlie range of physical conditions which pre\^ail 
in the universe as a whole." The theory sometimes advanced that 
life may have been transferred from another planet does not help us 
much, for we still have to account for life's origin. As has been so 
well said of life on Mars, which of any of our planetary neighbors 
has concUtions the most possible for supporting life, "Man recon- 
structed to walk on Mars would be crushed to death by his own 
weight on the eartli." Special creation as advocated by the early 
Church does not help the scientist very much, for it still leaves life 
to be accounted for. It allows of no scientific investigation and so 
it cannot be used by the biologist. 

Probably the theory which has the most hope of ultimate solution 
is the belief that at some time life arose by a chance combination of 
chemical elements of which the earth is made. Evidence found in 
the rocks indicates that the earth is much older than its inhabitants. 
Professor Henry F. Osborn pointed out the striking similarity of the 
salts found in the blood and those found in sea water. He made the 
suggestion that life might have originated in some pool in which the 
saUne contents contained the life elements found in protoplasm. 
Would it be too much to speculate on the origin of some simple form 
of life by allowing a flash of lightning to release the pure nitrogen 
H. w. H. — 27 


of the air in some form of nitrate which would combine with the life 
elements found in sea water and the carbon dioxide of the air ? This 
theory is in reality a refurbished concept of spontaneous generation. 
In discussing it two points should be kept in mind. First, if sponta- 
neous generation of this sort did occur at one time, the contrast between 
the physical environments of the past and present would be great. 
Second, even if conditions were right for the similar production of 
life today, it appears likely that such simple beginnings would be 
almost immediately destroyed by better established forms of life. 
Both serve as explanations of why we do not have life produced 
spontaneously today. 

Life Produces Life 

Since the time of William Harvey, court physician of Charles I of 
England, the statement "Omne vivum ex ovo" has been used. Living 
things come from other living things, not always from eggs, as Harvey 
said, but in the case of unicellular animals and plants by the cell 
dividing to form two. 

Each organism, plant or animal, has a definite life cycle, a series of 
changes which it goes through from its simplest form as an egg to its 
ultimate adult structure. More than this, sooner or later it will die. 
In some unicellular forms the life cycle takes a very brief period 
indeed, but in the elephant it is over a hundred years, and some 
trees, like the giant sequoias, live thousands of years. Sooner or later 
life activities cease and the Biblical statement of "dust to dust" is 
justified. Death comes as a final close of all activity and normally 
after the animal or plant has produced offspring. 

New individuals, whether complicated mammals or simple protozo- 
ans, arise from the same kind of pre-existing organisms. The exact 
method of reproduction, however, varies markedly in different 
groups. Protozoa, at one end of the scale, produce new individuals 
by the simple process of cell division, while the mammals, at the other 
extreme, show evidence of considerable division of labor with special 
organs involved in the production and functioning of the highly 
specialized sex cells. In order to understand these various processes 
it is desirable to summarize the different reproductive devices which 
appear in the animal kingdom. 


The replacement by an organism of lost or injured tissue is included 
in this discussion of reproduction on the ground that such a phe- 



iiomenoii, involving the creation of new cells by cell division, is a 
fundamental type of growth. The ability to regenerate lost parts 
seems to be correlated inversely with the degree of specialization 
and the extent to which division of labor appears. For example, an 
unspecialized sponge when pressed through silk bolting-cloth into 
small fragments will reproduce new individuals. Other more highly 

tyo rzev rays 

two newm^s 

WTone old 

two new Tays 

tliree old rays ' ^^^^ 


Examples of regeneration in representatives of four different phyla. How may 

such phenomona be explained ? 

specialized forms show less ability to regenerate so completely, but 
many of the coelenterates as well as certain worms and echinoderms 
possess this facility of regeneration to a high degree. Starfish, long 
the enemy of oysters, have increased rapidly in part due to the care- 
less practice of oystermen who tore them apart and left the frag- 
ments in the water. It is now known that such disjointed parts, if 
containing portions of the central disk, are capable of regenerating 
into new individuals. 


Lobsters, crabs, spiders, and some insects have tiie uncanny ability 
of breaking off an injured appendage near its base, a phenomenon 
known as autotomy. In such instances new appendages are usually 
regenerated and the animal emerges as a successful contestant in 
another skirmish in the struggle for existence. Vertebrates, how- 
ever, show but slight ability to replace lost parts. Of course a 
break in the skin is soon healed by regeneration, although more 
extensive damage to the part results merely in the elaboration of 
some connective tissue and skin and not in complete restoration. 
A crushed toe, for example, usually necessitates an amputation, for 
in such cases one never finds a new toe replacing the old. 

It is a rather striking fact that the more limited type of regeneration 
common among the higher vertebrates is almost indistinguishable 
from the normal metabolic processes so characteristic of growth and 
repair. It is only a step from such methods of growth to the highly 
specialized type known as reproduction. 

Asexual Types of Reproduction 

Budding and fission, or simple cell division, comprise the usual 
asexual methods of reproduction. A brief consideration of these 
methods at this point will serve to link regenerative processes with 
those of higher types of reproduction. The former may be thought of 
as reproduction by an unequal cell division, a mode of division not 
infrequently found among one-celled organisms. In more complex 
organisms, as Hydra, repeated divisions of totipotent cells may occur 
to produce a bud. Fission merely involves the division of an organ- 
ism into two or more, usually approximately equal parts. 


Organisms which undergo budding might easily be confused with 
those exhibiting regeneration. These phenomena closely resemble 
each other, the chief difference being that budding, unlike regener- 
ation, does not typically result from injury. It is, moreover, an 
important type of reproduction occurring quite generally in plants 
as well as widely throughout the lower animal kingdom. 

The fresh-water sponge reproduces by means of two kinds of buds, 
the first type being liberated to take up a separate existence while 
the second remains as a kind of internal bud, called a gemmule. It 


has been previously shown that in Hydra the new bud extends out 
from the body, developing tentacles, mouth, and hy[)ostome at the 
distal end of the organism. After growing sufficiently the base 
constricts and the two animals, parent and offspring, become sepa- 
rated, each taking up an independent existence (page 184). 

In the higher worms such as the palolo worm and the Naididae, a 
type of budding occurs which might be described as fragmentation. 
The number of fragments apparently depends upon tlu; size of the 
worm, each piece usually producing all of the missing parts. 


This variety of asexual reproduction is the most common. The 
one-celled protozoa rely almost exclutijvely upon this type of develop- 
ment, seldom resorting to the more complicated "sexual" methods. 
In binary fission the nucleus appears to take the initiative, since it 
divides first and is followed by the division of the cytoplasm of the 

Fission is rather closely allied to budding. Many of the turbel- 
larian and nemertin(\an flatworms utilize this method, as, for example, 
the turbellarian, Microsiomum, which often divides into two, four, 
or even sixteen pieces. These parts produce all of the necessary 
structures except eye-spots and often remain attached in chains for 
long periods of time. 

Sexual Reproduction in the Invertebrates 


Sexual reproduction involves the union of two cells produced usually 
by two animals of different sexes. This phenomenon appears in 
practically every group of the animal kingdom. Even in the protozoa 
there are two types of reproduction which may be thought of as 
initiating the sexual method. In the first type there is either a 
complete union of two individual cells of equal or of unequal size, 
or there may be specialized cells called gametes. Many variations 
of this type are to be found among different species. 

The second type of sexual reproduction occurring in the protozoa 
is called conjugation, which has already been described (page 161). 
Briefly, conjugation means that two single-celled organisms come 
together temporarily, form some sort of protoplasmic bridge, exchange 


nuclear material, and finally separate. If the conjugating forms are 
of equal size, as in the case of Paramecium, both usually survive and 
continue to reproduce, by asexual means. On the other hand, when 
the conjugants are of unequal size it frequently happens that the 
smaller, or micro-conjugant, degenerates soon after conjugation. 

Other Invertebrates 

As division of labor among the cells of an organism progresses there 
is increasing evidence of a gradual but none the less clear demarcation 
into two sorts of cells, the soma or body cells, and the germ or sex cells. 
These groups are separated early in the development of the individual, 
the former being burdened with the responsibilities of movement, 
protection, securing food, and in some cases caring for the young. 
The second, comprising the germ cells, is solely concerned with the 
elaboration of highly specialized cells adapted for the production of 
new individuals, and so serving for the maintenance of the race. 

Since sexual reproduction undergoes many modifications in the 
invertebrates, it appears logical to consider some of these phenomena 
before undertaking a detailed study of sexual reproduction in the 
higher vertebrates. 


Many of the lower invertebrates exhibit a kind of sexual reproduc- 
tion in which both the male and female organs are found in the same 
individual. A complete set of male and female reproductive organs 
occurs, for example, in a single Hydra. In this genus the syermary 
producing the spermatozoa is situated closer to the tentacular region 
than the ovary which is located near the foot. These gonads rupture 
when mature, and one of the liberated spermatozoa finally fertilizes 
the ovum contained in a disrupted ovary. When both gonads are 
functional on the same individual self-fertilization may occur. 

The earthworm likewise contains a complete set of male and female 
reproductive organs in the same individual, but here, as in many of 
the trematode flatworms, copulation takes place between two separate 
individuals. In such cases the exchange of spermatozoa results in 

While hermaphroditism is unusual in the vertebrates, it is believed 
to occur normally in a few instances such as certain hagfishes (cyclo- 
stomes) which are known to be hermaphroditic. In these forms, 


however, cross-fertilization occurs, since the ova and spermatozoa 
mature at different times. Reported cases of functional hermaphro- 
ditism among mammals appear to be highly doubtful. 


The development of an egg without fertilization by a sperm occurs 
quite commonly under natural conditions in some invertebrate forms. 
Usually there is a cessation of activity on the part of the males for a 
period of time when ova, produced by the females, develop into 
apparently normal individuals. In some few instances males are 
permanently absent. The rotifers, water fleas (Cladocera), and plant 
hce (aphids) all exhibit this type of development at times. In cladoc- 
era, of which Daphnia is a well-known example, the females produce 
parthenogenetic eggs during the warm weather. From two to twenty 
eggs, depending upon the species, are deposited and nourished in the 
brood-sac. Usually several generations of females will be produced 
in this fashion. Eventually male as well as female daphnids are 
produced, and the eggs from this generation of females must be 
fertilized by the males. When fertilization occurs, the eggs are 
covered by the highly resistant protective portion of the brood-sac 
(ephippium) which enables them to withstand desiccation and the 
rigors of winter. 

Numerous experimenters have been interested in attempts to induce 
artificial parthenogenesis in various invertebrate eggs by means of 
chemical or physical stimuli ranging all the way from simple salts 
and complex fatty acids to mechanical means, such as pricking with a 
needle, shaking, or raising the temperature of the water surrounding 
the experimental organisms. Mead first successfully induced arti- 
ficial parthenogenesis with the ova of annelids and Loeb extended 
the experiments to include starfishes, sea urchins, molluscs, and even 
frogs, which underwent at least partial development by means of 
various chemical or physical stimuli aptly described as parthenoge- 
netic agents. 

Most of the experimental efforts to induce parthenogenesis in 
vertebrates have been rewarded by failure. In a few instances tad- 
poles have been produced through mechanically initiating cleavage 
of the egg by pricking with a needle and introducing a small amount of 
blood serum at the same time. Pincus has also been able to carry a 
mammal embryo through early developmental stages after partheno- 
genetic stimulation. 



Reproduction by immature individuals is called paedogenesis. 
As it is rarely encountered in the animal kingdom, only two examples 
need be mentioned. The first occurs in the trematodes, where imma- 
ture larval forms, such as sporocytes and rediae, appear to produce 
the next generation parthenogenetically. These in turn often give 
rise to another generation through paedogenetic reproduction. In 
the vertebrates the best known example is the Mexican axolotl, a 
urodelous amphibian. This interesting animal, while still remaining 
in its larval form, reproduces its kind sexually without undergoing 
metamorphosis or losing its external gills. 

Alternation of Generations 

Alternation of a sexual with an asexual generation is called metagen- 
esis, or simply alternation of generations. Several of the invertebrates, 
especially the coelenterates, normally exhibit metagenesis. In the 
hydroid Obelia, for example, the asexual generation is represented 
by a sessile, colonial hydroid and the sexual generation by the mature, 
bisexual medusa buds (see page 185). 

Sexual Reproduction and Development in the Vertebrates 

Germ Cells versus Soma Cells 

The early growth and later development of the embryo and its 
systems, organogeny, are to be considered in some detail. To com- 
plete the picture it is necessary to envision the continued growth of 
the organism until it matures, reproduces its kind, and dies. The 
life of every organism, whether plant or animal, is involved with the 
mathematical concepts of division, multiplication, addition, and 
subtraction. In the formation of a new individual by two parents, 
two germ cells are added together {fertilization). In order that the 
hereditary genes thus united may not be disastrously doubled in 
each generation, one half of those present from each contributing 
parent are subtracted by the elimination of either the maternal or the 
paternal member of each chromosome pair just prior to maturation. 
Thus, a constant number of chromosomes with their respective genes 
is maintained in each body cell of any species. After this preliminary 
process of subtraction and addition has been accomplished, the newly 
combined germinal cell, that is, the fertilized egg, or ovule, initiates 



an exhaustive series of divisions, whereby each cell repeatedly becomes 
two (growth). The result of these successive divisions is an enormous 
multiplication of differentiating cells to form the entire body of the 
individual (development) . 

In the present connection it is only desirable to emphasize that 
this complicated process of cell-division (mitosis) has been exhaus- 
tively studied, so that its essentials are now well known. In a word, 
the end result is the final distribution, to every one of the innumerable 
cells that form the individual, of equal germinal contributions from 
the two parents in the form of gene-bearing chromosomes. 






axial — 

>|^^ animal pole 


encL — J 
pieces I 





veg'eLal polt 

Generalized diagram of spernialozoan (left) and ovum (right) ready for 
fertilization. Note the two views of the spernialozoan. The head contains 
much nuclear material plus the acrosome. The middle piece contains two disk- 
like centrosomes, twisted milochondria and cytoplasm, while the tail has an outer 
sheath and axial filament. Eggs are always larger than spermatozoa and con- 
tain varying amounts of reserve food. Yolk settles toward the vegetal pole. 
(After McEwen.) 

Sexual reproduction in the vertebrates is essentially identical 
regardless of the group considered. In every case there is a special 
organ in the male called a testis, or spermary, for the production of 
sperm, and an ovary in the female in which eggs are elaborated. Each 
sperm or ovum is a single cell. Both kinds of germ cells differ in 
shape and size throughout the vertebrate series. 

The tadpole-shaped spermatozoa are always much smaller, quite 
active, and lack nutrient material within their bodies, as contrasted 
with the sedentary ova in which food is stored for the prospective 
embryo. Sperm may be divided morphologically into three parts, 



^ first and. 
^ ,® SeconoC 


the head, middle, and tail pieces. 
The head is composed chiefly of 
chromatin and is usually more or 
less pointed. The middle piece con- 
stitutes the general region imme- 
diately posterior to the head and 
contains cytoplasm, m,itochondria, 
centrioles, and the axial filament, 
while the tail piece appears to be 
primarily a locomotor device. 

Ova, on the other hand, are always 
non-motile and much larger than 
the sperm, due primarily to the fact 
that ova contain nutritive material, 
or yolk, which is utilized after fertil- 
ization. The amount of yolk present 
in eggs of the various classes of ver- 
tebrates differs widely. In all forms 
in which the eggs develop outside 
of the body, as, for example, the 
fish, amphibians, reptiles, and birds, 
there must be enough nutritive mate- 
rial present in the form of yolk to 
supply the embryo until it hatches 
and can feed itself. 


Fertilization consists of the union 
of a sperm and an ovum. This fusion 
may occur either outside of the body 
of the female, as in the case of most 
of the teleost fishes and other water- 
inhabiting animals, or within the 
oviduct of the female. Literally 
millions of sperm are liberated, but 
usually only a single sperm enters an 

Generalized diagram of fertiliza- 
tion. (I) shows the formation of the 
first polar body, the maturation 
spindle of the second maturation 
division (see p. 429), and the pene- 
tration of the spermatozoan. The 
second polar body is formed by the 
second maturation division and the 
egg nucleus starts towards the cen- 
ter of the egg. The sperm nucleus, 
or male pronucleus, starts towards 

the center (II) via the entrance path, but turns (III) toward the center on its 
copulation path to meet the egg nucleus and be arranged on the equatorial plate 
(IV) for the first cleavage division. Note that the centrosomes for this division 
are supplied by the male pronucleus. (After McEwen.) 


egg and in any event only one normally effects fertilization. The 
head and middle pieces usually become separated from the tail piece 
as penetration is effected, leaving the tail at the p(>rii)hcry of the 
ovum in much the same way that sandals are left at the portal of a 
Japanese house. The continued penetration of the remainder of the 
sperm is made possible through movements of the cytoplasm within 
the egg. The male element, which is now known as the male pro- 
nucleus, absorbs water, enlarges, and finally becomes arranged on 
the equatorial plate with the female -pronucleus of the ovum, and the 
initial cell division follows. 

Results of Fertilization. The more important effects of ferti- 
lization may be briefly summarized as follows : (1) Reproduction. 
This is accomplished by restoring the normal (diploid) number of 
chromosomes and by so doing producing a new center of cell division. 
(2) Variation. As will be seen later, the whole phenomenon of 
maturation of the germ cells and the consequent reduction of chromo- 
somes to the haploid number makes possible new combinations and 
variations between fertilized ova, or zygotes, upon which natural 
selection may act. (3) Rejuvenescence. For years fertilization and 
the concomitant stimulation of protoplasm have been thought neces- 
sary to revivify an organism. Data have been collected both in 
support of and in contradiction to this theory. Endomixis, as shown 
by Woodruff (page 161), apparently acts as the rejuvenating agent in 
nonconjugating strains of protozoa. 

Early Cleavage and Variations Caused by Yolk 

Once fertilization has occurred, cell division proceeds rapidly and 
the zygote gives way to the early cleavage stages. In tlie simplest 
types each plane of cleavage typically passes at right angles to the 
preceding plane, the cells multiplying from the two-celled to the four- 
celled stage, and so on up imtil the number in a given cleavage stage 
cannot easily be determined. 

The amount of yolk present in the egg affects the cleavage rate and 
even the pattern of development, since yolk is denser than typical 
cytoplasm and, therefore, settles toward the lower side of the egg. 
Its presence affects the rate of cell division by slowing it down. If 
yolk is present in large amounts as in bird and reptile eggs, it tends 
to occupy most of the available space in the ovum. In such ova 
the embryo develops in the upper polar area, or in a restricted disk 
called the blastoderm lying on top of the yolk mass. The ova of 



amphioxus and of mammals contain but a small amount of equally 
distributed nutritive material, while a third type of distribution 
occurs in some insect eggs where the yolk is concentrated in the center 
of the ovum. 


In isolecithal eggs, in which the yolk is distributed throughout the 
egg, the cells produced by successive divisions are all of approxi- 
mately the same size, and cleavage progresses with regularity until 

, , /Polair body, 

polar Dod^ ^ -^ 

" :rivi tell ins 


arcbenteron -^ 
ectoderm^ blcrstocoel 


blastopore ^' 

ET .. . ^ 

Cleavage in Amphioxus. Note fertilization membrane (I) and decrease in cell 
size as blastulation occurs (II-IV). Gastrulation (V, \T) follows with a reduction 
of blastocoel and formation of gut {archenleron). (After Conklin.) 

the embryo is a mass of increasingly smaller undifferentiated cells. 
A central cavity is produced as soon as the scanty yolk is used 
up to furnish fuel for cell division. As a result the entire mass re- 
sembles a rubber ball with the surface representing the layer of out- 
side cells and the cavity inside of the liall forming the hlastocoel. 
This stage is called a hlastula, and the process whereby it is formed 
is known as blastulation. 


As mitosis continues after blastulation, the cells on the side con- 
taining the yolk gradually become larger and eventually are pushed 
inward much as one would push in the side of a hollow rubber ball 
with the finger. The new cavity thus formed represents the primitive 
gut, or archenleron, and the embryo is now spoken of as a gastrula. 



Thus far two germ layers 
can be differentiated, an outer 
layer of ectoderm and an inner 
one of endoderm which lines the 
archenteron, while the dimin- 
ishing remains of the blasto- 
coel lie between. This stage is 
suggestive of those organisms, 
like the coelenterates, which 
characteristically possess only 
two germ layers even in the 
adult condition, and are there- 
fore designated as diplohlastic. 

Mesoderm Formation 

The details of the further 
development of the embryo 
vary considerably, depending 
upon the form studied, but 
all of the higher forms above 
the coelenterates produce a 
third germ layer called the 
mesodcrjyi. The elaboration of 
mesodermal tissue may come 
from either, or possibly both, 
of the primary germ layers. 
In all of the vertebrates, two 
sheets of mesodermal cells are 
formed, an inner splanchnic 
layer associated with the inner 
tube, or developing gut, and an 
outer so7natic layer, which is 
contiguous with the ectoderm. 
Loosely scattered mesodermal 
cells {mesenchyme cells), de- 
rived from these more compact 
layers, fill in the narrow spaces 
between the gut and splanch- 
nic layer and between the 
somatic layer and ectoderm. 

mccUxllarx plate 

.Tn€.ciunary fold 




YnedujcWaxy fokL 






>ieura.l cocnal 
ocnd, tijcbe. 


Somatic and. 




Diagram of a generalized vertebrate to 
show the origin and early differentiation of 
the ectoderm, endoderm. and mesoderm. 
(I) shows the mesoderm arising by means 
of the enterocoelic pouches budfling off 
from the archenteron. Above and between 
these pouches lie the beginnings of the 
notochord. In (II) the medullary plate has 
formed the neural tube and the mesoderm 
has become differentiated into regions 
which will form somites (myotomes), kid- 
neys, and linings of the body cavity. This 
differentiation goes still further in (III). 
(After McEwen.) 




Diagram to show 
the closure of the 
blastopore in a frog. 
Figures I to III are 
views from the vege- 
tal pole. The rota- 
tion so typical of de- 
veloping amphibian 
eggs has been started 
in III and completed 
in IV. The view in 
IV is from the poten- 
tial ventral side of 
the embryo. (After 

Early Differentiation of the Embryo 

It must be borne in mind that the changes out- 
lined follow a definite pattern and that some of 
them are going on simultaneously. One of the 
first changes after gastrulation is a gradual in- 
crease in the length of the embryo due largely 
to the rapid cell divisions about the lips of the 
blastopore, which forms the exterior opening of 
the archenteric cavity. The result is a gradual 
fusion by a backward growth of the lips of the 
blastopore, which thus produces an elongated 
line, the primitive streak. This is one of the best 
known embryological landmarks. Anterior to 
the primitive streak there soon develops, partially 
produced by a sinking of the ectoderm, two 
closely associated parallel folds of ectoderm, 
which extend anteriorly forming the walls of the 
neural groove. Gradually an anterior-posterior 
fusion of the walls of the groove produces the 
central nervous system, a dorsal tubular structure 
characteristic of the vertebrates. Sheets of meso- 
derm likewise grow anteriorly and laterally from 
the region of the primitive streak, soon splitting 
distally to form the splanchnic and the somatic 
layers. Meantime beneath this the gut is form- 
ing and being pinched off from the yolk beneath. 
In its anterior part, the pharyngeal gill-pouches 
and later the gill-slits appear, together with out- 
growths which form the lining of the thyroid and 
thymus glands. Posterior to this region there 
soon develops a ventral out-pocketing of the gut, 
which later forms the lungs in land animals, while 
still further posteriad lie the forerunners of the 
liver and pancreas. 

The degree of closure of the gut along the 
ventral surface of the embryo is largely depend- 
ent upon the quantity of yolk present in the egg. 
An egg containing little or a moderate amount of 
yolk, as in Amphioxus or the frog, respectively, 



has the ventral body wall completed early in development. In such 
forms the yolk that remains is carried within the body of the embryo 
and is accessible as fuel for further metabolism. 



IbU. isoWcifhal 

ed'tf . amphioxa? 

f "blastopore 



egig. amphibicin 

ectoderm ^Wc. 



eg"g - toiT'd 

Diagram showing effect of yolk on the formation of the gastrula. Read text p. 420 
and attempt to describe the etfect of yolk on gaslrulation. (After Patten.) 

In many of the fishes that are relatively large-yolked forms, develop- 
ment is similar. Young fry of the small-mouthed bass carry around 
enough yolk to maintain their " flame of life " for about two weeks, 
after which they begin feedhig on the usually plentiful plankton 
organisms. Whereas in a macrolecithal type with an abundant 
supply of yolk, such as a bird's egg, the gut fails to close until a 
much later date, the embryo literally floating on top of the mass of 
potential food. Even as development continues there is such a vast 
quantity of yolk present that it appears impossible for the embryo 
to complete the ventral body wall until much of the potential food 
material has been absorbed. As this process takes some time the 
embryo remains independent of other sources of food material until it 
gradually depletes the supply, and surrounds the remainder of the 
yolk with the continued outgrowth of the gradually extending germ 


Tissue Formation 

Each of the three primary germ hiyers produces a number of 
different tissues that in turn form the various organ systems. Briefly 
summarized, the ectoderm forms all of the nervous tissue, which in 
turn makes up the nervous system, as well as the organs of special 
sense that are developed in connection with it. The ectoderm also 
gives rise to the epidermis of the integument and its various derivatives 
such as scales, hair, horn, nails, feathers, and the enamel of the teeth. 
In addition the linings of the mouth, anus, and nasal passages also 
come from the ectodermal epithelial tissue. 

The endoderm forms the epithelial tissue lining the digestive tract 
with the exception of its extremities which come from the ectoderm. 
Many zoologists believe that all the various outgrowths from the 
digestive tract, for example, the lungs, air tubes, and liver, as well 
as various out-pocketings from the pharynx such as the thymus and 
thyroid, contain a significant endodermal contribution. In some 
chordates, the notochord buds oE from the endoderm. It should be 
noted, however, that in the case of the lungs and liver considerable 
amounts of mesodermal tissue also enter into the formation of these 

The mesoderm is the largest contributor to the tissues and different 
systems of the body. The circulatory tissue is derived from the 
mesenchyme of the mesoderm, while both skeletal and muscular 
tissues and frequently the notochord come from this germ layer. 
Likewise, both the excretory and reproductive systems are derived 
from the mesoderm, which also makes some contribution to the 
respiratory system. Finally the derma of the skin, cartilage, con- 
nective tissues, such as ligaments and tendons, and the peritoneal 
lining of the coelomic cavity, may be classified as mesodermal 

Protective Devices for the Embryo 

Egg Shells 

Various and sundry varieties of protective envelopes for ova are 
found throughout the animal kingdom. Although protozoa do not 
have eggs, encysted forms are protected from unfavorable environ- 
mental conditions by hard coats analogous to shells. For example, 
the cyst of Endameba histolytica, the causative organism of amebic 



dysentery, passes from the alimentary canal of man safely protected 
by a thick, hyaline coat, until such time as ingestion by a suitable 
host brings about its dissolution in the host's stomach. The eggs of 
some of the tapeworms and roundworms are surrounded by dense 
impervious shells, rendering them viable, in the of Ascaris, for 
five or six years. Some of the parasitic roundworms are ovovivipa- 
rous, retaining the eggs 
within the body of the 
parent until thoy are 
nearly ready to hatch. 

A few fishes, like some 
of the skates, produce 
an egg surrounded by a 
hard, leatherlike, 
which is drawn out into 
entangling tendrils 
that readily become 
enmeshed in seaweeds, 
thus affording protec- 
tion to the egg. Most 
of the fresh-water fishes 
and amphibians, how- 
ever, lay eggs which are 
protected by nothing 
more than a gelatinous 
mass which .swells after 
the eggs are laid in the 
water and are fertilized 

by the sperm. Among the reptiles and birds a hard shell is usually 
produced which gives protection to the enclosed ovum with its 
stored food. Only one small group of mammals, the monotremes, 
lay eggs, all others being viviparous. 

The Yolk Sac 

Among the fishes which lay telolecithal eggs containing enough 
yolk to render the cleavage pattern irregular, a mass of undivided 
yolk accumulates beneath the developing embryo. Soon, however, 
the blastoderm upon which the embryo lies grows down over the 
yolk, eventually enclosing it. This mass of tissue is composed of an 
inner layer of endoderm and an outer lining of mesoderm and is called 
H. w. H. — 28 

Embryo and egg case of skate. Such cases afford 
protection against wave action. What other types 
of adaptations are there for the protection of eggs 
and embryos.^ (After Walker.) 




^pirzccL CarcC 


the yolk sac. Gradually 
blood vessels develop 
in the mesenchyme of 
the yolk sac, facilitat- 
ing the transportation 
of food to the develop- 
ing embryo. 

Amnion and Chorion 

In addition to the 
protection afforded by 
egg membranes or shells 
and the yolk sac, the 
higher vertebrates, 
namely, the reptiles, 
birds, and mammals, 
elaborate additional 
embryonic membranes 
that serve not only as 
supplementary protec- 
tive devices to keep the 
embryos from mechan- 
ical injury but also tem- 
porarily handle the problems of respiration, excretion, and nutrition. 
In order to understand their functions, and the fact that their 
evolution is intimately tied up with that of the land-inhabiting 
reptiles, birds, and mammals, one must trace their embryological 

As long as organisms returned to the water during the breeding 
season, as the amphibians still do, the exchange of gases and elimination 
of wastes takes place directly, since the surrounding water not only 
contains sufficient dissolved oxygen but also it soon dissipates waste 
products which are passed through the egg membranes and elimi- 
nated. With the acquisition of a land habitat, the inability to return 
to the water to spawn presented new problems, centering about the 
control of metabolism in the embryo. These needs were met through 
the elaboration of a series of embryonic membranes, which were 
apparently developed to facilitate the carrying on of normal metabolic 
processes through a permeable egg shell. They occur in modified 
forms in all land vertebrates. 



Diagram of a developing fish embryo. Note the 
" contained " yolk sac. What is its ultimate fate ? 



The first of these new membranes to be considered are the amnion 
and chorion. They may be best understood by studying their origin. 
It will be recalled that in telolecithal eggs the endoderm does not 
succeed at once in growing ventrally to meet, and so to close, the 
digestive tube. Instead the unclosed tube lies flat upon the surface 
of the yolk. Both the ectoderm and mesoderm grow laterally over 
the endoderm dii'ectly over the yolk on the inner layer of the blasto- 
derm. The mesoderm as a whole divides into three portions, the 






allantoic cavity 
amnion \ aWantois 








chorion! - 





,ai-nniotic Cavity 






.^ allantoic cavity^^^^.^ sWlc 
' ,">tolk scut. 





Development of the extra-embryonic membranes in the chick. State the 
contribution of each germ layer to the amnion, chorion, and yolk sac. (After 

first of which is the upper epimere part immediately flanking the 
developing neural tube and producing the somites. Beneath the epi- 
mere lies a small mesomeral portion that later develops the excre- 
tory and reproductive systems from a ridge lying in the dorsal wall of 
the coelom. The mesoderm below the mesomere is the hypornere, 
which soon divides into an outer somatic and inner splanchnic layer 
of mesoderm. In large-yolked eggs this hypomeral portion extends 
laterally over the endoderm which is covering the surface of the 
yolk. In all of the higher groups, beginning with the reptiles, the 


superficial ectoderm and the outer or somatic mesoderm are con- 
tiguous, and together are called the somatopleure. They grow up 
from the surface to produce folds known respectively as head, tail, and 
lateral folds, and these folds in turn grow up and over the embryo from 
the head posteriorly until they meet and fuse. Upon dissolution 
of the wall at the point where these folds meet, two new complete 
layers covering the embryo are produced, the inner layer of somato- 
pleure being known as the amnion, and the outer as the chorion. The 
amniotic cavity between the amnion and the embryo is lined with 
ectoderm and becomes filled with a shock-absorbing amniotic fluid 
which serves the additional function of keeping the embryo moist. 
Outside the amnion is the chorionic cavity which is lined with somatic 

All of the time that the head, tail, and lateral folds of the amnion 
are developing, the yolk is being reduced and the splanchnopleure, 
composed of the endoderm and splanchnic mesoderm, is growing down 
and around it to complete the yolk sac. The outer margins of the 
somatopleure at the base of the developing amniotic folds likewise 
continue to grow down and around the yolk sac until they finally meet 
ventrally. This new layer may really be called a continuation of 
the chorion, while the cavity lying between the outer surface of the 
yolk sac and the inner side of the chorion is in reality but a continu- 
ation of the body, or coelomic cavity. Because of its position this 
portion of the coelomic cavity becomes known as the extra-embryonic 
coelom. It will be seen from the figure (page 425) that the chorionic 
cavity is nothing but an outgrowth from this. 


A yolk sac is developed in all of the egg-laying types of reptiles and 
birds. Even in the mammals, it is present in a reduced form. Rep- 
tiles, birds, and mammals, however, develop a fourth embryonic struc- 
ture called the allantois, which serves as an excretory and respiratory 
organ. While the yolk sac is attached by a yolk-stalk to the mid- 
gut region, the allantois develops as a diverticulum from the ventral 
surface of the hind-gut. Its growth does not start until after the 
amnion and chorion are in the process of formation. Almost at once, 
however, this out-pocketing encounters the inner layer of mesoderm 
so that the allantois comes to be lined by endoderm on the inside and 
covered by splanchnic mesoderm on the outside. The outgrowth 
continues, extending out into the extra-embryonic coelom and up 



into the chorionic cavity. Thus the allantois in reptiles and birds 
comes to He close to the porous shell, where it is well supplied with 
blood vessels and so readily becomes a membrane through which 
oxygen may be secured and the various waste products of metabolism 


In all mammals except the egg-laying types and the marsupials, 
who bring forth their young in an immature stage of development, a 
new mechanism, the placenta, is evolved to supply the metabolic needs 

yolk $ojt 

allantois .r> 







Fallopian tube 

tfie uterus 

mucus plug 

muscular volls oj^lctefifS- 

Diagrammatic sagittal section of human uterus. What devices do you find for 

protection and nourishment .^ 

of the embryo. Other important changes are associated with the 
formation of this structure. In the first place the developing embryo 
reaches the uterus and becomes implanted in the uterine mucosa at 
about the time of gastrulation. The amnion is formed and serves the 
same protective function as in the lower types, while the chorion is 
intimately associated with the maternal tissue lining the uterus and 
so becomes concerned with respiration, excretion, and nutrition. 
Blood vessels invade this modified chorion, extending from it down the 


umbilical cord to the embryo. From the surface of the chorion 
fingerhke projections, or villi, push out which interdigitate with 
similar fingerhke processes of the uterine wall, thus facilitating the 
maintenance of metabolism. This portion of the chorion together 
with the wall of the uterus in which the embryo is embedded is usually 
designated as the placenta. While there is no exchange of blood 
between the parent and embryo, their two blood streams in the case 
of the primates are separated only by the lining of the fetal capilla- 
ries, the connective tissue surrounding them, and the epithelial layer 
on the surface of the chorionic villi. While the allantois does develop 
in the mammalian embryo, it is incorporated into the growing placenta 
and in primates is really functionless, except for the proximal portion 
which is transformed into the urinary bladder of mammals. As the 
embryonic membranes are not permanent structures they are dis- 
carded at birth. 

Elaboration of Germ Cells, or Gametogenesis 

It should be borne in mind that the germ cells themselves can be 
traced back in the developing embryo only to a certain point which 
varies in different groups. In the chick, for example, the germ cells 
may be traced to the anterior margin of the blastoderm. In some 
invertebrates, such as Ascaris megaloccphala hivalvens, it has been 
shown that the germ cells may be detected at the thirty-two cell stage. 
In the latter instance the primordial germ cell may be readily detected 
by its size. 

While the primordial germ cells are present early in the life of the 
individual, it frequently happens that the organism does not mature 
for some time and consequently the development, or maturation, of 
functional germ cells is delayed. Usually the maturation process 
covers a considerable period of time which, in the case of a male, 
terminates in the elaboration of sperm. Hence the entire process is 
called spermatogenesis, while in the female the production of ova is 
known as oogenesis. Both phenomena may be spoken of collectively 
as gametogenesis. 

Formation of Sperm — Spermatogenesis 

The primordial germ cells of the male undergo an extended period 
of division, the resulting cells of which are designated as sperma- 
togonia. These reproduce other spermatogonial cells by normal 



mitotic cell division, and when ready for the final maturing stages 
they first undergo a period of growth in which the cells increase some- 
what in size. At this point one must look inside the cell to see what is 
happening within the nucleus. Here the chromosomes are paired. 
Each member coming from the male or from the female parent, re- 
spectively, is identical as to shape and size with the exception in 

polar hsdy 



primordiaX "penod. of 
germ cells rnitoU'c 

drowth period 
synapsis and. 
tatrods formed. 

first msiotjo ^eixnd moXicnz^ 
ciivision. meioticdiViSiai germ cells 



Diagram illustrating meiosis and the maturation of the germ cells. Explain 
how a constant number of chromosomes is maintained for a given species. (After 
Curtis and Guthrie.) 

certain cases of the pair of so-called sex chromosomes. The sperma- 
togonium has now been transformed by this process into a primary 
spermatocyte. When mitosis takes place each chromosome instead of 
being split longitudinally as in the case of normal mitosis becomes 
separated so that one entire member of each pair of homologous 
chromosomes is passed to each daughter cell. This brings about an 
actual reduction of the numbers of chromosomes present in each 
daughter cell by one half. This division (meiosis) is spoken of as the 
reduction division and the number of chromosomes as the haploid 
number in contrast with the normal or diploid number found in nil 


other cells. Each of the daughter cells is now a secondary spermatocyte 
producing two spermatids by the next division in which each of the 
remaining chromosomes, as in usual mitosis, splits longitudinally in 
half, thus maintaining the haploid number in each cell. Each sperma- 
tid eventually undergoes a metamorphosis into an active sperm with- 
out further cell divisions. Thus, each primary spermatocyte pro- 
duces four functional sperm. 

Formation of Ova — Oogenesis 

Oogenesis differs from spermatogenesis only in certain essential 
respects, although the corresponding stages must necessarily be 
designated differently. Thus the primary germ cells produce oogonia 
which in turn produce primary and secondary oocytes, polar bodies, and 
finally ova. In the period of growth intervening between the oogonium 
and its transformation into a primary oocyte there is a large accumu- 
lation of stored food and an accompanying increase in size. In the 
next stage, when the primary oocyte undergoes its reduction division 
the resulting cells are of unequal size, one becoming much larger 
than the other, having monopolized all of the yolk. The smaller 
one is in reality an aborted secondary oocyte and is called the first 
polar body. The second maturation division again results in the 
formation of a relatively large egg and a tiny second polar body. 
Sometimes the first polar body likewise undergoes division, formmg 
a total of three small polar bodies and one large ovum. 

The process of fertilization brings together the male and the female 
pronuclei, each of which contains the haploid number of chromosomes. 
By this means the diploid number, or full complement of chromosomes, 
is restored. Each chromosome, moreover, is composed of a number 
of genes arranged on it like a string of beads. The manner in which 
this mechanism functions in bringing about variations in the offspring 
will be considered in the unit on genetics (page 457) . 

The New E)mbryology 

The question as to just how far back one can trace the develop- 
mental pattern of an embryo is one which has long fascinated the 
zoologist. Great strides along this line have been made in recent 
years by the students of experimental embryology. We know that 
fertilized ova develop with great rapidity into well-formed embryos, 
characterized first by germ layers, later by tissues, and finally by 


systems of organs. The modern experimental cmbryologist raises 
the specter of the old controversy of rpigenesis or preformation, by 
inquiring into the question of how much of the development is depend- 
ent upon the contents of the fertilized egg and how much is due 
to environmental factors. 


All of the evidence which has been gathered to date indicates that 
the development of an embryo is a highly complicated process. As 
a starting point one might mention the character-controlling genes 
of the chromosomes that are brought together in the formation of a 
zygote. The vital part w^iich these play in altering developmental 
patterns has been clearly demonstrated many times. 


The second important factor is the environment. Changes in the 
normal environment frequently result in abnormalities. It is well 
known that temperature is a vital factor, since in all except viviparous, 
warm-blooded forms, a change in temperature will affect the rate of 
development. Under some conditions, for example when gastrula- 
tion is occurring, atypical forms may result. Likewise variations in 
temperature may produce apparent changes in the genes themselves. 
When certain kinds of fruit flies are kept at a higher temperature, 
there is a decrease in the number of ommatidia produced in each eye. 
Subsequent breeding experiments and a lowering of the temperature, 
however, result in a return to the original type. Another example of 
the environmental influence which upsets the normal metabolism of 
the embryo so that abnormalities result may be seen in the alter- 
ation of the oxygen, or food supply. The introduction of poisons 
also has similar effects. 

Changes in the metabolic rate of an organism are definitely cor- 
related with environmental factors as shown by the work of Child 
and his associates, who demonstrated the presence of definite "meta- 
bolic gradients." The axial gradient theory accounts for differences in 
dominance of certain areas in the developing organism, beginning 
with the axis occurring between the two poles of an egg. The dorsal 
lip of the blastopore soon becomes established as the region of greatest 
metabolic activity and so determines the rate of development of the 
other parts. It is at this region of highest metabolic activity that the 


head develops. Such differences in metaboHc rates between differ- 
ent parts of an organism have been demonstrated experimentally 
and it is probable that they are related to differences in the oxygen 

Natural Potencies 

Great differences normally occur between the so-called "potencies" 
of various species of eggs. Some species of animals produce toti- 
potent eggs. These are eggs in which the formative material is equally 
distributed throughout the component cells, or hlastomeres during 
early development. The resulting cleavage is called indeterminate 
because all cells up to a certain stage are totipotent, a condition 
that may be demonstrated by separating the various blastomeres, 
for example, from the two-celled to the sixteen-celled stage in some 
of the jellyfish, and securing normal, though perhaps dwarfed, indi- 
viduals from each. Cleavage in man is apparently of this type, and 
is the logical explanation of the production of identical twins. 

In the case of non-totipotent species the cleavage pattern is said to 
be determinate. There is little doubt that many of the determinative 
factors are already present in the cytoplasm of an egg before it is 
fertilized. In such forms as the mollusc, Dentalium, or the tunicate, 
Styela, the cytoplasm of the egg itself appears to be arranged in a 
definite pattern with respect to its future development. In such 
cases the early separation of blastomeres results in the formation of 
partial embryos. 


Certain parts of embryos are called organizers because they appear 
to be more or less directly responsible for the development of other 
closely associated regions. Much experimental work has been done 
abroad by Spemann and his co-workers, and in this country by 
Harrison and his students, all of which demonstrates the presence of 
such organizers. Perhaps one of the most important organizers is the 
dorsal lip of the blastopore. That this region is normally associated 
with the development of a neural plate may be demonstrated by 
transplanting it to a region beneath the ventral ectoderm of a frog's 
gastrula, where one would normally expect the formation of epidermis, 
but instead an aberrant neural plate appears. Such experimental 
evidence has been most carefully checked and rechecked by all manner 


of transplantation experiments. Naturally the stage of development 
reached at the time of transplantation affects the results obtained. 
Much work, however, remains to be done in this fascinating field. 


Huxley, J. S., and DeBeer, R. G., Elements of Experimental Embryology, The 

Macmillan Co., 1934. 

Scientific but readable account of modern embryology. 
McEwen, R. S., Vertebrate Embryology, rev. ed., Henry Holt & Co., 1931. 

A standard elementary text for reference. 
Morgan, T. H., Embryology and Genetics, Columbia University Press, 1934. 

Popularly written attempt to tie up modern embryology and genetics. 
Patten, B., Early Embryology of the Chick, 3rd ed., P. Blakiston's Son & 

Co., 1929. 

Excellent account of avian development. 
Richards, A., Outline of Comparative Embryology, John Wiley & Sons, Inc., 

1931. Pp. 20-90. 
Wells, H. G., Huxley, J. S., Wells, C. P., The Science of Life, Doubleday, 

Doran & Co., 1934. Pp. 150-159. 

Popular account of human development. 



Preview. Seed and soil • Independence of the germplasm • Lines of 
approach • The experimental method : The usefulness of hybrids ; Mendelism ; 
what Mendel did ; monohybrids, dihybrids, trihybrids, and other crosses : 
Unit characters and factors, modified ratios, different kinds of factors • 
Practical breeding : Selection, mass selection, pedigree breeding, progeny 
selection; inbreeding and cousin marriage; outbreeding and hybrid vigor; 
asexual propagation ■ The germplasmal method : Chromosomes ; genes ; 
linkage and crossing-over; chromosome maps • The role of cytoplasm • 
Sex in heredity • Suggested readings. 


"Now these are the generations of Pharez : Pharez begat Hezron, and 
Hezron begat Ram, and Ram begat Amminadab, and Amminadab begat 
Nahshon, and Nahshon begat Salmon, and Salmon begat Boaz, and Boaz 
begat Obed, and Obed begat Jesse, and Jesse begat David." 

As will be remembered, along came Ruth at the Boaz stage and 
injected a welcome bit of romance into these dry statistics. It is not, 
however, the vivid story of this Moabite woman, who was in her day 
so young and charming, that is the reason for introducing this quo- 
tation from the Book of Ruth, but rather the bare record of names 
in itself, together with the indispensable "begats," that claims our 
immediate attention now. The generations of mankind have always 
been hooked up in this chainlike fashion. The spark of life has 
always been borne forward for certain intervals of time by indi- 
viduals, and then transmitted to individuals of another generation to 
carry on. This is the Great Relay Race, participated in alike by all 
human beings, lower animals, and plants. It depends upon the 
co-operation of long lines of separate mortal individuals who play 
their temporary part and then inevitably die, while the immortal 
enduring line of life itself persists. The science of genetics attempts 
to explain how such a relay race is run. 

A single microscopic streptococcus, a solitary wandering housefly, 
or a chance weed pulled up from the wayside, each can boast of a 
longer pedigree than can the King of England. This universal 



principle of continuous inheritance, although not always recognized, 
has been used and practiced as an art from the begiiuiing, not only in 
the case of man himself, but also with domestic animals and cultivated 
plants. The real factors of heredity, however, together with the 
orderly "laws" which indicate their manner of working, have not 
been analyzed and made into a science until within comparatively 
recent times. The very word "genetics" was first employed by 
Bateson in 1906. 

To agree in advance to conduct any would-be excursionist down the 
rapidly flowing genetic river to a definite landing place is both pre- 
sumptuous and unwise, for there are at present too many long, un- 
charted stretches and too much that is unknown to make positive 
textbook promises of this kind probable of fulfillment. Nevertheless, 
the general direction in which the river of genetics flows, in spite of 
its shifting changes, is plain to all, and the tales of returning travelers 
invite us to intellectual adventure. Students in this field today, 
however, must make up their minds at the start to be alert explorers 
and ambitious pioneers, rather than passive, personally conducted 

Seed and Soil 

In the relay race of heredity the continuous thing that is handed on 
from generation to generation is not the lighted torch, but rather 
something that corresponds to a box of matches with which another 
torch may be lighted. Biological inheritance, unlike legal inheritance 
by which material possessions are transferred from parents to children, 
consists in the transmission of genes, or ultra-microscopic chemical 
units possessing the uncanny capacity, under suitable conditions, of 
expanding into visible structures or traits that resemble those in the 
parental make-up. 

Heredity binds the generations together and is absolutely essential, 
but in itself it is not enough. The potent genes, which are the 
determiners of heredity, must have a suitable setting in which to 
unfold their potentialities. This necessary setting is called the 
environment. It expresses and represents the spread that occurs 
within the Hmits of the hereditary possibilities, for the hereditary 
pattern may be enhanced or dwarfed in its expression by the action 
of the environment. Stated another way, the environment does not 
change the quality of hereditary characters, although it makes possible 
either a greater or a lesser development of them. 


Long ago Semper demonstrated, for example, that the size to which 
fresh-water snails will grow is somewhat dependent upon the spacious- 
ness of the aquarium in which they are kept, and Baur has shown 
that red-flowering primroses may be made to produce white flowers 
if subjected to continuous high temperature (30° C.) for a week or so 
immediately before blooming. 

The heredity factor is so important, nevertheless, that organisms 
can after all breed only their own kind, regardless of the environment 
in which they are placed. It is quite as futile, therefore, to argue the 
relative importance of heredity and environment as it would be to 
debate which of the two surfaces of a sheet of paper is more essential 
in making it a sheet of paper. Naturally the biologist is impressed 
with the contribution which heredity makes in the formation of a new 
individual, while the sociologist, as would be expected, emphasizes 
the environmental factor. Although no seed is so poor that it may 
not be improved by good soil and nurture, and no seed is so good that 
it will not imperfectly develop in poor soil, yet it is not within the 
capacity of tares under any circumstances to produce wheat, nor can 
we expect dogs to engender cats. Former President Lowell of Harvard 
once said, "There is a better chance to raise eaglets from eagle eggs in a 
hen's nest, than from hen's eggs in an eagle's nest." Neither heredity 
nor environment is effective alone. In the formation of any individual 
organism, the environment is the force that works from without in, 
while heredity works from within out. Both are as indispensable in 
producing a plant or an animal as land and water are in the formation 
of a shore line. 

Moreover, there is extra-biological or social inheritance to reckon 
with, that makes us the "heirs of the ages." CiviHzation in itself 
may be regarded as the collective achievements of mankind, and 
as time goes on these environmental collections multiply and accu- 
mulate. We live today, for example, in a world of skyscrapers, 
automobiles, stock exchanges, airplanes, chain-stores, movies, ocean 
liners, and radios, the acquisition of which our ancestors of three 
hundred years ago never even dreamed of. If we may seem to have 
a larger horizon and to sec farther than our ancestors, it is not so 
much because we are taller than they were, as it is because we stand 
on their shoulders with respect to these extra-biological acquisitions. 

There is no doubt that the environment of mankind has undergone 
more modification than human heredity has. When we consider, for 
example, the intellectual and artistic output of ancient Greece, a small 

THE GREA-'r 1 11:1. W llACIi; 





country in classical times with restricted environment, and contrast 
it with the corresponding output of the whole enlarged modern world, 
with its highly elaborated setting, there is occasion to wonder whether 
the intrinsic capabilities of man have increased as much as his oppor- 
tunities. It has always been easier for man to modify his surroundings 
than to control his own heredity. To quote Joseph Jastrow, the 
psychologist, "The fact that modern schoolboys are far better 
equipped to withstand, utilize, and control the forces of nature than 
was Aristotle, is not due to the superiority of the schoolboys, but to 
the contributions of the Aristotles of past generations." 

Furthermore, the range of hereditary possibilities, particularly in 
the case of man, may be considerably influenced by training or educa- 
tion, which is a hopeful factor that perhaps cannot be entirely ac- 
counted for either by heredity or environment. Education in itself 
forms no part of the hereditary stream, since it is only the capacity 
to acquire education in a yroper environment that can be handed on 
from parent to child. In the 
case of plants, and those ani- 
mals whose automatic in- 
stincts make it unnecessary 
for them to learn how to live, 
the factor of training or edu- 
cation does not play as domi- 
nant a part as in man. 

In the accompanying 
diagram an attempt has been 
made to indicate the mutual 
dependence of heredity and 
environment, in the forma- 
tion of three different hypo- 
thetical individuals. A, B, and C, represented by the rectangles in the 
figure. When the parallel edge indicating the environment is shoved 
back and forth, like a slide rule, different-sized rectangles result. 
The act of shoving, particularly when the slide rule is shortened and 
the "rectangular individual" is consequently enlarged, is much like 
the process of education or training. In each case it will be noted 
that neither the whole of the hereditary nor the whole of the envi- 
ronmental edge is involved in the resulting individual. This cor- 
responds with our common observation and conviction that neither 
our capacities nor our opportunities are all ever entirely utilized. 






A "slide-rule" diagram, showing how the 
interplay between heredity and environment 
may result in different individuals, A, B, 
and C. 



Independence of the Germplasm 

The germplasm, or the sexual cells that carry the load of hereditary 
possibilities, and the somatoplasm, which makes up the body of the 
individual, although to a certain extent dependent upon each other 

in a nutritional way, are 
remarkably independent. 
Despite the popular idea to 
the contrary, it is extremely 
improbable that changes 
wrought by, or impressed 
upon, the somatoplasm ex- 
ercise any modifying influ- 
ence upon the accompanying 
germplasm. The somato- 
plasm is simply like a casket 
in which the jewel of germ- 
plasm reposes. No decora- 
tion or elaboration of the 
casket will have any material 
effect upon the jewel within. 
This point has been con- 
vincingly brought out, along 
with other cumulative evi- 
dences, in a critical experi- 
ment performed in 1911 by 
Castle and PhiUips. These 
investigators successfully 
transplanted the ovaries of 
a black guinea pig into a white guinea pig whose own ovaries had 
been removed. Later, after recovery from the operation, when 
this white female with the borrowed ovaries of the black female was 
mated with a white male guinea pig, the offspring were all black, 
although both their parents were white, and under ordinary circum- 
stances would produce only white offspring. This shows that 
temporary residence within a white somatoplasm did not in any way 
affect the character of the black-producing germplasm that had been 
grafted into the white body. 

The establishment of the fact of the practical ineffectiveness of 
somatic influence upon the germplasm has far-reaching applications 

Diagram of ovarian transplantation experi- 
ment by Castle and Phillips, to show the lack 
of somatic influence on the f^erniplasm. The 
ovaries of a black guinea pig were engrafted 
into a female albino whose ovaries had been 
removed. Upon recovery this female was 
mated three times with an albino male. All 
the progeny were black. (From Walter, 
Genetics, by permission of The Macmillan 
Company, publishers.) 


in any theory of heredity. It means tliat modifications acquired 
within the Hfetime of the individual are not transferred to the parental 
germplasm, and do not consequently reappear as hereditary charac- 
ters in the next generation. If this conclusion seems perhaps dis- 
couraging to prospective parents who would gladly have whatever 
success in the building of character, the development of intelligence, 
or the attainment of artistic or other ability that they have been able 
to bring about in their own lifetime perpetuated in their children, 
they may well be reminded of the other side of the picture, namely, 
that parental failures in accomplishment during life likewise form no 
part in their children's biological inheritance. Each child, therefore, 
starts out with his ancestral biological inheritance unimpaired by 
either parental failures or successes. In any case, the honest scien- 
tifically-minded person is bound to accept the facts whatever they are, 
if they can be ascertained, regardless of the conclusions to which they 
lead, rather than to place dependence upon unproven propositions 
that, with wishful thinking, he would like to believe are true. 

It should be pointed out clearly that the only biological opportunity 
where it is possible to improve the germinal chances of the next genera- 
tion is not after the germinal equipment has already been assigned 
to the prospective parent from his ancestors, but at the critical time 
of mating when two streams of germplasm are selected for combina- 
tion. Picking out the right mother is the most important contribu- 
tion which any man can make for his future children. 

Thus, the individual somatoplasm is simply the guardian and 
executor of the germinal possibilities committed to its care. Heredi- 
tary possibilities do not come directly from the parents, but through 
them down the long ancestral line. When and how remote ancestors 
have picked up the gifts of biological inheritance which they present to 
posterity forms one of the most intriguing riddles in the science of 
genetics. It is encouraging to know that the results of modern 
researches have hopefully opened up the way to a possible answer to 
this question, which may be more suitably developed later on. 

Lines of Approach 

There are two fundamental lines of approach to genetics : first, by 
way of the more visible so7natoplasmof organisms, and second, thegerm- 
plasmal approach, which involves recourse to microscopic technique. 
The former approach may be subdivided into at least three lines of 
attack, namely, observational, statistical, and experimental. 

H. w. H.— 29 


The observational method has been practiced from time immemorial, 
and to it is due most of the accumulations of our general knowledge 
concerning heredity up to about the turn of the present century in 
1900. The phrase "like produces like" expresses the general impres- 
sion that is gained from observation, although there are plenty of 
exceptions to the apparent rule. We say that children in a general 
way "take after" their parents, although there are conspicuous in- 
stances when it becomes necessary for parents to "take after" their 
children, in order that they may be made to conform to a family 
tradition, whatever it may be. It is repeatedly observed that not 
only individuals of one generation may be in general like their pred- 
ecessors, but that certain noticeable characteristics in the make-up 
of an individual may occur more often in some family lines, breeds of 
animals, or strains of plants than in the general population of which 
they are a part. Whenever this is so we are led to suspect, even when 
we may not be entirely convinced, that such characteristics are 
hereditary. General but more or less vague observations of this sort, 
while useful in establishing the simple fact of inheritance, do not go 
very far in determining and analyzing the causes of heredity and the 
laws of procedure that underlie the mechanism of inheritance, which 
it is necessary to know in order to establish a real science of genetics. 

The statistical method recognizes the desirability of arranging quali- 
tative data in quantitative terms, as a necessary process in reducing 
random observations and guesses to definite scientific form. Recourse 
must always be made to mathematical treatment in formulating any 
science, and genetics is no exception. Mathematics, however, is 
simply a useful tool to be employed in arranging the facts and in 
bringing them together in convenient form for interpretation. There 
are repeated occasions when it is not only desirable but indispensable 
to focus isolated and scattered facts into a single comprehensive pic- 
ture which can only be accomplished by statistical treatment. Statis- 
tics, however, to be of value in solving problems of heredity, must be 
based upon careful observations and accurate measurements pre- 
viously obtained. Biometry, the science of measurement when ap- 
plied to biological data, is powerless to extract true conclusions out 
of faulty observations or findings. 

The biometrical approach is about the only way available in which 
to investigate the problems of heredity as applied to mankind. It is 
obviously not feasible, even if it were desirable, to plan and execute 
controlled experiments in human breeding, of sufl&cient magnitude 


and duration, to be of general significance in establishing the laws of 
inheritance. Not only would any such ambitious program take too 
many generations to reach any satisfactory conclusions, even if it 
were possible, but also it would involve too many insuperable social 
diflSculties. In the case of mankind, therefore, we are forced to 
resort to experiments in marriage and other sexual relations that have 
already been made in the past, for collecting data, and this type of 
investigation demands the technique of statistical treatment. 

The third method of approach in storming the citadel of genetics is 
the ex'perimental method. This has proven to be very successful. By 
controlling breeding of animals and plants and observing the outcome, 
which is not open to the objections encountered when human material 
is employed, it has become possible to find out much concerning the 
modus operandi of inheritance. The same biological laws and pro- 
cedures that are found to be true of plants or animals may then, to a 
large extent at least, be applied to man. This method will be elabo- 
rated somewhat in the following sections. 

All of these methods, namely, observation, statistical treatment, 
and experimental breeding, are concerned primarily wnth somato- 
plasms. The germplasmal method of approach, on the other hand, is 
concerned with the concealed beginnings of the life story, rather than 
with its visible sequel in the bodies of organisms. The germplasmal 
approach has to do wdtli the astonishing behavior of the genes, which 
are the determiners of subsequent somatoplasmal manifestations. 
This underground phase of the heredity problem is proving in recent 
years to be most illuminating, and some consideration of it, together 
with the experimental method just mentioned, will make up the 
essential remaining part of this section on genetics. 

The Experimental Method 

The Usefulness of Hybrids 

In order to learn the secrets of inheritance by the controlled crossing 
of plants and animals, it is necessary to use parental stocks that differ 
from each other in some of their characteristics. When this is done, 
hybrids are produced in which the respective contributions to the 
offspring from the two parents may be determined, and thus the 
first steps made in the analysis of the problems of inheritance. 

If both parents and the consequent offspring are alike, then a color- 
less monotony results that gives no differential clue as to how heredity 


works. Just as in the evolution of species during long periods of 
geological time, variation must somewhere have entered in to make 
it possible that an elephant and a mouse could have arisen from a 
common, remote ancestor, so in the relay race of heredity we cannot 
picture the details of how a succession of generations comes about, 
when all the individuals concerned are alike. The uniform bulk of in- 
heritance passes unnoticed. It is only the " sore thumbs " of variation 
that stand out, for although hereditary succession may and does occur 
in the absence of variation it is only when a visible variable is intro- 
duced from one parent or the other, that we can see how the inherit- 
ance of a characteristic jumps from one side of the house to the other, 
skips a generation, doubles up, or behaves in some other manner. 
One outstanding way in which hybrid variation is brought about in 
nature is by sexual reproduction, in which two different streams of 
germplasm unite to form a new generation. 

Pure hereditary strains, on the other hand, are probably not nearly 
as common in nature as are hybrids. In self-fertilized plants, for 
example, we may not expect to find much in the way of hereditary 
variation, since no different outside germplasmal potentialities have 
been introduced in the production of offspring. Likewise, in par- 
thenogenetic organisms, which develop progeny without any contribu- 
tion from the male parent, as well as in all kinds of asexual propaga- 
tion, where a fragment of the parental body gives rise without germinal 
modification to a new individual, one may expect to encounter 
monotony so far as hereditary variations are concerned. Transient 
variations that are induced by environmental causes, like the tanned 
skin of a lifeguard at the seashore, or the luxuriant growth of a 
pigweed on a manure pile, do not carry over in heredity. That 
hereditary variations frequently do appear in the absence of hybrid 
combinations is to be accounted for by the occurrence of mutations, 
or spontaneous hereditary variations, which are mentioned in the 
section on Evolution. 

In the early days of the nineteenth century, certain scientifically- 
minded botanists in Europe began to explore the possibilities of 
hybridization by artificially crossing plants. Koelreuter (1733-1806) 
and Gaertner (1772-1836) in Germany, Naudin (1815-1889) in 
France, and Knight (1759-1839) in England were conspicuous 
pioneers in this field of experimentation. It remained, however, for 
Gregor Johann Mendel (1822-1884) of Austria to become the master 
hybridizer of them all, and to carry his experiments through to results 


and conclusions that mark him as the patron saint of the modern 
science of Genetics. 


Gregor Johann Mendel, with peas and arithmetic, not only demon- 
strated the existence of an orderly system of inheritance that bears 
his name, but was himself a living example of the extent to which 
innate hereditary ability can dominate an environment none too 
favorable. He was an Augustinian monk, attached to a monastery 
in Briinn, Austria (now Brno, Czechoslovakia), where, with ordinary 
garden peas, he carried through a remarkable series of breeding 
experiments extending over several years. During the first part of 
his career, when working on these famous experiments, he was 
handicapped by having only a small patch of a cloister garden in 
which to operate. Later on, when he finally became abbot of the 
monastery and could control garden space at will, he was necessarily 
so occupied with the administrative duties of his office that he did 
not have much time to devote to scientific pursuits. Yet, in spite of 
these limitations, and regardless of the fact that his associates were 
not particularly sympathetic with his unpriestly avocations, he 
carried to completion by himself this remarkable piece of fundamental 
investigation which insures for him a permanent place in the biological 
Hall of Fame. 

His results were finally published in 1866 in the obscure "Pro- 
ceedings" of a small, unimportant local Natural History Society. 
They did not at the time gain appreciative attention and were 
promptly forgotten, due in part perhaps to the preoccupation of the 
scientific world at the time with the newly launched Thconj of Natural 
Selection (1859) of Charles Darwin. Unrecognized and unknown, 
Mendel died in 1884, with the confident declaration on his lips, 
"Meine Zeit wird schon kommen ! " Some years later this prophecy 
came true when, in 1900, three scientists, Correns in Germany, von 
Tschermak in Austria, and DeVries in Holland, independently 
rediscovered Mendel's forgotten contribution, and because of it, 
initiated the remarkable era in the study of heredity that has resulted 
in establishing the science of Genetics as we know it today. 

What Mendel Did 

Mendel's genius is shown by the fact that he did not make his 
experiments blindly, but set for himself the clearly defined problem 


of reducing the phenomena of inheritance to a measurable mathemat- 
ical basis. For this purpose he wisely chose for experimentation gar- 
den peas, which not only are easily grown, but also possess readily 
recognized constant characteristics. Since peas are normally self- 
fertilized, they represent at the start comparatively pure hereditary 
strains. Moreover, hybridization in peas can be controlled from 
contamination by insects. Since fertilization occurs before the flowers 
open, the hooded structure of the flowers is such that interference from 
their chance visits is prevented. As is well known, insects may carry 
on involuntary hybridization experiments of their own in connection 
with many plants, by transferring pollen grains from the stamen of 
one flower to the stigma of another. 

Instead of considering the whole complex individual in the light of 
a "hybrid" unit, as former hybridizers had done with much result- 
ing confusion, Mendel focused his attention upon single alternative 
characters, one pair at a time, that were unlike in the two contributing 
parents. This simplification of the problem made the collection of 
data less complicated, and the analysis of results possible. Finally, 
he not only combined single pairs of characters into hybrids, but he 
went further and followed up the results obtained by breeding these 
known hybrids together through several generations, meanwhile tak- 
ing meticulous pains to account for all the offspring of whatever sort 
in each case, so that ratios of relative occurrence could be computed. 
For example, he dealt with seven pairs of alternative characteristics 
found in different strains of peas, as follows : ^ 

1. Smooth seeds or wrinkled seeds ; 

2. Yellow seed-coats or green seed-coats ; 

3. Tall vines or dwarf vines ; 

4. Colored flowers or white flowers ; 

5. Axial flowers or terminal flowers ; 

6. Inflated pods or constricted pods ; 

7. Green pods or yellow pods. 

In every case when these pairs of characters were put together 
the hybrids thus produced were not intermediate in appearance, but 
were alike, and resembled one of the parents and not the other. 
When these hybrids in turn were interbred with each other, or allowed 
to be normally self-fertilized, which amounts to the same thing, the 

' Dr. O. E. White, of the Brooklyn Botanical Garden, as early as 1917 reported thirty-four pairs 
of hereditary characters in peas on which determinative experimental studies have been made. 



progeny always fell into two groups in appearance like the two grand- 
parents, in the ratio of 3:1. Thus, when smooth peas were arti- 
ficially crossed with wrinkled peas, the hybrids were all smooth peas, 
and when these smooth hybrids in turn were allowed to cross inter se, 

Parents CP) 


Hqbrid children (f,) 


Diagram of the ancestry and progeny of a typical monohybrid, formed from 
smooth and wrinkled garden peas. The inner circles represent germplasm, en- 
closed in the outer circles, or somatoplasm. S, determiners of smooth peas; 
s, determiners of wrinkled peas. 

the resulting grandchildren could be grouped in the ratio of three 
smooth peas to one w^rinkled pea. These results are indicated dia- 
grammatically in the accompanying figure, with the smooth charac- 
teristic represented by a single letter as *S, and the wrinkled kind by 
s. The germinal make-up of each individual is thus represented by 
two letters, since it is always derived from two parental gametes. 

If smooth and wrinkled gametes come together in the same indi- 
vidual, the smooth determiner covers up, or "dominates," the 
wrinkled one, and is consequently called a dominant, while the 
wrinkled gamete recedes from visible expression for the time being, 
and is designated as a recessive. Which one of the alternate pair of 
parental characters will be dominant and which recessive in the 
offspring in any given case cannot be learned in advance by inspec- 
tion. It is, therefore, necessary to resort to the breeding test in order 
to make the determination. 

Further crosses on IVIendel's part showed that SS peas were pure 
stock, like one of the grandparents with which he started, and when 
interbred produced only SS peas, although coming from impure or 
hybrid parents. Similarly the ss peas were also pure like the other 
grandparent, and likewise always gave rise only to ss peas when 
allowed to inbreed with their own kind. The hybrid Ss peas, on the 



other hand, being constituted like their hybrid parents, when interbred 
furnished again the typical 3 : 1 ratio. In Mendel's original experi- 
ments there were actually obtained from the Ss peas 5474 smooth 
and 1850 wrinkled peas, which is very near the expected 3 : 1 ratio. 
Such pure SS peas and hybrid Ss peas are said to be phenotypically 
alike and genotypically different. That is, they look alike, but have 
different possibilities when it comes to producing gametes. The 
way to distinguish the one from the other is to breed them hack with 
the recessive ss peas, which can conceal nothing, and observe the kind 
and proportion of the offspring produced. SS X ss gives 100 per cent 
Ss (phenotypically smooth), while Ss X ss gives 50 per cent Ss 
(phenotypically smooth) and 50 per cent ss (phenotypically wrinkled), 
as shown in the checkerboard below, in which the gametes of the two 
sexes are placed outside the double lines, and the resulting kinds of 
individuals are represented by double letters within the squares. 














Sometimes dominance may be incomplete, in which case it is not 
necessary to back-cross with the corresponding recessive in order to 
determine which are pure and which are hybrid dominants. The 
four-o'clock {Mirahilis jalapa), as pointed out by Correns, furnishes 
a well-known demonstration of this point, for the hybrid produced by 
red X white flowers is not dominant red, as might be expected, but 
pink. The pink hybrids give in turn the proportion of three colored 
flowers (one red and two pink) to one white. 

Mendel carried through the same hybridization procedure and sub- 
sequent follow-up, with each of the seven pairs of contrasting char- 
acters, and found that the approximate 3 : 1 result always obtained, 
regardless of whatever other characters were present in the individual 
plants. Each pair of characters, in other words, behaved inde- 
pendently of every other pair. This is called the principle of inde- 
pendent assortment. 

It is apparent, moreover, that the determiner for each character 
retains its integrity, reappearing in the next generation true to 
itself, regardless of the company it has been keeping within the germ 
cell. This integrity of the hereditary determiners, together with the 



uncontaminated reappearance of the character in the next generation, 
is termed the principle of segregation. 

Thus, out of simple but perfectly controlled experiments with garden 
peas, Mendel was able to lay down three "laws," namely, dominance, 
independent assortment, and segregation, which together constitute the 
essential features of what is known as "Mendelism." These funda- 
mental laws have been confirmed many times over, in a great variety 
of plants and animals by a host of critical investigators, and their use 
now makes possible a precise prediction of results in experimental 
breeding that was quite impossible before their formulation. 

Monohybrids, Dihybrids, Trihybrids, and Other Crosses 

The fundamental Mendelian laws as illustrated by a monohyhrid, 
that is, a hybrid with respect to a single pair of characters, are com- 
paratively simple. When two monohybrids are bred together, as 
shown in the preceding paragraphs, the resulting progeny occur in the 
phenotypic ratio of 3 : 1, and the genotypic ratio of 1 : 2 : 1. Dihy- 
brids, trihybrids, tetrahybrids, etc., are increasingly comphcated, 
but are quite understandable when it is remembered that they are 
nothing more than combinations of monohybrids, resulting from the 
independent assortment of the characters involved. The expecta- 
tions for such crosses are show^l in the following table : 

Number of 

Pairs of 

Possible Combi- 
nations When 

Number of 

Phbnotypes in 


Number of 

Genotypes in 







Dihybrid .... 





Trihybrid .... 





Tetrahybrid . . . 










As an example of the way in which a dihybrid works out, black 
color in horses is dominant over chestnut color, and trotting gait over 
pacing. These two pairs of characters are independent of each other, 
so that when a black pacer is mated with a chestnut trotter, all the 
offspring of the hybrid generation will be black trotters, since black 
color and trotting gait are dominant characters. Then when such 
hybrid black trotters are mated together there will be sixteen possible 




combinations, falling into four phenotypic groups, and nine genotypic 
groups, as shown in the following checkerboard, in which the double 
gametes of the dihybrid parents are represented outside the double 
hnes, and their combinations in the offspring indicated within the 
sixteen squares. The arbitrary symbols used are, B (black) ; b (chest- 
nut) ; T (trotter) ; t (pacer). It will be seen from the checkerboard that 
DIHYBRID CHECKER- ^^^^ chances out of sixteen are possible that 

a black trotter will result, since both B and T 
are present at least once in their make-up. 
There are three chances that a black pacer 
(Bt) will occur, three chances for a chestnut 
trotter (bT), and one chance in sixteen that 
the dihybrid parents will produce a chestnut 
pacer (bt) . Thus, the phenotypic ratio in the 
case of a dihybrid is typically 9:3:3:1. The 
checkerboard further shows that the sixteen 
possibilities fall into nine genotypic, or ac- 
tually different, groups represented by dif- 
ferent combinations of the four symbolic letters within the squares. 
The expectation when two trihybrids are crossed is shown by 






























































































































Mendel's garden peas, in which three alternative pairs of characters are 
selected, namely, yellow (Y) and green (y) peas; tall (7') and dwarf 
(0 vines; and axial (A) and terminal (a) flowers. The trihybrids in 
this case will have the genotypic formula YyTtAa, and will be pheno- 
typically yellow, tall, and axial. Such hybrids, because of the inde- 



27 VTA (yellow, tall, axial) 

9 YTa (yellow, tall, terminal) 

9 YtA (yellow, dwarf, axial) 

3 Yta (yellow, dwarf, terminal) 

9 yTA (green, tall, axial) 

3 yTa (green, tall, terminal) 

3 ytA (green, dwarf, axial) 

1 2fta (green, dwarf, terminal) 


Kinds of trihybrids 

pendent assortment of their characters, can produce eight possible 
kinds of gametes, or mature germ cells, each carrying three charac- 
ters, as follows: YTA, YTa, YtA, Yta, yTA, yTa, ytA, yta. When 
these triple gametes unite, there are sixty-four (8 X 8) possible com- 
binations, as shown in the accompanying checkerboard, which will 
fall into eight different phenotypic groups in the ratio of 27 : 9 : 9 : 3 : 
9:3:3: 1 (total 64), and they may be further classified into twenty- 
seven genotypically different groups, represented above as three 
monohybrid ratios combined. 



In actual practice, if a combination of three or more characters 
is desired, one character at a time in either pure dominant or recessive 
form is obtained. By this method, since the expectation of either a 
pure dominant or a pure recessive in a monohybrid is one out of four, 
early reahzation of the desired combination is likely. 

Unit Characters and Factors 

A great deal has been learned about heredity through the experi- 
mental breeding of plants and animals since Mendel's laws became 
available. Many of the facts gained, however, are at first sight in 
apparent contradiction to these laws, but the value of the fundamental 
concepts of dominance, independent assortment, and segregation in the 

^ ,. interpretation of inherit- 

nSreQIiary OOmailU wcice remains unques- 

Determiners Characters tioned. Any adequate 

\_) .«...___^ consideration of the ap- 

parent departures from 
the clear-cut conclusions 
of Mendelism would re- 
quire many more pages 
than are available in this 

For one thing, Men- 
del's experiments led him 
to the idea of Unit 
Characters, each spon- 
sored by a single germinal 
determiner. There is 
now abundant evidence 
that whatever it is in the 
germplasm that, under suitable environmental conditions, becomes 
eventually expressed as a single character, it is often made up of more 
than one unit. This discovery has led to the development of the factor 
hypothesis, which implies that there is usually, if not always, an in- 
terplay between different hereditary factors in determining the con- 
tribution which inheritance furnishes to the formation of a character 
in an individual. Moreover, a constellation of interacting hereditary 
factors may be responsible, in certain instances, for the expression of 
more than one visible character. 

Modified Ratios. The existence of factors, or fractional rather 


Diagram of the relation between hereditary 
determiners and resulting somatic characters. 
A, three or more determiners may combine to 
produce a single visible character, or B, a single 
hereditary determiner may find expression in a 
number of difTerent somatic characters. 



than unit determiners, is particularly apparent when, for example, the 
typical dihybrid ratio of 9 : 3 : 3 : 1 becomes modified into other than 
the usual phenotypic groups. The following ratios have been dem- 
onstrated in various dihybrid crosses: 3:6:3:1:2:1, 9:3:4, 
10 : 3 : 3, 12 : 3 : 1, 9 : 6 : 1, 9 : 7, 10 : 6, 13 : 3, and 15 : 1. In each of 
these cases it is still a dihybrid, made up of two monohybrids and 
totaling sixteen possibilities involved. 

To work out a single illustration of how the factor idea gives rise 
to a modified phenotypic ratio, let us take Bateson's famous case of 
sweet peas, that resulted in the 9 : 7 ratio of flower color. Bateson 
dealt with two different strains of white-flowering sweet peas that 
bred true to the white color as long as they were not out-crossed. 
When the two white strains were artificially crossed with each other, 
however, all the progeny in the first generation produced purple 
flowers. This purple color was found to be due to the combination of 
two factors, which may arbitrarily be designated as A and B, one of 
which was furnished by each parental strain. Neither factor alone 
could produce the purple color since the parents were both white. 
When the purple hybrids in turn formed their possible kinds of 
gametes and were crossed with each other, there resulted the custom- 
ary sixteen combinations of a dihybrid, as shown in the checker- 
board. AAhh (white) X aaBB (white) = AaBh (purple). Gametes 
from AaBb = AB, Ab, aB, ah. 

























Of the sixteen possibilities, the nine possessing at least one A factor 
and one B factor produced purple flowers, while the remaining seven, 
which did not possess both the A and B factors, were whilse. It will 
be seen that the seven phenotypically white-flowering possibilities 
fall into three genetically different groups, namely, 3 AAhh or 3 Aahh, 
3 aaBB or 3 aaBh, and 1 aahh. By breaking up the seven kinds 
of white-flowering sweet peas into the genetically different groups 
3:3: 1, and adding them to the nine purple-flowering kinds, the 
underlying Mendelian dihybrid ratio of 9 : 3 : 3 : 1 is restored. This 


is a case of complementary factors, because one factor is required to 
complement the other in order to bring the character into expression 
while neither is effective alone. 

Different Kinds of Factors. There are also supplementary 
factors, where one factor alone may produce a visible effect, but a 
second factor may change its manifestation ; or inhibiting factors, 
where the expression of a factor is prevented by the interference of 
another; or duplicate factors, where separate "doses" of the same 
thing combine to produce a cumulative effect ; or lethal factors, which 
are so disharmonious that if they arrive together from both parental 
sources, the unfortunate individual sooner or later dies, although able 
to survive when only a single lethal factor comes from one parent ; 
or sex-linked factors, that are tied up with either the maternal or the 
paternal side of the house. In all these cases the factors in their 
behavior obey the fundamental Mendelian laws, although the resulting 
ratios furnish intriguing complications that Mendel himself did not 

It is hoped that the reader will be stimulated to explore in books 
devoted primarily to Genetics (see bibliography) further than the 
general survey presented in this chapter. 

Practical Breeding 


Long before Mendel pointed the way by which to control the 
operations of heredity, man was active in fixing desirable characters in 
animals and plants by means of artificial selection, and in doing this 
was only following in the footsteps of Mother Nature, who has been 
exercising ''natural selection" from time immemorial. Many of the 
forms selected and nurtured by man never could have survived if left 
to the more exacting demands of nature. 

We know today, thanks to Mendel, that phenotypes do not always 
reproduce their own kind, and that the genotype is the all-important 
thing to get at in heredity. It must be admitted, however, that in 
spite of difficulties encountered, our pre-Mendelian forebears, in estab- 
lishing lines of domesticated animals and cultivated plants by the 
method of blind selection of phenotypes, attained a remarkable degree 
of success. Even the ancient lake-dwellers of prehistoric Switzerland, 
it is said, developed ten different kinds of cereals from wild plants. 

There are three different methods of phenotypic selection which are 


still practiced with gratifying results by practical breeders, namely, 
mass selection, pedigree breeding, and progeny breeding. 

Mass Selection. In mass selection a general population, exhibit- 
ing desirable qualities on the average, is drawn upon to furnish pro- 
genitors for the following generation in the faith that "like produces 
like." There are two ways in which a desirable population to breed 
from may be obtained. A crop, for example, may be grow^n under 
the most favorable conditions of cultivation and environment and 
the improved individuals resulting chosen as seed. This method of 
procedure is based upon the questionable belief that acquired charac- 
ters reappear in the next generation. Or the same crop may be grown 
under adverse conditions and those individuals which are pheno- 
typically most promising chosen, with the idea that, since they have 
made good in spite of unfavorable surroundings and poor nurture, 
they must obviously possess desirable inherent or hereditary qualities. 

The limitations of this common practice of mass selection lie in the 
fact that selection must be made over and over again, since nothing 
dependable has been established. Moreover, the best individuals in 
this wholesale procedure are often swamped by the average ones, so 
that all are reduced to a mediocre level. 

Pedigree Breeding. Pedigree breeding, based likewise upon the 
fallacy that like always produces like, narrows selection definitely to 
single individuals or lines, rather than hopefully employing a confusion 
of many unknown lines. It is a method that has been particularly 
successful in breeding race horses and various kinds of domestic 
animals, and depending upon stud-books and zealously recorded pedi- 
grees. Even human beings are known to indulge in "blue books" 
and proud genealogical records that characterize pedigree breeding. 

Progeny Selection. Progeny selection depends upon the princi- 
ple that the only way to determine the character of the essential 
germplasm in plants and animals is to see what kind of somatoplasms 
it produces. In the poultry pens at the Massachusetts Agricultural 
Station at Amherst, for example. Hays and Sanborn established a 
strain of hens in which the annual egg production was raised from 145 
to 235. This was done by selecting cocks that bred pullets which 
made good by producing an increased yield of eggs. Thus it was 
demonstrated that the male has a hand in the heredity of egg pro- 
duction, although it is the female that does the real work. 

In similar fashion, bulls siring heifers that prove to be high milk- 
producers are selected for building up a herd of dairy cows. Bulls 


cannot produce milk but they can sire heifers that do. In these 
cases, instead of predicting what the offspring will do by observing 
the parental performance, the offspring themselves are taken to show 
what their parents can do in producing desirable progeny. Mendelism 
has shown that selection of any kind, in order to be effective, must 
deal with genotypes rather than phenotypes, and that the material 
from which selection is made must be hybrid rather than pure in its 
composition if progress is to result. 

Inbreeding and Cousin Marriage 

Inbreeding in various degrees of consanguinity or blood relationship 
tends to produce uniformity, or purity, in the hereditary stream. 
Notwithstanding popular opinion to the contrary, inbreeding in itself 
is not harmful. It simply tends, in the case of hybrids, to bring 
recessive traits out into the open, and these are in many instances 
less desirable than dominant characters. Cousin marriage in highly 
hybridized human stocks is a potent way of unearthing "skeletons in 
the closet," for cousins, being of approximately the same hereditary 
make-up, are apt to carry concealed the same recessive characters, 
which thus have a Mendelian chance of getting together and becom- 
ing somatically visible. On the other hand, when people not closely 
related are mated together, their undesirable recessive traits, being 
different in each 'parent, are likely to remain concealed or covered up 
by corresponding dominants contributed by the other parent. For 
example, \i Aa and Aa represent two similar related individuals of 
the same make-up so far as the characteristics A and a are concerned, 
there is one chance in four, according to the Mendelian monohybrid 
ratio, that the undesirable combination of aa will appear in the off- 
spring. If, however, two unrelated individuals, Aa and Bh, carry 
undesirable gametes represented by the small letters a and h, there 
is only one dihybrid chance in sixteen that the individual showing the 
undesirable recessive combination aahh, with no concealing dominant 
to interfere, will appear, and there are only three additional chances 
each out of sixteen that either the aa or the hb recessive characteristic 
will come to light. (See checkerboard on page 451.) 

In nature there are many instances where inbreeding is enforced. 
Wheat, and cereals generally, as well as the legumes to which Mendel's 
peas belong, are habitually self-fertilized, and this is even closer 
inbreeding than brother and sister mating, to say nothing of the 
pairing of cousins. 


Outbreeding and Hybrid Vigor 

Outbreeding, on the contrary, introduces variety and tends to 
cover up recessive defects by the introduction of new dominant char- 
acters, although it does not permanently eliminate the former. 

In nature probably most animals and plants outbreed. Even 
hermaphroditic animals such as earthworms and snails, in which both 
sexes are included in one individual, usually mature their eggs and 
sperm at different times, as already noted, thus insuring outbreeding. 
The same thing is true to a large extent of the great array of plants in 
which both pollen grains and ovules are housed in the same flower. 

One of the beneficial results of outbreeding is hybrid vigor, which 
usually accrues to the first generation of hybrids. This result may be 
accounted for as the summation of desirable dominant characters 
from the two diverse parents. The advantages gained by this type 
of cross, however, do not endure in successive generations, when 
inbreeding comes in with its leveling effects. The former confusion 
and uncertainty about the consequences of inbreeding, outbreeding, 
and hybrid vigor is straightened out when one goes behind the scenes 
with the insight made possible by Mendel's laws. 

Asexual Propagation 

Another practical way of maintaining desirable hereditary quali- 
ties, particularly in plants, when once they have been obtained, is 
by asexual propagation through slipping or grafting. This is the 
method employed in maintaining strains such as navel oranges, w^hich 
produce no seeds, and also in plants which do produce seeds whose 
phenotypes are known desirable somatoplasms but whose genotypes 
are hidden in unknown problematical seeds. By this procedure of 
asexual propagation the desired combination is continued, without 
the introduction of any disturbing germinal modification from the 
outside. Many of Luther Burbank's famous plant "creations," 
such as the spineless cactus and the white blackberry, have been 
established and made available by this method. 

The Germplasmal Method 

The foregoing somatoplasmal methods of approach in studying 
heredity, although to a remarkable degree successful, are at best only 
indirect. It is more and more apparent that the most hopeful line 
H. w. H. — 30 


of future advance is concerned with the direct analysis of the germ- 
plasm itself, that is, of the basic chemical materials (genes) out of 
which somatoplasms are derived. This has been made all the more 
possible within the last half century by the increased efficiency of 
greatly improved microscopes and microtomes, and through the 
development of staining technique by means of aniline dyes which 
render visible and differentiated microscopic details of structure that 
were formerly unseen. 


Every germ cell, as well as each of the somatic cells that are the 
building stones of the body, contains a nucleus, within which, at 
certain times in the life cycle of the cell, chromosomes may be seen. 
These structures stain more deeply with certain dyes than do other 
parts of the cell, thus becoming visible under the microscope. 

It is doubtful that Mendel ever saw chromosomes, for it was not 
until the late seventies, after his scientific career was practically over, 
that the invention and development of aniline dyes made possible 
their discovery. Each pair of chromosomes has a characteristically 
different shape and size, whereby it is usually possible to distinguish 
them from every other pair. Chromosomes, moreover, retain their 
specific identity, in spite of the fact that they may change their form 
temporarily, or for a time disappear entirely from view. When 
germ cells undergo maturation to form their gametes as a preliminary 
to fertilization, the total number of chromosomes in each cell is 
reduced to one half. An entire pair is never normally eliminated, 
although this sometimes occurs under abnormal circumstances 
{non-disjunction). The result is that ordinarily there is left behind 
one complete outfit of all the chromosomes characteristic of the 
species, with their determinative genes, both in the mature egg and 
the mature sperm. As pointed out previously, fertilization restores 
pairs of chromosomes and then ever afterwards, by means of the 
meticulous machinery of mitosis, these pairs are handed on to all 
subsequent cells of the body that arise from the fertilized egg. 

One of the evidences that chromosomes play an important part in 
heredity lies in the fact that they are the only parts of the germ cells 
in which the two sexes contribute equally to the formation of the 
fertilized egg in animals, or ovule in plants, that initiates a new 
individual. It is common observation that each parent in the long 
run is equally responsible for hereditary traits in the offspring, and 


this agrees in general with the fact of equal contributions to the 
following generation of chromosomes from each parent. It has been 
repeatedly shown by experiment, as, for example, with the eggs of 
sea-urchins, that when more than one sperm enters and fertilizes an 
egg, thus involving the presence of an atypical number of chromo- 
somes, the resulting larvae are monstrous, or at least abnormal, and 
do not long survive. Evidently this is a case of too much father! 
The conviction of the responsibility of chromosomes in heredity is 
further strengthened by a very large number of remarkably ingen- 
ious investigations made in the last twenty years, centering about 
the occasional abnormal behavior of chromosomes during the matura- 
tion divisions, particularly with the much-studied banana-fly Droso- 
phila, maize, and the jimson-weed Datura. It is not possible in this 
limited summary to do more than to call attention to this brilliant 
and complicated work, which goes far in confirming the importance 
of chromosomes in heredity. It is earnestly hoped, nevertheless, 
that the reader may eventually have the opportunity to explore this 
fairyland of fact. Although it involves a somewhat discouraging 
array of strange technical terms, such as non-disjunction, transloca- 
tion, coincidence, inversion, duplication, deficiency, deletion, interfer- 
ence, and ploidy, it turns out that the terms used are not at all 
formidable upon closer acquaintance. 


Although the chromosomes of the male and female germ cells 
unite to build the "imponderably small" bridge over which the 
hereditary load passes from one generation to another, they are not 
in themselves the actual units of heredity. These ultimate bearers 
of inheritance, which are borne by the chromosomes, are known as 
genes, a name given them by the Danish botanist Johannsen (1859- 
1927). Dr. W. E. Castle has defined a gene as "the smallest part of 
chromatin capable of varying by itself." In other words, genes are 
the ultimate invisible hereditary units and as such form the essential 
subject matter of genetics. 

That no one has ever surely seen genes under the microscope does 
not lessen the fact of their reality. Like the atoms of the chemist and 
the electrons of the physicist, of whose reality there is no doubt, they 
are too small to be seen by any means at present at our disposal. 
We know next to nothing about the structure and chemical composi- 
tion of these ultimate hereditary units ; nevertheless, we already know 


a good deal about their behavior, although the scholarly attack upon 
the gene in the light of what is sure to follow can be said to have 
hardly begun. 

It is plain that there are many more distinguishable traits and 
characters present in an organism than there are chromosomes. In 
Drosophila, for example, which has only four pairs of chromosomes to 
a cell, over five hundred hereditary differences have been accurately 
identified. Consequently, many determining genes must be located 
in each pair of chromosomes. What has been found to be true of 
Drosophila in this respect, is undoubtedly true of other organisms. 
So much of our knowledge of genes in general has been acquired by 
investigations upon the ubiquitous banana-fly that genetics stands in 
some danger of becoming Drosophiletics. These tiny flies, that have 
never even heard of birth control, lend themselves very favorably to 
the study of genes. Within a month a single pair can become grand- 
parents of so many grandchildren that it is difficult to keep track of 
them. Millions have actually been experimentally bred and critically 
examined one hy one by different workers within the past three decades 
since their scientific usefulness has been discovered. They even 
gained the Nobel prize award (1934), with the aid of Dr. Thomas 
Hunt Morgan and his associates. 

It has not only been possible for the investigators of Drosophila 
to determine more than five hundred determining genes in these flies, 
but also even to locate these several genes definitely in particular 
pairs of chromosomes, and to arrange them with reference to each 
other at definite distances apart within a single chromosome. All 
that has been learned by the followers of Mendel about the interaction 
of what are termed "factors" appUes to the invisible genes. For 
example, it is not likely that single genes, any more than single 
factors, "determine" single somatic traits or characteristics. Rather 
the genes must work together to bring about visible results, since 
"genie balance" is essential to somatic success. 

Linkage and Crossing-over 

Although there is, as Mendel demonstrated, independent assortment 
between different chromosomes during the formation of the gametes, 
the genes that are located in any single chromosome tend to hang 
together in succeeding generations and not to become separated from 
each other. This is called linkage. By means of it, whole blocks of 
genes may act together as a unit in heredity. 


Mendel did not hit upon linkage, because it fortunately so happened 
that the determiners of the seven characters (page 444) with which ho 
dealt were each located in separate chromosomes, of which there are 
known to be seven pairs in garden peas. This was a happy accident, 
for if Mendel had chanced upon genes linked together in a single 
chromosome, he might never have been able to establish the law of 
independent assortment, which is so essential in determining the 
Mendelian ratios. 

In mitosis it sometimes happens, however, as shown by the sub- 
sequent breeding results, that chromosomes emerge which contain a 
different combination and arrangement of genes than that in the 
originals from which they came. In other words, linkage is broken 
up. The way this comes about is as follows. During the process of 
the preparation of the germ cells for sexual union (mciosis), as has 
been repeatedly observed, the maternal and paternal chromosomes in 
each pair of egg or sperm come to lie close together side by side. 
They may even twist around each other. This intimate contact of 
homologous chromosomes from the two parents is called synapsis. 
It will be recalled how later the still entire chromosomes separate or 
unwind from their mates and migrate to opposite poles of the germ 
cell, during the unique reduction division, thus forming two new cells 
each containing but half the normal number of chromosomes in each 
cell. This means that either the maternal or the paternal chromosome 
of each pair is missing in the resulting daughter cells, while the end 
result of ordinary mito.sis, or cell division, is the production of two 
new cells, each with a complete equipment of chromosome pairs 
representing the maternal and paternal contributions. 

After synapsis, the two chromosomes in each pair may separate and 
go their different ways with all their genes linked together exactly as 
they were before intimate contact with each other, or during synapsis 
they may stick temporarily together and then later break into frag- 
ments and become reassembled in a new relationship, with a part of 
a paternal chromosome attached the supplementary part of a mater- 
nal chromosome. When such an interruption of linkage occurs it is 
termed crossing-over. It is as though, following an ardent embrace, 
Jack's head should be found perched on Jill's shoulders, and in 
exchange, Jill's head should turn up on Jack's shoulders. That this 
extraordinary kind of performance actually does happen with the 
chromosomes has been amply demonstrated over and over by observ- 
ing the ratios in which the offspring appear following a dihybrid cross. 


An illustrative case may serve to make both linkage and crossing- 
over plainer. In corn, colored kernel (C) is dominant over colorless 
kernel (c), and plump starchy grains (S) are dominant over wrinkled 



Diagram of the steps in crossing-over. A, an allelomorphic pair of chromo- 
somes, with genes represented as soHd or open circles ; B, synapsis, or the con- 
tact of homologous chromosomes ; C, breakage of chromosomes at the point of 
contact ; D, reassembly of chromosome fragments, resulting in a cross-over of 
genes, making a new combination. 

sugary grains (s). Thus, when pure colored-starchy corn (CCSS) is 
crossed with pure colorless-wrinkled corn (cess), the resulting hybrid 
will be colored-starchy like the dominant parent in appearance but 
with the genotypic formula of CcSs. When in turn these hybrids are 
back-crossed with the recessive parent (cess), in order to reveal the 
different kinds of offspring which they are capable of producing, the 
expected result, if there is independent assortment, would be the ratio 
of 1 CS :lCs: IcSilcs, as shown below. 

Hybrid Gametes 





Recessive gametes cs 





In an actual experiment, however, when the hybrid was back-crossed 
to the recessive parent, the offspring were phenotypically 4032 
CS : 149 Cs : 152 cS : 4035 cs. This is approximately the ratio of 
48 : 2 : 2 : 48, instead of the expected 1:1:1:1. The explanation 
of this result is that out of a total of 8368 cases there were 8067 
instances in which the characters CS and cs, that entered into the 


hybrid combination together, stayed together in linkage, while in the 
remaining 301 cases out of 8368, crossing-over occurred between the 
colored (C) and the starchy (*S) genes derived originally from one 
grandparent, with the corresponding colorless (c) and wrinkled 
(s) genes furnished by the other grandparent. These cross-overs were 
a new combination in corn, namely, colored-wrinkled (Cs), and 
colorless-starchy (cS). 

Chromosome Maps 

By experiment, particularly with Drosophila, which lends itself 
especially to this kind of investigation, varying percentages of crossing- 
over between different pairs of genes located in the same pair of chro- 
mosomes have been determined. This method of taking advantage 
of the occurrence of crossing-over has led to the determination of the 
distance hetween individual genes in particular chromosomes, depending 
upon the principle that the nearer together two pairs of genes are, 
the more likely they are to remain linked when the chromosomes twist 
about one another and subsequently break and rejoin, while the 
farther apart they are, the more likely they are to shift from one 
chromosome to the other during synapsis. 

For example, if the percentage of crossing-over, as shown by the 
results of breeding, between the hypothetical genes Aa and Bh, is five, 
and that between Bh and Cc is twenty, then the cross-overs between 
Aa and Cc ought to be twenty-five (5 + 20) if the order of the genes in 
the chromosomes is A-B-C, or fifteen (20 - 5) if the order of arrange- 

15 /^-5^B 


I I I I I 
-15 ^^ 25 

The determination of the order of genes on a chromosome. 

ment is C-A-B. This kind of confirmation has been repeatedly 
verified in actual breeding experiments. 

By an extension of this technique it has been possible to construct 
chromosome maps, in which the location of the different invisible genes 
in the various chromosomes can be determined with astonishing 
accuracy. Such a map of the four different chromosomes in Droso- 
phila, as far as it had been completed in 1926. when Morgan published 
"The Theory of the Gene," is shown on page 462. Today the chro- 
mosome map of Drosophila, like a recent map of the world as com- 



pared with one of Marco Polo's day, shows many new additions, 
thanks to the patient and tireless labors of the small army of Droso- 







0.0 i, J hairy-wing 

0.+J\ /{scute 

0.3 -^i 

=^~ kthal-1 

0.0 1 


0.6 <^- 

1.0 yr 

1-5 '//Z 

'^ broad 
~\ V prune 
"V\\ whilp 


- — star 

3.± — 




6.* — 

"-" extended 




W^ abnormal 


Vo echirttts 
\ bifid 

12 + — 


13.7 V- 

i6.± y- 

-\\ruby , 
-^^ cross-veinless 

13.0 -: 

- — truncate 

-^ dack^ous 

\ streak 

20.0 —Z 



^^ singed 



27.7 ---: = 


33.0 ^_ 




36.1 \ 

^ miniature 




-~^^ dusky 

38. ± ^- 


43.0 \_ 

^^ sable 


-^ jammed 

44.4 --- 

— garnet 


-^ minute-e 

48.5 \_ 



\ jaunty 

54.2 V 

^ small-wing 

54.5 -i: 
56.5 \_ 





57.0 -'- 



■ cinnabar 


59.0 y 
59.6 y- 
62,± y- 

65.0 ' 

~\^ small-eye 
_ \^ fused 


— — safranin 

\y beadez 
\ \ minute-^ 


64.i — 


68. ±''- 


— — vestigial 
^ telescope 

70.0 / ^bobbed 


. lobe 





83.5 — 

90.0 humpy 


. arc 
/, plexus 

99.5 \ 



106. ± . — purploid 

107.0 J7 \Xspeck 
107.5' \ balloon 

6.01^- -^jbroum 
S.±J\ /{blistered 

0.0 — -y — roughoid 

20.0 — — divergent 





40.1 \ 

40.2 -^: 





49.7 y 


58.2 ~. 
58.5 ^y. 
58.7 </ 
59.5 -V" 
62.0 V' 
63.1-^ . 
69.5 v.^ 

70.7 -;: 

72.0 ■^' 




93.0 — 


0.0 --^— bent 


^.< ^shaven 

9.0 ' \ eyeless 

;■— -sepia 
^ hairy 

— rose 

. — crcam-111 

^ minute-h 
:^ tilt 




.^ pink 
' Y^ maroon 



hairy-wing sup, 
: — stubble 
;^ sj)ineles3 

\ bitkorax 
' V> stripe 
\ glass 

■^ delta 

^^ hairless 
'■ — ebony 

\ band 
:-— cardinal 
\ white-ocelli 

• — rough 
• — crumpled 

V^ beaded 
\ pointed 

^ claret 

\ minute 

100.7 v^ 

101.0 ^' 

106.2 — -*- — minute-g 

(After Morgan) 

" When it is remembered that Drosophila is a very tiny fly ; that paired 
reproductive organs occupy only a small part within its abdomen ; that each 
of these reproductive organs in the male is made up of several tubules ; that 
within these tubules may eventually be found the sperm cells with plenty of 
room in which to move about ; that within each sperm cell is a nucleus ; 



that after half the contents of the nucleus has been disposed of there remains 
four chromosomes; that within each chomosome there are, beyond the 
range of vision, hundreds of genes ; and that it is possible within a single 
chromosome to determine not only the relative arrangement of the many 
genes, but also to find out the relative distance between any two of these 
genes, it wUl be realized that the analysis of the germplasm has gone a long 
way." ' 

The Role of the Cytoplasm 

In spite of the demonstrated importance of the chromosomes and 
their genes in the mechanism of heredity, they are not the whole story. 
There is the cytoplasm to be reckoned wdth, particularly in the egg. 
In no cell can either the nucleus or the cytoplasm lead an independent 
existence. Each depends upon the other. Hence, while the un- 
doubted significance of the chromosomes is being emphasized, it is 
well to remember also the indispensable cytoplasm. Is there such 
a thing as cytoplasmic inheritance, in addition to that of the genes ? 

In answer to this question it is necessary in the first place to dis- 
tinguish the part that cytoplasm plays in development as well as its 
possible function in hereditary transmission. 

The nuclear membrane separates the chromosomes from the sur- 
rounding cytoplasm during the resting stage of every cell cycle, 
resulting in some degree of temporary independence. However, 
every time mitosis is repeated this protective membrane vanishes for 
the time being, leaving the chromosomes directly exposed to the 
cytoplasm. Here, then, is furnished an opportunity for exchange of 
materials between chromosomes and cytoplasm, and this exchange 
does undoubtedly occur. During mitosis, it will be remembered, 
each chromosome splits lengthwise, and the half chromosomes thus 
formed, mingling freely with the cytoplasm, migrate to their respec- 
tive poles. Meanwhile they are restored to their original dimensions 
by the intake of material from the cytoplasm itself. Thus a part 
of the cytoplasm of the cell becomes made over into chromosomal 

In the long series of successful mitoses by means of which the 
zygote eventually becomes an adult individual, the chromo.somes in 
each newly formed cell still maintain their original genetic make-up as 
to form and numbers of pairs. The cytoplasm of these various cells, 
on the other hand, undergoes transformation to constitute the different 

' From W'alter, H. E., Genetics. By permission of The Macmillan Cominiiiy, publishers. 


tissues of the body. In other words, while there is accompUshed an 
equal distribution of chromosomes, an unequal distribution and 
elaboration of the cytoplasm takes place. It is plain, therefore, that 
in this process the genes not only take in material to be elaborated 
from the cytoplasm, but that in turn something must go out from 
them to bring about the differentiation of the surrounding cytoplasm. 
That there is a chemical difference between what is in the chromo- 
somes and what is outside of them is proven by the differential way 
in which these substances respond to certain stains. Apparently 
there is carried on from generation to generation throughout life an 
elaborate and extensive performance of "give and take" between the 
germplasmal chromosomes and the somatoplasmal cytoplasm of the 
cells. Dr. H. S. Jennings states the matter in the following words : 

" This process of changing the cytoplasm by the action of the genes is the 
fundamental thing in development. The genes repeat this process over and 
over again, taking in cytoplasm, modifying it, giving it off in changed condi- 
tion, and leaving the genes themselves unaltered." ^ 

In the light of the intimate relationship between genes and cytoplasm, 
and recognizing the dominant part taken by it in developmental 
processes, can we assign any truly hereditary role to the cytoplasm 
itself, except as it is first taken in and made a part of the chromosome 
complex ? 

It is common knowledge that apple blossoms, when fertilized with 
foreign pollen, produce only apples like the maternal parent because 
the apple itself is merely an elaboration of the maternal tissue of the 
ovary, determined in its character before the ovule in the ovary is 
fertilized. Seeds of such apples, however, grow into trees that 
produce fruit showing paternal as well as maternal characters. This 
sort of "maternal inheritance" suggests the presence of some heredi- 
tary factor outside the genes that keeps an apple a sweet apple, for 
instance, although its blossom is fertilized by pollen from a sour 
apple tree. It is only necessary, however, to remember that the 
cytoplasm of the sweet apple is already determined by the germinal 
contributions of the preceding generation, both maternal and paternal, 
rather than by the fertilizing pollen in the present case, in order to 
find a satisfactory explanation that does not involve cytoplasmic 

' From Jennings, H. S., Genetics, p. 233. By permission of W. W. Norton & Company, publishers. 


In practically all groups of plants there are certain structures 
embedded in the cytoplasm called plastids, which are centers of meta- 
bolic activity. They are composed of packets of various materials 
essential to plant life, such as starch grains, chlorophyll, oil droplets, 
and the like, having a definite chemical composition and easily visible 
under the microscope. It is generally agreed that ])lastids are 
derived from preceding plastids, quite as chromosomes are from 
preceding chromosomes, and that they are not formed anew in each 
cell. Unlike chromosomes, however, they do not undergo orderly 
mitosis when they divide, thus securing in daughter plastids an 
accurate halving of material as in the case of chromosomes and 
genes, nor do they always follow the Mendelian laws in their redis- 
tribution. A case in w^hich the chromosomes and genes do not 
apparently play their usual equal parts, but in which it looks as if the 
inheritance is through self-perpetuating plastids in the female cyto- 
plasm and never through the male gametes, is found in plants with 
variegated or striped leaves. In these plants, the cells with plastids 
carrying chlorophyll {chloro-plasts) determine the green areas in the 
leaf, while cells with plastids that lack the chlorophyll (leucoplasts) 
account for the white areas. Branches and flower buds occur with 
either chloroplasts or leucoplasts. When crosses are made between 
flowers borne upon a green branch, and those from a white branch of 
such variegated plants, the resulting offspring are white or green 
according to the kind of plastids present in the maternal parental 
branch, irrespective of the kind of pollen employed. The grand- 
parental genes determine the character of the maternal plastids, which 
in turn cause the new branch or plant to be white or green. 

Current opinion about the whole matter is summarized in the 
statement of Dr. E. M. East to the effect that "though the nucleus 
and cytoplasm co-operate in development, the only ascertained agent 
of heredity is the nucleus." What the future may disclose still 
remains a question unanswered, but at present it appears that 
"cytoplasmic inheritance" is unproven. 

Sex in Heredity 

While it is quite possible for one generation to arise from another 
by various asexual methods, yet it is evident that the whole mech- 
anism of heredity has been revolutionized by the rise of sex. 

As previously pointed out in the section on "The Usefulness of Hy- 
brids" (page 441), in the study of heredity so long as level uniformity 



characterizes the succession of generations, there is no way by which 
the laws of inheritance may be detected. Distinctive alternative 
characters must be introduced from unlike parents and combined in 
various ways in order to make the manner of inheritance in the 
progeny recognizable. Transitory environmental variations, since 
they play no part in inheritance, only cloud the picture. It is germ- 
plasmal variations alone that can be of service in inheritance, and 
such variations are provided in double measure by the device of 
sexual reproduction. Thus sex is not only the major means by which 
inheritance is effected but it also furnishes the key that unlocks the 
mystery of how evolution is brought about. 

The way in which sexual recombination can change the flow of 
germplasm from one generation to another is suggested in the figure, 

Two different biparental streams of germplasm, A and B, may form four new 
different biparental streams of germplasm, a, b, c, and d, in the next generation. 

which reduces the matter to terms so simple that it is consequently 
entirely inadequate to represent the actual complexity and possible 
rearrangement accompanying sexual reproduction. 

Although Mother Nature's children, that is, plants, animals, and 
even mankind, have successfully utilized the mechanism of sex for 
an incomprehensible span of time, it is only in recent years that man 
has come to understand, with anything like scientific accuracy, the 
way in which it works. 

In the eighteenth century, the "ovists" held that the egg was the 
all-important factor, and that the sperm simply served to start the 


egg on its developmental way. An opposing school of "spermists" 
maintained that the egg was only useful as a means of food storage 
for the essential sperm. Notwithstanding the fact that the ancient 
Assyrians were well aware that date palms would not mature fruit 
unless pollen from male trees was dusted on the blossoms of the 
female trees, it was less than a century ago that it was finally estab- 
lished by Leuckhart (1822-1898) that both egg and sperm are homol- 
ogous partners in heredity. 

It was not until the beginning of the present century, after Mendel's 
laws had been re-established and chromosomes had been discovered, 
that sex was recognized as a hereditary trait in itself, dependent 
principally upon genes. That other factors besides genes may con- 
tribute to the determination of sex is no doubt true. For example, 
Dr. Oscar Riddle, of the Carnegie Institution of Washington, has 
advanced a well-grounded theory of the metabolic determination of sex, 
based upon exhaustive experiments extending over many years, in 
breeding doves at Cold Spring Harbor, Long Island. Other inves- 
tigators have emphasized the modifying influence of the external 
environment, and of the internal hormones, but no one denies the 
action of the genes as the primary effective factor in sex determi- 

The theory most generally accepted today to account for the 
approximate equality of the sexes in the offspring of any species is 
that of Correns, who postulated that the gametes of one parent are 
of two kinds, male-producing and female-producing, while the gametes 
of the other parent are alike so far as sex determination is concerned. 
This idea has been amply substantiated by the discovery in many 
forms of plants and animals of what has subsequently been designated 
as sex chromosomes. 

As has been repeatedly emphasized, chromosomes occur in homolo- 
gous pairs, one member from each parent. McClung in 1902, dis- 
covered that in the germ cells of the male locust, Xiphidium fasciatum, 
there occurred an odd chromosome without a mate while in the 
female immature germ cells every chromosome was supplied with a 
corresponding mate. Consequently, this being the case, when the 
members of the chromosome pairs, following synapsis, separate to 
form the gametes, the odd chromosome joins one group of daughter 
chromosomes, leaving the other group one chromosome short. The 
former sort of gametes, carrying the odd chromosome, upon union 
with a normal female gamete having a full quota of chromosomes, 


forms a zygote that will produce a female, while the latter sort without 
the odd chromosome, when uniting with the normal female gamete, 
produces a zygote that is destined to become a male. Thus, if XX 
represents the sex chromosomes of the female, and XO those of the 
male, the result is diagrammatically as follows : 

Germ cells XX XO 


In many instances it has been observed that the formula XY, instead 
of XO, represents the male sex chromosome pair, while the female 
remains XX. That is, instead of an odd unpaired sex chromosome, 
there is a mismated pair. The accompanying figure, showing the 
chromosomes in Drosophila, serves as an example of, such a case. It 
will be seen in this figure that in the male there are present three pairs 
of chromosomes in which the mates are alike, but that one chromo- 
some of the fourth pair is rodlike, while its mate, the F-chromosome, 
has a bent tip. By substituting Y for the in the preceding diagram, 
the same explanation for the equality in number of the sexes among 
the offspring is reached, as in the case of McClung's locusts. In 
both of these examples it is the number of X-chromosomes present, 

that is, one or two, that deter- 
V CT mines the sex of the offspring. 

Other variations of this funda- 
mental idea have been found in 
the copious investigations which 

nj C have been made on the heredity 

f • of sex, but all agree with Correns' 

^ ^ original interpretation of unlike 

The four pairs of chromosomes in Dro- ^^^ gametes in one parent and 
sopiiila melanogaster. (Alter Morgan.) . ° . ^ 

like gametes in the other. 

The great majority of plants and animals that have been examined 
show that the male ordinarily is the sex that produces two kinds of 
sex-determining gametes. Birds, butterflies, and moths form an 
exception to this general rule, for in them all the sperm gametes are of 
one kind, while two kinds of mature eggs, male-producing and female- 
producing, occur. The result of approximate equality of the sexes in 
the progeny, however, is the same as in the former instances. 

-^.^ -^..^ 



In mankind there are twenty-four pairs of chromosomes, of which 
twenty-three pairs, common to both sexes, are called autosomes, and 
to these is added one pair of sex chromosomes, designated XF in the 
male and XX in the female. A curious fact about F-chromosomes in 
general is that, with few exceptions, breeding experiments prove them 
to be devoid of genes. They play a dummy hand. Thus the 
F-chromosome exerts the same non-contributory role in heredity as 
the element does in the XO combination. The X-chromosome, on 
the other hand, not only plays a part in sex determination, but it also 
harbors additional genes that control the appearance of other traits 
and characters. These are called sex-linked traits. Their existence 
is demonstrated in the male because there is nothing in the F-chromo- 
some mate to conceal them. 

This point may be made clear by citing Morgan's now famous case 
of the white-eyed Drosophila. Many years ago in one of his cultures 
of normal red-eyed flies, there appeared a single white-eyed male 
mutant individual. The conjunction of Professor Morgan's seeing 
eye with the white eye of this particular tiny fly marks an event in the 
history of genetics comparable to what happened to the science of 
physics when the falhng apple and Sir Isaac Newton's head came 
together. In both cases an exceptional brain was fortunately 
stimulated, with far-reaching benefits to science. When Morgan's 
unique white-eyed male fly was mated with a normal red-eyed female, 
all the offspring were red-eyed, thus showing the dominance of the 
red-eyed character over white-eye. When these red-eyed hybrids 
were mated together, the expected Mendelian ratio of three reds to 
one white resulted, but all the males were white-eyed. Omitting the 
autosomes and representing only the sex chromosomes, the matter 
may be diagrammed as foUow^s. (The underscored A" indicates that 
red-eye color is linked with the sex chromosome. The absence of 
underscoring means white-eye.) 








Fj offspring 





Fg offspring 


— xa: ■ 






In order to obtain a white-eyed female, it was necessary to mate a 
wliite-eyed male to a hybrid red-eyed female, which works out as 
follows : 



/ \ 


X- ___ A' 


F, offspring 


In this type of sex-linked inheritance, the paternal character may 
be transferred directly in 50 per cent of the cases from father to 
son and from mother to daughter. There is another type of sex- 
linkage, as exemplified by some kinds of color-blindness in man, in 
which the inheritance is never direct from father to son and from 
mother to daughter, but indirect, or zigzag, as from father through 
daughter to grandson. This is called criss-cross inheritance. Thus, 
when a female, normal for color-blindness, is mated with a color-blind 
male, the trait skips a generation before it reappears. 



F. children 


color-blind (>^ 

Gametes ■^" 

F2 grandchildren XX 

normal ( 

It will be seen that in addition to regular Mendelian inheritance, 
which has to do with the genes located in the various autosomes and 
which results in the typical 3 : 1 ratio when the hybrids are bred 
together, there are two other types of inheritance, involving the sex 
chromosomes. One of these is the direct type in which the character 
may be handed on from father to son or from mother to daughter, and 
the other is the indirect type of criss-cross inheritance in which the 
father cannot give the character to his son, but may pass it along to 
his grandson by way of his daughter. 

In drawing this section to a close, it is worth while to quote the 
opinion of the eminent English geneticist, C. C. Hurst, who says, 
"Perhaps there is nothing which has helped the study of genetics 
more than the existence of sex." It would take us too far afield to 
follow out the enticing vistas of heredity opened up by the phe- 
nomenon of sex. Some of the many aspects of heredity which might 


be considered in this connection are suggested by such terms as sex 
hormones, sex determination, sex reversal, parthenogenesis, hermaph- 
roditism, gynandromorphs, gonad transplantation, sterility, free- 
martins, and identical twins. In order to go on, the interested student 
must have recourse to books and source material devoted entirely to 
genetics. Even with such aids much that is new and illuminating in 
this rapidly developing science will be found wanting, 


Castle, W. E., Genetics and Eugenics, 3rd ed., Harvard University Press, 1924. 

An authoritative summary by a pioneer in genetics. 
Crew, F. A. E., Animal Genetics, Edinburgh, 1925. 

The way a brilliant Scotchman sees heredity. 
Dunn, L. C, Heredity and Variation, The University Society, 1934. 

Brief and very readable. 
Jennings, H. S., Genetics, W. W. Norton & Co., 1935. 

Particular emphasis upon the chromosomal aspect. 
Morgan, T. H., The Theory of the Gene, Yale University Press, 1926. 

The statement of a Nobel prize winner. 
Schwesinger, G. C, Heredity and Environment, The Macmillan Co., 1934. 

Emphasis upon the genesis of psychological characteristics. 
Sinnott, E. W., and Dunn, L. C, Principles of Genetics, 2nd ed., McGraw-Hill 

Book Co., 1932. 

A widely used text. 
Snyder, L. H., The Principles of Heredity, D. C. Heath & Co., 1935. 

A very excellent up-to-date book. 
Walter, H. E., Genetics, 3rd ed.. The Macmillan Co., 1930. 

An elementary presentation. 
Wilson, E. B., The Cell in Development and Heredity, The Macmillan Co., 


A masterly storehouse of reliable information. 

H. W. H. — 31 





Preview. The stretch of time • Measures of time • Kinds of fossils • 
Fossils as time markers • The testimony of extinct types • The role of pale- 
ontology • Suggested readings. 


There are two things with which living creatures are inseparably 
involved and from which there is no escape, space and time. 

Although everyone has a working idea of what is meant by these two 
common words and uses them freely and constantly in all sorts of con- 
nections, it is somewhat surprising how difficult it is to define them 
satisfactorily without making use of other words that require definition 
as well. Try it ! Just what is time ? Do not resort to the dictionary 
until you are willing to give up. You will probably find the dictionary 
disappointing. Is time, perhaps, that particular bit of eternity to 
which we can set limits? If so, what is eternity? 

There are two sciences in this connection that are profitable to 
explore, if only to enlarge our intellectual sky lines. The first and 
older science is Astronomy, which serves to expand our ideas of space, 
and of which man alone can have any inkling. The second is Paleon- 
tology. Although this has been developed more recently, it is never- 
theless concerned with very old things. One benefit to be gained from 
the study of paleontology is that it stretches, and makes more spa- 
cious, our concept of time. 

It is not the purpose of this section to present an outline of paleon- 
tology, but simply to consider very briefly the relation between time 
and living things. The role of living things with reference to space 
has already been touched upon in unit II under the title, "The 
Biological Conquest of the World." 

The Stretch of Time 

Whatever time is, the geologist has plenty of convincing evidence 
that an enormous amount of it already has been spent upon this earth 
since it became the earth, for time was passing, ''with no vestige of a 




beginning and no prospect of an end," even before the " everlasting 
hills " were born. The geological evidences of the passage of time are 
plain and unmistakable to everyone. 

A visit to the Grand Canyon of the Colorado, for example, and an 
inspection of the gigantic stone book there revealed, with its leaves 
of stratified rock piled one upon another, must impress even the most 

flippant traveler with the 
record of time spent that 
is there displayed. Strati- 
fied rocks made out of 
sediment such as those 
which form the walls of 
that stupendous gorge 
were built up first some- 
what slowly through the 
erosion of land masses, 
then the sediment was 
collected and borne away 
by flowing streams and 
finally deposited bit by 
bit in horizontal beds 
under water. These sedi- 
ments were subsequently 
compressed, cemented, 
and hardened into layers 
of stone, varying in thick- 

Sooner or later there 
might follow the gradual 
shifting of the levels of 
land and water, possibly 
caused by the aging and consequent wrinkling on a large scale of 
the earth's crust. At any rate, whatever the cause, there has resulted 
an eventual submergence here and there of what was once land, as 
well as a slow up-thrust of the neighboring ocean bed to form newly 
emerged land. 

Meanwhile rain fell, not continuously in delugelike floods, but 
from time to time just as it does at present, with considerable intervals 
between the rainy spells. In fact, there is every reason to believe 
that all the processes leading to the formation of sedimentary rocks 

U. S. Geological Survey 

Erosion in the Grand Canyon of Colorado has 
laid bare stratifications of soil formation de- 
posited in centuries past. 


in the past were gradual and time consuming, exactly as they are seen 
to be before our very eyes today. 

Such repeated rainfalls drain down the slopes of the newly emerged 
land, and, after countless contributions from lesser streams, combine 
into rivers which cut slowly into the elevated accumulations, of sedi- 
mentary rock and wear it away. Thus, in the course of long eons of 
time, a river with its abrasive sediment scours out and fashions a 

In the case of the Grand Canyon the rushing Colorado River, now 
down a mile deep from the rim of the gorge, is still grinding away 
unceasingly at its uncompleted task of recording spent time. What 
a majestic open diary of the passage of time ! 

Measures of Time 

The biologist finds it not only convenient but indispensable to 
establish some sort of foot-rule by means of which the continuous 
and incomprehensible past may be divided into understandable por- 
tions. To this end, the stratified or sedimentary rocks of the geologist 
prove to be of the greatest use. Even so, only through much persist- 
ent study by experts has anything like a satisfactory time-scale been 

Sedimentary rocks, for example, sandstones, limestones, and shales, 
do not envelop the entire earth in continuous layers in the way that 
an onion is made up. They occur only in patches here and there, 
where once was water in which they could be deposited from the 
surrounding land masses. However, when the various patches of 
sedimentary rocks the world over are examined and compared, it is 
quite possible to piece them together, like a jig-saw puzzle, into a total 
column of layers one above the other. 

For purposes of identification and description, the time consumed in 
the formation of this column may be divided into eras and sub- 
divided into periods. While the opinion of experts may differ with 
respect to the limits and details of these arbitrary divisions of past 
time, there is universal agreement as to their orderly sequence. 
Such a time-scale of eras and periods is given on the following page. 

In this time-scale stratified rocks can be employed as a standard 
of estimation for only approximately the last half of known time, i.e., 
45 per cent. The rocks of the Proterozoic and Archeozoic eras that 
characterize the older approximate half, i.e., 55 per cent of the time- 
scale, are either of the original fire-fused sort which has never been 






Estimated Percentage of 
All Known Time 


Recent (Post-glacial) 


Pleistocene (Glacial) 














Permian (Glacial) 












subjected to erosion and stratification, or those which, even if they 
may once have been sedimentary, have lost their stratified character, 
due to crushing pressure or to transforming association with vulcanic 
forces. Marble, for example, laid down originally in layers following 
the disintegration of calcareous skeletons, or by the deposition of 
dead shells of myriads of microscopic marine organisms, is metamor- 
phosed sedimentary limestone, while quartzite and gneiss are rocks 
that, by the action of heat and pressure, have been made over out of 
sandstone, which was also once stratified. 

Sedimentary biology, or the horizontal arrangement of fossil remains 
in sedimentary rocks, practically begins with the Paleozoic era, 
although there are shadowy evidences, such as the graphite traces of 
primitive seaweeds, showing that life occurred as far back as the 
Archeozoic era. In the rocks of the Proterozoic era have also been 
found scanty traces of calcareous algae, primitive sponges, and shells 
of radiolarians, but most of the remains of life during this enormous 
expanse of time have been obhterated. Only a part of the Proterozoic, 
and some of the Paleozoic, era are represented in the famous walls of 
the Grand Canyon of the Colorado. 



The Pleistocene period, in which modern man finally made his 
appearance, and which probably does not include more than 50,000 
or 100,000 years, is such an insignificant fragment of the whole that 
it is scarcely worth while to attempt to include it in a percentage 
column of known time. 

Kinds of Fossils 

A fossil is an indication of past life, not of recent past life but of 
something that lived so long ago that ordinarily it would be forgotten 
and disregarded entirely, except for the interest of the curious inquir- 
ing paleontologist. 

Fossils are of many kinds. They may be the actual remains of 
organisms preserved indefinitely from decay, as, for example, mixed-up 
bones of struggling animals caught in the ancient asphalt pits at 
Rancho La Brea in California ; insects imprisoned in transparent 

Courtesy of Los Anueles Mixscum 

Skulls and bones of bison, horse, and dire wolf are recognizable in this mass of 
fossil bones ready for removal from one of the tar pits at Rancho La Brea, 



Mammoths preserved in arctic ice. 

amber, which is hardened Oligocene pitch ; or mammoths frozen 
centuries ago in arctic mud and ice, with no opportunity since then 

to thaw out, of which 
at least a score of 
authentic instances 
are known. 

Petrifactions of bone 
or shell or wood are 
another kind of fossils, 
formed by the filtra- 
tion of dissolved min- 
erals into spaces left 
after the decay of the 
original organic mat- 
ter. In such fossils 
the inorganic part has 
resisted disintegration 
long enough to serve as a matrix or a mold, and thus to preserve the 
original shape. Sometimes the mineral replacement of minute parts 
may be so gradual and complete that the bone or shell or tree-trunk 
is said to be "turned to stone," often with histological details faith- 
fully retained. Limestone is often composed of innumerable shells 
of minute organisms, such as foraminifera and the skeletons of corals 
that extract from the water the necessary calcareous materials. 

Still other types of fossils are 
casts and molds in which the 
organisms or parts of them re- 
main undestroyed long enough 
to permit the taking of a perma- 
nent death mask of some kind, 
which is then all that is finally 
preserved. Some beautiful ex- 
amples, which may reproduce in 
great detail the character of the 
original, are impressions of ferns 
and leaves, or of insect wings, 
occasionally to be found when 
shale or slate rock is split open. 
Under favorable circum- 
stances, tracks and trails left by A Paleozoic fernlike plant. 



Milloii li. Wtid 

Dinosaur footprints in Connecticut. 

animals may be preserved, showing that the animal in question was 
once a going concern. Just as rabbit tracks in the snow register 
the fact that a rabbit has 
passed that way, so the 
many stone footprints 
which Professor Hitch- 
cock of Amherst College 
originally cHscovered up 
and down the Connecticut 
Valley are dinosaur auto- 
graphs, signed in the great 
stone book, that record 
who were once travelers 

Particularly curious fos- 
sils are the so-called copro- 
lites, which are hardened 
feces of animals. These, in some instances, by their twisted form, 
give a hint as to the structure of the vanished soft parts of the 
posterior part of the intestine, which were able to shape excreta in 
such a fashion. Some coprolites, furthermore, even enable the 

paleontologist to deter- 
mine the bill-of-fare of 
an animal that lived 
perhaps a million years 

Finally, coal and oil 
deposits, wherever found 
in nature, mark the place 
and time of former vege- 
tative life. 

In all these cases what 
we call a fossil is a 
truthful and undeniable 
witness of the former 
existence of a living thing. 
They are not to be con- 
fused with artifacts which 
are structures fashioned 
by the hand of man. 


( '. .s. Geological .Surrey 

Folded beds of limestone on the south coast of 


Fossils as Time Markers 

Just as the inclusion of contemporary documents of various sorts 
within the corner stone of a building, or the carving of a date over 
the door, indicates the time when the building was erected, so the 
presence of fossils, found embedded within a particular layer of sedi- 
mentary rock, serves to fix the approximate time when the sedimenta- 
tion occurred. 

Since fossils succeed each other over long reaches of time in a cumu- 
lative series, they aid in establishing the date when a particular layer 
of the earth's crust was formed, as was first pointed out by William 
Smith (1769-1839), who succeeded in homologizing certain scattered 
rock formations in England by means of typical key fossils found in 
them. Moreover, the kind of stratified rock in which fossils are 
found in turn helps to determine when the organisms which resulted 
in fossils lived. Thus, the confirmation works both ways. This is 
not as much of a vicious circle as it may seem to be, for the progres- 
siveness, or upward evolution, of organic forms is not taken advantage 
of in estimating the relative ages of different strata until after the 
strata themselves have been surely arrayed, by painstaking obser- 
vation and interpretation, in their unmistakable natural order of 

The Testimony of Extinct Types 

A ruined castle on the Rhine, with broken battlements and tumbling 
towers, is a mute witness to many years employed first in building, 
followed by a probably extended period of occupation, and by a final 
period of gradual decay and abandonment. It is quite unlike the 
flimsy tent of the camper, which is quickly put up at night and taken 
down in the morning. The castle stands for the lapse of time. The 
tent does not. The same story of the flight of time is told more 
emphatically by fossil animals and plants. 

While there have been innumerable individual animals and plants 
that have lived and died in the past, usually without leaving any trace 
of their former existence, there are also whole groups of organisms, 
that is, species, genera, families, orders, and even classes, which have 
likewise become entirely extinct, and are now known to have existed 
only because of the occasionally fossilized remains of their representa- 
tives. To have developed these extensive groups by any process of 
evolution, and then to allow time enough for the bringing about of 



their gradual downfall and elimination, naturally calls for more than 
the work of a day. 

When one visits a museum, like the American Museum of Natural 
History in New York City, and there encounters the unbelievable 
genuine framework of some towering dinosaur, he is compelled to 
admit that it must have taken a great deal of time to evolve such a 
creature by any possible process of slow successive adaptations. 


Comparative sizes of man and dinosaur. 

Moreover, not one kind of dinosaur alone but many diverse kinds, 
which have taken time enough to branch o& from the original stock, 
whatever that was, have, without the least shadow of doubt, also 
lived and died. Probably the slow processes that have led up to such 
bizarre manifestations of former life in many cases ran concurrently, 
like jail sentences, but even so, enormous quantities of time must 
have been demanded for the accomplishment of these known results. 
It does not seem likely that a sane and reasonable Creator ever made 
one of these dinosaurs de novo, "out of whole cloth." They bear every 
mark of having been repeatedly cut over and reassembled out of 
preceding garments. There is no evidence, moreover, that dinosaurs 
came to a sudden catastrophic end all at once. It took long periods of 
additional time finally to undo the gigantic task, and to bring about 
the wreckage and gradual extinction of these elaborate creatures. 

The age-long episode of the rise and fall of the dinosaur dynasty, 
for example, which endured for some millions of years, has been 


repeated over and over again in the case of other animal and plant 
groups. Thus, not only is the fact of the existence of all sorts of 
fossils, marking various remote stages of past life, evidence of the 
vast extent of known time, but also the slow rise and fall of plant and 
animal groups as a whole emphasizes the same point. 

The Role of Paleontology 

To learn the kinds of animals and plants that have lived in former 
times ; to determine just when they lived and what they did ; and 
to find out which lines failed to maintain survivors down to the 
present time, and why, are some of the concerns of paleontology. 

There is a seductive lure in fossil hunting, like that which stimulates 
the prospector for gold, only in the case of the paleontologist it is 
intellectual gold that he is after, the acquisition of which is of much 
more inestimable value than the discovery of the yellow metal. 

For those who care to look into this matter of past life, and for those 
who would like to share some of the joys of the exploring paleon- 
tologist, there follows a short list of books, which is recommended 
to point the way. 


Lucas, F. A., Animals before Man in North America, D. Appleton Co., 1902. 

An excellent popular presentation of ancient animal life. 
Lull, R. S., Fossils, The University Society, 1931. 

A short stimulating introduction to the life of other days. 
Merriam, J. C., The Living Past, Charles Scribner's Sons, 1930. 

On the subject of earth history, the President of the Carnegie Listitu- 

tion can converse with the young as well as with the old. 
Shimer, H. W., An Introduction to the Study of Fossils, The Macmillan Co., 


A valuable textbook of paleontology interpreted through the study of 

existing forms. 
Sternberg, C. H., The Life of a Fossil Hunter, Henry Holt & Co., 1909. 

The adventures of a typical American who hunted fossils when railroads 

were new in Kansas, Texas, and the Dakotas, and Indians were more in 

evidence than automobiles. 



Preview. The universality of change • Adaptations • Making the best 
of it • Kinds of organic adaptations : Structural ; embryological ; physio- 
logical ; psychological ; genetical ; ecological ; physical ; biological • Evo- 
lution • Evolution and miraculous creation • The nature of scientific 
evidence • Evidence from comparative anatomy ; the key to comparative an- 
atomy is organic evolution • From embryology • From classification • From 
distribution • From fossils • From serology • From human interference • 
Environmental theory of Lamarck • Natural selection theory of Darwin: 
Variation ; overpopulation ; struggle for existence ; survival and elimination ; 
inheritance ; isolation • Mutation theory of DeVries • Germplasm theory of 
Weismann • Other theories • Conclusion • Suggested readings. 


Observable inborn as well as acquired CHANGES in animals aiid 
plants, and in their surroundings, necessitate ADAPTATIONS and ad- 
justments on the part of organisms which, if inherited, residt in 

To challenge, analyze, and expand the ideas contained in this 
statement is a large order. It will require full and willing co-operation 
on the part of the reader, who is expected to think as he reads of 
cases from his own observations and experience that bear upon the 
general propositions advanced. 

It is freely admitted that, with such an ambitious thesis as this, 
one is tempted to take now and then to the aerial route of specula- 
tion, and to generalize with panoramic views of the w^liole, when to 
particularize with illustrative details might be more illuminating and 
to the point. The contributing reader is consequently hereby warned 
in advance to keep one eye at least on the solid ground of fact below, 
whenever, by flights of fancy and theory, he finds himself being 
hurried to his destination by the more rapid and less substantial air 
route of speculation. 

The Universality of Change 

It is a matter of common experience that everything which we can 
observe about us eventually undergoes change. 



Although it may be necessary to extend the duration of observa- 
tion in order to detect the occurrence of something different as 
happening, nevertheless, it always comes about in the end. The 
apparently stationary hour-hand of a clock, for example, is known to 
shift its position during the day, in spite of its appearance of stand- 
ing still. 

That no structure or action remains constant and enduring is 
particularly evident in living things, in which change is inevitable 
from the cradle to the grave, not only in mankmd but also in the 
daily life of every animal and plant. 

Furthermore, if it were possible to take a complete census of all 
the different kinds of organisms represented on the earth today, for 
comparison with similar censuses taken during the different geologi- 
cal periods, sweeping changes in the character of whole groups would 
at once be apparent. Even with the partial census which biologists 
have been able to make of organisms laiown to have existed in the 
past, as contrasted with the catalogue of living forms thus far dis- 
covered, it is proven without a doubt that the Law of Change is now, 
and always has been, everywhere in constant operation. 

The causes, or sequences of events, leading up to all sorts of changes 
are naturally diverse and numerous. With organisms they may be 
inborn, that is, genetic in character, or largely external and environ- 
mental, but in any case, the fact of change with its consequences 
is observable and can be analyzed, even though the underlying 
causes that bring these changes about are often uncertain and un- 

Sometimes changes, in themselves slight, may have far-reaching 
consequences. For example, when single-celled organisms, accus- 
tomed from time immemorial to divide periodically each one into 
two individuals, discovered the great advantages of partnership and 
remained attached to each other after fission occurred, instead of- 
separating and going their independent ways, right then was born 
the pregnant idea of tissues and organs. The device of cell multipli- 
cation opened up consequent possibilities of the working together of 
parts for greater effectiveness, through the fertile principle of "divi- 
sion of labor." This change was a great historical event in the world 
of life, with extensive sequels. 

Again, when by gradual changes the shift from a single parent to 
the sexual method of two parents came about, another great bio- 
logical epoch began in which,' by utilizing two hereditary streams 


instead of one, the possibilities of the offspring of successive genera- 
tions were more than doubled. 

Take one more illustration of a series of changes that has altered 
the whole course of subsequent biological events. The bilateral sym- 
metry of locomotor animals, that is, those having a head end and 
right and left sides to the body, was preceded by the radial symmetry 
of attached forms like Hydra, an arrangement making it possible 
from a point of anchorage to explore the surroundings for food in 
every direction without the machinery of locomotion. When ani- 
mals with radial symmetry become free-swimming, like jellyfish 
and sea-urchins, they go at random in any direction. It is only 
after one definite part in the circumference of a radially symmetrical 
animal constantly takes to leading the way that a head end is initi- 
ated, with a bram center to direct the increasing activities of the 
changing animal. The connecting hnks in this chain of changes 
between radial and bilateral animals are to be seen in certain tur- 
bellarian flatworms, whose fundamental plan is the same as that 
which would be formed if a radial jellyfish were stretched out length- 
wise with one horizontal axis elongated, thus forming a polar arrange- 
ment with head and tail ends. It may be a long call to return thanks 
to these lowly creatures for discovering the advantages of a head with 
a directive brain in it, but perhaps it is not too late at least to register 
oiu' gratitude. 

There is much variation in the degree of plasticity shown by 
organisms and in the range of changes which they undergo. Some 
forms, like certain brachiopods and shell-bearing protozoans of the 
deep sea, are so well fitted to the constant habitat in which they 
five, where there is no variation of pressure or temperature, and 
where no day or night intrudes upon their tranquillity, that those 
living now have not changed perceptibly in appearance from their 
extremely ancient forebears. 

On the other hand, in the strenuous environment of the tidal zone, 
where land and restless waters meet, the inhabitants are kept con- 
stantly busy and alert in matching structural and functional changes 
with insistent and recurring changes in their surroundings. Thus it 
is that changes in the environment necessitate continuous adjust- 
ments on the part of plants and animals. Whatever may happen 
meantime to the individual actor, the show must go on. This tradi- 
tion of the dramatic stage is quite as true also for the larger stage 
of changing life. 


Making the Best of It 

Adaptations are biological charigcs that organisms make in adjusting 
themselves to physical changes which constantly occur in the environ- 
ment. Here are two variables to analyze and consider, namely, 
changes made by the organism, and those that come to pass in the 
environment of the organism. How do they interact? 

Organic adaptations vary widely in the degree of perfection. They 
may be incipient, partial, and ineffective, or, at the other extreme, 
they may have gone so far as to result in overspecialization. This 
latter condition is frequently dangerous to its possessor, since the 
specialist, having all his eggs in one basket, cannot help sacrificing 
some of the saving adaptability necessary to meet a changing turn 
of the environmental wheel. This is particularly true in human 
affairs. The hobo who is limited to snow-shoveling is unemployable 
in the summer season. 

Many instances of adaptation are entirely obvious. Others are 
obscure and speculative, but in any case the extensive gallery of 
common adaptations furnishes abundant and intriguing material for 
the biologist. "Maeterlinck's essay on the adaptations of the bee," 
says Henshaw Ward, "makes the Arabian Nights seem flat." 

Kinds of Organic Adaptations 

The classification which follows is entirely arbitrary and by no 
means complete. It is simply an attempt to arrange certain cate- 
gories of adaptation temporarily for purposes of description. There 
is such a wealth of illustrative material that it is almost hopeless 
to attempt to pick and choose. Consequently, resort will be made 
more to suggestion than to detailed elaboration of particular cases. 
Here is an excellent opportunity for the reader to fill in omissions 
with supplementary material of his own. 

Structural Adaptations 

The elaborate mouth-parts of insects are all designed apparently 
on the same fundamental plan, but this plan is carried out quite 
differently in the "tobacco-chewing grasshopper," which feeds on 
\'egetation, and in the prodding mosquito, that sucks blood out of 
protesting humans. 



The size of an animal may be in itself a structural contribution to 
success in life. A single horse that weighs a ton and a ton of mice 
both require in general the same sort and amount of food, but the 

.-..secor^d y77axilla 

maxillary I 


— )T7axillar/ palp 

Second, maxilla 

The cockroach (left) and the female mosquito 
(right) inherit a homologous set of mouth parts, 
which have become considerably modified to meet 
the conditions imposed by different functions. 

mice stand the better 
chance of getting about 
and securing food 
necessary for main- 
taining a ton of proto- 

Some other random 
suggestions of examples 
of structural adapta- 
tions are radial sym- 
metry in sessile ani- 
mals, the histological 
structure of leg bones 
adapted to bear body 
weight, the handy, 
prehensile tails of South 
American monkeys, the 
sharp claws of certain bloodthirsty carnivores, the sticky protrusible 
tongue of ant-eaters, the snowshoelike feet of the Mexican jacanas, 
which get their insect food while skipping lightly over floating lily- 
pads, the elongated snouts of chestnut weevils that have the problem 
of spiny burrs to solve, and the shoelike hoofs of heavy ungulates. 

Embryological Adaptations 

Reptiles and birds, that hatch by breaking through an enclosing 
eggshell after making a preliminary start in life, and mammals, which 
go through the early stages of their development in safety within 
the mother's body, have to be fitted successively for two quite dif- 
ferent sets of conditions. Such embryos, during the period of their 
imprisonment, employ two notable adaptive devices, the amnion and 
the allantois, which are discarded upon emergence. The amnion is 
an enveloping antenatal robe, filled with fluid, within which the deli- 
cate, rapidly growing embryo floats, protected from mechanical shock 
and from growth-checking exposure to a dry world. It is an adapta- 
tion to land life quite unnecessary in the case of fishes and amphibians, 
whose usually shell-less eggs are deposited in the water during the 
period of their preliminary development. The allantois is a make- 
u. w. H. — 32 


shift respiratory device, effective within the eggshell, or, in the case 
of mammals, in the uterus of the mother, before it is possible for the 
lungs of the young individual to take over the task of respiration. 

Curiously, the embryologist often has to describe a different organ 
from that which the anatomist cites for the accomplishment of the 
same function in the animal body. An adult anatomical structure 
over and over again succeeds a transitory embryonic forerunner. 
Thus, temporary nephroi are followed by permanent kidneys ; downy 
lanugo is replaced by hair, more or less permanent ; the gauzy em- 
bryonic covering of epitrichium gives way to the adult skin ; there is 
a succession of teeth ; the intestine replaces the yolk sac ; the primi- 
tive vitelline circulation gives over its temporary emergency service as 
the systemic circulation arises ; the two-chambered, fishlike, embry- 
onic heart of the mammal becomes replaced by the three-chambered 
amphibian stage before the final four-chambered heart is established ; 
while for the embryonic vertebrate skeleton, patterned largely in 
cartilage, there is eventually substituted a more efficient bony frame- 

All these illustrations and many more indicate adaptations to 
adult life, following the different preliminary conditions imposed by 
embryonic existence. 

Physiological Adaptations 

When for any reason one kidney is removed, or put out of com- 
mission, the remaining kidney assumes the double task and increases 
correspondingly in size. This is a physiological adaptation. 

The apparatus of the sweat glands is a physiological device enabling 
mammals to adjust themselves to the greater variation in tempera- 
ture which occurs on land, as contrasted with that to which sweatless 
water animals are exposed. 

A grasshopper, with its large immovable compound eyes facing 
everywhere except below where the mouth is located, is not able 
to see the food that it is eating, so tactile palps, that are sensory 
modifications of the mouth-parts, become adapted to function in- 
stead of eyes in the examination of food. 

Darwin cites the strange case of a certain species of parrot in New 
Zealand which, after the introduction of large herds of sheep into 
its habitat and after somehow getting the taste of blood, gave up 
its former vegetarian habit of life and became a murderous, blood- 
thirsty carnivore, living entirely on the flesh of sheep. 



The activation upon occasion of the mammary glands, as well as 
the formation of antitoxins, and the acquisition of immunity to cer- 
tain disea"ses, are further examples of physiological adaptation to 
bodily needs. 

Psychological Adaptations 

Patterns of instinctive behavior which adapt an untaught cater- 
pillar to spin a cocoon of a definite sort, or direct an insect to lay 
its eggs upon a particular food-plant specific for its offspring which it 
will never see, as well as the inner urge that causes birds, and certain 
other animals, to migrate periodically, may possibly be cited as ex- 
amples of psychological adaptation. At any rate, the exercise of the 
nervous system that enables actors to repeat their lines unconsciously 
in dozens of plays, and by which musicians are able upon repetition 
to perform complicated and extensive scores without conscious effort, 
comes close to being an adaptation of a psychological nature. 

Genetical Adaptations 

Adaptations frequently work for the benefit of the species rather 
than for the welfare of the individual. 

The clever dandelion grows close to the ground in a flat rosette. 
This habit enables it to escape from browsing animals to a consider- 
able degree and to with- 
stand trampling. Its yel- 
low blossom lies low and 
bides its time until all is 
ready and then, just at 
the critical time, the hol- 
low stem shoots up like a 
fire ladder into the air 
almost overnight, bearing 
a cluster of white-tufted 
aviating seeds that are 
all prepared for distribu- 
tion. They are so deli- 
cately poised, pincushion- 
fashion, in their elevated position that the slightest breeze is 
sufficient to waft them on their way. 

All the many reproductive modifications, both structural and func- 
tional, which are involved in the fertilization of animal eggs as well 

^ — "€_. 

The adaptable dandelion, providing for itself 
and for its progeny. 


as in the formation of spores and seeds of plants, make up a world 
of adaptations in themselves that, since they have to do with the 
maintenance of species, may be considered as genetic in character. 

Another genetic adaptation, that is so universal as to be properly 
regarded as a law of nature, is shown in those animals and plants 
whose reproductive products are particularly exposed to great perils, 
and which in consequence produce a correspondingly larger number 
of eggs or seeds than do those whose offspring are better safeguarded. 
Nest-building in all its diverse forms, as well as the multitudinous 
devices employed by plants to secure pollination and the dispersal 
of seeds are further examples of genetic adaptation. 

Ecological Adaptations 

Any group of varying organisms, adjusted more or less imperfectly 
to a certain habitat, tends in the course of time to branch out and 
to occupy different neighboring habitats. Adaptation to new habi- 
tats is ecological adaptation. This type of adaptation has been 
somewhat amplified in unit II, on the "Biological Conquest of the 
World," and innumerable examples of conditions met in the great 
primary habitats of water, land, and air will come to the mind of 
every observing reader. If animals could talk and had intelligence 
enough to know what to say, think what tales they could rehearse 
of the troubles they have known and the satisfactions they have 
experienced in becoming adapted to their particular niches in nature ! 
Imagine, for instance, a Thousand and One Nights spent in listening 
to such representative spokesmen as the hermit crab, the nocturnal 
earthworm, the carrion beetle, the golden plover, the sperm whale, 
the liver fluke, the snake in the grass, and the bullfrog on the bank. 
Even the plant world could be profitably admitted to take part in 
such a symposium. For example, what might the northern pine 
and the southern palm, the roadside weed and the head of rice, to 
say nothing of the bacteria of " Typhoid Mary," have to tell us of 
ecological adaptation ! 

Physical Adaptations 

Certain factors in the make-up of the physical environment, such 
as temperature, pressure, and light, set limits within which organ- 
isms must adapt themselves in order to live. The range of livable 
possibilities imposed by these physical factors varies greatly with the 
organism. Professor Brues of Harvard reports that there are algae 



. /(/// "' \nturiil Histnry 

A deep-sea group of fish : left, Macrurid, 
and right, Brotulid. 

and the larvae of certain insects 
that are adapted to live in hot 
springs, the temperature of which 
is sufficient to coagulate the pro- 
toplasm of most organisms. Some 
animals, frogs for example, can 
survive a degree of freezing that 
would be fatal to others. Trees 
and woody shrubs can successfully 
withstand low temperatures that 
cause most of the less woody 
plants to succumb. The varying 
range of frost and heat to which 
plants of different sorts are sus- 
ceptible is common knowledge to 
every farmer. 

Adaptive devices, such as gem- 
mules of fresh-water sponges, the 
winter eggs of daphnids, and the 
statoblasts of certain bryozoans, 

carry these lowly animals through the freezing winter into another 
summer quite as effectively as the various coats and shells of seeds 
and nuts. Again, warm-bloodedness is an adaptation fitting birds 

and mammals to cope successfully 
Avith great and often sudden shifts 
in temperature on land, to which 
the cold-blooded inhabitants of 
water are not subjected. 

Pressure is another physical 
factor to which every organism, 
in order to live, must be adjusted. 
Most animals and plants living on 
the surface of the earth, beneath 
a uniform blanket of atmosphere, 
are not subjected to much differ- 
ence in pressure, but deep-sea 
fishes, with an additional weight 
of superimposed water, have quite 

American Museum of y.aturnl Histunj ^ ^ ; -i 

Oceanic angler fish, Linophryne. The a different problem to meet. This 
beard is probably luminous. particular form of adaptation 


consists not so much in protective envelopes of one kind and another, 
that must inevitably be crushed, as in the development of easily per- 
meable tissues through which the pressure is equalized. 

Light is essential to photosynthetic plants. These exhibit many 
adaptations by way of the arrangement and form of their leaves to 
secure adequate exposure to light. There are many kinds of animals 
on the other hand, Uke cave-dwellers and deep-sea forms, as well as 
fungi among plants, that can dispense with light entirely. In such 
animals, the eyes and other adaptations to a world of light and shadow 
are either entirely wanting or have become degenerate. To com- 
pensate animals that live in darkness for their loss of light, tactile 
devices of various sorts develop, enabling them to find their food 
and to accomplish the business of living, while in the case of plants 
the saprophytic method of living a chlorophyll-less life is adopted. 

The reflex mechanism in the iris of the eye by which the size of 
the pupil is made to vary and the amount of light admitted to the ret- 
ina is regulated is a beautiful adaptation of the organism to amount 
of light. In fact, the whole vertebrate eye is an exquisite example of 
cumulative organic adaptation to the environmental factor of light. 

Biological Adaptations 

The association of organisms with each other gives rise to a great 
variety of biological adaptations, such as symbiosis, commensalism, 
saprophytism, parasitism, gregariousness, and social life. 

Flowering plants evolve ways of attracting the visits of insects 
and of inveigling them to transfer pollen in the production of seeds. 
Insects in turn are so modified as to take advantage of what the 
flowering plants offer them by way of nectar and other desirable 
forms of food. It is significant that flowering plants did not develop 
in geological time until after insects appeared. 

Carnivorous hunters are fitted to pursue their prey, and the hunted, 
by developing speed in flight or by wits with which to outguess the 
pursuer, are adapted to escape. Mother Nature impartially gives 
both the hunter and the hunted a sporting chance. 

Through protective coloration and camouflage, by bluffing with 
warning colors, or by intimidating behavior, some animals escape 
their enemies, while others are blackmailed into surrendering to their 
captors a part of themselves and escaping with a viable residue, 
having in reserve the adaptive resource of regeneration of lost parts. 

There is a curious European toad, Bombinator igneus by name. 


that has developed a bright scarlet belly and a taste nauseous to 
birds, in the course of its adventures in adaptation to an environ- 
ment unfortunately shared with toad-devouring storks. When a 
stork by chance seizes a bombinator, the victim is usually ejected 
because of the acrid taste produced by the skin-glands of the toad. 
Neither stork nor toad gains anything by this performance, and, to 
lessen the likelihood of its occurrence, whenever a stork swoops down 
upon a pond where bombinators are socially congregated around 
the margin, the little animals quickly flop over and expose their 
conspicuous scarlet bellies to view, thus furnishing red-light signals 
for the stork to "stop," before an accident happens that both would 

A great variety of defensive devices, such as armor, shells, spines, 
fangs, horns, hoofs, and stingers, have been developed in different 
animals. The nonchalant skunk is so well assured by its defensive 
fire extinguisher mechanism that it does not run away from danger. 
This may be why so many of them, upon the intrusion of the jugger- 
nautlike automobiles into their habitat, are run over and killed, 
while the more cautious rabbits and other wayside animals escape. 

Plants display a great range of biological adaptations in the attempt 
to defend themselves against browsing herbivores and devouring 
insects. Some plants have bitter or unpalatable chemical substances 
lodged in their tissues. Cacti and thistles bristle with discouraging 
spines, while shrubs and trees are provided with tough resistant bark. 
Desert plants develop fuzzy hairs that hinder transpiration, or are 
coated over with an impervious varnish that tends to prevent the 
loss of water from their tissues. 

There may be still other categories of adaptations, and some of 
the foregoing examples could perhaps be assigned to other classifica- 
tions, but the undeniable fact remains that adaptations of infinite 
variety characterize the living world about us. 


Evolution and Miraculous Creation 

Perhaps two of the most famous scholars of the ancient English 
universities of Oxford and Cambridge were John Milton (1608-1674) 
and Charles Darwin (1809-1882). Each wrote an immortal book 
upon the same epic theme of how living things in this world came 
to be as they are. Mi