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Marine Biological Laboratory 

R...,veH Jnly 19, 19 47 

Accession No. 

^. n McGrsYZ-Hiil Book Co., Inc. 

^'"'" ^y— ^ Ngv r York City — 




A. FRANIvLIN SHULL, Consulting Editor 


Selected Titles From 


A. Franklin Shull, Considtiyig Editor 

Baitsell • Human Biology 

Breland ■ Manual of Comparative Anatomy 

Burlingame ■ Heredity and Social Problems 

Chapinan ■ Animal Ecology 

Clausen ■ Entomophagous Insects 

Frost • General Entomology 

Goldschmidt • Physiological Genetics 

Graham ■ Forest Entomology 

Haupt ■ Fundamentals of Biology 

Hyman ■ The Invertebrates: Protozoa through Ctenophora 

Johannsen and Butt ■ Embryology of Insects and Myriapods 

Metcalf and Flint ■ Insect Life 

Mitchell ■ General Physiology 

Mitchell and Taylor • Laboratory Manual of General Physi- 

Pearse ■ Animal Ecology 

Reed and Young ■ Laboratory Studies in Zoology 

Riley and Johannsen ■ Medical Entomology 

Rogers ■ Textbook of Comparative Physiology 

Laboratory Outlines in Comparative Physiology 

Senning ■ Laboratory Studies in Comparative Anatomy 

Shull ■ Evolution 


Principles of Animal Biology 
Shall, LaRue, and Ruthven ■ Principles of Animal Biology 
Simpson and Roe ■ Quantitative Zoology 
Snodgrass ■ Principles of Insect Morphology 
Storer ■ General Zoology 

Laboratory Manual for General Zoology 
Van Cleave • Invertebrate Zoology 
Welch • Limnology 
Wieman ■ General Zoology 

An Introduction to Vertebrate Embryology 
Wolcott ■ Animal Biology 

There are also the related series of McGraw-Hill Publications in 
the Botanical Sciences, of which Ednuiml W. Sinnott is Consulting 
Editor, and in the Agricultural Sciences, of which Leon J. Cole is 
Consulting Editor. 


^ ?z 





Professor of Zoology in the University of Michigan 



Professor of Zoology in the University of Michigan 


President of the U niversity of Michigan 


Sixth Edition 
Second Impression 



principles of animal biology 

Copyright, 1920, 1924, 1929, 1934, 1941, 1946, by the 
McGraw-Hill Book Company, Inc. 

printed in the united states of AMERICA 

All rights reserved. This book, or 

parts thereof, may not be reproduced 

in any form without permission of 

the pubiishers. 

To the Teachers of 



The changes introduced in this sixth edition are more than usually 
varied. While none can be regarded as radical, they affect in important 
ways nearly every part of the general plan. The book is still devoted to 
principles; indeed, the changes appear even to emphasize its devotion to 
fundamental concepts. 

If any one statement can be made which would characterize much of 
the alteration now made, it is that the treatment of function has been 
increased or clarified or thrown into relief by emphasis. Such changes 
relate, among others, to enzymes, photosynthesis, oxidation, muscle 
action (including cardiac), breathing movements, transfer of respiratory 
gases, blood composition, the clotting process, kidney function, vitamins, 
endocrines, the placenta, and reflex arcs. The authors have not hesitated 
to give chemical formulas and reactions that beginning students need not 
be expected to remember or reproduce, because these exact forms of 
expression carry conviction concerning the precision of present knowledge 
Avhich no more general statement can produce. 

Greater clarity of exposition has been sought at many places by illus- 
trations and slight changes of language or inclusion of features not 
heretofore expressly described. Comparisons that were formerly illus- 
trated by figures borrowed from research contributions are now in several 
instances portrayed by simplified diagrams placed side by side with the 
contrasts indicated. Among the phenomena thus treated are symmetry, 
centralization and cephalization of the nervous system, endocrine secre- 
tions, the hydroid metagenetic cycle, and the evolution of living things 
in geological time. More explicit description is the method adopted for 
the types of circulatory and excretory systems, for the operations of the 
kidney, for the biogenetic law, and others. 

Order and emphasis have occasionally been changed at the suggestion 
of teachers elsewhere, even when the authors were not quite convinced 
that the new method was an improvement but could see no objection to 
it. Molecules and atoms have been introduced before protons, neutrons, 
and electrons. The names of the phases of mitosis have been restored 
in the belief that under the guidance of a good teacher cell division will 
still be conceived as a continuous process. Biological terms have been 
introduced in a number of places with the conviction that names some- 
times clarify ideas, simply because terms must have definitions. Yet the 
glossary is today shorter than in the early editions. 



In the treatment of genetics the simple phenomena have been described 
in less space than formerly but, it is believed, with greater clarity. 
Description of the mechanism in advance of its operation, a method 
used with success in genetics courses, should contribute to this result. 
The two linkages have been restored to the general text — at the request 
of teachers and in conformity with the authors' preference. If the work 
in genetics is to be shortened in any institution, this can still be done by 
omitting the later parts of the chapter, for the topics are treated in the 
order of their importance and desirability for beginning students. The 
problems in genetics have been modified to call for precise (usually 
numerical) answers, not for charts or discussions. There is no reduction 
in the thought or organization required of the student in solving them; 
he merely gives one specific part of his conclusion instead of all of it, 
which should facilitate checking his accuracy. 

Among the more general of the other changes should be mentioned the 
addition of marine habitats to the chapter on ecology, a considerable 
extension of the historical treatment in zoogeography, and an enlarge- 
ment of the account of prehistoric man. 

To compensate in part for the increase of space that many of the 
foregoing revisions entail, omissions and condensation have been effected 
elsewhere. The authors will be interested to learn whether the omissions 
are missed. 

One incidental consequence of these extensive revisions is the removal 
of some distinct contrasts of literary style, which are seemingly unavoid- 
able results of joint authorship. While the present style may not be 
better, uniformity of style is surely to be desired. • 

As usual, the authors' colleagues who use this book in an elementary 
course have been generous with suggestions for improvement. Among 
teachers in other institutions who have furnished ideas, special mention 
should be made of Prof. Roy D. Shenefelt, whose recommendations could 
have come only from a well-considered philosophy of teaching. 

A. Franklin Shull. 

Ann Arbor, Mich. 
May, 1946. 


-♦ g^ ^ CONTENTS 


Preface ix 


1. The Growth and Scope of Biology 1 

2. Primary Organization of Living Matter 23 

3. Some Fundamental Physics and Chemistry 30 

4. The Functions of Protoplasm and Cells 39 

5. Cell Division 55 

6. From One Cell to Many Cells 64 

7. Basic Organization of the Metazoa 77 

8. Physical Support and Movement 87 

9. Sources of Energy and Materials 100 

10. Respiration and Release of Energy 113 

11. Transportation System 122 

12. Disposal of Wastes 133 . 

13. Integration of Activities 140 

14. Reproduction 159 

15. The Breeding Behavior of Animals 177 

16. Embryonic Development 193 

17. Genetics 222 

18. Principles of Taxonomy 244 

19. The Groups of Animals 259 

20. Animals and Their Environment 281 

21. Geographic Distribution 307 

22. Fossil Animals 325 

23. Modification of Species 349 

Glossary 369 

Index 407 





When knowledge can be classified and organized on some basis 
which exists in nature, not just in the minds of men, the body of knowl- 
edge so arranged is science, or a science if the field is limited. The devotees 
of science seek to discover the natural principles which control the 
phenomena they observe. The more fundamental the ascertained 
principles are, the more significant the science is. The first step in dis- 
covering principles is usually observation of the facts or phenomena 
which require explanation. Very often the second step is experiment, 
or interference with natural events, with possible alternative outcomes 
in mind. Finally, there is the logical consideration of all facts to see 
what relation exists among them. 

When the phenomena studied are those of living things, the organized 
knowledge is called biology. It is not distinct and separate from other 
sciences, for all life processes are fundamentally physical and chemical. 
Indeed, no science is a province unto itself, for the constitution of matter 
and energy, with which they all deal, is everywhere the same. Each 
division of the field of science emphasizes certain types of phenomena, 
but the more the various fields can be intertwined the more fruitful 
scientific work becomes. 

The several sciences have also been interrelated as they developed 
over the centuries. Let us see how biology has shared in the early stages 
of this growth and what it has come to be in later times. 

Ancient Civilizations. — Among primitive peoples knowledge and 
superstition regarding life came chiefly from three sources: from their 
wonder and awe at the phenomenon of death and customs relating to 
the preservation of the bodies of the dead, from their fear of the great 
wild beasts, and from their attempts to cure disease and heal injury. 
The earliest known civilization is that of Babylon. Medical science, 
which is the form that early biology most often took, made some progress 
there. Clay models of various organs of the human body have been 
preserved, and Babylonian writings show that two kinds of blood, light 
and dark, were recognized. The heart, however, was regarded as the 
seat of intelligence. In Egypt, another very old civilized country, 



embalming of the dead led to a knowledge of anatomy, and an art of 
healing based not upon superstition but upon observation was developed. 
The Israelitic tribes borrowed their scientific knowledge from other 
peoples and clothed it with a religious significance but added nothing to 
the store of biological information. The other great peoples who might 
have contributed to early biological knowledge were interested in other 
branches of culture — the Hindus in mathematics, the Chinese in ethical 
and social problems. 

Early Greeks. — It was among the Greeks, therefore, that biology 
received its first great impetus. The passion of these people for intel- 
lectual inquiry was due partly to their innate qualities but in part to 
the practical absence of powerful restrictive governmental and religious 
organizations. The Ionic tribes, coming into contact with the cultivated 
peoples of the East, through their colonies in Asia Minor, developed the 
earliest natural philosophers. One of these was Thales (about 650-580 
B.C.), who, though he left no writings, is reputed to have regarded water 
as the source of all things, including life. Anaximander (about 611- 
546 B.C.) entertained a theory of the origin of the universe from a vague 
something which he called "apeiron," but his chief concern with biology 
was his supposition that living things arose from mud. First, he thought, 
came the lower animals and plants, and then human beings; but the 
latter were in the form of fish and lived in water. Later these human 
beings cast off their fish form and lived on land. This view of the origin 
of living things was adopted by Diogenes of Apollonia (not the famous 
cynic Diogenes), who conceived that the agent which brought forth 
living things out of the earth was solar heat. Diogenes was the author 
of the earliest known work on anatomy, fragments of which are still 
preserved, and his ideas of human embrj^onic development give evidence 
of being based on dissection. 

Some of the more important of the remaining Greek natural phi- 
losophers came from the colonies of the west. Xenophanes, who had 
wandered to southern Italy, is chiefly noted for his discover}^ of fossils, 
his recognition that they were animal remains, and his conclusion there- 
from that in some cases what are now mountains were once under the 
sea. He died about 490 b.c. Another western Greek was the braggart 
Empedocles, in Sicily, who lived about the middle of the fifth century 
before Christ. Among the many things which he boasted of doing, he 
appears actually to have rid a neighboring town of malaria by draining 
the district. On the theoretical side of his biology, he conceived living 
things to have arisen out of the earth, plants having come first. Animals 
arose in the same way, but in pieces. Separate limbs, trunks, etc., arose, 
kept apart by the force of hate. When love triumphed, these members 
joined in accidental manner. Some such combinations were malformed 


monsters incapable of life; others, more fortunately constructed, survived 
and gave rise to the animals of today. The blood he regarded as the seat 
of intelligence, the eye he likened to a lamp, and respiration he thought to 
occur partly through the skin. 

Democritus. — More important for natural science than any of his 
predecessors was Democritus (Fig. 1) who was born about 460 B.C. 
Chaste in morals and temperate in habits, he lived to the ripe age of a 
century. Curious about the world, Democritus spent his patrimony in 
travel, then lectured for pay to avoid the serious Greek charge that he had 
wasted it. His interests were exceedingly inclusive, and he is best known 
for a materialistic ("atomic") theory 
of the universe, some features of which 
have a distinctly modern flavor. 
While it was through his general 
philosophy that he most influenced 
subsequent thought, not a few strictly 
biological concepts are found in his 
writings. He distinguished types of 
animals differing in the quality of their 
blood, a basis of classification later 
adopted by Aristotle. In embryonic 
development, he supposed the external 
organs arose first, the internal struc- 
tures later. He knew that mules are 
sterile and conceived an anatomical 
reason for it. He regarded the brain as 
the organ of thought, the first of the natural philosophers to do 
so. In his more subtle theoretical ideas, Democritus was strictly 
materialistic; even the soul was regarded as a material thing, consisting 
of globules of fire which impart movement to the body. He represents 
the climax and close of the first scientific period of Greek philosophy, 
which was an era of search for purely natural causes. 

Hippocrates. — A contemporary of Democritus was Hippocrates, 
the Father of Medicine. What Hippocrates actually wrote is not cer- 
tainly known. A collection of about a hundred works has been attributed 
to him, but many of these were probably not his. His interest was 
scarcely scientific, but rather in the healing of men; yet in one of the 
works on diet in the collection is a reference to an attempt to classify 
animals. While the study of medicine is biology, Hippocrates treated 
it as an art; his descriptions of operations are models of clarity. The 
social and moral responsibilities of physicians engaged his attention, 
and a famous oath administered to medical graduates was based on his 

Fig. 1. — Democritus. 


Aristotle. — A reaction set in against the materialistic conceptions 
of Democritus and others. Philosophy came to be dominated by 
Socrates, who was interested in ethics, and by Plato, who found true 
reality in the world of abstract thought. The latter says expressly 
that no true knowledge is to be attained through observations of the 
senses. One leading philosopher who came under Plato's influence was 
Aristotle (384-322 b.c.) (Fig. 2), the greatest of the early biologists, to 
whom the essence of living things was their form. Everything that 
happens, he taught, is due to a supreme intelligence, everything is done 

Fig. 2.— Aristotle, 384-322 b.c. {From Heklcr, "Greek and Roman Portraits," G. P. Pul- 

nam,'s Sons.) 

for a purpose, and the primary purpose in nature is the development 
of a higher form. As a result of this continuing purpose, there has been 
an evolution from lower types to higher ones. 

Despite his leaning to supernatural causes, Aristotle made some excel- 
lent observations in biology and sought to organize them wherever possi- 
ble. He classified animals according to their mode of life and their 
structure and knew over five hundred kinds, all Greek; those from other 
countries he knew only from descriptions. He insisted that the study of 
anatomy should be comparative, which is a fruitful procedure at the 
present time. The heart was regarded as the organ of the soul and 
intelligence; here Aristotle drops behind his predecessor Democritus. 
Digestion was to him a process of ''cooking." Nerves were confused with 
tendons; the brain was thought to be cold and the spinal cord hot. Fleas 


and mosquitoes were held to arise by spontaneous generation out of putre- 
fying substances, while other insects originated through sexual reproduc- 
tion. His descriptions of the embryonic development of animals, mostly 
the chick and certain marine forms, are rather accurate. He devised an 
ingenious scheme of heredity and regarded temperature as a sex-determin- 
ing agent. He believed that the future of a man could be read from the 
lines of his palms and that flat-footed people have treacherous disposi- 
tions. Indeed, a curious mixture of truth, error, and superstition! 

Aristotle's greatness in biology lay not so much in his discoveries as 
in the fact that he devised a system of thought that dealt with the entire 
realm of living things. He has long been credited with insisting upon 
the inductive method, in accordance with which one first collects facts 
and then draws conclusions based upon them. Other philosophers had 
been prone to reach a conclusion first and then to decide what the facts 
must be to accord with the adopted principle. Aristotle did more than 
urge the inductive method, he used it — part of the time. In general, his 
work in natural history followed this method. For his scheme of the 
universe, however, he had not enough facts at his disposal, and here he 
drew upon fancy. As a consequence, his concept of the cosmic system 
had what modern biologists consider a serious fault in that it called for 
the guidance of nature by an outside intelligence. Democritus had come 
nearer than he to the modern scientific view in that he postulated a natural 
necessity which determined the course of events; but Democritus had no 
inclusive theory relating to living things in particular. 

Pliny. — At the time of Aristotle's death, Greek culture was already 
declining, so that the accomplishments of this naturalist-philosopher 
represent the highest attainment of antiquity in most fields of science. 
His successors and followers include Theophrastus, generally regarded as 
the founder of botany, and a number of others by none of whom was any 
notable advance made. Specilized phases of biology fared a little better, 
particularly anatomical studies at Alexandria. 

Rome did not advance far until a much later time. Her chief biolo- 
gist of this period was Pliny (a.d. 23-79), who is best known through his 
"Natural History" of 37 volumes. This work was a curious compilation 
of all the stories of nature which the author was able to gather. Nothing 
appears to have been rejected, so that fantastic fables abound, along with 
reliable accounts of the habits of animals, their utility, the particulars 
of cattle husbandry, etc. Pliny had recourse to two thousand books 
in the preparation of his "Natural History," and for fifteen centuries 
thereafter this work supplanted all of them in the popular mind as the 
source of information regarding natural objects. The author did not, 
however, add anything of importance to the store of knowledge by his 
own observations. 


Galen. — Rome, though succeeding to a dominant position in world 
affairs, did not foster learning in scientific fields. Instead of an intellec- 
tual revival during her period of prosperity, there was a notable decline. 
Pliny lived in the midst of this decline. The last great biologist of 
antiquity was Galen (131-210?), a physician living in Rome but of Greek 
parentage. He dealt mostly with human anatomy and reveals a pro- 
found admiration for the creator of so marvelous a mechanism. Every 
organ had its use and was constructed on the plan best calculated to 
serve that end. He was obliged to study these organs mostly in other 
animals, for dissection of human bodies, once permirjsible, was in Galen's 
time forbidden. When he describes the human hand, it is obvious that 
the object before him is the hand of an ape. His errors are mostly 
traceable to this necessity of using other animals. 

His accomplishments are numerous, such as his proof that the arteries 
and the left side of the heart contain blood, instead of air as others sup- 
posed, and his inference that the arteries and veins must be connected. 
He seems not to have been fully appreciated in his own time, yet Galen's 
books were for many centuries thereafter the standard of reference. 
They were used in the medical schools, where anatomy was taught from 
the desk with little or no demonstration, and modern criticism has given 
to him a high measure of praise. 

The Dark Ages. — The thousand years and more following Galen's 
time constitute the dark ages for biology as for other fields of learning. 
Among the Arabs, who were dominant in the East, mathematics, astron- 
omy, and chemistry made some advance, but writings in the field of 
biology were mostly commentaries on the works of Aristotle and of Galen. 
The division of the Roman Empire and the ravages of migratory peoples 
in the West were not conducive to learning. Universities arose beginning 
about the eleventh century, but these came to be controlled by religious 
orders. The churchmen, finding a powerful ally in Aristotle's conception 
of the earth as the center of the universe and his belief in a dominating 
intelligence directing natural phenomena, turned the reverence in which 
ancient philosophy was held to their own advantage. It took little guid- 
ance from them to ensure that biological inquiry should consist merely 
of commentaries on the writings of Aristotle, with no effort to ascertain 
facts afresh. The views of the Greek natural philosopher were accepted 
as correct even where simple observations could easily have proved them 
wrong. The few books about animals which appeared in this era, aside 
from the commentaries mentioned, contained only entertaining stories 
and notes on the usefulness of animals to man. 

To deliver biology from the dominance of Aristotle, it was necessary 
to destroy his system of thought. Aristotle, as was pointed out earlier, 
based his theory of a universal order on an outside intelligence which 


directed the transformations of matter. This outside intelligence was 
naturally not subject to inquiry, and it was this feature of the Aristotelian 
doctrine which won to him the support of the conservatives of the Middle 
Ages. The uprooting of this system of thought required time, and it was 
not until the seventeenth century that other well-defined systems of 
philosophy replaced it. In the meantime biology was struggling up 
out of the inaction of the Middle Ages, through the period of the 

The Revival. — In the early part of the period of renewed interest in 
learning, several works on natural history appeared, which showed they 

Fig. 3. — Andreas Vesalius, 1514-1564. {From Garrison, "History of Medicine," W.B. 

Saunders Company.) 

were based in part upon observations made by their authors. The leader- 
ship in the revival, as far as it concerned biology, was taken by Andreas 
Vesalius (1514-1564) (Fig. 3), an anatomist. Born at Brussels, he went 
to Paris at the age of eighteen to study medicine and there showed great 
independence and force of will. After several years of practice he was 
called to the University of Padua, in Italy, where everything was favor- 
able to his work. In his teaching he first followed Galen but soon found 
the latter incomplete and in places self-contradictory. He then realized 
tha.t he must teach from his own observation and, to make this possible, 
published two anatomical works which were masterpieces. His over- 
throw of Galen infuriated conservative anatomists, including Vesalius's 



revered teacher Sylvius, himself an anatomist of high reputation. Vesa- 
lius was charged with all sorts of crimes, from being godless and sordid 
to dissecting men alive. This persecution finally drove him to resign 
his professorship, after which he was physician to Emperor Charles 
V. Upon the succession of the less liberal Philip II, Vesalius found small 
opportunity for creative work. He left the court and tried to regain 
his old post at the university but died on a journey to Jerusalem before 
the appointment was made. His ideas of anatomy, and particularly 
of the functions of the organs, were not wholly correct. Some of them 

were borrowed from Galen, whom 
he still admired, and now seem 
absurd. His great contribution 
was his overthrow of authority 
and his return to firsthand obser- 
vation as the basis of knowledge. 

Harvey and the Circulation of 
the Blood. — One of the sharpest 
reactions against the authority of 
antiquity, and one of the most 
hotly contested, was the recogni- 
tion of the circulation of the blood. 
Against the prevailing early view 
that the arteries conveyed air, 
Galen had held that they carried 
blood; but he was never clear how 
the arterial blood became converted 
into venous blood, and in the veins 
he definitely supposed the blood 
to flow in both directions alternately. His views on this question 
were still accepted in the sixteenth century. 

The first recognition that the entire course of the blood is a circulation 
is found in the works of William Harvey (1578-1G57) (Fig. 4), of England. 
He proved that the wall of the heart is muscular and that its contraction 
drives the blood forward into the arteries; in the old theory the heart 
was regarded as passive. By a simple calculation he demonstrated that 
the quantity of blood passing through the heart in a very short time 
exceeded the weight of the whole body and reasoned that new blood could 
not be produced at such a rate. He showed by the swelling of the veins 
below a ligature, and by the point of exit of blood at a wound, that blood 
flows toward the heart in veins and away from it in arteries. He con- 
cluded as a logical necessity that there must be a connection between 
arteries and veins, but without a microscope he could never visually 
demonstrate the capillaries. 

Fig. 4.— William Harvey, 1578-1657 
{From Garrison, "History of Medicine.") 


Besides correcting an ancient mistake, Harvey performed a service to 
biology in making it an experimental science. While others before 
Harvey had occasionally used experiments, he gave the method a strong 
impetus. But while Harvey was modern in his method of solving 
problems, at the same time his concept of life and its manifestations in 
general was no more advanced than was that of Aristotle. 

The Seventeenth and Eighteenth Centuries. — The two centuries 
following Harvey mark a distinct phase in the development of biology. 
The lethargy of the Middle Ages had been definitely cast off, and the 
spirit of inquiry was again prevalent among intelligent people. Two 

Fig. 5. — Two early microscopes. Left, that used by Robert Hooke; right, from eight- 
eenth century. (From "Educational Focus," Bausch & Lomb Optical Co., and American 
Museum, of Natural History.) 

general concepts of natural phenomena arose, one of them mechanistic, 
the other mystical ; and the history of biology ever since has been in part a 
conflict between these two systems of thought, with the former steadily 
gaining ground. The science of chemistry was coming to the aid of 
biology by enabling physiology to seek for purely mechanistic explana- 
tions of life processes. Following Harvey's proof of the circulation came 
the dicovery of the lymphatic system of vessels carrying digested food 
from the intestines to one of the larger veins. The nervous system was 
more thoroughly studied, and the functions of the divisions of the brain 
began to be understood. The contraction of muscles was explained by 
fermentation — incorrectly, but it is significant that the role of chemistry 
in living matter was recognized. However, the early advances were 
mostly in the field of morpholog}'', the science of structure. 


The Microscope. — One important aid to the mechanistic theory of 
living matter was the invention of the compound microscope. The 
refractive power of glass had long been known, and simple lenses had 
come to be used in the sixteenth century for spectacles and as scientific 
toys. The combination of two or more lenses in a tube to form a com- 
pound microscope is generally attributed to Zacharias Jensen and is 
said to have been first used about the year 1591. During the following 
century considerable improvement of these instruments was effected. An 
early microscopist, Robert Hooke (page 15), described the one at the 

Fig. 6. — Marcello Malpighi, 1628-1694. {From Garrison, ''History of Medicine," after the 

painting by Tabor, Royal Society.) 

left in Fig. 5, while a moderately improved one is on the right. Almost 
no further improvement was made thereafter for a century and a half. 

The founder of microscopic anatomy was Marcello Malpighi (1628- 
1694), of Italy (Fig. 6). He studied the lungs and observed the capil- 
laries, thus confirming the theory that blood circulates through them. 
He also examined various glands, the embryo of the chick, the structure 
of the silkworm, and the tissues of plants. His work on plants was 
extensive, and, with Nehemiah Grew (1628-1712) of England, he became 
the founder of plant anatomy. Anton van Leeuwenhoek (1632-1723) 
(Fig. 7), of Holland, stepped out from behind his dry goods and notion 
counter often enoiigli to become one of the most skillful of the makers of 
lenses; one of his lenses, still in existence, magnifies two hundred and 
seventy times. He made these for his own use, never sold one, and never 


loaned one. Everything that could be observed with a microscope 
became an object of his study. The biological objects included were the 
blood capillaries, red blood cells, spermatozoa (male germ cells), striated 
muscle, the crystalline lens of the eye, the eggs of insects, and minute 
organisms in pond water. Another Dutchman, Jan Swammerdam (1637- 
1680), besides some work on gross anatomy, studied the minute anatomy 
of insects and snails and the development of the eggs of various animals. 
Microscopes existed in America in the seventeenth century, but no 
important use of them in biology appears to have been recorded. 

Fig. 7. — Anton van Leeuwenhoek, 1632-1723. {From Garrison, "History of Medicine.") 

Classification of Animals and Plants. — One of the early trends away 
from structure was the series of attempts to classify living things. Efforts 
to systematize the listing and arrangement were made in very early times 
by Plato and Aristotle. These were very simple; Aristotle mentions by 
name only two ranks, which correspond roughly to the species and family 
of our present classification. When the great geographic discoveries of 
the sixteenth and seventeenth centuries were made, and many new 
animals became known, such simple groupings were of little use. The 
first classification worthy of note was that of John Ray (1627-1705), 
in England. Ray's idea of the species was very similar to that of the 
present time. He grouped similar species into a genus, but his genera 
were much more inclusive than at present. Anatomical likeness was the 
basis on which species were grouped together, though he allowed old 



prejudice to prevail in some cases, as when he included the whales with 
the fishes despite his knowledge that they more closely resemble the 

It was Carolus Linnaeus (1707-1778) (Fig. 8), however, who made the 
greatest advance in classification. Of a Swedish family and trained to be 
a physician, he yielded to his interest in natural history and was even- 
tually named professor of botany in the University of Uppsala. He had a 
passion for arranging all sorts of natural objects into groups on the basis 
of like qualities. The choice of qualities to form the basis of this clas- 
sification was sometimes arbitrary, especially in his earlier years, as 

Fig. 8. — Carolus Linnaeus, 1707-1778, in Lapland dress at the ago of thirty. 

of New York Botanical Garden.) 


when he classified plants according to the number of stamens and pistils 
in their flowers. In later life he recognized that likeness in a single 
character, in the absence of other similarities, was not a safe ground on 
which to group organisms. He followed Ray at first in assuming that 
species have now the characters with which they were created, and in 
general he held to the "fixity" of species. Yet in his later writings he 
(juestions whether the several species belonging to one genus ma}^ not 
have evolved, l^y change, from a single origin in creation. One of 
Linnaeus's greatest services was the introduction of two terms in the 
name of a species — the first the name of the genus, the second that of the 
species — a method which is used at the present time. It was fully 
developed in his great work, the "Systema Naturae," in which all the 


animals and plants which Linnaeus knew are described and named. 
So accurate are the descriptions that many of his species are recognizable 
today, and his names for them are still applied. 

Foundations of Modem Biology. — Naturalists of a certain stamp have 
always found the classification of objects a fascinating occupation, and 
Linnaeus had many followers. For the most part they were less able than 
he, and their labors often degenerated into an attempt to discover and 
name as many species as possible. Because of this tendency, classifica- 
tion suffered a degree of disrepute. Moreover, there were many other 
features of living things to engage attention. Discoveries were made 
and theories formulated in nearly all the fields of biology. The phys- 
iology of sense organs and the nervous system was studied. Embryology, 
the science of development of the individual, was greatly advanced. 
The process of fertilization of eggs by spermatozoa came gradually to 
be understood, and it was found that some eggs could develop without 
the intervention of the male cells. The existence of sex in plants was 
recognized, and some crosses were made to ascertain the course of 
heredity. Mutilated animals were observed to regenerate their missing 
parts. Comparisons of the structure of various animals foreshadowed 
the comparative anatomy of the next century. The behavior of the 
castes of social insects was studied, marking the beginning of animal 
psychology. In the sister science of chemistry, the nature of oxygen 
and carbon dioxide was discovered, and naturalists began to see their 
relation to the respiration of animals. Vague ideas of change of species, 
implying concepts of evolution, began to be put forth. 

With this increase in the factual phase of biology, philosophy declined ; 
and with the rising tendency to limit theory to what could be reasonably 
supported by the ascertained facts, biology entered upon what may be 
regarded as its modern period. This period corresponds roughly to the 
nineteenth and twentieth centuries. It witnessed the rise of comparative 
anatomy, the discovery of cells, the development of embryology and 
cytology, the general acceptance of the evolution doctrine, the rapid 
increase in the use of the experimental method, research in heredity, the 
study of the general physiology of protoplasm, and specialization in 
several of the narrower fields of biology. 

Comparative Anatomy. — The earliest well-defined modern trend was 
in the field of comparative anatomy. The founder of this branch of 
biology was Georges Cuvier (1769-1832) (Fig. 9). Cuvier possessed a 
natural interest in living things and, being a clever draughtsman, had 
made pictures of many of the animals he studied. Some of these pictures, 
exhibited in Paris, won him a professorship of comparative anatomy there. 
His rise was rapid, and mmierous honors were bestowed upon him. 
Cuvier's comparative anatomy differed from all previous brands in that 



the standard of comparison was not man but the lower animals. He 
had begun his biological career by studying marine animals; and, while ho 
later went over almost wholly to the vertebrates, he never, as did the 
medically trained anatomists before him, adopted man as the starting 
point for comparison. Paleontology also traces its origin to Cuvier, 
since his comparative studies were extended to fossils, especially to the 
elephantlike forms, the mastodons. 

It is curious that Cuvier, who was forcibly brought face to face with 
the evolution theory, never saw fit to embrace it. His discoveries in 
comparative anatomy are now regarded as indicating kinship of various 

Fig. 9. — Georges Cuvier, 1769-1832. {From Locy, "Biology and Its Makers.") 

animals, and the fossils he studied clearly demonstrate that living things 
of successive ages were of very unlike kinds. Cuvier chose to explain 
these successive types of beings by catastrophes, which destroyed all 
life, and subsequent recreation of new kinds of beings. He was not 
merely passive in rejecting the evolution doctrine but actively opposed 
it. In a series of discussions participated in by him and Geoffroy St. 
Hilaire before the French Academy of Sciences in 1830, his opposition was 
repeatedly stated. Cuvier, who was an excellent debater and very 
influential, was then generally held to have won this debate. 

The Cell Theory. — The comparative method of study was applied to 
smaller and smaller objects as rapidly as moans of doing so were available. 
Further progress in the improvement of the microscope (such as the first 
production of achromatic lenses about 1827), after a period of nearly 



a century and a half in which Uttle change took place in these instruments, 
led to the discovery of the universal occurrence of cells. The credit for 
this discovery belongs to no one person. Hooke had seen the boxlike 
cavities in cork in 1665, and Malpighi observed those of other plant tissues 
in 1670. Lamarck and Mirbel taught, early in the nineteenth century, 
that plants and animals are composed of "cellular tissue." The nucleus 
was sporadically seen and in 1833 recognized by Brown as of regular 
occurrence in plants. His observation was verified by Schleiden, and 
Schwann (Fig. 10) extended it to animals. The universal occurrence of 
cells in living things was recognized by Dutrochet and Purkinje (Fig. 11), 

Fig. 10. Fig. 11. 

Fig. 10. — Theodor Schwann, 1810-1882. Fig. 11. — Johannes Evangelista Purkinje, 

1787-1869. {Both from Garrison, " History of Medicine.") 

and a formal statement of that universality was published by Schwann in 
1839. Knowledge of the nature of cells was gradually accumulated 
through the work of various biologists, culminating in the convincing 
proof by Max Schultze, about 1861, that the essential feature of living 
things is the jellylike substance called protoplasm, which was at first 
regarded as merely incidental. 

This knowledge of cells had a profound influence upon further 
advances in morphological biology. The study of tissues, begun several 
decades before, now became a study of like cells grouped together. 
Embryology was pushed back to the very beginning of development, 
to the egg cell, and the so-called germ layers (of cells) in the embryo of 
the chick were discovered. Unfortunately, knowledge of the minute 
structure of cells was not sufficient until much later to influence physiolog- 



ieal work appreciably. The theoretical and natural history phases of 
biology also went on quite unaffected, for the time, by cell discoveries. 

Modem Physiology. — Physiological investigations were much more 
dependent upon the advances being made in animal chemistry than upon 
cell studies. Knowledge of the composition of all sorts of animal struc- 
tures was strengthening the belief that life is a group of chemical phe- 
nomena. Studies of function necessarily made use of the experimental 
method, which once more became one of the most valuable tools of 
biology. One of the leaders of this period in physiology studied the 

Fro. 12. — Jean Baptiste Lamarck, 1744-1829. (From Locy, " Biolosjij and Its Makers" 

and Thornton, " British Plants." } 

processes of nutrition (particularly the role of the liver), the production 
of sugar in animal bodies and the influence of the central nervous system 
upon this process, the secretion of the pancreas, and the effects of poisons. 
Another studicxl sense perception and the function of different kinds of 
nerve cells, while a third worked on reflex actions. But all this was done 
without particular reference to cells. It was t)nly much later that the 
physiology of the cell was recognized as lying at the foundation of all 

Evolution. — Another of the great developments of the nineteenth 
century which occurred quite without reference to the knowledge of cells 
was the growth of the evolution doctrine. The idea of evolution, or 
change of species, was briefly and crudely stated or suggested in the writ- 
ings of the early Greeks, Empedocles in particular. Linnaeus, in the 



eighteenth century, betrayed a sUght loaning to t,he possibihty of evohi- 
tion in his later writings when he conceived that the species belonging to 
the same genus might have had a common origin. His contemporary, 
I3utfon, speculated more openly upon the origin of the various life forms 
and was unwilling to accept the notion of independent creations. It was 
not until the time of Lamarck (Fig. 12), however, that any general theory 
of evolution was proposed. Lamarck observed the great variation exhil/- 
ited by animals and conceived that it was due to the effects of use or disuse 
of the various organs by the animals. He supposed that the changes thus 
induced were inherited, thus becoming permanent — a view that has been 

Fig. 13. — Charles Darwin, 1809-1882. (From University Magazine. 

Leonard Darwin.) 

Photograph by 

abandoned by most biologists since then. These views of Lamarck were 
expressed most fully about 1809, at the beginning of what may be 
regarded as the modern period in biology. As has been pointed out in an 
earlier section, Cuvier opposed the evolution doctrine, notably in the 
series of discussions in the French Academy of Science in 1830, and his 
great personal influence determined the attitude of French biologists 
toward the new doctrine. 

It was in another land, therefore, that the chief modern development 
of the evolution idea had its origin. To Charles Darwin (1809-1882) 
(Fig. 13), of England, is due the credit of convincing the thinking world 
that change of species has taken place throughout the whole history of 
living things. This he did partly by marshalling such a mass of evidence 
in favor of evolution that there was no rejecting it, partly by devising a 


theory — natural selection — to account for it, so plausible that acceptance 
of the fact of evolution was rendered easy. Within a few years of the 
publication of Darwin's "Origin of Species" in 1859, the supporters of the 
evolution idea far outnumbered its opponents in intellectual circles. 
Naturalists everywhere were busy finding examples of apparent evolution 
and striving to fit the observed facts into the natural selection theory. 
The whole course of development of biology was modified by this prev- 
alence of evolutionary speculation during the two or three decades after 

Not all discussions of evolution were wholly speculative; some were 
founded on detailed facts which were gained by hard labor. An example 
is the expansion of work in comparative morphology in Germany. This 
science became distinctly evolutionary; the comparisons were made with 
an eye to kinship and became some of the most important of the evidences 
of evolution. Embryology, too, profited by the idea of kinship of animal 
forms and in turn furnished much of the evidence on which the evolution 
theory is based. Only among the French, of the great intellectual 
peoples, was the acceptance of the evolution doctrine long delayed; and 
when the idea finally triumphed there, it was rather in the form proposed 
by their countryman Lamarck (as a consequence of use and disuse) than 
in the Darwinian form (as guided by natural selection). 

Genetics. — In one respect in particular did enthusiasm for the 
evolution theory overreach itself. Since evolution can consist only of 
hereditary variations, it would be supposed that any information regard- 
ing the phenomena of heredity would be promptly seized upon as of 
importance to evolution. Darwin himself did strive to learn from 
practical breeders and others what was known of these phenomena. 
His feeling of their importance was not shared sufficiently by biologists 
in general, so that when in 186G Gregor Mendel (Fig. 14), an Austrian 
monk, published some experiments dealing with inheritance in garden 
peas, they attracted no attention. Mendel's work lay unnoticed until 
1900. By that time the ardor of the natural selectionists had cooled 
enough that the futility of attempting to discover the course of evolution 
by speculation alone was duly recognized. Realizing that in a knowledge 
of heredity lay the best hope of explaining evolution, various biologists 
had resumed the study of inheritance by means of experiments. Plants, 
being simplest, yielded the first results, and in 1900 three European 
botanists, working independently, publislunl at about the same time 
accounts of their crosses, from which they derived the same conclusion as 
Mendel had derived before them. Fortunately they also discovered 
Mendel's old paper. These experiments were capable of being explained 
in so simple a manner that a great impetus was given to the experim(;ntal 
study of heredity. Hundreds of plants and animals have been shown to 



follow the fundamental rule laid down by Mendel. His principles have 
undergone some modification, as a result of the investigations of T. H. 
Morgan and others, so that the known operations of heredity are no longer 
so simple as Mendel's statement. Further complexities are still being 
discovered, but with few exceptions they form a harmonious whole, and 
genetics at the present time approaches more nearly the condition of an 
exact science than any other division of biology. 

Fig. 14. — Gregor Johann Mendel, 1822-1884. {From a photograph taken about 1880. 
Reproduced from the report of the Royal Horticidtural Society Conference on Genetics, 1906, 
by permission of the President and Council.) 

Cytology. — The handmaiden of genetics in all this advance has been 
the science of cytology, which deals with the very small structures of the 
cell. Advance in this field beyond the stage to which Max Schultze 
brought it has depended upon further improvement of the microscope, 
the discovery of dyes or stains by which these minute objects could be 
made more readily visible, and the invention of mechanical devices for 
cutting cells into very thin sections. These improvements in technique 
led early to an understanding of cell division (in the eighteen seventies) 
and later of the ripening of the germ cells. While cytology has been 
concerned with all sorts of cell structures, the chromosomes, minute 
objects in the cell nucleus, have long been regarded as of chief importance. 
It is the chromosomes that have allied cytology so closely with genetics, 
for the machinery of heredity is found in the chromosomes. At first, in 
this alliance of genetics and cytology, the latter took the lead. Chromo- 


somes were observed (1880-1910) to behave in certain wa3\s before their 
genetic significance was understood. Later the order of discovery was 
reversed; the demonstrated workings of heredity required that the 
chromosomes should operate in a certain manner, and in many cases their 
behavior has been subsequently found to coincide with the theoretical 

General Physiology. — While stains, smears, section-cutting apparatus, 
and improved microscopes have been the traditional tools of the cytolo- 
gists, recent work in that field has dealt with living cells and has included 
minute dissection of cells by means of ingenious devices which can be 
operated under the microscope. This phase of cytology borders closely 
upon general physiology, which deals with fundamental activities of 
protoplasm. General physiology is concerned with chemical composition 
and reactions of living matter, permeability, viscosity, colloid structure, 
electrical charges, transformations of energy, etc., in an attempt to relate 
these conditions or processes to the phenomena of life. The material 
used in such studies is partly a host of one-celled organisms, partly the 
eggs of various aquatic forms, and partly the specialized masses of cells, 
or tissues, of higher animals. Although these cells differ much in 
appearance and in their ultimate fate, they must do certain fundamental 
things in common. It is in the province of general physiology to discover 
these common processes. This development is comparatively recent, 
and a large number of biologists at the present time are engaged in this 
type of work. 

Change in Content of Biology. — It will have been observed that 
throughout the development of biology, from the early Greeks to the 
present time, the bulk of what was known regarding living things con- 
cerned their structure. This branch of biology is known as moryhology . 
At first little else was known, and in the Middle Ages the continuity of 
biology hung on the one thread of anatomy. Only gradually did the 
functions of organs come to be of much interest, and William Harvey, in 
the seventeenth century, is often regarded as the founder of -physiology. 
At first a study of mechanics, physiology later became concerned with 
the principles of organic chemistry. Attempts were made to apply 
these principles not only to the workings of the organs of the adult but 
to the processes of embryonic development. Embryology thus became 
physiological as well as morphological, and modern work in embryology 
is chiefly of the former kind. 

Simultaneously with physiology there grew up the science of classifica- 
tion, or taxonomy. At first, as developed by Linnaeus, classification was 
arbitrary. Though similar animals were grouped together, their similar- 
ity was not held to have any significance. A century later, when evolu- 
tion was generally accepted, the basis of taxonomy came to be kinship. 


Similar animals were grouped together because they were believed 
to be related through common descent. Concepts of evolution and 
hence of taxonomy were altered in quite recent times by increasing 
knowledge of genetics which lies at the foundation of both of the sciences 
just named. 

These five sciences, morphology, physiology, taxonomy, evolution, and 
genetics, are the main fundamental divisions of pure biology. Because 
they are all concerned with living things, they necessarily overlap. 
Evolution and genetics have much in common, as have both with taxon- 
omy. Physiology and morphology are not wholly separable, since 
function cannot exist apart from structure. Yet there is considerable 
independence among them. It is possible to study morphology without 
being concerned with the function of the structures involved. One may 
study genetics without knowing or caring what bearing the discovered 
facts have on evolution. Taxonomy may — and did for a century — 
proceed without any relation to evolution, even though that kind of 
taxonomy would be regarded now as without significance. 

Composite Biological Sciences. — There are several divisions of biology, 
however, which do not possess this degree of independence, but which are 
only special phases or combinations of the five named above. One of 
these is paleontology, the science of extinct animals. Paleontology is 
only a specialized form of zoology, limited in its scope because it is con- 
cerned only with fossil types, not with living animals. It deals largely 
with morphology, chiefly of external features, though internal anatomy is 
sometimes preserved in fossils. Taxonomy is quite feasible in paleon- 
tology, since external form of fossils, taken in connection with similar 
kinds of living animals, is sufficient to indicate probable kinship. Evolu- 
tion is clearly shown by the differences between fossils of successive 
geological periods. However, the physiological processes of extinct 
animals can only be inferred from their structure, and knowledge of 
genetics is impossible in the absence of detailed comparisons of parents 
and offspring. Paleontology is thus a limited sort of zoology. 

Ecology, which is a study of the relation of living things to the environ- 
ment, is likewise a composite of the fundamental biological sciences. 
The ecologist strives to discover in what ways organisms meet the condi- 
tions imposed by the world around them. He learns in what situations 
animals live, and why they are there. He studies the interplay of 
processes within organisms and processes occurring outside. To some 
extent this relation to the environment is purely structural ; very largely 
it concerns function. So far as ecology concerns the organisms them- 
selves, therefore, it is but a combination of morphology and physiology. 
The other things with which ecology has chiefly to deal concern the 
organization of the environment. This latter phase of ecologj^ is not 


really biology at all, except as the environment of one animal is made up 
of other living things ; but it is as essential to ecology as is a knowledge of 
physics and chemistry in general physiology. 

Somewhat related to ecology is the geographic distribution of animals, 
or zoogeography. Ecology relates partly to local distribution of organisms, 
as determined by environmental conditions. Zoogeography also involves 
these questions of local distribution, since no species can live where the 
conditions are not suitable, and wrong conditions constitute barriers to 
distribution. However, no kind of animal is found in all the places on 
the earth where conditions suitable for it exist. The absence of a species 
from some regions entirely capable of supporting it is accounted for by 
such things as the place where the group originated and the time of its 
origin. These things are historical; ecology has nothing to do with them, 
but they are an important part of zoogeography. The latter science is 
therefore morphology and physiology, as far as the fitness of species to 
occupy certain regions is concerned; and it is evolution and geolog}^ 
whenever absence from a given region is explained by the time or place 
of origin of the species. 

Too much emphasis should not, however, be placed upon the clearly 
composite nature of these several biological sciences. All the divisions 
of biology overlap to some extent; indeed, the unity of them all, which 
makes them biology, would not exist but for such overlapping. Plants 
share this unity with animals. There is a morphology, a physiology, a 
taxonomy of plants. These sciences differ from the corresponding ones 
for animals in the objects with which they deal, but not greatly in the 
principles involved. Each of the other divisions of biology discussed 
above relates to plants as well as to animals. It is traditional to separate 
botany from zoology, but there is scarcely more difference between plants 
and animals as they relate to one of these sciences than there is between 
some of the more extreme animals. 


LocY, W. A. Biology and Its Makers. Henry Holt & Company, Inc. (Especially 

Chaps. I-IV, VI, VII, XI.) 
LocY, W. A. Growth of Biology. Henry Holt & Company, Inc. (Particularly 

Chaps. II, IV, IX, X.) 
MiALL, L. C. The Early Naturalists, Their Lives and Work. The Macmillan 

Company. (Sec. V, minute anatomists; Sec. VIII, part on Linnaeus.) 
NordenskiOld, E. History of Biology, .\lfred A. Knopf, Inc. (Especially Chaps. 

I, II, V, VII, VIII, XIV of Part 1. ^Fhe rest of the book will be better appre- 
ciated after several advanced courses in biology.) 
OsBOHN, H. F. From the Greeks to Darwin. The Macniilian Company. (Hi.story 

of the evolution idea over the period indicated.) ^ 

Singer, C. Biology: History. Medicine, History of. Articles in Encyclopaedia 



No feature of organisms has so many and such varied consequences 
as the fact that they are composed of protoplasm which is usually 
arranged in the form of cells. If a bit of animal tissue, cut thin, be 
it from muscle, gland, skin, brain, or sense organ, is examined under 
a microscope, it is found to be blocked off in small areas, all of which 
resemble one another in certain respects and some of which ai-e alike in a 
great many ways. These are the cells. We have seen (pages 14, 15) how 
the existence of cells gradually became known, and how much this dis- 
covery influenced work in different fields of biology. The authors of the 
•ceil theory, as it was first formulated, were content to claim that all things 
are composed of these units. Its immediate effect was therefore only on 
the structural side of biology, as has already been related. Had the 
theory developed no further, it would have continued to affect only 
morphology. When, however, the chemical and physical composition of 
the protoplasm was studied, and when the minute structure of the parts 
of the cells began to yield to the microscope, it became apparent that the 
existence of cells was highly important in physiology, heredity, and 
evolution. A knowledge of cells therefore lays a foundation for much of 
the rest of biology. 

The Size of Cells. — It is surprising to find how much difference there 
is among cells with respect to size. The radius within which the various 
activities of cells must occur should be of some significance. Each cell 
consists typically of a nucleus lying within a bit of protoplasm which is 
the cell body or cytosome. Important reactions take place between 
the different parts of the cell. Since the nearness of these parts to one 
another must influence the ease with which they work together, the 
size of the cell should be of some importance. Yet cells show very great 
differences in this respect. Some bacteria are so small as to be almost 
invisible even with a good microscope; somewhat larger are most tissue 
cells, which are quite easily seen when thus magnified but cannot be seen 
without such aid; but all these are topped by the egg yolks of the larger 
birds, which are 2 or 3 inches in diameter. Nerve cells often have great 
length, particularly those which extend from the spinal cord to the ends of 
the extremities in man or the other large mammals, but are quite slender. 
Sometimes these great differences in size fit the cells for their particular 




functions, but in most cases no such explanation is known. When cells 
that are presumably alike in their origin and function show great differ- 
ences in volume, as when one unicellular animal (Paramecium, for 
example) is several hundred times as large as another 
of the same species (Fig. 15), it is probable that 
differences in the environment have caused part, though 
not all, of the contrast. 

The size of cells bears no constant relation to the 
size of the animals or plants in which they are found. 
In very many kinds of animals, large individuals have 
more, but not larger, cells than do small ones. In 
others, the number of cells in each individual is 
always the same, and in them large size is attained 
only by the growth of each cell. In salamanders in 
which, through some abnormal step in cell division, 
the cells have extra chromosomes, the cells are larger 
but the body is not: such animals simply have fewer 
cells. Sluggish animals like frogs generally have larger 
cells than active ones such as birds, and there is 
presumably some important connection between these 
Gross Shape. — The shape of cells is also very variable. Some cells, 
owing to surface tension, are typically spherical; but that shape is 
attained, even approximately, only in free cells, such as eggs and a iew 
of the one-celled organisms. Cells take on other forms for various 
reasons. Amoeba and other related protozoa may actively change their 

Fig. 15. — Ex- 
treme difference of 
fsize in otherwise 
similar cells; two 
members of same 
species of Para- 
mecium, one 300 
times as large as 
the other. 

Fig. 16. — Change of shape in amoeba. Half-ininuto interval between first and second, 
five mjnutes between second and third. {CoiirUsy of Gcnenil Biological Supply House.) 

shape by thrusting out portions of the body into fingerlike pseudopodia. 
Such an animal is seldom of the same shape for any considerable time 
(unless it goes into a "resting" state, in which it is apt to be nearly 
spherical), and it may even be changing every instant (Fig. 16). Other 
free-living cells, of more or less constant form, are kept constant by a wall 
or pellicle that the cells themselves have secreted (Fig. 17). These 



pellicles may be flexible but firm, so that while the shape of the body 
may become temporarily distorted it is characteristic of the species. 

Cells that exist in groups usually have their form altered by the 
mechanical pressure of the cells around them. When this pressure is 
the only factor altering their shapes, the cells are irregular polyhedrons. 
Other factors, such as unequal growth in different directions and perhaps 
inequalities of surface tension, combine to produce cells of a great variety 
of shapes. They may be box-shaped, as in plants; long cylinders, as in 
voluntary muscle; greatly flattened cells with their largest sides polygons, 
as in the outer layer of frog skin; somewhat flattened elliptical cells, as in 
the blood of many animals; circular and flattened, as in human blood; 
narrow and spindle-shaped, as in involuntary muscle; or finely branched, 

'i«Aj ^"-~_-^-^^^==^ 

Fig. 17. Fig. 18. 

Fig. 17. — Various forms of ciliated protozoa whose body shape is kept fairly constant 
by a surrounding pelhcle. Though this shape may be altered by pressure, it is restored 
when the pressure is removed. Cilia project from the surface. 

Fig. 18. — Various forms of nuclei in cells. A, part of muscle cell with multiple ellip- 
soidal nuclei; B, gland cell of butterfly with branching nucleus; C, marrow cell of rabbit with 
ring nucleus; D, Epistylis with curved rodlike nucleus; E, Stentor with beaded nucleus; 
F, Trachelocerca with distributed nucleus. (5, C, and F after Wilson, courtesy of The Mac- 
Millan Company.) 

as in pigment cells of the skin of frogs and salamanders, or bone and nerve 

The Nucleus. — The most important part of a cell is its nucleus. 
This body is ordinarily located somewhere near the middle of the cyto- 
some but may be crowded to one side by other structures and may move 
from one place to another. It is most often spherical, owing to the ten- 
sion of the very thin film, or nuclear membrane, which surrounds it, but 
other shapes may be impressed upon it or it may actively take other 
forms. In long narrow cells the nucleus is generally elongated (Fig. 
18A), and in flat cells it is disk-shaped. Physiologically very active 
cells often have branched or lobed nuclei {B, C) ; and in certain unicellular 
organisms the nuclei may be of odd shapes — ropelike, beaded, or broken 
up into many small bits (D-F) — characteristic of the species but without 
any known significance. The red cells of human blood are devoid of 
nuclei, a condition generally held to be due to degeneration of the nuclei 
which they possessed in young stages. 



The importance of the nucleus derives from a substance known as chro- 
matin which it contains. This substance, as will appear in later chapters, 
exercises some control over physiological processes, development, and 
heredity. It owes its name to the fact that it colors deeply in most 
ordinary dyes such as are used by cytologists to make it conspicuous 
enough for study. The chromatin is collected into a number of distinct 
masses, the chromosomes, but these bodies are so diffuse in their structure 
that they cannot usually be recognized as separate objects except at the 
time of cell division. During the periods between cell divisions one com- 
mon form in which chromosomes exist is that of distended bags, the walls 
of which contain the chromatin itself, while the interior is filled with a 





Cell wall- 


Golgi body 

-4 Cenlriole 
— Cenfrosphere 



Fig. 19. — Generalized cell. 

semiliquid substance called the nuclear sap. The chromatin is thus 
greatly thinned out, though quite irregularly so, for there are little knots 
and branching strands of it thick enough to be seen when stained. Some 
chromosomes are shown in Fig. 38 (page 59), gradually experiencing this 
expansion at the end of cell division. When the chromosomes in this 
distended form are packed closely together in a nucleus, it is usually quite 
impossil)le to see the outlines of the chromosomes, but the kn-ots and 
strands of thicker chromatin are visible, together gi^^ing the appearance of 
a network (Fig. 19). In other cells the chromosomes appear to be in 
the form of branched threads rather than bags, but the resulting appear- 
ance of the nucleus is still that of a chromatin network whose spaces are 
filled with nuclear sap. 

Some nuclei contain, in addition to the sap and the network of 
chromatin, a nucleolus. Two or more nucleoli may be present. They 
are rounded bodies that stain readilv, but in a manner different from 


the chromatin. Nucleoh are therefore not to be confused with bunches 
of chromatin, which have sometimes been called nucleoli. The nature 
and function of the nucleolus, when it is present, are not understood. 
Some biologists have regarded it as a waste product; others have held it 
to be a reserve supply of materials used in cell division, since it dis- 
appears during that process; and it has been regarded as a reserve food 
supply for the nucleus. 

The Cytosome. — The body of a cell is seldom uniform in composition 
but includes a number of different structures. The more common ones 
are here described, though very few cells have all of them. At the surface 
there may be a definite cell wall which is lifeless, not composed of proto- 
plasm but secreted by the cell. It is very common in plants, where it is 
composed mostly of cellulose, one of the principal components of wood. 
Some animal cells have such a lifeless covering, but in them it is often 
made of other materials. Sometimes the cell is covered by a much 
thinner and more flexible coat, the pellicle, as are the cells of Fig. 17. 
Beneath the cell wall, or at the surface of the cell if there is no other cover, 
a somewhat firmer layer which may be called the cell membrane is formed 
out of the protoplasm itself in about the same way that water forms a 
film at its surface. 

Within the cytosome, plastids are common. In the higher plants 
they are universal and are usually green. Some are of other colors, 
as in fruits and flowers, and some are colorless. In animals, plastids are 
found chiefly in certain classes of protozoa (one-celled animals) where 
they are mostly colored. 

Vacuoles are vesicles of liquid enclosed in the protoplasm. They 
may be permanent or temporary. In the protozoa, temporary vacuoles 
are common. They usually either enclose bodies of food in process of 
digestion, in which case they are called food vacuoles, or disappear at 
intervals by ejecting their liquid contents through the surface layer of 
protoplasm into the surrounding medium. The latter kind is called a 
pulsating or contractile vacuole. In some cells a centrosphere is found, 
usually near the nucleus. It is a mass of somewhat differentiated proto- 
plasm, containing a minute body that stains deeply, the centrosome 
or centriole. When present, the centrosphere takes a conspicuous though 
probably unimportant part in cell division, as described in another 

Structures known as mitochondria (Fig. 19) are found in many kinds 
of cells, perhaps in all cells. They are of various shapes — rods, threads, 
granules — and occur almost anywhere in the cytosome. Many conjectures 
regarding their function have been made, but little is definitely known 
regarding it. An object known as the Golgi apparatus, of various forms, 
often a conspicuous network, occupies various positions, usually near the 


nucleus and in some cases characteristically near the centrosphere. The 
function of the Golgi apparatus is still unknown, though there is some 
indication that it takes part in the process of secretion by gland cells. 

Besides all the above structures which serve, or ma}^ serve, some func- 
tion in the cell, and which may therefore be regarded as cell organs, there 
are often lifeless matters enclosed in the protoplasm. These may be 
grains of starch, or oil or fat globules, which the cell has produced and 
which are stored as future food. Or the lifeless objects may be undigested 
remains of organisms taken as food, or even objects picked up incidentalh' 
along with food or otherwise. These nonliving objects may be spoken of 
as cell inclusions. 

Polarity. — Beside the differentiations described above, cells may 
possess another type of organization which is termed polarity. One por- 
tion is destined to perform certain functions, another portion othei- 
functions, even when these portions are visibly alike. In a develop- 
ing egg one part will become the nervous system and associated sense 
organs, another part the digestive tract. In the ordinary course of 
development these parts are not interchangeable. This evident arrange- 
ment of parts, as shown l)y their future activities, is the phenomenon 
which is called polarity. Examples of polarity are found in the eggs of 
insects, in which one end of the egg, in some way different from the other 
end, always becomes the head. Other cells than eggs are commonly 
polarized. Thus, cells bearing cilia (hairlike projections) on one end are 
polarized. So also is the connection (synapse) between nerve cells, since 
nerve impulses travel over it in only one direction. Many gland cells 
receive materials from the blood on one side and after working them o^-er 
extrude the product into a chamber on the opposite side. When long 
slender cells standing on end are crowded together to form a layer covering 
the surface of some organ, the nuclei of the cells are usually near the lower 
end. These are all polarized cells. In some cases the polarity is visible; 
but, before the structures indicating the polai-ity were developed, there 
was presumably an invisible difference in the proto])lasm. The nature 
of this organization is not known, and there is much disagreement as to 
whether it is inherent in the cells or is impressed on them by external 

Structural Relation to Other Cells. — When cells are free-living and 
independent, as in the protozoa, they may have little or no influence 
upon one another. When they are aggregated into masses, as in the 
multicellular animals, there is always the possibility that each cell may 
be modified, and its activities guided, by the cells around it. Often such 
interdependence must follow merely from the diffusion of fluids from cell 
to cell, or from electric phenomena. In some cases, however, proto- 
plasmic connections extend from one cell to another. These have been 



demonstrated in the skin of the salamander, are conspicuous in Volvox, 
and have been described for many kinds of animal cells (Fig. 20). In 
plants, cell bridges are usually present, the fine protoplasmic filaments 
passing through minute pores in the cell walls. Presumably these 
l)ridges are lines of communication between cells, but they are not 

Fig. 20. — Iiiteicellular l)riclges; left, highly- thicketied human ei)itlieliiiiii; right, ik r. iiinuon. 
{Courtesy of General Biological Supply Hous,.) 

essential, since cells in contact with one another are capable of passing 
litiuids or electric currents from one to another without such connections. 


Shari', L. ^^^ An introduction to Cytology. 3d Ed. McGraw-Hill Book C*oni- 

pany, Inc. (Chap. II; details of parts of cells, Chaps. III-VII.) 

Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan 

Company. (Chap. I.) 


In the activities of cells, great importance is to be attached to that 
very fine, mostly invisible, structure which inheres in the chemical 
composition and minute physical constitution of the protoplasm itself. 
These features of protoplasm are appropriately discussed along with 
the physiological processes which depend on them. Such processes 
should next engage our attention. Since, however, an understanding of 
this minute structure presupposes a knowledge of elementary chemistry 
and physics, it is advisable to pause a moment to acquire some of the 
more important ideas in that field. 

Composition of Matter. — The physical substance of which objects 
are composed is called matter. Matter exists in a number of different 
forms called elements. An element is a svibstance possessing a character- 
istic structure which is different from that of every other element and 
which cannot be broken down into substances different from itself (that 
is, into other elements) by ordinary chemical means. The stipulation 
"ordinary chemical means" is intended to exclude radioactivity and 
powerful electronic machines. Among the more common elements 
entering into the composition of living things are carbon, nitrogen, 
oxygen, and hydrogen. 

The elements may exist by themselves, chemically separate from 
other elements, as do oxygen and nitrogen in the air. More often they 
exist in compounds; these are distinct substances, made up of two or 
more elements, joined in definite proportions and with characteristic 
internal structure. Carbon dioxide is a very stable compound, made 
of carbon and oxygen, which is eliminated as a waste product by all 
living things. Calcium carbonate, of which bones are largely built, is a 
compound composed of three elements: calcium, carbtm, and oxygen. 

Both elements and compoimds are divisible into molecules. These 
are the smallest imits of a substance in which its characteristic chemical 
structure is maintained. The molecules are likewise the smallest units 
which exhibit the chemical properties of that substance. If a molecule 
is divided or broken up, its parts no longer have those properties. The 
elements which enter into a compound are present in each molecule in 
the same proportion as in large masses of the substance. Each molecule 
is exactly like every other molecule of the same substance, not only in 



the quantity of its elements but also in their structural arrangement. 
The molecules are completely separable from one another; in a solution 
of sugar in water, the molecules of sugar float singly, and in air the 
molecules of oxygen or of nitrogen are free from other molecules. 

The molecules of many substances are in turn composed of atoms. 
These are defined as the smallest divisions of matter that may exist, 
either singly or in combination. Some molecules consist of only one 
atom, as in the gas helium. In such substances there is no distinction 
between molecule and atom. In oxygen, however, the molecule is 
composed of two atoms. Here the atoms have properties veiy dif- 
ferent from those of the molecules ; the atoms enter into chemical reactions 
much more readily than do the molecules. 

Protons, Neutrons, and Electrons. — Even the atoms are not the 
ultimate units in the structure of matter, for they are made up of protons, 
neutrons, and electrons. These entities may be spoken of as particles, 
though they may be such only in a very special sense. The astounding 
feature of these units is that they are the same in all kinds of matter. 
The protons of an atom or molecule of oxygen are exactly like the protons 
of chlorine. Similarly the neutrons are everywhere the same, in all 
elements, and the electrons are the same in all. 

The protons have mass, and each of them bears a positive electric 
charge. This positive charge is a unit which is the same in all protons. 
Neutrons have mass, practically identical with that of the protons, but 
they carry no electric charge. Electrons are units of negative electric 
charge; their mass is negligible. Atoms and molecules of all substances 
are made up of these units. The mass (weight) of an atom is dependent 
almost entirely on the protons and neutrons it contains, while its volume 
is determined mostly by the electrons. These relations will be made 
clear by an examination of the structure of the atom in several elements. 

Structure of the Atom. — An atom of any substance consists of a 
central nucleus, around which one or more electrons are distributed. The 
nucleus of an atom contains one or more protons, and usually one or 
more neutrons. Since the protons bear positive electric charges, the 
nucleus of an atom is always positively charged. How great a charge 
it carries depends on how many protons it contains. Both protons and 
neutrons contribute to the mass of the nucleus, but only the protons 
furnish the charge. This positive charge of the nucleus is balanced by 
the negative charges of the surrounding electrons. There are as many 
electrons around the nucleus as there are protons in it, so that the atom 
is neutral. 

Structure of the Elements. — With this knowledge of the fundamental 
similarity of all matter let us return to the elements. The number of pro- 
tons and neutrons in the nucleus varies considerablv, as does also the 



Fig. 21. — Diagrams of atoms 
of hydrogen (left) and helium 
(right). The central black spot 
is the nucleus; the concentric 
circles mark oflf the shell of 
negatively charged electrons. 

number of electrons surrounding it. In hydrogen (H), which is the 
simplest and lightest of the elements, the nucleus consists of just 1 proton, 
no neutron, and the atom has just 1 electron (Fig. 21, left). The single 
unit of positive charge furnished by the proton is neutralized by the 
negative charge of the electron. Helium (He), has 2 protons and 2 

neutrons in the nucleus, which is therefore 
four times as heavy as the hydrogen 
nucleus, but it bears only twice as great a 
positive charge. To balance this positive 
charge, there are 2 electrons in the atom 
(Fig. 21, right). Carbon (C) has 6 protons 
and 6 neutrons (in the nucleus) and 6 
associated electrons; oxygen (O) has 8 
protons and 8 neutrons in the nucleus, with 
8 electrons'; while chlorine (CI) exists in 
two forms, one of which has 17 protons and 18 neutrons, the other 
17 protons and 20 neutrons in the nucleus, with 17 surrounding 

The details of these particular elements are not important to the 
biologist, but the fact that they are composed of identical kinds of 
units and that they differ only in the number and arrangement of these 
units should be understood. Every element has a different number of 
protons and electrons from every other element. From the lightest 
element, hydrogen, which has 1 
proton and 1 electron, to the one 
long believed heaviest, uranium 
(U), which has 92 protons and 92 
electrons, there should be 92 ele- 
ments. All but two or three of 
these have been obtained in 
chemical laboratories. News- 
papers occasionally report the 
discovery of one or more of the missing elements, which await confirma- 
tion by other investigators. In the construction of the atomic bomb two 
elements with 93 and 94 protons, respecti\'ely, were produced. 

The chemical properties of an element, the ways in which it reacts 
with other elements, are determined by the electrons surrounding the 
nucleus. When these electrons are numerous, they are arranged in shells, 
some near the nucleus, others farther away. The 2 electrons of lu^ium 
(Fig. 21) constitute such a shell, and a similar inner shell of 2 is in all 
elements heavier than helium. Outside this is a shell which may contain 
from 1 to 8 electrons. Oxygen has 6 electrons in this outer shell, as 
diagrammatically indicated in Fig. 22. When the number of electrons 

Fig. 22.- -Atom of oxjgcn; two wajs of 
representing its two shells of electrons. 



is greater than 10, the additional ones are in a shell outside of a first 
shell of 2 and a second shell of 8. 

It is only the electrons of the outermost shell which enter into ordinary 
chemical reactions. Different elements having the same number of 
electrons, similarly placed, in this outermost shell tend to possess similar 
properties and enter into similar reactions. A number of families of 
elements are thus recognized whose properties are much alike, such as 
the halogen family which includes fluorine (F), chlorine (CI), bromine 
(Br), and iodine (I), in which there are 7 electrons in the outer shell — but 
a different shell in each of these elements. 


f ^ 






]/ - 



Fig. 23. — Two atoms of oxygen combined to complete their outer shells of electrons. 

Chemical Reactions. — Some elements react more easily than others. 
The difference between them in this respect lies in their outermost shells 
of electrons. -In each shell of an atom there is a maximum possible 
number of electrons. An element which has this maximum number 
of electrons in its outer shell does not react I'eadily; the inert gases, such 
as helium used in balloons and neon in electric signs, are in this state. 
Most elements, however, have less than the maximum number of electrons 
in the outer shell, and it is easy for such elements either to complete that 
shell or to lose the electrons which are already in it. Because of this ease 
of reaction, two atoms of the same element sometimes join to complete 
their outer shells. Oxygen, as already stated, has six out of a possible 
eight in its second (outer) shell. If one atom shares two of its electrons 
with the other atom, and in turn accepts two electrons from the latter, 
each has a complete shell of eight electrons (four of them in common) 
and the two atoms are combined (Fig. 23). A molecule of oxygen is thus 



formed. In chloi'ine, which has seven of the possible eight electrons in its 
outer shell, two atoms combine by sharing two electrons (one furnished 
by each atom, Fig. 24), thus making a molecule of chlorine. 

Two atoms of different elements may combine, for the same reason, 
and thus a compound is produced. Sodium (Na), for example, has just 
one electron in its outer (third) shell, which it readily gives up to any 
other atom capable of accepting it. Chlorine, as just explained, has 
seven in its outer shell and readily accepts an electron from an outside 
source. The two atoms perform these easy reactions by combining; 
they form a new substance, sodium chloride (NaCl). 

Valence. — The number of electrons which an atom readily gives up 
or acquires constitutes its valence. Sodium has a valence of one, since 

Fig. 24. — Two atoms of chlorine combined to complete their outer shells of elections. 

it easily loses but one electron. Magnesium easily loses two electrons, 
because that is the number in its outer shell, and its yalence is two. 
These valences must be matched when compounds are formed. Thus, 
while one atom of chlorine (whose valence is one) matches one of sodium, 
it requires two atoms of chlorine to take up the two extra electrons of 
magnesium and form magnesium chloride (MgCU). 

Ions. — When a sodium atom gives up its one outermost electron to 
some other atom, its electric balance is disturbed. It has lost one unit 
of negative electric charge; hence the net charge of the remainder is 
positive. Such a positively charged body is no longer the element 
sodium, it is not even an atom; it is instead an ion (Na+). Similarly, 
when a chlorine atom acquires one extra electron (which is, of course, 
negative), its electric balance is disturbed and it becomes negative. It 
is no longer chlorine, no longc i- an atom, but a chloride ion (Cl~). An 






ion may be defined as part of a molecule, consisting of one or more atoms 
with an electric charge. Ions are either positive or negative, depending 
on whether the atom has lost or gained electrons in producing them. 

When sodium and chlorine combine to form sodium chloride, which 
is common table salt, a crystal of the salt is supposed to have the lattice 
structure shown in Fig. 25. There is no sodium in the crystal, no chlo- 
rine, but only sodium ions and chloride ions. There are not really any 
sodium chloride molecules, since each chloride ion (observe the central 
white one in the figure) is surrounded by six sodium ions at equal dis- 
tances, and each sodium ion is surrounded by six chloride ions at eqi'.al 
distances. One cannot say which 
negative ion neutralizes a given 
positive one, so that no specific 
pair of ions can be said to form a 
molecule. A molecule can hardly 
be said to exist in a sodium chlo- 
ride crystal, but only positive and 
negative ions. 

Radicals. — In all the above 
examples, the units of chemical 
reactions have been atoms of ele- 
ments or ions derived from them 
by transfer of electrons. Very 
often such reaction units are 
formed of two or more different 
elements. Sulfur (S) and oxygen, for example, may unite in the 
proportion of one of the former to four of the latter. In this propor- 
tion, however, their electric charges are not balanced, and the group bears 
two units of negative electric charge — that is, two extra electrons. They 
constitute a negative ion. In this form they act as a unit in combining 
with atoms which have lost electrons (positive ions). Potassium (K) 
may unite with them, but it takes two potassium ions to balance them, 
and K2SO4 (potassium sulfate) is formed. A group of atoms acting as a 
unit, as do the sulfur and oxygen (SO4") in this example, is called a radical. 
Other groups of atoms (radicals) are positively charged (as NH4+), 
forming positive ions. 

Acids, Bases, and Salts. — When a hydrogen atom (see Fig. 21) gives 
up its electron, only its nucleus remains. This nucleus is a proton and is 
positively charged: it may also be called a hydrogen ion (H+). Certain 
substances in water readily yield up these protons to other substances, 
and they possess certain properties as a consequence. They have a sour 
taste, color litmus paper red, and do a number of other characteristic 
things. SuTJStances which readily donate hydrogen ions (protons) are 

Fig. 25. — Crystal of sodium chloride, 
showing lattice arrangement of sodium ions 
and chloride ions. 


called acids. Other substances which easily accept protons are called 
bases. They do so through the formation of negative ions consisting of 
oxygen and hydrogen (0H~), known as hydroxyl ions. Bases in solution 
have the properties of lye, are said to be alkaline, and are recognized by 
the blue color they confer on litmus. 

It will be observed that the characteristic positive ions (H+) of acids 
and the characteristic negative ions (0H~) of bases together contain the 
components of ordinary water (H2()). Now water is an exceedingly 
stable compound. It is to be expected, therefore, that when an acid 
and a base are brought together in a solution the above ions will promptly 
unite to form water. This they do. But what becomes of the other 
radicals that belong to the acid and the base? They also combine in the 
sense that sodium ions (Na+) and chloride ions (Cl~") combine to produce 
sodium chloride. What they produce depends on what the other 
radicals of the acid and base were, but in any ease the product is called a 
salt. A salt is defined as a substance which produces, or is a combination 
of, positive and negative ions other than H+ and 0H~. 

If the acid used was hydrochloric (HCl) and the base was sodium 
hydroxide (NaOH), the solution containing the former would contain 
hydrogen ions (H+) and chloride ions (Cl~), while the latter in solution 
would consist of sodium ions (Na+) and hydroxyl ions (0H~). When 
these two solutions are mixed, the hydrogen ions (H+) and hydroxyl 
ions (0H~) promptly unite to form water. The ions of the other two 
kinds, Na+ and Cl~, do not actually unite, but they form a solution of 
sodium chloride. If such a solution is dried up, crystals of sodium chlo- 
ride having the lattice structure shown in Fig. 25 are formed. The 
sodium chloride is a salt. 

If sulfuric acid (II2SO4), in which there are hydrogen ions (11+) and 
sulfate ions (S04=), is mixed with potassium hydroxide (KOH). in which 
there are potassium ions (K+) and hydroxyl ions (()H~), water is again 
formed by the H+ and 0H~ ions. This leaves the potassium ions (K+) 
and sulfate ions (S()4=°) to form potassium sulfate (K2SO4). The potas- 
sium sulfate is likewise a salt. 

Salts may be obtained in other -ways than by mixing acids and bases. 
Mixing two salts gives rise to two other diffei-ent salts. Thus, if a 
solution of soduun chloride is mixed with a solution of potassium sulfate, 
the combined solution contains two kinds of positive ions (Na+ and K+) 
and two kinds of negative ions {C\- and SOr). While the ions do not 
join in solution, it is just as correct to regard the solution as containing 
potassium chloride (KCl) and sodium sulfate (Na2S04) as the original 

Electrolytes. — Ions, because of their charges, are able to carry an 
electric current when they are free to move. The sodium and chloride 


ions in a crystal of common salt are too rigidly held to move, but if the 
crystal is dissolved in water they are free. If into different parts of such 
a solution wires from the two poles of a l^attery are placed, a current of 
electricity is carried through the solution from one pole of the battery 
to the other (Fig. 26). The positive ions (Na+) go toward the negative 
pole and, by taking up electrons from it, becomes ordinary neutral 
sodium (Xa). Removal of electrons from the negative pole reduces the 
negative charge conferred upon it by the battery and sets up a current 
in the wire. The negative ions (Cl~) pass in like manner to the positive 
pole, where they deposit their surplus 
electrons on that pole, forming neutral 
chlorine (CI). Sodium is thus col- 
lected about one pole of the battery, 
where it reacts with the water; chlo- 
rine collects about the other pole and 
escapes as a gas. Decomposition of 

a substance in this manner is known t. „^ t.- ^ . , ■ . 

biG. 2b. — Diagram of electrolysis of 

as electrolysis. In the metal mdus- sodium chloride in solution. Chloride 

tries this process is used to separate \'^l\^^'''' *° *^^ "^^*' ^°'^'"'" '""'' *° 
certain metals from their ores. Sub- 
stances which, like sodium chloride, form ions in solution and are thus 
capable of carrying a current are called electrolytes. Most of the salts 
are good electrolytes. 

Energy. — Energy is the capacity to do work, that is, to produce 
change. The arrangement of the electrons and protons in an atom 
involves energy. Changing that arrangement either requires that 
energy be expended upon the change or releases energy no longer needed 
in the new arrangement. Both types of change are exceedingly common. 
Of the common elements about us near the earth's surface, oxygen is by 
far the most abundant, making up nearly half of the total. It is also 
very common in living things. Since oxygen is a fairly active element, 
some of the most frequent chemical reactions are the combinations of 
oxygen with other substances. These changes are called oxidation. 
The rusting of iron and the burning of wood or coal are examples. An 
important feature of oxidation is that it releases energy. Use is made 
of this fact in industry, when the energy of steam engines or electric 
current is furnished by burning coal, and in plants and animals whose 
activities depend on energy obtained by oxidizing food. The energy 
which is tied up in the composition of chemical substances, whether foods 
or any other, is called potential energy. When converted by a chemical 
reaction into the energy of heat or of movement, it becomes kinetic energy. 

Applications to Biology. — The examples used in this chapter to illus- 
trate chemical principles have been taken mostly from inorganic chemis- 


try because of their simplicity. The examples therefore need hardly be 
remembered if the ideas they represent are mastered. The principles 
have been kept at a minimum but should suffice for a fair understanding 
of the simpler operations of protoplasm. Living things are essentially 
chemical and physical laboratories, with this distinction, that the chemical 
substances are not limited to a few reagent bottles on the shelves nor 
the physical apparatus to a few resistance boxes and potentiometers in 
the cabinets; instead these things constitute most of the building itself. 
Changes are going on in them everywhere and all the time. It is of 
these chemical and physical processes that life consists. As explained 
in other parts of this book, the common physiological processes of 
digestion and respiration are chemical reactions and physical phenomena 
that are fairly well understood. Not so well known but assuredly 
chemical and physical are muscular contraction and elimination of wastes. 
Even growth, the development of the embryo or young stages, and the 
conduction of impulses by nerves must be largely physicochemical. 

It is important to know, in connection with all these life processes, 
that substances react as they do because of their electronic structure. 
This structure is, in most protoplasmic substances, enormously com- 
plicated by radicals of complex design. Their reactions and structure 
are for this reason not easy to discover, but there is every reason to 
assume that their physiological behavior is quite as dependent upon their 
architecture as are the reactions of the simplest inorganic compound. 
Valence determines the proportions of different substances which will 
unite in protoplasm as certainly as in the salts. Electric phenomena 
result from electronic reactions in living things just as in batteries. 
Energy, one of the most important requirements of animals and plants, 
flows from chemical combination as abundantly and as certainly in 
protoplasm as in a test tube or an engine. It seems likely that life 
consists entirely of physical and chemical changes. 

With this equipment of elementary knowledge in a pair of sister 
sciences, and an understanding of the extent to which these sciences 
underlie all knowledge of biology, we may notv return to the operations 
of cells. 


Partington, J. R. A Textbook of Inorganic Chcnustry. 5tli Ed. The Macmillan 

Company. (Pp. 428-430; 446-453; 466-473.) 
Smith, A. W. The Elements of Physics. 4th Ed. McGraw-Hill Book Company, 

Inc. (Chaps. 60, 61 ; structure of atom, nuclear physics.) 
TiMM, J. A. An Introduction to Chemistry. McGraw-Hill Book Company, Inc. 


The living substance whose functions we are to study differs from 
nonhving matter in certain characteristic ways. It has certain types of 
chemical structure, not easily defined, but not duplicated in inorganic 
bodies. It is arranged in unit masses, the cells, which are usually 
recognizable by their form and such nearly universal features as the 
nucleus. This living matter moves spontaneously, that is, from causes 
arising within itself. It grows by taking up new material throughout 
its interior, not just by additions on the outside. It is irritable; that is 
it responds in some way to changes in the environment, or changes 
within itself, which are great enough to act as stimuli. And finally, 
individual living things are capable of producing other individuals of 
their own particular kind; that is, they reproduce. 

These statements are not intended as a definition of life, or of living 
things, because there are exceptions to them, or situations in which the 
criteria could not be practically applied. They are meant merely to 
indicate the general types of functions which must be examined in a 
survey of life activities. 

Protoplasm is not a chemical compound, the structure of Avhich may 
be expressed by a chemical formula, but is an elaborate mixture of 
chemical compounds in water. A bit of protoplasm large enough to 
analyze, from any source, always yields carbon, hydrogen, nitrogen, 
oxygen, phosphorus, sulfur, sodium, potassium, magnesium, calcium, iron, 
and chlorine. Additional elements that frequently occur in such analyses 
are aluminum, silicon, manganese, copper, fluorine, bromine, and iodine. 
Naming these elements tells very little, however, concerning protoplasm, 
since it does not suggest the manner in which the elements are combined, 
and it is the compounds, not the elements, that are of real importance. 
These compounds in protoplasm are of a variety of kinds, which are 
partly organic (produced in living things) and partly inorganic. The 
latter are described first; they are water and the various salts. 

Water and Salts. — Water is the most abundant constituent of proto- 
plasm, making on the average about 80 per cent of the total mass. The 
properties and activities of protoplasm are quite as dependent upon the 
remarkable properties of water as upon the properties of its other con- 
stituents. Some of these properties of water are its power to absorb or 
give off great quantities of heat without changing much in temperature, 




its capacity to dissolve many ditferent substances, and the free movement 
which it permits in the ions of salts dissolved in it. These features of 
water enter into so many of the living processes that life without water, 
if it could exist at all, would have to be of a very different sort from any 
that is known. 

Dissolved in this water of protoplasm are the salts. The commonest 
ones have sodium and calcium as their positive ions, but potassium, mag- 
nesium, iron, and manganese are also present in this positively charged 
state. The negative ions are the chloride ion and the radicals known as 
carbonate, nitrate, sulfate, and phosphate. These ions of salts dissolved 
in water give protoplasm certain electrical properties. Inorganic salt 
make up about 1 per cent of average protoplasm. 

The Organic Compounds. — There are three principal classes of 
organic compoimds, the carbohydrates, lipids, and proteins. The carbo- 
hydrates are the sugars, starches, celluloses of plant walls, glycogens or 
animal starches, and some others. They constitute less than 1 per cent 
of most protoplasm but are important out of proportion to their quantity. 
They are composed of onty three elements: carbon, hydrogen, and 
oxygen. The hydrogen and oxygen are always in the ratio of 2:1; that 
is, there are twice as many atoms of the former as of the latter, just as 
in water. In most carbohydrates the carbon atoms are in multiples of 
six. A simple sugar has only six carbon atoms and is known as a mono- 
saccharide. Glucose, one of the most common of them, is present in 
nearly all cells. Other simple sugars are fructose (fruit sugar) and 
galactose. The formula of all these simple sugars is CeHigOe, but there 
are differences between them in internal ai-rangement. When two 
molecules of a monosaccharide are combined into one (with loss of 
water) the combination is a disaccharide. Sucrose (CioHooOh), the 
ordinary cane or beet sugar of table use, maltose (malt sugar), and 
lactose (milk sugar) are of this type. When many molecules of simple 
sugar are combined (with more loss of water), a polysaccharide is pro- 
duced. The starches (of plants), glycogens (animal starches), and 
celluloses (of cell walls) are of this kind. The polysaccharides are prac- 
tically insoluble in water, so that the starches and glycogens are excellent 
food-storage forms. None of the carbohydrates forms ions when dis- 
solved; hence they play no role in electrical i)hen()mena. They contain 
a gi'eat deal of potential energy, which may be released by oxidation. 

either reservoirs of stored energy are the lipids. The physical 
properties of these substances are very characteristic, including the non- 
evaporating grease spots which they make and their insolubility in 
water. This insolubility is what makes them good storage products. 
The lipids constitute about 3 per cent of oi-dinary protoplasm, though 
stored lipids may be many times that fra(;tion of an animal's body. 


Among the lipids are the true fats, such as butter fat, oHve oil, and 
the fat of beef or pork. True fats are composed entirely of carbon, 
hydrogen, and oxygen, with the proportion of oxygen much lower than 
in carbohj^drates. The natural fats have large molecules — around 50 
atoms of carbon, double as many of hydrogen — but only 6 atoms of 
oxygen. They are a combination of 1 molecule of glycerol (commonly 
called glycerin) with 3 molecules of fatty acid. There are a number of 
different fatty acids characteristic of different fats, some of them used 
commercially in water emulsions to produce the brushless kinds of 
shaving cream. 

In other types of lipids there may be more than the three elements 
which true fats contain. Lecithin, which includes phosphoric acid and 
another substance in place of one of the fatty acid molecules, is abundant 
in egg yolk and is probably present in all cells as part of the proto- 
plasmic structure. Cholesterol, which is foiuid in bile and is a source 
of gallstones, consists only of carbon, hydrogen, and oxygen, but the 
carbon in it forms a "skeleton" j^n rings instead of straight chains as in 
the fats. 

Most significant of the organic compounds are the proteins, because 
it is they that make one kind of living thing so sharply and definitely 
different from others. Aside from water, they are the most abundant 
substances in protoplasm — about 15 per cent of the total mass. Proteins 
are especially characteristic of lean meat (muscle) but are distributed 
through all cells. They do not diffuse readily through other substances 
but alloAV some, though not all, other substances to diffuse readily 
through them. Chemically they are compounds of the amino acids, a 
group of 25 different organic acids. A generalized formula of amino 
acids is R— CH(NH2)-C00H, in which R stands for the "body" of the 
molecule, different in each of the 25 acids. The rest of the formula 
applies to all of them. The COOH makes them organic acids, the NH2 
makes them amino acids. In the simplest amino acid, glycine, R is 
simply an atom of hydrogen, H; in the next simplest, alanine, R is the 
radical CHg. These amino acid molecules may be joined with one 
another, as carbohydrate molecules are joined, with the loss of a molecule 
of water at each junction. The more complex of these combinations 
are the proteins. The molecules of proteins are relatively huge, con- 
taining hundreds or even thousands of atoms. With such large molecules, 
which may include varying proportions of most of the amino acids, and 
frequently carbohydrates or lipids, there may be an enormous number 
of kinds of proteins. 

Enzymes. — Many chemical reactions are greatly hastened by the 
presence of certain chemical substances which do not enter into the 
reaction in a definitive way. Hydrogen peroxide (H2O2) is stable enough 


to last for months in a bottle; but if a pinch of manganese cUoxide (Mn02) 
is added, the extra oxygen of the peroxide comes away so rapidly as to 
produce a froth. The manganese dioxide acts as a catalyst, which is the 
name applied to inorganic accelerating agents. Now, many living tissues 
are constantly producing hydrogen peroxide, but it is promptly decom- 
posed. Something in the cells does what manganese dioxide does in the 
bottle. That something is called catalase. It is one of many organic 
accelerators called enzymes. 

For the first time in 1926 an enzyme was isolated, and now some 30 
of them have been purified. All of these are apparently proteins or 
protein compounds. Some of them work in the cells; others, as the 
digestive enzymes, are extruded from the cells and do their work outside. 
They work best at temperatures of 30 to 40°C., are inhibited by tempera- 
tures around 50°, and destroyed by prolonged exposure to this tempera- 
ture. Each enzyme accelerates some particular reaction, and all cells 
possess a wido variety of these agents. Theoretically an enzyme may 
accelerate a reversible reaction in either direction, and the direction is 
dependent on other conditions. Actually, however, the other conditions 
in living things are usually such that the enzyme works only one way. 
Som.e enzymes ordinarily break down substances (for example, the 
digestive enzymes); others build up materials into more complex sub- 
stances. The destructive type may be extracted and work in about the 
same way under artificial conditions. Those of the constructive class, 
however, seldom work outside of cells. Perhaps protoplasm could be 
manufactured in the lal^oratory if constructive enzymes worked as Avell 
in test tubes as the analytical or destructive ones do. 

Physical Structure of Protoplasm. — No matter how smooth and 
structureless protoplasm may look to be in a microscope, it is far from 

homogeneous. In general, it consists 
of particles of various sizes, mostly 
very minute, distributed through a 
supporting liquid substance. In the 
terms of physical chemistry, proto- 
plasm is a ''system" consisting of two 
"phases," of which the particles are 
'^gQS:C);#BlQ%MlC the "dispersed" phase and the sup- 
,, ^ ^^. , , porting liquid is the "continuous" 

i'lG. 27. — Diagram of an emulsion, i i- 

illustrating the physical structure of a phasc. In SO far as thc dispersed 
very common kind of protofjiasm. particles are liciuid and large enough 

to be visible in a microscope, such 
a mixture is an emulsion (Fig. 27). If the particles are submicro- 
scopic in size and liquid, as they usually are, the mixture is an emulsoid. 
Material in such a finely divided state is also said to be colloidal, or, 


though somewhat improperly, such substances are called colloids. The 
existence of invisible particles may be detected and they may be counted 
with the ultramicroscope against a dark background. Some of them 
may be photographed by means of the electron microscope. Even the 
fine particles are mostly larger than molecules and so may be composed 
of more than one substance. Their composition cannot be precisely 
known, but they must be relatively insoluble in water in order to main- 
tain themselves as particles. There are indications that the particles 
are surrounded by a lipoid film, which may have something to do with 
their insolubility in water. 

This whole structure is, of course, permeated with water, and there 
are always salts, and usually sugars, in solution. The particles of these 
dissolved substances, being either ions or single molecules, are much 
smaller than the dispersed emulsoid particles and confer very different 
properties on the protoplasm. 

Diffusion and Osmosis. — The molecules and ions of a substance in 
solution engage in continual spontaneous movement. So do the mole- 
cules of the water or other liquid in which the substance is dissolved. 
The particles bombard one another and the walls of the containing 
vessel if there is one. The direction of movement of individual particles 
is entirely impredictable. Yet if a substance is more concentrated in 
one part of a solution than in another, the particles spread more 
from the place of high to the place of low concentration than in the 
opposite direction. The spontaneous random movement of the particles 
in a solution is known as diffusion, and it tends to equalize the concen- 
tration in all parts. Protoplasm is the scene of constant shifts of this 
kind. The elimination of the waste product carbon dioxide is effected 
by diffusion from a place of high concentration in a cell or tissue to a 
place of low concentration in the surrounding air or water. The entrance 
of oxygen into the cell is dependent on the same principle. Rapid 
entrance of water into single-celled animals, requiring its elimination by 
pulsating vacuoles, is practically simple diffusion. There are many 
situations where an important physiological process is merely diffusion. 

There are places, however, in which the diffusion of different sub- 
stances is quite unequal. The membrane of a cell — not the dead wall 
or the secreted pellicle, but the outer film of protoplasm itself — exercises 
a selective influence on the passage of substances through it. Some 
substances pass through it readily, others slowly, still others practically 
not at all. The membrane is said to be semipermeable. The exchange 
of particles between two solutions on opposite sides of a semipermeable 
membrane is known as os77iosis. In general, the gases (carbon dioxide 
and oxygen) and water pass through a cell membrane rapidly. Simple 
sugars (glucose), the amino acids (components of proteins), and glycerol 



and fatty acids (components of fats) pass through slowly. The ions of 
inorganic salts, and the disaccharides (sucrose, etc., page 40) penetrate 
the membrane very slowly, and the proteins, polysaccharides, and fats 
practically not at all. For some of these substances the inability to 
traverse the membrane is explained by the large size of their particles. 
For the ions of salts it is probal)ly their electric charges which keep them 
out. The cell membrane itself has a charge, usually negative, which 
repels ions of like charge; and since the oppositely charged ions cannot 

part company, both are excluded. There are 
probably other reasons, not yet understood, for 
the retardation of passage of particles through 

The result of osmosis is easily illustrated by 
tying a piece of bladder tightly over a thistle 
tube, filling the tube with sugar solution, and 
immersing the expanded end in a dish of piu-e 
water (Fig. 28). After a short time it is found 
that the sugar solution in the tube has risen to a 
higher level, but that it is not so concentrated 
as at first. Water has obviously passed through 
the bladder into the sugar solution. A little, 
but not much, of the sugar has also found its 
way through the membrane into the Avater. 
The molecules of water are in constant motion, 
striking the walls of the dish, the membrane, and 
other molecules of water. Their impacts against 
the membrane drive some of them through. 
Now the water inside the thistle tube is also in 
motion, and some of its molecules pass out into 
the water of the larger vessel. But there are 
fewer molecules in a given volume of the sugar 
solution because the sugar molecules take some 
of the space, and their movement is less vigorous 
owing to hindrance by the sugar. Hence fewer 
molecules of water get out /of the thistle tube than would do so if the 
sugar were not there. Water is thus passing through the membi-ane 
in both directions, but more of it goes toward the sugai- solution than 
away from it. The sugar solution thus rises in the tul)e but becomes 
more dilute. 

Surface Phenomena. — An important consequence of the colloidal 
structure of protoplasm is the enormous surface exposed by the dis- 
persed particles. Extremely finely divided ])articles present a greater 
surface relative to their volume than do larger particles. This great 

Fig. 28. — Diagram of 
apparatus used to illus- 
trate osmosis. T , inverted 
thistle tube covered with 
animal membrane and con- 
taining a solution of sugar 
in water; V, vessel of 


surface increases the rate of chemical and physical activity at every face 
of contact between the two phases of the system. These activities have 
been called surface 'phenomena. Some surface phenomena are surface 
tension, adsorption, and various electrical phenomena. 

Surface tension is exemplified by the film at the surface of water, the 
external membranes of cells, the membrane of the nucleus, and the films 
that surround vacuoles. A considerable pull is exerted by these films. 

Extremely finely divided solids or those with extremely fine pores tend 
to condense on their surfaces anj^ gases or vapors or other substances 
with which they are in contact. Such substances are said to be adsorbed. 
The thin films of these adsorbed substances are held so tenaciously that 
great pressures are required for their removal. A gas mask removes 
gases from the air because of the great adsorptive power of charcoal, and 
the clarification of sirups and sugars is accomplished by making use of 
the adsorption of coloring matter by bone black. Certain properties of 
living matter are best explained on the basis of adsorption. 

Electrical properties are conferred on protoplasm by its ionized salts. 
Ions are capable, as explained in Chap. 3, of conducting electricity but 
in protoplasm are more important because they are probably adsorbed 
upon the surfaces of the colloidal particles. These particles thereb}^ 
acquire an electric charge. Through the interior of the cytosome the 
particles appear to carry positive charges, but in the nuclear sap they are 
negative. The surface of a cell as a whole seems, as stated before, to be 
negatively charged. The occurrence of like charges on the interior 
particles causes mutual repulsion and is probably the chief reason why 
these particles do not adhere to one another. If they did adhere, the 
protoplasm would coagulate or harden. 

Changes in Viscosity. — Viscosity is the resistance which the particles 
of a substance otfer to movement upon one another. The viscosity of 
light liquids like water or gasoline is low, while that of thick sirup — or 
still more so of solids — is high. When a bit of fresh meat is subjected 
to pressure while still warm, even if it be from an organ which like the 
liver has no conspicuous fibers in it, it appears to be highly viscous. The 
resistance is offered mostly, however, by the cell membranes. These are 
firm enough, like well-filled bags of wheat, to tend to preserve the shape 
of the cells. The interior protoplasm of a cell, at least of those which 
have been studied in this respect, turns out to be quite liquid. In one 
kind of cell the protoplasm is only about ten times as viscous as water 
and only about one one-hundredth as viscous as ordinary glj^cerin. 

This fluid state is probably maintained, as indicated in the preceding 
section, by the like electric charges on the colloidal particles in the proto- 
plasm, causing these particles to repel one another. The viscosity 
changes frequently, however, for reasons not yet understood. Such 


changes occur regularly during cell division, the protoplasm being firmer 
at the beginning of division, more liquid (less viscous) later on. 

Metabolism. — The protoplasm of a cell carries on all the general 
processes of any living body. Within it occurs a multitude of complex 
chemical reactions by which the protoplasm maintains and renews itself 
and produces more protoplasm. Protoplasm digests food and for this 
process secretes various chemical substances. When food is broken down 
into simpler substances during digestion, it is absorbed and built up into 
the living substances itself or perhaps is combined with oxygen for the pro- 
duction of heat and motion. Protoplasm also respires, gets rid of waste 
materials by the process of excretion, grows, is capable of movement, 
and responds to changes in external conditions, or exhibits irritability. 
The chemical processes involved in all these activities of protoplasm are 
included under the term metabolism. 

Metabolism may be defined as the sum of all the chemical and physical 
processes carried on within the protoplasm. It consists of two phases, 
namely, the constructive phase or anahoUsm and the destructive phase 
or catabolism. Anabolism includes all the processes concerned in the 
growth and repair, or upbuilding, of protoplasm. It includes all processes 
by which substances are transformed into reserves of food. Catabolism 
includes all those processes opposed to anabolism. These are the proc- 
esses by which protoplasm is broken down and the waste products 
eliminated. Both anabolism and catabolism are continous processes. 
As long as anabolic processes are in excess of catabolic processes, growth 
occurs; but when catabolic processes are in excess a diminution in size 
takes place. 

So far as metabolism of animals relates to food, it pursues the following 
cycle in the economy of living things collectively. Organic food is first 
made out of inorganic matter through the process of photosynthesis in 
plants. These organic substances become the food of animals which 
arc unable to subsist on inorganic food. The animals digest these foods, 
and from the simpler digestive products build up their protoplasm 
through the process of assimilation. To supply the energy required for 
all this work the animal must secure oxygen by respiration. Waste 
materials produced along the way are eliminated by excretion, and useful 
products accessory to the general processes are elaborated by secretion. 
One of the products of the food cycle is commonly growth. All these 
processes are part of metabolism; they are described in the next seven 

Photosynthesis. — Tlu; things which i)hints may take in are water 
and salts from tiic soil, and oxygen and carbon dioxide (COo) from the 
air (or water, in the case of aquatic plants). The fii-st three of these 
are utilized in about the same way in plants as in animals. The carbon 



dioxide and some of the water, however, are put to a totally different use. 
Carbon dioxide is a by-product of the burning of coal or wood or the 
decay of dead animals and plants or of anything else composed partly of 
carbon. It is constantly' being thrown off as a waste product by animals 
and by plants, except as they use it in the process about to be described. 
Plants absorb the carbon dioxide into their leaves or other green parts 
and there combine it with water to form one of the simple sugars, glucose. 
The final results of this reaction are indicated by the equation 

6CO2 + 6H0O + energy^CeHijOe + 6O2 

Tn words this means that six molecules of carbon dioxide and six of 
water are decomposed and their parts recombined to form one molecule 
of glucose and six molecules of oxygen. The energy 
expended in bringing about this change comes from 
sunlight, hence the process is called photosynthesis, 
literally construction by light. In most plants 
production of glucose can occur only in the pres- 
ence of chlorophyll, the green substance in their 
plastids, and certain enzymes. The energy of the 
sun in this reaction appears not to affect the car- 
bon dioxide directly, but to decompose the water. 
The hj'drogen set free from the water is picked up 
by other substances which then, without any aid 
from light, proceed to attack the carbon dioxide. 
The oxygen that is liberated is not produced 
directly by the decomposition of the original raw 
materials; it comes from a peroxide which is an 
intermediate product. That oxygen is liberated 
may be demonstrated by an experiment with water 
plants. In such an experiment the cut ends of a 
Avater plant, as Elodea, are inserted in a test tube 
filled with water, the plant and tube are immersed 
in water, and the tube is inverted (Fig. 29). 
When the plants are placed in sunlight, bubbles of gas escape from their 
cut ends and collect in the tube. Suitable tests show the gas to be 

Photosynthesis is not absolutely limited to plants, for there are some 
simple animals which contain chlorophyll, and in these glucose is pro- 
duced in the same way as in plants. Nor are chlorophyll and light 
always necessary for the production of glucose, since some colorless 
organisms are capable of doing this in darkness. 

Plant Products as Food of Animal Cells. — Inasmuch as most animals 
are incapable of producing carbohydrates directly from inorganic com- 

FiG. 29. — Method 
of collecting oxygen 
produced by the 
aquatic plant Elodea 
during photosynthesis. 
The oxygen rises from 
the plant into the 
closed end of the test 


pounds or the simple elements, they must get them from plants. Plants 
store any excess of carbohydrates above their immediate needs, in some 
insoluble form, usually starch or some similar substance. Animals, from 
the simplest one-celled ones up to the most complicated, use these stores 
of plant starch for food. Out of these plant carbohydrates the charac- 
teristic components of animal protoplasm are made. Glucose is to be 
had by merely breaking down the starch. Glucose can be converted, 
mostly by rearrangement, into glycerol and fatty acids; from these, fats 
may be formed. 

For one of the essential parts of animal protoplasm, however, the 
plant starches will not suffice; that is the highly important class of 
proteins. Animals in general cannot make proteins out of inorganics 
substances. Only a few can make proteins out of carbohydrates. There 
is something lacking in the physiology of most animals which prevents 
them from making this particular synthesis. The missing thing is 
probably an enzyme or a set of enzymes. Animals must therefore get 
their proteins, as well as their carbohydrates, either directly or indirectly 
from plants. They may obtain these proteins from other animals, as 
the carnivorous animals almost exclusively do, l^ut these other animals 
must get the proteins ultimately from plants. • 

Conversion of Food. — Very little of the food which animals take can 
be utilized at once for its ultimate object, unless water and oxygen be 
considered food. Most of the food has to be worked over in some way. 
Glucose and other equally simple sugars are ready to use, but these 
constitute only a very small fraction of the food of animals. One of the 
chief reasons why other foods cannot be used at once is that they are 
not soluble. The starches, lipids, and proteins must all be converted 
into some form that will diffuse through protoplasm. This conversion 
is effected in the process of digestion. 

Digestion is essentially the same process everywhere but will be con- 
sidered here chiefly as it occurs within cells rather than in the cavities of 
large organs like the stomach. Unicellular animals take in small organ- 
isms and surround them with a droplet of water containing one or more 
enzymes, thus forming a food vacuole. All such animals can produce 
enzymes that will digest proteins, many can digest starches, most of 
them can digest fats. Proteins are dismembered to yield their amino 
acids; fats are split up into glycerol and fatty acids; starches are con- 
verted into simple sugai's. The final products named in each case are 
all soluble in water and can diffuse through protoplasm. 

In this soluble form they pass to every part of the cell, or from cell 
to cell. Oxidation of them may occur if energy is needed. The deriva- 
tion of energy from oxidation of glucose is represented by an equation 


which is just the reverse of that by which ghicose is formed in photo- 
synthesis, namely, 

CeHizOe + 6()2->6H20 + 6(X^2 + energy 

This equation says, in words, that one molecule of glucose and six 
molecules of oxygen are recombined (in combustion) to form six molecules 
of water and six molecules of carbon dioxide, with the release of energy. 
Even some of the transitory steps involved in this reaction are reversals 
of those occurring in photosynthesis. 

If new protoplasmic structure is required, the soluble products of 
digestion are available for this purpose. If the digested foods are in 
excess of the requirements for these two purposes, they may be stored; 
but in this case they must be rendered insoluble again, for otherwise they 
could not be retained. If carbohydrates are to be stored in animals, the 
glucose is commonly converted into animal starch or glycogen. Glycerol 
and fatty acids are again converted into fats, although the fats are likely 
to be of different kinds from those which were taken as food. The pro- 
duction of these insoluble storage products is done by enzymes, and the 
same enzyme may work in both directions, that is, either break down 
substances (starches, for example) or build them up. 

Little is known about the construction of new protoplasm out of 
digested foods. The name assimilation is given to the process, and it 
seems certain that enzymes are engaged in the work, but of its nature 
we are mostly ignorant. 

Respiration.— To provide energy or new protoplasm, all living things 
require oxygen. Land animals and plants get it from the air, submerged 
aquatic ones from the oxygen which is dissolved in water. There 
are, however, some kinds of animals and plants that normally live in 
situations devoid of oxygen, and some of these organisms would die 
if brought into contact with free oxygen. Such organisms require 
oxygen in their metabolism, but they secure it from compounds in 
which it occurs. 

The combination of oxygen with protoplasm and foods results finally 
in the formation of water and carbon dioxide, as indicated by the equation 
in the preceding section. The carbon dioxide must be eliminated. The 
absorption of oxygen and the elimination of carbon dioxide are together 
called respiration. 

In simple animals and plants, dissolved oxygen diffuses directly 
through the surface of the organism into the protoplasm. Thence by 
diffusion and protoplasmic currents it is carried to all parts of the cell. 
In many small multicellular animals and plants with few layers of cells the 
oxygen may readily diffuse through the intervening cells to those which 


lie deeper. In larger organisms, however, a transport system is required, 
as discussed in Chap. 11. 

Excretion. — Metabolism results in the formation of various gases, 
water, and other compounds, which are of no value in the body or would 
be harmful if allowed to accumulate. The process of their elimination 
is called excretion. Gases resulting from metabolism are eliminated 
along with carbon dioxide in respiration. Other waste substances pass 
through the cell membranes to the exterior, or in some of the protozoa 
they are collected by the contractile vacuoles, along with excess water, 
and voided through the outlets of these organs. In higher animals 
excretions are taken up by the blood and lymph, from which they are 
then separated by special organs. 

Secret;;ion. — All cells produce certain chemical compounds which may 
be used in the processes going on within the cell or in cavities adjoin- 
ing the cells. Such products are called secretions. They differ from, 
excretions in that they are used in performing some function. Many of 
the secretions which are discharged from the cells are first stored in the 
cells as granules, which finally break out of the cell and then become 
gaseous or liquid. Other secretions produced as liquids within the cell 
diffuse out and escape as rapidly as formed, are absorbed by other cells, 
or are carried in the blood stream. Such secretions may perform their 
functions at a considerable distance from the cells where thej' were 
elaborated. Secretions are very diverse in their uses. Some aid in 
digestion, others give protection because of their odor or because oi 
poisonous properties, some serve as lubricating material, others oxidize 
readily with the production of light as in fireflies. 

Growth. — Growth is caused by the conversion of foods into proto- 
plasm at a more rapid rate than protoplasm is being broken down. 
Increase in the size of cells may not be wholly due to increase in the 
quantity of protoplasm. Fat cells increase in size because of the depo- 
sition of globules of fat, a process which may be continued until there is 
much more fat than protoplasm. In plant cells and certain animal cells 
volume may be increased by the imbibition of water which may be 
stored in vacuoles. In such extreme cases as those mentioned, the 
quantity of protoplasm may be actually decreased, although the cell 
may be larger. 

Reproduction. — Reproduction, or the formation of new individuals, 
is likewise characteristic of living beings. In unicellular organisms, and 
only in these, reproduction is equivalent to cell division. In higher 
organisms, reproduction usually involves the formation of special cells, 
the germ cells, which by their division, with rearrangement of the result- 
ing cells, give rise to new organisms. Here reproduction involves cell 



division too. Cell division is described in Chap. 5, reproduction in 
Chap. 14. 

Fig. 31. — Fibrillar structure of 
cilium of Stylonychia. {From Del- 
linger in Journal of Morphology.) 

I'lG. 30. - Locomotion in an amoeb;i with sevcM :il psoudopodia, which rest on the substratuni 
only at their tips. (From Dellinger in Journal of Experimental Zoology.) 

Protoplasmic Movement. — One of the attributes of living organisms 
usually distinguishing them from nonliving matter, is the power of 
independent motion. Most animals at some stage in their existence, 
many plants of the lower orders, and the 
swarm spores of other low plants are 
motile. Higher plants are not capable of 
locomotion, but within their cells the 
protoplasm may undergo movement. 

In many cells the protoplasm frequently travels as if in channels, 
particle following particle, carrying plastids, food vacuoles, and cell 

inclusions along with it. When 
an amoeba (a one-celled animal) 
moves, it thrusts out one or more 
lobelike processes, called pseudo- 
podia. Then the body is pulled 
forward or flows forward. Some- 
times there is only one pseudo- 
podium, and the amoeba just 
flows along. In other kinds of 
amoeba there are several pseudo- 
podia at one time, and only their 
tips touch the substratum, in 
which case the animal may almost 
be said to walk (Fig. 30). A 
pseudopodium is extended appar- 
ently because of a local increase 
of viscosity in the outer layer of 
protoplasm at some part of the 
cell, carrying with it a slight contraction which forces the protoplasm else- 
where to protrude; but how the change in viscosity is effected is not clear. 

Fig. 32. — Form of cilium during strokes; 
forcible stroke at left, return stroke at right. 
Numbers show successive positions, indicate 
direction of movement. 



Many of the simple unicellular animals and some of the multi- 
cellular ones perform movements by means of cilia or flagella. The 
cilia are minute hairlike projections capable of rapid vibration. 
Each cilium has an elastic outer layer containing one or more con- 
tractile threads within it, as in Fig. 31. Contraction of the threads 
on one side bends the cilium in that direction, and elasticity of the sheath 
causes it to return. 

In the vigorous stroke of a cilium, it is extended and moderateh' 
stiff, so as to catch much liquid; on the return stroke it bends limply 
nearer the surface of the cell (Fig. 32). Neighboring cilia usually beat 
in unison or in waves. 

Flagella differ from cilia chiefly in their greater length and are few 
in number (usually one to eight per cell). Sometimes the flagellum is 

surrounded by a vaselike collar (Fig. 33). 
Flagella may beat regularly in one plane, as do 
cilia, or they may have a rotary motion. The 
whole flagellum may move, or only the free end of 
it. The flagellum is composed of an elastic per- 
ipheral layer within which are several contractile 
threads (Fig. 34), and the movement is due to 
'^^s the contraction of these threads. Flagella give 
a motile cell a jerky erratic movement; cilia cause 
it to glide. 

Fig. 33.— Portion of ReSDOnses to Stimuli. — A characteristic pro- 

cross section of the sponge ,• 1- • J, ■ -i 1 •!•, J 1 i 

Grantia. cc, collared cells perty ol livmg matter is its ability to respond to 
of endodenn; ect, ecto- stimuli. A stimulus is anv influence of sufficient 

derm; fl, flagellum of col- • i , \ • , i 

lared cell; mes, mesogioea; magnitude to cause a change m protoplasm. 

sp, spicule (portion only), ^j-^g stimulating agent may be external to the 
organisms, such as changes in light, temperature, chemical substances, 
sound, pressure, or electric current; or it may originate within, through 
osmosis, electric charges, chemical substances, pressure, or nerve impulses. 
To be a stimulus, the modification must have a certain degree of 
suddenness. A very gradual change in the intensity of light may have 
no observable effect, while a sudden change of the same amount produces 
a marked reaction. 

Responses are of very different sorts. Muscle cells and others con- 
tract; gland cells produce secretions. Pigment cells in the skin of a 
frog, which are highly branched and have their pigment distributed 
throughout all parts of the cell when at rest, contract their pigment into 
a small compact mass in response to light, thereby changing the animal's 
color. Streaming of protoplasm in plant cells stops in response to an 
electric current. A chemical substance in the retina of the eye of 



vertebrate animals is decomposed by light. The electric organs of 
certain fishes produce a series of discharges. 

The nature of the response is determined by the nature of the respond- 
ing protoplasm, not by the kind of stimulus. A muscle cell contracts, 
whether the stimulus be chemical or electrical. A gland cell secretes, 
and its product is always the same, regardless of what started its 

The extent of a response is, in general, rather definitely fixed for any 
given cell. If the cell responds at all, it does so to its full capacity. An 
organ made of many cells may respond in various 
degrees, depending on whether few or many of its 
component cells join in the response. How many 
cells respond depends on the intensity of the stimulus. 
Each individual cell, however, follows the all-or-none 
rule of acting either at its maximum capacity or not 
at all. 

What Is Living Matter? — The characteristics of 
living matter enumerated in the opening paragraph of 
this chapter do not constitute a criterion which would 
enable even an expert to say in a specific instance 
whether a bit of matter were alive. Application of 
the rules would occasionally be futile. The chemical 
composition of recently killed protoplasm would, on 
analysis, be indistinguishable from that of living proto- 
plasm; but something intangible would be gone from 
it. Spontaneous movement and change of shape may 
occur in a drop of liquid, under certain circumstances, 
because of changes in the surface film. Moreover, 
living things in the form of resting spores exhibit no 
detectable movements over long periods of time. A crystal may be made 
to convert part or all of itself into a flock of smaller crystals, in a way 
that would be hard to exclude in a definition of reproduction. Finally, 
metals respond to things in the environment, such as a magnet or 
electric potential. 

A definition of life which lists the ordinary activities or conditions of 
Uving things is feasible; but it could not be used practically for a 
complete classification of all objects into two categories, living and 

A B 

Fig. 34.— Fla- 
gellura ifl) of Eu- 
glena, showing 
(right) contractile 
threads within it. 
(B after Dellinger 
in Journal of 


Heilbrunn, L. V. An Outline of General Physiology. W. B. Saunders Company. 
Marsland, D. Principles of Modern Biology. Henry Holt and Company. 


Mitchell, P. H. Textbook of General Physiology. McGraw-Hill Book Company, 
Inc. 3d Ed. (Chap. VII, the chemistry and physiology of proteins, lipids, and 
carbohydrates; Chap. VIII, the salts; Chap. IX, water and electrolytes.) 

Rogers, C. G. Textbook of Comparative Physiology. McGraw-Hill Book Com- 
pany, Inc. 2d Ed. (Chap. Ill, diffusion and osmosis; Chaps. IV and V, struc- 
ture and properties of protoplasm.) 

Verworn, M. General Physiology. The Macmillan Company. (Part II of 
Chap. II, contrast of living and lifeless.) 


When cells .were first discovered, and even after it became fairly cer- 
tain that all organisms were composed of them, no one appreciated how 
fundamentally the cells Avere involved in the constitution of living things. 
They were thought, for example, to be of secondary origin; that is, 
animals and plants were believed to possess a formative or nutritive 
substance without any particular organization or structure, and out 
of this the cells were supposed to be formed. While all organisms were 
found to contain cells, it was not thought that these cells had any neces- 
sary function in the production of new cells out of the formative material. 
Gradually, however, the idea gained ground that the origin of new cells 
occurred by division of old cells, a doctrine which in 1855 was expressed 
by the famous pathologist Virchow in the words omnis cellula e cellula — 
all cells from cells. While the origin of cells from cells was thus early 
recognized, the mechanism by which cells originated from other cells 
was not known until twenty or thirty years later. It was not until 
1873 that the common method of cell division — resolution of the chroma- 
tin into distinct separate bodies and the formation of a spindlelike 
mechanism manipulating these bodies — was discovered. The same 
method was soon witnessed in a variety of plants and animals and is 
now found to be nearly universal. To this method of cell division the 
names mitosis and karyokinesis are applied. The latter is the more 
descriptive, but the former is more often used. 

Interphase. — A cell not in division is said to be in interphase. In 
such a cell the chromatin is so diffuse as to present the appearance of a 
network (Fig. 35 A). Actually, in most cells, this chromatin exists in 
a number of distinct portions, the chromosomes; but the threadlike form 
which these chromosomes take in most animals makes it impossible to 
distinguish them. In a few organisms (some grasses among them) the 
chromosomes are more condensed and are separately visible even in 
the interphase. In some special tissues, such as the salivarj^ glands of 
flies, the chromosomes are greatly enlarged and are more easily recog- 
nizable in interphase than in any cell division. The chromosomes of 
these glands also have a pattern by which they can be distinguished; 
and every nucleus has a set of chromosomes identical in pattern with 
those of any other nucleus. The individuality of the chromosomes which 




is so evident in these glands undoubtedly exists elsewhere. One indi- 
cation of this is found in animals which have different shapes and sizes 
of chromosomes. At every division there is the same number of chromo- 
somes of a given shape and size, which could hardly be true unless the 
chromosomes maintained their identity in the intervening interphase. 
Also, chromosomes may be broken up by X rays, and reconstituted in 
new sizes and shapes, and these new chromosome forms appear again 
after cell division and in later generations. Obviously chromosomes 

Fig. 35. — ^Mitotic cell division. A, cell not in division; B, centrioles move apart; C, 
distinct chromosomes formed; D, nuclear membrane dissolved, spindle completed; E, F, 
equatorial plate, side and end view, with chromosomes duplicated; G, H, chromosomes 
move apart; /, /, division of cytosome and construction of new nuclei. (A, interphase; 
B-D, prophase; E, F, metaphase; G-I, anaphase; J, telophase.) 

maintain their individuality in the interphase, even though it cannot 
be observed. 

Prophase. — Mitosis is nearly enough alike in most cells to make 
possible a general account of the process. Starting with a cell in inter- 
phase, in which the centriole is already divided into two parts, one of 
the early signs of division is the condensation of the chromatin into 
distinct threads tangled about in the nucleus (Fig. 35/?). In whatever 
way the chromosomes (page 26) are spread out through the nucleus, 
they now contract into smaller compass, usually in the form of slender 
strings or ribbons. The parts of the centriole' separate and move toward 
opposite sides of the nucleus. Sometimes between them a few threadlike 


lines are stretched, and around each one radiating Hnes may develop, 
giving the appearance of a star. The contraction of the chromatin 
continues, and before the process is more than well under way the chro- 
mosomes are distinguishable as separate bands or ropes (C). The 
entire scattered chromatin of the interphase nucleus is now collected 
into these conspicuous bodies. Though the chromosomes have been 
separate bodies all the time, it can now be seen for the first time that they 
are distinct. While this change in the chromatin has been taking place, 
the nucleolus, if one was present, has disappeared. The centrioles have 
moved around to opposite sides of the nucleus, and very distinct threads 
from them appear to be pushing against or even into the nucleus. The 
membrane of the nucleus then dissolves away, leaving the chromosomes 
free in the general protoplasm. Some of the threads from each centriole 
quickly pass through the space formerly occupied by the nucleus and 
connect with the other centriole, establishing a complete spindle between 
them. Other threads go only halfway and end at the chromosomes. 
The chromosomes shorten still further and thicken to form definite 
bodies, often of very different shapes and sizes within the same cell. 
The chromosomes are placed where the nucleus was, without any par- 
ticular arrangement. The changes so far described, including stages 
B to D in the figure, are collectively called the prophase, though the 
plural form would be more accurate. 

Metaphase. — The chromosomes then move, probably are drawn, into 
a flat group across the middle of the spindle. In this position they 
form what is called the equatorial plate. Seen from the side of the 
spindle they appear as in E, but viewed from one of the centrioles they 
are as in F. This stage of mitosis is called the metaphase. It is of very 
brief duration, so that it appears less often in preparations than the 
other stages do. Either in the metaphase or at some earlier time, 
the chromosomes become double structures. This doubling is usually 
described as a division, but it may equally well be conceived as a dupli- 
cation, that is, the formation of a second chromosome just like the 
original. It is not important to decide at this point which of these 
methods is employed, since in either case two identical chromosomes 
exist where only one of that kind existed before. The chromosomes 
are shown thus duplicated in E, less clearly so in F because of the direction 
from which they are viewed. 

An important feature of this division is that the two chromosomes 
produced from one are, in all significant features, identical with each 
other and with the original chromosome which produced them. To 
understand this fact one must know that the chromosomes have a 
longitudinal pattern. They contain different substances at different 
points in their length. A longitudinal division of the chromosome 


divides all the different components, so that the resulting two chromo- 
somes have the same pattern as the original one. 

Anaphase. — From their position in the equatorial plate the two 
chromosomes, formed from one, move or are drawn toward opposite 
ends of the spindle. This stage is known as the ana-phase. The shapes 
of the chromosomes often indicate that they are being pulled. Thus, in 
Fig. ?>bG, the long chromosomes could be given their V shape by being 
pulled from their middle points toward the centrioles. Moreover, some 
of the so-called fibers extending out from the centrioles may often be 
seen to attach to the chromosomes at these points. Consequently, the 
fibers are often thought of as pulling the duplicated chromosomes apart. 
Whether they actually pull or not is uncertain. The fibers may be only 
lines of flow in the protoplasm, that is, courses along which the fluid 
protoplasm is moving. Whatever causes this flow could drag the chromo- 
somes along. If the middle parts of the long chromosomes were caught 
in this current, the characteristic V form of such chromosomes would 
still result. 

Whatever the cause of their movement, one chromosome of each 
pair of duplicates goes to each end of the spindle {H). Here they collect 
in two close groups (/), ready to form two new nuclei. In the meantime 
the cytosome narrows between the retreating groups of chromosomes 
(//, 7) and finally constricts in a sharp furrow (/) which eventually cuts 
the cell completely in two (J). In many cells, about this time, the 
centriole divides in two, as if in preparation for the next division (7), so 
that during the whole ensuing interphase the centriole is double. 

This separation of the daughter chromosomes has as important a 
consequence as does the longitudinal duplication of each one. ' The 
chromosomes are of different kinds; they contain different things. Each 
cell possesses a complete set of the different kinds of chromosomes. The 
accurate separation of the sister chromosomes, one going to each pole 
of the spindle in the anaphase, insures that the two daughter cells will 
likewise have a complete set of chromosomes. All the cells of a multi- 
cellular animal thus have identical chromosomes in them. 

Telophase. — The remainder of the process of cell division consists of 
th(^, restoration of the chromosomes to the diffuse state in whicli they 
existed before division began, and the disappearance of all remnants of 
the divisi(m apparatus from the cytosome. The chromosomes become 
diffuse either by becoming filled and distended with a fluid or by spinning 
out their chromatin into fine, perhaps branching, threads, as explained 
on page 26. Some particulars of this process are given later. By either 
method the chromatin comes to be scattered in irregular knots or strands, 
giving the appearance of a network. A membrane is formed about the 
whole grou]) of chromosomes (./) and the reconstruction of the nuclei 



approaches completion. During these changes the new nucleus may 
rotate considerably in the cytosome, as it is shown to have done in the 
illustration. In the figure (J) the two cells are shown in different stages 
of the reconstruction process. This is done merely to illustrate the 
steps involved, for as a rule they transform at about the same speed and 
are at all times in about the same stage. 


A B C 

Fig. 36. — Chromosomes of various shapes and sizes shown just before they are arranged 
across the middle of the spindle. A, oogonium of the beetle Dytiscus (from Dehaisieux 
in La Cellule); B, spermatogonium of arrow worm Sagitta (from Bar das in La Cellule); 
C, egg of hellbender (from B. G. Smith in Journal of Morphology and Physiology) . 

The principal other features of the reconstruction are the loss of the 
remaining spindle fibers in the cytosome and the formation of a nucleolus 
if there was one prior to division (J) . When these steps have been taken, 
two new cells of smaller size, essentially identical with one another, have 
been produced from one older cell. 

Variations Relating to Chromosomes. — While the foregoing account 
represents a fairly typical mitosis, there are many variations in the 
process. The number of chromosomes differs greatly in different species. 

Fig. 37. Fig. 38. 

Fig. 37. — Splitting of the chromosomes before the equatorial plate stage; peritoneum 
of the salamander Ambystoma. (From Parmenter in Journal of Morphology.) 

Fig. 38. — Reconstruction of nuclei through imbibition of liquid by the chromosomes 
to form vesicles. A and B, early and late stages of vesiculation in the egg of the sea urchin, 
in which the vesicles fuse. (From Danchakoff in Journal of Morphology.) 

In the parasitic worm Ascaris megalocephala each cell has 4 chromosomes; 
in the vinegar fly Drosophila melanogaster the number is 8; and man 
has 48 chromosomes. Most of the numbers from 4 to 60 are found in 
one or more species, and there are some numbers above and below these 
limits. The number differs in the two sexes in some animals, being 
usually more numerous in the female when there is such a difference. 



The chromosomes differ greatly in size in different organisms, and 
often in the same cells. Two sizes of chromosomes are shown in Fig. 35, 
and further differences are represented in Fig. 36. 

The time of duplication or splitting of the chromosomes varies con- 
siderably. In some cells, as in Fig. 35, the chromosomes do not duplicate 

Fig. 39. Fig. 40. 

Fig. 39. — Vesiculation of chromosomes by formation of protoplasmic film around each 
chromosome; A early, B late stage. The vesicles do not fuse. 

Fig. 40. — Interphase nucleus of the hellbender, showing the chromosomes distinct and 
separate as vesicles, in which, however, the chromatin is very diffuse. {From B. G. Smith 
in Journal of Morphology and Physiology.) 

themselves until they are in the metaphase. In others they are doubled 
while still in the long ropelike stage before taking their places on the 
middle of the spindle (Fig. 37), that is, in the prophase. 

Fig. 41. Fig. 42. 

Fig. 41. — Mitosis without centrioles in a cell of the root tip of the hyacinth. {From 
Dahlgren and Kepner, "Principles of Animal Histology.") 

Fig. 42. — Dividing cell with conspicuous spindle in whitefish embryo. (.Courtesy of 
General Biological Supply House.) 

The expansion of the chromosomes to form new nuclei at the close 
of division differs in different animals and plants. In some species there 
is a very plain formation of vesicles by the accumulation of liquid \\ithin 
each chromosome (Fig. 38.4). Then the vesicles fuse to form one largo 
vesicle {B), though it is still quite likely that the chromosomes maintain 



their individuality in and at the wall of this vesicle. In other organisms 

a film of protoplasm forms around each chromosome, 

and within the vacuole so created the chromatin spreads 

out in diffuse form (Fig. 39). In the example used in 

this figure these vacuoles do not fuse but remain 

separate. A third way of rendering the chromosomes 

indefinite in appearance, hinted at in the preceding 

account and earlier on page 26, is to have their chromatin 

spin out into fine threads, often branching, without the 

formation of vesicles in or around them. This method 

is combined with vesicle formation in the generalized 

illustration Fig. 35 (/, left). Occasionally, even when 

chromosome vesicles are formed, they are distinguishable 

as separate objects during the interphase (Fig. 40). 

Variations in Spindle and Cytosome. — A striking 
variation in the spindle is the lack of any centrioles in 
the cells of flowering plants (Fig. 41). In animal cells ^ ,^ ^ ^ 

. ^ \ o / Fig. 43. — Intra- 

they may be very minute but are usually present. The nuclear spindle in 
rest of the spindle, that is, the fibers and radiating lines 1,^®, piotozoon 

•^ ' ' ° Euglypha. {From 

about the centrioles, may or may not be conspicuous. Wilson," The Cell," 
In Fig. 42 the spindle fibers and the rays around the ^^^^^ Schevnakoff.) 
centrioles are very conspicuous. But in the very fiat cells in the outer 

layer of the skin of salamanders there is 
little or no sign of a spindle, even though 
the chromosomes are sharply defined. 

The place where the spindle forms is 
different in different organisms. In Fig. 
35 it is shown forming outside, but near, 
the nucleus. This is its usual origin. But 
in certain protozoa and some multicellular 
animals it forms wdthin the nucleus. In 
^f^^^^^^mlKi^M^^ such animals the spindle may be well 

developed and the chromosomes arranged 
on it, or the chromosomes may even be 
moving toward the ends of the spindle 
(Fig. 43), before the nuclear membrane 

With respect to the cytosome, the prin- 
,,,...,, .p cipal variation is the way in which the two 

the beginmng of the process, {rrom ^ -^ 

Dahigren and Kepner, "Principles cells produced by division are separated 
of Animal Histologyn f^^^ ^^^ another in plants as compared 

with animals. Instead of dividing by means of a furrow around the 
cell, plant cells form a group of nodules on the middle of the spindle 

Fig. 44. — Formation of the cell 
plate in a dividing cell of the root 
tip of the hyacinth. The thicken- 
ings on the fibers of the spindle are 



(Fig. 44). These lumps increase in size until they coalesce into a plate, 
which forms a new wall dividing the cell into two. 

Amitosis. — Amitosis is a type of cell division which involves no com- 
plicated visible mechanism. The word means, literally, not mitosis. 
Many supposed examples of amitosis are merely distorted forms of 
mitosis, the distortion being due either to faulty preparation or to natural 
degenerative changes in the cells. Preparations of cells have in some 
cases been so defective that cell division was at first regarded as amitotic, 
but better technique revealed some of the features of mitosis. Also, in 
certain degenerate animals it appears that the process itself has become 
so modified that even the most perfect preparations of dividing cells 
resemble amitotic division very closely. 

Fig. 45. — Amitotic division of the nuclei in the follicle cells of the cricket's ovary. 
Various stages of nuclear division are shown. (From Conklin.) 

Confusion has arisen from the fact that the nucleus of a cell may 
divide without any subsequent division of the cell body. This division 
is often called amitosis of the nucleus, but it is not amitotic cell division. 
Follicle cells in the cricket's ovary (Fig. 45) show nuclear division of 
this sort. 

Genetic Significance of Mitosis. — The longitudinal duplication of the 
chromosomos and the equal distribution of sister chromosomes to the 
cells in division, to which attention has been called, has a greater sig- 
nificance than has yet been indicated. The chromosomes contain the 
units of heredity, which are called genes (Chapter 17). It is these genes, 
more than anything else, which are arranged in longitudinal pattern in 
the chromosomes. In the division of the chromosomes, the greatest 
imi)ortance attaches to the duplication of the genes. The necessity of 
distributing a complete set of cihromosomes to each cell rests on the 


necessity of having a complete set of genes in each cell. Incidentally 
it is the genes in the chromosomes which make the word duplication 
preferable to division in describing the formation of two chromosomes 
from one, for the genes may be single protein molecules. As such they 
could not be divided and retain their identity; they could, however, be 

How the cells in different parts of a multicellular animal become and 
do different things when they contain identical chromosomes and genes 
is a question which must be postponed until embryonic development is 
studied. The even greater importance of genes and chromosomes in 
reproductive cells, and a different type of cell division which manipulates 
the genes in germ cells, must likewise await the discussion of embryology. 


Calkins, G. N. Biology of the Protozoa. Lea & Febiger. (Pp. 208-245, types of 
division in unicellular organisms.) 

Dahlgren, U., and W. A. Kepner. A Textbook of the Principles of Animal His- 
tology. The Macmillan Company. (Chap. V.) 

MiNCHiN, E. A. An Introduction to the Study of the Protozoa. E. J. Arnold & Son, 
Ltd. (Chap. VII.) 

Sharp, L. W. An Introduction to Cytology. 3d Ed. McGraw-Hill Book Com- 
pany, Inc. (Chap. VIII.) 

Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan 
Company. (Chap. II.) 


Knowledge of the structure, function and multiplication of single 
cells should pave the way for an understanding of the more intricate 
structure, function, and interrelations of the complex animals or metazoa. 
As a step toward such an understanding it will be useful to reflect upon 
some of the consequences of the differences between the complex and 
the simple. 

Insight into the nature of multicellular organisms would be furnished 
by some certain knowledge of how they became multicellular. It seems 
clear that living things have not always existed in the highly complicated 
form that many of them now show. There must have been an origin of 
complex beings from simpler ones. This conclusion is often couched in 
the statement that multicellular organisms must have arisen from 
unicellular ones, but it would be somewhat safer, as we shall see, not to 
imply that cells were involved in the change. Some biologists hold that 
a step comparable to a change from one cell to many was made before 
these living things had arrived at a genuinely cellular constitution. But 
whatever the origin of multicellular organisms was, if we knew that origin 
we should have an important clue to some of their other characteristics. 

Relation of Parts to the Whole. — Two schools of thought have arisen 
concerning the relation between multicellular animals and the cells of 
which they are composed. One school has held that the whole is the 
sum of its parts; hence that many-celled organisms are what their cells 
make them. If cells of a certain structure and certain capacities are 
assumed, any body composed of them will have the combined structures 
and ca])acities of those cells. The other school has regarded the whole 
as superior to its parts. A living thing is a whole first of all; its parts are 
secondary. Animals and plants are not determined by the cells com- 
posing them. Instead, they impress upon their cells certain properties 
because the parts of the given whole must have those properties. Tlio 
former view is the more easily understood of the two, though probably 
only because in the physical and industrial world about us we see many 
examples of construction of wholes, such as buildings, out of units, such 
as bricks, whose properties are predetermined and do not change, just 
as bricks do not change when they are set in a wall. We are not accus- 



tomed to building materials whose nature depends on the kind of struc- 
ture to which they contribute. 

Two Contrasted Theories of Multicellular Origin. — In consequence of 
these two views of the relation of parts to wholes, two general theories 
of the origin of multicellular organisms have been entertained. Accord- 
ing to one theory, parts have joined to make wholes; cells have joined 
to make many-celled bodies. According to the other theory, wholes have 
been divided into parts. Organisms became complex, then divided into 
cells whose qualities were dictated by the nature of the whole from which 
they were produced. 

Which of these theories contains the greater element of truth it is 
impossible to say. As applied to the origin of metazoa, both have 
received ardent support from biologists. Both have certain physiological 
facts in their favor. On the one hand, as a purely logical deduction, it is 
obvious that the function of an organ is the sum of the things which its 
component cells do. But that deduction means nothing if the single 
cells are doing things which are dictated by the whole. On the other 
hand, it is known from the development of embryos that cells become 
certain structures because they occupy a certain place among their 
fellows. But there is no certainty that this is in any sense a consequence 
of a property of w^holeness in the embryo. The two theories must be 
left, therefore, with the mere statement of their import, without any 
attempt to judge between them. 

When, however, one considers the step-by-step consequences of the 
possible evolution of higher organisms by the one or the other of these 
general methods, the two concepts rest on different planes. Biologists 
have usually held that, in the evolution of any line of descent, many 
branches of the group have arisen, some of which have advanced farther 
than others. If all of these branches could be collected, they could be 
arranged in such an order as to give at least a hint of the steps by which 
the evolution of the most advanced branches had reached their ultimate 
condition. The less advanced types might, of course, become extinct 
and so destroy the evidence of the successive stages, and in actual 
evolution it is certain that such extinction has often occurred. On the 
chance, however, that some of them have survived, biologists have fre- 
quently sought among existing relatively simple organisms approximate 
representatives of the conditions through which the more complex ones 
have gone in their evolution. The attempt to reconstruct lines of descent 
by means of series of modern organisms must be done with caution, and 
no very close correspondence between modern forms and ancestral types 
can be expected. 

In a reconstruction of the origin of the metazoa by means of a series 
of modern organisms supposed to represent the evolutionary steps, the 



two theories of the relation of parts to wholes fare very unequally. Only 
a few modern representatives of the one type of change may be selected, 
while very many are available to represent the other. 

The Organismal Theory. — The organismal theory is that which treats 
living things primarily as wholes, to which the parts are subordinate. 
In accord with this theory, the evolution of complex organisms from 
simple ones should start with an increase in complexity in some animal or 
plant while it is still a single cell. Much differentiation in the structures 

of the cytosome must have occurred. It would 
be expected also that the nucleus would have 
divided into many nuclei without corresponding 
divisions of the cytosome. That is, a multi- 
nucleate cell would have arisen out of a uninu- 
cleate one. Protoplasm containing many nuclei 
without separating cell membranes is known in 
a number of animals and plants and is called a 
syncytium. Voluntary muscle cells (page 95) in 
the higher animals have many nuclei, and the 
developing eggs of insects (Fig. 172) pass through 
a stage in which there are many nuclei before 
cell membranes begin to appear. One cannot, 
however, think of these very complex metazoan 
structures as remnants of an evolutionary stage 
which most of the other metazoa have passed. 
To have any possible significance as representa- 
tive of a step in evolution, the syncytium should 
be some rather simple organism. Vaucheria 
(Fig. 46), one of the simple plants, is syncytial, 
and there are several other plants. Good ex- 
amples are lacking among animals. The organismal theory is thus 
not well supported by living representatives of the stages for which 
it calls, though this lack can hardly be regarded as a fatal objection to 
the theory. 

The Colonial Theory. — If one regards organisms as the sum of their 
component parts, the natural supposition is that multicellular animals 
and plants arose through some form of colony formation. Cells multi- 
plied by division and then, instead of falling apart as they do among the 
protozoa, they clung together in groups. Such colonies could be formed 
before any of the cells became any more greatly differentiated than the 
single cell had been. The differentiation and increase in complexity 
could then follow in a succession of steps. The multicellular condition 
comes first, the complexity later, rev(n-sing the order expected from 
the organismal concept. This way of deriving the metazoa has the 

Fig. 46. — Vaucheria, a 
simple plant illustrating 
a syncytium or multinu- 
cleate cell. {From Sharp, 
"Introduction to Cytology.") 



advantage — if advantage it be — of being capable of illustration by 
organisms now living. The series of types used to illustrate it must 
still show considerable gaps, and the representation is sure to be only 
approximate; but the imagination can easily fill the vacant 
places. Let us consider what these representative living 
organisms may be. 

Types of Colonies. — The adherence of the two cells 
produced by division should require no more explanation 
than the physical connection and the mode of separation 
seen in mitosis in multicellular animals. The fact that 
protozoan cells should regularly separate is quite as remark- 
able as that metazoan cells should regularly cling together. 
Protozoan species in which the cells remain attached exist 
in colonies. Sometimes no more than two cells adhere; 
sometimes the number is thousands. The manner of 
adherence varies. An envelope of jelly may help hold 
the cells together, or they may be joined by stalks, or 
the cells may cling to one another merely by small areas of 

Colonies take various forms. In Ceratium (Fig. 47), the cells are in 
a single row, making what is called a linear colony. This type is rare in 
animals but common in the simple plants (algae), in which cylindrical 
cells are set end to end in long fine filaments. In some species the cells 
do not touch one another but are joined by branching stalks (Fig. 48), 

Fig. 47.— 
A linear 
colony, Cera- 
tium cande- 

Fig. 48. Fig. 49. 

Fig. 48. — Codosiga cymosa Kent. A, treelike colony; B, individual cell in detail. 
Fig. 49. — A gregaloid colony, Microgromia socialis. {From Calkins, "The Protozoa," 
The Macm,illan Com-pany.) 

forming a treelike or dendritic colony. These branching colonies are of 
many degrees of complexity, from those in which two cells fork off from a 
single common stalk to ones in which the stalks branch and rebranch 
and end in hundreds of cells. These organisms are all aquatic. The 
branched stalks and cells may be quite exposed to the water, so that 
currents of water pass freely among them, or they may be imbedded. 


stalks and all, in a mass of jelly. Such colonies may be as large as 
walnuts, or even baseballs. 

A third type of colony is the gregaloid, in which the cells are irregularly 
placed in a mass of jelly. These cells may be loosely arranged and in 

Fig. 50. Fig. 51. 

Fig. 50. — A gregaloid colony, Proterospongia haeckeli. {From Hegner's "College 
Zoology," The Macmillan Company.) 

Fig. 51. — Pandorina morum, a spheroid colony. 

contact with one another by means of fine processes branching out from 
them (Fig. 49), or they may be quite separate with only the jelly to hold 
them together (Fig. 50). 

Somewhat more compact and more regularly arranged are the spheroid 
colonies. In these there is usually a mass of jelly nearly spherical in 

A B 

Fig. 52. Fig. 5:3. 

Fig. 52. — A spheroid colony, Eudorini elegans. A, adult colony, X475; B, daugh- 
ter colony, X7.30. {From West after Goehd.) 

Fig. 53. — Anthophysa vegetans. Spheroid colonies arranged on a branching stalk, 
thus combining two typos of colonies. {After Kent.) 

shape, in which cells are imbedded in a layer near the surface, but none 
is in the center. The cells may be actuall\' in contact, or nearly so 
(Fig. 51), especially in young colonies (Fig. 52/i), or widely separated, 
as in most such forms when older (Fig. 52A). 



In some organisms two of these types of colonies may be combined. 
The cells may be in globular masses (spheroid type), though not iml^edded 
in jelly, and several of these masses joined by a branching stalk (Fig. 53). 

Choice of Colony to Illustrate Metazoan Origin. — If it is assumed, in 
tracing probable lines of descent, that the colonial theory is correct, 
which of these colonial types is most likely to represent the early evolution 
of the metazoa? The massive compact form of most of the metazoa 
suggests that the linear and dendritic colonies may be left out of consider- 
ation. Of the other two types, each has something in its favor. The 
fact most favoring the gregaloid colony is that in one of 
the best known organisms of that kind, Proterospongia 
(Fig. 50), each cell at the surface bears a delicate proto- 
plasmic collar around its one flagellum. Such a collar, 
surrounding a flagellum, is found on certain internal 
cells of the sponges (Fig. 33, page 52), which constitute 
one of the simplest groups of metazoa. Some biologists 
have inferred from these collared cells that the earliest 
metazoa may have been in some degree spongelike and 
that they came from colonies somewhat like present-day 
gregaloid colonies. 

The spheroid type of colony is favored by its greater 
abundance at the present time. Most of the spheroid 
colonies consist of cells bearing flagella, and many 
students of protozoa have held that the flagellate forms 
are the most primitive of the single-celled animals, 
which is another pair of facts in favor of the spheroid 
colony. Furthermore, the spheroid colonies lead directly 
to other forms that may, as we shall see, be used to illustrate later steps 
in the evolution process. 

This reasoning may not be correct, biit many biologists in the past 
have followed it and concluded that the metazoa probably arose from 
a single-celled organism, bearing some resemblance to modern flagellates 
(Fig. 54), through the formation of colonies. 

The First Differentiation. — In all the colonies described, the cells of 
one group are all alike, at least potentially. In Proterospongia (Fig. 50) 
they may seem to be of two kinds, since the cells in the interior of the 
jelly mass do not have collars. This is not a real difference, however, 
for the cells take turns coming to the surface, where they feed, and while 
at the surface develop a collar and flagellum, which they lose when they 
retreat to the interior. 

Now, the chief distinguishing mark of the metazoa is that their cells 
are not all alike. In the evolution of the multicellular organisms there 
must have been a differentiation of the adhering cells into two or more 

Fig. 54 .— 
illustrating a 
primitive type of 
organism from 
which colonies 
and later met- 
azoa may have 



kinds, if the colonial theory of origin is correct — or a differentiation of the 
parts of the cell which later became distinct cells, if the organismal 
theory is correct. Following only the colonial origin, what differentiation 
shall we expect? 

A B 

Fig. 55. — Pleodorina illinoisensis , consisting of 28 reproductive and 4 sterile cells. A, 
young organism; B, reproductive stage. The sterile cells may be regarded as the beginning 
of a soma. 

If we are to draw our answer to this question from the animals and 
plants that live at present, we should look for those in which there has 
been only one differentiation — in which, as a consequence, there are only 
two kinds of cells. The only organisms which exhibit a single differentia- 
tion among their cells are those in which some cells have lost the power of 
reproduction, while others retain it. Pleodorina is an example. In one 

Fig. 50. Fig. 57. 

Fig. 56. — Pleodorina californica, with small sterile cells almost as numerous as large 
reproductive ones. 

Fig. 57. — Volvox wcismannia, with 10 reproductive cells and thousands of sterile cells. 
{From Powers, in Transactions of American Microscopical Society.) 

of its forms (Fig. 55), which may be only a variety of Eudorina elegans 
(Fig. 52), it consists of 32 cells in a jelly matrix. Four of these cells, 
placed at that side which moves foremost as the organism swims, are 
smaller than the rest. These 4 cells are sterile, while the remaining 28 
may reproduce. Any of the 28 larger cells may divide to form a group of 
32 cells which escape from the jelly and lead an independent existence 


or form special cells which reproduce the group in another way; but none 
of the 4 small cells can do this. In another species (Fig. 56) the cells are 
more numerous, and the sterile and reproductive cells are more nearly 
equal in number; but they are again of two sizes, the smaller ones being 
sterile. Volvox (Fig. 57) is another, though much larger, form in which 
there are sterile and reproductive cells; but here the sterile cells greatly 
outnumber the reproductive. The two Pleodorinas and Volvox, taken in 
the order in which they are used here, show an increasing number of the 
sterile Cells. 

The existence of such forms as these suggests that the earliest differ- 
entiation between the cells of a colony, on its way to becoming a met- 
azoon, was the loss of reproductive powers by some of the cells. The 
group of sterile cells in these organisms corresponds to the soma, or body, 
as contrasted with the germ cells, or reproductive cells, of the metazoa. 

Further Differentiation. — In the organisms just studied, all the sterile 
cells are alike in structure and function, except in Volvox, in which the 
cells on the front side, as the organism swims, differ 
slightly in color and the size of certain of their structures 
from those on the rear side. This is quite at variance with 
the higher metazoa, in which the cells of the soma are of 
very many markedly different kinds. There is no way of 
knowing which of the manj^ types of somatic cells originated 
earliest; hence no clue as to what kind of modern animal Fig. 58. — 

we should look for to illustrate that step. The best we ^^ '^senemi 
can do, if we are to pursue this plan of choosing present- form of body, 
day representatives, is to select some animal in which the ^' 
differentiations among the somatic cells are not too numerous. A 
suitable form is the fresh-water Hydra, in which half a dozen kinds of 
somatic cells are found. A brief description of the body as a whole must 
precede the study of these cells. 

The form of Hydra is essentially cylindrical (Fig. 58) when extended 
and more or less globular when contracted. Ordinarily the body is 
attached 'by one end, the foot, to a solid object. At the tip of the free 
end of the body the mouth is located. Near the mouth is a circlet of 
long contractile tentacles which have arisen from the body by an out- 
pushing of the body wall. By means of the tentacles Hydra captures and 
thrusts into its mouth minute aquatic animals. The conical eminence 
between the mouth and the tentacles is the hypostome. 

The body of Hydra is hollow (Fig. 59), the interior space being a 
digestive cavity. Its wall is composed of two layers of cells, the outer 
known as the ectoderm, the inner as the endoderm. The endoderm cells 
are all essentially alike, being tall and slender and bearing flagella. Their 
function is the digestion of food. The ectoderm has differentiated into 



Fig. 59. 

-Hydra, dia- 

several kinds of cells. The bulk of that layer is made up of nearly cubical 

cells called the epithelial cells. Some of these epithelial cells, at the side 

toward the endoderm, are drawn out into long slender processes which 

serve both to contract, like muscles, and to convey impulses, like nerves. 

They are accordingly called neuromuscular cells 
(Fig. 60). Between the bases of the epithelial 
cells are numerous smaller rounded cells which, 
from their location, are named subepithelial cells. 
These give rise, at intervals, to very specialized 
cells, the cnidohlasts, which travel toward the 
surface of the ectoderm and produce within them- 
selves a threadlike stinging apparatus called a 
nematocyst. As the nematocysts are consumed 
in attacking other animals or in defending the 
Hydra, other cnidoblasts migrate to the surface 
and produce new stinging threads. At the foot 
of the animal the epithelial cells have the ability 
to produce a sticky substance by which the body 
is made fast to other objects and may therefore 
be called gland cells. 

Hydra has also reproductive cells, which are 
included in the ectoderm layer and which at inter- 
vals develop into the mature cells, eggs and 

spermatozoa. The former, which are the female cells, raise the ectoderm 

into a rounded lump called the ovary (Fig. 59ov) ; the latter, the male 

cells, elevate the ectoderm into a conical mound called the testis (ts). 

Hydra also reproduces by buds (Fig. 596 1, 62), into which all the various 

body cells in the region of the bud enter. 
It is thus apparent that Hydra, like 

Pleodorina and Volvox of the preceding 

section, possesses germ (reproductive) and 

somatic (sterile) cells. The existence of a 

budding process in Hydra, by virtue of 

which the somatic cells may share in the 

production of new individuals, does not 

alter the fundamental contrast between one 

class of cells which retain the typical mode 

of reproduction and another class of cells 

which have lost that power. Unliko> Pleodorina and A'olvox, however, 

Hydra has not stoi)ped with this one differentiation. It has gone farther 

and differentiated its somatic cells into five or six different kinds. 

Parallel between Foregoing Series and Individual Development. — 

Some biologists have favored tiie foregoing series of colonial i)i'otozoa 

tion of a lengthwise sec- 
tion, bi, b2, buds in dif- 
ferent stages of growth; 
ec, ectoderm; en, endo- 
derm; /, foot; gvc, gas- 
trovascular or digestive 
cavity or coelenteron; 
TO, mouth; ov, ovary; t, 
tentacle; ts, testis. 

Fig. go. — Neuromuscular cell 
II.Nilra. (From Schneider.) 






and simple metazoa as representative of the course of evolution of the 
metazoa because it finds a parallel in the development of the individual 
among the metazoa. Whether this 
parallel has any particular significance, 
or is of interest only as part of the 
historical development of evolution 
theory, is uncertain, but the comparison 
is interesting. 

Individual development begins with 
a single cell, the egg, which is com- 
parable to the supposed protozoan 
ancestor of the metazoa. This egg 
divides repeatedly (Fig. 61//-/F) to 
form a group of cells, which may be 
likened to the protozoan colony. As 
the division of the egg proceeds farther, 
it yields a hollow ball of cells, the 
hlastula (V, VI), which has a form very 
much like that of Pleodorina and 
Volvox. It will be recalled that in 
these organisms the cells are all near 
the surface, no cells being at the middle 
of the jelly. The next step in develop- 
ment is the indentation of one side of the 
blastula to form a two-layered embryo, 
the gastrula (Fig. 62A, B). When a 
diagram of Hydra is placed beside 
a diagram of a gastrula (B and C, 
Fig. 62), they are seen to be built 

on the same general plan — that of a two-layered sac open to the exterior 
at one end. At a stage quite as early as these, some animals show the 

Fiu. 61. — Early metazoan develop- 
ment. I, undivided egg; II-IV, 
successive segmentation stages; V, 
blastula, exterior view; VI, blastula in 
section to show hollow interior or 
blastocoele. (From Wilder, ''History 
of the Human Body," Henry Holt and 
Company, Inc.) 





Fig. 62. — Gastrula compared with Hydra. A, beginning of gastrula formation; B, 
completed gastrula; C, diagram of Hydra; ap, animal pole; ar, archenteron; b, blastocoele; 
bp, blastopore; ec, ectoderm; en, endoderm; g, gastrovascular cavity; m, mouth. 

distinction between germ and somatic cells (Fig. 63), just as Pleodorina, 
Volvox, and Hydra do. The germ cells are usually larger than the somatic 


cells, when they can be distinguished at all, and sometimes contain granules 
of a peculiar sort. Finally, to complete the comparison, development of 
the embryo need be followed only a short way to ol^serve differentiation 
of the somatic cells into at least as many kinds as Hydra possesses. 

A Conclusion, and Caution in Adopting It. — The principle of using 
embryonic development to discover the course of evolution is known as 
the biogenetic law. According to this generalization, the development of 
an individual repeats the history of its race. This law is seriouly ques- 
tioned by many biologists and vigorously opposed by some. Also, the 
^■r^f^;:y,-;^!-s'yf"'£r-<:S:^\:-f/:m:::M use of scrics of modern organisms to illustrate 

what may have taken place in evolution must 
])e made, if at all, with great care. Both of 
these comparisons have been made, however, 
by l)iologists in the past, using the organisms 
referred to in the preceding pages. The con- 
clusion to which they lead is that metazoa 
have arisen through (1) the adherence of pro- 
FiG. 63. — Posterior end tozoan cclls to fomi a colony, (2) the loss of 
of developing insect egg. reproductive powers by some of the cells of 
ductive cells, all others this colony, and (3) the differentiation of these 
somatic. sterile cells into a number of kinds. These are 

the fundamental steps; the details of cell structure and the general form 
of the colony are immaterial. 

This conclusion, it will be observed, accepts the colonial and rejects 
the organismal theory. It rather favors spheroid colonies over the 
gregaloid type because the modern organisms availal)le for a series of 
representative types are spherical, and because the blastula of embryonic 
development is a hollow ball. Many biologists hesitate to recognize 
these reasons, and reference to them here is in no sense a pronouncement 
in favor of the mode of origin of the metazoa which they appear to 
indicate. Nevertheless, that origin is not improbable. And even if the 
scheme of evolution described should be far from correct, a consideration 
of it has led to an understanding of the relation of parts to wholes and a 
glimpse of some of the situations which many-celled organisms have to 

What Is a Colony, What an Individual? — When any change is effected 
by a number of graduated steps, as the origin of metazoa from simpler 
organisms must have been, it is difficult to say just wluui any stage that 
may be named is reached. When, for example, has a metazoon been 
evolved out of a protozoon? How far must the change go to be recog- 
nized as having reached that goal? No matter what process led to 
the metazoa, the answer to this question must be a matter of definitions. 
If the organismal theory is correct, was the animal with numerous 


nuclei but no cell membranes around them a metazoon? If not, was 
it a metazoon as soon as the cell membranes were formed? If not 
then, was it a metazoon after some differentiation among those cells 
had occurred? If the colonial theory is correct, was the first group of 
adhering cells a metazoon or only a colony of single-celled animals? 
Would a group of a thousand cells be a metazoon, while a group of 
four was only a colony of protozoa? If number makes no difference, 
would differentiation among the cells constitute the mark of a metazoon? 

Whatever the event that marks the advent of a metazoon, the 
organism that has experienced that event is an individual. Without 
that characteristic, it is a protozoon or a colony of protozoa, depending on 
the nature of the origin of the metazoa. Biologists have differed in their 
definition of the individual. To some, a group of cells that shows any 
differentiation becomes a metazoan individual. Since in actual cases 
when only one type of differentiation exists it is that between reproductive 
and sterile cells, as in Pleodorina, defining the multicellular individual as 
any group of cells in w^hich differentiation exists is equivalent to saying 
that the individual is any group in which sterile cells are set apart from 
reproductive cells. Other biologists have insisted that a group of cells is 
not an individual unless its sterile cells are differentiated into several 
kinds, as in Hydra. Under the former definition Pleodorina and Volvox 
are individuals; under the latter they are colonies of unicellular organisms 
exhibiting division of labor, since some reproduce and others do not. 

The distinction between reproductive and sterile cells is more funda- 
mental than the distinctions among several kinds of sterile cells. In this 
respect the former definition has the advantage. It is also preferable for 
the reason that the criterion of individuality is, according to it, always the 
same thing — loss of the capacity to reproduce by some of the adherent 
cells — while under the latter definition the criterion of the individual 
would presumably be a different distinction between sterile cells in every 
line of descent. But definitions are arbitrary, and there is no tribunal 
except usage which can choose among them. 

Further Organization. — Beyond the stage at which they are barely 
"entitled to be called metazoa, most of the higher animals have gone long 
distances. They have increased the number of their cells so that even 
a moderate-sized animal contains literally billions of these units. With 
increase in size, they have usually developed a framework or shell of some 
sort wdiich provides protection or aids locomotion. Special devices are 
created for the providing of food and the elimination of waste materials. 
With large volume, they have had to provide means of communication by 
which substances may be quickly transported from one part to another. 
Structures of different sorts capable of effecting movement have arisen. 
Unified control and the harmonious working together of the various parts 


have been provided in different ways. So multifarious are these char- 
acteristic developments that a group of chapters, immediately following, 
must be devoted to them. 


Lankester, E. R., editor. A Treatise on Zoology. A. & C. Black, Ltd. (Vol. 1, 

Fascicle I, Introduction.) 
Sharp, L. W. An Introduction to Cytology. 3d Ed. McGraw-Hill Book Company, 

Inc. (Pp. 20-24, 435-43G, for Organisnial Theory.) 


Beyond the evolutionary stages traced in the last chapter, ending 
with the differentiation of the somatic cells into a number of kinds, 
the metazoa have gone various ways in great groups. Within each group 
there is much in common, both in structure and in physiology; but 
between groups there are many differences. Scarcely anything is com- 
mon to them all. There are a few features, however, that are character- 
istic of several or many of the great groups. Some of the more important 
of these frequent structural conditions should be passed in review. 

Symmetry. — Symmetry is an arrangement of parts in relation to 
planes, straight lines or points. A point is a position in space; it has 
no dimension or size. A straight line is the shortest distance between 
two points; it has only one dimension, length. A plane is, in common 
words, a flat surface; more precisely it is a geometric figure of two dimen- 
sions — length and breadth but no thickness — such that if any two 
points in it be connected by a straight line that line is everywhere within 
the figure. 

Symmetry is defined as a correspondence in shape or arrangement of 
parts on opposite sides of a dividing line or plane, such that if the portion 
on one side were viewed in a mirror it would appear identical with the 
part on the other side. A symmetrical surface is divided into the corre- 
sponding parts by a straight line; solid (three-dimensional) objects, 
including animals, are divided into their equivalent parts by a plane. 
The plane which divides a body into its corresponding halves is called the 
plane of symmetry. Objects have difi'erent types of symmetry (Fig. 64) 
depending on the number of planes of symmetry which may be passed 
through them. If only one such plane is possible, the symmetry is 
bilateral. Most animals (including all the higher ones) are bilaterally 
symmetrical. They possess anterior and posterior ends which differ, 
right and left sides which are alike except for the reversed order, and a 
dorsal side (at or toward the back) and a ventral side (literally per- 
taining to the belly, hence opposite to the dorsal side). The plane of 
symmetry passes through the two ends, through the dorsal and ventral 
surfaces, and between the right and left halves. 

Some animals possess a number of planes of symmetry. If these 
planes all have a certain straight line in common, that line is the axis 




of symmetry. An axis is a line around which something rotates, or 
around which things are placed. The planes of symmetry may be 
thought of as rotating on the axis of symmetry. Symmetry of this sort 
is known as radial. In one of the major groups of animals (Fig. 65) the 





Fig. 64. — Types of symmetry illustrated by familiar objects. 

bodies regularly possess radial symmetry. The arms or tentacles of the 
animals of that group limit the number of planes that divide them 
symmetrically; in practice the symmetry is called radial if there are two 
or more such planes all having the axis line in common. 

Sometimes there are many planes of symmetry having, not a line, 
but a point in common. Symmetry is then said to be universal, and 
the common point is the center of symmetry. In a sphere, any plane 
that passes through the center is a plane of symmetry. Not many 
animals have a spherical form, but Eudorina (Fig. 52) approaches it. 

An object may possess symmetry of two types. A football, for 
example, has radial symmetry around its long axis, but bilateral sym- 
metry in relation to the plane halfway between its ends. Some cells 
have approximately that form, as do also some protozoan colonies. 



In general, animals which move rapidly or are capable of well coordi- 
nated movements are bilateral. The radial animals are usually slow 
movers and frequently are attached to fixed objects. Universally sym- 
metrical animals are aquatic and progress with a rolling movement. 

Asymmetry. — Any object which cannot be divided into corresponding 
halves by any plane is said to be asymmetrical. Many of the protozoa 
are made asymmetrical by a groove running spirally part way round the 
body. The coiled shell of a snail is asymmetrical. 

Fig. 65. — ^Various coelenterates, showing their radial symn^etry. A, sea anemone; 
B, group of coral polyps; C, the medusa, Mitrocoma cirrata, ventral view. D, polyp of the 
hydroid, Perigonimus serpens. {A and B after Jordan, Kellogg, and Heath; C after Mayer; 
D after Allman.) 

Many animals which are externally symmetrical may have their 
internal structures arranged on an asymmetrical plan or on a plan of 
symmetry different from the external plan. Examples are the heart, 
stomach, and other parts of the alimentary tract and the lobes of the 
liver in man, which are arranged asymmetrically. Many animals which 
exhibit asymmetry in certain of their adult organs are symmetrical in 
early stages of development. The flatfishes (halibut, floimder, and sole) 
which have two eyes placed on one side of the head, are in their early 
embryos bilaterally symmetrical, but one eye migrates through the head 
to its new^ position. 

Metamerism. — Animals exhibiting metamerism are composed of a 
linear series of body segments fundamentally alike in structure. These 
units are called somites or metameres, and animals so constructed are 
said to be metameric. In simple metameric animals the somites closely 
resemble one another in size, form, and the arrangement of organs. In 
no animal, however, are all somites entirely alike because some of them 
have become specialized and perform special duties. 

The common earthworm (Figs. 135, 137) is a metameric animal. 
It is composed of a series of ringlike somites outwardly much alike. 
The limits of the somites are marked on the outside by grooves, and on the 
interior by the septa (cross partitions) which lie immediately under the 
grooves. The segmental arrangement extends to both external and 
internal structures and involves organs of locomotion and excretion, 
muscles, blood vessels, and the nervous system. The sexual organs also 
have a segmental arrangement, although they are limited to a few somites. 



Certain other organs are repeated in only a few segments, but in general 
the earthworm's structure is that of a metameric animal. 

In complex animals the metameric arrangement has often become 
obscured through fusion of somites, loss of organs, and centralization. 
The primitive arrangement, however, is readily seen in the embryos of 
such animals. Thus the embryos of the vertebrates generally reveal a 
well-marked metamerism in certain organs (the muscles, for example), 
in which this arrangement is later partly or completely lost. Not all 
metamerism has been lost even in the adult vertebrates, however, for 
it may be seen in the vertebrae and ribs (Fig. 79), spinal nerves and 
ganglia (Fig. 117), and branches of the dorsal artery. 

Body Cavities. — Most of the higher animals have a cavity of some sort 
in their bodies, but these cavities are of several kinds. In Hydra (Fig. 59) 






Fig. 66. — Relations of body cavities (enteron and coelom). At left, the earthworm; at 

right, cross section of a vertebrate animal. 

there is but one cavity, which is open at one end, the mouth, and closed 
at the other end. A cavity so constructed is called a coelenteron, though 
in Hydra, in recognition of its function of digestion and its assumption of 
some of the tasks of a blood system, it is often named the gastrovascular 
cavity. Flatworms also have a coelenteron. Undigested food must, in 
such animals, be ejected through the mouth. 

In most of the metazoa there are two cavities. One is in the digestive 
tract, the other lies between the digestive organs and the body wall. 
The digestive cavity in most complex animals is open at both ends and 
to distinguish it from the closed sac in Hydra is known as the enteron. 
The space between the digestive organs and the body wall is the coelom. 
These relations are shown diagrammatically for the earthworm in Fig. 66 
(left). In vertebrate animals (right) the cavities are in the same relative 
position; the coelom appears to be filled with many organs. These, 
however, arc merely pushed into it from the outside. Since some animals 



Fig. 67. — Connective tissue, 
consisting of cells, matrix, and 

(the lobster, for example) have irregular spaces among their organs, filled 
with body fluids, there is sometimes difficulty in deciding whether a 
cavity is a coelom or not. In general, the coelom must be lined by a 
definite layer of cells, the peritoneum, which is lacking around the spaces 
in the lobster, and the principal reproductive organs {gonads) are sus- 
pended from its walls. 

Tissues. — In practically all metazoa in which the several kinds 
of somatic cells are very numerous, those of any one kind are grouped 
together, not necessarily all in one place but 
usually in a number of places. In Hydra 
(Fig. 59), as we have seen, all endoderm 
cells together form a continuous layer con- 
stituting the inner part of the body wall. 
The epithelial cells are similarly placed 
together in the ectoderm. The secreting 
cells of the foot are together in a small 
group. The other somatic cells of Hydra 
are not conspicuously grouped, since the 
subepithelial cells and the cnidoblasts derived from them do not form a 
continuous layer. 

In most metazoa the somatic cells of any given sort are more con- 
spicuously assembled in layers or masses than in Hydra. Such groups 
or masses of like cells are called tissues. A tissue may be defined as a 
number of cells of the same kind forming a continuous mass. Ordinarily 
they perform some function in common, but it is not necessary to know 
their function to consider them a tissue. Tissues may be classified on 
the basis of both structure and function. In the vertebrate animals 
these classes are sustentative, epithelial, contractile, nervous, vascular, and 

Sustentative Tissues. — The sustentative tissues are primarily those 
which support. The typical sustentative tissue is ordinary connective 
tissue which binds the skin to the flesh beneath or holds the muscles 
of the thigh together in a mass or helps suspend the intestine from 
the body wall. It contains scattered cells (Fig. 67), but the serviceable 
part is made of things secreted by the cells. These things are a gelati- 
nous matrix, or ground substance, and large numbers of tough fibers 
imbedded in the matrix. It is the latter that give connective tissue its 

Certain connective tissues of very great strength are given special 
names. The ligaments binding bones together at the joints, and the 
tendons joining muscles to the bones which they move, are examples. 
The essential fea,tures of connective tissue — cells, matrix, fibers — are 
present in both, but the fibers far outbalance the other parts. 



Cartilage and bone are likewise specialized forms of sustentative 
tissue. They are alike in having their cells more or less scattered in a 
substance, the matrix, which the cells have secreted. In cartilage the 
cells are entirely separate from one another, though often placed in pairs, 
trios, or quartets (Fig. 68) resulting from recent divisions of an earlier 
cell. The matrix is firm or pliable, contains much gelatin, and is used as a 
buffer to absorb shock or in places requiring flexibility. In bone the 
cells possess numerous slender projections, some of which, probably, are 
always in contact with similar projections from other cells (Fig. 69). The 
hard bony material of the matrix, consisting largely of calcium carbonate 







Fig. 68. — Sections through cartilage. A, development of cartilage (top) from meson- 
chyme (bottom); B, hyaUne cartilage. {From Lewis-Stohr, '"Textbook of Histology," 
The Blakiston Company.) 

and calcium phosphate, is secreted by these cells; consequently there are 
always spaces in the bone for the cells and their slender processes. 

Fatty or adipose tissue is regarded as sustenfative, but rather because 
of its original similarity to connective tissue than from any mechanical 
function which it may serve. The cells are numerous and closely packed, 
not scattered as in other sustentative tissues. The fat itself is in globules 
of small or large size contained within the cells. It is reserve food; hence 
fatty tissue fluctuates greatly in \^olume, depending on the state of 
nutrition of the organism. Favorite places for the deposit of fat are in 
the abdominal wall and beneath the skin at many other places. 

In many embryos, and in the adult of certain lower animals, such as 
the flatworms, there is a tissue known as mesenchyme, which should be 
included with the sustentative tissue, though chiefly because of its struc- 



tural resemblance to some of the supporting tissues. It is a very loose 
tissue whose cells are irregular, often star-shaped. These cells are not 

closely packed but touch one another 
only by their corners or the tips of 
their projections (Fig. 70). Con- 
siderable space is thus left among the 

Fig. 69. Fig. 70. 

Fig. 69. — Section through bone, showing the stellate spaces in the matrix occupied by 
cells, and at left part of the space occupied by a blood vessel. (From Hill, "Manual of 
Histology and Organography,'' W. B. Saunders Company.) 

Fig. 70. — Mesenchyme from umbilical cord. {From Hill, "Manual of Histology and 
Organography," W. B. Saunders Company.) 



cells, which is filled with some more or less liquid substance, 
spongy structure is everywhere characteristic of mesenchyme. 

Epithelial Tissue. — An epithelium is a layer of cells covering some 
surface, either the outside of an organ or the lining of the wall of a cavity. 




Fig. 71. — Types of epithelium. A, columnar; B, cubical; C and D, squamous (side 
and surface views, respectively); E, ciliated; F, flagellate; G, collared; H, stratified; vac, 

The endoderm and ectoderm of Hydra, already described, are epithelia. 
Others likely to be observed in laboratory studies are the outer layer 
(hypodermis) of the body wall of the earthworm, the lining of the intestine 



of any animal, the peritoneum which covers the intestine and Hnes the 
abdominal cavity (coelom) of vertebrate animals, the outer layer (epi- 
dermis) of the skin, and the inner or secreting layer of any gland. 

An epithelium is designated cubical, columnar, or squamous, according 
to the shape of its component cells (Fig. 71A-D), the last term meaning 
flat and tilelike ; ciliated, flagellated, or collared, if the free ends of the cells 
bear any of the structures indicated by these words (E-G) ; and stratified, 
if the layer is several cells thick and the cells at different levels have 
different shapes (H). 

Fig. 72. — Types of secreting surfaces and glands. A, scattered gland cells (two 
goblet cells containing secretion in the darkly stippled goblets) ; B, gland cell enlarged and 
dropped below general level; C, group of secreting cells dropped slightly below the general 
level; D, a simple multicellular gland; E, alveolar gland with neck; F, tubular gland; G, 
compound alveolar gland; H, compound tubular gland; I, lumen; m, mouth; n, neck; v, 
acini. Secreting portions of the glands are stippled. 

Epithelia on the outer surfaces of organs are usually in some degree 
protective. When they line a cavity, they often have the function of 
secretion. The lining membrane of the intestine in vertebrate animals 
is secretory, and in all glands the secreting portion is epithelium. If a 
gland consists of a single cell, that cell is in an epithelial layer (Fig. 72A, 
B). If the gland is multicellular, its secreting cells may dip below the 
general level of the surface, but still it is part of the epithelium (C, D). 
When the secreting cells thus indented form a channel of nearly uni- 
form diameter, the gland is said to be tubular; if the deepest portion is 


expanded like a flask, the gland is alveolar. Such an indented epithelium 
may branch, that is, form subsidiary indentations {G, H), and then the 
gland is termed compound, as contrasted with simple glands in which the 
tube is not branched. Nearly all glands, in the higher animals at least, 
have other tissues, including blood vessels, collected around or spread 
among the epithelial part; but in every case it is the epithelium that 
does the actual secreting. 

The Other Tissues. — The two types of tissues described in the 
preceding sections are distinguished largely on structural grounds, while 
the functions performed by different samples of them may be quite 
unlike. The remaining tissues of those listed on page 81 are, however, 
highly specialized for specific functions. They are so much more impor- 
tant in connection with those functions than with respect to their struc- 
ture that descriptions of them are deferred to later chapters. Contractile 
tissue includes mainly the voluntary and involuntary muscles; nervous 
tissue comprises all the nerve, brain, and ganglion cells; vascular tissue 
includes the blood and the more fluid parts of the blood-producing organs 
(red marrow, spleen); and reproductive tissue consists of the germ cells 
and their forerunners. 

Organs and Systems. — An organ, generally speaking, is any structure 
which performs a given function. In this general sense, a single cell 
may be an organ, as in the case of single secreting cells scattered through 
an epithelium. Usually, however, cells that do a certain thing are 
grouped. Thus the secreting cells of Hydra which provide the adhesive 
substance that holds the animal fast to other objects are all located on 
the foot. Also, the stinging cells of Hydra show a tendency to be col- 
lected in patches, particularly on the tentacles. Where such patches are 
sharply marked off, as the glandular foot of Hydra, each group could be 
considered an organ. 

Some biologists, however, reserve the term organ for a collection of 
tissues acting together to perform some function. The stomach of a 
vertebrate animal is a suitable example. The inner epithelium, just one 
cell thick, does the secreting of the digestive fluid or fluids. Outside this 
layer is a connective tissue layer rich in blood vessels and lymph spaces 
by which the materials for secretion are brought in and the digested foods 
are carried away. Covering this layer are two layers of muscles, running 
in different directions and together serving to churn up the contents of 
the stomach and mix them with the digestive fluids. The several tissues 
are structurally unlike, but each contributes in some way to the digestion 
of the food. The stomach is thus an organ in this more restricted sense. 

When a number of organs are occupied with different phases of a 
complicated general process, they constitute a system of organs. The 
mouth, esophagus, stomach, intestine, and several glands associated 


with these organs are all concerned in some way with digestion. They 
constitute the digestive system. The heart, arteries, veins, and capil- 
laries propel or convey the blood and so make up the circulatory system. 
In like manner the brain, spinal cord, ganglia, and nerves compose the 
nervous system. The term system is sometimes applied to a group of 
organs of a single kind, when these are the only organs concerned with 
that function. Thus, as will be explained in a later chapter, the excretory 
organs of some of the simple animals (the earthworm, for example) 
are all alike, but there are many of them. There is no objection to 
speaking of these organs collectively as a system; but in all the more 
complex animals the systems are everywhere made up of unlike parts, 
each contributing a different portion of the general process. 


Dahlgren, U., and W. A. Kepner. A Textbook of Principles of Animal Histology. 

The Macmillan Company. (Chap. VI, epithelium; Chap. VII, supporting and 

connective tissue.) 
KiNGSLEY, J. S. Textbook of Vertebrate Zoology, Part I. Henry Holt & Company, 

Inc. (Pp. 9-16 for tissues.) 
Storer, T. I. General Zoology. McGraw-Hill Book Company, Inc. (Pp. 17, 51-58 

for tissues; Chap. 4 for organs and systems.) 


In many animals the characteristic activities could be performed only 
in the presence of hard parts which may collectively be termed the 
skeleton. A skeleton is any more or less firm framework on or within 
which the softer fleshy parts of the body are placed. The services per- 
formed by the skeleton are chiefly of three types: (1) it provides support 
for soft organs whose relations to one another could not otherwise be 
maintained; (2) it protects delicate structures; and (3) it furnishes a 
mechanism through which different types of movement may be executed. 
Skeletons are widespread, from the protozoa to the largest mammals. 
Such prevalence is testimony to their usefulness; yet some large groups 
of animals (fiatworms, roundworms) and some members of other groups 
(jellyfishes) get along without them. 

Support Furnished by Skeleton. — It is not practicable to separate 
mere mechanical support from protection in many cases, though an 
attempt will be made to choose examples 
where this may be done at least in principle. 
Sponges of all kinds possess narrow channels, 
lined in places by collared cells (Fig. 33, 
page 52) which take in food. Currents of 
water are constantly maintained in these 
channels by the flagella of the collared cells, Fig. 7.3. — e i e m e n t s of 

and it is essential that the passages be XTLttZX'o.S'T,^. 
prevented from collapsing. While conceiva- spicules of different types. 
bly the canals might be kept open by cells of ^^''"^ "^"^ ^^"''' '"^''^ Hertwig.) 
firm consistency, they actually are kept open by means of a skeleton. 
In the so-called bath sponges, this skeleton is a network of horny material; 
in other kinds the skeleton is made of numerous limy or siliceous rods or 
variously shaped objects called spicules (Fig. 73). 

Fresh-water mussels and marine clams bear on the outside of their 
bodies a bivalve shell, consisting of two saucerlike pieces hinged together 
at one edge and opening like a book. Between the edges of these pieces, 
at certain places, water must enter and leave by fixed routes in order to 
bring the animal its food and oxygen and remove its wastes. The actual 
channels for the water are formed by the fleshy parts of the mussel, but 
these fleshy parts must be kept in their proper positions. In many of 



the mussels they are too soft and deHcate to do so unaided, and it is the 
shell which holds them in place. 

The importance of the skeleton is closely related to size of body and 
the place where the animal lives. A large animal may exist in the sea 
and, because the body is of about the same density as the surrounding 
water, be buoyed up in such a way as to allow its parts to function. 
Cuttlefishes, for example, lead active lives in marine waters but washed 
up on shore are helpless and shapeless. On land, however, even moder- 
ate-sized mammals, because the medium around them, the air, is so much 
lighter than themselves, would be unal^le to maintain the physical rela- 
tions of their parts to one another sufficiently to enable them to function 
if they were made of mere protoplasm. Some form of mechanical sup- 
port other than a skeleton might have been evolved; but large size with- 
out such support, along with physiologies of the general sort exhibited 
by modern land animals, would have been out of the question. 

Skeletons and Protection. — Nearly every skeleton may be regarded 
as a source of protection, though often there is little definite information 
to show what injuries might result in the absence of the skeleton. Those 
sponges which have a skeleton of limy spicules generally bristle all over 
with long shafts projecting from the surface cells (Fig. 74). How much 
they are thus protected from predatory animals can only be 
conjectured. In some marine animals known as hydroids, 
having the general structure of Hydra but existing in 
branching colonies, there is a horny tubular-sheath covering 
the various branches and main stem of the colony. This 
skeleton enables the hydroids to stand out more or less 
firmly instead of being lashed against other objects by the 
Fig. 74.— waves. In insects, crayfishes, spiders, and their allies 
simp e i\^Qy.Q jg Q^ skeleton of a horny substance known as chitin 

sponge. -^ 

{From Heo- which covers the entire l^ody on the outside. This does 
^Zooloav " The ^^^ protect them from predatory animals, since members 
Macmillan of this group, particularly the insects, are abundantly 
ompany.) eaten by other animals; but it must serve to ward off 
mechanical injuries of other kinds. The limy wall, or test, of sea urchins 
and the shells of clams are presumably likewise protective structures. In 
the vertebrate animals some of the most delicate and vital organs are 
within bony cases — the brain within the skull, the spinal cord in a canal 
running through the backbone, the heart within the framework of the 
chest, and such sense organs as the cars and eyes either imbedded in 
solid bone or set in among projecting ridges or other prominences. 

Skeletons which serve only the functions of supi)ort and protection 
may often be rigid one-piece structures. Some of the protozoa have a 
solid limy shell sm-rounding the whole cell, and corals rest in limy cups 



which they have secreted. Most skeletons serving other functions are 
either flexible or jointed. 

Function of Hard Parts in Movement. — Only occasionally are the 
hard parts of much service in movement among the simpler animals. 
One of the best examples of such use 
is the earthworm, which is provided 
with a number of spines, or setae, 
projecting from the body in each 
segment except a few at the ends. 
These setae are operated by muscles 
attached to their inner ends and 
sloping off in different directions 
(Fig. 75), like the ribbons of a May- 
pole, to the body wall. When the 
worm crawls forward, the outer end 
of the seta is tilted backward, so 
as to catch the soil, and in crawling 
backward or holding fast in the worm's burrow the seta points forward. 

Sea urchins also have movable hard parts, which, however, are not 
precisely a part of the locomotor equipment. The fleshy parts are 
enclosed in a round shell, or test, the surface of which is studded, porcu- 
pinelike, with a host of spines (Fig. 

76). These spines are capable of cr-lki M^iB—:-f 

movement in any direction and, when ^^?Q|/ Vf^l 

the animal is thrust over on its side 

Fig. 75. — Seta and muscles in the 
earthworm, drawn from a longitudinal 
section anterior to the clitellum; cm, 
circular muscles, and Im, longitudinal 

.A B 

Fig. 76. Fig. 77. 

Fig. 76. — A sea urchin, covered with a test and spines. {From Haupt ," F iindamentals 
of Biology.") 

Fig. 77. — Relation of muscle to hard parts in appendages of insect and man. A, leg 
of insect; B, leg of man; /, femur; fs, skeleton of foot; i, insertion of muscle; m, nmscle; 
o, origin of muscle; ta, tendo-Achilles; ti, tibia. {A after Berlese; B after Hesse and Dofiein.) 

or back, may give it an irregular motion that helps it right itself. 
But the main movement is effected by fleshy tubes ending in suckers. 

The fullest use of skeletal parts for movement is found in the insects 
and their allies and in the vertebrate animals. In both groups the hard 
parts are joined by curved surfaces, which permit free movement of one 



upon another. Sometimes these curved surfaces are such as to permit 
movement only in one plane, as in a hinge, while other joints allow a 
rotary motion. The skeleton of insects and that of vertebrates differ, 
however, in one important respect. In the insects it is on the outside, 
covering all the fleshy parts, and here is known as an exoskeleton. In 
vertebrate animals the skeleton is on the inside, everywhere covered by 
flesh, hence of a type called an endoskeleton. The muscles which operate 
the movable parts must work from the inside in the former but from the 
outside in the latter (Fig. 77). 

Skeleton of Vertebrates. — To illustrate the main features of a typical 
skeleton, that of the vertebrate animal is chosen. This skeleton is 
composed of bones and cartilages united partly by ligaments, is covered 


Fig. 78. — Regions of the vertebrate skeleton (cat). {From Jayne, "Mammalian Anatomy.") 

by the soft parts of the body, and is supplied with blood vessels and 
nerves. It may conveniently be divided into regions as indicated in 
Fig. 78. On more fundamental anatomical grounds it is also subdivided 
into the axial and the appendicular skeleton. The former lies in the 
longitudinal axis of the body, and to it the latter is appended; hence the 

Axial Skeleton. — The axial skeleton (Fig. 79) is made up of the skull, 
hyoid apparatus, vertebral column, ribs, and sternum. The skull furnishes 
a case for the brain, capsules for the organs of hearing and smell, and 
orbits for the eyes. It also includes the bones of the jaws. To it is 
attached the hyoid apparatus which is a bony or cartilaginous support 
for the base of the tongue. 

The vertebral column is a jointed structure composed of a number 
(different in different species) of vertebrae placed end to end. Together 
they form a tube enclosing the s])inal cord, and their outer surfaces 
form attachments for ligaments and muscles. The vertebral column 



is structurally differentiated into five regions, the cervical, thoracic, 
lumbar, sacral, and caudal (see Fig. 79). The plan of a vertebra is shown 
in Fig. 80. It is composed of a heavy ventral portion, the centrum, from 
which arises a bony arch, the neural arch. The latter encloses the neural 
canal which is occupied by the spinal cord. From the sides of the arch 
two transverse processes project, and from the apex of the arch arises the 




Fig. 79. — Axial skeleton of the cat. (From Jayne, ''Mammalian Anatomy.") 

neural spine. One pair of articular processes or zygapophyses projects 
anteriorly and another posteriorly from the sides of the arch. The 
relations of the anterior and posterior zygapophyses and the articular 
faces of the centra of adjoining vertebrae are made clear in Fig. 80 (right). 
The forms of the vertebrae in different regions of the vertebral column 
are very different, as shown in Fig. 79. In the thoracic region of an 

Fig. 80. — Diagram of a typical vertebra viewed from in front or behind and from the 
left side: az, anterior zygapophysis; c, centrum; /, intervertebral foramen through which 
nerves and blood vessels pass; ic, intervertebral cartilage; na, neural arch; nc, neural canal; 
ns, neural spine; pz, posterior zygapophysis; tp, transverse process; z, zygapophysis. 

animal having ribs the vertebrae have faces for the articulation of the ribs. 
In the sacral region the vertebrae in some animals are considerably thick- 
ened without great change in form, while in others they are much flattened 
and more or less fused into a platelike structure, the sacrum. In the 
sacral vertebrae the neural canal is reduced in size and in the caudal 
vertebrae it is entirely absent. The spinal cord does not pass into the 
latter region. 



Vertebrae articulate with each other chiefly by means of the centra. 
The articular surfaces of the centra may be concave or convex. Com- 
monly one of the surfaces of a centrum is concave and the other convex, 
the convex surface of one vertebra fitting into the concavity of the next. 
But in some vertebrae both surfaces are concave and the space between 
the centra is filled with a lens-shaped pad of cartilage. Biconcave verte- 
brae are called amphicoelous (amphi = both and koilos = hollow). In 

the concavoconvex type of vertebra, 
if the concavity is directed toward 
the head, the vertebra is said to be 
procoelous, but opisthocoelous if the 
concavity is directed posteriorly. 
These types of vertebrae are illus- 
trated in Fig. 81. 

Ribs are usually attached to the 
vertebrae in such a manner that they 
can be moved. Some of the hinder- 
most ribs are free at their ventral 
ends, while others are connected to 
the sternum or breast bone more or 
less directly by means of cartilage. 
The sternum is a bony or cartilaginous 
structure which lies in the median 
ventral part of the thorax. The 
number of pairs of ribs varies in different species, being 12 in man. 

Parts of Appendicular Skeleton. — The appendicular skeleton consists 
of the shoulder or pectoral girdle, the hip or pelvic girdle, and the fore 
and hind limbs. The generalized plan of the girdles and limbs of animals 
higher than the fishes is shown diagrammatically in Fig. 82. In these 
appendicular skeletons each of the girdles is composed of three pairs 
of bones which are similarly arranged in the two gii-dles. Each side 
of the pectoral girdle is composed of a flat bone, the scapula, or shoulder 
blade, directed dorsally, a coracoid bone connecting the scapula and the 
sternum (the latter not shown), and a clavicle which in some vertebrates 
also connects the scapula and the sternum. There may be a cartilage, 
the precoracoid, affixed to the posterior edge of the clavicle. A cavity, the 
glenoid fossa, located at the junction of scapula and coracoid, serves as 
the surface of attachment of the fore limb. Each side of the pelvic 
girdle consists of an ilium, ischium, and pubis. These three bones in a 
generalized skeleton are arranged similai-ly to the bones of the pectoral 
girdle. The cavity at the junction of the three bones is the acetabulum. 
In it is seated the head of the femur (thigh bone). 

The bones of the arm and leg or fore and hind limbs are arranged 

A ' \ u \ I c 

Fig. 81. — Three types of vertebrae. 
Only the centra and lateral processes are 
shown. Upper end is anterior. A, pro- 
coelous; B, opisthocoelous; C, amphi- 



according to the same plan and may be compared bone for bone, humerus 
with femur, radius and ulna with tibia and fibula, respectively, carpal 
(wrist) bones with tarsal (ankle) bones, metacarpals with metatarsals 
(body of hand and foot, respectively) and phalanges (bones of the digits) 
of the hand with those of the foot. Vertebrates with primitive limbs have 
five digits on fore and hind feet, but the limbs of specialized animals 
have undergone more or less extensive modifications from the original 
five-fingered and five-toed plan. In them usually the number of digits 
has been reduced. 








Fig. 82. — Diagrams of generalized fore {A) and hind (B) limbs with limb girdles: 
acth, acetabulum; CL, clavicle; en. 1, en. 2, centralia; COR, coracoid; dst. 1-5, distal row 
of carpals and tarsals; FE, femur; FI, fibula; fi, fibulare; gl, glenoid fossa; I-V, digits; 
HU, humerus; IL, ilium; int, intermedium; IS, ischium; mtcp. 1-5, metacarpals; mtts. 1-5, 
metatarsals; ph, phalanges; p.cor, precoracoid; PU, pubis; RA, radius; ra, radiale; SCP, 
scapula; TI, tibia; ti, tibiale; UL, ulna; ul, ulnare. (From Parker and Haswell, "Textbook 
of Zoology.") 

The Motive Power. — The movement of structures in the higher 
animals, whether these structures contain parts of the skeleton or not, 
is all effected by muscles. Protoplasm in general has the power of 
contracting, and in the protozoa there are motile structures, the cilia and 
flagella, which have already been described (page 51). The muscles are, 
however, much more specialized than any of these. 

In general, the muscles are arranged in opposing pairs or sets. In 
the earthworm, in which crawling is effected by alternate contraction 
and expansion of the length of the animal, there is one set of muscles 
running lengthwise, another passing circularly around the body. With 
the front end of the worm holding to the soil with its sloping setae, a wave 
of contraction of the lengthwise muscles draws up the rest of the body. 



Then the circular muscles contract, while the longitudinal ones relax. 
Since the body cavity (coelom) is filled with a fluid and cannot reduce its 
volume, contraction of the circular muscles forces the body to elongate, 
thus pushing the front end forward to take a new hold upon the soil. 
The setae, as previously explained (page 89), are tilted forward or back- 
Avard by opposing muscles. In vertebrate animals, bones are moved by 
muscles and tendons placed on opposite sides of the bones at or near the 
joints. The arrangement at the knee joint in man is shown in Fig. 83. 
The flexor muscle bends the joint, the extensor straightens it. When 


Fig. 83. — Diagram of knee joint in man, illustrating opposed muscles. 

one of these muscles contracts, the other must relax if movement is to 
be produced. If both contract the leg is merely made tense. The area 
of attachment of the less movable end of the muscle (usually that nearest 
the body) is called the origin of the muscle, that of the more movable 
end its insertion. In such boneless movable parts as the eyelids and lips, 
one set of muscles, operating to pull radially away from the openings 
which these structures surround, is opposed by circular bands of muscles 
which close the openings. The stomach and intestine of vertebrate 
animals possess longitudinal and circular muscles which operate much 
as do those of the earthworm. Everywhere muscle is opposed by muscle. 

Fig. 84. — Smooth-muscle cells. 

The necessity of this arrangement arises from the fact that, while muscle 
contracts vigorously, its expansion is entirely passive. It can force move- 
ment in one direction but can only permit it in the opposite direction. 

Muscle. — Muscles constitute the contractile tissue referred to in the 
preceding chapter (page 81). They are nearly always plates or bundles 
of cells, not single cells. Three types of muscle cells in vertebrate 
animals may be recognized, known respectively as smooth, striated, and 

Smooth muscle is composed of cells each of which is provided with a 
single nucleus. The cytosome contains well-marked longitudinal fibrils. 
These cells (Fig. 84) have the form of slender spindles with unbranched 
tips or in certain organs the tips may be branched. They are found in 



the walls of the digestive tract, urinary bladder, gall bladder, arteries 
and veins, and in certain glands and their ducts. 

Striated muscle differs greatly in its structure from smooth muscle. 
For one thing, it has many nuclei in each cell. The cells of an embryo 
from which striated muscle cells develop have only one nucleus apiece, 
but after a time the nucleus divides a number of times without an 
accompanying division of the cell body. Many nuclei are thus present 
in the muscle cells of the adult. The striated muscle cell is roughly 
cylindrical in form and usually very long. It is covered by a firm mem- 
branous sheath, the sarcolemma. Within this is the rather liquid proto- 



A B C 

Fig. 85. — General appearance of striated muscle. A, part of a muscle fiber of a frog; 
B, part of a fiber teased out to show myofibrils; dh, darli bands; lb, light bands;/, myofibril; 
n, nucleus; s, sarcolemma; C, a myofibril, diagrammatic; dh, dark band; Ih, light band with 
a thin band of dark material dividing it into two portions. (A and B from Parker and 
Haswell, " Textbook of Zoology.") 

plasm called the sarco'plasm. Imbedded in the sarcoplasm, and forming 
a large part of the bvilk of the cell, are numerous slender strands, the 
contractile myofibrils (Fig. S5B,f). Each myofibril consists of alternate 
segments of different substances, light and dim in appearance. In the 
muscle cell these myofibrils extend parallel to each other and to the long 
axis of the cell and are so aligned that the dim segments are side by side, 
and light segments are side by side. Collectively they give the whole 
cell the appearance of being marked by light and dark transverse bands 
(Fig. 85A). These are the marks to which the term "striated" refers. 
Little is known of the chemical or physical properties of the substances 
in the light and dim bands, but when they are examined with polarized 
light it is found that the dark substance is doubly refractive. 



Cardiac muscle is found only in the heart of vertebrate animals. It 
contains fibrils somewhat resembling those of striated muscle, and has 
cross striations which these fibrils confer on it. However, the strands 
of heart tissue interconnect in a network, and there is little or no blocking 
off of the protoplasm into cells. The heart is thus practically a large 
syncytium (page 66). 

The actions of the three kinds of muscle are very different. Smooth 
muscle is capable of only relatively slow movement. It is not directly 
subject to the will, hence is sometimes called involuntary muscle; but 
this is not a distinctive designation, since the heart is also free from 













^ \ 



Fig. 86. — Curve illustrating the course of a single muscle twitch. 

conscious control, and even striated muscle sometimes acts involuntarily. 
Striated muscle acts very strongly and very rapidly; and since its move- 
ment is regularly initiated by act of will, it is known as voluntary muscle. 
Heart muscle acts without control of the will, as do other vital organs. 
Its rhythmic action can be maintained for long periods after removal of 
the organ from the body, as determined by a mechanism to be described 
in a later chapter. Because of its syncytial nature, waves of stimulation 
pass rapidly over the whole heart, and the organ tends to act as a single 

Muscle Contraction. — In the living animal, contraction is stimulated 
only by nerve impulses, though in laboratory experiments artificial 
stimuli can be given. A single nerve cell may govern only a few muscle 
cells, or as many as 150. The group of muscle cells controlled by one 
nerve fiber constitutes a motor unit. It is characteristic of motor units 
that, if they (contract at all, they do so to their fullest capacity, in accord- 
ance with the all-or-none law already stated (page 53). Since muscles 
are made up of many motor units, some contracting, others usually not, 
an entire muscle may experience many degrees of contraction. How 
many motor units act depends on the intensity of the nerve stimulus, a 
strong stimulus activating many of them, a \veak stimulus few. 

A single stimulus to a striated muscle results in a single quick twitch 
of the muscle. If tlic muscle is attached to a movable pointer, which 



traces a line on smoked paper on a revolving drum, the single twitch is 
recorded by a curve of characteristic form (Fig. 86). The twitch as a 
whole lasts about 0.1 second in the frog. It takes a very short time 
(0.01 second) for the muscle to start to contract. This brief period of 
inaction is known as the latent period; by the time it is ended the change 

Fig. 87. — Curves of jcontraction of muscle in response to repeated stimuli. Rate of 
stimulation is slow at the bottom, but gradually increases toward the top. {From Howell, 
"Textbook of Physiology," W. B. Saunders Company.) 

of electric potential which is the sign of stimulation has usually reached 
a peak and subsided. Then the muscle contracts for about 0.04 second, 
and the succeeding relaxation lasts about 0.05 second. Following the 
tw'itch there is a period of recovery lasting a number of seconds in which 
the muscle returns to its previous condition. If stimuli are applied 
repeatedly before the recovery is complete, the muscle shows fatigue 


and its responses are weaker. Smooth muscle, as in the intestine, reacts 
much more slowly, the contraction lasting about 20 seconds. The relax- 
ation of any muscle is purely passive; the ends of the muscle fibers do 
not push. 

Single twitches are not, however, the commonly observed type of 
muscle action. During ordinary contraction, nerve impulses are deliv- 
ered in rapid succession, beginning, say, at 4 or 5 per second and increas- 
ing in frequency to 40 or 50 per second. These rapidly repeated stimuli 
may be shown experimentally to be the most effective method of getting 
strong and sustained contraction. The nature of the contraction result- 
ing from stimuli repeated at different rates is shown in Fig. 87. In the 
lowest curve the stimuli were given at a slow rate, and after each one 
the muscle relaxed almost to its former state. But when the stimuli 
were given more and more rapidly, as in the remaining curves of the 
figure, complete relaxation did not have time to occur between them, 
and the total contraction gradually increased. 

In striated muscle the cells act separately and do not communicate 
stimuli to surrounding cells. In smooth muscle, however, stimulation 
at one point may lead to a wave of contraction passing over a whole 
sheet of muscular tissue, showing that the stimulus is communicated 
from cell to cell. 

The efficiency of muscle, that is, the ratio of work done to energy 
consumed, is rather high. For a single twitch, including the recovery 
period following, this ratio is about 50 per cent. For sustained contrac- 
tion, however, the efficiency is much less — around 25 per cent. 

Chemistry of Muscle Contraction. — Just what happens in a striated 
muscle when it contracts is only partially understood. It is the myo- 
fibrils that do the contracting, but the important thing to know is the 
set of physical or chemical conditions which cause them to shorten. 
Clues have been furnished by chemical analysis of fatigued muscle. 
Most of the glycogen, which in rested muscle amounts to about 3 per 
cent of the weight, has disappeared in fatigue, as has also much of the 
oxygen. At the same time the inorganic phosphates (produced out of 
organic phosphates) have considerably increased; so also has carbon 
dioxide. If imder experimental conditions oxygen is excluded there is 
also an increase of lactic acid. How the glycogen is lost is known; 
combining with water, it is converted into glucose and lactic acid. 
Something must also have been oxidized to account for the increased 
carbon dioxide. Under ordinary conditions the lactic acid does not 
persist, for part of it is oxidized to obtain energy with which the rest 
of the lactic acid is reconverted to glycogen. Formerly it was thought 
that the breaking down of glycogen or the oxidation of one of its products 
furnished the energy for muscle contraction; yet conversion of glycogen 


may be prevented by certain poisons, and the muscle still be able to 
contract. It seems necessary to conclude that the energy comes from 
decomposition of organic phosphates; such phosphates are known to 
release energy with almost explosive speed when they are decomposed. 
The organic phosphates must be reconstituted, ready for the next con- 
traction, and the energy for this reconstitution comes from oxidations. 
The oxidations are thus accessory phenomena; instead of furnishing the 
energ}^ for the contraction itself, they provide for the restoration of the 
phosphates, and the latter on decomposition furnish the energy for 

The mechanism of the contraction itself is probably the sudden 
folding of long protein molecules arranged lengthwise in the myofibrils. 
Since the most abundant protein in muscle is myosin, this may be the 
responsible agent. Myosin extracted from muscle exercises a strong 
catalytic action on the decomposition of organic phosphates, and this 
action may be a part of the contraction process. 

A muscle in which there is no more organic phosphate nor glycogen, 
and in which much lactic acid has been accumulated, is incapable of 
contraction; it is "fatigued." In living animals as distinguished from 
laboratory preparations, however, the common source of fatigue is 
not in the muscle itself, but between the muscle fibers and the nerve 
which delivers the commands to contract. Some substance there, at 
the junction of nerve with muscle, experiences a change in response 
to repeated stimulation such that it no longer transmits the stimulus 
or does so more weakly. The nerve fiber still conducts, and the muscle 
is still able to contract. The nature of the failure of the junction is not 


Carlson, A. J., and V. Johnson. The Machinery of the Body. University of 
Chicago Press. (Pp. 345-360.) 

Mitchell, P. H. A Textbook of General Physiology. McGraw-Hill Book Com- 
pany, Inc. 3d Ed. (Chap. II, muscle contraction.) 


Muscular action and the other activities of an organism entail destruc- 
tion of living substance, which must be steadily replaced. In growing 
animals, not only are repairs necessary, but provision must be made 
for new construction. The general source of material for growth and 
replacement is food. How this material is utilized in single cells has 
already been described; how it is transformed in multicellular animals is 
now our concern. 

Since most food is not in a form that can be transported through 
protoplasm, it must usually be converted in some way. In large part 
the conversion consists of making it soluble. But even some soluble 
foods are unable to pass through tissues, because of the selective action 
of protoplasm which will receive some substances and not others. The 
conversion is accomplished by the process of digestion which, in multi- 
cellular animals, is carried on in some sort of digestive system. 

The Locus of Digestion. — In the protozoa digestion is an intracellular 
process. Amoeba engulfs food by flowing around it at any part of the 
cell. Paramecium takes the food in at a particular place, through a 
permanent gullet. In either case the food is surrounded by a droplet 
of liquid, which is acid in reaction at first, and presumably enzymes are 
secreted into this fluid. The food vacuole thus formed is the digestive 
apparatus. These features of protozoan digestion were described earlier 
but are repeated here in the first two parts of Fig. 88 for contrast. Among 
the multicellular animals, sponges retain the intracellular type of diges- 
tion. Through the channels and cavities which are characteristic of 
sponges, water flows, kept in motion by the flagella of collar-bearing 
cells in some of the channels (Fig. 33). From the water the collared 
cells seize organisms, after the manner of Amoeba, and digest them. 
Products of this digestion are passed on to other cells by diffusion or 
osmosis, so that nutrition in sponges is on a cooperative basis; but just 
as in protozoa, digestion is done within the cells. 

In all metazoa other than sponges digestion is performed partly, even 
chiefly, in cavities of organs — -surrounded by cells, but not in cells. The 
process is at least bcgvm in these cavities, and in the higher animals is 
almost completed there. The more complicated types of food are 
rendered quite simple before they leave these cavities. Some foods are 




rendered completely soluble and immediately ready to enter into the 
metabolism of protoplasm. Other foods leave the digestive cavities 
lacking still one or two of the simplifjdng steps which are necessary. 
The cells which receive these incompletely digested foods finish the 
process themselves. Indeed, all cells which use these kinds of foods in 
their metabolism must have the power of taking these last digestive 
steps. Thus some of the primitive digestive activities characteristic of 
protozoa are not lost b}'' any active cells in any organism. 















^-^S' LARGE 

ccT,K. ?-~0"^ INTESTINE 







Fig. 88. — Diagrams of several types of digestive systems in metazoa, compared with 


Simple Digestive Systems. — ^The simplest system in which digestion 
occurs in a cavity is that known as a coelenteron. Hydra (Fig. 88) has 
such a system. A coelenteron has only one opening to the outside, 
usually called the mouth, although besides taking in food that opening 
must also be the place of exit of undigested matter. The coelenteron 
of Hydra is in the main a simple sac, though it is branched into the 
ring of tentacles near the free end of the body. A less diagrammatic 
representation of Hydra's coelenteron is given in Fig. 59, where it is 
labeled the gastrovascular cavity and the cells forming its wall are the 
endoderm. Flatworms also have a coelenteron. In some of them (Fig. 
89, above) it is as simple as in Hydra, but the simplicity is not primitive; 




Fig. 89. — Digestive system 
(coelenteron) of a rhabdocoele 
flatworm (above) and a triclad 

it is a result of degeneracy. Other flatworms have a three-branched 
coelenteron, each part of which is extensively branched (Fig. 89, below). 
As animals rise in the scale of complexity the digestive system becomes 
a tube open at both ends. One end is the mouth, which ingests food, 
the other end the anus through which undigested, mostly indigestible, 

matter is ejected. In the course of the 
tube it is differentiated into organs. In 
the earthworm (Fig. 88), following the 
mouth, there is a short buccal cavity, a 
-pharynx with strong muscular walls, an 
esophagus^ a croy in which food may be 
stored, a gizzard with thick muscular 
walls and a chitinous lining by means of 
which food may be finelj^ ground, and an 
intestine with secreting and absorptive 
cells. An internal ridge, the typhlosole, 
formed by an infolding of the dorsal wall 
of the intestine (Fig. 66), gives increased 
surface. About the exterior surface of 
the intestine is a layer of brown cells, the 
chloragogen cells, which have been thought to serve as a digestive gland, 
possibly as a liver. 

Digestive Systems in the Vertebrates. — In the vertebrates the diges- 
tive system reaches its highest development. Here it consists not only 
of an alimentary canal, subdivided into regions, but also of highly 
developed glands which produce digestive secretions. A diagram repre- 
senting vertebrates in general fairly well, but more particularly the 
mammals, is at the bottom of Fig. 88. The system in the frog is slightly 
more simple (Fig. 90, left). In the mouth the upper jaw bears teeth 
which serve to hold the prey when caught. Attached to the anterior 
portion of the floor of the mouth is a prehensile tongue which is provided 
with many glands that produce a sticky secretion. The buccal cavity or 
mouth cavity leads backward into the short broad esophagus through a 
distensible opening, the pharynx. The esophagus leads into the muscular 
stomach which in the frog, as in most vertebrates, is a curved organ 
usually lying somewhat to one side of the middle line. The walls of 
both the esophagus and stomach are provided with highly developed 
glands which secrete digestive solutions. The stomach opens into the 
small intestine through a muscle-encircled passage, the pylorus. The 
small intestine of vertebrates is usuall}^ subdivided into three portions 
named, respectively, the duodenum, jejunum, and ileum. Of these the 
duodenum and ileum alone are recognized in the frog. These regions 
as a rule merge imperceptibly into one another, yet each shows certain 



characteristic structural features and each occupies a certain portion of 
the intestine. The duodenum receives the secretions of two large diges- 
tive glands, the liver and the pancreas. In the frog the secretions of 
these two glands are discharged through the common hile duct into the 
middle region of the duodenum. A reservoir, the gall bladder, attached 
to the liver and connected with the bile duct, serves as a storage place 
for the hile, one of the secretions of the liver. The small intestine is 
connected at its posterior end with the large intestine which in the frog 
is subdivided into two portions, namely, the rectum and the cloaca. The 
term cloaca is used to designate that portion of the large intestine 
which is used as a common passage for undigested materials from the 

Fig. 90. — Digestive systems of the frog (left) and nlan, somewhat simplified. 

alimentary tract, for urine, and for reproductive cells from the urino- 
genital system. It occurs in a few mammals and in most other verte- 
brates. The large intestine opens to the exterior by means of the anus. 

The human digestive system differs little enough from that of the 
frog that the illustration in Fig. 90, right, should suffice without further 
description. That figure, however, omits the mouth and its associated 
salivary glands, and the small intestine is greatly shortened. 

It is worthy of note that the intestine of the frog is relatively short. 
This condition is found in flesh-eating animals in general. Animals which 
feed entirely or largely upon vegetable food have long intestinal tracts 
and frequently have a large caecum (a blind pouch) at the junction of 
the small and large intestines. The rabbit and muskrat have a large 
caecum with a vermiform appendix at its end; the chicken and dove have 
two caeca. In man the caecum is small, rudimentary, with a vermiform 


appendix. Highly specialized modifications of the stomach occur in 
ruminants (animals which chew the cud) and in seed-eating birds. 

Digestion in Man. — Inasmuch as the digestive process as it occurs in 
man has been much more intensively studied than in any other animal, 
the discussion of digestion which follows will be based on the human 
system. In the mouth, food is broken up, during which process the three 
pairs of salivary glands pour out their secretion (saliva) which is mixed 
with the food. The saliva contains an enzyme, ptyalin, which is al^le to 
transform starch, particularly cooked starch, into certain sugars. The 
breakdown of starch occurs by degrees, the intermediate products being 
various dextrins, but in no case does the digestion in the mouth go 
farther than to maltose, which is not one of the simple sugars. It is still 
a disaccharide (page 40) and not readily diffusible through protoplasm. 
Ordinarily, because of the short sojourn of the food in the mouth, little 
starch digestion actually takes place there ; and since ptyalin acts only in 
an alkaline medium, its action is stopped by the acid of the stomach 
when the food reaches that organ. 

In the stomach, the food is acted upon by the secretion of the gastric 
glands which are small branched or simple tubular glands located in the 
inner layer of the stomach. The movement of the muscles of the 
stomach mixes the food with the gastric secretion, which contains hydro- 
chloric acid and two important enzymes, pepsin and rennin. The hydro- 
chloric acid affords a suitable medium for the action of the enzymes 
and incidentally stops the action of the ptyalin descending from the 
mouth. The rennin coagulates milk, a fact made use of in cheese fac- 
tories where a preparation of rennin made from calves' stomachs is used 
to separate the curd from the whey. Pepsin as it comes from the gastric 
glands is in an inactive state in which it is called pepsinogen. Pep- 
sinogen is activated (converted into pepsin) by the hydrochloric acid, 
which is secreted in a concentration of about 0.4 to 0.5 per cent. Pepsin 
acts only on proteins, converting them to peptones and proteoses, 
which are also proteins but simpler than most proteins taken as food. 
Ordinary fats are not acted upon in the stomach. 

Absorption of foods in the stomach is negligible. Alcohol is absorbed 
there, which may account for its quick action on mental and other 
physiological processes. 

Secretin. — When the acid stomach contents are ejected through the 
pylorus, the acid acts upon a substance in the lining epithelium of the 
duodenum and changes this substance to secretin. The secretin is 
absorbed by the blood and is carried to the pancreas and liver which are 
thereby stimulated to secrete their fluids. Secretin belongs to a class of 
activators known as hormones. Normally, the pancreas and liver are 
also controlled in part by nerve impulses. Nevertheless, these glands dis- 


charge their secretions even after the nerves which innervate them 
are cut. 

The Pancreatic Juice. — The pancreas produces a thin watery secre- 
tion containing three enzymes, which act upon proteins, carbohydrates, 
and fats, respectively. The protein-spHtting enzyme is inactive when 
it emerges from the pancreatic duct and is then known as trypsinogen; 
but, when it comes in contact with the duodenal surface, it is quickly 
rendered active. The conversion of trypsinogen is initiated by the 
enzyme enter okinase , produced in the lining of the duodenum. This 
enzyme acts upon the inactive trypsinogen, changing it to the active form 
called trypsin. The trj^psin splits proteins, proteoses, and peptones from 
the stomach into simpler and simpler compounds. The end products of 
protein digestion are amino acids (page 41) and several other compounds. 
Trypsin works in alkaline, neutral, or even acid media. It completes the 
work begvin by the pepsin and works more rapidly and breaks up the 
protein more completely than does the pepsin. 

The carbohydrate-splitting enzyme of the pancreas is amylopsin. 
Unlike trypsinogen, it requires no activation. It converts starches, dex- 
trins, and complex sugars (with the aid of so-called inverting enzymes) 
into simple sugars (glucose and others), which are in condition to be 

The fat-splitting enzyme of the pancreatic juice is steapsin. Steapsin 
splits fats into glycerol (glycerin) and one or more fatty acids (page 41).. 
These substances are soluble and are absorbed in this condition. 

The Secretion of the Liver. — Bile, the secretion of the liver, contains 
no enzyme. It contains water, bile salts, and certain excretory materials. 
The discharge of bile is stimulated, as explained above, by the hormone 
secretin in the same manner as is the secretion of pancreatic juice. Bile is 
ordinarily stored in the gall bladder until the partially digested acid food 
is ejected by spurts from the stomach, but it has been shown in some 
animals that such a temporary storage place is not essential to the 
proper production and ejection of the bile. Each ejection of food into the 
intestine stimulates a flow of bile through the bile duct. The bile salts 
break up the fats into very fine droplets, thus greatly increasing the 
surface through which the fat-splitting enzyme may attack them. If 
the bile is artificially prevented from entering the intestine, a large 
share of the ingested fat is not digested and may be recovered in the 

The Intestinal Secretion. — The secretion of the small intestine is 
produced in small tubular glands which are local evaginations of the lining 
layer. This secretion consists of enterokinase, erepsin, several other 
enzymes, and secretin. Enterokinase, as stated above, converts inactive 
trypsinogen into active trypsin. Erepsin is a protein-splitting enzyme 


which, although unable to digest the original proteins, attacks the pep- 
tones which result from digestion in the stomach, reducing them to 
amino acids. It thus supplements the action of trypsin. The other 
enzymes convert maltose and the dextrins (resulting from the operation 
of ptyalin and amylopsin upon starches) into glucose and other simple 

Secretin, as indicated above in connection with the stimulation of the 
pancreas, is not an enzyme but a hormone. It exists in the wall of the 
duodenum as prosecretin which is stable and does not affect the pancreas. 
The acid from the gastric juice mixed with the food coming from the stom- 
ach changes the prosecretin into secretin which is absorbed and carried 
by the blood to the pancreas and the liver, which are thereby stimulated 
to secrete pancreatic juice and bile, respectively. 

Digestion in the Large Intestine. — The large intestine produces no 
enzyme. Water and some of the products of digestion are absorbed 
here. Bacteria flourish in the large intestine. Many of these attack 
proteins, while others attack the cellulose of plant cells and perhaps so 
break it down that some sugars are recovered from it. Bacteria which 
attack proteins are not numerous, however, when the products of protein 
digestion are removed with normal rapidity. Bacteria may also supply 
an important vitamin, as is indicated later. 

Absorption. — In the more complex animals absorption occurs along 
the portions of the alimentary tract. In such simple animals as Hydra 
all the endodermal cells are bathed in the products of digestion or carry 
on digestion in themselves, and through these cells absorption takes place. 
Some of this material not used by the endoderm is passed on by diffusion 
to the ectodermal cells. In animals with a circulatory system the sim- 
pler substances pass through the absorbing cells directly into the blood 

In man, as stated earlier, there is little absorption in the stomach. 
Most of it occurs in the small intestine, whose inner surface is enormously 
enlarged by the fingerlike protrusions called viUi (Fig. 91). Amino acids 
and simple sugars are absorbed directly into the blood, which carries them 
through the liver before delivering them to the general circulation. 
Glycerol and the fatty acids are absorbed, but in the process are at least 
partly reconverted into fats. Since fats are insoluble, they exist in the 
form of droplets and are delivered thus, not to the blood, but to the lymph 
vessels. However, since the lymph vessels empty into the blood stream 
(in the left shoulder, page 131), the entrance of fat into the blood is 
merely delayed. 

While absorption by the intestinal wall is partly simple diffusion, 
some selection is practiced by the absorbing cells, so that certain sub- 
stances are passed readily, others are retarded or rejected. This selecti\'e 



action may even send substances against the diffusion gradient — that is, 
cause them to go from places of lower to places of higher concentration. 
Storage of Food. — Carbohydrates, in the form of glucose or other 
simple sugars, are ordinarily present in the blood to the extent of less than 
0.1 per cent. After a meal they may increase perceptibly, but when they 
rise above 0.14 per cent they begin to be excreted by the kidneys and are 
lost. Protoplasm contains some glucose, mostly in combination with 
other compounds, and to that extent carbohydrates contribute to the 



capilla^^/ network 
in villus 



l^mph vessel 


FiG. 91. — Diagram of .section through wall of small intestine, showing two villi and their 
enclosed blood and lymph vessels. {From Stover, "General Zoology") 

architecture of the living substance. Much carbohydrate material is 
stored in the form of glycogen, which is made up of many molecules of 
simple sugars combined. The main storehouse of glycogen is the hver, 
from which it is withdrawn when current supplies are inadequate. 

Lipids enter into the construction of protoplasm, particularly at the 
surfaces of cells, where they play an important role in determining perme- 
ability of the cell membrane. Since the need of these materials is con- 
tinuous, while the supply from digested food is intermittent, lipids must 
be stored. The ones so deposited are chiefly fats. All cells store them to 
some extent, but connective tissues between skin and muscles and among 


the muscles, and the mesenteries of the intestine and other organs, are 
particularly devoted to this function. 

Storage of indiffusible substances such as glycogen or fats necessitates 
redigestion of them when they are to be used; consequently enzymes for 
carbohydrate and fat digestion must be produced or producible in all 
cells which store these products. 

Proteins are not stored in animals, as carbohydrates and fats are 
stored. The supply of protein foods must therefore be rather steady; 
that is, they should be included in the diet almost daily. Amino acids 
enter the blood after the digestion of protein foods and are taken up 
by the cells which require them. When the diet is deficient in proteins, 
requirements of amino acids in vital situations are supplied only by 
breaking down body proteins elsewhere, as happens in starvation. 

Energy Requirements. — Any balanced diet must provide two things, 
energy and materials. Energy is measured in the units known as 
calories, one calorie being the amount of heat necessaiy to raise the 
temperature of a kilogram of water 1°C. Each gram of a carbohydrate 
or protein food utilized in metabolism yields about 4 calories, a gram 
of fat about 9 calories. A relaxed, fasting human body of average size 
and shape, in prone position, requires about 1600 calories daily. More 
than half of this energy goes to maintaining the body temperature. 
The rest is expended by the vital organs such as the heart and the 
muscles performing breathing movements. If food is taken, so that 
muscles of the digestive tract are also active, the daily energy require- 
ment is about 1800 calories. For sedentary workers leading normal lives 
it rises to about 2400 calories, while manual laborers need 3000 to 5000 
calories, depending on how hard and long they work. If an average 
person consumes much more energy than is proper to his mode of life 
and occupation, he may have an overactive thyroid gland or a fever. 
If the energy consumption is much less than normal, the cause may be 
a deficient thyroid or pituitary or adrenal gland, or low nutrition. 

So far as mere quantity of energy is concerned, it may be obtained 
from any of the types of food. Pligh protein diet requires more work 
of the kidneys because of the increased nitrogenous wastes, but the 
kidneys are capable of much more than an average load if they are 
healthy. An excess of fat is objecticmable chiefly because fats do not 
oxidize very completely unless carbohydrates are being oxidized at the 
same time. To some extent the human body can alter the proportion 
of the different kinds of compounds derived from its food, for amino acids 
can be converted to glucose, and carbohydrates to fat; but there is little 
conversion of fat to carbohydrate, and only the simpler amino acids can 
be made from nonprotein foods. 

If the food currently taken does not provide the required energy. 


stored foods are consumed. The carbohydrates (glycogen of the Uver and 
muscles) are used first. Fats are used simultaneously with the carbo- 
hydrates but usually last until after the carbohydrates are exhausted. 
Then the materials of the protoplasm itself are used, first those of the 
less essential organs, then of the brain, spinal cord, and heart. Death 
usually follows quickly upon such extreme starvation. 

Materials Required. — Besides furnishing energy, food must also pro- 
vide materials with which to build protoplasm and such secreted products 
as the hard parts of bone and teeth. One of the most urgently required 
materials is water — 2000 cc. a day in an average person. Certain salts 
must be regularly supplied, since about 30 grams are lost per day, mostly 
in urine and sweat. Most ordinary foods contain about the right pro- 
portion of the various salts, though vegetable foods are deficient in 
sodium chloride (NaCl). This is the reason for the common use of 
table salt. Any one sweating profusely because of heavy labor in hot 
places must usually drink salt water to avoid muscular spasms. 

There are certain minerals which are necessary. The ones most 
likely to be poorly represented in the diet are iron, calcium, and iodine. 
The hemoglobin of red blood cells requires iron, and this is adequately 
provided in liver, meats in general, eggs, and many vegetables and fruits. 
Calcium is needed for bone and teeth, and is obtained from milk, cereals, 
peas and beans. Iodine is necessary for the hormone of the thyroid 
gland. It is abundant in sea foods; and in inland communities health 
authorities often require that potassium iodide be introduced into table 
salt. Other minerals, including copper, zinc, manganese, and cobalt, are 
essential for the production of important enzymes, but the amounts 
needed are exceedingly small and natural diets usually contain enough 
of them. 

For construction of protoplasm proteins are steadily required — a 
minimum of 50 grams a day for an average adult person. A variety 
of amino acids is necessary, and since only a few of the simplest ones 
can be synthesized from other substances, the others must be included 
in the diet. Foods which supply all the necessary amino acids are the 
proteins of eggs and lean meat, the glutenin of wheat, and the lactalbumin 
of milk and cheese. Most other protein foods lack, or include too small 
quantities of, certain amino acids. Some fat is also required; for though 
most of the fatty acids can be synthesized from carbohydrates, the ones 
which the human body can not synthesize are quite essential, and these 
must be received ready-made. 

Vitamins. — One group of required specific substances deserves sepa- 
rate treatment. It has long been known that a diet consisting of purified 
proteins, carbohydrates, and fats leads to serious trouble. Natural foods 
evidently contain something that does not occur in the purified foods. 



These essential substances were given the collective name of vitamins 
before anything was known of their identity. These substances, in 
small quantities, are needed for healthy activity or growth. If any of 
them is lacking, or present in too small amount, a deficiency disease results. 
The disease is specific for each of the vitamins. 

The earliest known and recognized of the deficiency diseases was 
scurvy. Before the end of the sixteenth century an officer of the English 

Fig. 92. — The need of vitamin A. Upper two dogs show xerophthahuia caused by 
deficiency of vitamin A. Lower figure, one of same dogs after 10-day treatment with cod- 
liver oil. {From Steenbock, Nelson, and Hart in American Journal of Physioloyy.) 

navy observed the bruised skin, bleeding gums, and general anemia of 
his crew after they had been many months at sea and fresh foods had 
been exhausted, and he discovered that these symptoms could be com- 
pletely prevented l)y giving his men a small amount of lime juice daily. 
The essential feature^ of the lime juice was long designated vitamin C, 
though its nature was unknown. In 1933 this vitamin was separated 
out in pure form, and was found to be ascorbic acid, of the chemical 
formula CeHgOe. It is abundant in citrus fruits (oranges, lemons, limes, 
grapefruit), many other fruits, tomatoes, and many vegetables. Diets 


which inchide raw plant food are generally adequate, but cooking in 
vessels exposed to air usually destroys much of the antiscorbutic effect. 

Vitamin A, itself colorless, can be split off, in the human body, from 
the yellow pigment carotene found in carrots and many yellow and green 
vegetables. Its formula is C20H30O. Severe lack of it in the diet leads 
to a dry, ulcerated condition of the cornea of the eye known as xeroph- 
thalmia (Fig. 92). Milder deficiencies cause abnormalities of epithelial 
membranes and retard growth. Vitamin A is also used by the retina 
of the eye in the synthesis of visual purple, one of the light-sensitive 
pigments, and was administered during the war to night-flying pilots to 
improve their vision. Being soluble in fats (as are two other vitamins, 
D and E), vitamin A is obtainable in liver oils and in such foods as milk, 
butter, and egg yolk. Manufactured butter substitutes are usually 
fortified by the addition of this vitamin. 

What was originally called vitamin B eventually proved to be a 
collection of different substances, enough alike to be hard to separate, 
and occurring mostly in the same natural foods. This group, consisting 
of seven or more vitamins, is now known as the B complex. Only the 
more important of these can be mentioned here. Lack of thiamin (Bi) 
causes polyneuritis, which in man is usually named beriberi. This 
disease involves degeneration of the nerves, causing progressive paralysis. 
Along with paralysis go retarded growth and loss of appetite and vigor. 
Intravenous injection of Bi into polyneuritic animals restores normal 
muscular movement in as short a time as one hour. The formula of 
thiamin is C12H16N4SO. One of its sources in food is in cereals, especially 
the outer seed coats. For this reason polished rice, in which the seed 
coats are removed, and highly refined wheat flours (as contrasted with 
whole wheat) are poor in thiamin. It is common practice now to add 
thiamin in the manufacture of white flour. Other natural sources of 
thiamin are meats, especially pork, and yeast. 

A second member of the B complex is riboflavin (C17H20N4O6), called 
also B2. It is found in the same foods as Bi and the other vitamins of 
this group. Lack of it induces a predisposition to cataract, loss of 
weight, and scaliness of skin around the ears and mouth. 

Closely associated with the other B vitamins is niacin (C6H5NO2), or 
nicotinic acid. Lack of it is the principal cause of pellagra, which is 
characterized by dermatitis (eruption of the skin) and diarrhea. As a 
pellagra preventive, niacin has come to be called vitamin P-P. The 
disease is still common in southeastern United States, where corn, 
molasses and meat are the staple diet. Niacin is manufactured and is 
available to prevent pellagra, but is not yet in sufficiently wide use. The 
dermatitis feature of pellagra may be due to lack of Be, or pyridoxin, 
which is frequently absent from the pellagra-producing diet. 


Rickets, the imperfect growth of bones and teeth, is caused by a 
deficiency of vitamin D. This substance is now known to be calciferol 
(C28H44O). It is produced from a closely related substance, ergosterol, 
regularly present in the skin, by ultraviolet radiation. In summer time 
the conversion of ergosterol to calciferol is usually adequate in most 
regions, but in winter it is often advisable to supply vitamin D artificially. 
The common foods containing it are butter, milk, and the oils of liver and 
other animal tissues. So well understood are the preventive properties 
of these foods, or the manufactured vitamin, that rickets, once a common 
disease, is seldom observed in most communities. 

Reproductive disturbances in some animals are caused by lack of 
vitamin E, a-tocopherol (C29H50O2). In its absence female rats do not 
retain the embryos in the uterus, and male rats do not produce functional 
spermatozoa. No such effects have yet been shown in man. Vitamin E 
occurs widely in plant and animal oils, particularly in the germ of wheat. 

Failure of coagulation of the blood may be caused by lack of vitamin 
K, whose formula is C31H46O2. In its absence the body does not pro- 
duce enough prothrombase, from which the clotting enzyme is produced 
at wounds. Vitamin K is regularly administered before child-birth, with 
a considerable decrease in mortality from bleeding in both the newborn 
children and their mothers. Natural food sources of the vitamin are 
leafy vegetables; it is prepared commercially from alfalfa. 

Vitamin P, not yet identified chemically, is closely related to ascorbic 
acid (C) and is involved in scurvylike weakness of the walls of blood 
capillaries. Its status is still unsettled. 

The necessary amounts of vitamins are so small (0.01 gram or less 
daily) that they cannot be regarded as sources of energy. They must 
be in some way essential in protoplasmic structure. Three of the 
vitamins, thiamin, riboflavin, and the antipellagra factor, are known to 
enter the composition of important oxidative enzymes; that is, they 
furnish the nonprotein part of the enzymes. What other structural 
contributions the vitamins make is not known. 

The need of vitamins in food differs greatly in different animals. 
Rats, for example, need no ascorbic acid in their diet, since they syn- 
thesize it in their metabolism; rats never have scurvy. Man can get 
along with little or no thiamin in his diet; but bacteria in his large 
intestine must then supply it. As stated above, man probably does not 
require vitamin E, or else produces it in normal metabolism. 

Carlson, A. J., and V. Johnson. The Machinery of the Body. University of 

Chicago Press. (Chap. VII.) 
Mitchell, P. H. A Textbook of Ceneral Physiology. 3d Kd. McGraw-Hill Book 

Company, Inc. (Chap. XVIII, digestion; Chap. XXI, respiration; pp. 745-772, 

vitamins.) , 


The total requirements of energy and the general source of it in the 
food have already been discussed in connection with nutrition. How 
energy is released from food is a separate problem. 

Derivation of Energy. — Ultimately most energy comes from sun- 
light. Many plants and a few of the simplest animals have chlorophyll, 
which utilizes solar energy to make sugars. In these sugars, energy is 
bound up in chemical structure. As sugars are converted into starches, 
or fats, or proteins, by combining them with other substances, still further 
energy is stored in these higher products. When plants are devoured by 
animals, the latter take possession of this potential or stored energy. 
So it is that all energy of life is traceable to sunlight. Indeed, most other 
energy in the world comes from the same source. Coal and oils used for 
fuel got their energy from ancient sunlight. Even the energy of water- 
falls came from the same source, for it was the energy of the sun which 
lifted the water to its higher level. About the only energy expended on 
the earth which is not traceable to sunlight is that of the tides. 

Animals derive some of their energy directly from the sun, for sunlight 
is one of the most potent of health-giving agencies. In the main, how- 
ever, they obtain it from food, and for this they are directly or indirectly 
dependent on plants. To get energy from foods, it is necessary that the 
latter be chemically decomposed. The foods must be changed into 
simpler substances whose content of potential energy is smaller. In 
general, complex substances with large molecules have more energy 
bound up in their constitution than do simple substances with small 
molecules. Nearly all chemical reactions which split up molecules into 
smaller and simpler ones may therefore be depended on to release a 
certain amount of energy. Proteins, carbohydrates, and fats, on being 
decomposed, even in the process of digestion, liberate energy. 

There is, however, one type of energy-yielding chemical reaction which 
is so much more abundant than any other that it is common practice to 
speak of energy as coming from that source. That type of reaction is 
oxidation (page 37), the union of oxygen with other elements. The 
commonest of these unions is that of oxygen with carbon, because carbon 
is abundant in all the classes of organic compounds — in proteins, but 
especially in carbohydrates and fats. Carbon dioxide, a very stable 



compound which ties up very httle potential energy, is a product of these 
oxidations, so that the amount of carbon dioxide which an animal pro- 
duces is often taken as an indication of the quantity of energy it uses. 

Respiration. — How is all the oxygen for these oxidations obtained? 
There is not enough of it in the substances to be oxidized. The common 
carbohydrates contain only about half enough oxygen to oxidize their 
own carbon, even if all their oxygen were available — which it is not — for 
that purpose. Fats, the other main source of energy, have even less 
oxygen than the carbohydrates. The oxygen must therefore be intro- 
duced from external sources. For land animals that source is the air, 
about one-fifth of which is oxygen. Aquatic animals of most kinds 
secure the oxygen which is dissolved in the water about them. 

The obtaining of oxygen is included in the process known as respira- 
tion. In small animals — unicellular and small multicellular ones — oxygen 
is absorbed more or less directly by the cells that use it. In the larger 
animals, those in which most of the cells are too far away from the surface 
to rely on this simple diffusion, respiration is a double process. That is, 
the oxygen must first be got into their bodies, a process known as external 
respiration, and then be conveyed to the cells where it is ultimately used. 
Its absorption by these cells, often far within the organism, is called 
internal respiration. In the protozoa, external and internal respiration 
are merged into a single process, to which neither name may be properly 

Whether an animal must have any special devices to carry on its 
external respiration depends on its oxygen requirement in relation to its 
surface. A large animal has much less surface relative to its volume than 
a small one has ; hence, in general, the larger animals must have structures 
which greatly increase their absorptive surfaces. Warm-blooded animals 
consume much more oxygen than do cold-blooded ones, and active 
animals much more than sluggish ones. Even as large an animal as the 
earthworm, which is cold-blooded and not very active, is able to absorb 
enough oxygen through its general surface. Many smaller animals, 
however, because they are active, require some sort of respiratory organ 
for their external respiration. 

Types of Respiratory System. — Probably the earliest external respira- 
tory organs devclopcxl in animals were gills. These may be employed by 
aquatic animals, and by aerial animals having some way of keeping them 
moist, for oxygen cannot bo absorbed through dry surfaces. A gill, 
like any other respiratory organ, must furnish a large surface, since the 
amount of oxygen taken in increases with increase of surface. It may 
consist of branching or treelike projections (Fig. 93), or of bunches of 
fine tubes, or of clusters of flat plates, or of numerous ridges or fingerlike 
projections, or of sievelike sheets through which water passes. In 



every such organ the first essential is an increased surface, and the 
different forms of gill merely represent various ways of attaining that end. 
Among animals that use gills are fishes, some salamanders, crayfishes, 
clams, some marine worms, and young stages of many insects. 

Lungs are internal cavities into 
which air is drawn for absorption 
of its oxygen. Notwithstanding 
their internal location, lungs are 
organs of external respiration, 
since the bulk of the oxygen they 
absorb is not used for energy 
rel6ase in the cells of the lungs 
themselves but is passed on to 
other cells of the organism. 

The lung in lower amphibians 
is a baglike organ with a large 
central cavity (Fig. 94a); but 
in higher amphibians it becomes more complex since its inner surface 
is thrown up into corrugations with cross corrugations forming boxlike 
spaces (b, c). These corrugations increase the respiratory surface. 
In higher vertebrates the lung (d) is entirely subdivided into minute 
air spaces which are in indirect connection with one another through 

Fig. 93. — External gills of the amphibian, 
Epicrium glutinosum. {From Wiedersheim 
after Sarasin.) 


Fig. 94. — Diagrams of types of lungs, a, amphibian lung with plain surface; b, amphib- 
ian lung with low folds making simple alveoli; c, amphibian lung with higher folds which 
are themselves folded making more numerous alveoli; d, human lung. 

large tubes, the bronchi, and their branches, the bronchioles. The 
bronchi unite in a single large tube, the trachea, which is present in 
the higher vertebrates, but absent in some of the lower forms, as 
the frog. The trachea opens into the mouth through a slitlike glottis. 
The trachea and bronchi have cartilage rings in their walls, so they 



do not collapse. The bronchioles end in expanded chambers, the 
alveoli, which are in close contact with blood capillaries. The aggregate 
interior surface of the alveoli in man (Fig. 94d) is more than 1000 square 
feet or about fifty times as great as the general surface of the body. 

In most insects, air is taken in by tracheae. These are tubes opening 
at the surface of the body at various points. The tracheae branch, tree- 
fashion, in such a way as to reach all parts of the body (Fig. 95). No 

part of any insect tissue is more than a 
few cells away from the nearest tracheal 
branch. Formerly it was thought that 
air pulsed back and forth, into and out 
of these tracheae. It is nOw known for 
some insects, however, that air goes in at 
certain tracheae, out at others, thus 
implying a circulation of the air. The 
tracheae are connected with one another 
by branches, so that such a circulation is 

The young stages of May flies, dragon 
flies, and some beetles live in the water, 
yet respiration is carried on by tracheae. 
Instead of opening at the surface of the 
body, such tracheae begin in fine closed 
branches which spread out in external 
gills (flat plates or tubes), from which 
they receive their oxj^gen by absorption. 
Such gills richly supplied with tracheae 
are known as tracheal gills. 

Breathing Movements. — Whatever 
mechanism an animal possesses for the 
absorption of oxygen, it is necessary that 
there be a continuous supply of oxygen 
to absorb. An animal that lives fully 
exposed but attached to some object in 
swiftly flowing water usually requires no 
special device to ensure that supply. But one that lives in still water 
and i-emains motionless soon absorbs all the neighboring oxygen; and 
since oxygen diffuses only very slowly through water, the supply is not 
([uickly renewed. Fishes swim about; but since the gills are under a 
protective plate (the operculum) at each side just behind the head, mere 
moving about does not suffice. Renewal of the oxygen supply next to 
the gills is effected by taking water into the mouth and then pumping it 
out through clefts among the gills. The opercula are raised to allow 

Fig. 95. — Tracheal system of an 
insect, a, antenna; b, brain; I, leg; 
n, nerve cord; p, palpus; s, spiracle; 
st, spiracular branch; t, chief 
tracheal trunk; v, ventral branch; 
vs, visceral branch. (From Folsom, 
"Entomology," after Kolbe.) 


the water to pass out but settle back immediately after, so as to prevent 
water from entering there. The action is repeated, and a pulsating 
current of water is kept up. Lobsters have a fanlike structure at one 
edge of the gill chamber, and by its movement a continuous stream of 
water is kept flowing over the gills. 

Land animals have various devices acting to the same end. Insects 
expand their chitinous exoskeleton by muscular movement, and air 
rushes in: the skeleton collapses, and the air is forced out. Valves at the 
entrances of the tracheae determine which ones shall receive air. In 
general the air chambers or passages have, of themselves, no power of 
either expansion or contraction ; they are manipulated by something else. 
The lungs in man are expanded at all times, to fill the cavity of the 
thorax, merely by the air pressure within them. If the chest expands, 
more air is forced in from the outside to equalize the pressure. In 
inspiration, the volume of the chest is increased by two means: (1) 
raising the ribs, and (2) lowering the diaphragm. The ribs are movably 
joined to the vertebral column, from which they slope downward both 
laterally and forward. The muscles between the ribs contract, so that 
all ribs are lifted, the lowest ones most of all. Since the ribs slope down- 
ward, elevating them pushes them outward (sidewise and to the front), 
thus enlarging the chest in both directions. The diaphragm, a muscular 
sheet across the bottom of the thorax, is convex like an inverted bowl. 
When its muscles contract, the diaphragm is flattened, thus further 
increasing the size of the chest cavity. Air pressure in the lungs is thus 
reduced, hence air is forced in to restore an equilibrium. In expiration, 
the rib muscles relax, and the ribs drop, largely by their own weight. 
Both width and depth of the thorax are thus decreased. When the 
muscles of the diaphragm relax, tension of the muscles of the abdominal 
wall presses the viscera up against it and the diaphragm rises. With 
the accompanying decrease in the size of the thorax, air is forced out 
of the lungs. 

All such movements designed to ensure a continuous supply of oxj^gen, 
whether in air or water, are termed breathing movements. To supply 
the right amount of air, these movements must vary in vigor as the 
animal's activities change. In man, the rate of breathing is controlled 
by a nerve center in the medulla, posterior division of the brain. The 
action of this center depends on the amount of carbon dioxide in the 
blood. If muscular activity increases, much more carbon dioxide enters 
the blood from the tissues; this extra quantity stimulates the respiratory 
center in the medulla, and breathing becomes more rapid. Panting is 
an extreme response to such stimulation. If the breath is voluntarily 
"held" for a short time, carbon dioxide accumulates in the blood to 
such an extent that restoration of breathing is forced. No will power 


can resist the urgent demand of the respiratory center that breathing 
be resumed. 

Mechanism of Oxygen Collection. — It has already been stated 
that oxygen does not spread through dry surfaces. This is because 
the movement of oxygen in entering an organism is a process of diffusion, 
which can occur freely only when the oxygen is in solution. Aquatic 
animals, except a few air-breathing types like whales and other swimming 
mammals, never meet oxygen except in solution. When air comes in 
direct contact with an animal, its oxygen cannot enter unless it is first 
dissolved. All that is necessary is to have the surfaces moist; oxygen 
dissolves in the film of moisture, then passes readily inward through 
the membranes. Lungs and tracheae have no difficulty in maintaining 
this moisture, since they possess internal cavities in which there can 
be little evaporation. Land animals with gills, however, must either 
live in places that are perpetually moist, such as swamps, or must prevent 
evaporation in some way. Land-dwelling crayfishes protect their gills 
from drying by means of chitinous flaps of the exoskeleton (page 
90) and have the habit of burrowing in the soil until moisture is 

The passage of oxygen through moist membranes depends on the same 
principle as that which causes water to flow down hill, or winds to blow 
from areas of high atmospheric pressure to those of low pressure. Oxygen 
goes from places of high oxygen pressure to those of lower pressure. This 
pressure is not entirely a matter of quantity, for a small amount of oxygen 
dissolved in a certain volume of water may exist at a greater pressure than 
does a greater amount in the same volume of air. When oxygen enters 
the gills of an aquatic salamander, it is because the oxygen in the water is 
at greater pressure than is the oxygen in the gills. In a land animal with 
liuigs, the oxygen in the air in the lungs is at higher pressure than in the 
tissue of the lungs. In the human lungs the air in the remote alveoli, 
being diluted with waste products there, exhibits an oxygen pressure 
somewhat lower than the oxygen pressure of open air; and yet it is nearly 
three times as great as the oxygen pressure in the tissues of the lungs; 
hence the transfer to the tissue. From the cells lining the alveoli of the 
lungs it is a very small step to the blood, for the capillaries are closely 
applied to the alveoli. Oxygen enters the plasma, the liquid portion of 
the blood, again in response to a pressure gradient : pressure is lower in the 
plasma. Pressure is constantl}^ kept lower in the plasma, because the red 
blood cells contain a protein which takes up (]uantities of oxygen in 
chemical combination. Moreover, the blood is circulating; blood that 
has absorbed oxygen is continually l)eing replaced by blood that has 
little of it. So a perpetual transfer oi (jxygen to the blood is set up in the 


Internal Respiration. — When the oxygen is finally presented to the 
tissues or cells in which it is to be consumed, its introduction to those cells 
is again dependent on relative pressures. Oxygen is at higher pressure 
in the plasma of the blood than in the adjoining tissue cells, which have 
used their oxygen. As the plasma gives up its oxygen to the cells its 
oxygen pressure is lowered; and in response to this reduction, oxygen is 
released from chemical combination in the red cells, and is dissolved in the 
plasma. The plasma thus maintains a higher oxygen pressure as long 
as there is oxygen in loose combination in the red cells; and before the red 
cells have lost all their loosely combined oxygen, the blood has passed 
on and been replaced by fresh blood which has not yet been called upon 
to give up its oxygen. So there is a continual diffusion of oxygen from 
the blood to the tissue cells. The transfer is very rapid, for the oxygen 
pressure in the blood is reduced by half in one second of time. The cells 
nearest the capillaries pass some of their oxygen on to cells farther away, 
again in response to differences in pressure but aided by a fluid (see next 
chapter) bathing the cells, and no cell is very far from the nearest blood 

Respiration Also an Excretory Process. — While we are not yet ready 
to discuss the general phenomenon of removal of Avastes, it should be 
pointed out in passing that certain wastes are removed in respiration. 
These wastes are carbon dioxide and a small amount of water. Carbon 
dioxide results from the very abundant oxidation going on everywhere 
in living things. It leaves the tissues where it is produced because its 
pressure is higher than in the near-by blood plasma. The resulting 
increase of pressure in the plasma causes the chief protein of the red cells 
to combine with carbon dioxide. Delivered by the blood to the lungs, 
the carbon dioxide is at greater pressure in the blood than in the air of the 
lungs; hence the plasma gives up carbon dioxide to the air on the other 
side of the two thin walls w^hich separate blood and air, and red cells 
yield more carbon dioxide to the plasma. Since the blood moves on, no 
equilibrium can be reached; always carbon dioxide passes from blood to 
air in the lungs. This elimination of carbon dioxide is regarded as part 
of respiration, even though it is also excretion. Excretion in general is 
treated in another chapter. 

Release of Energy. — Energy for all sorts of work in living things is 
obtained, as stated earlier, by combustion of foods. These substances 
are literally burned, just as coal is burned in a boiler, with the difference 
that combustion in living things is carried on at relatively low tempera- 
tures. The reason for the ability of animals to burn their fuel without 
great heat lies in their possession of enzymes. The burning is simple 
oxidation, and the enzymes serve to bring oxygen and the foods together 
in chemical reaction. One of the chief functions of respiration is to 


furnish oxygen, just as one of the principal ends of digestion is to provide 
foods, for this reciprocal reaction whose object is the release of energy. 

Carbohydrates require less oxygen from outside sources for their 
combustion, because they furnish some of their own. The carbon of the 
sugar molecules unites with the oxygen which the same molecules con- 
tain and with oxygen of respiration. Carbon dioxide, the end product 
of this combustion, contains little stored energy. Most of the energy 
residing in the sugar is thus liberated. 

Fats, which are also primarily fuels, are burned in the same way; 
but since they contain relatively little oxygen, more oxygen of respira- 
tion is required for their combustion. Again carbon dioxide is the 
energy-poor end product. As stated in the preceding chapter, fats are 
not readily burned unless carbohydrates are being oxidized at the same 
time; the reason for this connection is not known. 

Proteins, which are primarily material for construction, may also be 
burned. To some extent they are utilized as a normal source of energy, 
but in times of starvation this use is stepped up markedly. Since 
proteins are not stored to any extent in animals, combustion of them is 
at the expense of the body tissues. Animals literally burn themselves 
at such times. Part of the living organism is being destroyed to main- 
tain the rest of it. Proteins are intermediate l:>etween fats and carbohy- 
drates in the amount of outside oxygen they require for their oxidation. 

Heat. — One of the important uses to which energy is put in some 
animals is the development of heat. This heat comes mostly from 
oxidations occurring in muscle. If the amount of heat is regulated in 
some way, so that a fairly constant temperature is maintained, an 
especially advantageous situation is produced. Many physiological 
processes bear a time relation to one another, and the speed of most such 
processes is accelerated by high temperatures and retarded by low ones. 
If the speeds of various processes are not equally affected, a change of 
temperature destroys a nice adjustment among them. Hence a con- 
stant temperature is an advantage. 

Many invertebrate animals have no heat regulation; and, when their 
muscular movements are slight, as in clams and snails, their temperatures 
are almost identical with that of othcn* things around them. Such 
animals are said to be cold-blooded. Among the vertebrates, the fishes, 
amphibia, and reptiles are all regarded as cold-blooded because their 
temperatures rise and fall with changes in external temperature; but some, 
perhaps most, of them have temperatures somewhat above that external 
to them. 

The higher mammals, including man, are warm-blooded (as are also 
the birds) and have very marked regulation of temperatiu'e. The tem- 
perature of the human body in health seldom rises much above 38° or falls 


much below 37°C. Regulation works in both directions. When the 
internal temperature falls to a certain degree, shivering is caused, and 
heat is produced by the additional muscular movement. When the 
temperature rises too far, there are several ways of checking it. Rapid 
breathing serves to cool the lungs, and with them the whole body. More 
blood flows to the skin; hence there is greater loss of heat by radiation. 
And in man and horses, but not so much in many other mammals, sweat 
exudes upon the surface, where its evaporation serves to lower the tem- 
perature. In the dog there are no sweat glands except on the nose and 
on the foot pads. In this animal rapid ventilation of the lungs in pant- 
ing is the chief source of control; whatever cooling is caused by evapora- 
tion occurs in the open mouth and on the lolling tongue. 

Regulation of temperature is governed by a nerve center in the 
thalamus of the brain. When this center is warmed, the nerves going 
to the blood vessels in the skin cause the latter to enlarge, and the sweat 
glands are stimulated to excrete. On cooling the nerve center, these 
actions are reversed, and muscle tension is increased, all of which leads 
to a rise of temperature. 


Carlson, A. J., and V. Johnson. The Machinery of the Body. The University 

of Chicago Press. (Chap. VI.) 
Mitchell, P. H. Textbook of General Physiology. 3d Ed. McGraw-Hill Book 

Company, Inc. (Chap. XXI.) 


Only in small animals can oxygen be taken in, digested food distri- 
buted, and carbon dioxide and other wastes eliminated by mere diffusion. 
In large animals the distances are too great for these slow-moving pro- 
cesses. In such animals there must be a system of transportation con- 
necting all parts of the body. This communication is furnished b}^ the 
circulatory system. 

Open and Closed Circulatory Systems. — In crayfishes, insects, and 
their allies there is a heart which forces blood into a small number of 
major blood vessels. These vessels or their branches open into small or 
great spaces among the ceils and organs, so that the blood comes into 
contact with the tissues directly. Food is carried to the cells, and wastes 
are removed, by direct contact. From the intercellular spaces the blood 
is passed through the gills, and finally returns to the heart. Circulation 
in such an open system must be slow because of the resistance offered by 
the tissues. 

Any system of fluid communication must, like that of the crayfish, 
reach the cells rather directly. To retain this necessary direct contact 
and at the same time speed up the circulation, the vertebrate animals have 
evolved two separate yet cooperating systems: (1) a blood system in which 
there are smooth, closed tubular vessels in which the flow is very rapid, 
and (2) a lymph system in which movement is slow but the cells are 
reached directly. These systems are connected, and the fluid in the latter 
is derived largely from the former. 

The Blood System. — A closed blood system consists of a set of tubes 
which branch so extensively as to bring all parts of the body very near to 
the circulating liquid. The blood is propelled through these tubes by a 
contractile organ, th(i heart. In some animals the walls of the blood 
vessels are contractile, and waves of contraction pass along them in 
the direction of circulation. When these vessels arc especially large, and 
when their contraction is more mai'ked than those; of other vessels, as are 
those at the sides of the esophagus in the earthworm, they may properly 
be called hearts. In the higher animals, vessels conducting blood away 
from the heart are called arteries; those returning it to the heart are veins; 
and the fine tubes leading from the arteries to the veins are called capil- 
laries. The arteries have strong walls capable of withstanding consider- 




able pressure, and they are firm enough to stand open even when empty 
of blood. The veins are not called upon to endure such pressures as are 
the arteries; their walls are comparatively thin and collapsible. More- 
over, in the veins there are at invervals valves, consisting of membranous 
flaps directed forward (in the direction of flow), which close and stop 
the blood if it starts at any time to flow backward (Fig. 96). 
The capillaries are of various sizes, the smallest ones 
being just large enough to allow the blood cells to pass 
along single file. They have very thin walls, only one cell 
thick. Being thin, they are collapsible, and at times of rest, 
when the circulation is slow, many of them are closed. 

Blood is kept coursing through these vessels by the 
motive power of the heart. Any muscular activity is apt 
to exert pressure on near-by veins, and this in conjunction 
with the valves in the veins helps to keep the blood mov- 
ing; but the heart action is the main source of power. 

Chambers of the Heart and Course of Circulation. — 
The hearts of various vertebrates have two, three, or four 
chambers, and the course of the circulation is in part 
related to this feature of heart structure. A diagram of 
the circulator}^ system in the dogfish, an animal with a 
two-chambered heart, is shown in Fig. 97. This diagram indicates that 
the blood of animals with gills and a two-chambered heart passes from the 
ventricle of the heart through the gills and then forward to the head or 
backward through the dorsal aorta to the organs of the body, where it 
passes through capillaries and returns to the auricle of the heart by means 
of the veins. 


, H°^"° ^ I ) } I 

Fig. 96.— 
Vein slit open 
to show 
Course of 
blood is 






Fig. 97. — Simplified diagram of the circulatory system of the dogfish. 

Except for the fact that the blood in the arteries is distributed to 
different organs, from each of which it returns independently to the veins, 
the blood of a fish covers only one circuit. It passes through two sets of 
capillaries, one in the gills and another in the head or some body organ or 
tissue, and goes to the heart only once in each circuit. This course is a 
consequence of the two-chambered construction of the heart. 

In animals with lungs and a heart of more than two chambers the 
circulatory system is more complicated. The heart of amphibians and 



reptiles, except crocodilians, has three chambers in place of two as in the 
heart of fishes (Fig. 97), and the heart of mammals, birds, and croco- 
dilians has four chambers. The four-chambered heart is composed of 
two halves, right and left. Each half is made up of two chambers, a 
thin-walled auricle and a thick-walled muscular ventricle. There is no 
passage between the two halves of the heart but there is a broad passage 
guarded by valves connecting each auricle with the ventricle of the same 
side. The relations of the parts of a four-chambered heart may be 
understood from Fig. 98. 

The circulation in such an animal is a double one. Beginning at the 
left ventricle (see Fig. 99 for the human scheme), the blood is driven 
into the large artery which, with its divisions, leads to the body in 
general, including the head. In these parts the arteries divide into 

capillaries, which are collected again into 
veins. The veins gather into two large veins 
which enter the heart by the right auricle. 
The circuit just described from left ventricle 
through the body to right auricle, is called 
the systemic circulation. The blood now goes 
from the right auricle, through valves, to the 
right ventricle, thence is forced to the lungs. 
After passing through the capillaries of the 
lungs it returns by a large vein to the left 
auricle of the heart, thence to the left ventricle. 
The circuit through the lungs is called the 
'pulmonary circulation. In a complete circula- 
tion, therefore, the blood passes through the 
heart twice, once through the left side, once 
through the right. The blood has no alter- 
native in this course, except that in the 
systemic circulation it may go to any one of a number of parts of the 
head, trunk, extremities, or abdominal organs. When it has gone 
through the systemic circuit, it has no choice but to go to the lungs. 

The doubleness of this circulation is a consequence of the four-cham- 
bered heart, that is, of its complete separation into right and left halves. 
In animals with a three-chambered heart, as in a frog, this distinctness 
does not prevail, for while there are two auricles there is but a single 
ventricle. There is therefore some mixing of the l)lood in the ventricle; 
but the structure of the ventricle with its deep recesses and the operation 
of valves in the principal artery are such that the mixing of venous and 
arterial blood is partially prevented. 

In general, when the heart has four chambers, the blood passes 
through only one set of capillaries in each circuit. There is only one set 

Fig. 98. — Diagram of a 
four-chambeied heart. LA, 
left auricle; RA, right auricle; 
LV, left ventricle; RV, right 
ventricle; L\, vessel from 
lungs; L2, vessel to lungs; S\, 
vessel to system; Si, vessel 
from system. 



Fig. 99. — Diagram of human circulation: a, aorta; ca, celiac artery; ch, capillaries of 
head; ci, capillaries of intestine; clu, capillaries of lungs; civ, capillaries of liver; fv, femoral 
vein; hv, hepatic vein; ia, iliac artery; I, lacteals (intestinal lymphatics); lea, left carotid 
artery; Ijv, left internal jugular vein; Isa, left subclavian artery; Isv, left subclavian vein; 
ly, lymphatic capillaries; pa, pulmonary arteries; par, portal vein; pv, pulmonary veins; 
rca, right carotid artery; rjv, right internal jugular vein; rid, right lymphatic duct; rsa, 
right subclavian artery; s, subclavian vein; S7na, superior mesenteric artery; td, thoracic 
duct; vci, vena cava inferior; vcs, vena cava superior. 


in the pulmonary circulation, and for the bulk of the blood there is only 
one in the systemic course. There are, however, certain exceptions. 
The blood which traverses the stomach, intestines, pancreas, and spleen 
collects into a vein (Fig. 99 'por) leading to the liver; in the liver it passes 
through a second set of capillaries, then enters the large vein returning 
to the heart. A circuit beginning and ending in capillaries is known as 
a 'portal system, and that going from the abdominal viscera to the liver is 
the hepatic portal system. Fishes and amphibia have a portal system 
leading to the kidneys also, but that is lacking in man and mammals in 

It has been estimated that about IY2 pei" cent of the weight of the 
human body is blood. From the amount ejected from the heart at 
each beat, it may be calculated that the speed of the blood is such that 
an entire circulation, both systemic and pulmonary, requires on the 
average only about 23 seconds. 

Composition of the Blood. — The blood consists of a liquid known as 
the plasma and a number of kinds of cells or cell derivatives. The 


Fig. 100. — Formed elements of human blood. A, red corpuscle; B, C, two forms of white 

cell; D, platelets. 

plasma floats the cells, and in addition carries a number of kinds of 
substances in solution. Among these substances are some temporary 
ones such as the products of digestion (glucose, amino acids, neutral fats, 
glycerol, fatty acids), waste materials (urea, uric acid), the respiratory 
gases (oxygen and carbon dioxide), hormones (secretin and others), and 
various enzymes, which are introduced and removed at certain places in 
the system. Other substances are permanent. Of these, proteins make 
up about 7 per cent of the weight of the plasma; one of the proteins is 
fibrinogen which features prominently in the clotting of the blood. 
Inorganic salts are about 1 per cent of the weight of the plasma; an 
important one is a bicarbonate which carries carbon dioxide in its negative 
ions (HCOs"). Finally, there are antibodies which the tissues of the 
body have produced in reaction to and protection against foreign proteins, 
including disease-producing organisms. 

The visible objects in the blood are of three general kinds: (1) red 
cells, (2) white cells, and (3) platelets (Fig. 100). The red cells are flat 
disks, circular in form and thin in the center in man and most of the 
other mammals, but elliptical in other vertebrates. There are about 25 
trillion (25 million million) red cells in an average human being. The 


human red cell has no nucleus when in the blood, but in its develop- 
mental stages in the red marrow of the bones, by which it is produced, 
it has a nucleus. Red cells contain an important protein substance 
known as hemoglobin, which gives the cells their red color. From the 
rate at which hemoglobin is disintegrated in the liver, -it is estimated that 
at least 5 per cent of the red corpuscles are destroyed every day. In 
other words, more than 10 million of them disappear every second. Hence 
there must be a rapid replacement of them by the marrow. 

The white cells are of half a dozen kinds. Two-thirds of them belong 
to one type having an irregularly lobed or even divided nucleus (Fig. 
lOOB), the power of movement like Amoeba, and the ability to engulf 
bacteria. These cells may creep out of the capillaries, through small 
crevices between the cells of the capillary walls (Fig. 101). They emerge 
from the capillaries in great numbers at the site of an infection, to 
engulf the infecting organisms. In their battle with the bacteria many 
of the white cells are killed, and their bodies make up a large part of the 


Fig. 101. — Successive stages in the emergence of a white blood cell from a capillary. 

pus which collects in an abscess. White cells of this kind originate in bone 
marrow. The next most numerous kind, about one-fourth of the total, 
originate in lymphoid tissue (Ij^mph glands, spleen). The remaining 
types are recognized by different staining reactions as well as by their 
size and nuclear structure; some of these devour bacteria, others do not, 
but their functions are not well understood. All kinds of white cells 
together number about 30 to 40 billions in an average human being. 

The platelets are not cells, but pieces of cells. They come from 
certain large cells in the bone marrow by fragmentation. They dis- 
integrate so rapidly when the blood leaves the capillaries that it is 
difficult to count them. By special techniques it has been estimated 
that there must be from one to three trillion of them in a human being. 
Only the mammals are certainly known to have them. Their disinte- 
gration on leaving the blood vessels yields a substance which is important 
in the clotting of the blood. 

Regulation of Heart Beat. — Because the heart is histologically practi- 
cally a unit, it beats also as a unit. It is one of the best organs with 
which to demonstrate the all-or-none principle, because of this unity 



and the constant vigor of its contraction. Several other features of its 
beating are of the utmost importance. 

The heart has a long refractory period. Any muscle, after it has con- 
tracted, will refuse to respond to a subsequent stimulus until a certain 
time has elapsed. This interval of rest, known as the refractory period, 
is exceedingly short (0.005 second) in skeletal muscle, but very long in 
the heart. This prevents the heart from responding to any abnormal 
nervous condition by remaining continuously contracted. It contracts 
once, then must wait an appreciable time, during which it relaxes, 
before it can contract again. 

Contraction of the heart is initiated by a mass of rather embryonic 

tissue located in the right auricle, near 
the point where the great veins enter. 
This tissue is known as the sinus node 
(Fig. 102). When this node is stimu- 
lated, the right auricle starts to con- 
tract, and a wave of contraction spreads 
to the left' auricle. This wave is mo- 
mentarily blocked at the margins of the 
ventricles but is carried over to them by 
another node located on the partition 
between the two auricles, a bundle of 
whose tissue is distributed through the 
ventricle walls. 

The sinus node is the "pacemaker" 
of the heart. It responds to an increase 
of carbon dioxide in the blood by caus- 
ing the heart to beat faster. An in- 
crease of temperature, acting through 
the sinus node, also leads to faster 
beating. For both of these reasons, exercise accelerates the circulation 
of the blood. 

The pacemaker is in turn partly regulated by nerves. A pair of 
accelerator nerves comes to it from the spinal cord in the chest region 
and a pair of inhibitor nerves from the medulla of the brain. The 
inhibitors are working constantly, exerting a continual drag on the heart. 
Against this braking effect the accelerators act to variable degree. 
Excitement and various reflexes (page 146) stimulate heart beat through 
the nervous control of the sinus node. 

Blood Pressure. — Tli(> pressure of the blood against the walls of the 
vessels is greatest in the arteries near the heart, declines moderately 
in the more distant arterial branches, diops markedly in the minute 
arterioles and capillaries, then declines slightly in the veins (Fig. 103). 

Fig. 102. — Pacemaker of human 
heart, the sinus node (SN). AVN, 
auriculoventricular node, with its 
extension in auriculoventricular bun- 
dles (AVB). V, valves between left 
auricle and left ventricle. 



In the veins next to the heart it is on the average less than atmospheric 
pressure; that is, a "suction" is present there when the auricles relax. 

The high pressure in the arteries is necessary to drive the blood 
through the capillaries where the resistance is great. It is also needed 
to send the blood above the pumping organ, as to the head in man. 
Pressure drops in the capillaries because of the great increase in the 
aggregate cross section of these numerous vessels, but there must still 
be a small pressure beyond the capillaries to push the blood (against 
gravity in much of the system) on to the heart. 

Blood pressure is elevated if heart action is accelerated, also if resist- 
ance in the vessels is increased. This resistance depends on the diameter 







:2 90- 





_- \ / ^^ 1 ^\ i* j._ ^ 

<- y ems — • * 



• E 60- 

















q: so- 






—Arteries >■ 





Course of Circulation 

Fig. 103. — Curve showing decrease of blood pressure in course of circulation in man. 

of the vessels, which is under the control of a nerve center in the medulla. 
A sUght increase in the carbon dioxide in the blood stimulates this 
center, the vessel walls contract, and pressure is raised. However, in an 
active muscle, where the excess carbon dioxide is being produced, there 
is an opposite effect, a local dilation of the vessels, perhaps a response to 
higher acidity caused by the extra carbon dioxide or extra lactic acid. 
The net result is a shunting of the blood to the active organ where it is 

A special situation in the great artery from the left ventricle regulates 
the heart beat by stimulating it (through a nerve) when the pressure in 
the artery falls, depressing the heart when this pressure rises. Other 
stimuli are associated with these, but they all work together to check 
activity when it becomes too great, stimulate it when it lags. Highly 
adaptive controls thus depend upon automatic responses of organs to 
stimuli which the organs themselves Kelp to create. 



Coagulation. — One property possessed by blood, as a protection for 
its own operations and the life of the organism, is its power to clot. 
When blood vessels of small size are broken, the gap may be stopped by 
the coagulation of the blood, thus preventing loss of excessive amounts of 
blood. The clot consists of a tangled mass of threads of a substance 
known as fibrin, in which are trapped multitudes of red corpuscles. The 
fibrin is produced from fibrinogen, already mentioned as an important 
protein component of the plasma. Conversion of fibrinogen into fibrin 
is accomplished by the enzyme thromhase. This enzyme cannot exist in 
the blood during normal circulation, but its forerunner, called prothrom- 
base (page 112), is regularly present. The conversion of pi^othrombase 
into thrombase is induced by a substance known as thromboplastin which 
is liberated partly from the damaged tissue cells at a wound, partly 
from the blood platelets which promptly disintegrate in exposed blood. 
The chain of reactions here described in reverse quickly leads to the 
precipitation of the fibrin network. Some other things are necessary to 
that chain. Calcium ions must be present, and clotting may be pre- 
vented in shed blood by precipitating its calcium with an oxalate or 
citrate. Vitamin K (page 112) also aids coagulation. Clotting can 
be artificially checked in surgical operations by injecting something 
{heparin, for example, extracted from liver and muscle) wiiich inactivates 
thrombase. People afflicted with hemophilia have a very slow coagula- 
tion and bleed a long time from minor wounds. One feature of their 
blood is the slowness with which blood platelets disintegrate, so that 
production of thromboplastin is delayed, but there must be other factors. 

The fibrin network traps most of the blood cells, and as it contracts 
it squeezes out a clear yellowish liquid, the serum, which is nearly identi- 
cal with the plasma minus its fibrinogen. 

Lymph and the Lymphatic System. — As a means of fluid communica- 
tion between all parts of an animal, the blood system alone is not quite 
sufficient. The blood as a complete entity is confined to the blood vessels, 
and diffusion of substances held in it, even from the capillaries, is too 
slow to meet all needs. Moreover, the diffusion of water itself from the 
capillaries must be a one-way movement because of the pressure of the 
blood. Some of these inadequacies of the blood system are overcome 
by the second of the great networks of vessels (page 122), the lymph 

Because of the considerable pressures which are maintained in the 
blood, there is a tendency for any of its components to escape if the}^ can 
do so. The capillaries, with their thin walls, are the only place where this 
is possible. The liquid part, the plasma, filters out rather readily, passing 
into the spaces (Fig. 104) among the tissue cells. Some of the dissolved 
parts of the plasma (chiefly proteins) are held back by the walls of the 





Fig. 104. — Diagram showing 
lymph spaces adjoining capil- 
lary and among cells. 

' capillaries, as happens in osmosis, and some other things may be added to 
it by a sort of secretion as it passes through those walls. The white 
corpuscles may crawl between the cells and escape (Fig. 101), and now and 
then a red cell may also pass out. The fluid which escapes from the 
capillaries is thus very little different from blood minus its red corpuscles 
and minus about two-thirds of its proteins. 
It is called lymph. 

The lymph carries with it most of the 
blood substances which can be dissolved in 
water, including most of the digested foods 
and a small amount of oxygen. It bathes 
the cells, which take any of the substances 
that are required. These cells also lose to 
the lymph any of their soluble \A»stes, 
principally carbon dioxide and urea. There 
is some diffusion of the various substances 
directly through the protoplasm of the cells, 
so that lymph is not the sole means of communication between the blood 
capillaries and the surrounding tissues. 

Lymph cannot continue exuding from the capillaries unless it is some- 
how removed, and it cannot return to the blood vessels from which it 
came, because of the blood pressure. Instead, it is drained off by another 
set of vessels known as the lymphatic system. Very small 
lymph capillaries pass among the cells everywhere, and the 
lymph moves into them, mostly by diffusion, though 
minute solid particles are somehow able to get into them. 
These capillaries collect into larger vessels, which even- 
tually empty into a vein. In man there are two main 
lymphatic trunks, one which receives lymph from the entire 
lower portion of the body below the chest and from the left 
side above that level, the other from the right side of the 
chest and head and the right arm (Fig. 105). These large 
vessels empty into certain veins, one at the base of the 
neck, the other in the left shoulder (Fig. 99td, rid). The 
lymph is thus returned to the blood system from which it 
came. In the course of the lymph capillaries there are 
valves (Fig. 106) which prevent backward flow, and there 
are valves at the two points where the main lymph ducts 
enter the veins. While these valves, together with pressure exerted 
by muscles, help maintain the flow of the lymph, the main cause 
of movement is the pressure of the blood behind it, and that is 
furnished by the heart. Because the source of pressure is distant and 
the resistance is great, the flow of lymph is sluggish. It takes an hour 

Fig. 105. 
Very unequal 
portions of 
human body 
supplied by 
the two main 



or more to flow from the leg to the vein in the shoulder, a.s compared 
with less than a minute for the blood to make a complete circuit from 
heart to heart. 

Interrupting the lymph vessels are numerous enlargements made of 
connective tissue, called lymph nodes, which filter out or otherwise remove 

the solid particles in the lymph. In these nodes 
one of the kinds of white blood cells (Fig. lOOC) 
is created. In the nodes any bacteria which 
escape destruction by white corpuscles at the 
seat of infection are apt to be destroyed, and 
nodes are often swollen during an infection. 

Connected with the lymph vessels is a set of 
tul:)es Avhich originate in the walls of the small 
intestine. These are the lacteals (Fig. 990, 
which are part of the lymphatic system. They 
extend into the minute fingerlike projections 
(the villi, Fig. 91) in the walls of the intestine 
and are especially useful in absorbing digested 
fats. These lacteals collect into larger vessels 
and finally merge with the lymph vessels of the lower part of the body, 
at a point shortly below the lowest rib. Their contents are thus 
disgorged into the left one of the veins which receive lymph. 

Fig. 106. — Lymph cap- 
illary, diagram of short 
segment above, photo- 
graph of single valve be- 
low. {Photograph by cour- 
tesy of General Biological 
Supply House.) 


Carlson, A. J., and V. Johnson. The Machinery of the Body. University of 

Chicago Press. (Chaps. III-V.) 
Mitchell, P. H. General Physiology. 3d Ed. McGraw-Hill Book Company, Inc. 

(Chap. XIX, chemistry of blood and lymph; Chap. XX, the circulation.) 
Rogers, C. G. Textbook of Comparative Physiology. 2d Ed. McGraw-Hill 

Book Company, Inc. (Deals largely with invertebrate animals: Chaps. X and 

XI, the blood; Chap. XII, coagulation; Chap. XIII, circulation; Chap. XIV, 

heart action.) 


Substances which cannot be built up into protoplasm, or do not 
yield energy when decomposed, or do not act as vehicles for important 
substances, or do not stimulate cells to activity can be of little use to 
animals. Such substances must be eliminated if they are incidentally 
acquired, as are the indigestible parts of various foods, or if they are 
produced as a consequence of physiological processes. Indigestible 
portions of objects taken in as food are removed as feces by the digestive 
tract itself. Those which result from the life processes are thrown off 
by the general process of excretion. It is only the latter group, the wastes 
which originate within the organism, that are dealt with in this chapter. 

Origin of Wastes. — Since oxidation (page 37) is the main source 
of energy in living things, some of the principal wastes result from that 
process. Carbon is abundant in all protoplasm and in all the classes of 
organic foods (proteins, carbohydrates, lipids). Oxidation of these 
things results therefore in quantities of carbon dioxide (CO2). This sub- 
stance, as previously explained, is very stable and contains very little 
potential energy, besides being toxic in large quantities; hence it is waste 
matter. Water must be taken in as a vehicle for other substances, but 
in larger quantities than can be retained; the excess is waste. Destruc- 
tion of proteins, whether those of protoplasm or unutilized food, must 
yield some nitrogenous wastes, the principal one being urea. There are 
minor substances of many kinds, but these three — carbon dioxide, water, 
and urea — form the bulk of the material that has to be removed. 

Gaseous Wastes. — The removal of carbon dioxide has already been 
mentioned (page 119) as part of the process of respiration. Cells accumu- 
late quantities of this substance as a result of their own oxidations and in 
man usually contain it at a pressure equivalent to about one-fifteenth of 
an atmosphere, or more. Since this pressure is double the pressure of 
the same substance in the blood of the capillaries, carbon dioxide diffuses 
from the cells into the blood. In the lungs, the pressure of the carbon 
dioxide in the blood is distinctly greater than in the air of the lungs; 
hence diffusion is outward. Gills operate in the same way as lungs, but 
the differences in pressure are smaller; hence the rate of elimination of 
carbon dioxide is slower. 

Small quantities of other gases, especially those arising from bacterial 
action in the intestine, or from defective digestion, are also removed by 




the lungs. Considerable water (about one-tenth of the total water loss 
of the human body at rest) is also there removed in the form of vapor. 
Excretion through the lungs, therefore, involves only gaseous wastes. 

Water and Urea. — Urea is a solid substance ; hence by most organisms 
it can be excreted only in solution. Many other substances besides urea 
contain nitrogen and are produced by decomposition of proteins, but 

nearly all of them are solids that 
require to be eliminated in dissolved 
form. As just stated, only about a 
tenth of the excess water taken in by 
man can be removed as vapor, so that 
the bulk must leave as a liquid. 
These two groups of wastes may thus 
be removed by a single operation. 
The urea and the other nitrogen-con- 
taining substances are dissolved in 
water, and all are eliminated to- 
gether. The amount of these wastes 
is much greater than that of all other 
wastes combined, and their removal 

Fig. 107. — Portion of a protonephridial sjstem from the tapeworm Taenia crassicollis. 
f, flame cell; n, nucleus of excretory tubule; tu, excretory tubule. {From Hesse and Doflein 
after Bugge.) 

Fig. 108. — Flame cell of a protonephridium of a flatworm: ci, cilia within funnel- 
shaped cavity of flame cell; n, nucleus. {From Hesse and Doflein after Lang.) 

is the chief task of what is called the excretory system of the multicellular 
animals. The excretory system is often aided by the skin, and there are 
other minor ways of removing water. 

Excretory Systems of Invertebrate Animals. — The excretory system 
varies considerably in different animals. In tlui flatworms and some 
others it consists of protonephridia, which are fine tubes rising in flame 
cells and discharging to the exterior. A portion of such a system is shown 
in Fig. 107, and the structure of a flame cell in Fig. 108. The flame cell is 



around tubules - 



somewhat stellate or irregular in shape, hollowed out to form a funnel- 
shaped cavity within itself. A number of long, slender cilia (the ' ' flame ' ' ) 
take their origin from the body of the cell and hang freely into the funnel- 
shaped cavity. In life, the cilia beat continuously and by their beating 
cause currents in the liquid wliich is excreted into the funnel by the cell. 

Nephridia. — In the annelid worms each segment or somite (with some 
exceptions) is provided with a pair of more or less coiled tubes, the 
nephridia, which have a ciliated opening, the funnel or nephrostome, 
which projects through the septum into the cavity of the somite ahead. 
There it opens directly into the body cavity or coelom. The other end of 
the coiled tube is connected to the body 
wall where it has an opening to the exterior, 
through the nephridiopore (Fig. 109). 
Through much of its course this tube is 
surrounded by a network of capillaries, a 
feature of the excretory organ of all the 
higher animals. In its operation, the 
nephridium takes in fluid from the coelom 
through the nephrostome. This fluid con- 
tains wastes exuded into it by the various 
tissues, but it also contains some usable 
substances, one of them being glucose. As 
the fluid passes along the tube, the glucose 
and other useful substances are absorbed by the tubule walls and are 
carried away in the capillaries to be used elsewhere. Excess water is 
also thus reabsorbed into the blood, and the fluid finally ejected at the 
nephridiopore is highly concentrated. 

Kidneys. — In embryos of the higher animals the excretory system 
starts in a form which is comparable to a row of nephridia in the earth- 
worm. It consists of a series of uriniferous tubules, a pair in each segment, 
the inner ends of which open into the coelom. The outer ends, instead 
of opening to the outside independently, all empty into a pair of tubes, 
one on each side, and these open to the exterior. In the course of develop- 
ment the coelomic openings, with a small portion of the tube, are closed 
off. Minute networks of blood capillaries are pushed into the sides of the 
tubules near the coelomic ends, and in the adult organ the tubule ends 
at that point. The tubule wall has grown almost completely around the 
invading group of capillaries, to form a double-walled cup through the 
open interior of which a blood vessel passes. This cup and the blood 
vessels in it are together known as the renal corpuscle (Fig. 110). The 
^\alls of the cup are Bowmaii's capsule, and the contained blood vessels 
are the glomerulus. The renal corpuscles with the uriniferous tubules 
are the essential excretory units in the vertebrate animals generally. 


Fig. 109. — Nephridium of 
earthworm. (From Storer, "Gen- 
eral Zoology.") 



In the lower vertebrates (up to the amphibians) much of this embryonic 
state is retained in the adult, particularly the repetition of the tubules 
in a serial arrangement. In the higher vertebrates the segmental 

arrangement is completely lost in the gross 
form of the system. Yet in all of them the 
uriniferous tul)ule with its renal corpuscle is 
the functional unit. 

The adult kidney in the frog, in cross sec- 
tion, is arranged as in Fig. 111. The renal 
corpuscles are located toward the ventral side. 
The uriniferous tubules from them pass up- 
ward, downward, and upward again, with 
many convolutions, and empty into collecting 
tubules, a number of which traverse the 
kidney near the dorsal surface. The collect- 
ing tubules begin in Bidder's canal, which 
extends along the median (inner) edge of the 
kidney, and end in the ureter, which extends 
along the lateral edge of the kidney, and then 
on to the cloaca and bladder. At the ventral 
side are nephrostomes, remnants of the 
embryonic openings into the coelom, but end- 
ing blindly in the adult. An important addi- 
tional feature of the kidney is the abundant 
supply of blood vessels ; the tubules are every- 
where in close contact with capillaries. 
The corresponding system in man is shown in Fig. 112. The kidney 
is bean-shaped, with the ureter emerging from the "eye" of the bean 


Fig. 110. — Structures 
from vertebrate kidney, dia- 
grammatic. A, renal cor- 
puscle; B and C, cross-sec- 
tions of uriniferous tubules at 
different levels; av, afferent 
vessel; be, Bowman's capsule; 
cap, capillary; cil, cilia (found 
in amphibia, not man) ; ev, 
efferent vessel; gl, glomer- 
ulus; ut, neck of uriniferous 

Fig. 111. — Diagrammatic representation of a cross section of the kidne.\- of a frog. 
B, Bidder's canal; C, collecting tubule; D, dorsal, L, lateral margin of kidney; AI, renal 
corpuscle; A'^, neijhrostoinc; 7', uriniferous tubule; U, ureter; V, renal portal vein. (Modi- 
fied f 10771 Holmes, " Biol 00 U of the Froy.") 

and discharging below into the bladder. A copious blood supply is 
furnished by l)ranches of the main artery and veiij. Inside the kidney 
are typical uriniferous tubules. Their renal corpuscles are massed toward 



the convex outer surface of the organ (Fig. 113). From there the course 
of the tuliules is in general two convohited stretches, with a more or less 
straight-limbed loop between them. The collecting tubules into which 
they empty converge toward the branches of the ureter, in pyramid- 
shaped groups. The ureters empty into the bladder, and this discharges 
through the urethra. 

Excretion by the Kidney. — The elimination of w^aste by the kidney 
involves two general processes: (1) filtration of a great deal of liquid under 
pressure from the blood in the glomerulus into the tubule at the renal 
corpuscle, and (2) resorption of the greater part of this liquid by the 
uriniferous tubules in the rest of their course. The liquid forced out 
of the glomerulus, through the inner wall of Bowman's capsule into the 


'A^\ illl'i, -^^cc^^^y^ coniaining 


Fig. 112. — Excretory system in man. 

renal corpuscles 

pyramid of medulla 
with collecting 

renal artery 
renal vein 
pelvis of kidney 


Fig. 113. — Human kidney, bisected. 
{From Storer, "Ge7ieral Zoology.") 

tubule, consists of water, urea, glucose, amino acids, and the salts of 
the blood plasma, in about the same proportion as these things exist in the 
blood. The proteins of the blood, however, are not allowed to pass ; nor 
are the other colloidal substances, such as the lipids, nor the blood cells. 
These are all retained in the blood vessels. The amount of fluid thus 
filtering into the tubule is about 1 per cent of the liquid of the blood 
passing through the glomerulus. 

Then the resorption of much of this material occurs as the liquid 
passes along the tubule. The glucose in it is taken back into the blood 
capillaries, unless there is already too much glucose in the blood. The 
salts are also partially resorbed, not necessarily in equal fractions, but in 
proportion to the need of them in the blood. Amino acids return in like 
manner to the blood; so also does about 99 per cent of the water. What 
remains in the tubule is therefore a rather concentrated solution of the 
waste substances, mostly urea and uric acid. This liquid is the urine. 
About 1500 cc. of it leaves the kidneys daily in an average adult person 



under average conditions. Urine consists of about 96 per cent water, 2 
per cent urea, 0.5 per cent uric acid, and 1.5 per cent inorganic salts. 

A small amount of waste material may be added to the forming urine 
in the tubules, by excretory action of the cells of the tubules; but this 
addition is unimportant in relation to the amount filtering in at the 
renal corpuscle. 

The Skin as Excretory Organ. — Excretion in the skin is done by the 
sweat glands, of which there are about two millions in man. These 
glands are of the simple tubular type (page 84), the deeper portion of 
the tube being closely coiled, while the outer part forms a duct which 
empties on the surface. Around the coiled bottom is a network of 

capillaries (Fig. 114). The amount of sweat excreted 
varies greatly with the temperature and the amount 
of muscular exertion; in mild weather and with 
moderate or slight exercise, about 600 cc. may be 
produced in a day, but five times that amount is not 
uncommon in hot weather and with great exertion. 

Sweat is much more dilute than urine, about 99 
per cent of it being water. Of its solids, sodium 
chloride is the most important. Urea is not very 
abundant; at the minimum production of sweat (600 
cc. per day) only about 1.5 per cent of the total 
urea is lost through the skin in man. Other soluble 
wastes, of the same kinds as are eliminated by the 
kidneys, are found in the sweat, but in much smaller 
amounts. Since the sweat evaporates as rapidly as 
it is formed under ordinary conditions, these solids 
dry on the surface of the skin. As is pointed out on 
page 121, in connection with heat regulation, many 
mammals have only a few sweat glands, or none at 
all. In them the kidneys bear the whole burden of 
The sweat glands even in man are not an important 
Their chief service is regulation of temperature. 

The liver shares in the excretion of urea. 

Fig. 114.— Hu- 
man skin, dissected 
to show sweat gland. 
At left, complete 
gland, much coiled 
at bottom. At 
lower right, network 
of capillaries from 
the midst of which 
the coiled portion of 
another gland has 
been removed. 

urea elimination, 
excretory device. 

Other Means of Excretion, 
since it helps convert protein wastes into urea. When proteins are 
broken down, ammonium salts are among the products. These salts 
are converted into urea partly in the liver, but the actual excretion is 
elsewhere. The liver performs, however., a primary act of excretion in 
the removal of the hemoglobin of worn-out red l)lo<)d corpuscles. The 
bile pigments are produced from this hemoglobin and are eliminated with 
the bile into the intestine, where they eventually pass out with the feces. 
Cholesterol is another waste substance excreted by the liver and elimi- 
nated into the intestine with the bile. ' 


Other glands producing liquid secretions have some chance of casting 
out soluble wastes. Thus in the saliva there are traces of urea; but since 
most of the saliva is retained within the body, the occurrence of urea in it 
hardly amounts to excretion. Drugs injected into the veins can often 
be tasted owing to a similar exci'etion of them in the salivary glands. The 
wall of the large intestine is able to excrete small amounts of unusual 
foreign substances occurring in the blood or of ordinary substances when 
present in excessive amounts, as calcium and magnesium sometimes are. 
These substances are removed from the intestine with the feces. 

None of these other excretory organs is important as a substitute for 
'the kidneys; not even all of them combined could take over the job 
of the kidneys. Fortunately the kidneys have a wide margin of safety, 
for a kidney and a half may be removed and the necessary excretion still 
go on. There is no recovery, however, from overdestruction of kidney 
tissue, for the renal tubules do not regenerate. 

Some organisms, principally plants, excrete wastes by simply render- 
ing them insoluble and then retaining them within or between the cells. 
Insoluble substances can do no harm and, when they are not abundant, 
are not greatly in the way. Among animals, sea urchins are said to 
store insoluble excretions. 


Baitsell, G. a. Human Biology. McGraw-Hill Book Company, Inc. (Chap. VI.) 
Carlson, A. J., and V. Johnson. The Machinery of the Body. University of 
Chicago Press. 


When many different operations are performed by the same machine, 
it is essential that they bear some definite relation to one another. Living 
organisms are subject to the same necessity. Their processes must dove- 
tail into one another. When unusual exertion increases consumption of 
energy and output of carbon dioxide, it would be disastrous were the 
circulation not speeded up to provide oxygen and remove wastes. When 
the circulation is accelerated, it would be inefficient not to hasten the 
breathing movements to introduce more oxygen. In the digestive system 
it would be wasteful to have saliva, bile, and other digestive fluids 
secreted all the time, yet they must be produced when foods require 
digestion. If in warm-blooded animals the temperature increases above 
the most favorable point,* it is important that the sweat glands of the skin 
or the breathing movement act to stop the rise. Even so simple an act as 
walking involves so many muscles that cooperation among the several 
units is necessary. The various organs cannot simply be wound up and, 
clocklike, run at the same speed, thereby ensuring proper timing, for 
many activities are carried on in response to external conditions and these 
change at irregular intervals. 

Some means of coordination is necessary. Animals in general have 
contrived two devices — one nervous, the other chemical — to serve this 
end. The former has assumed the larger burden, but both are essential. 
While it has been necessary, in describing the action of the heart, the 
respiratory movements, and the production of digestive fluids, to refer 
to the controls which keep these processes in tune with the rest of the 
organism and with the environment, it is desirable now to examine the 
mechanisms of control more specifically. 

Rise of the Nervous System. — The advantage or necessity of a 
nervous system is attested by its very general presence in widely different 
animals. Only a few groups are without it. It is made up of specialized 
types of cells, whose arrangement in the body exhibits an increasing 
complexity as other anatomical features become more complicated. 
Animals which have simple systems of other kinds have, in general, 
simple nervous systems. 

The simplest form of nervous system is that of Hydra. The cells 
which are specialized for conduction in this animal ha\e long, slender 




projections, usually branching (Fig. 115) and joining one another to 
form a network. The spread of these cells through the ectoderm is 
fairly uniform, though they are slightly more abundant at the foot and 
among the bases of the tentacles and around the mouth. Hydra's close 
relatives, the jelly fishes, have a ring of nerve cells around the edge of 
their cuplike bodies, with a loose network over the remainder. 

Animals successively higher than the jellyfishes show a progressive 
tendency to collect their nerve cells into masses or strands. In the 
flatworms there is a mass of them, which may be called a ganglion, in 
the anterior region (Fig. 116), and from this mass two long strands or 
cords pass back on either side of the body. From both the ganglion 

Fig. 115. — Nervous mechanism of Hydra. The long fibrils in the background are the 
contractile parts of neuromuscular cells lying in the mesogloea. {From Schneider.) 

and the cords slender threads called nerves extend to all parts of the 

Invertebrate animals above the flatworms generally have two longi- 
tudinal nerve cords, but these are usually joined into a single cord in 
which the two components are still easily recognizable. In the earth- 
worm (Fig. 116) these cords separate in the anterior region, pass upward 
around the digestive tract in the form of a collar, and become enlarged 
above the tract to form the bilobed brain. The rest of the double cord 
in the earthworm is swollen into a moderate ganglion in each segment, 
and from this ganglion two pairs of nerves emerge. The ganglia of the 
main nerve cords are much larger in the crayfish (Fig. 116) and its allies, 
with the larger ganglia located toward the front. 

The tendency to mass the nerve tissue in a head region is carried 
much farther in vertebrate animals. In them there is always a dis- 
tinctly enlarged brain. In the frog it is moderately larger than the cord 
behind it, which in the vertebrates is known as the spinal cord. The 



relative size of the brain increases up through the vertebrate group, 
reaching its maximum in man, whose brain includes more nerve tissue 
than all the rest of his nervous system together. 

There is thus a tendency, in the animal scale, for complexity in 
general to be accompanied by a massing or centralization of the nerve 
tissue, and to emphasize this massing in the head region. The sug- 
gestion is near that somehow a concentrated system is better fitted to 
serve as a mechanism of control of a complex body than is a diffuse 
system. Additional reasons for reaching this conclusion will appear as 
the arrangement of cells in the larger masses of the system are examined. 





Fig. 116. — Diagrams of nervous systems illustrating centralization and massing in the 

head region. 

The large masses of the nervous system, particularly the brain and 
spinal cord, constitute the central nervous system. The position of the 
central system in the body, and its structure, constitute fundamental 
differences between vertebrate and invertebrate animals. In the inverte- 
brates the nerve cord is below the digestive tract, in the vertebrates 
above it. The cord is a double one (or there are two separate cords) in 
the invertebrates, single in the vertebrates. Finally, the cords are 
solid in invertebrates, hollow in vertebrates (resulting fi-om the system's 
embryonic origin as a groove in the ectoderm which is pinched off below 
as a tul)e). 

Peripheral Nervous System. — The nerves which pass out fiom the 
central system and branch to all parts of the organism are collectively 
called the peripheral nervous system. Of the principal nerves, a number 
(10 in amphibia, 12 in the higher animals) arise from the brain within 
the cranium; these are called cranial nerves. From the spinal cord there 



emerge, between the vertebrae, pairs of spinal nerves (31 of these in man). 
Each of the spinal nerves arises from the cord by two roots, a dorsal 
and a ventral, which join in a single nerve trunk a short distance from 
the cord (Fig. 121). The dorsal root includes a ganglion which contains 

a host of nerve-cell bodies. The 
relation of the peripheral to the 
central system in the frog is illus- 
trated in Fig. 117. 

A special part of the peripheral 
system is known as the autonomic 


T^\^J XandX 


Fig. 117 Fig. 118 

Fig. 117. — Nervous system of frog, ventral view. I-X, cranial nerves; a, autonomic 
system; cer, cerebrum; n, nasal sac; op, optic lobe; spc, spinal cord. (After Wiedersheim.) 

Fig. 118. — Diagram of a typical neuron, ax, axon; d, dendrite; ms, medullary sheath; 
mu, muscle; n, node; ne, nerve endings; nu, nucleus of cell of neurilemma. 

nervous system because of its control, entirely free of the will, of many 
vital functions. It consists visibly of a pair of ganglionated cords on 
either side of the vertebral column, lying exposed in the body cavity. 
The ganglia are connected with the spinal cord by nerve fibers passing 
through the ventral roots of spinal nerves. In the extreme anterior and 
posterior parts of the autonomic system, however, there are nerve fibers 
which pass directly from the central nervous system to the organs con- 
trolled without connections in centrally placed ganglia. 

Unit of Structure of Nervous System. — The unit of structure of the 
nervous system is the neuron. The neuron is a cell possessing a number 



of fine projections which sometimes extend to great lengths. The cell is 
compact in the embryo like most other cells, and the processes can be 
seen to grow out from it, passing among other cells and dodging obstacles, 
until they reach the organ to whose action they are to be related. These 
projections are of two kinds, distinguished from one another not by struc- 
ture but by their normal functioning. Those which normally conduct 
impulses toward the body of the neuron are called dendrites; those which 
convey impulses from the body of the neuron are axons. Figure 118 
diagrammatically represents the parts of a typical neuron, and three 
very different kinds of neurons are sho^vn in Fig. 119. 

These cells, which are strictly speaking the only constituents of the 
nervous system, are bound together by connective tissue, and the masses 
thus formed are supplied with blood vessels. 

Fig. 119. — Three kinds of nerve cells. A, from ventral horn of spinal cord of an ox; 
B, from cortex of cerebrum of a cat; C, Purkinje cell from cerebellum of a cat; d, dendrite; 
neu, axon; nu, nucleus; ntic, nucleolus. {B and C from Golgi preparations.) 

Functional Unit. — In the operations of a nervous system, the func- 
tional unit is a group of neurons called a reflex arc. These neurons are 
so related to one another that, following a stimulus or excitation, they 
induce some sort of action. One end of the arc is in some tissue or 
organ capable of receiving a stimulus, the middle of it is in the central 
nervous system or an associated ganglion, and the other end of the arc 
is in a tissue or organ capable of responding, such as a muscle or gland. 
The arc consists of at least two neurons-, usually more. Leading from 
the sense organ is a nerve fiber (neuron) which, on lacing stimulated, 
conducts an impulse toward the central nervous system. This neuron 
is called an afferent fiber, the name meaning literally "bearing toward" — 
that is, toward the central system. It is also appropriately called a 
receptor neuron; very commonly, also, it is called a sensory neuron, 
though the result of the impulse it carries is not always sensation. The 
opposite end of the reflex arc consists of a neuron whose tip is applied 



to a muscle, or gland, or some organ capable of responding to a stimulus. 
This neuron carries the impulse away from the central nervous system, 
hence is designated an efferent fiber. It is also called an effector neuron, 
often a motor neuron though the action produced may be something else 
than movement. 

If the reflex arc consists only of an afferent and an efferent fiber, these 
two neurons are in contact with one another by a minute surface known 
as a synapse. The axon of the afferent touches a dendrite of the efferent, 
and the surface of contact is the synapse. An arc of this simple two- 
neuron type is represented above, at the right, in Fig. 120. The afferent 



X ; 


■''0^' AFFERENT 






> • 

I < 

Fig. 120. — Diagram of simple reflex arcs in the vertebrate nervous system. 

neuron enters the spinal cord through the dorsal root of a spinal nerve, 
in Avhose ganglion the body of the neuron lies. Within the spinal cord 
the axon synapses with the dendrite of another cell whose body lies 
within the cord. The axon of the latter cell passes out through the 
ventral root of the spinal nerve, and its tip is applied to the responding 
organ (muscle in the diagram). 

Most reflex arcs consist of more than two neurons. The extra ones 
are interpolated between the receptor and effector neurons. These con- 
necting neurons are kno\vn as intermediate or association neurons. The 
spinal cord is the seat of vast numbers of them. The association fibers 
are especially useful in carrying the arc over considerable stretches of 
the central system. In the lower right half of Fig. 120 is a reflex arc 



whose afferent fiber enters the cord by one spinal nerve, while the efferent 
fiber leaves it by way of the nerve next below. The lower level is 
reached by an association neuron between the receptor and effector. 
This same receptor is represented as connected also with an effector 
neuron on the opposite side of the spinal cord. A second association 
fiber establishes this connection. Some association neurons take the arc 
through the brain, across a number of cells, and back down the spinal 
cord again. Many arcs much more complicated than these exist. In 
all cases the first neuron in the chain is an afferent, the last one an 
efferent. All the contacts between any of the neurons are synapses, 
axon touching dendrite. 

The response to a stimulus carried over a reflex ai'c is called a reflex 
action. Many of these actions are inherited. The vital organs in the 
chest and abdomen are controlled by innate reflexes, as are also the con- 

FiG. 121. — Chain of efferent neurons in human- autonomic system, in chest region, n, the 
neurons; g, gangUon. Dotted lines represent neurons of ordinary spinal reflex arc. 

traction and dilation of blood vessels and the action of sweat glands. 
Other reflexes are learned — "conditioned" is the usual descriptive term 
applied to them. Habitual movements of all sorts are conditioned 

Functions of Autonomic System. — The reflexes for which the auto- 
nomic nervous system is responsible are of such vital importance and 
are related to one another in so remarkable a manner as to call for 
separate description. Attention will be directed only to the efferent 
fibers of the reflex arcs, because it is their control of the vital organs 
with which we will be concerned. The system in man is the one used 
for illustration. 

The neurons of the autonomic system lack a myelin sheath. Between 
the central system and the organ innervated there are always at least 
two, and often only two, neurons. In the chest region the bod}^ of the 
first neuron of such a chain is in the lateral part of the Il-shaped gray 
matter of the cord (Fig. 121), and its axon passes out through the ventral 
root of one of the spinal nerves. It leaves that root, however, close to 
the cord and enters a special ganglion. Here the first neuron terminates, 
its axon synapsing with the dendrite of the second neuron of the chain. 



This second neuron may then join the mixed spinal nerve at the same 
level of the cord, or pass up or down to nerves at other levels, in which 
it goes out to the organ which it controls. 

The autonomic system is divided functionally into two major regions. 
One centers in the middle portion of the spinal cord (chest and small 

Fig. 122. — Autonomic nervous system of man, in part, showing double innervation of 
each organ and the action of each nerve. Organs on the left, iris of eye, rectum, and 
bladder; on the right, heart, stomach, and small intestine. Small circles are ganglia. 

of back) and may be called the thoracolumbar system. The other has its 
center partly in the brain, partly in the lower end of the spinal cord, and is 
called the craniosacral system (Fig. 122). The chain of neurons described 
above belongs to the thoracolumbar. The ganglia of the craniosacral 
system lie in general much farther from the spinal cord, sometimes 
actually in the organ that is controlled. 


Each organ governed by the autonomic system is innervated twice, 
one nerve coming to it from the thoracolumbar system, one from the 
craniosacral. One of these nerves is an activator, the other a depressor. 
Each organ is thus accelerated by one of the major divisions of the 
autonomic system, inhibited by the other; but neither division is exclu- 
sively excitatory or wholly inhibitory, each division exciting some 
organs, depressing others. The thoracolumbar system accelerates the 
heart but inhibits movement of stomach and intestine. The iris of the 
eye is constricted by the craniosacral, dilated by the thoracolumbar. 

The excitation or inhibition is apparently accomplished by producing 
a chemical substance, and the organ responds to this substance. Accord- 
ing to current theory, all the nerves belonging to the craniosacral system 
produce the same substance, which is probably acetylcholine. In like 
manner, the thoracolumbar nerves produce one substance which has 
been called sympathin. Acetylcholine inhibits the heart, increases 
stomach movement and secretion, contracts the rectum and urinary 
bladder, dilates the vessels of the salivary glands, and constricts the iris 
of the eye. Sympathin produces the opposite reaction in each of these 

Nerve Impulse. — The impulse which is carried along a neuron like 
that in Fig. 118 travels at a speed of about 120 meters per second in mam- 
mals, about one-fourth of that velocity in a frog. The rate is in some way 
related to the presence or absence of a sheath around the branches of the 
cell, and to the structure of that sheath if one is present. The axon 
of the cell in Fig. 118 is surrounded by a white layer of noncellular fatty 
substance known as the myelin (medullary) sheath, which is divided into 
segments by irregularly placed nodes. Not all neurons possess such a 
sheath. Those of the autonomic system do not, and in them the impulse 
travels much more slowly — only 10 or 12 meters per second. Among 
myelinated nerve fibers, those with the longer segments of myelin between 
nodes conduct, in general, more rapidly than those with short segments of 
the sheath. There is some reason from experiment to believe that the 
impulse jumps from node to node; the longer the segments between nodes, 
therefore, the faster the impulse travels. 

According to present view, the nerve impulse is a surface phenomenon. 
The membrane of a nerve fiber — not the cellular covering or neurilemma 
and not the myelin sheath, but the outer film of the nerve cell itself — is 
charged positively on the outside, negatively on the inside. The charges 
are really borne by ions, which are located on opposite sides of the some- 
what impermeable membrane. This membrane keeps them apart and so 
prevents them from neutralizing one another (Fig. 123). The impermea- 
bility prevents neutralizing, and the separation of the ions in turn is 
supposed to hel]) keep up the impermeability. If, now, something (a 



stimulus of some sort) destroys the impermeability of the membrane at 
one point the polarization there is lost; the ions get together and neutralize 
one another. Such neutralization could then proceed to adjoining parts 
of the nerve fiber as rapidly as the impermeability is lost. No material 
thing moves along the nerve, but a wave of neutralization and permea- 
bility proceeds at considerable speed. 

Waves of some sort pass over other organs, as over the heart when it 
contracts, over skeletal muscle, and over glands. It seems likely that 

+ + 

+ + + + 'f ++ + + + + + 

FiG. 123. — Propagation of nerve impulse, a wave of permeability associated with neutraliza- 
tion of positive and negative ions. Dotted lines, permeable membrane. 

in all these structures essentially the same changes in polarization of 
surface membranes are taking place. 

Initiation of and Response to Nerve Impulses. — Though the impulses 
carried by all nerves are the same, no matter where they begin or end, 
the things that start them and the actions they induce are quite different. 
The impulse is initiated by a receptor of some kind, that is, a specialized 
nerve ending which is exceptionally sensitive to some one sort of stimulus. 
In the retina of the eye the receptors (rods and cones) are sensitive to 

Fig. 124. — Various receptors: left to right, rod and cone of retina of eye, taste bud of tongue, 
olfactory cells of nasal lining, and cold, touch, and pain endings in skin. 

light, the taste buds of the tongue and the olfactory cells in the lining of 
the nose (Fig. 124) are sensitive to chemical substances. Certain nerve 
endings in the skin are sensitive to cold, others to touch, still others lead 
to pain; the several kinds are structurally different from one another. 
These receptors are not interchangeable, each does its own work, no other. 
If a cold spot on the hand is stimulated in some other way than by low 
temperature — mechanically, for example — the sensation is still that of 


The response which a nerve impulse eUcits depends on the nature of 
the structure to which it is dehvered. An impulse delivered to a motor 
unit in a muscle causes contraction; an exactly identical impulse carried to 
a gland causes secretion. It is probable that in each instance a chemical 
substance is produced at the nerve ending, and that it is this substance 
rather than the nerve impulse itself which really stimulates the responding 
organ. At least that is true of responses of some of the internal or 
visceral organs. 

An impulse from one of the sensory endings in the skin leads to a cer- 
tain center in the brain, and the appropriate sensation is there produced. 
The nerve fibers from the retina go to one region of the brain, neurons 
from the olfactory area in the nose go to another, fibers from the pain 
endings in the skin lead to a third. These regions of the brain are indi- 
cated more fully later; the important point nov: is that for each activity 
there is a special kind of receptor, located at a particular place or places, 
and a certain organ or region of the nervous system where the appropriate 
response is given. The nerve impulse which goes from the place of 
stimulation to the place of response is everywhere the same. 

Direction of Impulse. — When a neuron is stimulated at its receptor 
ending, the impulse thus started travels toward the other end; there is no 
place else to go. Experimentally, however, and sometimes in special 
situations naturally, a neuron may be stimulated in the middle of the 
length of its axon or dendrite. When this happens, impulses travel in 
both directions to the limits of the neuron itself; but in one of the direc- 
tions it goes no farther than the end of that particular neuron. The 
difference lies in the synapses at the ends . of the axon and dendrite. 
Each synapse is a one-way conductor. An impulse can go over it 
from axon to dendrite but never from dendrite to axon. This is the 
reason why nerve impulses alwa3\s go in one direction over such a chain 
of neurons. As stated above, when a neuron is stimulated somewhere in 
its middle, the impulse moves in both directions from that point to both 
ends of that neuron. In the "forward" direction, arriving at the termi- 
nus of the axon, it goes over to the dendrite of the next neuron and 
continues the propagation, Ijocause the synapse there permits passage in 
that direction. But in the "backward" direction the impulse is blocked 
when it reaches the tip of the dendrite because the synapse will not carry 
it over to the adjoining axon. 

What gives the synapse this power of distinguishing direction? While 
the answer to this question is not certainly kno\\'n, a possibility is sug- 
gested l)y what is kno^vn of responses to stimuli in general. We are 
familiar with the control of su(!h organs as the heart by a double innerva- 
tion, one nerve acting to stimulate, the other nerve to inhibit. Each 
nei've pi-esumably produces a chemical substance to which the oigan 



directly responds. It is not unlikely that an impulse arriving at a 
synapse from an axon produces an activating substance so that the wave 
is initiated anew in the adjoining dendrite, while an impulse going back- 
ward over a dendrite to a synapse produces an inhibiting substance so 
that further propagation is prevented. 

Fig. 125. — Functional areas of human cerebrum. Above, lateral surface from left 
side. Below, median surface viewed from left. The olfactory area, because it is dis- 
continuous, is dotted. All boundaries are only approximate. 

Localization in Brain. — It is more difficult to ascertain the function of 
different parts of the brain than to determine the role of nerves, because 
those parts cannot be isolated and experimented upon wholly separately. 
Knowledge of the regions where different brain functions are performed 
comes from destruction of certain areas in laboratory animals, artificial 
stimulation of brain areas in anesthetized animals, the consequences of 


lesions due to accident or disease in man, and, recently, the study of 
"action potentials," which mark the path of nerve impulses from the 
point of stimulation to their center. The latter method is particularly 
useful in locating functional areas in the cerebrum. To understand 
what follows, it is necessary to know the general structure of the brain. 

As it originates in the embryo, the central nervous system is a tube, 
wider in front where the brain develops (page 208), narrower behind 
in the spinal cord. The brain tube enlarges moderately in three regions 
known as the fore-, mid-, and hindbrain. This tubular structure remains 
in the adult as the "brain stem," but the forebrain expands enormously 
upward, laterally, and backward, to form the cerebrum (divided into two 
hemispheres), while the hindbrain develops the cerebellum. Behind the 
latter is the medulla oblongata, which is usually counted a part of the 
brain but is really the somewhat enlarged anterior end of the spinal cord. 

The cerebrum has a gray surface layer, the cortex — gray because 
of the cell bodies which it contains — which in man and the mammals 
generally is greatly increased in extent by folds and furrows. It is 
the cortex which has been the subject of much of the localization study, 
because it is the seat of those psychic qualities which tend to distinguish 
man from the beasts. By the methods outlined above, the functions of 
various parts of the cerebral cortex have been found to be roughly as 
portrayed in Fig. 125. The best established of the areas there shown 
are the motor area and the area of skin sensation which together form a 
transverse band halfway between the front and rear, the areas for hearing 
at the sides, and that for vision at the extreme posterior part. The rest 
of the cerebrum is largely given over to what may be termed associations, 
some of the particular forms of which are indicated in the illustration. 
The association areas deal with integration of individual sensations into 
a whole. The cortex is not responsible for pain except to localize it, 
and it is not concerned with any viscer^il sensations such as hunger and 
thirst. Pain is a function of the thalamus, in the stem region of the 

The cerebellum serves to coordinate muscular actions. Destruction 
of it results in irregular, jerky, fumbling, or reeling movement, or in 
thick slurred speech. The middle portion influences muscles of the 
trunk, neck, and head; each side of the cerebellum acts on muscles of the 
same side of the body, but there is not much other known localization. 

The more important functions of the medulla in controlling the heart 
and digestive canal, the contraction and dilation of blood vessels, and the 
movements in breathing have already been described in this and earlier 

Chemical Regulation. — The control of vital actions by the medulla 
is exercised partly at the behest of accumulated carbon dioxide. It has 






been necessary in earlier chapters to point out some of the initiatory- 
actions of this substance which may be here recalled. Increased con- 
centration of carbon dioxide in the blood causes centers in the medulla 
to increase breathing movements and to contract the blood vessels. 
Here the effect is produced through the nervous system. Sometimes 
carbon dioxide may act directly, without mediation of nerves, as when 
it stimulates stronger heartbeat by direct action on the sinus node, 
and almost directly when, perhaps by increasing acidity, it locally 
causes dilation of blood vessels. There is thus an important chemical 
regulation of muscle action, partly through, partly independent of, the 
nervous system. Coagulation of the blood is also initiated by chemical 
substances liberated from disintegrating platelets and injured tissue cells, 
in conjunction with certain substances in the blood plasma. There are 
some physical agents, also, which exercise regulatory control either 
directly or through the nervous system. 
Thus slightly higher temperature of the 
blood, warming the thalamus of the fore- 
brain, starts activity of the sweat glands, 
which lowers the temperature; and 
higher blood pressure in the great 
arteries, acting through nerves, slows 
down the heartbeat. And, finally, 
greater warmth of the blood, influencing 
the sinus node directly, not through 
nerves, accelerates heart action. All 
these influences have been discussed 

Besides these chemical and physical 
agents, which are all part and parcel of 
the general physiological mechanism of 
the higher animals and which mostly 
serve other ends besides regulation, 
there is a group of chemical substances 
which have no other known function 
than to exercise control over something. 
These substances are know^n as hor- 
mones. In general they are produced at 
one place, but stimulate action at another, to which they have been 
carried by the blood. One of the earliest of these substances to be 
discovered was secretin, whose action in stimulating the pancreas and 
liver has been described (page 104). 

While it is possible that most tissues produce substances that have 
some influence elsewhere, the marked and well-known instances of 



(in female) 

(in mole) 

Fig. 126. — Location of endocrine 
glands in human body. Dotted lines 
represent kidneys (above) and ovi- 
ducts and uterus (below) to show 
positions of glands. 


hormone action are those exhibited by certain definite glands. These 
gland.s do not have ducts, or, if they do, the hormone is not ejected 
through the duct. All hormones diffuse directly into the blood. Such 
ductless glands are known as endocrine glands, and the hormones are also 
called endocrine secretions. The best-known hormonal actions are 
those of man, so the account here given must draw heavily upon the facts 
ascertained for human endocrine glands. The names and locations of 
most of those which are known or believed to be endocrine are shown in 
Fig. 12G. 

Endocrine Glands and Their Work. — One qf the best-known hor- 
mones is that of the thyroid gland, a bilobed structure lying beneath and 
beside the trachea in the neck in man. Its hormone, called thyroxin, 
has been isolated and has the formula C15H11O4NI4. The direct effect 
of thyroxin is to increase the rate of metabolism. Deficiency of this 
hormone in children or young animals retards their development. If 
this influence starts early enough it leads to cretinism, in which body and 
limbs are dwarfed and distorted, and mental development is arrested. 
Some regions of the Avorld have little iodine in the soil, hence little in 
crops, and the inhabitants are finable to produce adequate thyroxin, 
which includes that element. Cretins were common in such regions 
until public health measures, such as the requirement that potassium 
iodide (KI) be added to table salt, were adopted. Deficiency of thyroxin 
in adults often causes endemic goiter, a swollen condition of the thyroid 
caused by an accumulation of a colloid fluid in the capsules of the gland. 
A more serious effect of deficient thyroid is myxedema, with its low 
metabolism, a state of lethargy, and puffed skin. Excessive thyroxin 
commonly causes exophthalmic goiter, with its increased metabolism, 
high blood pressure, and protruding eyeballs; removal of part of the 
thyroid, the proportion depending on how much the metabolic rate has 
been raised, is one of the cures. 

Closely associated with the thyroid (imbedded in it in man) are the 
parathyroids. There are four of these bean-shaped bodies in the human 
thyroid. Separate experimentation with them has been hindered by 
their position. Their primary effect is upon calcium and phosphorus 
metabolism, and the calcium deposit in bones is reduced when the para- 
thyroids are deficient. Complete removal of the glands causes violent 
muscular convulsions. 

The adrenal glands rest on the kidneys (above them in man). They 
consist of a central part or medulla, which arises in the embryo as an 
outgiowth of the nervous S3^stem, and an outer part or cortex, which 
(;omes from the lining of the coolom. The two parts produce different 
hormones, that from the cortex being the more critically important. 
About one-fifth of the cortex suffices for normal processes, but if the whole 


cortex is removed pro»stration and death soon follow. Deficiency of its 
hormone interferes with carbohydrate metabolism, and the blood loses 
most of its glucose. Sodium chloride is also lost from the plasma, the 
osmotic properties of the blood are changed, and so the volume of blood 
is diminished and blood pressure falls. Development of reproductive 
cells is also stopped, and Addison's disease is partly caused by a cortical 
defect. Many substances have been extracted from the cortex, the 
potent ones all being chemically related to one another. The name 
cortin has been given to the active principle, but it has not been 
identified or isolated. 

The adrenal medulla produces the well-known adrenalin (C9H13O3N). 
This hormone has been synthesized artificially. Its effect is to strengthen 
and accelerate heartbeat, increase the glucose in the blood, whiten the 
skin, dilate the pupils of the eyes, and erect the hair. In general its 
action is the same as that of the thoracolumbar part of the autonomic 
nervous system. One theory of adrenalin is that it is a stand-by for 
emergencies. By its control of glucose in the blood, it has been supposed 
to increase muscular power and resist fatigue. In fear and rage and great 
excitement, adrenalin is increased, and the body is supposed to be able 
to perform feats under such emotions which it could not normally do. 

The pancreas, though a digestive gland whose digestive secretion 
flows through a duct, also produces a secretion which must diffuse out 
to the blood. This secretion is called insulin. It is produced in certain 
groups of cells, the islands of Langerhans, which in the embryo were 
budded off from the digestive tubules but which lose all connection with 
the duct. The function of insulin is to control sugar metabolism. 
Failure of the supply of this hormone causes the disease known as 
diabetes mellitus, excess of sugar in the blood and hence its presence 
in the urine. The disease may be relieved by administering insulin 
extracted from other animals, but it has to be injected into the blood 
vessels, not taken by mouth, for insulin is destroyed by the digestive 
enzymes. Also, its ^effect lasts only a few hours, hence it must be used 

The pituitary gland, at the base of the brain, consists of two parts. 
The anterior lobe is derived in the embryo from the roof of the pharynx, 
the posterior lobe from the floor of the brain. The connection with the 
pharynx is lost in the adult, but that with the brain persists. The ante- 
rior lobe produces a variety of hormones, one affecting growth, several 
affecting the sex organs, others acting on the thyroid, adrenal cortex, and 
mammary glands. Because of this multiple activity, particularly in 
control of other endocrine glands, the anterior pituitary is sometimes 
spoken of as the "master gland." The growth hormone was first isolated 
in 1944 as a pure protein. Oversuppiy of this hormone produces giants — 


8- or 9-foot stature with disproportionately long limbs. Too little of it 
produces midgets, with disproportionately short limbs. The hormones 
related to the sex organs and mammary glands are to be described in a 
succeeding section. The hormones affecting the thyroid and adrenal 
cortex have not been isolated ; but in an animal whose pituitary has been 
removed these glands experience degenerative changes; and when 
additional pituitary extract is injected, the thyroid and adrenal cortex 
are enlarged. 

The posterior lobe of the pituitary produces at least two substances, 
one of which stimulates contraction of the uterus in the reproductive 
system, the other constricts the smaller arteries and so raises blood pres- 
sure. Neither of these substances has been isolated. Injury to the 
posterior lobe also deranges the uriniferous tubules of the kidneys, so 
that they no longer resorb the great quantities of water from the filtrate 
entering through Bowman's capsule. A large volume of dilute urine is 
produced under these circumstances. 

The primary reproductive organs, ovaries and testes, produce hor- 
mones which are responsible for the development of the secondary sex 
characters, such as the beard and baritone voice in man, long tail feathers 
in cocks, and the contrasted features of the females. They also govern 
the mating behavior, and determine parental instincts. The principal 
hormone in the male is testosterone (C19H30O2), isolated as a crystalline 
compound. It is produced by the interstitial cells of the testis, not by the 
germ cells nor the tubules which produce germ cells. The corresponding 
hormone of the ovary (sometimes called estrogen though the name has 
varied) is produced by the follicles, blastulalike spheres of cells surround- 
ing the mature eggs. 

Other hormones may be produced by the pineal body above the 
brain, which regulates the speed of sexual development, and the thyrnus 
in the upper part of the chest, which is in some way related to sex develop- 
ment and appears to control the production of the hard shell on birtl 
eggs. Both of these organs are present in children, but the former degen- 
erates into a fibrous structure and the latter disappears in youth. 

Reproductive Cycle. — The influence of the pituitary on other endo- 
crine glands, mentioned al)Ove, hints at interrelations much more exten- 
sive. Presumably not all the interrelations between the glands are 
known, but one group of them has received considerable attention because 
of its bearing upon medical practice. This is the group of glands and 
other secreting structures which control the reproductive cycle in female 

These females show a rhythmical change in their behavior, in that 
periods of sexual excitement occur at regular intervals, separated by 
periods of apathy. This rhythm of behavior depends on an alterna- 


tion of production and disappearance of certain hormones; to understand 
these, it is necessary to know the operations of the female reproductive 
system. The following account is limited to the mammals. 

The female reproductive cells, in different stages, are contained 
in the ovary. Each cell is surrounded by liquid enclosed in a layer 
of cells known as the follicle. The cells (one or more at a time) ripen 
with considerable regularity, every 5 days in the rat, each 28 days in 
man, tmce a year in the dog. In the maturing of a cell the follicle grows 
and approaches the surface of the ovary (Fig. 127). The follicle is there 
ruptured, and the egg escapes into the open end of the oviduct. The cells 
of the broken follicle become converted into a yellowish mass called the 
corpus luteum, while the egg moves down the oviduct. If the animal 
has mated, spermatozoa may have moved through the uterus and into 
the oviducts, and the egg may be fertilized there. If it is not fertilized, 

Fig. 127. — Human ovary to show follicles and corpora lutea. At left, surface view, 
with two follicles of different ages protruding. At right, section showing cla, two degener- 
ating corpora lutea of different ages; civ, fresh corpus luteum;/, follicles; o, ovum. 

the egg disintegrates or passes out to the exterior. If it is fertilized, it 
sinks later into the wall of the uterus and proceeds to form an embryo. 
To receive the fertilized egg, the wall of the uterus must become thickened, 
glandular, and supplied with an extra amount of blood. This prepara- 
tion is all wasted if the egg is not fertilized, for then the uterine wall 
recedes to its "resting" condition. The corpus luteum degenerates 
(in about 2 weeks in man) if the egg is not implanted in the uterus but 
continues throughout pregnancy if implantation occurs. 

What governs all these events, to ensure that they occur in the proper 
relation to one another? In general, it is an interplay of hormones from 
the reproductive organs and the pituitary gland, one gland stimulating 
the other and then being inhibited when its product increases to a certain 
concentration. The pituitary, by means of a hormone, stimulates the 
growth of the egg follicle; the follicle then produces a hormone which 
induces the thickening of the uterus just described. When the follicle 
is ruptured, its hormone is no longer produced, but another hormone is 
produced by its successor, the corpus luteum, which continues the 
preparation of the uterus. No other follicle is growing in the meantime, 
for the hormones of the follicle and corpus luteum inhibit the pituitary, so 


that no follicle-stimulating hormone is forthcoming. If the egg is not 
implanted in the uterine wall, the corpus luteum degenerates, and its 
hormone is no longer produced. The thickening of the uterus conse- 
quently disappears, and the pituitary is relieved of its inhibition. The 
latter gland therefore begins to produce its follicle-stimulating hormone, 
and the cycle is started all over again. 

Why the corpus luteum persists if the egg is implanted is not entirely 
clear, but its hormone is essential to the continued development of the 
embryo, and the pituitary gland is in some way responsible for its 
persistence. Some have supposed that a hormone from the placenta 
guides the pituitary in this particular function, but this is not established. 
Increase in the size of the mammary glands during pregnancy, with 
their secretion of milk at birth, is also caused by a hormone of the pitui- 
tary, but the persistent corpus luteum seems to be the mentor of the 
pituitary in this control. 

The cycle in other vertebrate animals is likewise controlled by hor- 
mones, but, since their young are developed outside the mother's body 
and are not nourished with milk after birth, much of the complexity of 
the reproductive rhythm is wanting in them. In the amphibia, the repro- 
ductive cycle is an annual one. Eggs ripen during the winter and are 
laid in early spring. During the summer the ovaries are small flabby 
organs, in which the oocytes gradually increase in size into the fall, 
but normally none is liberated until the next spring. If, however, an 
extract of the anterior lobe of the pituitary gland is injected into one of 
these animals in the fall, eggs are released from the ovary in three or four 


Carlson, A. J., and V. Johnson. The Machinery of the Body. The University of 
Chicago Press. (Pp. 360-533.) 

Corner, G. W. The Hormones in Human Reproduction. Princeton University 

Mitchell, P. H. A Textbook of General Physiology. 2d Ed. McGraw-Hill Book 
Company, Inc. (Chap. IV, reflexes; Chap. V, correlating action of nervous sys- 
tem; Chap. VI, receptors.) 

Rogers, C. G. A Textbook of Comparative Physiology. McGraw-Hill Book Com- 
pany, Inc. ' (Includes many invertebrates: Chap. XXVIII, nervous system; 
Chap. XXV, hormones.) 

Sherrington, C. S. The Integrative Action of the Nervous System. Charles 
Scribner's Sons. (Chap. I, simple reflexes.) 


A new animal or plant comes into existence only by the transfor- 
mation of some part of a previously existing organism. While repro- 
duction must have been understood for man and his domesticated animals 
from time immemorial, it is not so long since it was popularly believed 
thei-e were other ways whereby new individuals could arise. Among the 
ancient Greeks it was common belief that leaves could be converted into 
fish or birds, mud into frogs, dead flesh into bees. In the Middle Ages 
barnacles were thought to be transmuted fruit of a tree, and to give rise 
in turn to geese. As these notions were abandoned, the idea was trans- 
ferred to the smaller organisms which improved microscopes were 
revealing. It was only comparatively recently that the view that 
bacteria arose de novo from nonliving matter was given up. The sup- 
posed origin of living things from nonliving matter was called abiogenesis 
or spontaneous generation. While in the evolution of life there must 
once have been a beginning of the living out of the lifeless, it is not 
likely that such changes are happening now. Certainly there is no pro- 
duction, from nonliving substance, of new individuals belonging to 
recognized present-day species of animals or plants. 

Increase in numbers of individuals, or replacement of losses, is pro- 
vided for by a variety of reproductive methods which fall into two 
general categories, namely, sexual and asexual reproduction. Sexual 
reproduction as a rule involves two parents and the union of two germ 
cells, or of two cells of some kind, or of two nuclei of different cells. 
Asexual or nonsexual reproduction includes all forms of reproduction 
not involving germ cells or any of the unions just named. 

Sexual Reproduction. — Sexual reproduction is a well-nigh universal 
method of reproduction. It is employed by representatives of every 
great group of animals and by many of them to the exclusion of the 
asexual method. It is also used by the plants, except the bacteria. 

In one of its very common forms, sexual reproduction is the union of 
two cells to form a single cell, the zygote, which by its subsequent divisions 
produces a new individual (in the metazoa) or a new series of individuals 
(in the protozoa). Not all cells are capable of uniting in this way, and 
cells which are capable of this act are called gametes. Certain gametes 
are relatively large, contain a considerable amount of nutritive material, 




and are nonmotile; these are called ova (singular, ovum), or eggs. Other 
gametes are minute, often a very small fraction of the size of the ova of 
the same species. These are poorly supplied with nutritive material, 
have a very small cytosome, and usually are motile; they are kno^\^l as 
spermatozoa (singular, spermatozoon). The individuals in which eggs 
develop are females, and those in which spermatozoa develop are males. 
Sexual Reproduction in Metazoa. — In metazoa the germ cells (ova 
and spermatozoa) are the only cells which retain the power of uniting 
to initiate the development of a new metazoan individual. All other 
cells have completely lost this power. As the time for sexual repro- 
duction draws near, the germ cells undergo a certain process of develop- 

A B 

Fig. 128. — Sperm cell and ovum. A, spermatozoon of rabbit; B, fertilized ovum of 
Nereis with two polar bodies, ph. (B from Wilson, " The Cell.") 

ment or of preparation for the sexual act. This preparatory process is 
described in detail in Chap. 16, but its essentials may be stated here. 
In the ovum it consists in the main of two cell divisions by which three 
or four cells are produced. Of these cells one is much larger than the 
others, and its nucleus has one-half the usual number of chromosomes. 
The small cells are called polar bodies and are nonfunctional. In the 
sperm cell the process does not differ essentially from that in the ovum, 
except that it results regularly in the formation of four relatively small 
cells of about equal size, all of which are usually functional. Like the 
eggs they have half the usual number of chromosomes. The male germ 
cells must then be transformed, by a striking change of shape, into 
spermatozoa. A sperm cell and an ovum with polar bodies are illus- 
trated in Fig. 128. 

When mature spermatozoa and eggs of the same or closely related 
species are brought together, the actively motile spermatozoa meet and 



penetrate the eggs. Usually but one sperm cell can enter an egg. After 
its entrance other spermatozoa are excluded, either by a change in the 
surface of the egg or by some other mechanism. The spermatozoan 
nucleus and egg nucleus arrange themselves 
side by side; and, as the zygote begins to 
divide in development, the chromosomes 
of the two nuclei mingle in such a way that 
their separate sources are as a rule com- 
pletely obscured. A new cell has arisen 
from two cells, and out of it comes a new 
individual derived from two parents. 

Sexual Reproduction in Protozoa. — In 
some of the protozoa, sexual reproduction 
involves union between two cells that are 
alike, which are accordingly known as iso- 
gametes (Fig. 129). In other unicellular 
organisms the cells that unite are neces- 
sarily of different kinds and are then 

known as anisogametes. In Eudorina elegans the difference is one of 
size; fusion is always between a large cell and a small one (Fig. 130). 
These might at first seem comparable to the egg and spermatozoon of 

Fig. 129. — Isogamy in Heteromita 
lens. {After Kent.) 

Fig. 130. — Reproduction in Eudorina elegans Ehrenberg. A, adult colony X 475; B, 
daughter colony produced by division of one of the cells of such a colony as in yl, X 730; 
C-E, development of spermatozoa from a mother cell; F, separate spermatozoa. {From 
West after Goelel.) 

metazoa, but both the large and the small gametes in Eudorina have 
flagella and are therefore motile. In Volvox and Pleodorina there arc 
likewise differences in size, and the large cells are nonmotile. Still, the 
parallel between these large cells and the eggs of metazoa is not complete. 



because in Volvox and Pleodorina the reduction in the number of chro- 
mosomes occurs, not just before the cells are ready for reproduction, but 
a long time earlier. Indeed, all the cells of these organisms have the half 
number of chromosomes; only the zygote from which they spring has 
the full number. In Pandorina morum (Fig. L31) there is a curious com- 
bination of isogamy and anisogamy; it has reproductive cells of two sizes, 
and union may occur between two small ones or between a large and a 
small one, but not between two large ones. 

In the foregoing examples, union of gametes is a fusion of whole 
cells. In some of the ciliated protozoa, however, it is only the nuclei 
of the cells which fuse. In the species in which this occurs, there are 

Fig. 1.31. — Reproduction in Pandorina morum Borg. A, vegetative colony; B, asexual 
reproduetion; C, gametes (y) ; D-E, union of gametes to form zygote {z) ; F-H, development 
of zygote. {From West after Pringsheim.) 

two nuclei in each individual, a large one called a macronucleus and a 
small one, or micronucleus. Only the micronuclei are involved in the 
union, and it is justifiable to regard these nuclei, rather than the whole 
cells, as the gametes. To effect this union, the cells must come together 
temporarily and make an exchange of nuclei. Temporary union of 
two protozoan individuals for exchange of nuclei is called conjugation. 
Since the process is rather complicated, it is best illustrated by a specific 
example, for which Paramecium is selected. 

At the time of conjugation (Fig. 132.1) two individual paramecia 
come together with their oral surfaces in contact. They are held in 
this position for a time because of the sti(;kiness of the protoplasm 
on those siu'faces. While they continue to swim about, internal changes 
in the micronucleus and macronucleus of each individual take place. 
The micronucleus of each Paramecium divides by mitosis {B, C, D), 



and then each half divides again. Thus each micronucleiis gives rise to 
four micronuclei (Fig. 132E'). Of these micronuclei, three undergo 
degeneration, and the one remaining in each Paramecium divides again 
into two parts, usually of unequal size (F). The smaller micronucleus 
of each individual now passes over into the other individual {G), while 






Fig. 132. — Diagram of the process of conjugation in Paramecium. The reference to 
maturation in the figure will be clear only after a perusal of the section on oogenesis in 
Chap. 16. {From Calkins, "Biology of the Protozoa.") 

the larger one is retained. The two pieces, one derived from each indi- 
vidual, now fuse to make the fusion micronucleus {H). During these 
stages of the process the macronucleus has been undergoing fragmentation 
and sooner or later its parts degenerate completely. Soon after the 
exchange of micronuclei the individuals separate and the process of 



conjugation itself is completed. Fusion of the micronuclei, however, 
initiates a series of changes covering a long period. These processes in 
one of the exconjugants are essentially as follows. The fusion micro- 
nucleus divides three times (Fig. 132/-M), resulting in the formation 
of eight micronuclei. Of these, four enlarge and become macronuclei, 
while the other four remain micronuclei. The exconjugant then divides 
twice (N-P), each new individual receiving one micronucleus and one 
macronucleus. After a growth period each cell divides by fission (page 
1G9) in the ordinary manner and at intervals of 16 to 24 hours thereafter 
for a considerable period, when again conjugation usually occurs. The 
part of this process which corresponds to fertilization is the exchange of 
micronuclei and the formation of a new nucleus from the two parts. 

■*» ^• 

■ft **' 

". ^ 



Fig. 133. — Conjugating strains of paramecia: at left, single strain, no conjugation; 
middle, two strains mixed ; right, clumps sorted out, mostly into pairs. {From Wichterman 
in Turtox News.) 

The repeated divisions of the cells following conjugation arc to be likened 
to segmentation of the fertilized ovum of the metazoa. 

A most interesting fact is that there are diffei'ent strains of Para- 
mecium, so organized physiologically that members of the same strain 
will not conjugate with one another, but all of them will conjugate with 
those of certain other strains. When members of two strains which 
will conjugate are mixed, they first form large clumps (Fig. 133). These 
aggregations slowly disintegrate and after a few hours are sorted out, 
mostly into pairs. Some biologists have been tempted to regard this 
distinction between .strains as sex, despite the difficulty of deciding 
which of two conjugating strains is female, which male. Since each 
member of a pair receives a micronucleus from the other, they would 
seem rather to be hermaphrodites (see page 166). 

Parthenogenesis. — In an earlier paragraph it was said that sexual 
reproduction usually involves two parents and the fusion of two germ 


cells. It is not uncommon, however, to find species of invertebrates 
among which, for considerable periods of time, no males can be found. 
The females produce eggs which develop into new individuals like the 
parent, although fertilization by spermatozoa does not occur, since no 
males are present. By their origin and division and nuclear changes 
the cells giving rise to new individuals are ova; hence the method is 
regarded as a sexual one. Development of an egg without fertilization 
is known as parthenogenesis. There are many animals which employ 
parthenogenesis. Some insects which do so are the plant lice, or aphids, 
and many ants, bees, and wasps. The method has also been observed 
in a few moths, a few of the scale insects, and commonly among the 
flower-inhabiting insects known as thrips. 

The females of many parthenogenetic species produce, for a number 
of generations, only females. At intervals, frequently in the fall, males 
are also produced which fertilize the eggs. These zygotes usually differ 
from the unfertilized eggs in being provided with hard shells and in being 
resistant to the rigors of a winter season. The fertilized eggs hatch in 
the spring into parthenogenetic females which repeat the cycle as out- 
lined. Many species of aphids and of the lower Crustacea have cycles 
of this type. In certain insects the bisexual reproductive phase is 
apparently entirely omitted, and reproduction is exclusively partheno- 
genetic. Thus the black flower thrips Anthothrips niger, the brown 
chrysanthemum aphid Macrosiphum sanhorni, many species of scale 
insects, and some gall-producing and parasitic insects never produce 
males. In the ants, bees, and wasps, both males and females are usually 
produced. The female lays both fertilized and unfertilized eggs, in some 
way controlling fertilization of the eggs by the release or retention of 
spermatozoa stored in the seminal receptacles. Among bees the males 
(drones) are derived from unfertilized eggs, the females (queens and 
workers) from fertilized eggs. 

Fertilization, where it occurs, has a dual function, that of (1) stimu- 
lating the egg to develop, and (2) introducing the hereditary properties 
of the male parent. In parthenogenesis there is only one parent; hence 
no paternal qualities can be transmitted, and the eggs are able for 
some reason to start development without any stimulus from a 

Parthenogenetic development has been induced in the eggs of a 
number of animals which ordinarily require fertilization. The methods 
have been various. Bathing the eggs with weak solutions of chemical 
substances, shaking them vigorously in a bottle, heating them, or pricking 
them with a fine needle, all have started division in certain eggs. Most 
of the individual animals whose development was started in this arti- 
ficial way have died in early stages, but a few frog eggs pricked with a 


needle and moth eggs raised to a high temperature have yielded adult 

Paedogenesis. — Although sexual reproduction is usually carried on 
only by adults, this is not always the case, for there are certain species 
whose members have the remarkable power of reproducing sexually while 
they are in the larval ^ state. This reproduction by a larval animal 
is called -paedogenesis. Paedogenesis may be either parthenogenetic or 

Parthenogenetic paedogenesis occurs in certain species of flies. The 
larvae in these species (Fig. 134) produce ova which develop by partheno- 
genesis into larvae before the oviducts are present. The latter generation 
of larvae escapes from the parent larva by rupture of the body wall. This 
results in the death of the parent. Several generations may be produced 

in this fashion; then the larvae of one 
generation pupate and emerge as normal 
adult male and female flies. 

Fig. 134. — Paedogenesis in the Paedogenesis of the bisexual type 

fly Miastor. The parent, itself a -iU iii lii/ii. 

larva, contains a number of larval OCCUrS m the Wcll-knOWn axolotl Amhy- 

ofTspring. {From Folsom after stoTYia Hgrinum, ov tiger Salamander. 

Pagenstecher.) tt i j. • j-i- au* • i 

Under certam conditions this animal 
attains sexual maturity and breeds while it is still in the larval form 
having gills. In some of the Mexican lakes this is said to be the 
usual occurrence, while in Kansas and Nebraska it is rare, and in many 
localities it probably does not occur at all. 

Hermaphroditism. — Most animals — a very great majority of the 
metazoa — possess either male or female organs of reproduction but not 
both. Species which have the sexes thus separate are said to be 
dioecious (living in two houses), while those species whose individuals 
produce both eggs and spermatozoa are called monoecious (living in one 
house). Individuals with both male and female organs are said to be 
hermaphrodites.'^ Two common species of Hydra are hermaphroditic, 
as are most of the flatworms, most snails, and some roundworms. In 
many monoecious species the spermatozoa are produced first and later 
the ova, but in some species this condition is reversed. By developing the 
sexual products at different times, cross-fertilization, that is, fertilization 
of eggs by spermatozoa from another individual, is assured. In the earth- 
worm, eggs and spermatozoa are produced in the same individual and 

' A larva is a young independent individual which differs from the adult in the 
possession of organs not possessed by the adult, or in lacking certain organs which arc 
present in the adult (for example, a frog tad{K)le). 

"^ The word monoecious is also applied to individuals, and is then synonymous 
with hermaphrodite; but the corresponding word dioecious cannot well be applied 
to individuals. 



at the same time. Cross-fertilization is assured in this case by the 
arrangement of the generative organs and by the method of mating. In 
mating, the bodies of two worms are closely applied by their ventral 
surfaces, the heads pointing in opposite directions and the thickened band 
or clitellum of each worm approximately opposite segments 7 to 12 of the 

Fig. 135. — Copulation of earthworms. (Courtesy of General Biological Supply House.) 

other worm (Fig. 135). In this position each worm secretes a slime tube 
(Fig. 136) which sheathes its body. Spermatozoa are discharged into 
the space between the slime tube and the body of the worm, are carried 
backward within the slime tube by the muscular contractions of the body, 
and finally are picked up by the seminal receptacles of the other member 

Fig. 136.- 

-Slime tube and cocoon of earthworm: above, in process of formation; below, 
after slipping off the worm. (After Foot.) 

of the pair. A cocoon is secreted around each worm, and eggs are laid 
in it. The cocoon with the eggs in it is then slipped off over the head 
end, along with the slime tube, and spermatozoa are discharged into it 
as it passes the seminal receptacles (see page 168). Fertilization occurs 
in the cocoon. 



To work in this manner, the ducts discharging the germ cells must be 
in front of the clitellum, by which the cocoon is secreted. Their arrange- 
ment is shown in Fig. 137. The male organs are two pairs of testes, 
three pairs of seminal vesicles, and one pair of vasa dcferentia. Male 
germ cells are originated by the first of these organs, are developed in the 
second, and are discharged thrqugh the third. The same worm also 
possesses a set of female reproductive organs consisting of one pair 
each of ovaries, ovisacs, and oviducts and two pairs of seminal receptacles. 
The eggs, after leaving the ovaries, are held temporarily in the ovisacs 
and then discharged through the oviducts. The seminal receptacles 
receive spermatozoa from another worm and hold them until a cocoon 
passes by their openings. 

Fig. 137. — Reproductive organs of the earthworm, schematic representation of the 
side view: IX— XV, numbers of somites; cm, circular muscles; ep, epithelium; /, funnel 
of vas deferens; Im, longitudinal muscles; ov, ovary; ovd, oviduct; ovs, ovisac; rs, recep- 
taculum seminis; ts, testis; vd, vas deferens; vs, vesicula seminalis; vsb, base of vesicula 
seminalis; 9, opening of oviduct; cf, opening of vas deferens. {Modified from Hesse.) 

While in the earthworm and in some other hermaphroditic species 
an elaborate mechanism ensures cross-fertilization, in other hermaph- 
roditic species no such devices exist and, indeed, self-fertilization (fertili- 
zation of eggs by spermatozoa of the same individual) is well known either 
as a regular or occasional occurrence. Some plants as wheat and beans 
regularly self-fertilize. Other plants as the violet produce some flowers 
which are regularly cross-fertilized and others which can only be self- 
fertilized. Among parasitic flatworms (tapeworms and flukes) and 
among snails both cross- and self-fertilization have been observed. 

As stated in an earlier section, Paramecium is to be regarded as 
hermaphroditic. One individual conjugates with another for exchange 
of micronuclei. Besides this, at intervals there is, without conjugation, a 
reorganization of the nuclei of a single individual which results in rein- 
vigoration, but which seems not to correspond to self-fertilization since 
there is no fusion of nuclei. 



Asexual Reproduction: Fission. — Fission is a common reproductive 
method among the protozoa, and occurs less commonly among the 
metazoa. The essentials of fission are that the parent cell or the parent 
body (if a metazoon) be divided into approximately equal parts, each of 
which grows and regenerates the misvsing parts and thus comes to resemble 
the parent. The parent disappears as an individual and two new indi- 
viduals take its place. The plane of fission may be longitudinal or 


\ < 





4 % 


t. "i 






Fig. 138. — Successive stages in the fission of a single Paramecium. (Courtesy of Ralph 
Wichterman and General Biological Supply House.) 

transverse. Transverse fission, the more common type, is illustrated in 
Fig. 138, which shows, step by step, the division of Paramecium cauda- 
tum. Structures which extend across the plane of fission are divided, 
and the missing portion regenerated. Other structures go with that 
portion in which they are located before fission, and corresponding 
structures arise anew in the other portion. Thus in a Paramecium with 
two contractile vacuoles, one placed anteriorly, the other posteriorly, 
one vacuole goes to each new individual and a second vacuole arises anew 
in each, usually before division is completed, as in the figure. In forms 
which have both macro- and micronuclei, both nuclei elongaste and finally 



divide, a half going to each new individual. After the separation into two 
individuals, regeneration is completed and each individual grows in size. 
As stated on page 164, fission occurs every 16 to 24 hours in a healthy line 
of paramecia. 

In the reproduction of certain parasitic protozoa the nucleus of a 
large cell may divide many times without the division of the cytosome. 
Later the cytosome divides, not by successive equal fissions but by 
many simultaneous divisions, into as many pieces as there are nuclei, 
thus forming a number of small cells at the same moment. This process 
is sometimes called multiple fission and sometimes sporulation. It 
occurs regularly in the complicated life history of the organism of malaria. 

Budding. — When an organism divides unequally, the reproduction is 
termed budding. The larger portion may be regarded as the parent, 
the smaller one as the offspring. Usually, also, there is a definite protru- 


Fig. 139. — Gernmule of fresh-water sponge (left), and young sponge recently emerged from 
gemmule: os, osculuni; sp, spicule. {Gemmule after Hesse and Doflein.) 

sion of the bud, which is small at first but grows larger. The bud usually 
develops organs similar to those of the parent and either becomes inde- 
pendent of, or remains attached to, the parent. Budding is a rare 
reproductive process among the protozoa but is common in certain 
groups of the metazoa. 

In the metazoa the budding may be either internal or external. In 
the former, the buds are formed somewhere within the body substance ; 
in the latter, they are on the surface. 

Internal Budding. — In fresh-water sponges, masses of cells collect 
in the jellylike middle layer of the body wall. Hundreds of cells are in 
each mass, and around them is a horny layer which often contains many 
spicules. Such a reproductive body is called a gemmule (Fig. 139, left). 
The gemmules are not shed, but when the parent's body disintegrates at 
the end of the season, they are left exposed on the log or stone to which the 
sponge was attached. They may remain there, or they may be trans- 
ported considerable distances by water currents or perhaps by the feet 
or beaks of birds. 

With the return of favorable (conditions the bud enclosed within the 
outer coating. of the gemmule begins to develop. There is an opening at 



one side of the geramule (above in the figure), which is plugged shut 
during the resting stage. This plug is removed by the developing sponge, 
which then creeps out. It is greatly distorted while crawling out, for the 
aperture may be so small as to permit the sponge to pass only several cells 
abreast. Once out, however, it quickly takes on the form of a sponge 

Fig. 140. — Diagram of bryozoan with statoblasts (s) ; also photograph of animals and (at 

right) a statoblast. 

(Fig. 139, right). Gemmules allow sponges to live through winter and 
permit them to be carried to other bodies of water. 

In the Bryozoa, or moss animals, the internal buds are called stato- 
blasts. They appear at first as white or yellow spots along a stalk which 
joins the stomach of the animal to its body wall (Fig. 140). The oldest 
statoblasts are next to the stomach. In forming them a mass of cells 

Fig. 141. — Statoblasts of several Bryozoa: a and b, two views of that of Cristatella; 
c, Pectinatella; d, Lophopus; e and/, floating and sessile types, respectively, of Plumatella; 
g, J'redericella. (a and b from Sedgwick after Allen; c-g, from Ward's Natural Science 

comes to be enclosed in a horny cover consisting of two valves, like two 
cymbals pressed together (Fig. 141a, b); or they may be of other shapes 
(e-g). These statoblasts escape by the degeneration of the body or 
some part of it. Some possess floats so that currents of water carry 
them, and some have hooks which tend to hold them fast to fixed objects. 
Some germinate late in the summer of the year in which they are pro- 
duced; others remain undeveloped over winter. They endure long 



freezing with impunity, but complete drying for a few days usually kills 
them. When they germinate, the two valves are forced apart but may 
remain attached to the growing animal for a long time. 

External Budding. — In external budding, the body wall is pushed 
out at some point and develops the characteristic features of the animal. 

Fig. 142. — The hydroid, Bougainvillea ramosa, portion of a colony at left; medusa at right: 

mb, medusa bud; p, polyp. {After Allman.) 

In some species the bud is eventually pinched off, as it is in Hydra (Fig. 
58, page 71). This is doubtless the original method. In other species 
the buds remain attached, and colonies are produced, as is common in the 
hydralike animals called hydroids. A typical one of these, Bougainvillea 
ramosa (Fig. 142), forms a colony with a much-branched coenosarc (inte- 
rior cellular portion) bearing at the ends of the branches flowerlike 

Fig. 143. — Diagram of structure of polyp (left) and medusa, with imaginary intermediate 
form between; the plan of structure is the same. 

zooids, called polyps or hydraiilhs. Each polyp is provided with a 
hypostomc, a conical projection at the distal end, around which is a 
circlet of lentades. The coenosarc is surrounded b> the pcrisarc, a tough, 
lifeless cuticle secreted by the cells of the coenosarc. The colony arises 
from a branched rootlike structure, the hydrorhiza, which is attached 
to a solid body such as a rock or log. This colony is produced by budding 
without a separation of the l)uds from the parent. From the stalks 
of many of the polyps, medusae (jellyfishes) are formed by budding. 



Medusae are bell-shaped individuals (right in figure) which after maturity 
become separated from the colony and swim freely in the water by means 
of rhythmic contractions of the bell. Each medusa produces eggs or 
spermatozoa. The fertilized egg develops into a ciliated free-swimming 
embryo which eventually attaches itself by one end to a rock and develops 
into a polyp. This polyp puts forth buds and thus a new colony is 
formed. Though polyps and medusae are so different in gross form as to 
have been regarded as different species before the production of one 
by the other was known, yet the general plan of their bodies is the same. 
In Fig. 143, by turning the polyp upside down, and introducing an 
imaginary form between, the scheme of structure is showm to be ahke 





Fig. 144. — Diagram of life cycle of Obelia, illustrating metagenesis. 

in both. The cavity or enteron does not always enter the tentacles; 
in some polyps and medusae the tentacles are solid chains of cells. 

Obelia forms a colony somewhat resembling Bougainvillea. In 
Obelia, however, the medusa buds are produced by budding from the 
gtalks of certain individuals (blastostyles) which, unlike polyps, have no 
tentacles. Each blastostyle is enclosed in a swollen chitinous sheath, 
the gonotheca. Blastostyle, attached medusa buds, and gonotheca 
together are often designated the gonangium. Obelia is thus composed 
of three types of individuals, two of which are sessile and incapable of 
sexual reproduction, w^hile the other is a sexual free-swimming form 
(Fig. 144). 



Species which, hke Obelia, exhibit several forms of body are said to be 
polymo7-phic (hterally of many forms) ._ In ObeHa, as in many other 
hydroids, polymorphism is accompanied in the life cycle by an alter- 
nation of asexual and sexual reproduction. The medusae, which are 
of separate sexes, produce eggs and spermatozoa. The fertilized egg, 
after fertilization, produces a larva or planula (Fig. 144). This settles 

yC?y7 eumatophore 


Sensor y 


I'lti. 145. — Diagram of a siphonophore colony composed of six kinds of individuals. 

fied from Fleischmann.) 


down, grows into a polyp which buds off other polyps. The colony 
thus formed also buds off gonangia, whose contained blastostyles bud 
off medusae. These medusae are set free, and the cycle starts o\-er again. 
In this cycle the medusae reproduce sexually; all other reproduction in 
it is budding, that is, asexual. It should be noted that the sexual indi- 
viduals have a very different structure from the asexual ones. Such a 
combination of polymorphism with sexual and asexual reproduction is 
called metagenesis. 



Extreme Polymorphism. — Remarkable examples of colony formation 
and metagenesis, accompanied by division of labor among the types of 
individuals that reproduce asexually, occur among' the marine animals 
known as siphonophores, which have a structural similarity to Hydra. 
The siphonophores are free-swimming colonies of varying complexity. 
Each colony (Fig. 145) consists of a common tube of coenosarc which 
bears at one end a pneumatophore 

■ z i 


or float and along its length zooids 
of various forms. The float is 
the expanded end of the coeno- 
sarcal tube. It generally con- 
tains gas and serves to support 
the colony which hangs freely in 
the water. Near the float is a 
group of swimming bells {nedo- 
calyces) which resemble medusae 
and whose function it is to propel 
the colony through the water by 
their alternate contraction and 
expansion. At intervals l:)elow 
the swimming bells occur bracts, 
or covering scales ; feeding polyps 
which ingest the prey and digest 
it for the entire colony; sensory 
polyps which in some species at 
least also serve as digestive or- 
gans; tentacles (defensive and 
offensive individuals) provided 
with nematocysts (page 72) ; and 
gonopJiores (reproductive zooids) 
with or without bells. A first 
examination of a siphonophore 
might lead to the conclusion that 
it is a complex individual with 
half a dozen kinds of organs. By 
a careful study of selected forms, 
however, and by means of a 
comparison of these with such forms as Obelia, it may be determined 
that most of the structures which in a siphonophore resemble and 
function as organs are really much modified individuals, either polyp 
or medusa (Figs. 143, 144). In certain species the bracts contain 
remains of radial canals which are characteristic of medusae. The 
bracts, swimming bells, and gonophores are constructed on a medusoid 


Fig. 146. — Physalia, the Portuguese man- 
of-war, drawn from live animal floating on the 
surface of the sea. cr, crest; p, polyp; pn, 
pneumatophore; t, tentacle. {From Parker 
and Haswell, " Textbook of Zoology,'^ after 


plan, while the feeding polyps, sensory polyps, and tentacles are con- 
structed on the polyp plan. In a few species the gonophores may 
separate from the colony, as do the medusae in typical hydroids, but 
usually they remain attached. 

The Portuguese man-of-war Physalia (Fig. 146) differs from the 
generalized form described above in possessing a float which sits high 
above the water and serves as a sail. It has no swimming bells or bracts. 

Origin of Colony Formation. — Among the metazoa the formation of 
colonies, the integral union of individuals of the same species, occurs 
only in those groups which employ an asexual mode of reproduction such 
as budding or fission. Animals which employ the sexual method of 
reproduction alone do not form colonies. Colony formation, especially 
when it involves polymorphism and division of labor, may have made 
for greater efficiency in the performance of certain functions, but it should 
not be considered that efficiency is a goal toward which species have 
striven. It seems rather to have been an accident made possible by the 
existence of an asexual method of reproduction and to have been due 
to a failure of the mechanism by which budding or fission is normally 

Limits of Asexual Reproduction. — ^Asexual reproduction occurs only 
among the lower forms of life. It never occurs among vertebrate animals, 
and there are a number of great groups of invertebrate animals which 
never employ it. Even in those groups in which it occurs, there are 
many species which never use it. Nevertheless, asexual reproduction is 
very widespread. Because its mechanism is less complicated than that 
of sexual reproduction and because it is employed chiefly by animals of 
simple structure, it is regarded as the primitive method of reproduction. 
Animals must have reproduced asexually for ages before even the simplest 
arrangement for reproductive cooperation of two cells or individuals 


Hegner, R. W. The Germ Cell Cycle in Animals. The Macmillan Company. 

(Especially (!liaps. I and 11.) 
LocY, W. A. liiology and Its Makers. Henry Holt & Company, Inc. (Abiogenesis, 

pp. 277-293.) 
LoEB, J. Artifi('ial Parthenogenesis and Fertilization. Chicago University Press. 

(Chap. I, history of attempts to initiate development artificially.) 
MiNCiiiN, E. A. Introduction to the Study of the Protozoa. E. J. Arnold & Son, 

Ltd. (Chap. VII, fission; Chap. VIII, conjugation and sex.) 
Thomson, J. Akthur. The Study of Animal Life. John Murray. (Chap. XIV.) 
Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan 

Company. (Chap. III.) 


Reproduction in which both sexes are involved is dependent upon the 
uniting of the germ cells, proper conditions for the development of 
the fertilized egg, and conditions suitable for the development of the 
immature animal. The parents often do more or less to ensure these 
events and conditions, to guard against accident to the immature off- 
spring, and to help it over the period of its own helplessness. These 
services of the parents are habitual and are known collectively as breed- 
ing behavior. 

Breeding behavior in the animal kingdom is exceedingly varied. 
There are two apparent reasons for this variety. First, different forms 
have different modes of life, and the breeding habits must be suited to the 
manner of living if they are to accomplish their purpose. Second, the 
increasing complexity attained in the higher forms of life apparently 
necessitates in them a longer period of prenatal development. At least, 
the development before birth or hatching is longer in the complex forms 
than in the simpler ones. The differences in behavior are not characters 
that distinguish large groups of related animals from one another, for 
within these groups there is considerable dissimilarity in breeding habits. 
Even closely related tree frogs, for example, may employ very different 
means of assisting the processes of reproduction and development. 
Because of this diversity no attempt will be made to describe in detail 
the various breeding habits of animals, but rather to classify and sum- 
marize and to introduce just enough detail to illustrate in concrete 
manner the several types of breeding behavior. 

Urinogenital Systems. — Since some features of the breeding habits 
of animals are dependent upon the structure of their reproductive organs, 
these must first be examined. In vertebrate animals the reproductive 
and excretory systems are intimately connected and together they com- 
prise the urinogenital system. The excretory system of the frog has 
already been described (page 13G). In both sexes of the frog the gonads 
(meaning testes or ovaries) develop ventrally to the kidneys and here they 
hang suspended in sacs of peritoneum. This relation is most plainly seen 
in the male and in young females whose ovaries have not yet become 

The oviducts are coiled tubes passing by the ovaries (Fig. 147, left). 
Each oviduct takes its origin in a ciliated funnel which lies near the 




heart and at the extreme anterior end of the coelom or body cavity. The 
posterior end of each oviduct is transformed into a thin-walled distensible 
bag, the uterus, which is connected by means of a narrow passage with the 
cloaca, in the same region as the opening of the ureter. The walls of the 
uterus and the ureter become united side by side in their lower courses, 
but their cavities remain distinct. Eggs are released into the body 
cavity by ruptures in the peritoneum covering the ovaries. They are 
carried forward to the funnels of the oviducts by the general body 
movements, assisted by pressure of the fore arms of the clasping male. 

Fig. 147.— Uriuogenital system of female (left) and male frog. Kidney at left in male 
is in surface view, that at right dissected to show internal tubes. A, anus; BT, Bidder's 
tube; CL, cloaca; CT, collecting tubule; CV, postcaval vein; F, funnel of oviduct; FB, fat 
bodies; K, kidney; LI, large intestine; MD, Muellerian duct; OV, ovary; OVD, oviduct; 
SI, small intestine; T, testes; UB, urinary bladder; UR, ureter; UT, uterus; VE, vasa 

Currents are also produced by the strong beating of the cilia which line 
the funnels of the oviducts. These currents sweep the eggs and other 
matter into the open funnels and down the oviducts. The remainder 
of the path to the exterior is indicated by the structure and arrangement 
of the organs. 

In the male frog (Fig. 147, right) the testes are connected to the 
kidneys by means of fine ducts, the vasa efferentia. These fine ducts 
penetrate into the kidney and open into a longitudinal canal (Bidder^s 
canal), which is a long tube running lengthwise of each kidney near its 
median border. Bidder's canal is connected with the lu'eter by means 
of a series of collecting tubules into which the uriniferous tubules (page 
135) also open. Spermatozoa in the frog must therefore pass through 
the vasa efferentia, Bidder's canal, the collecting tubules, the ureter. 



and the cloaca on their way to the exterior. In some species of frogs, 
the lower end of the ureter in the male may be expanded into a seminal 
vesicle in which spermatozoa are stored until they are emitted at the time 
of breeding. 

A comparison of the reproductive systems of male and female frogs 
reveals that in the male the reproductive organs are more intimately 
connected with the excretory organs than in the female. In reptiles and 
birds, the genital system, especially in the male, is mare distinct from 
the excretory system, though in 
both of these groups, as in the 
frogs, both excretory and genital 
systems discharge into the cloaca. 

In most mammals, the genital 
and excretory systems open to the 
exterior through a common open- 
ing which Is separate from the anal 
opening. That is, there is no 
cloaca. In the female, the funnel 
of the oviduct is close to the ovary 
but is not connected with it. The 
oviduct opens into the uterus in 
which the young are retained and 
nourished until birth. The form 
of the uterus differs in the different 
groups of mammals. That illus- 
trated in Fig. 148 is common 
among the carnivores, rodents, and 
others which bring forth young in 
litters. The uterus is connected 
to the exterior by the vagina which 
is the copulation passage. The 
urinary bladder, which belongs to 
the excretory system, is connected 
to the lower portion of the vagina 
by means of the urethra. In the 
male the testes are connected by 
means of the vasa deferentia (singular, vas deferens) with the urethra, 
which extends from the urinary bladder through the penis. 

Methods of Ensuring Fertilization. — In chronological order, the first 
event of the breeding process in bisexual animals is fertilization of the 
germ cells. From the nature of their reproductive systems it might be 
expected that this event would occur differently in hermaphroditic 
animals and those with the sexes separate, for in hermaphrodites self- 

Fig. 148. — ^Urinogenital system of a fe- 
male mammal having a bicornuate uterus, 
somewhat schematic, bl, urinary bladder; 
A;, kidney; od, oviduct; ov, ovary; sug, uri- 
nogenital sinus; ur, ureter; ut, uterus; ng, 
vagina; *, position of embryos. {Modified 
from Wiedersheiyn.) 



fertilization is conceivable. This expectation is not usually realized, 
however. Relatively few animals are hermaphroditic, among them being 
some sponges, Hydra and a few animals similar to it, worms, and snails. 
Though hermaphroditism is often described in various kinds of vertebrate 
animals, the condition so named is usually merely the existence of egglike 
and spermlike cells in the same gonad. Since usually only one, if either, 


Fig. 149. — Genital organs of a hermaphi-odite animal, a common land snail Polygyra 
albolabris (Say). Note that some of the organs are characteristic of a male, others of a 
female. 1, atrium; 2, penis; 3, prepuce; 4, vagina; 5, spermatheca; 6, vas deferens; 7, free 
oviduct; 8, uterus; 9, spermatic duct; 10, talon; 11, hermaphroditic duct; 12, hcrmaphioditic 
gland; 13, penis retractor; 14, albumen gland. 

of these kinds ever reaches maturity and since appropriate ducts for 
leading off both kinds of cells are not often present, such animals are not 
functional hermaphi-odites at all. The reproductive system of a really 
hermaphroditic animal, a snail, is shown in Fig. 149. 

Most hermaphroditic animals have some way of avoiding self-fertiliza- 
tion. In some of them, though l)oth kinds of germ cells are produced, 
eggs predominate in some individuals, spermatozoa in others, and mere 
chance favors cross-fertilization. In other animals, the two kinds of 



germ cells are produced at different times, eggs first in some, spermatozoa 
first in others. Obviously there can be no self-fertilization under these 
circumstances. In still others, the mating habits prevent self-fertiliza- 
tion, as has been described for the earthworm in the preceding chapter. 
Some of the roundworms, however, and Sacculina, which is a parasite 
attached to crabs, fertilize their own eggs regularly. No special act is 
necessary to bring eggs and spermatozoa together in these self-fertilizing 
forms, since they mingle freely within the body. Sometimes self-fertiliza- 
tion may occur accidentally, as in Hydra whose sperms are shed into the 
water, where they penetrate eggs still located in the ovaries. Since the 
spermatozoa find the eggs largely by chance, they may reach either an 
egg in the same individual or one of another individual. 

Fertilization in Dioecious Species. — In many aquatic animals with 
separate sexes the sexual elements, or at least the spermatozoa, are 
simply discharged into the water and the germ cells come together by 
chance. Thus in the jellyfishes the spermatozoa are liberated into the 
water and may or may not happen to meet the eggs, which are retained 
in the ovaries of the females of some species just as in Hydra. In other 
animals there is congregation of the sexes at the breeding time, and the 
eggs and spermatozoa are liberated in 
proximity. Starfishes and sea urchins 
periodically congregate in this manner. 
This close association of the sexes un- 
doubtedly greatly favors the meeting of 
the germ cells but still leaves to chance 
an important role, and many eggs are 
never fertilized. The hellbender (a sala- 
mander) is a form that congregates with 
its fellows at the breeding season. In 
certain other salamanders the male 
deposits the spermatozoa in a naked, 
nearly spherical mass resting on a 
gelatinous stalk which is attached to a 
leaf or some other object in the water. 
This structure, including the stalk, is 
called a spermatopho)-e (Fig. 150). The mass of spermatozoa at its top is 
subsequently removed by the female with the lips of the cloaca, and the 
eggs are fertilized within her body. 

This last way of bringing eggs and spermatozoa together can be 
adopted only by animals that fertilize their eggs internally. In frogs 
and toads, fertilization occurs outside the body, and in these forms special 
behavior is designed to bring the sex cells together. In addition to 
congregating at the breeding season, the males practice clasping. The 

Fig. 150. — Spermatophore of No- 
tophthalmus viridescens viridescens 
(Raf.), the common newt of eastern 
North America. The stalk is a clear 
gelatinous substance; the apical mass 
(dotted in the figure) is a snowy- 
white mass of seminal fluid contain- 
ing spermatozoa. {After B. G. 



male grasps the female, with his forelegs around her body (Fig. 151), 
and pours out the fluid containing the spermatozoa as she lays her eggs 
in the water. One of the salamanders, Notophthalmus viridescens, shows 
a curious combination of methods, the value of which is obscure ; the male 
first clasps the female, but instead of pouring out the spermatozoa into 
the water, he then deposits them in spermatophores, from which the 
female takes them into her cloaca as just described. 

In many other animals the spermatozoa are introduced into the 
body of the female by direct act of the male, a process known as copula- 
tion. Fertilization then occurs internally. This method is employed 

Fici. 151. — Clasping in a species of toad, Bnfo typhonius (Linnaeus). The small individual 
is the male, the larger the female. {Photograph by A. G. Ruthven.) 

by some parasitic worms, snails, fishes, and amphibia, and by all insects, 
reptiles, birds, and mammals. 

Place of Development. — From the methods of ensuring fertilization, 
it will be seen that the eggs may be fertilized either before or after they 
are laid. That is, fertilization is either internal or external. When 
fertilization is internal the eggs may be retained for a long time after 
fertilization, or they may be laid very soon thereafter. Whatever period 
of time the eggs remain in the organs of the female after fertilization is 
utilized in development, so that the embryo may be far advanced before 
it is separated from the mother, or it may have attained only an early 
stage of development, or development may scarcely have started. Thus, 
in most of the insects and in all the birds the eggs are laid soon after 
fertilization. In these cases only a few divisions of the egg, or of its 
nucleus, have taken place at the time of oviposition, or it may not have 
divided even once. On the contrary, development may proceed until a 





well-formed embryo is produced, and then the eggs are laid; this occurs 
in some of the salamanders. Usually, if the eggs undergo more than a 
few cleavages while within the mother, they remain until a rather late 
larval stage, or until the form of the adult is attained. Some insjects, 
some snakes, and the true mammals are of the last-named type. 

Source of Nourishment of the Embryo. — Animals that lay their eggs 
are said to be oviparous; the eggs may be laid before fertilization, or, if 
after fertilization, while the embryos are still incapable of existence out- 
side the egg membranes. Animals that retain the embryos until with 
proper care they are capable of independent existence are designated 
viviparous. Of viviparous species there are two general types. In one 
of these, the eggs are large and laden 
with yolk, from which the embryo 
derives its nourishment, just as in 
oviparous animals. The mother 
serves, in such cases, chiefly as a 
nest in which the eggs may develop. 
Viviparous animals in which prac- 
tically the whole nourishment of the 
young lis furnished by the egg itself 
are said to be ovoviviparous. Some 
reptiles are ovoviviparous (Fig. 152), 
the embryos being held in the oviduct 
of the mother until they are far 
advanced but receiving the food from 
the egg. The second type of vivipa- 
rous animal is that in which the 
nutrition of the embryo is obtained 
from the mother, whose reproductive 
system is then of the general type 
represented in Fig. 148. The embryo, 
resting in the uterus, has as intimate a relation with the mother's blood 
vessels as do the mother's own tissues. Blood vessels of the embryo 
extend out through the umbilical cord, and branch profusely at the end 
in a highly vascular structure known as the placenta (Fig. 153, left). 
The placenta is furnished partly by the embryo, partly by the uterus of 
the mother. In it the blood of the mother and that of the embryo, while 
never joining in the same vessels, are separated only by the thin walls of 
their respective capillaries. In the human placenta the connection is 
even closer, for the walls of the maternal vessels become eroded away, so 
that the blood comes to lie in large sinuses, resembling the open blood 
spaces of the crayfish or insect circulatory system (page 122). In this 
lake of maternal blood the capillaries of the fetal system (branches of the 

Fig. 152. — Urinogenital system of a 
lizard. B, bladder; CI, cloaca; K, kid- 
ney; 0, ovary; Ov, oviduct; Ow^, cloaca! 
opening of oviduct; Ov^, abdominal open- 
ing of oviducts; R, rectuni. The lizards 
are oviparous or ovoviviparous. 



umbilical vessels) are bathed, as shown by the diagram at the right in 
Fig. 153. The physiological operation of the two blood systems is 
precisely like that of blood and adjoining tissue. Digested food and 
oxygen in the maternal blood are transferred to the fetal blood, because 
they are at higher pressure in the former. Accumulated urea and carbon 
dioxide go in the opposite direction because they are at greater pressure 
in the embryo. The fetus is thus being fed, and its wastes removed, 
as efficiently as if it were a rapidly growing tissue in the mother's own 
body. No blood cells are transferred in either direction, however; the 
exchange is entirely a process of diffusion and osmosis. 

Forms in which the embryo is connected with the maternal uterus 
by a placenta are spoken of as truly viviparous. Hydra and some of 








Fig. 153. — Position of fetus in uterus (left), with its attachment by umbilical cord and 
placenta to uterine wall: F, fetal placenta; 0, opening of oviduct; <S', maternal placenta. 
Rectangle shows approximate location of dissection of human placenta at right. {Left 
after Kingsley, " Vertebrate Zoology," Henry Holt and Company, Inc.) 

the jellyfishes, among aquatic animals, exhibit something like viviparity, 
since only the spermatozoa are shed into the water. The spermatozoa 
in these forms find the eggs, largely by chance, while the eggs are still in 
the maternal ovary and penetrate the eggs in that situation, and the fer- 
tilized eggs develop there for a time. In these cases the eggs are large 
and presumably contain much of the necessary nourishment. 

Intermediate between ovoviviparous and vivipai-ous forms are those 
in which the young develops for a considerable time in the egg and later 
becomes attached to the body of the mother. C^ertain sharks (Fig. 154) 
exemplify this intermediate condition. The expanded end of the yolk 
sac becomes attached to the wall of the uterus, terming an organ like 
the placenta of mammals. The young receive nourishment through 
it from the mother. 



No Evolutionary Sequence. — Since some of these types of breeding 
behavior are plainly much more specialized than others, one might be 
tempted to suppose that they exhibit some sort of evolutionary sequence. 
That is, it might be thought that the simpler habits would be employed by 
the more primitive groups of animals, while the complicated methods 
would be adopted by the higher forms. Such appears not to be the case, 
however, either as to assurance of fertilization or as to place of develop- 
ment. Thus, copulation, which is a specialized habit, is employed by 
some parasitic worms, some snails, the insects, reptiles, birds, and mam- 
mals. These groups are so diverse in structure that it is impossible to 
regard them as all primitive or all 
highly developed. Furthermore, 
most of the fishes and amphibia 
use either external fertilization with 
clasping or internal fertilization 
without clasping, while some mem- 
bers of each of these groups employ 
copulation. In general, the same 
breeding habits may occur in 
animals of widely different groups, 
and animals of the same group often 
have very different habits. The 
principal generalization concerning 
fertilization is that among aquatic 
or amphibious forms the habit pre- 
vails of depositing the spermatozoa 
and eggs freely in the water or 
in immediate proximity to each 
other, or of depositing the sperma- 
tozoa so that they can be secured 
later by the female; while in 
the groups composed mostly of land forms the habit of introducing them 
into the body of the female predominates. The latter method is essen- 
tial to most land forms, since air is fatal to the delicate sexual cells, 
whereas in aquatic forms the eggs (at least after fertilization) can endure 
the water for a prolonged period. 

In the method of bearing the young, also, there is no evolutionary 
sequence. Oviparity and viviparity are found in the vertebrates and the 
invertebrates. Certain conditions of reproduction itself, however, make 
one generalization possible. The forms in which the eggs are fertilized 
outside the body of the mother are necessarily oviparous; and it is only 
among forms mth internal fertilization that viviparity, ovoviviparity, 
and the laying of fertilized eggs can occur. As a result, viviparity, 

Fig. 154. — Embryo sharks of a vivip- 
arous species, Mustelus mustelus (Lin- 
naeus), attached to the wall of the uterus, 
which is here dissected open. {After 



ovoviviparity, and the laying of fertilized eggs prevail among land 
forms, where protection against evaporation of the eggs is necessary; and 
the habit of laying eggs before fertilization is mostly found among the 
aquatic species and the amphil^ious forms which lay their eggs in water. 
Care of Fertilized Eggs. — Among oviparous species the methods of 
caring for the fertilized eggs are almost endlessly varied in their details. 

FxG. 155. — Nest of the Australian brush turkey, consisting of Utter in which the eggs 
are buried to be hatched by the heat of tlie decomposing vegetable debris. The nest is the 
heap of debris in the lower half of the photograph. {Photograph by E. R. Sanborn, loaned 
by the New York Zoological Society.) 

There are many animals which give no care whatever to the eggs. This 
is particularly true of aquatic species which pour the eggs and sperma- 
tozoa freely into the water to come together by chance. The starfishes 
and sea urchins and many other marine animals exhibit this lack of 
parental care. Other forms merely put the eggs in places where develop- 
ment is facilitated. Thus toads and certain salamanders which live 
on land in the adult stage lay the eggs in the water. Aquatic turtles 
come to land to lay eggs in the warm sand which hastens their develop- 



ment. Digger wasps, ichneumon flies, and certain other insects deposit 
their eggs in various places and provision them with Hving or dead animal 
food. Birds of one group, the megapodes, lay the eggs in a pile of 
decaying vegetation, the decomposition of which liberates heat that aids 
in development (Fig. 155). Again, many animals build nests. These 
nests may be very simple in construction. In the fishes, for example, 
many species merely hollow out a small area on the bottom of the stream 
by pulling out the pebbles and heaping them up on the downstream side 
of the nest. The eggs, when laid, drop into this hollow and among the 
loose stones. Birds build nests of a great variety of forms, from the loose 
collection of grass or straw put on the ground by the killdeer, or the 

Fig. 156. — Blue-tailed skink, Eumeces fasciatus (Linnaeus), with eggs. This lizard 
buries its eggs (the white mass in the middle foreground) in decaying wood and stays with 
them until hatched. The curved white streak to the left of the center of the picture is the 
tail (blue in life) of the parent, and a part of the striped body can be seen to the right of the 
center. {Photograph by A. G. Ruthven.) 

insecure litter of twigs set in the branches of trees by the mourning 
dove, to the elaborate hanging basket of the orioles. Still other forms 
enclose their eggs in cases, as was pointed out for the earthworm in the 
preceding chapter and as is true also of the leeches and some insects, 
snails, and spiders. 

Among the nest-building forms the habit of caring for the eggs has 
usually been developed ; that is, one or both of the parents in many species 
remain with the eggs until they are hatched. The habit of remaining 
with the eggs may ensure incubation, or the elevation of the temperature 
to a point at which development will proceed. Incubation by the parents 
is necessary in most birds and is an aid in some other animals. Remain- 
ing with the eggs does not, however, necessarily imply incubation. For 
example the common skink is a "cold-blooded" animal which remains 
with the eggs (Fig. 156). Its temperature is so nearly that of the sur- 



rounding air that the development of the eggs can scarcely be affected 
by the presence of the parent. Some other species apparently incubate 
the eggs to a small extent. The python, for example, coils about its 

Fig. 157. iio. i5<s. 

Fig. 157. — Hyla fuhrmanni Peracca, a South American tree frog that has the habit of 
carrying the eggs on the back. The female carries the eggs. (Photograph by A. G. 

Fig. 158. — A marsupial frog, Gastrotheca monticola Barbour and Noble, from Peru. 
The opening of the pouch and a protruding egg may be seen in the lumbar region. (Photo- 
graph by G. K. Noble.) 

eggs, and as the temperature within its coils is a few degrees above that 
of the surrounding atmosphere, development is thereby probably some- 
what accelerated. 

The habit of carrying the eggs attached to the body is found in several 

groups, among both nest-building forms and 
others that build no nests. Thus, the female 
crayfish carries her eggs attached to the swim- 
inerets under her abdomen, where she waves 
them back and forth. The movement of the 
eggs increases aeration, which is perhaps neces- 
sary. Fresh-water mussels keep their eggs in 
the chambers of the gills of the female, where 
they are furnished oxygen by the water that is 
constantly passing through the gills. In spiders 
the silken egg case mentioned earlier is often 
carried about by the mother. Certain frogs 
(Fig. 157) and insects bear the eggs glued to the 
back of one sex or the other. In other frogs 
the eggs are attached to the belly, or the egg 
masses are wrajiped around the hind legs of the 
male or are held in the vocal sacs. One frog, the 
marsupial frog (Fig. 158), has a pouch formed of a fold of the skin on the 
back in which the eggs are carried. This habit is again found in the pipe- 
fish and sea horse (Fig. 159) which carry the eggs in a ventral pouch.^ 

Fig. 159. — Hippo- 
campus, the sea horse, 
male specimen showing 
brood pouch: hr. ap, 
branchial aperture; brd. 
p, brood poucli; df, dorsal 
fin; op, opening of brood 
pouch; pet. f. pectoral fin. 



Eggs thus carried in pouches may perhaps receive oxygen from the parent, 
but little is known on this subject. Either the male or female may carry 
the eggs, but usually only one sex does this in any given species. 

Care of the Young after Birth or Hatching: Birth Stages. — After 
birth in viviparous forms and after hatching in oviparous species, the 

Fig. 160. — The black swamp wallaby. The young are born in a very immature stage 
and are carried in a pouch (marsupium) on the ventral side of the mother. {Photograph 
loaned by the New York Zoological Society.) 

young may or may not require protection and assistance in getting 
food. This is partly dependent upon the stage of development which the 
offspring has attained at the time of birth, but not entirely so. 

The animal may leave the egg complete in all its parts and needing 
only the growth of the body and the maturity of the sex cells to attain 
the climax of its development. Among these forms the young may 



receive little or no parental care or they may be fed and cared for for 
many weeks or even months. Among the reptiles, for example, the 
young are left to their own devices as soon as they hatch or are born. 
Most fishes and invertebrates also throw off all parental solicitude after 
their offspring leave the eggs. Most birds, on the contrary, must feed 
and protect their young for a period of days or weeks; and mammals care 
for their offspring for weeks or years. In these cases, how long the 
young must receive aid depends on how far they develop before birth. 

Fig. 161. — Recently hatched young of the chimney swift, Chaetura pelagica (Linnaeus), 
left, and spotted sandpiper, Adiius macularia (Linnaeus), right. These are examples, 
respectively, of altricial and precocial birds. 

There are great differences in birth stages even in the same group. Thus 
among mammals the marsupials (opossums and kangaroos) give birth 
to young in a very immature state and carry them in a pouch (Fig. IGO) 
until they are well formed; mice are born blind, hairless, and very helpless; 
rabbits are born blind but covered with hair; and guinea pigs are born in 
such an advanced stage that they are very shortly independent of the 
mother. Among birds are to be distinguished altricial and precocial 
forms (Fig. 161), the former usually, although not always, hatched blind 
and practically without feathers, thus requiring longer parental care; the 
latter covered with down and with the eyes open, requiring shorter care. 



The common song birds are all altricial, while domestic fowls, partridges, 
most wading birds, and the various ducks are precocial. 

There are also animals which escape from the egg so early that they 
lack important organs and must undergo extensive changes to attain 
the adult form. Or they may possess 
organs which they must lose before *they 
become adults. Young animals, leading a 
separate existence but lacking certain organs 
of the adult or possessing organs not found 
in the adult, are known as larvae. The 
offspring of jellyfishes emerge from the ovary 
of the mother, where in some kinds as stated 
earlier the eggs are fertilized, as a simple 
ball of cells, almost at the beginning of 
development. They receive no care what- 
ever thereafter. The embryos of sponges 
escape at a stage almost as early, as the 
jellyfishes. The developing embryos of 
starfishes, sea urchins and their allies (Fig. 
162), and marine worms are also capable of 
free-swimming existence at a very early 
stage. In the frogs and toads the tadpole is a larval form (Fig. 163), but 
it hatches at a much later developmental stage than do the larvae of the 
several preceding examples. 

Early development, may be direct or indirect. In direct develop- 
ment the embryo develops directly toward the sexually mature condition, 
the organs being outlined and developed one after the other. In indirect 

Fig. 162. — Free-swimming 
larva of the holothurian Syn- 
apta, leading an active inde- 
pendent existence at a very 
early stage of embryonic de- 

Fig. 163. — Tadpole of frog, illustrating a larval form. Organs are present that are lacking 
in the adult, and some organs are missing which the adult possesses. 

development, on the contrary, organs belonging only to the immature 
stages and for that reason called larval organs are first formed and later 
destroyed. Thus the caterpillars (larval stage) of butterflies are dis- 
tinguished from the adult not only by the absence of wings and compound 
eyes but also by the presence of anal feet and spinning glands which 
are absent in the adult butterfly; and tadpoles of toads and frogs (Fig. 


163) are distinguished from the adult frog not only by the absence of lungs 
and legs but also by the presence of gills and tail. The transformation 
by which the larval organs disappear and the missing organs are con- 
structed is kno^vn as metamorphosis. The more numerous the larval 
organs the more pronounced the metamorphosis becomes. This phe- 
nomenon is further described in Chap. 16. 

Relation of Birth Stages to Parental Care. — That birth at an early 
stage of development necessitates parental care would seem at first con- 
templation to be obvious. That is not usually true, however, except 
for the animals of common daily observation. It cannot be said for 
animals in general that the stage of development at birth determines 
the amount of parental care necessary, for many of the lower invertebrates 
with incomplete larvae and many fishes which have very immature young 
give no care to the offspring, while other invertebrates with feeble young 
(for example, the ants) carefully guard and feed them. But it is note- 
worthy that, where no care is exercised, the young born in early stages are 
usually those of aquatic or amphibious forms, while the young of ter- 
restrial forms are mostly born in relatively advanced stages or receive 
parental care. Furthermore, while many aquatic forms give some atten- 
tion to the young, it is among the terrestrial forms that the greatest 
development in the habit of caring for the offspring is found. It may 
thus be concluded that, when aquatic animals, or amphibious forms 
with aquatic young, deposit the eggs or young in suitable habitats, they 
have done much to facilitate postembryonic development, but that 
land forms must usually give birth to young in an advanced stage of 
development or exercise parental care in proportion to the helplessness 
of the offspring. 


The minimum accomplishment of the reproductive processes is the 
formation of germ cells. With the aid of breeding behavior these germ 
cells are brought together in a favorable environment, where they are 
gradually converted into new organisms. Into this period of transfor- 
mation of the fertilized egg into an active independent being is crowded a 
multitude of changes — analyses, reconstructions, rearrangements, growth, 
and differentiations — which constitute embryonic development. Embry- 
ology may properly treat of many of the things already described as 
breeding habit or reproduction; but there is left for examination in this 
chapter the whole series of structural changes and the chains of physi- 
ological events which lead to the formation of the new individual. The 
story may begin with the reorganization of the reproductive or germ cells. 

Maturation of the Germ Cells. — The germ cells in a very young 
animal may remain for a long time in a relatively undifferentiated con- 
dition. Often it cannot even be stated whether they will become eggs 
or spermatozoa, yet in most animals, despite theu' lack of recognition 
marks, they are irrevocably destined to become the one or the other. 
During this time they divide frequently by ordinary mitosis, thereby 
multiplying in number. In this apparently unspecialized condition the 
reproductive cells are called, in a male animal, spermatogonia (singular, 
spermatogonium), in a female, oogonia. 

As the time of reproduction approaches, the spermatogonia and 
oogonia undergo a series of remarkable changes called spermatogenesis and 
oogenesis, respectively. These changes consist typically of two rapidly 
succeeding cell divisions, in one of which the number of chromosomes is 
reduced to half. There are many variations in the process in different 
species, but the fundamental features are the same for nearly all the 
higher animals. 

Spermatogenesis. — As soon as the spermatogonia reach the end of 
their multiplication period, that is, as soon as they have divided by 
ordinary mitosis for the last time, the cells are known as primary sperma- 
tocytes. The history of these cells in their further development is illus- 
trated in Fig. 164, to which constant reference should be made throughout 
the following account. 

During all of their history up to this time, the germ cells contain the 
same number of chromosomes as any other cells of the body. That 




number, barring differences in the sexes, is constant for the species. 
In an animal descended from two parents, these chromosomes, with cer- 
tain exceptions that may for the present be ignored, come in equal num- 
bers from the father and the mother. Half of the chromosomes in any 
cell may therefore 'be designated paternal, the other half maternal. 
These chromosomes may look precisely alike and may in fact be exactly 








Isf Polar Body. 



^ad Polar Body. 

Mature tqq. 

Pert/I/zed Egg. 

First Ckai/age. 

Fig. 1G4. — Spermatogenesis and oogenesis diagramniatically represented. The black 
chromosomes may be assumed to be of paternal origin, the white ones maternal. 

alike; the terms paternal and maternal refer only to their source, not to 
their nature. 

The spermatocytes grow considerably in volume, and at the same 
time their chromosomes come together in pairs. Each pair is composed 
of one paternal and one maternal chromosome. The pairing is not a 
purely fortuitous occurrence, for each paternal chromosome meets a 
particular maternal chromosome. As a result of this union of the 
chromosomes there are, of course, half as many pairs as there Avere 
chromosomes before. 



While the chromosomes have been coming together, they may also 
have become duplicated; that is, each chromosome is in some way con- 
verted into two. Each pair thus comes to consist of four half chromo- 
somes, and the quadruple body formed is called a tetrad. Owing to 
its origin, two of the parts of each tetrad are maternal, the other two 

The Divisions in Spermatogenesis. — In the two divisions that follow, 
the tetrads are divided in two planes, first into double bodies called 
dyads, next into their single components. A spindle is formed on which 
the tetrads take their place. How the tetrads are divided depends 
on the way they are placed on the spindle. In some animals the tetrad 
may be turned so that its maternal half faces one end of the spindle, 
the paternal half the other end. In other animals the maternal and 
paternal halves of the tetrad may be turned toward the sides of the 
spindle (Fig. 165). In either posi- 
tion the tetrad is cut in two in such 
a way that the two parts facing an 
end of the spindle go to that end in 
the cell division. In Fig. 164 it is 
assumed that the tetrads were so 
placed that the maternal half was 
separated from the paternal half. 
It is a matter of chance, however, 
whether the paternal half is turned 
toward one end of the spindle or 
toward the other. It may happen, 
therefore, that all the paternal dyads go into one cell and all the maternal 
dyads into the other or, as in the figure, part into one cell and part 
into the other. The cells produced by this division are called secondary 

It 'is important to note that in the division just described no chromo- 
some has divided. The tetrads have divided, but merely by the sepa- 
ration of the two chromosomes which had previously come together. 
Such a division is called a reduction division, or meiosis;^ it never occurs 
in divisions of somatic cells. 

The secondary spermatocytes now divide by a mitosis in which the 
dyads are divided into two components. The resulting cells are called 
spermatids. A given spermatid may contain only paternal chromosomes, 
or only maternal, or both paternal and maternal in any proportion. 
The number of these chromosomes is only half that of the original 

^ The term meiosis is sometimes applied to the whole process of spermatogenesis 
and oogenesis, including both divisions. 

Fig. 165. — The t'^^-o possible positions 
of a tetrad on the spindle of the first 
division in spermatogenesis, and the kinds 
of cells resulting from them. 



By a transformation in shape, the spermatid becomes a mature sfer- 
Tnatozoon. This cell consists usually of a head and a whiplike tail, but 
the forms are very different in different animals (Fig. 1G6). The chromo- 
somes are all contained in the head, the tail being merely a motile organ. 

Oogenesis. — The ripening of the female germ cells is in most respects 
similar to that of the male. The early germ cells or oogonia undergo a 
period of multiplication in which they divide by ordinary mitosis. 
Eventually this ordinary division ceases, and the cells are ready to initiate 




Fig. 166. — Different forms of .spermatozoa: A, badger; B, slieldrake; C, sturgeon; 
D, flycatcher; E, opossum; F, lobster; G, crustacean Polyphemus; H, crab Droniia; /, crab 
Porcellana; J , crustacean Ethusa. (A-D after Ballowitz; F after Her rick; G after Zacharias; 
H-J after Grobben. From Wilsori, " The Cell in Development and Heredity." Courtesy of 
The Macmillan Company.) 

the maturation process. They are now known as primary oocytcfi. 
These oocytes grow rapidly to many times their original volume, the 
growth being much greater than in the male. 

During growth the chromosomes meet in pairs, each pair, as in the 
male, being composed of one maternal and one i)aternal chromosome. 
Each chromosome may divide or be duplicated as they come together, so 
that the pair presents a quadruple body, the tetrad. 

Divisions in Oogenesis. — These tetrads are divided in the remainder 
of the process, first into dyads, next into their single components, in a 
manner strictly comparable to the divisions in the male. When a sjjindle 
is formed for the first division, it appears not in the center but near the 



surface and is placed approximately perpendicular to the surface. The 
tetrads take their place on this spindle, again with their maternal and 
paternal halves either toward the ends of the spindle or toward its sides 
(Fig. 167). What kinds of dyads go into the two daughter cells depends 

Fig. 167. — Two possible positions of tetrad on spindle of first division in oogenesis, and 

the kinds of cells resulting from them. 

on which of these two positions the tetrads take. In Fig. 164 the tetrads 
are assumed to have been turned with their maternal half toward one 
end of the spindle, the paternal half toward the other, so that the first 
division was a reduction division. Each dyad formed is thus either 
wholly maternal or wholly paternal, although of the 
dyads in a given cell some may be paternal, some 

The two cells are of very unequal size. One 
contains nearly all the protoplasm of the primary 
oocyte, the other very little indeed. The disparity 
between them is much greater than Fig. 164 indicates; 
the correct sizes for one animal are shown in Fig. 168. 
The larger cell is named the seco7idary oocyte. The 
smaller cell is never functional and is called the 
first polar body or first polocyte; it eventually 

In most animals only the secondary oocyte undergoes further division. 
In some species the first polar body also divides, and, to complete the 
comparison with the male, this occasional division is represented in Fig. 
164, but the resulting two polar bodies are not functional. 

Fig. 168.— Star- 
fish egg with 
polar body above. 
(Courtesy of General 
Biological Supply 



The division of the secondary oocyte involves the division of the 
dyads into their halves. The division of the cytosome is again very 
unequal, so that one small cell, the second polar body or second polocyte, 
and one large cell are produced. The large cell, unlike the final cells 
in the male, does not undergo any change of shape ; its maturation is fin- 
ished when the second division is completed, and it is therefore a mature 


Comparison of Oogenesis and Spermatogenesis. — Comparison of the 

maturation of spermatozoa with that of eggs reveals that with respect to 

Fig. 169. — Homolecithal egg of the sand worm Nereis. C, cytosome; /, fat droplets; 
m, egg membrane; ri, nucleus; nl, nucleolus; y, yolk spheres. (After Wilson. Courtesy oj 
The Macmillan Company.) 

the chromosomes the two processes are parallel. The chromosomes unite 
in pairs and are often at the same time duplicated so as to produce tetrads. 
Two rapidly succeeding divisions divide the tetrads into dyads and 
then single chromosomes. 

The final cells contain half as many chromosomes as did the reproduc- 
tive cell before the process began. These chromosomes may be paternal, 
or maternal, or paternal and maternal mixed in any pro{)ortion. 

The striking feature in which the processes differ in the two sexes 
concerns the cytosome. In the female the divisions are very unequal, 
so that from each original cell there are produced not four functional 
cells as in the male but only one functional cell and two or three degener- 
ate ones. 



The Eggs. — The eggs of animals are typically spherical or nearly so. 
Often, however, one diameter is much greater than the others, or the 
egg may be elongated and curved, as in many insects. Internally the 
substance of the egg is in some way differentiated so that opposite 
sides or poles do different things. One 
side is known as the animal pole, the 
opposite side as the vegetative pole. 

The food, or yolk, stored in an egg may 
be very meager and is in such instances 
rather uniformly distributed through the ^ 
protoplasm. Sea urchins, marine worms 
(Fig. 169), and mammals have such eggs. 
In fishes, reptiles, and birds, and less 

Fig. 170. Fia. 171. 

Fig. 170. — Generalized egg of telolecithal type, a, animal pole; c, cytosome; m, 
second spindle in oogenesis; p, first polar body; s, spermatozoon; v, vegetative pole; y, yolk 
crowded toward vegetative pole. 

Fig. 171. — Centrolecithal egg of the fly Musca, in longitudinal section, cy, cytosome; 
em, egg membrane; m, micropyle; 7i, egg and spermatozoan nuclei; pb, three polar bodies; y, 
yolk. (From Korschelt and Heider, after Henking and Blockmann. Courtesy of The Mac- 
■millnn Company.) 

strikingly so in frogs, the yolk is crowded toward the vegetative 
pole, so that most o*f the protoplasm is at the animal pole (Fig. 170). 
In insects the yolk is in the central part, with a principal layer of 
protoplasm at the surface (Fig. 171). Eggs with little yolk are said to be 
alecithal or, from the uniform distribution of the yolk, homolecithal. Eggs 
with much yolk aggregated toward the vegetative pole are telolecithal; 
those with the yolk in central position, the protoplasm in a surface 
layer, are centrolecithal. 


Eggs are very often enclosed in a membrane or shell, particularly 
among species that lay their eggs on land where evaporation must be 
retarded. These envelopes may be of a chitinous nature, as among 
insects, or composed of keratin which resembles chitin, or they may be 
impregnated with calcium salts. The shell of the egg of the domestic 
fowl is composed of three layers. The inner layer is composed of limy 
particles with conical faces pointing inward. These particles do not 
fit closely, and air may pass between them. Outside this layer is a com- 
pact sheet of calcareous strands which also permits the slow passage of 
gases. On the outer surface of the shell is a third layer, the cuticle, 
which appears to be structureless except that it is penetrated by pores. 
Within the shell is a membrane consisting of two layers of fibers crossing 
one another in various directions. The envelope as a whole is calculated 
to prevent excessive evaporation, and yet it permits the passage of gases 
necessary for the respiration of the egg and embryo. Indeed, air begins 
to penetrate the shell soon after the egg is laid and accumulates in a space 
between the two layers of the membrane within the shell at the large end 
of the egg. 

Time and Mechanism of Fertilization. — Eggs and spermatozoa are 
brought together in fertilization by breeding behavior or some sort of 
affinity, as described in the preceding chapters. The time of their 
union, particularly in relation to the stage of oogenesis, is very variable. 

In Ascaris megalocephala, a roundworm parasitic in the intestine of 
the horse, the spermatozoon enters the oocyte about the time of the 
formation of the spindle of the first division. It remains in the oocyte 
during the succeeding divisions. In the frog, rabbit, and some others 
the spermatozoon enters after the first polocyte is formed but before 
the second. In the sea urchin the spermatozoon does not enter until 
after both divisions. 

In eggs having a shell at the time of fertilization, there is an opening 
through which the spermatozoon enters (Fig. 171m). In naked eggs,, 
the spermatozoon may enter anywhere. Usually only one male cell 
penetrates an egg. Some change of a chemical or physical nature takes 
place in the protoplasm of the egg when a spermatozoon unites with it, 
such that no other spermatozoa can be drawn in. When by accident 
two or more spermatozoa gain entrance at the same time, al')normalities 
of development are likely to occur. However, in some animals numei'ous 
spermatozoa regularly enter the egg; but the imcleus of only one of them 
unites with the egg nucleus. 

Cleavage. — Shortly after fertilization, within a time measured by 
minutes or hours in most animals, the fertilized egg begins to divide. 
This division, which is repeated in rapid succession until the egg is con- 
verted into many cells, is called cleavage or segmentation. In the follow- 



ing account of cleavage the egg may be likened to the earth with its two 
poles, so that a plane passing through the animal and vegetative poles 
may be spoken of as meridional, other planes aii equatorial or parallel 
to the plane of the equator. 

In alecithal eggs the early cleavage is very regular (Fig. 172, above). 
The first cleavage plane is meridional, passing through both animal and 
vegetative poles and dividing the egg into two approximately equal cells. 










Fig. 172. — Cleavage of eggs, in relation to the amount and distribution of the yolk in them. 

The second cleavage is also meridional and perpendicular to the first 
plane; four cells are thereby produced. The third cleavage is nearly 
equatorial, resulting in eight cells. 

After the third cleavage there are two or more cleavage planes at the 
same time. The fourth cleavage passes through two planes, both of 
them meridional, and perpendicular to one another. The 16 cells thus 
formed then divide into 32, and so on. Up to the 32-cell stage, in such 
an egg, the divisions usually take place at the same time in all the cells; 
but irregularities occur later, and some cells divide earlier and more 
rapidly than others. By this cleavage the single cell (fertilized egg) is 
converted into hundreds of cells forming a nearly spherical mass, with a 



liquid-filled cavity in the interior. The whole embryo is now designated 
a hlastula, the cavity within it the hlastocoele. 

In telolecithal eggs, cleavage is considerably modified. In general, 
the third cleavage is elevated toward the animal pole, so that the upper 
quartet of cells is smaller than the lower. Also the divisions occur 
earlier and require less time near the animal pole than at the vegetative 
pole, with the consequence that the smallness of the upper cells is accen- 
tuated. In some way connected with this difference between the poles, 
the blastocoele is eccentric in position, nearer the animal pole. All 
these features are sho^vn in the frog cleavage (Fig. 172, second row). 

In fishes, reptiles, and birds there is so little protoplasm in the yolk- 
laden vegetative part of the egg that no cleavage occurs there at any stage. 
Only the cap of protoplasm above the yolk segments and the blastocoele 
is bounded by a layer of cells above and by undivided yolk below (Fig. 
172, third row). In the bird egg in the figure the animal pole is in the 
center of the first three illustrations, but at the top in the fourth. 

Fig. 173. — Cleavage in arrowworm Sagitta, showing x body (x), which identifies the germ 


In insects, cleavage is limited to the surface of the egg, where most 
of its protoplasm is located. A layer of cells is formed there (Fig. 172, 
below), while in the interior of the egg is undivided yolk. There is no 
hollow interior corresponding to the blastocoele at this stage in the insect 

First Differentiation during Cleavage. — Later stages of embryonic 
development are replete with diffenniiiations of cells. Far in ad\'an(;e 
of them is a most important differentiation, that between sterile cells 
which go to form the body {somatic cells) and those which retain their 
reproductive powers and give rise to the germ cells. In some animals 
this distinction arises during cleavage, ev(Mi in Acry early cleavage. 

In the arrowworm Sagitta the egg contains a small object, the x body, 
which in the first six divisions goes undivided into one of the cells (Fig. 
173). Thus in the 64-cell stage only 1 coll contains an x body. This is 
the forerunner of all the germ cells, the other 63 are somatic cells. After 
the sixth cleavage, the x body divides at each cell division, and every 
germ cell contains it. 



In Ascaris megalocephala (page 200) the first distinguishing mark of 
somatic cells is their early division. In the second cleavage of the 
fertilized egg, one cell divides earlier than the other. Thus in Fig. 
174A, B the left cell is ahead of the right in division, and it gives rise in 
later cleavages only to somatic (sterile) cells. The cell which lags 
behind gives rise to both somatic and germ cells. As the 4 cells derived 
from this cleavage begin to divide to produce 8 cells, a second mark of 
somatic cells becomes evident (Fig. 174C). The middle portion of each 
of their chromosomes breaks up into many small pieces, which continue 

c D. 

Fig. 174. — Cleavage of the fertilized egg in Ascaris megalocephala, showing distinction 
of somatic and germ cells. A, second cleavage, in which cell on left, in more advanced 
stage of division, is somatic; B, later stage of second cleavage, with cells in same relative 
positions and same relative states of advancement; C, third cleavage, with chromosomes of 
three cells (somatic) fragmenting and losing their ends, those of the fourth remaining 
intact; Z), fourth cleavage, with chromosomes fragmented in six cells (one hidden), becom- 
ing fragmented in one (middle right), and remaining intact in one (upper right). All germ 
cells are descended from the last-named cell. (Schematized from account by Fogg in Journal 
of Morphology and Physiology.) 

as chromosomes, while the ends of the original long chromosomes are 
thrown off into the surrounding protoplasm where they degenerate. 
Three of the 4 cells lose chromatin in this way, and all these give rise 
later only to somatic cells, while the one which retains its chromosomes 
intact (upper right in C) produces both germ and somatic cells. In 
each of the next two cleavages, in one of the cells that had retained 
whole chromosomes, these chromosomes break up into small fragments 
and lose their ends in the cytosome (D). Thus at the 32-cell stage 
there is only 1 cell with long chromosomes like those of the fertilized egg. 
In subsequent divisions of this cell there is no further loss of chromatin, 
and all its descendants become germ cells. The other 31 cells have 
fragmented chromosomes, and all their descendants are somatic cells. 



In insects the germ cells usually either are larger (Fig. 175) or contain 
certain granules not found in somatic cells. In vertebrate animals the 
distinction between somatic and germ cells is not recognizable until a 
much later stage. In the embryos of a number of forms the germ cells 
are found as large cells in the lining of the digestive tract (Fig. 176), 
whence they migrate up through the mesentery and out to the place 

where the gonads subsequently develop. 
Whether germ and somatic cells have 
existed as distinct entities through the 
earlier embryonic stages is not known. 

Gastrulation. — When the blastula is 
well formed, it is converted into a two- 
layered embryo. The process by which 
this conversion is effected, already briefly 
outlined in C'hap. G, is called gastrulation. 
The simplest form of invagination takes 
place in those animals whose eggs have a 
small amount of yolk evenly distributed, 
that is, in alecithal or homolecithal eggs. 
In these the vegetative side of the 
blastula becomes flattened, then in- 
turned (invaginated) (Fig. 177, above). 
The invagination proceeds until the 
inturned cells are in contact with the 
opposite side of the blastula wall. The 
embryo now has two layers of cells, an 
outer or ectoderm and an inner or etido- 
dcrm. The blastocoele has been obliter- 
ated, but a new cavity, the archcntcron, 
lies within the endoderm. This cavity 
has been pushed in from the outside, 
with which it is still connected by a 
small opening called the blastopore. The two-layered embryo of this 
stage is known as a gastrula. The endoderm of the gastrula becomes 
the lining layer of cells of the digestive tract of the adult. 

The blastula produced from a mildly telolecithal egg could not well 
be invaginated directly from the vegetative side by flattening and 
infolding, because the layer of cells there is so thick. In such a blastula 
the invagination begins about midway between the ar\imal and vegetative 
poles, where the cell layei- is thinner (Fig. 177, below). It is mostly the 
cells above the blastopore which are invaginated, though there is some 
withdrawal of the whole yolk-laden mass of lower cells into the interior. 
The end result is, as in the alecithal embryo, a two-layered gastrula. 

Fig. 175. — Early recognition of 
germ cells (gc) in the development of 
the egg of the fly Miastor, showing 
also the cleavage cells (cl) at the 
periphery and the yolk globules (y). 
(After Hegner in Journal of Alor- 


In the frog, whose gastrulation is of this type, the invagination appears 
from the exterior as in Fig. 178. The cells are inturned along a short 
crescent-shaped line, which becomes extended into a marked U, and 
finally completes a circle which diminishes in size to a mere pore as the 
yolk-filled cells are withdrawn inside. 

Gastrulation in strongly telolecithal embryos, like those of birds, 
reptiles and most fishes, is so modified as to require an interpretation 
o[ events too difficult for presentation here. In insects there is an 
infolding which is usually called gastrulation, but the tissue turned in 



Fig. 176. — Earliest recognition of the distinction between somatic and germ cells in a 
vertebrate animal. Diagram of cross section of the body of the embryo, showing germ cells 
in the endoderm of the intestine and their path of migration (shown by arrows) to the site 
of the reproductive organs. How much earlier than this stage the somatic cells have lost 
their reproductive powers is not known, c, coelom; en, endoderm of intestine; gc, germ 
cells; ge, germinal epithelium which later covers the gonads and from which the germ cells 
issue; i, intestine; m, myotome, or muscle segment; ms, mesentery; nc, neural crest, from 
which nerves and ganglia develop; nd, notochord, forerunner of the backbone; s, spinal cord. 

becomes not just the lining of the digestive tract but the whole internal 
structure of the body. These two types are omitted from the com- 
parisons in Fig. 177. 

Mesoderm Formation. — At the end of gastrulation at least two 
layers of cells, ectoderm and endoderm, are present. In most multi- 
cellular animals a third layer, the mesoderm, if not already present is 
soon formed between these two. In the fishlike amphioxus, a classical 
form in biology, the upper portions of the endoderm (Fig. 179) are 
turned outward in the form of grooves, shown dotted in cross section 
in the illustration {A). The edges of each groove meet and fuse, and 



the groove now in the form of a tube is completely separated from the 
endoderm {B). The two tubes thus formed are the mesoderm, and the 
slender openings in them constitute the body cavity, or coelom. In later 
stages of development the tubes expand, as in C, shown black. One 
side becomes a thin layer of cells applied to the digestive tract, while 
the other side lines the inside of the ectoderm. 







Fig. 177. — Gastrulation of embryos, in relation to the quantity and distribution of yolk in 


In the frog the mesoderm is formed simultaneously with the endo- 
derm — indeed, almost before endoderm — <iuring gastrulation. The cells 
which turn in over the dorsal rim of the curved blastopore, in its middle 
portion, form the mesoderm directly (Fig. 180). A band of these cells 
migrates forward from the blastopore, above the archenteron, to the 
front end of the future embryo. In late stages of gastrulation, cells 



Fig. 178. — Gastrulation in, frog, external view. 

invaginated at the lower margin of the (now circular) blastopore also 
contribute to the mesoderm. The endoderm below the main sheet of 
mesoderm is in the form of an open trough. The upper rims of this 
trough (a) were originally continuous with the edges of the mesoderm, 
but they break loose and curve up under the mesoderm. They meet 
at the top, to enclose a tube which becomes the intestinal tract. 

Subsequent Development of the Vertebrates. — The three layers of 
cells, ectoderm, endoderm, and mesoderm, are often called gerin layers. 
They are so designated because certain organs are normally derived 



from each one, so that the layers may be thought of ais containing the 
germs of those organs. They are not irrevocably destined to form these 
organs, for, as we shall see later, their fate may be experimentally altered 
in a variety of ways. 

From the ectoderm ordinarily arise the epidermis of the skin, reptilian 
(but not fish) scales, feathers, hair, nails and claws, the nervous system 
including nerves and their endings, and some glands which discharge at 
the surface. From the endoderm comes the lining of almost the whole 
digestive tract and of all the organs which branch off from it, such ai. 
the lungs, liver, and pancreas, and of the thyroid gland which, though 
wholly separate in the adult, is an out-pocketing of the digestive tract 


en — 

Fig. 179. — Mesoderm formation in the amphioxus, in cross section. A, evagination of 
ridges (dotted) at upper lateral regions of endoderm; B, these ridges pinched off as tubes; 
C, mesoderm (black) expanded so as almost to surround the digestive tract; c, coelom; dg, 
digestive tract; dm, dermatome; ec, ectoderm; en, endoderm; m, mesoderm; mc, myocoele; 
mp, mesodermal pouch; mt, myotome; n, neural plate; nd, notochord; nt, neural tube; soin, 
somatic layer of mesoderm; spl, splanchnic layer of mesoderm covering the digestive tract; 
vm, ventral mesentery. (A and B after Hatschek.) 

in the embryo. From the mesoderm are derived muscle, bone, connec 
tive tissue, blood vessels, and the thick inner layer of the skin. 

The development of the several organs from the ectoderm and endo- 
derm is in its early stages a bending or folding of these layers, which \6 
called invagination or evagination according as the sheets of cells are 
bent into, or out from, some enclosed space. The following account 
of their origin is limited to the vertebrate animals. 

The Early Embryo. — Several of the chief systems of organs are laid 
down at a very early time. One of the first changes visible externally is 
the appearance of two prominent ridges, close together, along the 
dorsal side of the future embryo. These extend lengthwise and are 
roughly parallel except at the anterior end where they diverge from 
one another (Fig. 181). In a cross section of the frog these ridges 
appear as in Fig. 182, nf. They are the neural folds, the beginning 
of the central nervous system. Where these folds are near one 
another, the spinal cord develops; the divergent folds in front form 



the brain. These ridges approach one another and fuse along their 
upper surfaces (Fig. 18 IB), cutting off a tube beneath the ectoderm. 
In longitudinal vertical section at this time, the nervous system appears 
as in Fig. 183. 

Fig. 180. — Mesoderm formation in frog. First three figures, median sections; last 
figure, cross section. The hne-shaded cells on the outside, as they turn in, become the 
mesoderm, a, edges of trough of archenteron. 

Beneath the nervous system a cylindrical rod of cells, the notochord, 
is formed out of the middle portion of the inturned mesoderm. Around 

it later is formed the backbone. 
The digestive tract has been pres- 
ent, as the archenteron, ever 
since gastrulation took place. At 
first it is usually enlarged in front 
and narrowed behind. These parts 
correspond roughly, in the frog, to 
the stomach and intestine. Poster- 
iorly the intestine opens to the 
outside through the anus, which in 
some animals is the same opening 
as the blastopore but in others a passage produced anew aftei- the 
blastoi)or(i has closed. 

As indicated earlier (page 20()) and in Fig. 182, the mesoderm is early 

Fig. 181. — Neural folds of frog em- 
bryo. A, folds still separate, brain above, 
spinal cord below; B, folds fused, produc- 
ing neuial tube beneath surface. 



divided into two layers, one applied to the inside of the ectoderm, the 
other covering the endoderm. The peritoneum, which occupies approxi- 
mately the corresponding positions in later stages, is derived from these 
layers. Above the endoderm, between it and the notochord, two layers 
of the mesoderm approach one another and form the mesentery (Fig. 176, 
page 205) which later suspends the digestive tract in a trough of peri- 
toneum. In the longitudinal section (Fig. 183) the mesoderm is not 
represented above the digestive tract, since the section passes exactly 
through the median plane. But below the intestine the mesoderm occurs, 
divided into its two characteristic layers. 

Fig. 182. — Cross section of the early embryo of a frog, diagrammatic, c, coelom; dig, 
digestive tract; ec, ectoderm; en, endoderm; ms, mesoderm; nc, neural crest; nd, notochord; 
nf, neural fold; ng, neural groove. 

Anterior Digestive Tract. — The gill pouches, represented as seen from 
above in Fig. 184, are evaginations of the endoderm in the sides of the 
pharynx, or anterior part of the gut. Typically there are five of these 
protrusions on each side, but some of them are often rudimentary, or 
two of them may be nearly combined, so that the number frequently 
appears to be less. Successive stages in the evagination of the gill 
pouches are shown in A, B, C. They finally reach the ectoderm, with 
which they fuse. In fishes and usually in amphibians the ectoderm 
and endoderm both break open at the point of fusion, so that the pharynx 
is open to the outside. These openings are the gill clefts. They serve 
as channels for the passage of water, which enters at the mouth (not 
shown in the figure since it is at a lower level). The course of the water 



is indicated in the figure by arrows. In the fishes and in at least the 
young stages of amphibians, gills (organs of respiration) are developed 
upon the tissue {gill bars) between the gill clefts. 

In the higher vertebrates the gill pouches do not open to the outside 
at all or do so only temporarily. They are to be regarded as to some 
extent vestigial organs, an inheritance of an ancestral condition in which 
functional gills were present. However, some of them are regularly 
converted during embryonic development into other functional or non- 


Fig. 183. — Longitudinal section of the early embryo of a frog, diagrammatic, an, anus; 
br, brain; c, coelom; ec, ectoderm; en, endoderm; int, intestine; li, liver; ms, mesoderm; 
nd, notochord; sp, spinal cord; st, stomach. 

functional organs. Thus the first pouch becomes part of the Eustachian 
tube and middle ear. Certain of the bars share in the production of the 
tonsils, the thymus, and the parathyroid glands. 

The mouth starts as an invagination of the ectoderm from the outside, 
as in Fig. 185m. For a time it is separated from the rest of the digestive 
system by a membrane composed of an outer layer of ectoderm and 
an inner layer of endoderm. This membrane later breaks, and part 
of the fore end of the gut is incorporated in the mouth cavity. That 
part of the mouth derived from the external invagination is of course 
lined with ectoderm. 

Outgrowths of the Digestive Tract. — The liver appears at an early 
stage as an evagination from the lower side of the intestine just behind the 
stomach. In the frog the liver is present shortly after the fusion of the 
neural folds (see Fig. 183, li). An early indication of the liver is also 
shown in Fig. 185, li. This pouch grows in extent and soon becomes 
branched. One branch at the posterior side of the liver forms the gall 
bladder (gb). The rest are bound together by mesodermal tissue which 
collects about them, forming part of the body of the liver. The undivided 



basal portion of the original pouch remains as the bile duct (bd), through 
which the secretions of the liver are conveyed into the intestine. During 
all this development the liver has been covered by the layer of peritoneum 
(mesodermal) which invests the entire digestive tract. The adult liver 
is thus covered by peritoneum and suspended by mesenteries formed from 
the same layers of mesoderm. 

The pancreas originates from two pouches evaginated from the intes- 
tine (Fig. 185). One arises from the dorsal side of the intestine nearly 
opposite the liver (dp) ; the other springs from the angle between the liver 

Fig. 184. — Diagrams showing the early development of some of the organs of verte- 
brate animals, as seen in section from above. The stages here shown are not contempora- 
neous in all cases. A, B, C, successive stages; au, auditory vesicle; br, brain; cr, crystalline 
lens; ec, ectoderm; en, endoderm; eu, Eustachian tube; gb, gill bar; gp, gill pouch; gs, gill 
slit or cleft; me, endodermal portion of mouth; olf, olfactory pit; opn, optic nerve; ops, optic 
stalk; ph, pharynx; ret, retina; sp, spinal cord; ty, tympanum or middle ear. Arrows in C 
denote current of water through mouth, pharynx, and gill slits. 

and the intestine (vp). The two pouches intertwine their branches to 
form the pancreas, which is likewise invested with peritoneum. 

The lungs take their origin from a protrusion from the ventral side 
of the gut some distance in front of the stomach (Fig. 185, Ig). This 
pouch is at first single (Fig. 186A), but soon divides into two parts 
(B, C, D). As these grow in size they become branched. The undivided 
stalk of the lung rudiment is the trachea, the two principal branches are 
the bronchi, and the finer divisions are the air passages and alveoli 
within the lungs. Mesoderm is constantly pushed before the growing 
lung rudiments, so that the adult lungs are invested with a peritoneum. 
Other mesodermal tissue is incorporated in the lungs among the air pas- 
sages, where blood vessels are abundantly developed. 

It should be borne in mind that Fig. 185 is diagrammatic and does 
not represent a condition prevailing at any one time in embryonic 
development. For the sake of compactness, organs have been shown 
in the same figure in stages which do not occur simultaneously. 



Nervous System. — It has already been pointed out, and shown in 
Figs. 182 and 183, that the early central nervous system is a tube formed 
by the fusion of two folds or ridges of the ectoderm. This tube is wide 
in the anterior region, where it 
forms the brain, and narrow pos- 
teriorly, where it produces the 
spinal cord. The thickening and 
folding of the walls of this tube, 

Fig. 185. — Diagram representing the development of .some of the organs of vertebrate 
animals, early and later stages. The figures are a trifle to one side of the median plane. 
The stages shown are not necessarily contemporaneous, an, anus; bd, bile duct; hr, brain; 
c, coelom; dp, dorsal rudiment of pancreas; gh, gall bladder; int, intestine; Ig, lung; li, liver; 
m, mouth; ms, mesoderm; nd, notochord; pc, pericardial chamber; rt, root of tongue; sp, 
spinal cord; st, stomach; t, tongue; vp, ventral rudiment of pancreas. 

especially in the formation of the lobes and cavities of the brain, are 
very complicated processes. 

The nerves extending from the spinal cord take their origin in part 
from the neural creds. These crests are masses of cells budded off from 
the inner surface of the ectoderm at or near the region of the neural folds, 
as indicated in the cross section of the frog (Fig. 182, nc) and in Fig. 17(5. 

As was pointed out in Chap. 13, the large nerves arising from the 
spinal cord are connected with the cord by two roots. The dorsal root is 
composed of afferent fibers and the ventral root of efferent fibers. The 
dorsal root is enlarged to form a ganglion. 

The dorsal ganglion is in each nerve developed from one of the neural 



crests. The dorsal root is completed bj'' processes of nerve cells growing 
inward from the neural crest and entering the dorsal part of the spinal 
cord, and by other processes growing outward from the same cells in the 
neural crest toward the periphery of the body, forming the afferent part of 
the spinal nerve. The ventral root fibers grow out from the ventral part 
of the spinal cord and join the fibers of the dorsal root at a point beyond 
the ganglion. The nerve fibers from these two roots remain distinct from 
one another but are enclosed in the same connective tissue coverings. 

Sense Organs. — The principal sense organs are developed either as 
outgrowths from the central nervous system or as ingrowths, chiefly from 
the ectoderm, which come secondarily into connection with the nervous 

Fig. 186. — Successive stages in the development of the lungs. The esophagus is 
shown in A, B, and C, but not in D. As the lungs grow the mesoderm is pushed before them 
and thus comes to invest the adult lungs and to make part of the lung tissue, br, bronchus; 
es, esophagus; I, lung; m, mesoderm; tr, trachea. 

system, or by a combination of these two modes of origin. The eye 
begins as an evagination from the side of the brain (Fig. 184^). This 
protrusion elongates and at the same time expands at its outer end into 
a hollow bulb. The bulblike expansion flattens on its outer side and is 
then invaginated to form a double-walled cup resembling a gastrula (Fig. 
18-1:5, C). The inner layer of this cup becomes the visual part of the 
retina, and the basal stalk on which the cup rests is the optic nerve. When 
the outgrowth from the brain comes near the ectoderm, the latter thickens 
and later invaginates, finally pinching off a rounded mass of cells {B, C). 
This mass becomes the crystalline lens of the eye. The ectoderm at the 
point where the lens was formed becomes transparent and with additions 
from the mesoderm in most vertebrates forms the cornea. The rest 
of the eye, including its muscles, is derived from the mesoderm. 

The ear begins its development in the surface ectoderm, not, as does 


the eye, from the central nervous system. A patch of ectoderm on each 
side of the head region thickens and then invaginates (Fig. 184A), 
producing a pear-shaped vesicle. The vesicle is pinched off from the 
ectoderm and comes to lie within. It changes its shape, producing the 
characteristic semicircular canals and the (sometimes) coiled body of 
the inner division of the ear. Nerve cells growing out from the ganglion 
of the eighth (auditory) nerve join the vesicle with the brain. 

The middle ear, which contains the bones of the ear, is derived at 
least in part from the first gill pouch (Fig. 184A, B, C). In the distal 
portion of this pouch the ear bones are developed out of mesoderm, while 
its connection with the digestive system, as already stated, forms the 
Eustachian tube which connects the middle ear with the pharynx. 

The olfactory organ, like the ear, is at first a patch of thickened ecto- 
derm on each side of the head far to the front. This ectoderm invaginates 
(Fig. 184, olf), but unlike the ear the pit thus formed does not close; it 
remains open to the outside as the nostril. The pit enlarges and protrudes 
inward to meet the front end of the digestive tract just behind the ecto- 
dermal part of the mouth. An opening is subsequently formed at this 
point of contact, and the nostril is thus connected with the deeper portion 
of the mouth cavity. Only certain parts of the ectoderm that forms the 
olfactory cavity become sensory. From these parts nerve processes 
grow toward the brain, thus forming the olfactory nerve. 

Metamorphosis. — Besides the usual course of development, which is 
in large measure the same for all vertebrate animals, some members of 
that group undergo an additional series of changes called metamorphosis. 
Animals that metamorphose are born or hatched with one or more 
organs which they will not possess as adults, or lacking organs that will 
be developed before they become adult. It is the process of losing the 
larval organs and of gaining the missing adult organs which is called 

The transformation of a tadpole into a frog (Fig. 187) or toad is the 
classical example. The readily visible changes are the degeneration 
of the so-called "sucker" or attaching organ beneath the head; the 
development of the legs; and the absorption of the tail, the material of 
which is probably used elsewhere for growth. The external gills, hidden 
under a fold of skin called the operculum, disappear early, to be replaced 
by internal gills which are developed on the endodermal lining of the gill 
slits. The internal gills are lost later, and their function served by lungs, 
which have all the while been developing. The jaws are provided with a 
horny armature, serving as teeth, but these are shed and the mouth 
increases greatly in size. The intestine, from the early tadpole stage a 
\(iiy long and much coiled tube, is greatly shortened. 

Other kinds of animals undergo metamorphosis, notably among the 



insects. In some kinds the changes are very small from stage to stage, 
as in the bugs (Fig. 188). These sucking insects shed their skins periodi- 
cally as they grow, and at each change they are a little more like the adult. 
At the very beginning, however, they are easily recognized as bugs. 
Such a series of changes is scarcely metamorphosis at all but is usually 

Fig. 187. — Metamorphosis of frog. 1-4, growth of gills; 5-6, covering of gills by 
operculum, degeneration of sucker; 7—10, growth of legs (9 shows greatly elongated intes- 
tine) ; 11, fore legs pushed through operculum; 12-14, degeneration of tail. {Rearranged 
from Newman, "Outlines of General Zoology,^' by perm.ission of The Macmillan Company.) 

called incomplete metamorphosis. Contrasted with this gradual trans- 
formation is the very marked one which flies, butterflies, bees, and beetles 
experience. In the flies (Fig. 189) the larva is a legless wormlike animal 
called a maggot. This changes, in a very brief operation, into a quiescent 
nonfeeding form, the pupa. After a definite (usually short) time there 
bursts from the pupa shell the adult insect. The development of the 



adult occurs gradually enough within the pupa, but the emergence of 
the fly is sudden. These marked and more sudden modifications make up 
complete metamorphosis. 

Fig. 188. — Incomplete metamorphosis of a bug. The most easily recognized change is the 
gradual growth of the wings. {From Frost, "General Entomology.") 

Problems of Development. — The question naturally arises, how are all 
these complicated developmental changes brought about? This is the 
general problem which experimental embryologists have set themselves. 
Some progress in solving it has been made, but much remains to be done. 
The knowledge already gained has to do with such questions as why the 
embryo is placed in a given position in the seemingly indifferent material 
of the fertilized egg; how a structure is stimulated to develop, and how it 
is guided so as to acquire its characteristic form; the time at which the fate 
of any bit of tissue is settled, and whether the decision at that time is 
final or revocable; whether development is a sorting out and the loss of 
capacities, or a gain of new ones; the importance of the mere position of a 

Fig. 189. 

-Complete metamorphosis of a fly. The successive stages shown are the larva, 
pupa and adult. {From Frost, "General Entortiolofjy.") 

piece of tissue in the embryo, in deciding what it shall become; whether 
a tissue is passively moved about l)y some force, or actively assists in the 
change; whether the agencies which direct development reside within 
the colls, or impose their control from the outside; and the duration of any 
influence in relation to the period within which it can normally be efTec- 


tive. Some of these topics will be considered in connection with concrete 

Orientation of Embryo. — The higher animals are all bilaterally 
symmetrical; yet they all develop from an egg which is apparently radial. 
The animal pole is differentiated from the vegetative, and it is clear why 
development should commence in the animal portion. But so far as can 
be seen in an unfertilized egg, the head of the embryo might be turned 
toward any point in the circumference of the circle of which the animal 
pole is the center. What decides the position which it 
actually does take? 

In the frog, the median plane of the future animal 
is fixed by the point of entrance of the spermatozoon 
in fertilization (Fig. 190). The first cleavage of the 
egg passes through that point and also thi'ough the 
animal pole and vegetative pole. Up to the time of Fig^~790 Sec- 

fertilization, any plane passing through the two poles tion through frog's 
may become the plane of symmetry. In some of the |^J^ cleavag!J!Thow- 
salamanders either the first or the second cleavage ing at right the 
plane may become the median plane, and the entrance T!^ ^ ^ J'o + ^^^-.T!!^ 
of the spermatozoon has nothing to do with fixing the (Modified from 
positions of these planes. In fishes, sea urchins, '^ " ^^ 
and some other animals there is no connection between the early 
cleavages and later symmetry, and in them it is unknown how the median 
plane is determined. 

After the position of the embryo is fixed, all later questions of orienta- 
tion are settled in relation to it. When, by artificial methods, a second 
embryo is made to develop at the surface of the same egg, it is roughly 
parallel with the first, with its head pointing in the same direction. A 
patch of ectoderm in which gills would normally develop at its anterior 
margin may be cut out, turned halfway around, and made to grow in 
place. The gills still grow in the anterior portion, but this was originally 
the posterior part. Also, if the regenerating stumps of cutoff arm and leg 
rudiments be removed and their positions exchanged in transplantation, 
the anterior one becomes an arm, the posterior one a leg, which is the 
reverse of their normal fate. 

Some biologists have suggested that a gradient of some sort is set 
up at the first orientation of the embryo. Perhaps a chemical substance 
occurs in gradually less and less concentration from front to back, or a 
physical phenomenon becomes less and intense in that direction, and 
the position of structures is governed by this gradient. Little is known, 
however, that would establish this supposition. 

Principle of Determination. — Another important question is why dif- 
ferent parts of an embryo produce different structures. In the majority 


of animals there is no fundamental difference between the cells of different 
regions. For example, the cells of a sea urchin embryo, in the two- or 
four- or eight-cell stage, may be separated from one another, and each 
becomes a complete, though small, larva. If left in contact with the 
other cells, each cell would have produced only certain parts of a single 
animal, but it obviously has the capacity to produce all of it. In a few 
animals, however, the cells are in some respect different, for, if the cleav- 
age cells are separated, each one gives rise only to a fraction of a larva. 
Animals of the former type are said to have indetermininate development, 
the latter kind determinate development. 

The cells of indeterminate embryos take on their specific destinies 
at a mut;h later time. This has been most completely shown for some 
of the salamanders. If, at a time shortly after gastrulation begins, bits 
of tissue are transposed, a group of cells that would normally become 
nervous system exchanging places with a group that would become epi- 
dermis, the fate of each is altered. The would-be part of a nervous 
system becomes epidermis, the prospective epidermis becomes nervous 
system. The exchange of regenerating stumps of fore- and hind limbs, 
described in the preceding section, is a similar example. The interchange 
of bits of tissue may be made between different species ^vith equal success. 
One such interspecific exchange was effected between species differing 
in color, one very light-colored, the other quite dark. The cells retained 
their color characters but produced strange organs. In one experiment, 
presumptive brain cells of a dark species were transplanted to the region 
on a light species where gills develop. Now these species differ not only 
in color; their gills are of different shapes. The transplanted dark cells, 
while being converted into gills instead of brain, produced gills of the 
form characteristic of the dark species. The general fate of the cells 
may be altered, but their specific performance within the general field 
remains unchanged. 

In all these examples the fate of the transplanted tissues had not 
yet been determined. For each of them, however, there comes a time 
after which such reversals of fate are no longer possible. After that time, 
transplanted parts become what they would have become in their original 
situation. If, for example, a patch of ectoderm including a portion of the 
neural folds (a stage shortly after the end of gastrulation) is placed on the 
side of the body, it Ijecomes nervous system despite its strange location. 
Something has hapi)ened to these cells during the process of gastrulation 
wliich has deprived them of the capacity to respond to their position in 
the embryo and has fixed their fate regardless of location. An area of 
such determined ectoderm may oven be cut out of the embryo and culti- 
vated by itself in a suitable salt solution, and it still develops the sort of 
organ (nervous system, for example) which it was destined to become. 



Organizers. — What induces this change in a tissue, destroying its 
apparent independence of action, and forcing it into a single further 
course? It is often some influence coming from other cells near it. In 
salamander embryos, the cells which roll over the dorsal lip of the blasto- 
pore and become the notochord and mesoderm (Fig. 180) exert such an 
influence. It is because of them that neural folds are produced in the 
ectoderm above the notochord. The mesoderm cells possess that power 
of inducing nervous system even before they are invaginated into the 
gastrula. This is beautifully shown by an experiment. If some cells 
are removed from the dorsal rim of the blastopore, before they are invagi- 
nated, and are inserted among the ectoderm cells of another embryo, at a 
place where only epidermis w'ould ordi- 
narily develop, they sink below the surface 
and are covered over by the ectoderm. 
From that ectoderm an additional nervous 
system is formed, so that the embr^^o has 
two nervous systems (Fig. 191). The 
transplanted cells would, in their own 
embryo, have been invaginated to form 
mesoderm and would have induced a 
nervous system in the ectoderm above 
them. That same influence they exerted 
on the strange ectoderm beneath which 
they were planted. 

In a similar way, the eye stalk pro- 
truding from the side of the brain (Fig. 
184), as it approaches the outer ectoderm, 
stimulates that layer to thicken and 
invaginate to form the crystalline lens of 
the eye. In some animals the ectoderm 
forms a sort of lens without such stimulus, as when the eye stalk is cut 
off; but the lens is seldom normal unless the optic stalk comes near it. 

Something issues from the prospective mesoderm and the eye stalk, 
in the above examples, which causes the ectoderm to develop a certain 
structure. This something, whatever it is, has been called an organizer. 

An important question arises, whether embryonic development is 
conducted by a series of such organizers, produced in succession in dif- 
ferent structures. May one organizer ensure the development of a cer- 
tain organ, and then a different organizer arise in that organ that would 
stimulate a third organ, and so on? Some slight indications of such 
chains may be found, but they are not general. The eye stalk often 
stimulates a lens, and the lens then helps to bring about the invagination 
of the optic cup to form the retina. A few other such chains of influences 

Fig 191. — Development of 
nervous system in response to trans- 
planted cells. Left, neural fold of 
salamander, Triton, developing in 
its normal situation. Right, op- 
posite side of same embryo, witli 
additional neural fold produced 
because cells from the dorsal lip of 
the blastopore of another embrjo 
were transplanted in that region. 
The transplanted cells were from a 
lighter colored species and form the 
pale streak in the middle. {Modi- 
fied from Spemann.) 


are known. In general, however, the events of early embryonic develop- 
ment appear to be more or less independent, though working in harmony. 
Probably they are helped to keep in the proper order of time by organizers 
that successively arise. 

Nature of Organizers. — These organizers are not specific, not effec- 
tive merely in their own species, since transplants between species show 
about the same consequences as those within species. This fact has 
encouraged a search for the nature of such influences, for the same ones 
must be fairly general and widespread. Almost certainly organizers are 
chemical substances. A number of organic acids have been shown to 
induce certain differentiations. Among them are several of the fatty 
acids, nucleic acid, and adenylic acid from muscle. There is some 
indication that the sterols (higher alcohols) have inductive powers. 
Glycogen is probably in some way connected with the power of induction 
in salamanders, for while the cells are being rolled over the rim of the 
blastopore during gastrulation (which is about the time at which these 
cells first acquire the power to induce nervous system), they rapidly 
lose their glycogen. What an organizer does to stimulate development, 
what happens between the stimulus and the response in differentiation, 
is unknown. 

The power of an organizer to induce a certain event usually lasts much 
longer than there is any need of it in ordinary development. Thus, 
notochord and mesoderm, taken from embryos in which nervous systems 
have long since been irrevocably established above them, are still capable 
of stimulating secondary nervous systems in younger embryos into which 
they are transplanted. The power of the tissues to respond to organizers 
is, however, not so persistent. Usually they must be stimulated at about 
a certain time, or they cannot respond at all. 

In general, it may be said that the inherent properties of the tissues 
to respond by developing arc more important than the stimuli received 
from organizers. 


Bailey, F. R., and A. M. Miller. Textbook of Embryology. William Wood & 

Company. (Chaps. I-VI.) 
Hegner, 11. W. The Germ Cell Cycle in Animals. The Macniillan Company. 

(Chaps. I and II.) 
Holmes, S. J. The Biology of the Frog. The Macmillan Conii)aiiy. (Chap. V.) 
Kellicott, W. E. a Textbook of General Embryology. Henry Holt & Company, 

Inc. (('haps. VI-VIII, cleavage to formation of germ layers.) 
Morgan, T. H. The Development of the Frog's Egg. The Macmillan Company. 

(Chap. V, early development.) 
Morgan, T. H. Experimental Embryology. Columbia University Press. (Chap. 

XV, fate decided before cleavage; ("laps. XVII and XVIII, partial embryos; 

Chap. XIX, fate determined by position.) 


Prentiss, C. W., and L. B. Arey. A Laboratory Manual and Textbook of Embry- 
ology. W. B. Saunders Company. (Chaps. I and II.) 

Spemann, H. Embryonic Development and Induction. Yale University Press. 

WiEMAN, H. L. An Introduction to Vertebrate Embryology. McGraw-Hill Book 
Company, Inc. (Chaps. Ill and IV, early development of the amphioxus, frog, 
and chick.) 


The object of the study of embryology, as outHned in the preceding 
chapter, is to discover how animals become what they are. Mere obser- 
vation shows by what steps the development proceeds, and experiments 
have revealed some of the physiological principles underlying these 
events. Embryology discloses these things satisfactorily for the major 
features of structure which are essentially alike in whole large groups 
of animals — satisfactorily, that is, if one does not require to know the 
fundamental causes of the different types and steps of development. 

There are, however, many minor features of organization which are 
just as definitely fixed parts of animals as their digestive and nervous 
systems are, but which are different in different individuals. Color of 
eye, shape of hair, dimples, stature, complexion, and talents are different 
in different people, yet all through their embryonic development it is 
quite settled what these characters are going to be. No embryologist 
could tell what the outcome would be in any of these traits in the adult, 
but the die would have been cast before cleavage of the egg had begun. 

These individual differences furnish another way of learning the rules 
governing the development of characteristics. This method consists 
of crossing individuals having different traits and observing the occur- 
rence of these different traits among the descendants. This is the method 
of genetics. It would be impossible to use it to discover much about 
the structures with which embryology deals, for there is no difference 
between individuals ^vith respect to the major features. Nevertheless 
when the mode of inheritance of minor characters has been discovered, 
it may be taken as certain that the inheritance of the major features 
follows the same scheme. Genetics uses minor features to discover the 
principles of heredity, with the conviction that the same principles apply 
to the major features as well. 

Genetics has the further advantage, in the study of origins, that it 
reveals more fundamental causes. While embryology, when it employs 
experiment, may reveal physiological processes (;ausing the developmental 
changes, genetics lays bare to some degree the causes of the physiological 
processes. It is today one of the biologist's most })otent tools in delving 
into the fundamental nature of living things. Embryology is an aid 
because it reveals some of the visible mechanism of heredity, particularly 



in the maturation of the germ cells, but the crossing of unlike individuals 
demonstrates the nature of much that is invisible. 

Modem Genetics. — The story of the Austrian monk Gregor Mendel 
as the leading figure in the beginnings of modern genetics has been 
recounted in the opening chapter (page 18). Before going into the 
details of hereditary transmission, it will be profitable to indicate briefly 
wherein his ideas of heredity differed from those which preceded him; 
for it must be remembered that Mendel was not the first student of 
heredity. Many before him had tried to solve its mysteries, and the 
mere fact of resemblance between parents and offspring, or even between 
more distant relatives, had been recognized from time immemorial. 

One of the chief distinctions of the Mendelian system was the recogni- 
tion that offspring do not necessarily inherit any particular character 
of either parent. Not only do the offspring not have to show such a 
character in themselves, they may even be quite incapable of transmitting 
it to subsequent generations. Prior to Mendel's time there had been a 
prevalent suspicion that any character which appeared in one or more 
individuals in a given line of descent might be expected at some future 
time to appear in any branch of their posterity. No one of the descend- 
ants was to be regarded as free from the possibility of that character's 
recurrence. According to this old notion, if in a given line of descent 
of horses there had once been a chestnut animal, there was a distinct 
expectation that some time or other the chestnut character would 
reappear in some individual of any branch of the descending family. 
According to the IMendelian scheme, it is now clear that this color may 
be bred entirely out of the descendants. It is almost certain to be 
bred out of some branches of the general relationship and may be lost 
to all of them; and chestnut is no more likely to occur after such elimina- 
tion than it is in a line which never had a chestnut ancestor. Later 
we shall see why this is true. 

Another distinctive feature of Mendel's contribution to knowledge 
of heredity was his discovery that characters may be transmitted quite 
independently of one another. Wing length is one character, eye color 
another, body color a third, and so on, each having its own inheritance. 
Because of their separateness, such characters have been spoken of as 
"unit" characters. Some degree of detachment of traits was, of course, 
popularly implied when it was pointed out that a child had its mother's 
eyes, its father's lips, and perhaps its grandfather's wavy hair. But 
complete scattering of one individual's characters in succeeding genera- 
tions was not previously thought to take place — certainly not as a regular 
occurrence. Before Mendel's time there was a strong tendency to think 
of heredity in terms of the totality of characters exhibited by an indi- 
vidual; by Mendel himself emphasis was put upon the single characters. 


Heredity, Mendel concluded, juggles characters, not individuals; it 
deals with traits, not ancestors and descendants. The complete inde- 
pendence which he supposed characters to have is illustrated by the peas 
which he studied. He found that shape of pod, color of seed, height of 
stem, etc., were entirely free to go to the various offspring without refer- 
ence to the other characters. Thus there arose different combinations 
of the characters in different plants. One would have constricted pods, 
green seeds, and tall stems; another inflated pods, green seeds, and dwarf 
stems; a third constricted pods, yellow seeds, and dwarf stems; and so on. 
This freedom of assortment proved later, in heredity in general, to be 
less than Mendel supposed, but it is very ^\idespread. 

Mechanism of Heredity. — How heredity operates will be more 
easily understood if its mechanism is known. Inherited characters are 
represented in the cells of an organism by minute bodies called genes. 
These genes are located in the chromosomes and are demonstrated in 
some animals and plants to be in a row, from one end of the chromosome 
to the other. There are two genes representing each character in each 
cell, one of them derived from the mother, the other from the father. 
These two genes must, from their source, be in two different chromo- 
somes, one of which has come from the individual's mother, the other 
from its father. The genes in one of these chromosomes all relate to 
the same characters as do the genes in the other chromosome. Two of 
the chromosomes in the cells of the vinegar fly Drosophila are diagram- 
matically sho^vn in Fig. 192. Two chromosomes having corresponding 
genes, as these do, are said to be homologous with one another (see 
page 252 for homology). The genes in them are likemse homologous; 
the gene for yellow body is homologous with the gene for gray body, 
white eye with red eye, complete eye with bar eye, and so on. 

All the chromosomes in a cell are members of such homologous pairs. 
One chromosome of each pair has come from the mother, the other from 
the father. The two homologous chromosomes come together in a 
pair in the oocytes and spermatocytes early in the maturation of the 
germ cells, as in Fig. 164. In the reduction division they are separated 
again, one going to each of the cells produced by that division. Since 
the genes are in the chromosomes, the two homologous genes of every 
pair part company at the reduction division, one gene going to each of 
the cells produced. At the end of maturation in the male, each sper- 
matozoon contains one gene of every pair, never both of any of them. 
In the female, each mature egg contains one gene of every kind, ncncr 
both. Polar bodies receive their share of the genes, but these genes are 
lost as the polar bodies degenerate. 

As a result of the reduction division, therefore, the mature germ cells 
have a single set of genes, one of every kind. Body cells, on the con- 



trary, have a double set, two genes for each character. When egg and 
spermatozoon unite in fertihzation, the zygote receives a double set of 
genes, and these are handed on as a double set to all the cells of the 
body of the individual produced from that zygote. With this under- 
standing of the mechanism of heredity we may now turn to some con- 
crete examples of its operation. 

Yell 01^ Body 
Nhife £ye 

I Oroi/ Body 
Red Eye 



Verm f a on 






Rud I men- 
fa r^l^in^ 


Ban £y6 

Fig. 192. — Diagrams of two homologous chromosomes of the vinegar fly Drosophila. 
Some of the genes are represented, and are in their proper order through the length of the 
chromosome. Homologous genes are located at the same level in the two chromosomes. 

Simple Inheritance. — Among guinea pigs there are different color 
varieties which breed true so long as animals of the same color are mated 
Avdth one another. One of these true-breeding strains is black (Fig. 193), 
another one is albino or wdiite, from the absence of all of the ordinary 
pigments in skin and hair and the iris of the eyes. If a black animal is 
mated with a white one, the offspring are all black. This result is 
described by saying that black is dominant, white recessive. The hybrid 



generation is known as the Fi generation (abbreviated from the words 
first filial). The white coat is not lost in the Fi animals, however, for 
when they are mated together they produce an F2 (second filial) gener- 
ation consisting of some blacks and some whites. In a large collection 

Fig. 193. — Black and white guinea pigs, with smooth coats. (Courtesy ofProfessor W. E. 

Castle and the Harvard University Press.) 

of such F2 families the black animals are found to make up about three- 
fourths of the total number, the wliites about one-fourth. 

These results are explained by the diagram in Fig. 194, where the 
genes involved are symbolized by letters — the white gene by w, the 
black gene by W. The two letters under each parent are its genetic 

formula, the single letter under these 





Black X 



F, V/w (Block) 

w>=^— ^ w 


the formula of the germ cells of that 
parent. The Fi generation has the 
formula Ww, and the animals are 
black because one gene W is capable 
of producing black pigment just as 
well as two IF's are. That ability 
of one gene to do the work of two is 
what is called dominance; W has 
that ability, w does not. 

When the reduction division 
occurs in Fi animals, two kinds of 
germ cells are produced because the 
two genes are different. Some eggs 
and spermatozoa contain W, some 
contain w; and the numbers of the 
two kinds are about equal. When two kinds of eggs, equally numerous, 
are fertilized at random by two kinds of spermatozoa, equally numerous, 
four combinations result, also about equally numerous. These combi- 
nations are WW, Ww, wW, and ww, as shown in the F2 line of the figure. 
The first three of these are black, the last one white — hence the 3:1 
ratio of blacks and whites. The two middle formulas are identical, and 

MWW /4Ww MwW A WW 

Black Black Black White 

Fig. 194. — Inheritance of black and 
white color in guinea pigs: W, gene for 
black; w, gene for white. 


would ordinarily be written the same way; they are written in opposite 
orders here merely to show that the gene which came from the egg in 
one came from the spermatozoon in the other. 

Choice of the letters W and w to represent this particular pair of 
genes is in accord with a generally accepted convention that the name 
of the newer character should suggest the symbol. Without much doubt 
there were colored guinea pigs before there were white ones, hence white 
is the newer color. In accord "with another convention the small letter 
is used for the recessive gene, the capital for the dominant. 

To describe other types of matings, it is desirable to provide names 
for certain of the genetically different types of individuals. An organism 
whose two genes for any particular character are alike (TFTl^ or ww) is 
called a homozygote; one whose genes are different (Ww) is a heterozygote. 
The same animal may be, and usually is, homozygous for some genes, 
heterozygous for others. 

F. Black X White Fj Black x Black 

Ww WW 

Eggs _3li;ir= W Sperm Eggs 



)iWw /iww 
Black White 


WW /^Ww 
Black Black 

Fig. 195 Fig. 196 

Fig. 195. — Backcross of a heterozygous black guinea pig with a white animal. 
Fig. 196. — Backcross of a heterozygous black guinea pig with a homozygous black 

Backcross. — Not always do e.xperiments proceed from an Fi gener- 
ation to an F2. A very useful kind of cross is that between an Fi animal 
and another like one of its parents. Such a cross is a backcross. Essen- 
tially it is a mating of a heterozygote with a homozygote. Such a cross 
might well be made between a heterozygous Fi black guinea pig and a 
white one exactly like the white parent. Figure 195 shows what happens 
when that is done. The heterozj^gous parent produces two kinds of eggs, 
in equal numbers, the white parent only one kind of spermatozoon (w). 
Consequently there are two kinds of offspring, heterozygous black (Ww) 
and white (ww) in equal numbers. 

The backcross may also be made betw^een an Fi and the black parental 
type, as in Fig. 196. There are two kinds of offspring as before, with 
respect to their formulas; but they all look alike (black). The difference 
between these two backcrosses is that one was made to the recessive 
parental type, the other to the dominant type. The former cross is 



made often, the latter seldom because its two kinds of offspring cannot 
be distinguished. 

Two Pairs of Characters. — Since every animal possesses probably 
thousands of different kinds of genes, any mating between individuals 
serves as a test of the mode of inheritance of any or all characters in 
which the two individuals differ. The experimenter may center his 
attention on as many or as few of these as he wishes. For most purposes, 
the smaller the number of characters studied simultaneously the better, 
for the interpretations are clearer. No more than two pairs of characters 
will be used in this book. For an example, Ave may add another pair of 
characters in guinea pigs to the black-w^hite contrast already presented. 

Ordinary guinea pigs have smooth coats of hair, since the individual 
hairs all slope in the same general direction in any part of the skin. 

Fig. 197. — Two guinea pigs with rough coats. The hairs are in many places arranged 
in whorls, sloping away from the central point. {Courtesy of Professor W. E. Castle and the 
Harvard University Press.) 

One variety, however, has a rough coat because, at a number of places 
on the body, the hairs slope outward in all directions from a central 
point like the radiating spokes of a wheel. These hairs push against 
other hairs sloping in other directions, producing an unkempt appearance 
(Fig. 197). Rough and smooth coat could be used in a single-pair cross, 
in which case rough would appear in Fx, and the F2 would be three- 
fourths rough, one-fourth smooth. That is, rough is dominant. 

The two pairs of characters, hair slope and color, can be combined 
in four ways, namely, rough black, rough white (these two in Fig. 197), 
smooth black, and smooth white (Fig. 193). To test the inheritance of 
the two pairs of characters simultaneously, the animals crossed must 
differ in both of them. Suppose one of the original parents is rough 
black, the other smooth white. The Fi generation is rough black, since 
these are the two dominant characters. When these hybrids, which are 
heterozygous for both pairs of genes, produce their germ cells, the genes 
of each pair separate from one another in the reduction division and go 
to different cells. The two pairs undergo this separation independently, 



for they are in different pairs of chromosomes. As a result of this 
independent distribution, four kinds of germ cells are produced, RW, 
Riv, rW, and rw. There are these four kinds of eggs, about equally 
numerous, and the same four kinds of spermatozoa, equally numerous. 
In fertilization, random unions take place between each kind of egg and 
each kind of spermatozoon — 16 combinations all told. 

Rough Black x Smooth White 

RRWW rrww 

RW rw 


F; RrWw fRough Black) 

Rw Rw 

rW rW 

rw rw 



























Fig. 198. — Inheritance of two pairs of independent characters in guinea pigs, black 
and white color, rough and smooth coat. The points on the backs of the animals indicate 
rough coat. 

To write these 16 combinations in the F2 generation without omission 
or duplication, it is convenient to use the Punnett square, so named 
from the English geneticist who devised it. Such a square is included 
in Fig. 198, which explains this cross. Each egg formula is written four 
times, in one of the columns of four spaces down the chart. They are 
put there to be fertilized by the four different kinds of spermatozoa. 


Then each sperm formula is written four times in one of the rows of 
spaces across the chart. In each space are the genes found in one of the 
sixteen kinds of F2 animals. They are written in the chart with the genes 
from the egg separated by a dot from the genes from the spermatozoon; 
but in other situations it is preferable to write the two genes of one pair 
together, followed by the genes of the other pair. Some of the sixteen 
formulas are identical Avith others, but they have been arrived at in 
sixteen different ways. 

It remains only to indicate the appearance of the guinea pigs having 
these genes. The little figures accompanying the gene formulas are 
intended to do this. Nine of the sixteen are rough black, three rough 
white, three smooth black, and one smooth white. It should be remem- 
bered that these numbers are a ratio, 9:3:3:1, not absolute numbers. 
They are so many sixteenths of the total number in F2. In a single 
litter the least frecjuent kind (smooth white, the double recessive) could 
easily be missing. 

Fig. 199. — Gray and ebony body and long and vestigial wings in Drosophila, combined in 

the four possible ways. 

A Two -pair Backcross. — ^As a basis of judgment of certain phenomena 
to be described later, a backcross involving two independent pairs of 
characters will be useful. The characters chosen for illustration are the 
color of the body and the shape and size of the wngs of the fly Drosophila. 
The body is normally of a brownish gray, but there is a very dark variety 
known as ebony. The wings are ordinarily long and lie flat over the 
back of the fly when at rest; but in one variation of them, called vestigial, 
the wings are small and crumpled and project obliciuely outward from 
the body. The vestigial wing is useless for flight; flies with such wings 
merely crawl or jump. 

The four combinations into which these characters may enter are 
shown in Fig. 199. Suppose that the cross be made between a gray long- 
winged fly and an ebony vestigial- winged one. The Fi generation is 
gray and long-winged, for these are the dominant characters of the two 
pairs. If these Fi flies, which are heterozygous for both pairs of genes, 
are mated with ebony vestigial flies, which are necessarily homozygous 
for the two recessive genes, all four of the kinds of flies illustrated in 
Fig. 199 are produced. Moreover, they are about equaUy numerous; 



about one-fourth of the backcross family are of each of these kinds. 
Figure 200 gives the explanation. The four kinds of eggs produced by 
the Fi flies are about equally numerous because the two pairs of genes 
are distributed at random in the reduction divisions of their germ cells. 
Whatever ratio exists among these eggs must also prevail among the 
backcross offspring produced from them — hence the equal numbers of 
the four kinds of flies in that generation. 














ev Soerm 


J^EeW i^Eevv J^eeVv J^eevv 





Fig. 200. — Inheritance of two pairs of characters in a mating between a double hetero- 
zygote and a double recessive. The characters are gray and ebony body and long and 
vestigial wing in Drosophila. 

Interactions of Genes. — The two pairs of genes studied in guinea 
pigs, and the two in Drosophila, appear to be entirely independent of 
each other in the production of their characters. An animal with gene 
W is black, regardless of the slope of its hair; and one whose formula is 
rr is smooth, no matter whether it is white or black. A long-winged 
fly may be either ebony or gray, and an ebony fly either long- or vestigial- 
^vinged. Very often, however, the action of one gene is modified by 
some other specific gene if they are both present in the same individual. 
A striking example is found in the combs of fowls. When a pea-combed 
fowl (Fig. 201, upper left) is crossed with a single-combed one (lower 
right), their offspring are pea-combed, and the F2 generation is three- 
fourths pea and one-fourth single. These results indicate that pea comb 



and single comb differ in just one pair of genes, with pea dominant. In 
like manner it is shown that rose comb (upper right) is dominant over 
single and differs from single by just one pair of genes. 

What should be expected, then, if fowls showing the two dominant 
characters pea and rose, respectively, are crossed? No one could pre- 
dict the result; it has to be determined by experiment. The hybrid 
proved to have a large rounded comb overhanging the base of the beak, 
as in the center of Fig. 201. From its shape this comb is called walnut. 
A clue to the nature of this remarkable character is obtained by breeding 
some of the Fi walnut fowls together. They yield four kinds of offspring 

Fig. 201. — Interaction of genes for combs in fowls. The gene for pea comb (upper 
left) interacts with that for rose comb (upper right) to produce walnut comb (center). Two 
of these Fi walnut-combed fowls, bred together, produce four types of offspring. Single 
comb (lower right) is produced when^ neither the pea gene nor the rose gene is present. 
{Rearranged from Punnett, " Mendelism." Courtesy of The Macmillan Company.) 

walnut, pea, rose, and single (lower row, Fig. 201). Very significantly, 
the ratio of these four types is 9 :3 :3 : 1 in the order named and pictured. 
This ratio indicates that two pairs of genes are involved. The pea- 
combed fowl must have had the formula PPrr, the rose-combed one 
ppRR, in which P is the gene for pea comb, p for no pea, R iov rose 
comb, r for no rose. Single, which is "no pea" and "no i-ose," is pprr. 
The student is encouraged to work out the gene explanation for the 
Fi and F2 generations; any individual possessing both dominant genes 
P and R will have a walnut coml). Whatever effect these genes have, 
singly, on the physiology of comb development, together they interact 
to produce a very different effect. 

Many other examples of interaction of genes belonging to different 
pairs have been discovered. Sometimes the relation is such that one of 
the genes in question cannot produce a visible result unless the other 


gene is present. Sometimes one gene suppresses the action of a gene of 
some other pair. Sometimes two genes, neither of which produces any- 
thing detectable by itself, combine to produce a visible result when they 
occur together in the same animal. When the interactions are between 
dominant genes, they result in Fo ratios which are some modification 
of the fundamental ratio 9:3:3:1. This ratio is changed because two 
or more of the classes of individuals appear alike. To describe details 
of such interactions would go beyond the scope of a first study. The 
complexity is considerably increased by interactions among three, four, 
or five different genes. So many examples of combined actions have 
been found that it seems probable that they are universal. That is, 
every gene probably interacts with some — even many, or all — other genes. 
The phenomena of heredity can be very complicated. 

#1% • %■ 

iw ^ i.»# 

' ' , / I I' 


9 k\4lA 


Fig. 202. — The divisions of the male germ cells of the bug Anasa: a, polar view of 
equatorial plate of first division; all the chroniatic bodies are double except one and there- 
fore represent 21 chromosomes, the somatic number; b, second division in side view, not all 
of the chromosomes shown; the single chromosome of a is going undivided to the lower pole; 
c and d, polar view of the two anaphase groups of the second division; 11 chromosomes go 
into one spermatid (female-producing), 10 into the other (male-producing). (After Wilson 
in Journal of Experimental Zoology.) 

Inheritance of Sex. — A special genetic situation exists in the dis- 
tinction between the sexes, for in a large number of animals and some 
plants the chromosomes of the male and female are in some respect 
unequal. Either one sex has one more chromosome than the other, or one 
of its chromosomes is larger than the corresponding chromosome of the 
other, or certain corresponding chromosomes are of different shapes. 
When the number of chromosomes is different, most species of animals 
have more in the female than in the male. An example of this condition 
is found in a species of bug whose chromosomes are shown in Fig. 202. 
The male has 21 chromosomes, the female 22. The figure show\s the 
reduction division of the spermatocytes of the male. At the left (a) 
are the pairs of chromosomes, mostly so closely united that their double 
nature is not revealed. At the bottom of a, outside the circle of other 


chromosomes, is the odd chromosome not paired with any other. This 
unmated chromosome is called an X chromosome. In the division 
which follows (b), the paired chromosomes separate (not all of them are 
shown), while the X chromosome goes undivided to one end of the 
spindle (the lower end in Fig. 202) . The two cells thus formed (c and d) 
have 10 and 11 chromosomes, respectively. These two numbers of 
chromosomes go into the final spermatozoa, so that there are two kinds 
of spermatozoa, one with 11 chromosomes (including an X), the other 
with 10 (without an X). 

Now, the female of this species has 22 chromosomes, two of which 
are X chromosomes identical in composition with the one X of the male. 
Her eggs ripen in typical fashion, and every egg has 11 chromosomes, 
including one X. When an egg is fertilized by a spermatozoon contain- 
ing an X chromosome, the fertilized egg has 22 chromosomes, two of 
which are X's, and it develops into a female. If an egg is fertilized by a 
spermatozoon without an X chromosome, the fertilized egg has only one 
X (21 chromosomes all told), and it becomes a male. 

Whether there is a definite gene, or perhaps several genes, for sex 
in the X chromosome is not yet certain. They are in any case not the 
sole determiners of sex, for in Drosophila the other chromosomes contain 
genes modifying sex. 

Sex -linkage. — If, in species in which the two sexes have unlike chro- 
mosome groups, there are genes for other characters in the chromosomes 
that are chiefly associated with sex, it is obvious that these characters will 
be difi^erently inherited in the males and females. When the female has 
two X chromosomes, and the male only one X chromosome without any 
mate, any genes contained in the X chromosome will come to the male 
from only one parent (his mother), while the female will receive them 
from both parents. Furthermore, such genes even if recessive will 
produce their character in the male, because there is no other gene of the 
same pair to be dominant over it. The same situation exists in species in 
which the nujnhcr of chromosomes is the same in both sexes, but the shape 
or physiological properties of one of them are different. In such species 
the male possesses what is called a Y chromosome corresponding to one 
of the X's of the female; that is, the male is XY, the female XX. The Y 
chromosome possesses few known genes and with respect to most charac- 
ters might as well be absent. 

Characters whose genes are in the X chromosome are said to be sex- 
linked. How sex-linked characters are inherited is shown in Fig. 203 
which illustrates Drosophila in which the males have the XY constitution. 
The Y chromosome in this fly is shaped somewhat like a letter J. The 
character involved is white eye as contrasted with red. In the first 
cross (left) the female is white-eyed (ww), the male red-eyed (IF). The 



female produces only one kind of egg {w); but the male, because of his 
Y chromosome which lacks any gene of this pair, produces two kinds of 
spermatozoa, one having the X chromosome (with w), the other the Y 
chromosome. The two combinations of eggs and spermatozoa produce 
the two sexes, respectively, of the Fi generation. The males of this 
generation are white-eyed because there is no red gene (W) to dominate 
over their white gene. In the F2 generation, as the figure shows, there 
are four combinations, two of which are red, two white. The marks of 
sex-linkage in tliis cross are (1) that the Fi generation is of two kinds. 

Fig. 203. — Sex linkage of eye color in Drosophila. Left, white-eyed female X red-eyed 
male. Right, the reciprocal cross. (Modified from Morgan, Sturtevant, Muller, and Bridges, 
"Mechanism of Mendelian Heredity," Henry Holt and Company, Inc.) 

instead of only the dominant type, and (2) that the F2 ratio of dominant 
to recessive is 1:1 instead of 3:1. 

If the cross is made with the red eyes in the female and white eyes in 
the male, the results shown are as indicated at the right in Fig. 203. Tht 
Yi males get their eye color gene from their mother as before but now are 
red-eyed, as are also the heterozygous Fi females. In the F2 generation 
there are again four combinations. Three of these are red-eyed; hence 
the F2 ratio is 3 red: 1 white. However, the white-eyed F2 f^ies are all 
males. This last feature is the only sign, when the cross is made this 
way, that the character being studied is sex-linked. 

Any animal or plant whose sex is determined by chromosomes, and in 
which, as a consequence of this chromosome relation, the male produces 
two kinds of spermatozoa, may be expected to show sex-linkage of the 
kind just i"'istrated. Man is one of these animals. A modified form of 



this same phenomenon is found in birds, butterflies, and moths, for in 
these groups the sex-determining chromosomes are so arranged that the 
female produces two kinds of eggs and the male only one kind of sperma- 
tozoon. The distribution of the sex-linked genes in these animals is 
precisely .like that in Drosophila except that the sexes are reversed. 
What is true of the male in Drosophila is true of the female in birds, for 
example. An opportunity to work out the situation in birds is afforded 
by one of the problems at the end of the chapter. 

Autosomal Linkage. — The chromosomes other than X and Y are 
known as autosomes. When two genes for different characters are located 

in the same autosome, they have a 
strong tendency to remain together for 
a while, going to the same germ cells. 
How many successive generations they 
stay together depends on how far apart 
the genes are in the chromosome. 
The chromosomes break more or less 
at random and homologous chromo- 
somes recombine their pieces in new 
ways. If the breakage occurs between 
two pairs of genes, the genes enter into 
new combinations. The genes which 
had been going to the same germ cells 
now go to different germ cells. Natu- 
rally the farther apart they are, the 
more often the breaks occur between 

Linkage operates to distort the 
expected ratios of different kinds of 
individuals. This effect is illustrated 
in Fig. 204. The two pairs of genes 
involved are v (vestigial wing) con- 
trasted with V (long wing), and h 
(black body) as against B (gray). The chromosome composition of the 
two flies is shown at the top of the illustration. The chromosomes in 
their respective germ cells are pictiu*ed between the parents, and the Fi 
female fly below. This Fi fly is hetei'ozygous for both color and wing 
length and affords an opportunity to discover the breakage of the chromo- 
somes. It produces four kinds of eggs, as shown at its right. The first 
two of these {Bv and hV) are produced if the two pairs of genes are not 
separated; and the genes are near enough together so that this happens 
in about 83 per cent of all cells. In the other 17 per cent the two pairs of 
genes are separated by breakage of the chromosomes, resulting in the 

Fig. 204. — Linkage of body color 
and wing length in Drosophila. Left, 
above, gray vestigial-winged male; 
right, black long-winged female. 
{From Morgan, "Physical Basis of 
Heredity," J. B. Lippincott Company.) 


other two kinds of eggs (bv and BV). The Fi female is represented as 
mated to a black vestigial male, whose spermatozoa are necessarily hv. 
These spermatozoa fertilize the four kinds of eggs and produce four kinds 
of offspring which should be in the same proportion as the kinds of eggs. 
The first two kinds (gray vestigial and black long) together make up 
about 83 per cent of the family just as the Bv and hV eggs made 83 per 
cent of the eggs. The other two classes (black vestigial and gray long), 
coming from eggs containing broken and recombined chromosomes, con- 
stitute about 17 per cent. If these two pair of genes had been in different 
pairs of chromosomes and so had been independent of one another, the 
last generation would have exhibited a 1:1:1:1 ratio, each kind making 
about one-fourth of the total, as in the two pairs of characters in Fig. 200. 
The distorted ratio is the evidence that the genes are all in one pair of 

Mendel's Law; Mendelian Heredity. — Gregor Mendel never stated 
his discoveries in the form of a concise principle, but this has been done 
by others since. Heredity as Mendel conceived it differed in two impor- 
tant respects from heredity as understood by his predecessors. A state- 
ment of these two differences is commonly spoken of as Mendel's law. 
Using present terminology, one might state this law as follows. The 
genes of any pair separate from each other in the production of the germ cells, 
so that each germ cell receives only one of them; and the distribution of each 
pair of genes to the germ cells is independent of the distribution of other pairs. 
The separation of genes of the same pair is effected by the reduction 
division in maturation. Independence of the genes of different pairs 
exists when the pairs of genes are in different pairs of chromosomes, 
since these pairs of chromosomes are independently placed on the spindle 
of the reduction division. As is indicated in the preceding section, this 
latter condition is not always met. Many pairs of genes are in the same 
pair of chromosomes. Autosomal linkage, which results from this 
association, is very common. Such linkage is a violation of the second 
part of Mendel's law. Apparently Mendel never witnessed this relation 
between any two pairs of genes. 

Despite the fact that Mendel's law as stated does not provide for 
linkage, all the phenomena so far described are still regarded as belonging 
to Mendelian heredity. The concept of Mendelism has been widened to 
include them. Any heredity is now considered Mendelian if it is depend- 
ent on chromosomes. Most heredity is so dependent. Yet in some plants 
the plastids go over directly from one generation to the next, and what- 
ever color characters these plastids determine are independent of chromo- 
somal genes. Heredity of plastid colors in such plants is not Mendelian. 
Possibly, even probably, there are some other structural units which are 
transmitted directly like plastids. 


Since the inheritance of Hnked characters is still called Mendelian, 
it would be better if the statement of Mendel's law could also be liberal- 
ized. A better formulation would be: The fundamental units of heredity 
are distributed hy means of the chromosomes. This would exclude' plastids. 
Also, to understand the law it would be necessary to know a good deal 
about chromosomes. 

The Nature of Genes. — It is practically certain that the genes are 
chemical substances and that it is through their chemical properties 
that they control the development of the characters they represent. 
Presumably they are protein in nature. One reason for considering them 
protein is that the chromosomes give protein reactions, and the genes 
make up a fraction of the chromosomes. Moreover, genes are highly 
specific in their action; that is, they do certain definite things with con- 
siderable precision, and not other things. Highly specific reactions are 
characteristic of proteins in general, which would help to explain the 
functioning of genes if these be protein. 

Moreover, genes are subject to change. Although any mechanism of 
heredity must have some degree of permanence — otherwise there would 
be no heredity — genes do not remain forever the same. One of the genes 
for red eye in Drosophila changed, and the eye color was then brown. 
A gene for gray body color in the same species changed, and yellow body 
resulted. A gene for uniform color in mice changed, and the mice in 
succeeding generations were spotted. Changes of this sort are known 
as mutations. They must be chemical changes of the genes, which would 
be not only possible but probable if the genes were proteins. The chemi- 
cal structure of proteins is very complex, and occasional permanent 
change is more likely in complex substances than in simple ones. 

It is a current concept that a gene may be a single protein molecule. 
One reason for so believing is the suddenness with which gene mutations 
occur. If a gene were composed of several molecules, any change in 
chemical structure would presumably, just as a matter of chance, affect 
only one of them. The argument is that, with a number of molecules to 
change, mutation might tend to be a gradual process. With only one 
molecule, any structural change must affect the whole gene at once. 

Practical Applications. — Knowledge of heredity has been used for 
centuries to improve the economic situation of the human race. The 
classical field in which that has been done is the in^provement of crops and 
farm animals. The knowledge upon which this improvement rested 
was, until comparatively recent times, little more than a knowledge that 
heredity existed. Its laws have been fairly well understood by breeders 
only in the present century, but by the year 1900 most of the develop- 
ment of domestic races had already been accomplished. The reason for 
this great success of the early breeders is that their method was practically 


the same as it is at present. That method is selection. Those animals 
and plants which were most valuable were selected for breeding, in the 
belief that their good qualities would be transmitted. If even only a few 
of these characteristics were inherited, long-continued selection would 
result in great improvement. 

The discovery of Mendel's principles thus found mankind already in 
possession of very valuable varieties of animals and plants. Man had 
attained this result without knowing very much about how he did it. 
Improvement has, of course, gone on since then. It is now considerably 
plainer why certain results are obtained, and these results often come 
more quickly. Among the important domestic animals, poultry have 
probably yielded more to the newer Mendelian knowledge than any 
others. Considerably less has been done with pigs and sheep, and little 
has been revealed about Mendelian behavior of the valuable characters of 
cattle and horses. Undoubtedly the cost of experimenting with these 
larger animals and the long time involved, when one generation requires 
several years, are responsible for the lag of knowledge concerning their 

Plants have revealed more of their hereditary constitution, partly 
because they are inexpensive to rear, partly perhaps because they are 
of simpler composition. The most important feature of most crops is 
yield, which is inherited, since varieties differ greatly in this respect. The 
principal factor contributing to yield which is being studied now more 
successfully than a generation ago is resistance to disease. The various 
bacterial and fungous diseases of the grains and fruits are receiving con- 
centrated attention at most of the experiment stations, and the results 
attained are very gratifying. 

Room exists for improvement of man himself, through the elimination 
or diminution of some of his defects. Every system of organs and every 
sense organ exhibits hereditary deficiencies in some individuals, such as 
feeble-mindedness, fragility of bones, a tendency to bleed, cataract of 
the eyes, atrophy of muscles, and baldness. Some of these defects are 
more important than others, but there is not one which the human race 
would not choose to banish if it could. The only method is to avoid 
reproduction by individuals possessing genes for the undesirable qualities. 
With respect to most defects, this avoidance must be voluntary, and it is 
uncertain how seriously men and women take their responsibilities. 
Some of the more serious defects, such as feeble-mindedness and epilepsy, 
are, however, frequently dealt with by law. At present 29 states of 
the United States have laws designed to prevent people afflicted with 
these infirmities from rearing families. 

Theoretically, man should be able to improve himself by favoring 
those qualities, talents of various sorts, which it is particularly desirable 


to possess. Unfortunately, too little is known of the heredity of these 
traits to raise the hope that such improvement is imminent. No one as 
yet knows the formula for the production of genius at will. 


1. A rose-combed fowl (Fig. 201, upper right) mated with a single-combed fowl 
produces only rose-combed offspring. If many of these offspring are mated together 
and produce an aggregate of 64 fowls, how many of the latter should be rose-combed? 

2. Tall peas are dominant over dwarf peas. What would be the appearance of 
a plant heterozygous for tall and dwarf? If such a heterozygote were self-fertilized 
and produced 30 dwarf offspring, how many tall offspring should it yield? 

3. Mating a red-eyed and a pink-eyed fly yields red-eyed offspring. If one of 
these red-eyed offspring is mated with its pink parent, and they produce 60 offspring, 
how many of these should be red-eyed? 

4. Brown color in mice is dominant over albinism. In a given cross between a 
brown mouse and an albino, 6 of the offspring were brown, 5 albino. What was the 
formula of the original brown parent? 

5. A long-winged fruit fly mated with one having vestigial wings (a recessive 
character) produced 28 long-winged and 23 vestigial offspring. What were the 
formulas of the parents? Of their long-winged offspring? Of their vestigial-winged 

6. Snapdragons with bilaterally symmetrical flowers, crossed with plants with 
radial flowers, produce only bilateral Fi. If an Fi plant is self-fertilized, what is the 
chance that one of its offspring selected at random will be radial? 

7. Shepherd's-purse with triangular seed capsule is dominant over the variety 
with spindle-shaped seed capsule. If a homozygous triangular is pollinated from a 
heterozygous triangular, and 20 offspring are obtained from them, how many of these 
should have spindle-shaped capsules? 

8. Starchy grain is dominant over sugary grain in corn. If, in a cross between 
these types, 58 of the progeny are sugary, how many of the progeny should be starchy? 

9. A certain white-fruited squash, self-fertilized, produced some white and some 
yellow offspring. If there were 21 yellows, how many white would be expected? 

10. Short hair is dominant over long hair in guinea pigs. A short-haired guinea 
pig, one of whose parents was long-haired, was mated with a long-haired animal. 
If, blindfolded, you selected one of their litter from the cage, what is the chance you 
would get a long-haired animal? 

11. The offspring of a brown mouse and an albino are all brown. If the hetero- 
zygous brown mice are mated together and produce 80 offspring, how many of these 
should be albino? How many of the brown ones should be heterozygous? How 
could you tell which browns were heterozygous? 

12. If gray color in an animal mutates to yellow, and in crosses between stocks 
of gray and yellow the offspring are yellow, what (according to accepted conventions) 
would be the symbol for the gray gene? For the yellow? 

13. If an animal having the formula Cc produces 100 eggs, how many of these 
eggs should luive the formula C? How many c? How many Cc? 

14. A family consisting of 17 red-eyed and 15 purple-eyed flies probably came from 
a mating of parents whose formulas were P and (Fill the blanks properly.) 

15. Applying the conventions relating to choice of symbols for genes, make a 
number of matings between trotting horses Pp X Pp, and obtain 24 foals. How 
many of these should be pacers? 


16. If two parents which haVe the same visible characters produce some offspring 
wliich are hke the parents, some different, write the formulas of the parents using any 
symbols you choose. 

17. In squashes, white fruit is dominant over yellow. From a certain cross 
between a white- and a yellow-fruited plant, 54 white and 59 yellow offspring were 
obtained. What were the formulas of the parents, if squashes were primitively yellow 
like pumpkins? 

18. One flower of a white-fruited squash plant A is pollinated from another white- 
fruited plant B, and both white and yellow progeny are produced. Another flower 
of plant A is pollinated from a yellow-fruited plant and produces 44 offspring. How 
many of these should be white? 

19. A third flower of plant A in problem 18 is self-fertiUzed and produces 44 
offspring. How many of these should be white? 

20. Two gray female mice are mated with a black male. In several litters the 
first female produces 12 gray and 10 black offspring, the second female 19 gray. 
What are the formulas of the two females? Use your knowledge of wild mice in 
determining part of your answer. 

21. Pink eye in mice is recessive to the wild-type dark eye color. From a certain 
mating between two dark-eyed mice some dark- and some pink-eyed mice are obtained. 
The male is then mated with a pink-eyed female, and they produce, in several litters, 
20 offspring. How many of these should be pink-eyed? 

22. Uniform or self-color in mice is dominant over spotting. A self-colored mouse 
is mated with a spotted mouse, and their self-colored offspring are mated together. 
All the offspring of these crosses are mated to spotted mice. Assuming all matings 
to be successful, and the resulting litters of equal size, what fraction of the mice from 
the last matings should be spotted? 

23. Mating a red-eyed fly with curved wings and a claret-eyed fly with straight 
wings yields an Fi all red-eyed and straight-winged. If the Fi flies are bred together 
and produce 96 offspring, how many of these should be claret-eyed and straight-winged? 

24. Frizzled feathers in fowls are turned up at the end, smooth plumage lies down 
flat. Pea and single combs are illustrated in Fig. 201. If a cross between single 
smooth and pea frizzled yields pea frizzled, and if these are mated together and pro- 
duce in the aggregate 48 fowls, how many of these should be single smooth? How 
many pea frizzled? 

25. Self-colored rats (color distributed over the body) are dominant over hooded 
(color only on head, rest of body white). Albino rat is recessive to gray. Crossing 
a homozygous gray hooded rat with an albino having a pair of genes for self-color 
(which, of course, cannot show in an albino) would produce what kind of offspring 
in Fi? If the Fi animals were bred together and produced 80 offspring, how many 
of these should be albino? How many gray hooded? 

26. Two walnut-combed fowls, mated together, produce 9 walnut-combed and 
3 pea-combed ofifspring, and no others. Assuming that no class of offspring is missing 
because of the small numbers, what were the formulas of the parents? 

27. In cattle, black (B) is dominant over yellow (fe), and polled (P) (hornless) is 
dominant over horned (p). If several homozj^gous black horned cows are mated with 
homozygous yellow hornless bulls, what will be the appearance of their offspring? 
If these offspring are mated with one another, and in a number of such matings 9 
yellow polled animals are produced, how many black polled ones would be expected? 
How many yellow horned? 

28. If a homozygous red mule-footed pig (toes grown together) is mated with a 
homozygous black normal-toed pig, their offspring are black and mule-footed. If 


the Fi animals are crossed with red normal-toed ones and produce 80 offspring, how 
many of these should be red and normal-toed? 

29. Black is dominant over white in sheep, and in certain breeds horns are domi- 
nant in males but recessive in females. A homozygous black hornless ewe of one of 
these breeds is mated with a homozygous white horned ram. If their offspring is 
female, what will be its appearance? If male, what appearance? If a number of 
Fi males and females from such parents are mated together, and produce 32 offspring, 
equally divided between the sexes, how many of these will be black horned females? 
How many white horned males? 

30. Red eye (B) is dominant over brown (6) in Drosophila, and pigmented ocelli 
(WO) dominant over white ocelli (wo). A certain brown-eyed fly with pigmented 
ocelli is mated with one having red eyes and white ocelli, and some of their offspring 
have brown eyes and white ocelli. What are the formulas of the parents? 

31. In Drosophila, gray body is dominant over ebony, and straight wing dominant 
over curved. A certain gray-bodied curved-winged female is mated to a gray straight- 
winged male, and they produce some ebony curved offspring. Out of a total of 
40 offspring, how many should be ebony straight? How many gray curved? 

32. Each cell of the muscles of a certain male bug contains 27 chromosomes. How 
many chromosomes in its spermatogonia? How many in its mature spermatozoa? 
How many chromosomes in the body cells of the female of the same species? How 
many in her mature eggs? How many in fertilized eggs? 

33. Can a male Drosophila be homozygous for a sex-Unked character? From 
which parent does a male Drosophila receive his sex-linked genes? To which sex 
among his offspring does he transmit his sex-linked characters? If a gene were 
located in his Y chromosome, to what offspring would he transmit it? 

34. Color blindness is a sex-linked recessive, and sex in man is determined essen- 
tially as in Drosophila. A girl of normal vision whose father was color-blind marries 
a color-blind man. What is the chance that their first child will be color-blind? 

35. A woman of normal vision, whose father was color-blind, marries a man of 
normal vision whose maternal grandfather was color-blind. Among their three 
daughters how many should be color-blind? 

36. A. color-blind boy's parents and grandparents all had normal vision. What 
was the formula of his maternal grandfather? Of his mother? Of his maternal 

37. Yellow body (y) in Drosophila is a sex-linked character recessive to gray 
body ( F). A certain gray female mated with an unknown male produced some yellow 
and some gray offspring of both sexes. What was the formula of the original female? 
What was the appearance of the male to which she was mated? 

38. A female fruit fly with sable body (s) is mated with a male having gray body 
(S). Their daughters are gray, their sons sable. In what cliromosomes are the 
genes S and s? 

39. A barred rock hen mated with a black cock produced black daughters and 
barred sons. Using B and b to represent the genes, give the formulas of the two 

40. The genes for purple eye (normally red) and curved wings (normally flat) in 
Drosophila are in the same pair of chromosomes, and the normal red eye and flat 
wings are dominant. A homozygous purple-eyed flat-winged fly is crossed with a 
homozygous red-eyed curved-winged fly. One of their daughters is mated with a 
purple curved male. What kinds of offspring will they produce, and in what pro- 
portions, assuming that 21 per cent of the pairs of chromosomes break between the 
genes for eye color and wing shape? 


41. The character known as speck (s), a spot near the base of the wing in Dro- 
sophila, is recessive to no speck (S), and plexus (p), a tangled patch of wing veins, is 
recessive to no plexus (P). A doubly heterozygous no-plexus no-speck female (PpSs) 
is mated with a plexus speck male (ppss). Of their 200 offspring, 10 are plexus 
no-speck. How do you account for the smallness of this number? How many of the 
offspring should be plexus speck? 

42. In four-o'clocks the red flower color is not wholly dominant over white, so that 
heterozygous flowers (Pr) are pink. What would be the appearance of the offspring 
of a self-fertilized pink-flowered plant? If the progeny of this plant numbered 100, 
how many red ones should there be? 

43. If a pink-flowered four-o'clock is pollinated from a red one, and they produce 
84 offspring, how many of these should be red? 

44. In shorthorn cattle, the hybrid between red and white is roan (having white 
hairs and red hairs intermingled). What would be the nature of the offspring of a 
roan and a white animal? The offspring of a roan and a red animal? 

46. In Drosophila, cinnabar eye is recessive to red, stripe (a mark down the back) 
is recessive to no stripe, and bent wing recessive to straight wing. A cinnabar stripe 
bent fly is mated with a homozygous red no-stripe straight (wild-type) fly, and their 
offspring crossed with cinnabar stripe bent flies. Of 288 offspring from this latter 
cross, how many should be red stripe straight? 

46. If two Fi flies from Problem 45 are mated together, and among their r2 off- 
spring there are 36 cinnabar no-stripe straight-winged individuals, how many wild- 
type flies would be expected in the F2 generation? How many red stripe bent flies? 


Bateson, W. Mendel's Principles of Heredity. Cambridge University Press. 

(Part II: biography of Mendel, translations of his papers.) 
Shull, a. F. Heredity. 3d Ed. McGraw-Hill Book Company, Inc. (Chaps. 

VII-XIII on simple heredity; rest of book deals with more complex inheritance 

and emphasizes applications to human affairs.) 
SiNNOTT, E. W., and L. C. Dunn. Principles of Genetics. 3d Ed. McGraw-Hill 

Book Company, Inc. (Chaps. Ill and IV for Mendel's laws.) 



Objects of all kinds that have ever interested civilized man have been 
classified by him as soon as they became numerous enough to show simi- 
larities amid differences. Animals have not escaped this human pro- 
pensity for cataloguing. Classification was not necessary when chiefly 
the large, conspicuous animals were known, and when travel and com- 
munication between regions was so meager that each naturalist knew 
only the beasts of his own land. But as knowledge enlarged through 
travel, and as microscopes increased the range of size of animals that 
could be observed, the method of describing animals and their habits 
and modes of life singly, without reference to other animals, became 
cumbersome. It was then that classification began. 

The classification of living things is known as taxonomy (from the 
Greek taxis arrangement and nomos law), which means literally an orderly 
arrangement. Both animals and plants are classified, and the principle 
on which their grouping is based is the same in both; but the schemes 
adopted for these two great kingdoms are somewhat different. Tax- 
onomy of animals is often called systematic zoology, that of plants sys- 
tematic botany. Only the plan adopted for zoology is considered in 
this book. 

Conceptions of Taxonomy. — An orderly arrangement of objects or 
facts presupposes a system of classification. The same objects or facts 
can usually be classified in different ways by the use of different charac- 
ters, qualities, or relations as a basis. What qualities are chosen to form 
the basis of classification depends on the importance attached to those 
qualities. If their importance is not known, the classification depends on 
the purpose or bent of mind of the classifier. It thus happened that 
in the early taxonomy of animals there were likely to be various schemes 
of classification, because no settled convictions existed regarding the sig- 
nificance of such grouping. Some of the first schemes arc described 
below, but it may be pointed out in advance that all but one of the sys- 
tems of classification that have ever been in use have been essentially 
devices to save confusion. Things were put upon shelves, figuratively, 
and labeled and catalogued. As long as prevention of confusion was 
the chief aim, classification might be artificial and arbitrary. The one 
exception to this arbitrary basis of arrangement is found in the system of 



classification that prevails at the present time. The modern system 
serves two purposes instead of but one. It has fitted admirably the 
modern evolution doctrine, according to which species of animals are 
related to one another through common descent. Classification may 
now afford the convenience that was desired in the earliest attempts at 
organization and at the same time express the kinship which the evolution 
doctrine implies. It is rather by accident than by design that the 
modern system is both a convenience and an expression of the course of 
evolution, because the author of it did not subscribe to the evolution 
doctrine. The system of classification is a branching one, and evolution 
results in a branching scheme of kinship. When the evolution idea was 
adopted, therefore, it was easy to adapt the branching classification to 
the portrayal of evolution. The scheme had the further advantage of 
being capable of expansion; the successive branchings could be as numer- 
ous as was required in any line of descent. A classification which 
expresses evolutionary development is called a genetic or natural sys- 
tem — genetic because ancestries are involved, natural because the basis 
of it exists in nature, not just in the minds of men. 

Ray and Linnaeus in Taxonomy. — It has been said that John Ray 
(1627-1705), an Englishman, was the first true systematist. Ray pro- 
posed a dichotomous systematic table of the animal kingdom, that is, 
a system which branched by twos. He used anatomical likenesses as the 
basis on which animals were grouped, and the soundness of his judgment 
of these characters is shown by the fact that several of his groups are still 
recognized as natural ones. It is Carolus Linnaeus (Fig. 205), 1707- 
1778, however, who is considered to be the real founder of classification. 
Linnaeus's most important work was the "Systema Naturae," which 
appeared in 12 editions between 1735 and 1768 and, after his death, in a 
thirteenth, edited by Gmelin. In this work Linnaeus completed a classi- 
fication which Ray had established in part, giving names to important 
groups that Ray had left without appellations and describing animals in 
language which, unlike many of the writings of his time, could not be 
misunderstood. Linnaeus also had the courage to defy prejudice in such 
details as removing the whales from the group of fishes, to which Ray also 
knew they did not belong, and placing them with the terrestrial hairy 
animals called mammals. For, in the Linnaean classification, structural 
characters, rather than habits or external forms, were used as a basis. 
Six classes were employed, four of them vertebrate (borrowed from Ray) 
and two invertebrate. These classes were divided into orders, the orders 
into genera, and the genera into species. The lesser groups were usually 
much more inclusive than the groups now given these same ranks. Thus, 
a Linnaean genus occasionally includes three or four orders, as these 
groups are now reckoned. Moreover, the genus often contained animals 



now placed in widely separated categories. One genus was erected to 
include certain sea cucumbers, a worm, a colonial jellyfish, and several 
primitive near vertebrates ; some of these are now placed near the bottom, 
others near the top, of the animal scale. 

Later Temporary Systems of Classification. — Following Linnaeus, 
many naturalists concerned themselves with systematic zoology. Some 
of them adopted the Linnaean system in general but altered it to suit their 
tastes, sometimes improving it but quite as often not. Others invented 
new classifications. Georges Cuvier (1769-1832) established four major 

Fig. 205. — Carolus Linnaeus, 

1707-1778, at the age of forty. 
Botanical Garden.) 

{Courtesy of New York 

groups, called branches, which he divided into classes, 19 in number; 
and some parts of his classification remained in vogue in his own country 
(France) for three-quarters of a century. De Blainville (1777-1850) in 
several instances happily discovered the structural characters that were 
of genuine importance in distinguishing natural groups. He proposed a 
classification involving three subkingdoms, distinguished by the arrange- 
ment of their parts about a center or axis. These subkingdoms weie the 
Artiomorphes, having a bilateral form like the majority of animals; the 
Adinomorphes, with a radiate form like a starfish; and Heteromorphes, 
animals having an irregular form (chiefly protozoa and sponges). 
Lamarck (1744-1829) devised a classification based upon nervous sensi- 
bility and proposed three principal groups: the apathetic animals, those 
without nervous systems or apparent sensation among the invertebrates; 


the sensitive animals, also among the invertebrates; and the intelligent 
animals corresponding to the vertebrates. Oken (1779-1851), who was a 
philosopher rather than a naturalist, advocated simultaneously at least 
two classifications, which were equally worthless. One divided animals 
into groups according to their systems of organs, as intestinal, muscular, 
sexual, respiratory, vascular, etc. His other classification was based 
on the senses. Thus, there were the Dermatozoa (literally, skin or touch 
animals), by which he meant the invertebrates; the Glossozoa (literally, 
tongue animals), the fishes; the Rhinozoa (nose animals) which included 
the reptiles; the Otozoa (ear animals), or the birds; and another class, 
which appears to have been called interchangeably the Ophthalmozoa 
(eye animals) or Thricozoa (hair animals), the mammals. It would be 
hard to name a set of distinctions less applicable as classification marks 
than most of these, but Oken did not engage in practical matters. Then 
there was a host of minor systematists the value of whose labors was 
diminished by attempts to force their classifications into some numerical 
system, as, for example, those who held that the number of orders in 
each class should be the same as the number of families in each order, 
or the number of genera in each family. The favored number was five 
in some classifications, less often three, four, or seven. 

These early modes of arrangement of animals have been described not 
for any value that may attach to them as classifications but to form a 
background for the one system that has survived. It should be obvious, 
from the brief statements made, that most of the plans used were totally 
unsuited to the requirements which later developments of zoology would 
have imposed upon them. The system of Linnaeus, however, was hap- 
pily capable of being adapted to the demands of the tenets of evolution, 
and it alone has persisted to the present time. 

The Linnaean System. — That the Linnaean system was rapidly 
adopted in advance of the general acceptance of the evolution idea is 
doubtless due largely to the fact that it introduced a sharply defined 
grouping, a definite terminology, and brief, clear diagnoses. It also 
permitted early naturalists to group those forms that resembled each 
other, which would be a natural tendency in any classifier. And then, 
as stated earlier, came the added advantage that it equally well per- 
mitted the classification of forms according to their relationships. As 
stated above, Linnaeus recognized groups of four different values — the 
class, the order, the genus (plural, genera), and the species (plural, 
species). To these categories have been added the phylum (plural, 
phyla) and subphylum (assemblies greater than the class), the subclass, the 
suborder, the family, the subfamily, the subgenus, the subspecies, and 
others. Of these additional groups the phylurn and family are now 
generally accepted, and every classification includes a named group of 


each of these ranks. So regular is this practice that if there were only 
one kind of animal in a phylum, it would probably be assigned also to a 
named class, an order, and a family, as well as a genus and a species. 
The other ranks named are used for some groups or by some naturalists. 
The rank of recognized categories may be expressed as follows: 

Phylum. Example, Chordata (the chordates) 

Subphylum. Example, Vertebrata (the vertebrates) 
Class. Example, Mammalia (the mammals) 

Subclass. P]xample, Eutheria (the placental mammals) 
Order. E>xample, Rode7itia (the rodents) 

Suborder. Example, Sciuromorpha (the squirrellike rodents) 

Family. Example, Sciuridae (the flying squirrels, marmots, squirrels, 
Subfamily. Example, Sciurinae (marmots, squirrels, chipmunks) 
Genus. Example, Sciurus (the arboreal squirrels) 

Subgenus. Example, Tamiasciurus (the red squirrels) 

Species. P^xample, hudsonicus (the Hudsonian red squirrel) 
Subspecies. Example, loqiiax (the southern Hudsonian 
red squirrel) 

In some grovips "divisions" or '^sections" are recognized by authors, 
but these categories have no definite place in the system; that is, they 
may be introduced to mark off a group of genera, an assemblage of orders, 

The Linnaean system designates the species by two Latin or latinized 
names, the generic name, a noun, and the specific name, usually an adjec- 
tive. Thus Natrix is the generic name of a group of water snakes, and 
Matrix rhomhifera and Natrix sipedon are two species of water snakes. 
This is known as the binomial system of nomenclature. When subspecies 
are recognized, three names are used — the generic, the specific, and the 
subspecific — thus: Thamnophis sirtalis parictalis. Subspecies must 
usually have somewhat separate geographic ranges, but they grade into 
the neighboring subspecies at their common l:)oundaries. The term 
variety, sometimes carelessly used synonymously with subspecies, often 
means only a genetically different type of individual not having geo- 
graphic separation, for which the word phase is a preferable designation. 
Thus, the cinnamon individuals that occur not infrequently throughout 
the range of the black bear, Euarctos americanus, to which species it 
Ix'longs, may be called a phase or variety. Such varieties are not ordi- 
narily named in the Linnaean scheme. However, the taxonomic rank 
of variety may be assignc^l to divisions smaliei- than subspecies, and in 
one group, the ants (family Formicidae), the systematists regularly 
recognize and designate divisions smaller than sul).spe('ies by name, using 
four names for each variety (for example, Camponotus hcrculeanus 
ligniperdus noveboracensis, the northern carpenter ant). 


Rules of Nomenclature. — The binomial and trinomial systems of 
nomenclature have been of great convenience to naturalists. Before 
their adoption, common names were in use in the scientific world and led 
to much confusion, the same animals being known by different names and 
different animals by the same name. To make certain that each animal 
shall have but one scientific name and that no two animals shall have the 
same name, rules of nomenclature have been proposed at different times 
for the purpose of determining which name shall prevail when several 
have been or are likely to be inadvertently proposed for the same form. 
Linnaeus seems to have appreciated the necessity for rules and to have 
proposed a set. These rules were not sufficient, and several other codes 
have been proposed, the more important of which are the British Associa- 
tion Code, the American Ornithological Union Code, the Code of the German 
Zoological Society, and the Code of the International Zoological Congress. 
The code now almost universally in use is the International Code of Zoo- 
logical Nomenclature, adopted by the International 2^oological Congress 
and governed through a Commission on Nomenclature created in 1898. 

The International Code. — Some of the essential features of the Inter- 
national Code are as follows. The first name proposed for a genus or 
species prevails on the condition that it was published and accompanied 
by an adequate description, definition, or indication, and that the author 
has appUed the principles of binomial nomenclature. This is the so-called 
law of priority. Duplicate names which have to be rejected because not 
prior are called synonyms. The tenth edition of the "Systema Naturae" 
of Linnaeus is the basis of the nomenclature. Names given earlier and 
not used in that edition are not recognized. The author of a genus or 
species is the person who first publishes the name in connection with a 
definition, indication, or description, and his name in full or abbreviated is 
given with the name; thus, Bascanion anthonyi Stejneger. In citations 
the generic name of an animal is written with a capital letter, the specific 
and subspecific name mth initial small letter. The name of the author 
follows the specific name (or subspecific name if there is one) without 
intervening punctuation. If a species is transferred to a genus other 
than the one under which it was first described, or if the name of a genus 
is changed, the author's name is included in parentheses. For example, 
Bascanion anthonyi Stejneger should now be written Coluber anthonyi 
(Stejneger), the generic name of this snake having been changed. It is 
common practice now for the author of a species to designate one par- 
ticular specimen as the type of the species, and to indicate the museum 
or other collection in which it is placed. If the species is later divided, 
the original name goes to that part of it which includes the type specimen. 
Also the specimen can be inspected in case of doubt regarding the identity 
of the species. One species constitutes the type of the genus. This 


decides, in case the genus is later divided into two genera, which group 
shall receive the original name. One genus constitutes the type of the 
subfamily (when a subfamily exists), and one genus forms the type of 
the family. The type is indicated by the describer or, if not indicated 
by him, is fixed by another author. No two genera in the whole animal 
kingdom may have the same name — a rule still occasionally violated 
because the interested taxonomists have not proposed corrected names. 
The name of a subfamily is formed by adding the ending -inae and the 
name of a family by adding -idae to the root of the name of the type 
genus. For example, Colubrinae and Colubridae are the subfamily and 
family of snakes of which Coluber is the type genus. Names of sub- 
families are accented on next to the last syllable, family names on the 
third syllable from the end. 

The Basis of Classification. — Early systematists largely employed 
superficial characters to differentiate and classify animals, and their 
classifications were thus largely artificial and served principally as con- 
venient methods of arrangement, description, and cataloguing. Since 
the time of the development of the theory of descent with modifications 
by Lamarck (1809) and Darwin (1859), as stated in an earlier section, 
there has been an attempt to base the classification on relationships. 
Very nearly related animals are put into the same species. They are 
related because they descend from a common ancestry. The common 
ancestry could not in most cases have been very ancient, otherwise 
evolution within the group would have occurred and the species would 
have been split into two or more species. Species that are much alike are 
included in one genus, being thus marked off from the species of another 
genus. The similarity of the species of a genus is held to indicate kin- 
ship ; but since there is greater diversity among the individuals of a genus 
than among the members of a species, the common stock from which the 
species of a genus have sprung must have existed at an earlier time, in 
order that evolution could bring about the degree of divergence now 
observed. In like manner, a family is made up of genera which resemble 
one another more than they resemble other genera, and their likeness is 
again a sign of affinity. But to account for the greater difference 
between the extreme individuals belonging to a family, evolution must 
have had more time; that is, the common source of the members of a 
family must have antedated the common source of the individuals of a 
genus. Orders, classes, and phyla are similarly regarded as having 
sprung from successively more remote ancestors, the time differences 
being necessary to allow for the differences in the amount of evolution. 
This statement is, however, only in a general way correct. Since 
evolution has probably not proceeded at the same rate at all periods 
or in all branches of the animal kingdom at any one time, the time rela- 


tions of the groups of high or low rank must not be too rigidly assigned. 
Thus certain genera in which evolution has been slow are probablj^ 
much older than some families in which evolution has been rapid. The 
genus Lingula (a burrowing marine brachiopod found between tide lines) 
has evolved very little. The modern animals differ only slightly from 
fossil Lingula of Ordovician time, estimated by some to be 400,000,000 
years old. This is an extreme instance of slow evolution: Lingula is 
probably the oldest living genus. Many families, even orders, and some 
classes must be younger than that. It is not improbable, also, that 
some genera are quite as old as the families which include them; but in 
no case can they be older. Furthermore, different groups are classified 
by taxonomists of different temperaments, so that groups of a given 
nominal rank may be much more inclusive (and hence older) in one 
branch of the animal kingdom than in another. On the whole, neverthe- 

FiG. 206. — Analogous structures; legs of several animals. A, kangaroo; B, crayfish; C, 
honeybee. {C from Met calf and Flint, "Destructive and Useful Insects.") 

less, the groups of higher rank have sprung from ancestry more remote 
than that of the groups of lower rank. 

Judging Kinship. — The means of recognizing the kinship implied in 
classification permit some differences of opinion. It is recognized that 
likeness in structural characters is the chief clue to affinities. However, 
similarity in one or several structures unaccompanied by the similarity of 
all parts is to be distrusted, since animals widely separated and dissimilar 
in most characters may have certain other features in common. Thus, 
the coots, phalaropes, and grebes among birds have lobate feet but, as 
indicated by other features, they are not closely related ; that is, the lobes 
on their feet are analogous, meaning that they serve the same function. 
Analogy is mdespread in the animal kingdom, since the same activities 
must be carried on by animals of very different structure. Locomotion, 
for example, is effected by legs of vei;y different kinds. The legs of a 
kangaroo, a crayfish, and a honeybee (Fig. 206) are analogous, but their 
structure is unlike. The skeleton is within the flesh in the first of these 
but on the outside in the other two, and the materials of the skeleton 
are different. The crayfish and the bee, though alike in the position of 
the skeleton, differ in the number and character of the segments of the 
leg. Another case of analogous structures is that of lungs and gills 


(Fig. 207). Both are used for absorbing oxygen but are wholly different 
in structure. 

The foregoing analogous organs are so unlike in structure that no one 
would be led to classify together the animals that possess them. Not 
always, however, are the structural differences so obvious externally. A 
whale swims by means of paddles and a flattened tail which greatly 
resemble fins, and the early naturalists regarded whales as fishes. Yet 
the whale is a warm-blooded air-breathing animal with a four-chambered 
heart and some hair on the skin and has also the other characters of 
mammals, while the fishes are cold-blooded and aquatic, and have a 
two-chambered heart and scales in the skin. A close resemblance is also 
exhibited by certain lizards (Amphisbaenidae) to a group of snakes 
(Typhlopidae), because the former are blind and legless and have a short 
tail. These external similarities have apparently arisen in evolution 

A B 

Fig. 207.— Analogous structures; respiratory organs: A, gills of salamander; B, lung of 

frog. {From Wienian, "General Zoology.") 

independently of one another and for that reason are not an indication of 

Homology. — In judging of kinship by means of structural similarities, 
therefore, care must be taken to employ only those structures that 
have had similar origins in evolution. It is sometimes difficult to deter- 
mine now whether similar structures in two groups of animals arose in 
evolution in the same way, or have converged for some reason from 
originally distinct beginnings. In general, if two or more groups of 
animals have one or a few structures in common while all others are 
different, it is safer to assume that the common structures arose inde- 
pendently, or at least that their recent evolutionary developments have 
been independent, and that the groups are therefoi'e not closely related. 
The lobate feet of the several groups of birds mentioned above fall in 
this category. If, however, a gi-eat many features of two groups of 
animals are closely similar, the probability is that such similarities could 
only have come from similar or identical origins in evolution. The work 
of the taxonomist therefore becomes, in large measure, the recognition of 
those characters in different animals whose similarities are due to com- 
mon evolutionary origin. 



Structures that arise in the same way in evolution are said to be 
homologous with one another or to exhibit homology. Homology means 
similarity of origin in evolution. Unfortunately for the taxonomist the 
early evolution of the structures on which his classification is based took 
place in many instances millions of years ago. How can he ascertain, 
under these circumstances, whether the evolution of structures in two 
animals was similar or not? The answer to this question must usually 
be arrived at indirectly. 

Homology Judged from Adult Structure. — The most reliable means 
of judging of similarity of evolution in two groups would be fossil mem- 
bers of those groups, if fossils could be obtained in sufficient numbers to 
establish a fairly complete history extending far back into their ancestry. 
Some such histories are given in Chap. 22. In most families of animals, 









bird bat whale 

Fig. 208. — Homology in the bones of the fore Umbs of vertebrates. Numbers 1-5 refer to 
digits, from thumb to Uttle finger. {From Storer, "General Zoology.") 

however, good fossil series are wanting, and the taxonomist must rely 
on what can be discovered from the living animals of today. In clear 
cases adult structure is sufficient, but only where many features are 
alike in the animals in question. A classical case of homology, judg- 
ment of which could safely rest on adult structure alone, is that which 
exists among the forelimbs of vertebrate animals (arms, wings, forefeet, 
etc.. Fig. 208). Although the external forms of these forelimbs differ 
greatly in birds, seals, horses, whales, bats, and man, their skeletons are 
found to correspond very closely, bone for bone, at most points. It is 
believed that so many similarities could not be the result of accident or of 
convergence from originally distinct sources and that the likenesses are 
a sign of similar evolutionary origins. The nervous systems of vertebrate 
animals are equally good examples. The parts of the brain in fishes, 
amphibia, reptiles, birds, and mammals have a very obvious correspond- 
ence, and the origin and distribution of the cranial nerves are very similar 
in all of them. It is scarcely conceivable that these nervous systems could 



be alike in so many respects unless their evolutionary histories were 
largely the same. 

Homology Ascertained from Embryonic Development. — Somewhat 
better evidence of homology than is afforded by adult structure can often 
be obtained from a knowledge of embryonic development. As was 
pointed out in Chap. 16, corresponding structures in vertebrate animals 
arise in essentially the same way in the embryo. The nervous system of 
one vertebrate begins with ridges that are much like those of another 
vertebrate embryo. The eye of a bird develops as does the eye of a frog. 
The early ear also is about the same, whether found in a reptile or a 

/7s/7 Salamander Tortoise Chick 

Hocf Calf Rabbit Man 

Fig. 209. — Homology of embryonic form, and particularly of gill clefts and bars, in verte- 
brates. {From Haupt, after Romanes, "Darwin and After Darwin.") 

mammal. This similarity of the first appearance of embryonic structiu"es 
occurs even when the adult organs are strikingly diffei'ent. The arm of a 
man and the wing of a bird are different from one another in the adult 
condition, especially in the hands; but in the embryo the earliest limb 
l)uds are almost identical. An even greater difference exists between the 
adult fore- and hind limbs of a bird. When compared, bone for bone, 
there is scarcely a point at which there is not a distinct difference. Yet 
the wing and leg could be interchanged in the early embryo, and few 
observers would detect the substitution. Even the general form of the 
whole embryo is similar in the several classes of vert(^brates (Fig. 209). 
This illustration also shows the common origin of gill clefts and gill bars, 
and their presence in the embryos of reptiles, birds, and mammals which 


do not have gills in the adult. Thus, on the whole, animals whose 
adult structure is similar resemble each other even more closely in 
embryonic stages. Similarity of embryos is particularly useful in taxon- 
omy in those instances in which the adult animals, though closely related, 
have become so changed as to lose all similarity. An example of this 
kind is found in the parasite Sacculina described in Chap. 23. Biolo- 
gists believe that similarity of structures in the embryo can be due only 
to similarity of the evolution of those structures ; and because resemblance 
in the embryo sometimes remains after adult similarity has been dimin- 
ished or destroyed, embryonic development is frequently better evidence 
of homology than is adult structure of the same animals. 

The only known phenomenon which could preserve the similarities 
possessed by different animals is heredity. The likenesses of present- 
day animals must therefore be inherited from like animals of the past. 
Since it is scarcely conceivable that two identical organisms ever could 
have arisen independently of one another, inheritance from like ancestors 
must ultimately be inheritance from the same ancestors. Animals of 
different modern groups are held to possess like features in both adult 
and embryo because of this descent from a common source. This is the 
argument upon which the taxonomist relies when he classifies animals 
on the basis of supposed homologies. 

Biogenetic Law. — The evident dependence of homology upon a com- 
mon descent led, in the last century, to a conception comprised under 
the term biogenetic law, sometimes called by the more expressive and 
less committal name recapitulation theory. According to this law or 
theory (already stated page 74), the embryonic or other early stages of 
individual animals of today represent the condition of successive ancestors 
of these animals. That is, early developmental conditions represent 
very remote ancestors, later embryonic stages represent more recent 
ancestors. Some biologists held that the early embryonic stages are like 
the adult ancestors; others believed merely that the embryonic stages 
of the present are like the embryonic stages of the ancestors. 

If this law were capable of rigid application, it would be easy to trace 
the evolutionary history of a race simply by studying the development of 
its individuals. In some cases this simple precedure is almost feasible. 
A series of fossil cephalopods (allies of the cuttlefishes) is a case in point. 
The fossil remains of these animals indicate that, in their racial history, 
their shells were at first provided with straight partitions, later with 
partitions whose edges were bent, crooked, and finally lobed in a very 
complicated manner (Fig. 210). Since in the fossils both the young and 
old stages of each individual shell are preserved, it is possible to compare 
the individual development with the racial development. When this is 
done, it appears that the individuals of the highly complex types passed 



through very similar stages, in which the partitions were first straight, 
then bent, crooked, and finally complicated. 

Another suggestive and perhaps significant individual development 
is that of the decapod Crustacea (lobsters, prawns, shrimps). The shrimp 
Penaeus hatches as a nauplius, and goes through several increasingly 
complex forms (Fig. 211), the last immature one being the so-called my sis 






Fig. 210. — Biogenetic law illustrated by fossil cephalopods. Edges of partitions of 
shells start nearly straight and become increasingly crooked, in both evolution of group and 
development of individual ammonites (like Phylloceras, E). {From Storer, "General 

stage. The appendages through all this development are two-branched. 
In the adult shrimp, however, the outer one of these branches on the five 
pairs of trunk appendages is considerably reduced. The lobster has 
shortened its individual development and hatches as a mysis which has 
two branches on all appendages; but the five pairs of walking legs have 
lost the outer branch completely in the adult. The support which these 

Nauplius Protozoea 2oea Mysis , ADULT 

P'iG. 211. — Larval stages and the adult of the shrimp Penaeus, jjeihaps illustrating 
biogenetic law. Numbcis refer to successive appendages. (From Storer, "General 

decapods give to the biogenetic law lies principally in the fact that there 
exists a present-day animal called Mysis (from which the larvae of other 
forms take their name) in which two well-developed branches persist 
on the trunk appendages of the adult. The lol)ster and shrimp thus 
pass through a developmental stage which resembles a supposedly more 
primitive animal. 

In most animals embryonic develoi)ment has undergone many 
changes, so that steps in development no longer represent accurately 


the steps in the evolution of their ancestors. That is, the biogenetic 
law is less generally applicable than it was formerly supposed to be. 
However, many important facts of evolution, of limited scope, have been 
discovered by an appeal to this law. A case in which the recapitulation 
theory is presumably correct is in the development of gill pouches in all 
the vertebrate animals. Gills are never developed in the reptiles, birds, 
and mammals ; but gill pouches are formed in the embryo, and these may 
actually open temporarily to the outside as gill clefts, between which are 
the gill bars upon which gills are developed in fishes and amphibia. The 
production of gill pouches and bars in the higher vertebrates as well as in 
the lower, besides indicating a common ancestry of all these animals, 
points to the conclusion that the ancestor was an aquatic animal that 
respired by means of gills. 

Practical Taxonomy. — The foregoing scheme of genetic classification 
is a goal toward which taxonomists in general strive. Application of it 
is attended with some difficulties. One obstacle is that before a satisfac- 
tory classification of even a small group can be made the species in it 
must be known. Judgment of kinships rests largely on a comparison of 
structures, and the characters of each species have an influence on one's 
judgment of the relationship among other species. Omission of some 
species tends to modify judgments concerning the whole group. Since 
there are usually many species in a family, or even a genus, the task of 
discovering and describing them is no small one. This work has been 
going on a long time, yet many species are still unknown. Every year 
many new species are described — few in the groups of large, conspicuous 
animals, but many in those less generally observed. Because of this 
still waiting task of describing species, many taxonomists, particularly 
in the past, have devoted their energies chiefly to naming and putting on 
record the newly discovered forms. They have had to concern them- 
selves with kinship to the extent of putting species in the right genera, 
etc., but they have conceived their main task to be filling out the record. 
More and more, however, the genetic classification will have to be their 

The large number of species in existence is also a difficulty. Among 
well over a million, possibly over two million, species no one person can 
be expert on any considerable fraction. Each taxonomist must limit 
himself to one group, perhaps an order, often only a family. Names are 
given to these specialists according to the phyla or classes in which they 
have competence. An entomologist deals with insects, though he is never 
an expert in all the orders ; a protozoologist studies the unicellular animals ; 
an ornithologist knows birds, a herpetologist reptiles or amphibia or both, 
a mammalogist mammals, etc. 

The other difficulties are mostly those which inhere in the animals 


studied. To know which characters best indicate kinship is the chief 
problem. In the higher ranks of the classification, those qualities which 
are constantly associated with one another are presumably best. Thus 
feathers are constantly associated with wings, a beak, claws, a four- 
chambered heart, and warm blood. These are the marks of one class, the 
birds. This principle may be pushed down to the lower ranks, the orders 
and families, but in less marked degree. When it is used for genera and 
species it is still valid but often difficult to apply. For species the uncer- 
tainties of its application are so great that some systematists have 
advocated abandoning it in favor of some more or less arbitrary scheme. 

Relations of Taxonomy. — Classification has wide connections with 
nearly all other phases of biology. In a practical manner every biologist 
has occasional or frequent use for the technical knowledge of the systema- 
tist, and this requirement is not a purely formal one. Many investiga- 
tions whose principal aim is entirely apart from classification must, 
nevertheless, constantly use the data of taxonomy. Thus the zoogeog- 
rapher, as will be apparent in Chap. 21, is not primarily interested 
in classification ; but in order to discover the principles which have guided 
migration or determined extinction in the past, he must be thoroughly- 
conversant with the taxonomy of the group whose distribution he studies. 
The paleontologist also requires a knowledge of classification not only 
of extinct forms but of their living relatives. The Work of the physiolo- 
gist frequently involves the question of relationship, as does that also 
of the ecologist. Indeed, every biological field is in very close connection 
with taxonomy. 

This intimate relation is not one-sided, for each of the phases of 
biology contributes to a knowledge of classification. Distribution and 
fossil forms supply information where morphology fails or may refute 
conclusions based on morphology alone. Physiological facts must be 
taken into account in explaining the formation of species. Ecological 
relations must be understood if certain taxonomic questions are to be 
correctly answered. In practice, this close relation between taxonomy 
and the other phases of biology is not always observed, but all of them 
suffer from its neglect. 


Gill, T. Systematic Zoology: Its Progress and Purpose. Annual Report of Smith- 
sonian Institution, 1907. (Pp. 449-471 for history of taxonomy.) 


In applying the principles of taxonomy systematic workers have 
often disagreed. This is inevitable because of the many judgments which 
must be made from meager evidence. When groups of facts seem to 
point to different conclusions, biologists may and frequently do weigh 
the conflicting data differently. Various schemes of classification have 
therefore arisen, all of them agreeing in many major features, differing 
from one another in less fundamental respects. The one here given may 
not be the best, but it is in common use. 

The principal groups of animals are given, with brief descriptions 
and some well-known examples. The definitions are necessarily incom- 
plete and are often not sufficient to distinguish all the members of one 
group from those of another. They will serve, however, to give a general 
concept of classification and a bird's-eye view of the animal kingdom. 

Phylum 1. Protozoa. — These are single-celled animals, mostly of 
microscopic size, though some are visible to the unaided eye. Some 
species are colonial, but in these the cells are usually all potentially alike; 
that is, ^here is no differentiation among the attached cells to form tissues 
or organs. Protozoa live in very varied situations but usually require 
moisture. Many of them live in the soil. They are exceedingly com- 
mon in ponds, streams, lakes, and oceans and may be attached to solid 
objects, be buried in mud or debris, or swim freely in the water. Many 
of them are parasitic in other animals. Some of the parasitic ones cause 
disease, as malaria, dysentery, and African sleeping sickness in man. 
Some protozoa live in other animals in a relation that is beneficial to the 
host as well as to the guests. A most remarkable example of mutual 
benefit is that received and conferred by certain protozoa in the digestive 
tracts of termites. These insects, whose food is wood, would be quite 
unable to digest the cellulose without the aid of the guest protozoa. 

Untold numbers of protozoa live in the sea, and lived there ages ago. 
The great limestone beds, chalk cliffs, and quartzite and flint deposits 
are made up of shells of ancient protozoa. Noctiluca is a marine proto- 
zoon which is responsible for some of the remarkable phosphorescence 
observable in disturbeji waters at night. 

There are three principal modes of locomotion. Some protozoa 
thrust out pseudopodia, projections of their protoplasm, and then flow 




into them. This is characteristic of the class to which Amoeba (Fig. 
212) belongs. Protozoa of this type have no constant characteristic 
form but are always changing. Others have at one end of the cell one 
or two long whiplike flagella whose lashing or sometimes wavelike motion 
propels the organism through the water. Euglena (Fig. 213) is one of 
these. Still others have the body covered by hundreds of cilia, short 

Fig. 212. — Amoeba. 

Fig. 213. 

Fig. 214.— 

Fig. 215. — Podophrya, 
one of the Suctoria. 

hairlike projections whose beating drives the body along, as in Para- 
mecium (Fig. 214). Some protozoa, particularly the parasitic ones, have 
no locomotor structures. The classification of protozoa follows. 

SUBPHYLUM I. Plasmodroma. Protozoa that never have cilia in any stage. 

Class I. Mastigophora. Protozoa with flagella, which serve for locomotion 
or for taking food. Euglena. (Figs. 34, 47, 48, 50, 51, 52, 53, 54, 129, 130, 131, 213.) 

Subclass I. Phytomastigina 
Order 1. Chrysomonadina 
Order 2. Cryptomonadina 
Order 3. Dinoflagellata 
Order 4. Eiiglenoidina 
Order 5. Phytomonadina 

Subclass II. Zoomastigina 
Order 1. Protomonadina 
Order 2. Polymastigina 
Order 3. Hypermastigina 
Order 4. Distomatina 
Order 5. Cystoflagellata 

Class II. Rhizopoda. Protozoa with pseudopodia or other changeable processes. 
Amoeba. (Figs. IG, 30, 43, 49, 212.) 

Order 1. 


Order 4. 


Order 2. 


Order 5. 


Order 3. 


Order G. 


Class III. Sporozoa. Parasitic; Protozoa, usually without motile organs or 
mouth, reproducing by spores. Malarial organism. 

Subclass I. Telosporidia 
Order 1. Coccidiomorpha 
Order 2. Gregarinida 

Subclass IT. Neosporidia 
Order 1. Cnidosporidia 
Order 2. Sitrcosporidia 
Order 3. Haplosporidia 



SUBPHYLUM II. CiLlOPHORA. Protozoa having cilia in some stage. 

Class I. Ciliata. Ciliophora with cilia throughout life. Paramecium. (Figs. 
]5. 132, 138, 214.) 

Order 1. Holotricha 
Order 2. Heterotricha 
Order 3. OHgotricha 

Order 4. Hypotricha 
Order 5. Peritricha 

Class II. Suctoria. Ciliophora with cilia in young stages, tentacles in adult. 
(Fig. 215.) 

Phylum 2. Porifera. — The sponges are roughly radial in form and 
always diploblastic (two-layered), though many wandering cells are 
found in a jellylike substance between the layers. The body wall is 
always penetrated by many pores, which give the phylum its name. 
These pores lead to chambers within, which may be single cavities extend- 
ing from outside to inside, or may branch or connect with other cavities 
in a complex system. The final opening through which the water leaves 

Fig. 216. — Elements of sponge skele- 
tons. 1, spongin; 2-7, spicules. 

Fig. 217.1 
A sponge. 

the body is called the osculum, and there may be many of these oscula. 
Some of the chambers are lined by collared cells (Fig. 33, page 52). The 
collared cells also possess flagella, by means of which a current of water is 
kept up continuously in the same direction. Food organisms and oxygen 
are brought, and wastes are carried away, by these currents. The 
collared cells seize the food, digest it, and pass along much of the nutrition 
to the other parts of the organism. 

The sponges all possess a skeleton, which in some consists of a host of 
limy or siliceous spicules, in others of a network of horny (spongin) 
threads (Fig. 216). It is this latter horny skeleton which makes the 
ordinary bath sponge. 

Members of this phylum are all sessile; that is, they are attached to 
other objects and do not move about. About a hundred and fifty species 
live in fresh water, where they sprawl in irregular form over twigs or 
logs. It is these fresh-water forms that reproduce by gemmules (page 
170). The bulk of the phylum is marine, and they are found all over the 

Courtesy of General Biological Supply House. 


A curious feature of the development of sponges is their "inside-out" 
gastrulation. It is the cihated cells of the blastula that are invaginated 
and form the endoderm, whereas other gastrulas, if ciliated at all, regu- 
larly bear the cilia on the outside. Sponges also have remarkable powers 
of regeneration. Their bodies may be crushed, the separated cells sifted 
through a bolting-cloth net upon a surface under water, and there the 
cells gradually collect into lumps from which new sponges grow. 

In the irregular, spreading, fresh-water and bath sponges, there has 
been some debate as to what constitutes the individual sponge. One 
concept is that each osculum is the center of an individual, and that 
the mass called a sponge is a colony. The boundaries of the individuals 
would then necessarily be indefinite, since all the oscula are parts of one 
system of canals. 

There are three classes of sponges: 

Class I. Calcarea. Sponges with spicules composed of calcium carbonate, mon- 
axon or tetraxon in form. (Figs. 74, 139, 217.) / 

Order 1. Homocoela Order 2. Heterocoela 

Class II. Hexactinellida. Sponges with spicules composed of silicon, triaxon in 

Class III. Demospongiae. Sponges with spicules composed of silicon, not triaxon 
in form, or skeleton composed of spongin, or with skeleton of both spicules and 

Order 1. Tetraxonida Order 3. Keratosa 

Order 2. Monaxonida 

Phylum 3. Coelenterata. — This phylum includes Hydra, the 
hydroids, jellyfish, sea anemones, and corals (Figs. 218, 219). Its 
members are radial in form and are all diploblastic. 
They possess a coelenteron (page 101), a cavity with only 
one opening, the mouth. There is no other body cavity. 
They have tentacles, and in the ectoderm are stinging 
cells used for offense and defense. Their nervous system 
is very diffuse, consisting of a network of scattered cells. 
While such a system provides for related actions through- 
out the body, the coordination is often imperfect and 

Fig. 218. ,, , 

-Hydra, rather slow. 

with buds. There are in general two forms of body: (1) the polyp, 

Carolina Bio- which is typically tubular and elongated with tentacles 
Logical Supply arouud One end, and (2) the medusa or ic^llvlish, which 
is ordinarily compressed into a hemisphere or flat 
disk with tentacles around the edge. Polyp and medusa are really 
built on the same fundamental plan, as is readily understood if the 
mouth and the center of the convex surface of the medusa be imagined 



drawn apart so that the body is a long cyUnder hke a polyp (also see 
Fig. 143). The medusa is regularly free-swimming, though because it is 
produced by budding from the polyp form it remains in some species 
attached to its parent. The polyp is usually sessile, though sometimes, 
as in Hydra, it may become detached from one object and loop along 
to a new situation where it again glues itself fast. 


Fig. 219. — Various coelenterates: A, Gonionemus; B, Aurelia; C, sea anemone; D, coral. 
{A-C from Carolina Biological Supply Co.; D from Wolcott, "Animal Biology.'') 

Colony formation is common. Most of the hydroids are branching 
colonies. The corals have massive stony skeletons which in the aggregate 
may form reefs and atolls or other islands. The sea pens are colonies 
resembling a quill pen, with the pointed end thrust into the sand. Many 
of the colonial types are gorgeously colored and are responsible for some of 
the brilliance of tropical seas. The siphonophores (Figs. 145, 146, pages 
174, 175) are free-swimming colonies. 

Coelenterates exhibit a great deal of polymorphism. The polyp 
and medusa have already been mentioned as generalized types. Each 
may be considerably modified in different species and 
modified in several different ways in the same species. 
In the hydroids the medusa shows more variation than 
the polyp. It is free-living in some species, perma- 
nently attached to the hydroid colony in others. When 
attached, it may suffer considerable reduction; that is, 
it does not develop the full medusoid structure, which 
-would be useless to an inactive individual. Sometimes 
the reduction of the medusa is so great that practically 
only the gonads are left (Fig. 220). Then the medusa 
looks like a reproductive organ belonging to a colony 
of polyps. 

Much more marked polymorphism is found in the siphonophores 
(page 174). In them there are usually several kinds of structures which 
betray, sometimes in vague but often in unmistakable ways, their 
medusoid architecture and several other kinds which, in development or 
adult anatomy, are more or less like the polyp. 

These polymorphic species often show that type of alternation of 
generations which is kno^vn as metagenesis (page 174). One or more 

Fig. 220.— Hy- 
droid, with re- 
duced medusae. 
{Courtesy of Caro- 
lina Biological 
Supply Co.) 


kinds of individuals reproduce by budding (asexually), another kind by 
eggs and spermatozoa. 

In some groups (Scyphozoa, Fig. 219B) only the medusoid generation 
exists, and in them its structure is different (see table of characterizations 

Corals are the skeletons of two kinds of coelenterates, the Hydrocoral- 
linae and the Madreporaria (see below), the latter being the more com- 
mon. Aside from their use as ornaments, corals are of interest because of 
the long debate concerning the origin of coral reefs and atolls. The theo- 
ries of their origin differ largely in whether the sea bottom on which they 
grew was assumed to be subsiding, stationary, or rising. 

Class I. Hydrozoa. Coelenterates without stomodaeum and mesenteries; sexual 
cells discharged to the exterior; life history including hydroid form, or medusa (with 
velum), or both hydroid and medusa in same species. Polyps (including Hydra), a 
few corals, small jellyfishes. (Figs. 58, 59, 65A, 142, 144, 145, 146, 218, 219^, 220.) 

Order 1. 


Order 4. 


Order 2. 


Order 5. 


Order 3. 


Order 6. 


Class II. Scyphozoa. Coelenterates with only the jellyfish, not hydroid form; 
velum lacking; notches at margin of umbrella. Larger jellyfishes. (Fig. 219-B.) 

Order 1. Stauromedusae Order 3. Cubomedusae 

Order 2. Peromedusae Order 4. Discomedusae 

Class III. Anthozoa. Coelenterates without medusoid forms, with well-developed 
stomodaeum and mesenteries. Sea anemones, most corals. (Figs. 65A, B, 219C, D.) 

Subclass I. 


Order 1. 


Order 3. 


Order 2. 


Order 4. 




Order 1. 


Order 4. 


Order 2. 


Order 5. 


Order 3. 


Order 6. 


Phylum 4. Platyhelminthes. — This phylum includes the planarians 
(Fig. 221), the flukes (Fig. 222), and the tapeworms (Fig. 223). The 
name of the phylum comes from the generally flat form of the bod}'-, and 
its members are commonly called flatworms even when the body is not 
flat. The body is bilaterally symmetrical, the only phylum so far men- 
tioned to possess this form. The animals are triploblastic, the third 
layer being mesenchyme (page 82), which makes up the bulk of the body. 
The digestive tract is a coelenteron (page 80), opening only at the mouth, 
and there is no other body cavity. Parasitic forms may, however, lack 
the digestive tract completely. The free-living species have cilia 
on the epidermis, but the parasitic ones lack them. The excretory sys- 
tem is of the protonephridial type (page 134) ending in flame cells. 



The planarias, which are free-hving, Hve under stones or logs in fresh 
water. They have remarkable powers of regeneration, and have been 
used by many investigators to study the physiology of development and 
growth. The theory of gradients (page 217) in embryonic development 
originally grew out of studies on planarias. 

The flukes are parasitic. Some of them are external' parasites, as on 
the gills of fishes or other aquatic animals. Others — and these are the 
menacing ones — are internal parasites. Some of the latter pass through 
very complicated life cycles, in which the successive generations are 
totally different in form. Usually these different types of individuals 
must live in different hosts, one of which is a snail, the others being 
usually arthropods (Phylum 9, below) and vertebrate animals. One such 
life cycle involves four dift'erent hosts, following one another in a certain 

Fig. 221. — Planaria. 


Fig. 222. — A fluke. 
{From Van Cleave.) 

Fig. 223.— a tape- 

order. Sometimes the host, of any of the several successive general 
types, must be a particular species — a certain species of snail, a specific 
arthropod, a definite vertebrate species; in other trematodes there is a 
choice of species for host, but usually only a very limited one. Some 
degeneration (loss of eyespots, reduction of sense organs and nervous 
system) has been permitted by the parasitic mode of life, but the repro- 
ductive system is highly developed and specialized. 

The tapeworms are parasitic in the digestive tracts of vertebrate 
animals. They consist of chains of rectangular individuals budded off 
from a small "head" which is attached to the intestinal wall of the host. 
There is no digestive tract, and no use for one since all food is absorbed 
already digested by the host. Longitudinal nerves and longitudinal 
excretory tubes pass along the margins of the "tape," common to all the 
individuals in it; but each individual has its own highly developed repro- 
ductive system which makes up most of the substance of the animal. 
Man gets his commonest tapeworms from insufficiently cooked pork; 
thorough cooking is the best guarantee against infection. 



Class I. Turbellaria. Free-living flatworms with ciliated epidermis. Planaria. 
(Figs. 89, 221.) 

Subclass I. Rhabdocoelida 
Subclass II. Tricladida 

Subclass III. Polycladida 

Class II. Trematoda. Parasitic flatworms without cilia but with a hardened 
ectoderm, usually parasitic and with attaching suckers. Flukes. (Fig. 222.) 

Subclass I. Monogenea 

Subclass II. Digenea 

Class III. Cestoda. Parasitic flatworms with the body diff'erentiated into a 
scolex, an enlargement usually provided with suckers and sometimes with hooks, and a 
chain of similar structures (proglottides), the whole being usually regarded as a colony. 
Tapeworms. (Fig. 223.) 

Phylum 5. Nemathelminthes. — These are elongated, bilaterally 
symmetrical animals, commonly called round- or threadworms. They 

A B 

Fig. 224. — Important Nemathelminthes: A, Trichinella encysted in muscle; B, hook- 
worm. (A from photograph by General Biological Supply House; B from Rivas, "Human 
Parasitology," W. B. Saunders Company.) 

are triploblastic, and there is a "coelom" in the middle tissue layer. 
The digestive tract, unlike that of the two preceding phyla, is not a 
coelenteron, for it opens at both ends. There are no cilia on any part of 
the body. The sexes are separate; that is, some individuals are males, 
some females, none hermaphroditic. 

This is probably one of the richest phyla in numbers of species, but 
its species are not proportionately well-known. Most of the members 
of this group are free-living, and they are found in all sorts of situations, 
in water or soil. Some infest plant tissues. Others are parasitic in 
animals. The dread human disease called trichinosis is caused by round- 


worms which are introduced in insufficiently cooked pork. The pigs get 
it by eating meat refuse or infested rats. The larvae get into the lym- 
phatic vessels or bore out through the intestinal wall and enter the mus- 
cles, where they become encysted (Fig. 224A). Government inspection 
of meats is carried out in a few countries, but in some of those with the 
most rigid inspection the incidence of trichinosis is high. The reason is 
the habit of eating rare pork in those countries. Thorough cooking is 
the safest preventive; once the larvae are on their way to the muscles there 
is no cure. Members of another family of roundworms may cause ele- 
phantiasis by clogging the lymph passages. 

The hookworm (Fig. 2245) of the southern states is also a member of 
this phylum. The larvae develop in moist soil. From there they enter 
the body through the skin of the feet, get into the blood, and thus reach 
the lungs and intestines. By feeding upon the blood and causing bleed- 
ing through an inhibition of clotting they produce an anaemic condition. 
Injury to the lungs predisposes the victim also to tuberculosis. The 
shiftlessness of the ''poor whites" in the South is attributed in part to 
hookworm disease. An important feature of preventive measures is 
proper disposal of human feces, so as to prevent pollution of the soil, thus 
stopping further infection. Curative treatments are also available for 
those already diseased. 

Phylum 6. Echinodermata. — Members of this phylum are radially 
symmetrical in the main, though usually some small feature is eccentri- 
cally placed so as to introduce slight bilaterality. Usually there are five 
rays, but the number may be very much greater. The skeleton consists 
of limy plates, either firmly joined into a globular shell or more loosely 
aggregated in the body wall so as to be readily movable on one another. 
There is a distinct coelom. Many echinoderms possess a peculiar method 
of locomotion by means of tube feet. These are hollow muscular tubes, 
ending in suckers and filled with water by which they are operated. The 
tube feet may be thrust out long distances by pressure on the contained 
water, attached to fixed objects by the suckers, then contracted, pulling 
the whole animal slowly along. Locomotion is more rapid in the brittle 
stars, since the slender arms of these animals can be bent rapidly and pro- 
vide a sort of walking or running movement. Some of the feather stars 
are sessile^ being attached by a jointed stalk to the bottom. All members 
of this phylum are marine. 

Starfishes (Fig. 225 A) have arms usually well marked off from the 
body disk. The brittle stars (B) have this distinction of arms from the 
body disk especially clearly marked. The name brittle star comes from 
the animals' practice of breaking off injured arms, which thereupon 

Sand dollars (D) have a nearly smooth margin, without division into 



arms. The sea urchins (C) are globular and without arms. Sea cucum- 
bers {E) have no arms, but around the mouth is a series of branched 
tentacles. The arms of the feather stars are branched like a feather, and 
the branches are featherlike. 

The starfishes have the peculiarity of digesting their food outside the 
body. They prey upon clams, forcing the valves of the shell open by a 
steady pull with the tube feet. The stomach is thrust out through the 
mouth, pushed between the separated valves, and wrapped around the 
exposed parts of the clam, which is then slowly digested. Oyster beds 
suffer considerably from these attacks. The other kinds of echinoderms 
take their food inside the body. 

A curious habit is that of the sea cucumbers, of eviscerating them- 
selves when irritated. If they are attacked, the body wall contracts so 
vigorously that it bursts, and a part (or even all) of the intestine is forced 

A B C D E 

Fig. 225. — Various echinoderms: left to right, starfish, brittle star, sea urchin, sand dollar, 
sea cucumber. {Courtesy of Carolina Biological Supply Co.) 

out, along with the branching respiratory organs that are attached to 
the cloaca. The tangled mass of viscera may so hinder (or perhaps 
appease) the enemy as to stop the attack. During a brief resting period 
the missing internal organs of the sea cucumber are regenerated. 

Echinoderms are invaluable subjects in experimental laboratories 
because of the abundance of their eggs and the ease with which they may 
be obtained. Hundreds of studies of cytology, physiology of fertiliza- 
tion, and embryology have been made on the eggs of starfishes and sea 
urchins, and sometimes the other groups of echinoderms. 

The relationships of echinoderms to the other phyla have been much 
debated because there is little clear evidence of them. Adult anatomy 
is entirely different from that of any other animals, and conclusions 
drawn from developmental stages have been various. There is less basis 
for establishing kinships of echinoderms than of almost any other group. 

Class I. Asteroidea. Free-livinfi;, typically peiitamerous echinodoniis with wide 
arms moderately marked off from disk and with ambulacral grooves. Starfishes. 
(Fig. 2251.) 

Class II. Ophiuroidea. Free-living, typically pentameroiis echinoderms with 
slender arms sharply marked off from disk and no ambulacral grooves. Brittle stars. 
(Fig. 225B.) 


Class III. Echinoidea. Free-living, pentamerous echinoderms without arms; 
test composed of calcareous plates bearing movable spines. Sea urchins, sand dollars. 
(Figs. 76, 225C, D.) 

Class IV. Holothurioidea. Free-living, elongated, soft-bodied echinoderms with 
muscular body wall and tentacles around mouth. Sea cucumbers. (Fig. 225E.) 

Class V. Crinoidea. Sessile echinoderms with five arms generally branched with 
pinnules, aboral pole usually with cirri, sometimes with jointed stalk for attachment 
to substratum. Feather stars, sea lilies. 

Phylum 7. Annelida. — These are the true worms, as distinguished 
from Phyhim 4 and Phyhmi 5 whose members are called flatworms and 
roundworms. The annelids are triploblastic, bilaterally symmetrical 
animals, with elongated body divided into segments. The segmentation 
is internal as well as external, for thin membranes divide up the body 
cavity or coelom. Corresponding with these segments, many of the 
internal organs are repeated in most of the segments, while some are 
repeated in only a few of the segments. The excretory organs, which 
are nephridia (page 135), occur in most segments; the nervous system, 
which is chiefly a long cord near the ventral side, typically has a ganglion 
and nerves in most of the segments; and the main blood vessels give off 
branches in each segment. Spiny projections or setae are common aids 
to locomotion. 

Some of the annelids are hermaphroditic but do not fertilize their 
own eggs. Some of them (Fig. 226) also reproduce by budding or unequal 
fission. Many of them have remarkable powers of regeneration if cut 
into pieces. Some curious results are obtained by cutting off the head 
end of an earthworm; at certain levels the head structures are regenerated, 
while if cut at other levels a tail is developed in place of a head. 

Despite the large size which many annelids attain, some of the larger 
ones respire only through the general body surface. Some others, no 
larger, have branched or filamentous gills which greatly increase the area 
through which oxygen is absorbed. 

Some theoretical interest attaches to the larval stage of many marine 
annelids, which is known as a trochophore. It is pear-shaped or nearly 
spherical, with a circle of cilia around its equator. Similar larvae are 
found among the clams and snails, and adult rotifers may have roughly 
the same shape. Many biologists have considered that some relation- 
ship among these phyla is indicated by the trochophore larva or trocho- 
phorelike adult form. 

Among the services to man performed by annelids may be mentioned 
the comminution and constant overturning of the soil by earthworms. 
These animals eat the soil, for whatever organic matter it may contain, 
and eject it from their digestive tracts. In making their burrows, 
much soil is brought to the surface from below. The burrows also leave 
the soil porous. Some annelids are also used for human food, notably 



the palolo of Samoa and other Pacific islands. These worms burrow 
in the coral reefs, and swarm in the open water in huge numbers just 
before the last quarter of the moon in October and November. They are 
captured in quantity by the natives at that time. 

Fig. 226. — Autolytus, a 
marine worm. 

Fig. 227. — Aeolosoma. 
a fresh-water worm. 

Fig. 228.— a leech. (From 


Some of the annelida live in the soil (earthworms), many live in fresh 
water (Figs. 227, 228), and many are marine. Some of the leeches (Fig. 
228) attach themselves to vertebrate animals by means of suckers and 
feed upon the blood of their host. Chaetopterus (Fig. 229) lives in a 

Fig. 229. — Chaetopterus. {Courtesy of Carolina Biological Supply Co.) 

U-shaped parchmentlike tube in the sand under marine waters. The 
tube is open at both ends, and circulation of the water in it is maintained 
by flat appendages at the sides of the body. 

Class I. Archiannelida. Marine Annelida with no setae or parapodia. 
Class II. Chaetopoda. Annelida with setae and a perivisceral coelom; marine, 
fresh-water, or terrestrial in habitat. Earthworms. 

Subclass I. Polychaeta. With many setae. Marine worms. (Figs. 226, 


Order 1. Phanerocephala 

Subclass II. Oligochaeta. 
worms. (Figs. 135, 136, 227.) 

Order 1. Microdrili 

Order 2. 
With few setae. 


Fresh-water and terrestrial 

Order 2. Macrodrili 

Class III. Hirudinea. Annelida without setae, and with anterior and posterior 

suckers. Leeches. (Fig. 228.) 

Phylum 8. MoUusca. — This group includes clams, snails, and cuttle- 
fishes. Their structure is so diverse that the phylum is difficult to 
define. Mollusks are triploblastic, unsegmented, and bilaterally sym- 
metrical, though their symmetry is disturbed by a secondary spiral 
winding in some kinds. They have a coelom of restricted extent and 
usually possess a shell. The name moUusk refers to their soft bodies. 

A structure called the foot is characteristic of the phylum but varies 
greatly in form. In the chitons (Fig. 230) it is like the sole of a shoe. In 



the clams and mussels (Fig. 231) the foot is a wedge which plows through 
the sand or mud. In the snails (Fig. 232) it is flat, and the animal creeps 
along on it, usually by rapid wavelike muscular contractions, but some- 
times by means of cilia. The foot of a snail may secrete a mucous 
substance along which the animal creeps; a vertical 
roadway may thus be erected directly through the water 
without any support except at one end. The foot of the 
squids (Fig. 233), cuttlefishes, and nautili is transformed 
into a circle of arms bearing suckers. 

The shell consists of two valves in the clams, oysters, 
and mussels; is spirally wound in the snails; is a row of 
movable plates in the chitons; is entirely embedded in the 
flesh in the cuttlefishes, squids, and most slugs; and is 
entirely lacking in certain marine mollusks called nudi- 
branchs, which bear some resemblance to snails, and in a 
few species of several other groups. 

The sexes are usually separate, but one class of snails is hermaphro- 
ditic, as are also some members of other classes. Among the latter, self- 
fertilization may occur, or two animals may mate. 

The mollusks began as marine animals, then began to invade fresh 
water, and finally the land. Only the snails have gone on land, however. 

Fig. 230. 
— Chiton. 
(.Courtesy of 
Carolina Bio- 
logical Supply 

Fig. 231. — A clam. 

Fig. 232. — A snail. 

Fig. 233.— a squid. 

and only the snails and clams into fresh water. The group as a whole 
has always been a successful one. It has maintained its abundance 
throughout geological time and is as well represented by number of species 
now as it ever has been. The evolution of mollusks has in general led to 

a reduction of the shell and the growth of the 
mantle over it, though one class has escaped 
these changes. 

Man has an economic interest in mollusks 
in several ways. Oysters and clams are im- 
portant articles of food, the former being 
extensively cultivated. Pearls are made up of layers of nacre secreted 
around some irritating foreign object by the epithelial cells of clams. 
Such objects are deliberately inserted by pearl raisers, and pearls of any 
desired shape may be obtained. The same substance, nacre, on the 
inside of clam shells constitutes mother of pearl, which is used for buttons 

Fig. 234.— Teredo, the 
ship worm. {Courtesy of 
Carolina Biological Supply 


and knife handles. On the debit side of the ledger, the shipworm Teedo 
(Fig. 234, a mollusk, not a worm) bores' into wharves and shipping and 
does considerable damage. 

Class I. Amphineura. Mollusca with obvious bilateral symmetry, sometimes an 
eight-parted calcareous shell and several pairs of gills. (Fig. 230.) 

Order 1. Polyplacophora Order 2. Aplacophora 

Class II. Gastropoda. MoUusks with a head and with bilateral symmetry usually 
obscured by a spiral shell of one piece. Snails. (Fig. 232.) 

Subclass I. Streptoneura Subclass II. Euthyneura 

Order 1. Aspidobranchia Order 1. Opisthobranchia 

Order 2. Pectinibranchia , Order 2. Pulmonata 

Class III. Scaphopoda. Mollusca with conical tubular shell and mantle. 
Class IV. Pelecypoda. MoUusks without a head, with bilateral symmetry, a 
shell of two lateral valves and a mantle of two lobes. Clams, mussels. (Fig. 231.) 

Order 1. Protobranchia Order 3. Eulamellibranchia 

Order 2. Filibranchia Order 4. Septibranchia 

Class V. Cephalopoda. MoUusks with distinct bilateral symmetry and a foot 
bearing eyes and divided into arms usually with suckers. Cuttlefishes, octopods. 
(Fig. 233.) 

Order 1. Tetrabranchia Order 2. Dibranehia 

Phylum 9. Arthropoda. — Members of this phylum have jointed 
bodies and jointed legs. Their skeletons are composed of a horny mate- 

FiG. 235. — A centipede. Fig. 236.— A beetle. 

(From Hegner, "College Zoology," The Macmillnn Company.) 

rial on the outside of the body. This horny shell is burst and shed at 
intervals, and replaced by a new skeleton beneath, as the animal grows. 
Examples of arthropods are crayfishes, shrimps, centipedes (Fig. 235), 
insects (Fig. 23(5), and spiders (Fig. 237). They are triploblastic and 
))ilaterally symmetrical. The blood system includes sinuses, which are 
merely spaces among the organs, into which the arteries open. The 
coclom is much reduced in size. 

The number of differcmt kinds of arthropods is almost unbelievably 
great. More known species belong to this phylum than to all other phyla 
combined. About half a million have been described, but the number 


must be several times as great. The insects furnish a greater share of 
these than all other arthropods together. 

Arthropods are found in practically all situations that support life — in 
fresh and salt water, in mud, burrowing in soil, on the surface of the earth 
where they feed on animal or plant food, flying in the air, boring in trees 
or herbaceous plants, and parasitic in or on animals. 

While most arthropods go through a fairly direct development, such 
that they are readily recognized at all stages even by the uninitiated, 
some of them, including many insects, have a striking metamorphosis 
involving larva, pupa, and adult. In the larva there are groups of cells 
forming the rudiments of the adult organs. These persist through the 
pupa, but the rest of the larval organization disintegrates into a milky 
mass which is doubtless partly used as nutrition for the growing adult 

Fig. 237. — A slider. Fig. 238. — A crab. Fig. 239.— A crayfish. 

(Courtesy of Carolijia (From. Van Cleave.) 
Biological Supply Co.) 

Among the interesting features of arthropods is the social organization 
among some of the insects. The bees, ants, wasps, and termites have 
structurally different types of individuals which are also distinguished as 
social castes, sometimes in a very complicated system. 

Worthy of note with respect to reproduction in the phylum is the 
rather frequent occurrence of parthenogenesis. In some of the smaller 
Crustacea there is diploid parthenogenesis, in which the eggs do not 
experience a reduction division. Such parthenogenesis may be repeated 
for many generations but is usually interspersed with bisexual reproduc- 
tion at intervals. In bees and many other insects there is haploid parthe- 
nogenesis, meaning that the egg which develops without fertilization has 
undergone chromosome reduction. The haploid individuals thus pro- 
duced are regularly males. 

Many members of this phylum are of economic importance to man. 
Lobsters, crabs (Fig. 238), in some regions crayfishes (Fig. 239), and 
shrimps are used as food, and bees collect honey in domestication. Small 
aquatic forms are common food for game fishes. Insects often pollinate 
flowers and are important to certain seed crops and fruits (figs). The 
silkworm moth is a valuable adjunct to the textile industry. Many 
species are injurious. They may destroy fruit or grain crops or shade 



trees or carry disease-producing organisms (mosquitoes, housefiies). 

Some of the insects are parasitic in domestic animals, and the mites may 

attack the skin of man ("jiggers"), poultry, 
or cattle. Barnacles injure bottoms of ships. 
Relationship of the arthropods to the 
annelids has often been suggested, largely 
because of the segmentation of the body into a 
longitudinal chain of metameres. In support 
of this idea is brought the annelidlike Peri- 
patus (class Onychophora, below). Peripatus 
has a body superficially like a worm and has 
segmentally arranged nephridia; but it has 
tracheae like insects, sinuses in the circulatory 

system, and no coelom. If Peripatus is a primitive form, which may be 

doubted, its value as a connecting link is considerable. 

Fig. 240. — A scorpion 
(left) and king crab. {From 
Carolina Biological Supply 


Class I. Crustacea. Arthropods 

breathing by means of gills, 

antennae. Crayfishes, crabs, and shrimps. 

(Figs. 238, 

239, 299, 300.) 

Subclass I. Branchiopoda 

Order 1. Phyllopoda 

Order 2. 


Subclass II. Ostracoda 

Subclass III. Copepoda 

Subclass IV. Cirripedia 

SxjBCLAss V. Malacostraca 

Order 1. Nebaliacea 

Order 6. 


Order 2. Anaspidacea 

Order 7. 


Order 3. Mysidacea 

Order 8. 


Order 4. Cumacea 

Order 9. 


Order 5. Tanaidacea 

Order 10. 


two pairs of 

Class II. Onychophora. Primitive air breathing arthropods with tracheae and 
nephridia. Peripatus. 

Class III. Myriapoda. Arthropods with tracheae, one pair of antennae, and 
many similar legs. Centipedes and millipedes. (Fig. 235.) 

Order 1. Pauropoda Order 3. Chilopoda 

Order 2. Diplopoda Order 4. Symphyla 

Class IV. 

Insecta. Arthropods with tracheae, one 

pair of antenn 

airs of legs. 

Insects. (Figs. 95, 203, 


236, 303.) 




Order 11. 





Order 12. 





Order 13. 





Order 14. 





Order 15. 





Order 16. 





Order 17. 





Order 18. 





Order 19. 


Order 10. 




Class V. Arachnida. Arthropods with tracheae, book lungs or book gills and no 
antennae. Spiders, mites, scorpions, king crabs. (Fig. 240.) 

Order 1. 


Order 6. 


Order 2. 


Order 7. 


Order 3. 


Order 8. 


Order 4. 


Order 9. 


Order 5. 


Order 10. 


Invertebrate Groups of Uncertain Position. — Certain groups of inver- 
tebrates have not been assigned a definite relation to other groups. 
Opinion differs so widely as to their affinities that they may well be 
kept out of the classification for the present. 

Mesozoa. Parasites apparently intermediate between the protozoa and metazoa. 
Not improbably degenerate relatives of the flatworms. , ;,». 

Nemertinea. Terrestrial, fresh-water, and marine animals resembling flatworms 
but with a proboscis, blood-vascular system, and alimentary canal with two openings. 

Nematomorpha. Long threadlike animals with the body cavity lined with 
epithelium, a pharyngeal nerve ring, and a single ventral nerve cord. 

Acanthocephala. Parasitic worms with spiny proboscis, a complex reproductive 
system, and no alimentary canal. (Fig. 241.) 

Fig. 242. — Arrowworm, Sagitta. 

Fig. 241. — Echinorhynchus, one of the 

Chaetognatha. Marine invertebrates with a distinct coelom, alimentary canal, 
nervous system, and two eyes. Arrowworm. (Fig. 242.) 

Ctenophora. Triploblastic animals; symmetry partly radial, partly bilateral; 
eight rows of vibratile plates radially arranged. Sea walnuts or comb jellies. (Fig. 

Fig. 243. 


{From Van 

Fig. 244. — A rotifer. 
{From Whitney.) 

Rotifera. Invertebrates with a head provided with cilia, usually a cylindrical or 
conical body often with a shell-like covering, and a tail or foot, bifurcated at the 
tip where it is provided with a cement gland. (Fig. 244.) 

Bryozoa. Mostly colonial invertebrates resembling hydroids in form, with dis- 
tinct coelom, and with digestive tract bent in the form of a letter U. (Fig. 140.) 

Phoronidea. A single genus of wormlike animals having tentacles and living in 
membranous tubes in the sand. 



Brachiopoda. Marine tentaculate animals with a calcareous shell, composed of 
two unequal valves, a dorsal and a ventral. (Fig. 245.) 
Gephyrea. Wormlike animals of doubtful affinities. 

Fig. 245. — A brachiopod. Left, the shell; right, the animal. 

Phylum 10. Chordata. — The animals of this phykim have at some 
stage a skeletal axis called the notochord, gill slits in the embryo or adult, 
and a nerve cord dorsal to the alimentary canal. (In preceding phyla 
when a nerve cord is present it is ventral to the alimentary tract.) This 

Fig. 246. — Balanoglossus. {From Carolina 
Biological Supply Co.) 

Fig. 247. — A tunicate. {From Carolina 
Biological Supply Co.) 

phylum includes a number of degenerate animals such as Balanoglossus 
(Fig. 246) and the tunicates (Fig. 247) which must be included here 
because of the presence of the notochord in the embryo. It also includes 
the amphioxus (Fig. 248), a fishlike animal in which the notochord is the 

^■.'J?:y* \^v>:9W:fi:m^ 

Fig. 248. — Amphioxus. 

Fig. 249.^ — Lamprey. {From Carolina 
Biological Supply Co.) 

permanent skeletal axis. The remaining chordates are called vertebrates 
from the fact that the notochord becomes invested with cartilage which 
is segmented to form a vertebral column. In some lower forms the carti- 
laginous vertebrae and the notochord which they surround persist 
throughout the life of the animal, but in the higher forms the cartilage is 
replaced by bone and the notochord disappears. 

Fig. 250. — Hagfish. {From Carolina Bio- 
logical Supply Co.) 

Fig. 25 L — A shark. {From Carolina Bio- 
logical Supply Co.) 

At the bottom of the vertebrate scale are the lampreys (Fig. 249) and 
hagfishes (Fig. 250) which are eellike in form but have no jaws and no 
lateral fins. The skeleton is made of cartilage only. Some of the lam- 
preys inhabit fresh water and lay their eggs in nests made by pulling up 
stones from the bottoms of brooks. Next above these in the scale are 
the sharks (Fig. 251), skates (Fig. 252), and rays, whose skeletons are also 
cartilaginous but which have jaws. Their skin is armored with a type of 
scale having a tooth or spine mounted on a flat base. The dried and proc- 



essed skin Avith these scales forms the leatherhke natural shagreen of 
certain costume accessories. 

Above these are five major groups, the true fishes, amphibia, reptiles, 
birds, and mammals. Differences among these are found in the hard 
parts of the skin, the form of the limbs, the structure of the heart, and 
the means of respiration. 

Fig. 252. — Skate. 


253. — A fish. {From Carolina Bio- 
logical Supply Co.) 

The fishes (Fig. 253) are aquatic, and respire by means of gills. The 
skin usually bears scales, but these are not toothed like the scales of 
sharks. The skeleton is at least partly of bone. Locomotion is effected 
by fins (and the bending of the body), and the heart consists of but two 
chambers (one auricle and one ventricle). 

Fiu. 254. — A saltiniander. 

Fig. 255.— a frog. 

The amphibia are the salamanders (Fig. 254), toads and frogs (Fig. 
255), and certain legless forms called caecilians. Their skin is smooth, 
nearly always devoid of scales, thovigh some fossil amphibia were heavily 
armored. They are nearly all aquatic at least in young stages, and some 
of them throughout their lives. Division of their habitats between water 

Fig. 256. — A lizard. {From Carolina Bio- 
logical Supply Co.) 

Fig. 257. — A turtle. {From Carolina Bio- 
logical Supply Co.) 

and land is what gives the class its name amphiliia. The heart is three- 
chambered — two auricles, one ventricle. Though the amphibia are of 
less value to man than are the fishes, frogs' legs are a table delicacy, toads 
devour many insects, and most orders have contributed material for 
important biological and medical investigations. 

Reptiles include lizards (Fig. 256), snakes, alligators, turtles (Fig. 
257), and such fossil forms as dinosaurs. Their skin contains scales or 


hard plates. They are cold-blooded in common with the fishes and 
amphibia but unlike the following two classes. They have no gills in any 
stage. The heart is three-chambered (approximately four-chambered in 
crocodiles, in which the ventricle is partially divided). Some snakes are 
poisonous, but most of them are beneficial to man (as are also the lizards) 
because they devour noxious animals. Some turtles are used for food. 

The birds are characterized by feathers, which grow from pits in the 
skin, forelimbs adapted to flight, a four-chambered heart, warm blood 
(warmer than that of the next class, mammals), and a beak with horny 
covering but no teeth. The bones of the skeleton are extensively fused, 
particularly in the wings. The body is made light for its size by large 
air spaces, variously placed, some of them extending into the cavities of 
certain bones. These spaces connect with the lungs, but their walls are 
not made of lung tissue, though doubtless they do effect some exchange of 
oxygen and carbon dioxide. 

Mammals are mostly quadrupeds. The skin is covered with hair 
— very sparsely in some. They breathe air even when they inhabit water. 
The heart is four-chambered, the blood warm. The red cells of the blood 
are devoid of nuclei except while they develop in the marrow. There is 
a muscular sheet or diaphragm between the thorax and the abdomen, 
important in breathing. The young are usually developed in the uterus 
of the female — a few lay eggs — and are nourished with milk from the 
mammary glands after birth. The most primitive mammals, the egg 
layers, live in Australia and neighboring islands. The marsupials, which 
give birth to their young in a very early stage and carry them for a long 
time in a pouch, are next most primitive. They live in the Australian 
region, in South America, and one kind (opossum) in North America. 

SUBPHYLUM I. Enteropneusta. Wormlike chordates of somewhat doubtful 
systematic position. (Fig. 246.) 

Order 1. Balanoglossida Order 2. Cephalodiscida 

SUBPHYLUM II. TUNICATA. Saclike marine animals with a cuticular outer covering 
known as a tunic or test. Tunicates. (Fig. 247.) 

Order 1. Ascidiacea Order 3. Larvacea 

Order 2. Thaliacea 

SUBPHYLUM III. Cephalochorda. Fishlikc chordates with a permanent noto- 
chord composed of vacuolated cells. Amphioxus. (Fig. 248.) 

SUBPHYLUM IV. VertebratA, Chordates in which the notocliord cither persists 
or becomes invested by cartilage, segmented, to form a vertebral colunui, or disap- 
pears, the vertebral column being made up of bony segments. 

Class I. Cyclostomata. l']ellike vertebrates without functional jaws or lateral 
appendages. Lampreys and hagfishes. (Figs. 249, 250.) 

Subclass I. Myxinoidea Subclass II. Petromyzontia 


Class 11. Elasmobranchii. Fishlike vertebrates without air bladder, with jaws, 
and with a cartilaginous skeleton and placoid scales. Sharks, rays, and skates. 
(Figs. 251, 252.) 

Subclass I. Selachii 

Order 1. Squali Order 2. Raji 

Subclass II. Holocephali 

Class III. Pisces. Aquatic, cold-blooded vertebrates breathing by means of gills, 
with air bladder, a two-chambered heart, and usualjy a dermal exoskeleton of scales. 
Fishes. (Figs. 159, 253.) 

Subclass I. Teleostomi. Fishes with a skeleton consisting wholly or par- 
tially of bone, breathing by means of gills. True fishes. 

Order 1. Crossopterygii Order 3. Holostei 

Order 2. Chondrostei Order 4. Teleostei 

Subclass II. Dipnoi. Fishes with a skeleton of cartilage and bone, and air 
bladder functioning as a lung. Lungfishes. 

Class IV. Amphibia. Cold-blooded vertebrates breathing by means of gills in 
some stage, skin usually not covered with scales, heart of three chambers. Sala- 
manders, toads, and frogs. (Figs. 93, 151, 157, 158, 163, 187, 254, 255.) 

Order 1. Caudata Order 3. Apoda 

Order 2. Salientia 

Class V. Reptilia. Cold-blooded vertebrates usually covered with scales, breath- 
ing throughout life by means of lungs. Lizards, snakes, crocodilians, turtles. (Figs. 
156, 256, 257, 279, 281.) 

Order 1. Testudinata Order 3. Crocodilini 

Order 2. Rhynchocephalia Order 4. Squamata 

Class VI. Aves. Warm-blooded vertebrates with the body covered with 
feathers, with the forelimbs usually modified as wings, and a heart of four chambers. 
Birds. (Figs. 161, 201.) 

Struthioniformes. Ostriches. 
Rheiformes. Rheas. 
Casuariiformes. Cassowaries, emus. 
Apterygiformes. Kiwis. 
Tinamiformes. Tinamous. 
Sphenisciformes. Penguins. 
Gaviiformes. Loons. 
Colymbiformes. Grebes. 
Procellariiformes. Albatrosses, petrels. 
Pelecaniformes. Pelicans, frigate birds. 
Ciconiiformes. Herons, storks. 
Anseriformes. Ducks, geese, swans. 
Falconiformes. Vultures, hawks, falcons. 
Galliformes. Pheasants, grouse, turkeys. 
Gruiformes. Cranes, rails. 
Charadriiformes. Shore birds, gulls, auks. 

Subclass I. 

Subclass II. 

Order 1. 

Order 2. 

Order 3. 

Order 4. 

Order 5. 

Order 6. 

Order 7. 

Order 8. 

Order 9. 

Order 10. 

Order 11. 

Order 12. 

Order 13. 

Order 14. 

Order 15. 

Order 16. 



Order 17. C-olumbiformes. Pigeons, doves, sand grouse. 

Order 18. Psittaciformes. Parrots, macaws. 

Order 19. Cuculiformes. Cuckoos, plantain eaters. 

Order 20. Strigiformes. Owls. 

Order 21. C'aprimulgifornies. Goatsuckers, oil birds. 

Order 22. Micropodiformes. Swifts, hummingbirds. 

Order 23. C'oraciiformes. Kingfishers, hornbills. 

Order 24. Piciformes. Toucans, woodpeckers. 

Order 25. - Passeriformes. Broadbills, ovenbirds, lyrel)irds, songi)irds. 

Class VII. Mammalia. Warm-blooded animals which are covered with hair at 
some stage, suckle the young, and have a diaphragm between thorax and abdomen. 
Mammals. (Figs. 92, IgO, 193, 197, 272, 294.) 


Order 1. Monotremata 
Subclass II. Metathkria 

Order 1. Marsupialia 
Subclass III. P]utheria. 

Order 1. Insectivora 

Order 2. Dermoptera 

Order 3. C'hiroptera 

Order 4. Carnivora 

Order 5. Pinnipedia 

Order 6. Menotyphla 

Order 7. Rodentia 

Order 8. Lagomorpha 

Order 9. Primates — With nails 

Egg-laying mammals. Monotremes. 
Pouched mammals. Marsupials. (Fig. 157. > 

Viviparous mammals. 
Order 10. 
Order 11. 
Order 12. 
Order 13. 
Order 14. 
Order 15. 
Order 16. 
Order 17. 
Order 18. 


True mammals. 
Hyracoidea > With 
Xenarthra \ hoofs 





CoMSTOCK, J. H. A Manual for the Study of Insects. Comstock Publishing Com- 

Pratt, H. S. A Maimal of Common Invertebrate Animals. A. C. McClurg & 

Storer, Tracy I. General Zoology. McGraw-Hill Book Company, Inc. For fuller 
account of various groups, also biological principles. 


Ecology deals with the relations of organisms to the environment. It 
has to do primarily with those relations to environment which determine 
the organism's characteristics, its success, its mode of life, and its distribu- 
tion. Ecology is also concerned with the environment itself. Since the 
things to which animals and plants are sensitive in the world about them 
are not everywhere the same, any organization which the environment 
may possess is of importance to living things. This organization is some- 
times very intricate, and many an ecological study has been directed 
toward an understanding of the environmental system, mthout imme- 
diate reference to any particular organism. 

The environmental relations of organisms may be approached from 
two different points of view: (1) that of the individual or single species, 
in which case ecology comes very near to a limited physiology, and (2) 
that of groups of species living in the same general situations and forming 
what are called associations or communities. These two points of view 
are successively adopted in this chapter. 

Temperature. — Each kind of animal is capable of carrying on its 
metabolism only within a certain range of temperatures. At some point 
within this range, usually above the middle but sometimes below, the 
physiological processes work best. For most animals the lower limit is 
slightly above freezing, while the upper limit is usually below 45°C. Fish 
eggs develop best a few degrees above freezing, birds' eggs at about 40°C. 
Some animals possess remarkable powers of adjusting themselves to tem- 
peratures outside their usual range. Thus some of the pj-otozoa which 
die when raised ^\dthin a short time to a temperature of 23°C. will endure 
70°C. if the temperature is raised very gradually. 

Since temperature varies irregularly on the earth's surface from the 
equator to the poles, with elevation above sea level, seasonally, and as 
between day and night, it is obvious that animals must be so located that 
their permissible temperatures are present and that their limits are not 
overstepped. Ordinarily, species with a low optimum temperature must 
live in temperate or cooler zones, those Avith a high optimum temperature 
in tropical regions. The factor of dormancy also enters into the deter- 
mination of geographic position. Most animals become torpid at suffi- 
ciently low temperatures, and some endure actual freezing. Many of 



them, however, cannot be dormant for any great length of time and still 
live. Such species have to live in tropical regions. 

Some animals avoid the dangers of extreme temperatures by special- 
ized habits. Certain wasps which dig in sand dunes cannot endure for 
long the high temperature at the surface of the sand on sunny days. 
They survive these temperatures by digging vigorously for a few seconds, 
then flying about a few inches above the sand where the air is much cooler, 
then returning to their digging for a brief period. 

The regulation of body temperature by birds and mammals has 
already been described (page 120). This physiological feature enables 
animals of these classes to range widely so far as temperature is con- 
cerned. Among cold-blooded animals there is occasionally the ability 
to regulate temperatures in groups. Honeybees can do this in colonies, 
though each individual bee cannot. A certain amount of heat is liberated 
in their metabolism; and if this is conserved in masses of bees, the tem- 
perature may be considerably raised. Temperatures in their hives are 
much higher than those outside in winter. 

One curious relation to temperature is the acceleration of metabolism 
by fluctuating as compared with constant temperatures. Grasshopper 
eggs develop much more rapidly at their optimum temperature if that 
temperature has been interrupted by a cold period. The acceleration is 
greater if the interruption by low temperature comes early than if it 
comes late in development. Eggs are laid by these insects in late summer 
and fall, over a period of many weeks. It would be expected that those 
laid early would be the first to hatch the following spring ; but all the eggs 
hatch about the same time. Those laid late in the fall enter the winter 
in an early embryonic stage but are accelerated enough more in the spring 
to enable them to overtake their older companions and emerge at the 
same time. This is an important reaction to temperature, for if any of 
the young grasshoppers emerged much later in spring or summer they 
would miss the most favorable period of the year for passing through 
their immature nymphal stages. 

Some anilnals change their reactions to other stimuli with changes of 
temperature. Thus one of the leaf-boring beetles studied by Chapman 
goes toward the light at high temperature and takes to flight if mechani- 
cally disturbed, but avoids light at low temperature and draws up its 
legs and falls if disturbed. These beetles live on a plant which grows in 
water, and during the warm part of the day they are out on the leaves. 
If their reactions were reversed and they were disturbed at this time, they 
would fall into the water, but instead they fly away. Disturbance during 
the cool part of the day, when they are hiding at the center of the plant, 
merely causes them to fall into the recesses at the bases of the leaves. 

The structure of an animal sometimes depends on the temperature. 


Among the aphids or plant lice some individuals have wings, other do 
not, and it has been sho^vn that temperature helps to determine whether 
wings develop. No general rule can be given for the control of wing 
production, since different strains respond differently, even within the 
same species. In one strain maximum wing production is obtained if 
the parents are reared continuously at low temperatures. Since the 
aphids generally alternate between two host plants during a season, and 
since ^\ings are the easiest means of effecting their migrations, it is impor- 
tant to them that uings develop in at least some individuals at the right 
time. Another insect that responds developmentally to temperature 
is the vinegar fly, Drosophila. One of its mutant varieties has vestigial 
wings (Fig. 204, page 236) which are quite useless for flight. At very 
high temperatures, however, the wings of this variety are nearly normal. 
This response happens to be of very little use to the flies for two reasons : 
first, the temperature which induces full wing development is so high 
that it is otherwise detrimental and flies seldom meet exactly that 
temperature, and, second, inability to fly is not this mutant's worst 
handicap, since it is physiologically weak and never matures so rapidly or 
in so large numbers as do the normal flies of the species. Color in butter- 
flies is likemse known to be affected by temperature, and it seems certain 
that the differences between the northern and southern varieties of a 
species are sometimes thus determined. 

Genetic and evolutionary effects of temperature are known in a few 
organisms. Mutations in Drosophila have been produced by heat in 
experiments by Goldschmidt and others. The amount of separation and 
recombination by characters in this fly due to breakage and reconstitu- 
tion of chromosomes by exchange of pieces (page 236) is increased by 
high temperature. And Seller has found that whether a given sex- 
determining chromosome in a certain moth goes into the polar body or 
remains in the egg at the meiotic division depends partly on the tem- 
perature. The effect is such that more females are produced at high 

Light. — The most obviously important influence of light upon the 
ecology of animals is its effect upon green plants upon which the animals 
feed. These plants are dependent on photosynthesis for their own 
nutrition and can maintain themselves only where sufficient light is 
present. Animals that live in caves must therefore subsist on plants 
that do not carry on photosynthesis or on other animals whose food 
chains do not end in green plants. In moderately deep lakes, as is pointed 
out later, green plants are limited to the surface water, if floating, and 
to a strip along the shore, if rooted (Fig. 258). Animals dependent 
on such plants for food must spend part of their time in the regions 



Another influence that may be indirectly important for animals is 
the effect of daily duration of light upon the reproductive processes of 
plants. Many plants mil come to flower only if they are exposed to a 
certain number of hours of light each day. . A certain range of duration 
is always permissible. If the daylight period is longer or shorter than 
this required range of hours, the plant may grow vegetativel}'^ even more 
vigorously than usual but will not bloom. Unless a plant has some 
satisfactory asexual method of propagation, it cannot maintain itself 
in a region not affording the right duration of daylight. Probably no 
plant whose required amount of daylight is yet known is the sole food 

l"'iu. 258. — Shore vegetation of lake, which is too deep elsewhere for rooted plants. {Photo- 
graph by F. C. Gates.) 

of any species of animal, but the possibility exists that the range of some 
animal is thus limited by the length of day. 

Of structural changes induced in animals b.y light, the most significant 
ecologically is probably the production of wings in aphids. In some 
sti-ains of aphids, light has an e\c\\ more important influence than has 
temperature, the effect of which is described above. In one of these 
strains, alternating light and darkness caused nearly every individual to 
be winged, provided the dark period was at least 12 hours long. Shorter 
periods of darkness, including continuous light, made most of the aphids 
of this strain wingless. Since temperate regions in summcn- do not have 
12-hour nights, wing production must be considerably cui'tailed in such 
strains. Other strains respond differently to light, some of them directly 
reversing the behavior just described. The importance of wings in 
the migration of these insects from one host plant to another has already 
been mentioned. 


The color of flatfishes, certain shrimps, and some other animals 
changes to correspond to the background on which they rest. When 
on a dark background, the pigment diffuses so as to fill the cells that 
contain it and in the aggregate makes the animal dark. When on a 
light background, the pigment collects into small knots, leaving much 
of the surface exposed; hence the animal is pale. These changes may be 
a concealing adaptation helping the animals to escape enemies. 

]Many animals respond to light with changes of behavior, some of 
which are of ecological significance. Isopods, the "pill bugs" or "sow 
bugs" that live under boards or stones or in other dark places, are driven 
into these places by their negative reaction to light. Such situations 
are generally moist, which is necessary for an animal which, like the pill 
bugs, respires by means of gills. Most other Crustacea live in water, but 
some of the pill bugs have taken to land and have done so by utilizing 
damp places. Their crevices also doubtless give them some useful 

Some animals change their response to light according to certain 
other conditions. A species of thrips, a minute flower-inhabiting insect, 
crawls away from the soui'ce of light when it is quiet but is positive to 
light when mechanically disturl^ed. Under ordinary conditions these 
reactions drive the insect into the flower (a clover head, for example) 
^vhere its food is; but if the flower is vigorously shaken, as by a grazing 
animal, it crawls out. Probably their lives are often saved by this 

A more complicated adaptation involving response to light is exhibited 
by a parasitic copepod (crustacean) named Lernaeopoda. This animal 
is free-living in its larval stage but must attach itself to the gills or some 
other part of the brook trout to complete its development. During 
the day the larval copepod, because it is positive to light, swims near the 
surface of the water, but at night it sinks to the bottom because it is 
heavier than water. The brook trout likewise swims near the surface 
in the daytime, either in response to light or in deliberate search for food 
organisms which are located there, but at night settles to the bottom 
because of its high specific gravity. Day and night, therefore, fish and 
copepod are brought together — an arrangement highly satisfactory for 
the parasite but not so advantageous for the host. 

Moisture. — All organisms contain in their protoplasm a certain 
amount of water, usually a very large amount. Without it they are 
unable to function as living things. Many of them are so constructed 
as to be unable to maintain this required water without living directly 
in water. Probably no animal can endure complete desiccation, though 
there are some that can exist for a long time in situations regarded as dry. 
Protozoa may secrete a thick wall (cyst) and lie in dry hollows (former 


ponds) or be blown about by the wind. Eggs of Crustacea and rotifers, 
similarly covered with heavy shells, may likewise be dried without all 
being killed. One family of rotifers may be dried in the adult stage, as 
may also certain roundworms. Earthworms burrow deeper in the soil 
as moisture disappears near the surface, and eventually they roll up in 
balls to conserve their moisture. 

Excess of moisture is often as injurious as dearth of it. Soil organisms 
may be drowned in wet seasons because the air is driven from the soil 
by water, and they are unable to obtain their required oxygen from 
water. The sugai'-beet root louse suffers most damage from excess 
moisture at the time of hatching from the egg and at the periodic shedding 
of the skin as it grows. So much damage is done at these times that the 
louse multiplies in dry soil more than fifteen times as fast as in soil 
moistened from below, and nearly thirty times as fast as in soil moistened 
by water falling from above. 

Insects that suck the sap of plants are more or less independent of 
moisture in the air around them, as long as their host plants can maintain 
themselves. Indeed, in such animals the water may be regarded as a 
waste material to be eliminated. The white fly, common on many 
greenhouse plants, ejects water from its rectum in frequent bubbles 
that burst and spray over the surrounding leaf surfaces at considerable 
distances. Aphids are similarly supplied with excess water. 

Among the higher animals, the water requirements differ enormously. 
Mammals that lose much water through sweat (man, horse) or con- 
siderable excretion of urine or milk (cattle) must make good the loss by 
drinking. Most mammals are included in one or more of these categories, 
but some manage to get along with very little water except that taken 
with their food. Camels are the classical illustration of the ability to 
do without water, since they can subsist a week with only dry food, and 
if they are fed green plants they can avoid other water for a month or 
more. Mountain goats, prong-horn antelopes, mule deer, jack rabbits, 
gazelles, jumping mice, and some of the ground squirrels are said to use 
only the water that is eaten with other food. Such animals are peculiarly 
fitted for regions where there are few or no bodies of water. 

One important ecological function of water in the protoplasm of 
animals is its modification of the effect of temperature. Relatively 
dry protoplasm endures high temperatures — even above that of boiling — 
without coagulation, and low temperature without freezing. It is not 
necessary that the water be actually removed from protoplasm to pro- 
duce this effect, but merely that the amount of free water be reduced. 
Thus, in the pupa of the polyphemus moth, which is covered with a thick 
horny coat, there is little actual evaporation of the water, but as winter 


approaches more of the water is adsorbed on the colloidal (page 42) 
particles in the pupal liquids, leaving less water free. As a consequence 
of this condition, the pupa endures winter freezing for months. 

Nutrition. — With very few exceptions, all of which are among the 
protozoa, animals are ultimately dependent on plants for their food. The 
green plants provide carbohydrates by photosynthesis, and a few micro- 
organisms; including those forming nodules on the roots of clover and 
other legumes, can utilize the nitrogen of the air to produce nitrites and 
nitrates. Out of these primary substances animals can make any com- 
pounds they require, but plants have to make the start. 

The manner of taking foods from plants is very variable. Many 
insects or their larvae eat the leaves or suck sap from the leaves, stems, 
or roots. Some eat the wood, though it is quite possible that fungi or 
other organisms growing on the wood or in the burrows form part of their 
food. Bees get carbohydrates (honey) from the flowers and proteins 
from pollen. Many animals grow on decaying logs or other plant matter, 
but it is likely that the microorganisms which are always present in 
such places constitute the actual food. Of the animals that do not feed 
directly upon plants but upon other animals, the larger ones usually, 
and the small ones often, kill their prey and eat its flesh. The larvae 
of the clothes moth eat hair or wool. Some insects live in the excrement 
of animals, but here again it is probably the microorganisms that furnish 
the food. 

A very special way of obtaining nutrition is through parasitism. 
The host is usually not killed — -at least until the parasite is past its para- 
sitic stage — but contributes some of its substance to the parasite. The 
flukes and tapeworms are regularly parasitic, as are some of the round- 
worms and some insects. Parasites show a tendency to be degenerate, 
which they can afford to be, since in their protected situations and with 
their food often digested (page 265) before they receive it many of the 
usual organs are unnecessary. The advantages of parasitism accrue only 
to the parasite, none to the host. 

Contrasted with this is the relation known as symbiosis, which is an 
association of two species with mutual benefit. A very striking example 
of symbiosis, in which food appears to be at least part of the advantage 
gained by both species, is that existing between termites (the so-called 
white "ants") and certain protozoa harbored in their intestines. The 
protozoa are so abundant that in some instances they weigh as much 
as the termite itself. The termites are wood-eating insects, and their 
normal food is cellulose. They are not themselves, however, able to 
digest the cellulose. This is done for them by the protozoa (page 259). 
These protozoa may be removed from the intestine artificially by high 


temperature or increatse of oxygen or starvation, and aftei* that the 
termites are no longer capable of .subsisting on wood. Also, the protozoa 
are unable to survive outside the termites. 

The amount of food and frequency of taking it vary greatly in different 
animals. A certain protozoon can swallow another protozoon ten 
times its own bidk, digest it in two hours, and be ready for anotlier; 
while some insect larvae may eat a hundred times their own weight daily. 
Cold-blooded vertebrates, on the contrary, subsist on small quantities 
eaten at long intervals. Certain birds may go without food for four or 
five weeks, a lobster for months. Some insects do all their eating in the 
immature stages and take no food when adult; certain butterflies and 
May flies are examples. Male rotifers get all their food by eating done 
a generation in advance; for they take no food after hatching, all their 
nutrition coming from material stored in the eggs from which they hatch. 

►Structural characteristics in a few animals are determined or modified 
by their food. In the honeybee, for example, any fertilized egg may 
tlevelop into a (jueen bee; l)ut to attain that end the lar\^ae must be fed 
on "royal jelly," which is predigested pollen jjrepared for and given 
to them by the workers. Other similar lar\'ae denied this food become 
workers. A certain predatoi\y bug acquires a yello^^' color l\v eating 
potato beetles, and the potato beetle gets the pigment from its food 
plant. The dependence of the (jueen bee on its food has an important 
ecological bearing, l>ut no such significance is known for the other exam- 
ples given. 

How serious a prol^lem food is in the ecology of a species depends on its 
food tolerance. An animal can live only where its food is obtainable, and 
it can be very successful onl.v if its food is rather abundant, iiut some 
animals are omnivorous, being capable of eating a wide variety of other 
organisms, while others are very specialized. AIan>' insect lar\'ae li^'e 
characteristically only on certain plants, numerous aphids are limited 
to two hosts (usually one at a time), and certain parasites are found 
only in on(^ kind of animal in each (or some) of their stages. Such animals 
lead a precarious existence unless their food is abundant or widespread, 
or both. 

Maintenance of Wumbers. — There are other factors wliich enter into 
the lives of animals that help to determine their success of their distribu- 
tion. Among them are altitude, as in mountainous regions, which affects 
temperature and density of the atmosphere, and pressure, as of the water 
in dee[) s(ias or lakes oi- of the aii' on mountains. The four discussed 
above are, however, among the most important, and they will suffice to 
illustrate the ecological situation of animals. 

Each species of animal has a certain capacity to maintain itself, and 
this capacity is matched against an environment made up of all the 


factors that influence the hfe of the animals. If the net result of all 
these elements favors the species, it is successful. 

An important part of the success of a species is the numl)er of indi- 
viduals it is prepared to pit against any unfavorable features of the 
environment. This number depends first of all on the rate of repro- 
duction. In this activity, animals differ greatly. The larger mammals 
produce as a rule only one at a birth, and the period of development is 
long, so that successive offspring are separated by wide intervals of time. 
Rate of reproduction is slow in such animals. Contrast with them the 
small mammals. A mouse produces half a dozen at a litter, and several 
litters in a year, at which rate only a few years would he rec^uired for 
the descendants of one pair to overrun the earth. A shad may lay 
100,000 eggs in a year, a tapeworm 100,000 eggs per day. A protozoon 
could, in seven years, produce a mass of protoplasm ten thousand times 
as large as the earth. One aphid could in a single summer gi\'e rise to 
500 thousand million million descendants. Punnett has calculated that 
a female rotifer^-which is parthenogenetic, lays 50 eggs, and requires 
only two days to reach maturitj^ — would be able to produce in a single 
year, if all its potential offspring survived, a mass of rotifers large enough 
to (ill the whole known universe and leave some over. 

In bisexual animals, the sex ratio is significant in the maintenance 
of numbers, since the number of offspring is determined primarily by 
the females. A species with many females has an advantage over one 
with few. A short life history also favors large numbers, because there 
w ill be more generations in a given time. 

Every species having great powers of reproduction is subject to 
enormous destruction. This is proved by the fact that it does not, in 
the long run, increase in numbers. Indeed, it may actually decrease. 
The rotifer for which the foregoing calculation was made, once an abun- 
dant object of biological experimentation, seems now to elude collection 
altogether; and the passenger pigeon, exceedingly abundant over most 
of eastern North America only a few decades ago, is now extinct. What 
keeps a species in check is not easily ascertained. Accidents reduce 
numbers to some extent, while predatory animals, disease, parasites, lack 
of food, and unfavorable climatic or other physical conditions must 
account for other extensive losses. The efficienc}^ of a species in over- 
coming these obstacles determines its success. Rapid increase is not 
always a sign of efficiency, for species which become especially abundant 
in one season or over a period of several years must usually suffer a 
reverse later; and there are circumstances (such as exhaustion of their 
food) in which the greater the increase the more severe the following 
decline. Greater safety lies in a steady maintenance of numbers. This 
principle is illustrated by one of the most successful of birds, the English 


sparrow. A census of this species in the north central states over a five- 
year period showed a minimum of 9 pairs and a maximum of 13 pairs 
per 100 acres of land. In the northeastern states it was almost equally 
steady at a lower level — 3 to 7 pairs per 100 acres. The number of eggs 
laid by the English sparrow is such that, starting with the normal 
number of pairs, about 260 individuals could have been produced in each 
100 acres in one year. But in the long run the numbers did not increase 
at all, and at no time mthin the five years were the sparrows excessively 
abundant. Casual observation indicates that this stability is common 
over longer periods. 

When some unusual event removes from the environment of a species 
one of its chief limiting factors, the number of individuals may increase 
enormously. Some of the best examples are found in the annals of eco- 
nomic entomology. An insect plant pest imported into a new region 
without the parasites which kept it in check at home may experience a 
remarkable outbreak. The end of such "explosions" has, in economic 
entomology, usually been brought about by introduction of the appro- 
priate parasites. How they might end in the absence of help from 
man is problematical. The pest might exterminate its only food plant, 
resistant strains of the food plant might be developed through selection, 
or some other parasite might find the newcomer a suitable host. 

Animal Communities. — Though the foregoing discussion deals mostly 
with single species in relation to their environment, more ecological infor- 
mation is often obtainable by a study of animal communities. A 
community consists of all the species living in one general situation. 
In a broad way, it is found that the species making up a community 
tend to be the same in many localities of the same kind. As will be 
seen later, similar ponds over a wide area have in part the same species 
in them; lakes of like size and depth not too far from each other are apt 
to contain many of the same species. These species are held together in 
a community by their requirement of practically the same set of environ- 
mental factors. Organisms requiring a given range of temperature, 
moisture, oxygen, and light herd together where these features are to be 
found. The constitution of communities is not rigid, for no two situations 
are exactly alike. One lake may have slightly more oxygen or lower 
temperature or clearer water than another. The difference may cause 
the communities of the two lakes to differ in certain species, tliough they 
are alike in most. Occasionally also two communities will differ in their 
component species by the mere accident that one or two species have been 
introduced into one but not into the other. 

Sometimes species are held together by some very specific relation 
between them This relation may involve merely the nutrition of one 
of the species. Many plant-eating insects favor, or are practically 


limited to, a single species of plant: for example, an aphid that lives 
almost solely on the chrysanthemum. Carnivorous animals are less com- 
monly or less rigidly limited; lady beetles nearly always feed on aphids, 
but accept a number of species, and can eat other small insects, such as 
thrips. They also devour insect eggs. 

A highly specialized interspecific relation is parasitism, which has 
already been mentioned as one means of securing nutrition. It is 
referred to here again as an example of interspecific relations, because 
of the great lengths to which life cycles of parasites have sometimes gone 
in affecting other species. 

In simple cases a parasite has only one host. The trematode Gyro- 
dactylus is parasitic on the skin and gills of the goldfish. When it 
reproduces, the offspring become attached to the same or another gold- 
fish. The liver fluke, however, employs two hosts. Its egg-producing 
stage is spent in the liver of the sheep, or certain other large mammals, 
but the offspring developed from these eggs must find a snail — any one 
of a number of genera. In the snail it undergoes a series of developmental 
changes, after which in a larval form it emerges from the snail and either 
floats in the water or becomes attached to grass. Here it is drunk or 
eaten by a sheep (or cow, or man) and the cycle is repeated. 

A parasite in the human lung passes through three hosts in its cycle. 
Escaping in the sputum into water, it enters a snail. Then at a certain 
stage of its development it emerges into the water again, and penetrates 
the body of a crustacean. If the crustacean is eaten raw, as is the cray- 
fish by people in Japan or sometimes shrimps in America, the human 
host is reentered and the cycle is concluded. And finally, the trematode 
Alaria passes through four hosts. From a carnivorous mammal, often 
a dog or a member of the mink family, it goes through a snail, then a frog, 
next a mouse or some other small mammal, and thence to a dog or 
other mammal which eats the mouse. 

Ecological Succession. — No community of organisms is in a stable 
condition. It is to be expected that the component species will vary 
in relative abundance seasonally and from year to year. Occasionally 
a species seems to disappear, perhaps to return later, and other species 
may be added from some outside source. While these frequent changes 
are of interest, they are far surpassed in importance by the alterations 
known as succession. Ecological succession is an orderly sequence of 
substitutions of species in a community. Certain species increase in 
numbers, become perhaps dominant members of the group, then decline or 
even disappear. Other species rise in succession, enjoy dominance for a 
time, and then recede. Were this succession a purely random change, it 
would have little more meaning than do the irregular seasonal and 
sporadic fluctuations mentioned above. But in ecological succession, the 



species in any given type of habitat tend to follow one another in a cei-tain 
order. This order of change of species is correlated with the order of 
change in the environment and in general is a change from instability 
toward a condition of equilibrium. 

Plant communities of certain kinds have advantages for a study of 
succession, because in them the remains of earlier species are preserved. 
Thus, when peat is dug from bogs for fuel, successive layers of the mate- 
rial are well enough preserved to indicate what plants produced them 
(Fig. 259). The general order in such places seems to be aquatic plants, 
sedges, grasses, bog shrubs or alders, bog trees (larches), dry-ground 

Fk;. 259. — Section through peat bed. The type.s of phmts that produeed it at the 
(iiffeieiit levels can be determined from the remains. {From Weaver and Clements, "Plant 
Ecology r) 

forest. Successions starting on bare land begin with herbs and pass 
through shrubs to forests. 

Animal successions are less easily ascertained and less simply described, 
l^rief cycles may be demonstrated, such as the succession of protozoan 
types in laboratory cultures. These cultures are at first dominated by 
flagellate protozoa, then several types of free-swimming ciliates (nearly 
always in a given order), then the stalked ciliate \'orticella, and finally 
the amoebalike species. Following these protozoa come the simple 
plants or algae. Another .succession much longer than the above, but 
short as compared with the plant series described, is that \\'hich is started 
by the wood-boring beetles that live in the trunks of living oak trees and 
gradually kill them. Larvae of another family, the long-horned beetles, 
enter the dying trees, utilizing the burrows of their predecessors. Dai-k- 
ling beetles come next and leave the bark separated from the wood by 

animal;s and their environment 


decaying material. Click beetles follow, bringing with them the wood- 
rotting fvmgi and bacteria. The trunk falls, and decay is gradually 
completed by various microorganisms. In the late stages of decay, 
spiders, small salamanders, and various other animals may use the log 
for shelter. 

The longer successions of animals are too complicated for description 
here. The number of changes and the number of species belonging to the 

i: .{■:.. \-:^:--;-r-y ■'■.■:■.::■ ■'rr^-'r:::-,-.-:::':.-;;.i)-i 


^' . * * * * - * * - • 1 * ■ • . • - ' - . m . . ,.. 


Fig. 260. — The filling of a lake wth peat deposits; the succession of land plants is also 
shown. {After Dachnowski in Bulletin of Geological Survey of Ohio.) 

successive communities are too great for simple summarizing. However, 
out of later sections of this chapter, in which various aquatic habitats 
are described, animal successions can be derived if the history of lakes 
is kept in mind. In general, lakes are being ftUed with washed-in 
soil and the remains of vegetation growing in them. They are generally 
converted into swamps or bogs, and finally dry land (Fig. 260). Under 
these circumstances, the animal communities characteristic of these 
situations may be placed one after another and together present a rough 
indication of the animal succession involved. 


Fresh-water Habitats. — Of the many situations occupied by animals, 
the aquatic habitats have many advantages for purposes of ecological 
illustration. They are amenable to inexpensive study and have accord- 
ingly been thoroughly explored. They present a considerable variety of 
physical features and so accommodate very diverse communities. More- 
over, they possess an organization which for orderliness is not easily 
matched in any terrestrial habitat. This organization depends on 
general principles which render aquatic situations capable of significant 

Besides the properties of water which make it an important constitu- 
ent of protoplasm (see page 39), an excellent heat reservoir, and a solvent 
of gases, salts, and other chemical substances, it has certain character- 
istics which pertain to it in the bulk. These qualities become the quali- 
ties of the various bodies of water. It may be turbid or clear, which 
greatly affects the penetration of light. It has considerable weight; 
hence objects located at great depths are subjected to high pressures. 
And lastly, being liquid, it is highly mobile and is subject to waves, con- 
vection currents, and horizontal currents. Most organisms living in 
water are influenced by one or more of these features, some organisms by 
all of them. 

The animals in water occupy different situations which are character- 
istic of different species. Some live on the bottom, others are free in the 
water and independent of the bottom. The latter include species that 
float passively or, if they swim, do so in an aimless, undirected fashion; 
and other species which, like fishes, swim actively and steer themselves in 
given directions. The other characteristics of aquatic animals are best 
described in connection with their several habitats. 

Ponds. — Ponds are shallow — usually not over 2 or 3 meters in depth — 
and heat from the sun penetrates through all the water. The tempera- 
ture is consequently nearly uniform from surface to bottom, though 
shading or resistance to currents by vegetation may cause differences 
of 5°C. between different parts of a pond. Although the water may 
be stirred completely by wind, wave action is so slight as to cause no 
important mechanical disturbance. The important gases are almost 
uniformly distributed through pond water; bright light and abundant 
algae may increase the oxygen content through photosynthesis, and, when 
crowded with animals, the water may contain excess carbon dioxide. 
Abundant rains dilute the chemical content and increase the turbidity 

Whether the bottom of a pond is covered with vegetation depends 
on its depth and tiu'bidity; even when the water is fairly clear, there are 
few plants beyond 3 meters in depth. In most ponds, however, this 
permits vegetation throughout their area (Fig. 261). These plants 



furnish additional surface to which aquatic animals may cling. One of 
the chief characteristics of pond life is that it must be prepared to dry up. 
Typical pond organisms are those which spend part of their life cycles out 

Fig. 261. — Ponds; large one above with vegetation practically throughout, snaall one 
below with heavy vegetation completely concealing the water. {Photographs by F. C. 

of water, either m a resting condition or actively in the air or on land. 
The larger ponds are permanent, and the organisms in these approach 
those of lakes in their characteristics. Ponds are temporary bodies also 
from a long-range point of view sinccj as explained above, they are being 




gradually filled in and converted into dry land; but that fact is of n 
importance in relation to the community of organisms existing in them 
at any one moment. 

Animals of ponds are of too many kinds to l^e named with any com- 
pleteness, but very characteristic ones are many protozoa; the fairy 
shrimp (particularly in temporary ponds) ; the immature stages of May 
flies, dragonflies, stone flies, and midges; mites or water spiders; snails 
and small bivalve mollusks; and often frogs, toads, and salamanders. 
Fish are uncommon; and occasionally muskrats build their houses in and 
over the water. Visitors are ducks, grebes, and other wading birds 
AA'hieh feed upon the pond animals. Pond animals must produce many 
offspring, for the environmental toll is especially heavy. 

Lakes. — Lakes differ from ponds chiefly in size, but this difference 
carries Avith it profound changes in all the principal factors of environ- 
ment — light, temperature, and dis- 
solved gases, with their effect upon 
nutrition. Lakes vary so much in 
their qualities, depending largely on 
size and geographic position, that what 
is said here will be limited chiefly to 
those of moderate size in the temperate 
zones. Two-thirds of the light falling 
upon a lake is absorbed by the flrst 
meter of water, and almost none pene- 
trates farther than 3 or 4 meters. The 
bottoms of most lakes are therefore in 
total darkness. The heat received 
from the sun and from contact with 
warm air in summer aftects only the 
surface water. The water near the 
A\'ind, and a layer of water of nearly 
down as far as wave action I'eaches. 




? 20 


S 25 






8 10 12 14 16 

18 20 2? 
Degrees Centigrade 

Fig. 262. — A thermocline; curve 
of temperature at different depths in a 
typical lake in summer. 

surface is stirred up by the 
uniform temperature extends 
In a lake of moderate size this surface layer is apt to have a temperature 
around 2()°C'. in late summer, and to be 10 or 12 meters in thickness. 
Below this depth the water becomes rapidly coldei' with increasing 
depth, as shown in Fig. 2G2. This layer of rapidly falling temperature 
is known as the thermocline, and in the lake represented in the figure it 
extends from about 11 meters to about 20 meters in depth. Ji(>low the 
thermocline the water continues to become colder at lower depths, but 
at a very much slower rate. Since the warm water above the thermocline 
is less dense than the cold water below it, there is practically no inter- 
mingling, and the water lielow is rather completely cut off from an>' com- 
munication with th(» world above. 



In the fall the surface water becomes cooler, and the thermocline 
gradually disappears. When the surface water is of about the same 
temperature (and hence density) as the bottom water, the wind is capable 
of stirring the water from surface to bottom. Then the bottom water, 
which is held captive during the summer, may escape to the surface. 
In winter the surface water usually freezes to a very slight depth, but 
the bottom never freezes. Indeed, the bottom may be only 2. or 3° 
colder than in summer. The sheet of ice, if one forms, prevents wind 
action; and besides, Avater at or near 0°C\ is not so heavy as that around 
4°. Consequently there is no intermingling of surface and bottom water 
during \nnter. In spring, however, as the surface water warms, there is 


Fig. 263. — Lake shore kept bare of vegetation by wave action. (Photograph by F. C 


again a complete circulation of the water under the influence of the wind. 
As summer advances, the surface water is heated, and the thermocline is 
again produced. 

Thus, twice a year, spring and fall, the water of moderate-sized lakes 
in the temperate zones circulates freelj' from surface to bottom; but at 
other times there is effective stratihcation, and surface and bottom waters 
do not mix. These facts have an important bearing upon the general 
ecological features of lakes. A lake is divided into regions whose proper- 
ties are distinctly different not only in temperature but in light, gas con- 
tent, and mechanical agents. 

The region above the thermocline in summer is relatively warm, is 
well lighted near the surface, is subject to mechanical disturbance by 
waves, has no fixed objects to which organisms may be attached except 
near the shores, is well svipplied with oxygen from the air (supplemented 
by that coming from green organisms carrjdng on photosynthesis), and 


is poor in carbon dioxide (a condition like^vise accentuated by any 
photosynthesis going on there). The temperature of the shallow water 
along the shore is likely to fluctuate greatly between day and night, 
especially on the side of the lake toward the prevailing wind and among 
vegetation, where there is little agitation; but out in the open water in 
the middle of the lake temperature is much more nearly constant. Shore 
regions exposed to the wind are subject to vigorous wave action which 
usually prevents vegetation from gaining a foothold (Fig. 263). 

Below the thermocline the water is always cold, often varying only 
3 or 4° throughout the year. It is always dark. There is no wave 
action, and almost the only mechanical disturbance is that occasioned by 
the complete circulation of the water in spring and fall. A solid sub- 
stratum is available for attachment. There is very little oxygen, some- 
times none at all, for whatever oxygen is brought in by the spring and 
fall overturn of the water is consumed by decay of dead organisms that 
fall to the bottom, and there can be no photosynthesis in this dark region. 
Carbon dioxide is always abundant, likewise because of the decomposition 
of organic matter, except temporarily at the times of the spring and fall 

The Organisms of Lakes. — It is obvious that the conditions described 
above have much to do with the types of organisms inhabiting lakes, 
and that different parts of a lake will have very different kinds. Plants 
can as a rule occupy only about 3 meters of the depth of a lake, owing 
to deficiency of light below that level. For plants springing from the 
soil, this means that they are limited to a narrow strip along the shore 
(Fig. 258). While a pond may have vegetation throughout, most lakes 
have plants over only 10 to 30 per cent of their area. This difference 
between ponds and lakes is indicated in Fig. 264. Since many animals 
depend on these plants, the abundance of the latter is important in the 
general ecology. Of the many animals found in such situations it is 
possible to mention only a few. In the shore region with the plants 
are usually snails and immature caddis flies, midges, IVIay flies, and 
dragon flies. Where there is little or no vegetation because of waves, 
there are often mussels and young insects \vith flattened bodies and 
clinging habits (certain May flies). In the open water of the middle of 
the lake are sometimes floating plants, chiefly algae, so abundant as to 
reduce very materially the amount of light that enters the water. With 
the algae, and often feeding upon them, are many small animals, chiefly 
Crustacea, protozoa, rotifers, and mites, abundant in numl)ers but not 
usually of many kinds in any one lake. 

On the bottom of a lake, below the thermocline, are found those 
organisms requiring no light and little or no oxygen. Characteristic 
examples are the minute plants known as diatoms, some of the annelid 



worms, small bivalve mollusks, and the larvae of midges and of the 
mosqiiitolike Corethra. Such a place would not seem favorable to much 
life, yet Juday has found these animals make up a mass of over three 
hundred pounds per acre on the bottom of a typical lake. 

The free-living population of a lake is subject to considerable fluctua- 
tion in amount and distribution. There is a daily variation in distribu- 
tion caused by the reactions of these animals to light. Since most of 
them are positive to light, they accumulate at the surface during the 
day and settle away from the surface at night. Reference has already 
been made to this reaction in one of the parasitic Crustacea (page 285). 
There is also great variation in the seasonal abundance of floating species. 
The algae generally have one maximum each year, occurring in midsum- 
mer, as have also certain protozoa. The diatoms, however, regularly 

Po/vD CommoNz if,^ Conditions 


Submerged aquafK regefafion 
Floating " •> 


Fig. 264.- 

' Emergent 

-Section through pond and lake, showing contrast in extent of vegetation due 
to difference in depth. (From Chapman, "Animal Ecology.^') 

have two maxima, in spring and fall, respectively. The animals which 
feed upon these minute plants are naturally influenced by this seasonal 

It will be observed that there is much overlapping in the general 
kinds of organisms living in lakes and ponds, respectively. This is 
largely due to the fact that the vegetated strip along the shore of a 
lake is not very different from many ponds. The most characteristic 
difference between lakes and ponds is in the swimming organisms. , Fishes 
are common in lakes, but there are few ponds that contain them, and then 
only certain species. Comparison of ponds of different ages shows that 
the older the ponds the fewer the fish they harbor. In some regions the 
amphipod crustacean Gammarus appears also to be a distinguishing 
mark of lakes as compared with ponds. 

Streams. — Water in motion has characteristics, as the habitat of 
animals, not possessed by standing water. The mechanical disturbance 
which it offers is very considerable in young streams (that is, those 
whose slope is steep) but much less in old streams. Soil may be carried 


in suspension; hence light penetration is periodically very low. Tempera- 
ture is nearly uniform at various locations in a stream at any one time, 
but its variation seasonally for the stream as a whole is often extreme. 
The oxygen content of s^viftly flowing water is generally near the satura- 
tion point, and most of the characteristic brook animals cannot be rearee. 
in a concentration of oxygen much below that level. 

Since the chief feature of streams, as distinguished from* lakes, is the 
movement of their water, consideration of their animals will here be 
limited to those whose currents are strong. This is the condition in 
most small streams or brooks. In such streams, animals have to be able 
to maintain their position; with the exception of the minute floaters, 
they cannot as a rule allow themselves to be carried along by the current 
and still be successful. One method of holding their places is to be 
attached to fixed objects. That is a feasible method in general, since 
animals do not have to travel in search of food, for it comes to them. 
In very swift water, one finds the larvae of the black fly, Avhich hold fast 
by adhesive organs at the posterior end, while their appendages are so 
construct;gd as to strain minute organisms out of the water that flows 
through them. Some of the caddis fly larvae spin nets on stones or other 
objects in rapids; they cling to the net, which also serves to catch food as 
the water goes through it. The other most abundant insects in brooks 
appear to be the larvae of midges, which live on or in the bottom, and May 
flies and stone flies of clinging varieties. In other animal groups there 
are snails, flatworms (planarians), amphipod Crustacea, and mites. 
Some algae form incrustations on rocks and other objects. In the same 
brooks, but in the quieter water, are miller's-thumbs (fish) lurking under 
overhanging banks, catfishes which lie close to the bottom, and darters 
(fish) which are strong sudmmers. The larger rivers contain larger 
animals, but they differ less from lake inhabitants. 

The rate of reproduction of brook dwellers must be high, since the 
risks of loss are large. An individual that loses its station, if dependent 
on attachment, is not likely to become reattached until it reaches slow- 
water, and there the conditions are not usually favorable. A single pair 
of midges, producing four generations a year, have a potentiality of 
nearly eight million descendants, but on the average only two are pro- 
duced and live, in each generation, to do what their progenitors did. 

Marine Conditions. — The oceans are so huge and are subject to so 
many variable influences in their various parts that no .simple descrip- 
tion of their environmental organization is possible. Their waters are a 
little heavier because of the salt content, hence offer more support to 
animal bodies than fresh water does. Deep water is at very high pres- 
sure, but this feature is of little significance to any animals except those 
fishes which have a closed swim bladder which is compressible. \'ery 



strong mechanical disturbance may be caused by waves and tides, and 
currents such as the Gulf Stream and the Labrador Current create 
special conditions of temperature and distribution. Temperature of 
surface waters varies little during the year in the open oceans (from 7 
to 12°C. in one situation), but considerably in stagnant seas and bays 
(3 to 18° in the Baltic Sea). Deep water is always cold. Light of 
sufficient intensity to aid plant growth penetrates the first 30 to 100 
meters of water but is detectable at 200 to 600 meters. Concentration 
of salt is increased ° by evaporation in the tropics, and diminished in 
summer in polar areas by the melting of ice. Ocean waters are also 
diluted by rivers, but these streams are more important for the materials 












--■ >- :^^^W CONTINENTAL 
-o^^^^ SHELF 



-'— -j^^^F Depf hs are exagger- 
— — -'^^^f o'^^<i' in relation +0 
~-^-^^f horizontal scale 



z~~^^ Temperatures and light in relation 
Tsrl'^m ■fo depth are very variable and 
^^-^M only approximote 


Fig. 265. — Vertical section through portion of ocean near the shore. Bottom fauna includes 
animals which are able to move briefly but must periodically come to rest. 

they bring in from the land. The Atlantic and Arctic Oceans receive 
by far the greatest contribution from rivers, while the southeastern 
Pacific receives the least. The solubility of oxygen in marine waters is 
about 20 per cent less than in fresh water, and cold water (either salt 
or fresh) dissolves more than warmer water does. Deep waters, which 
are cold and which are replenished by a circulation from the polar 
regions, therefore have a good oxygen supply. 

Ocean Bottom. — A great majority of marine animals live on, in, or 
near the bottom (Fig. 265). Near the shores the bottom is lighted; 
here it is that life is most abundant, and all groups of marine animals 
are found in this relatively shallow water (200 meters or less). The 
stock of animals in these coastal waters is generally regarded as having 
produced all the water-breathing aquatic forms, whether marine or 
fresh-water. Below the low-tide level there is abundant plant life if the 


bottom is one in which roots can take hold (clayey or sandy), and a 
rich animal population finds shelter, support, and oxygen among the 
plants. Where plants are lacking merely because shifting of the sand 
prevents their rooting, numerous animals (clams, worms, sea cucumbers, 
crabs) burrow in the bottom material and feed on remains of seaweeds and 
animals. Gravel bottom is practically without life, because movements 
of the pebbles in strong wave action destroy living things. Animals of 
this coastal area depend for food on the organic matter (largely dead 
bodies, including plant remains) brought in by rivers or produced in the 
coastal area itself. 

Between low and high tide there are fewer animals, yet some are 
able to endure the twice-a-day uncovering, exposure to the heat of the 
sun in summer and temperature extremes in winter, and dilution of the 

Fig. 266. — A burrowing animal between tide lines; the clam Scrobicularia. 

water by rains. The clam Scrobicularia (Fig. 2G6) burrows in the sand 
and with its long inhalent siphon explores the surface around it for 
food-bearing water. 

The ocean bottom below the effective penetration of light is less well 
populated. The organic remnants which serve as food here are the 
decaying bodies of swimming and floating animals and plants which 
settle down from above. The ooze thus formed has a pasty consistency. 
The most abundant bottom animals of the deep sea are sea cucumbers. 
Others are Crustacea (amphipods and isopods), hydroids, sponges, clams, 
and worms. In general, deep-sea animals are smaller than their relatives 
near the surface. Also they may be more delicately constructed (fragile 
skeletons, thinner shells) liecause there is little motion of the water. 

Animals of the Open Ocean. — Organisms of open water either swim 
or passiv-ely float. Floating life must have some way of reducing its 
specific gravity, since protoplasm itself is heavier even than salt water. 
One way is to take up much water, without the salt, into the tissues. 
Other ways are to develop fat, or gas chambers like those of the siphono- 


phores or the smm bladders of fishes. The air-breathing whales, seals, 
and turtles are floated by their lungs. Animals having no floating 
mechanism must actively swim, if they are to avoid settling on the 
bottom; among vertebrate animals only the powerful sharks and a few 
bony fishes without swim bladders are capable of the incessant exertion 
necessary to prevent sinking. 

Fewer groups of animals are represented in the open ocean than on 
the bottom. There are no sponges, no brachiopods, no bryozoans. 
Hydroids and other sessile coelenterates are missing, and there are few 
echinoderms (except larval stages), few worms, few clams and snails. 
The bulk of the sudmming animals (90 per cent) are copepod crustaceans. 

Ocean currents either come to an end by spreading out and slowing 
down to zero (Gulf Stream), or they form a closed circuit. The meeting 
of warm (Gulf Stream) and cold (Labrador) currents of the terminating 
type causes great mortality of organisms, and adds to the organic detritus 
used by bottom forms. The larger closed circuits take a year (North 
Atlantic) or two (South Atlantic) to bring their organisms back to any 
starting point. In the eddy enclosed by such a circuit there are often 
accumulations of seaweeds (Sargassum), perhaps torn loose by hurri- 
canes, and in these weeds is a characteristic animal community (certain 
fishes, crabs, shrimps, hydroids). An eddy of this sort is known as a 
Sargasso Sea, and each of the great oceans except the polar ones has one 
or more of them. 

Coral Reefs. — Coral reefs are built up from the bottom in tropical 
seas by two different groups of coelenterates, aided by a number of 
other lime-depositing organisms. They may be developed along the 
shore line (fringing reefs), out some distance leaving lagoons between 
them and the shore (barrier reefs), or at any distance from the mainland 
in the form of a ring or horseshoe (atolls). Various theories to account 
for reefs, beginning with those of Charles Darwin, have been proposed. 
The theories postulate the type of habitat in which corals will grow, the 
possible rise or fall of the land, differences in exposure to the open ocean, 
and long-time changes in the water level of oceans; but none of the 
theories is entirely satisfactory. About these reefs there are character- 
istic communities of other kinds of animals. 

Geographic Areas in the Oceans. — Swimming and floating organisms 
requiring moderate or relatively high temperatures are limited to their 
respective oceans, being cut off from other oceans by the continents 
which the}^ cannot pass around. Yet the animals of the Atlantic have 
a considerable likeness to those of the Indian and Pacific Oceans. In 
the coi^epods, even some of the species are identical. This likeness 
presumably resulted from the connection between the two areas across 
Central America in Tertiary time. 


The colder ocean waters, north and south, have fewer species of 
animals, but more individuals in a given volume, than do tropical 
regions. There is a striking similarity of arctic and antarctic animals, 
the same genera and even species occurring in l)oth oceans. This is 
presumably accounted for by the fact that there is a connection between 
them in the cold deep waters of the intervening ocean, which is kept 
cold by a north-and-south circulation of surface and bottom waters. 
Another possible explanation is that northern and southern species have 
evolved independently under the guidance of similar conditions. 

Soil. — Different types of land environment represent different stages 
in the evolution and concjuest of the earth. Starting with bare rock, 
the succession is roughly rubble, bare sand, sparse grass and other vege- 
tation, herbs, shrubs, and trees. The soil may thus be in a variety of 
conditions, since it develops by weathering and by the action and con- 
tributions of the vegetation. In texture it may range from very fine 
particles, as in clay, to coarse stones, as in gravel. In a good loam suit- 
able for plant growth, about half of the bulk of the soil is made up of 
spaces between the particles, and these spaces are occupied about equally 
by air and water. About 10 per cent of the solid matter is derived from 
plants; the rest is mineral. 

The temperature of the soil varies most at the surface and is nearly 
constant below a depth of 1 meter. Surface temperature fluctuates 
much more if the ground is bare than if it is covered by vegetation. In 
very cold regions the soil may freeze so deep in winter that it is never 
thawed out in summer; nevertheless, vegetation may gi-ow above this 
perpetual ice. 

Water may be held loosely in the larger spaces between soil particles, 
in which case it tends to drain away by its own weight, or it may be 
retained by capillary action between the fine particles. Even "dry" 
soil has some moisture adsorbed on the small particles. Silt and clay 
retain much more moisture than does sand or humus. 

The importance of the soil as an ecological unit may easily be under- 
estimated, unless it is remembered that most animals spend at least part 
of their life cycle in the soil. Some animals spend their whole lives 
there, such as earthworms, some roundworms, and protozoa. Some 
live in the soil during one stage, such as the grub of May beetles or the 
pupae of many other insects. Others make their homes in the soil but 
spend much of their time on or above its surface, as ants and termites. 
Burrowing in the soil is the common mode of life of moles and shrews, 
while homes are built in the ground by many other vei'tebrate animals 
(ground squirrels, ground hogs, mice, etc.). In numbers of individuals, 
the I'oundworms are the most abundant group, reaching as many as 
half a billion per acre. 


Most soil animals are near the surface, not deeper than 5 or 6 inches 
during the active season. Many species migrate downward annually 
to avoid frost and return in the spring. Earthworms have been found as 
deep as 6 feet, where they went to find moisture in dry seasons. Species 
which merely make their nests in the ground often go rather deep — 
gophers 2 feet, termites 5 feet, ants 9 feet, and the prairie dog as deep 
as 14 feet. 

As a special type of soil environment may be mentioned sand dunes. 
The chief physical characteristic of dunes is the extreme variation of 
their temperature and moisture. Even in moderately moist regions, 
rain water drains out of sand quickly; and in the heat of midday the 
temperature may rise to 50 or 60°C. The hottest part of a sand dune, 
when the sun has been shining upon it, is directly at the surface. The 
air a few inches above it and the sand at a depth of several inches are 
cooler. Certain wasps which dig burrows are among the most character- 
istic dune animals; and with them are certain other insects parasitic 
upon the wasps. Many other animals are occasional visitors but have 
no particular dependence on dime conditions. 

Associations in Vegetated Areas. — When vegetation has taken hold 
in the soil in abundance, the physical conditions are modified in several 
important ways. Sunshine is intercepted, thereby reducing the fluctu- 
ations of the temperature of both soil and air. The diminution of light 
by trees is much greater than that by shrubs or herbs, and the reduction 
by pine trees is much more than by larches or elms. In one forest it 
was found that the maximum daily temperature was 5 or 6° lower, and 
the daily minimum an equal amount higher, than in a near-by cutover 
area. Evaporation of water from the trees is one of the ways in which 
temperature is lowered. Some of this reduction of temperature is, how- 
ever, nullified by stoppage of the wind by trees, so that open spots sur- 
rounded by forest may, when the sun shines long upon them, become 
warmer than they would if there were no trees. General evaporation is 
also reduced by this retardation of the wind, beech-maple forest exercising 
a much greater control than cottonwood, for example. 

Introduction of vegetation modifies the characteristics of an area in 
very many ways, depending on what plants are present. As a conse- 
quence the animals become ex(?eedingly varied. The nature of an animal 
association is determined largely by the plant association. Insects 
which feed upon the leaves of plants often utilize only one or a few species. 
Those which produce galls on leaves are commonly limited to one species 
of plant. Wood-boring and bark insects prefer certain trees. Soil is 
altered differently by different plants, and root-feeding animals usually 
specialize in certain roots. Rotten logs in various stages of decay con- 
tribute to the variety of situations. The general effect of vegetation 


on temperature, light, and humidity, described above, introduces much 
diversity. As a consequence of this heterogeneity, it is impossible to 
regard vegetated areas as single ecological units. They consist of a 
number of types mingled with one another. Attempts have been made 
to classify them on the basis of predominant types of plants, but in an 
elementary discussion it is not practicable to follow any of these schemes 


Chapman, R. N. Animal Ecology. 2d Ed. McGraw-Hill Book Company, Inc. 
(Especially Chaps. XV, XVI, XVII on aquatic habitats.) 

Elton, C. E. Animal Ecology. Sidgwick & Jackson, Ltd. 

Hesse, R., W. C. Allee, and K. P. Schmidt. Ecological Animal Geography. John 
Wiley & Sons, Inc. 

Pearse, a. S. Animal Ecology. 2d Ed. McGraw-Hill Book Company, Inc. 
(Chap. IV, biological factors in ecology.) 

Semper, Karl. Animal Life. D. Appleton-Century Company, Inc. (Old, and 
lacks the modern organization of ecology but discusses, chapter by chapter, 
influence of food, light, temperature, water and air currents, etc., upon organisms.) 

Shelford, V. E. Animal Commimities in Temperate America. University of 
Chicago Press. (Chap. XV, general discussion.) 

Weaver, J. E., and F. E. Clements. Plant Ecology. McGraw-Hill Book Com- 
pany, Inc. 


The locations of species on the earth have been determined by two 
general sets of factors, the ecological and the historical. Animals must 
live in situations which are at least moderately favorable to them, but 
they are able to occupy suitable areas only if these are within reach. 
Many excellent sites are not occupied because they are far away, and 
there is no adequate means of transport. Moreover, most animals can- 
not be assumed to have purpose, and they cannot have knowledge of 
the conditions of life in other places. Accordingly, if they find new 
locations it must be as a result of normal activities, including some events 
which must be regarded as accidental. 

The purely local distribution of species, which depends on ecological 
factors, has been discussed in the preceding chapter. While it will be 
necessary to point out relations to the environment in this chapter also, 
only such relations as bear on the history of distribution will be included. 
Let us see how animals have come to be where they are. 

Interplay of Two Evolutions. — While present distribution of living 
things has often been used to prove that evolution has occurred, an 
understanding of zoogeography is most easily attained by reversing the 
arguments. If it be assumed that evolution has taken place, many 
peculiarities of distribution have a natural explanation. 

There are two of these evolutions, independent of each other in their 
origins, but with intert^vining results. One is the evolution of species 
of animals and plants, the other the evolution of the earth on which 
they live. New species have arisen out of older species, ever since life 
began. A group of individuals becomes different from their fellows, 
through mutation and recombination of genes and other events, and a 
new species is started. Usually the new species finds or at any rate 
occupies an area somewhat different from that of the other species. In 
time it gives rise to further new species, which take up their special 
locations. As more and more new species arise, there is a cleavage 
among them; some of them are much more alike, but differ strikingly 
from another group within which the species are rather similar. Genera 
are thus produced. As species change still more and more, there is 
cleavage among the genera, and families arise. Continued change of 
species results in divisions of higher ranks, the orders, classes and phyla — 



all of which was described in the chapter on taxonomy. The whole 
evolution process is a change of species, carrying with it necessarily the 
changes of genera and higher categories. 

An important feature of this process, as it relates to geographic dis- 
tribution, is that new species have been arising all the time since living 
things first existed. New species are originating at this very moment, 
and may be expected to continue to come into existence in the indefinite 
future. Also important is the fact that new species have taken their 
origin everywhere, in all parts of the earth which support life. Time 
and place thus enter in an important way into all questions of present- 
day distribution. The range occupied by each species becomes a center 
of dispersal from which its descendants tend to spread, and these centers 
have existed all over the earth and through long periods of time. 

Starting much earlier than the evolution of life was the development 
of the earth. With the early stages of this process we are not concerned; 
but those parts of the earth's evolution which were contemporaneous 
with the evolution of living things are very important in geographic dis- 
tributiom The changes which affected distribution have been largely 
the rise and fall or other shifts of position of the land, and changes of 
climate. The question of permanence of continents is an important one. 
Most zoogeographers have held that the continents have always been, in 
general, about where they now are; but there is another theory, that of 
continental drift, according to which continents, floating on the plastic 
interior of the earth, have moved horizontally. A common example of 
this drift is the alleged separation of Africa and South America. Many 
European and some American geologists have supported the drift theory, 
but distribution of animals has seemed to most biologists to call for more 
nearly permanent continents. 

Regardless of the general position of continents, their shapes have 
changed. Many areas of dry land teem with fossils belonging to classes 
which are strictly marine. Such areas must once have been under the 
sea. Michigan, for example, contains many extinct corals, though it is 
now hundreds of miles from any salt water. Even high mountains have 
arisen out of the ocean. Land has also sunk, and areas which were the 
shores of an ocean have become its bottom. Broken shore lines are a 
common result of the sinking of hilly or eroded land. 

Changes of climate have also been frequent. Michigan and most 
neighboring areas have been under glaciers more than once. At the 
other extreme, more northern regions have been tropical, as indicated 
by luxuriant plant growth preserved as fossils. Humid areas have 
become dry, swamps have become dry plains, forests have been con- 
verted into grasslands. These changes must have affected the distri- 
bution of animals profoundly. 


The timing of the changes of species and the changes of the earth 
must have had important consequences for hving things. When a 
group of animals experienced the genetic changes which might lead to 
the formation of a new species, any changes of the land or climate 
occurring at the same time and in the same region could spell the differ- 
ence between survival and destruction of the new group. When a region 
of the earth was undergoing a physical (perhaps climatic) change, any 
genetic change going on in a few individuals could decide whether any 
members of their species would survive the changes of environment. 
For the sake of emphasis these changes are described as sudden and 
radical; actually they have been very gradual. Interplay of the physical 
forces of the environment and the genetic forces of animal or plant life 
must have been crucial in the guidance of evolution, and in the deter- 
mination of the location of resulting species and higher groups. Let us 
turn to some of the facts of distribution, to see how they fit into the 
general scheme just outlined. 

Position of Ranges. — It is easily observed that species, families, 
orders, etc., have their characteristic places on the earth. With the 
exception of closely interdependent species, such as parasite and host, 
probably no two species have exactly the same range. The musk ox is 
arctic; the nine-banded armadillo ranges from Texas to South America; 
the North American alligator exists only in the extreme southeastern 
part of the United States. Among such vastly different groups, widely 
separated ranges do not occasion any comment. Within a single genus, 
however, the several species have their distinct areas. For example, in 
the genus of spadefoot toads, Scaphiopus couchii extends from Texas to 
Arizona, and into northern Mexico, including Lower California; S. ham- 
mondii ranges from Montana to Mexico and west to the Pacific states; 
8. holbrookii holbrookii occurs along the Atlantic from Massachusetts 
to Florida, and west to Louisiana, Texas and Arkansas; S. holbrookii 
albus is only in the Florida Keys, or possibly also the extreme tip of 
Florida; and S. hurterii is found only in Texas. 

The location of a species range depends primarily on where the 
species started. The present range must usually be around or near the 
point of origin. Looking backward, one sees the ''center of dispersal" 
of a species as some point in or near its present range. Most species 
have not lived long enough to have traveled far. Very old species, 
however, especially those which survive in only a few individuals, may 
be far from their places of origin. Such old and nearly extinct species 
may usually be recognized as such because no closely related species are 
anywhere near them. Species consisting of few individuals because they 
are very young are, on the contrary, surrounded by very similar types. 
Remnants of very old species are spoken of as relicts. Examples are the 


several species of Nautilus, sole survivors of a once flourishing family 
(the tetrabranchiate cephalopods), now found only at places in the 
Pacific and Indian Oceans. 

Size of Range. — Equally striking are the different sizes of ranges 
occupied by the various forms. When groups of high and low taxonomic 
rank are compared, as orders with genera, inequalities are to be expected. 
One simple reason is that the higher groups are made up of a number 
of lower ones. When those of the same rank occupy very unecjual areas, 
an explanation is not always easy. Particularly important in the theory 
of distribution are unequal ranges of species. Some ranges are very 
small. One species of ant is found only in the Garden of the Gods in 
Colorado, another species occupies much of North America. Kirtland's 
Warbler, not including its migration routes, exists only as a few indi- 
viduals in a very limited area, while the American Robin numbers 
millions of individuals and covers a continent. Among plants, a species 
of Oenothera includes only 500 to 1000 individuals and is known only 
in a mountain range in southern New Mexico. One of the spadefoot 
toads, already mentioned, occurs only in the islands off Florida and per- 
haps at the extreme tip on the mainland, while another species of the 
same genus has a range a thousand miles wide. 

When there appears to be no difference in the tolerance, rate of 
reproduction, or means of locomotion of two species, a tempting expla- 
nation is a difference in age. This is thought to be the reason for the 
very unequal ranges of four species of tree frogs (genus Hyla). Hyla 
versicolor is found from southern Canada to the Gulf states, and west to 
a line between Montana and central Texas; H. squirella extends from 
Virginia to Florida, west to Texas and Indiana; H. gratiosa from South 
Carolina to Florida and Mississippi; and H. evittala only along the 
Potomac and York Rivers in Virginia and New Jersey. The species 
believed to be the younger have the smaller ranges, and the explanation 
may be simply the shorter time they have had to spread. 

This idea has been developed as the "Age and Area" hypothesis, 
and has been applied more to plants than to animals. In accord with 
it is the fact that on the average groups of higher taxonomic rank (orders, 
for example) occupy areas larger than those belonging to groups of 
lower rank (genera, let us say). In general, the higher groups are older, 
and have had longer time to disperse. Some paleontological support for 
it is also claimed, for when the ages of taxonomic groups can be judged 
from the geological periods which furnish their earliest known fossils, the 
older ones again have the larger average ranges. 

There are known exceptions to the rule, however, and probably 
many which are not known. Two species of shophord's-purse differ in 
the number of chromosomes in their cells, one having just twice as many 


as the other, and it is fairly certain that the one with the larger number 
sprang from the other by a doubling of the chromosomes. This is a 
weh known method of origin of species in plants, and must apply to this 
example. However, the species with the double number of chromosomes 
(which must be the younger one) ranges much more widely than the one 
with the smaller number. One species is simply much more successful 
than the other. 

Continuity of Range. — Because a group of animals starts at some 
point, from which its members tend to spread until barriers are reached 
on all sides, ranges are expected to be continuous unless something 
happens to break them up. Taxonomic groups of as high rank as 
families and orders have usually been developing long enough for that 
"something" to take place. Ranges of such groups are large, and living 
conditions may change sufficiently to extinguish the animals across the 
middle of the area, thus dividing it in two. The camel family, for 
example, is represented by the true camels in Asia and Africa, and by 
the llama and its relatives in South America, wdth the great land gap 
of Europe and North America between. Fossil camels, however, are 
found in the area now vacant. The genus Alligator is composed of two 
species, one in central China, the other in southeastern United States. 
Extinct relatives of the alligators once ranged widely in North America 
and Europe, shoAnng how the modern range became discontinuous. 

When the range of a species is found to be discontinuous, which is 
rare, the reason is not easily found. The skink, Lciolopisma laterale, is 
found in southeastern United States, in China, and in certain of the 
southern Japanese islands. Why is it not in the areas between? Only 
if the species is an exceedingly old one would it be hkely that destruc- 
tion of its members over a large portion of its former range could have 
occurred. So improbable is the division of a species range by extinction 
that every example of it raises the question whether the species may not 
have developed independently in two places. Such an occurrence is not 
impossible. j\Iany mutations are known to be produced repeatedly, and 
among random recombinations of genes the same combinations could 
occur anywhere. If environment of a certain type tends to preserve 
certain genetic combinations, similar environments in two areas could 
guide evolution in the same direction. Such double origin of a species 
would not be a violation of the taxonomic concept that all members of a 
species are descendants of common ancestors, for the two groups from 
which the species arose would necessarily be much alike, both having 
come from the same ancestry. The common ancestry of a duplicated 
species would thus be simply pushed farther back. Nevertheless, this 
dual origin of a species is so unlikely that it is not to be lightly assumed 
as an explanation of discontinuity. 



Physical Conditions of Ranges. — Lest the ecological factors be I'or- 
gotten in the study of historical phenomena, it should be observed how 
different are the conditions obtaining in different ranges. A striking 
illustration of this is found in the distribution of vegetation. The general 
vegetation areas of North America are shown in Fig. 267. Coniferous 

J''i(j. 2G7. — Geuerul vegetation areas of North America. {From, liurlingarnc, Heath, Martin 
and Peirce, "General Bioloyy,'" Henry Holt and Comjmnij, Inc. Prepared by A. G. Vestal.) 

and deciduous foj'csts are sepai-ated l)y pliysical conditions, largely tem- 
perature, and they in turn determine the location of many animals. The 
eastern deciduous forests are the home of the opossum, gray fox, fox 
s(iuirrel, cardinal bii-d, (-arolina ^\'ren, and yellow-breasted chat. The 
northern coniferous forests shelter the snowshoe rabbit, pine martin, 



northern jumping mouse, three-toed woodpecker, and spruce grouse. 
The open treeless areas of the west are inhabited by the prong-horn 
antelope, bison, ground squirrels and many others. So nearly are many 
of these animals limited to one type of vegetation area that it is difficult 

iGlaucomys Sabrinus 
IGlaucoinys Volana 

Fig. 268. — Ranges of the North American flying squirrels, Glaucomys volaiis and G. sa- 
brinus. {After Howell, "North A7nerica?i Fauna," No. 44.) 

to avoid concluding that the conditions prevailing in such areas are 
paramount in their lives. Sometimes the maps of ranges of forest 
animals and of prairie animals appear to overlap, as if the vegetation 
were not of great importance. In one such case, however, the appear- 



ance of mixture was occasioned by the fact that a series of somewhat 
parallel streams with trees along their courses were separated by strips 
of grassland. The forest animals were along the streams, the prairie 


35 to 40 Inches. 
More than 40 

Fk;. 2G9. — Annual rtiinfall in the lower part of the state of Mieliigan. This illustrates 
the differences in physical conditions which may prevail even in relatively small areas. 
(After C. F. Schneider, Publication 9, Michigan Geological (iml Biological Survey.) 

animals between them; but on a map of moderate scale they appeared 
to be together. 

An actual example of species definitely related to I'orests is the genus 
of North American flying sciuirrels. As shown in Fig. 2(58, Glaucomys 



volans is in general limited to the deciduous forests, while the range of 
G. sabrinus approximates that of the coniferous forests. Limitations of 
these species to forests is mostly caused by their feeding on nuts and 
seeds, to a lesser extent by their habit of ''flight." Two animals that 
do not pay much attention to vegetation areas are Rana pipiens, the 
common leopard frog, which occurs all over North America east of the 
Sierra Nevada range; and the raccoon, Procyon lotor, which lives in 
deciduous forest areas and the prairie-plains region as well. The leopard 

Fig. 270. — Proximity of ranges of three varieties of one species of garter snake. West 
of the Mississippi and in Mexico, Thamnophis sauritus proximus; in Florida, Thamnophis 
sauritus sackeni; north of Florida, Thamnophis sauritus sauritus. {Modified from Ruthven.) 

frog ignores forests because of its semiaquatic habits, the raccoon because 
of its tolerance of various conditions. 

Besides vegetation, important physical conditions bearing on the dis- 
tribution of animals are temperature and rainfall. Even in a limited 
area the amount of rainfall differs greatly, as shown in the map of 
Michigan in Fig. 269. * 

These several factors are sufficient to illustrate that geographic dis- 
tribution is not wholly a historical development. Ecology and the time 
and place of origin of species have worked together. 







Proximity of Related Forms. — If species originate from other species 
it would be expected that a very young species would still be near its 
progenitor. It would not have had time to travel very far. If the 
youth of species and the sources from which they have sprung be judged 
from the similarity between species, this expectation is in general realized. 
Those species of a genus, or those subspecies, which are most nearly 
alike are found geographically near one another. An example is found 
in a group of garter snakes known as ribbon snakes. The forms in 

question are three subspecies of one 
species, Thamnophis sauritus. One sub- 
species, called proximus, occupies a range 
west of the Mississippi River and along 
the east coast of Mexico (Fig. 270); 
another, named sackeni, is in Florida and 
on the Gulf Coast east of the Mississippi ; 
the third, sauritus, is east of the Mississippi 
and north of Florida. The three ranges 
are practically in contact wdth one 
another; at any rate they are not sepa- 
rated by ranges of other garter snakes. 
Other species of garter snakes are at a 

The earliest of these subspecies, as 
judged from their characteristics, appears 
to be proximus; from it sackeni and 
sauritus must have sprung. The order 
in which the latter two forms arose is in 
doubt. The possibilities are portrayed 
by Fig. 271; proximus may have given 
rise to sackeni and sauritus separately, or 
it may have produced one of them (either 
one), and this in turn produced the other. 
The principle that nearly related (that is, similar) animals are geo- 
graphically near one another is illustrated also in the higher taxonomic 
categories. The genera of mammals east of the Rocky Mountains in 
the United States have more similarities among themselves, and the 
genera of the Pacific coast area more mutual likenesses, than do the 
eastern genera to the western genera. The principle holds even for 
continents. The animals of one continent are usually more alike than 
they^are like those of any other continent. Moreover, the faunas of 
neighboring continents are more alike than are those of more distant 
continents. The animals of North America and Eurasia are particularly 
good examples of this phenomenon. The similarity of these two faunas 






Fig. 271. — Three pcssible ori- 
gins of subspecies sauritus and 
sackeni from proximus, in a garter- 
snake species. {After Ruthven.) 


is believed to have been increased by a land connection between them 
across the Bering Strait and the adjoining Arctic Ocean, which would 
have permitted migration between them up to (geologically) compara- 
tively recent times. 

In all these instances the argument is that the similar animals have 
had more recent common ancestors, and there has been less time to 
migrate far away. The effect of a barrier, for example, the Rocky 
Mountains helping to keep eastern and western mammals apart, is 
merely to push back the time of the common ancestors of the less similar 
types, and so make their dissimilarities greater. 

Normal Migration. — So important in the explanation of these pecu- 
liarities of distribution are the abilities of the members of species to 
spread, and the time they have had at their disposal to attain their 
present locations, that the means by which they have become dispersed 
should be examined. By far the most important method is what may 
be called their normal migration. This is best seen in freely moving 
terrestrial forms. The individual seeks food or shelter, avoids enemies, 
seeks a mate. How rapidly it moves depends on its powers of loco- 
motion. Whether it goes alone or in flocks or herds depends on little 
understood psychology. These activities lead inevitably to the occu- 
pation of more territory, unless barriers forbid, and by a young species 
barriers are not as a rule reached very soon. 

This spread by normal migration is ordinarily very gradual. Under 
special circumstances, however, it may be greatly accelerated for a time. 
The potato beetle, Leptinotarsa decemlineata, was long restricted to the 
Rocky Mountains and the plains east, as far as western Kansas and 
Nebraska. It could go no farther because its natural food, a wild species 
of Solanum, did not exist east of that area. As the western part of the 
Mississippi valley became settled, the range of the cultivated potato 
(Solanum tuberosum) extended farther and farther west, until between 
1845 and 1850 it reached the range of Leptinotarsa. The beetle found 
the new Solanum a suitable food, so the eastern barrier was removed. 
In about 20 years it had reached the Atlantic seaboard, where it stopped 
until about 1918. Presumably in troop movements and shipment of 
food supplies in the war, the beetle was carried to Europe, where it has 
since existed despite efforts to eradicate it. 

How effective normal migration may be in spreading species is indi- 
cated by some computations. For the slow-moving earthworms, Gado iv 
calculates that if one pair produces enough offspring to occupy one square 
yard of soil in one year, their descendants in the time since the Ice Age 
(perhaps 30,000 years) would have choked the earth. Again, if a human 
family moved, gypsy fashion, only one day a week, and not more than 
three miles, then it would wander 156 miles each year; and the Mongo- 


lians, crossing Bering Strait, might at this rate have reached the Straits 
of Magellan in 50 years. 

Periodic Migration. — Not all the movements of animals are of the 
slow, steady, progressive type just described. Many species move in 
large numbers from one place to another at different times of the year 
or at different times in their life history. The southward migration of 
many birds in the fall and their return in the spring is an example of 
seasonal migration. The great majority of bird species which may be 
found in the course of a year at a given place in the middle of the north 
temperate zone, for example, are seen there only at certain times. A 
small number of species spend the summer there, building their nests 
and rearing their young but disappearing southward in the fall. A still 
smaller number are winter residents, some of which have come south 
from a more northerly summer range. A much greater number are 
migrants, going north in spring to their breeding range and returning 
southward as cold weather approaches in the fall. What causes birds 
to migrate is one of the great biological enigmas. ]\Iigration starts 
before the situation where they spend the winter or summer becomes 
unfavorable. In some species the migrating is done correctly by young 
birds without previous experience and without guidance. Individual 
birds have often been found to return to the same nesting place in suc- 
cessive summers, but the way in which they are guided to the spot can 
only be guessed. It has been suggested that endocrine secretions (page 
154), particularly those of the pituitary and of the gonads, and the 
duration of daylight may initiate migration, but how they could guide 
it is not clear. 

Some other animals migrate seasonally in search of food. When 
the bison was abundant in the Avestern plains, it wandered in droves 
north and south as grazing lands developed. The mule deer moves up 
and down the mountains likewise in search of vegetation. In these 
instances, however, there is no puzzle, for the animals move slowly, and 
they wait until the new feeding grounds are needed and are available. 
They do not anticipate events but direct their movements in relation to 
what can be actually seen. 

In a few animals the migration is not seasonal but occurs once each 
direction in a lifetime. The fresh-water eel migrates at times .separated 
by an interval of years. In its youth this animal ascends the rivei's 
from the sea and lives there for years but does not breed ; upon reaching 
maturity it returns to. the sea to breed. The Alaska salmon shows a 
similar migratory habit. 

Though periodic migration is important in the physiological cyQle 
of individuals and in the economy of species, i( is not know u to have any 
influence on species ranges. There is no known peculiarity of distribu- 


tion anywhere which seems to demand periodic migration as its explana- 
tion. The mere fact that the animals have to return from the place 
to which they periodically travel nullifies any effect which such move- 
ments might have on the size of the range. Migration would have to 
be accompanied by some physiological change in order to extend the 
area occupied. 

Sporadic Migration. — Somewhat allied to periodic movements pei'- 
haps are the sudden outbreaks or irruptions of a species that may occur, 
during which the range is widely extended. The classic example is that 
of the Lapland lemming, a small rodent. The migration of this species 
has been described by Lyell as follows. 

" Once or twice in a quarter of a century they appear in vast numbers, advanc- 
ing along the ground and 'devouring every green thing.' Innumerable bands 
march from Kolen, through Northland and Finmark, to the Western Ocean, 
which they immediately enter; and after swimming about for some time, perish. 
Other bands take their route through Swedish Lapland to the Bothnian Gulf, 
where they are drowned in the same manner. They are followed in their journey 
by bears, wolves and foxes, Avhich prey upon them incessantly. They generally 
move in lines, which are about three feet from each other, and exactly parallel, 
going directly forward through rivers and lakes ; and when they meet with stacks 
of hay or corn gnawing their way through them instead of passing around." 

Another case of sudden movements is afforded by Pallas's sand 
grouse. This species inhabits the steppes of central Asia, extending 
into northern China and the Kirghiz Steppes north of the Aral Sea in 
the winter. At least since 1859 the bird has been in a restless and dis- 
turbed state and great waves of individuals have moved out from the 
normal range. In an irruption in 1859 some of them reached Poland, 
Holland, and the British Isles. Another outbreak in 1863 apparently 
involved thousands of individuals, and the birds reached Italy and the 
Pyrenees in the south of Europe, Scandinavia and Archangel in the north, 
and the British Isles and the Faroes in the west. Still another wave 
occurred in 1888 and at this time flocks appeared in England, Scotland, 
and Ireland. After each wave the species soon disappeared from the 
invaded countries. The extinction may have been due to slaughter by 
man; but while some of the invaders bred the first year, they were not 
so well established that they could have reared young. 

Such sporadic outbreaks are apparently of the same nature as those 
which have been observed within the range of a species. An example 
is the mouse plague of 1907-1908 in the Humboldt Valley, Nevada. 
These mice (Microtus montanus), which live in scattered colonies in 
swampy places, are not usually abundant enough to attract notice. 
They produce half a dozen at a litter and four to six litters per year, 
but ordinarily are kept in check. In the year named, however, some 


element of control was removed, and the mice were produced in countless 
thousands. On some ranches there were as many as 12,000 per acre. 
Crops were destroyed, trees killed by injury to their roots, and banks 
of drainage ditches were riddled with their burrows. Great armies of 
mice moved on to new fields 5 miles or more from the point of first con- 
centration. Then their hordes disappeared even more quickly than they 
arose. In the course of three months they dropped to only 200 to 500 
per acre. No satisfactory explanation of either their increase or their 
disappearance was ever discovered. 

Apparently sporadic migration, as these irruptive movements may 
be termed, does not usually result in an extension of range, for the species 
in the cases observed have not been able to maintain themselves in the 
invaded regions. However, it is possible that at times such irruptions 
have brought species into regions where conditions were favorable and 
thus enlarged the inhabited area. Instances of widely discontinuous 
range have sometimes been explained, whether correctly or not, by appeal 
to sporadic migration. 

Accidental Dispersal. — Discontinuous ranges have been more often 
attributed to accidental dispersal than to sporadic migration. Animals 
are sometimes carried on rafts or floating logs or are blown by the wind 
beyond their normal range. Marine birds, such as the gannet, are occa- 
sionally during storms blown inland from the Atlantic Ocean as far 
west as Michigan, and a number of observers in the tropics have noted 
terrestrial animals on floating logs and rafts in the rivers and even out 
at sea. It has often been asserted that this method of dispersal is effica- 
cious in extending the range. Islands may have received certain forms 
by accident, but there are many difficulties in accounting for the entire 
faunas of islands in this way. Some of these difficulties are (1) the 
inability of some forms to survive a long sea voyage, (2) the fact that 
many island forms, such as the giant tortoises, could not possibly be 
carried on rafts or blown by the winds, (3) the necessity that in the 
higher animals at least a pair of individuals or a pregnant female be 
landed if the form is to be perpetuated, etc. But the greatest obstacle 
to the acceptance of accidental dispersal as an eft'ective method of 
extending ranges lies in the fact that actually observed cases of accidental 
dissemination beyond the range of a form are very few and mostly open 
to question. Possibly it may operate at rare intervals, for certain forms 
and over short distances. 

Man himself is responsible for the introduction of animals and plants 
to new regions in a few instances that are well known. Sometimes it 
was done by design, more often by accident as in the transport of rats in 
ships. The animals carried by man have sometimes succeeded much 
better in their new locations than in the original ones, witness the rabbit 


in Australia, the cotton boll weevil in southern United States, and the 
English sparrow in America. 

World-wide Scheme of Distribution. — Having so far examined some 
of the peculiarities of distribution, and the biological or geological 
processes needed to explain them, we may now attempt to see how these 
interlocking phenomena affect distribution on a large scale. One must 
usually limit such a study to a single major group of animals because 
of the different timing of evolutionary events in relation to changes in 
the earth. Zoogeographers have proposed different groups for this pur- 
pose. Mammals, snails, earthworms, birds, reptiles, insects, all have 
been urged as suitable. We shall use mammals, primarily because the 
different kinds are better known among nonbiologists, but partly because 
they are large, and the world has been explored enough to discover the 
location of most of them. They have one further advantage: their 
evolution has been rapid and recent, so that the effects of changes of 
the earth will be more readily discovered than in groups whose evolution 
has been slow and protracted. 

The bulk of the land area of the earth is in the northern hemisphere. 
With the connection which must have existed across Bering Strait, this 
land was formerly a continuous body. From this area there project 
southward three great continental masses, South America, Africa, and 
Australia. The last is believed to have been connected with Asia across 
the Malay Archipelago prior to Jurassic time. South America, though 
now connected with North America, is held to have been separated 
from it in early Tertiary time. This is indicated by similarity of the 
marine animals on the east and w^est coasts of Central America, as well 
as by geological evidences. 

Origin of Mammals. — Primitive mammals are believed to have arisen 
first in the northern continents. This conclusion flows partly from 
theory, since the great variations of environmental conditions character- 
istic of huge land masses should have been able to act selectively on 
almost any type of evolutionary change w^hich happened to occur in 
living things. The northern continental mass as the place of mammalian 
origin is supported, moreover, by the fact that the most primitive fossils 
of the group have been found there, though it must also be said that 
more explorations have been made in that area. 

These primitive mammals, resembling our monotremes and mar- 
supials more than true mammals, must have spread in all directions. 
To the north barriers were soon reached, but to the south the three 
great prongs of land provided ample room; and they had a geological 
age or two in which to enter these. 

Then the higher (true) mammals began to arise, also in the northern 
land mass. They proved to be superior to their predecessors, that is, 


more able to cope with the environment. This supposed superiority of 
the later mammals has been demonstrated in modern times by the intro- 
duction of northern true mammals into the southern areas, where they 
began to replace the primitive forms already there. This has happened 
very noticeably in Australia, where the dingo and rabbit were intro- 
duced. Something like a general principle must be involved here, for 
in other groups of animals northern forms have displaced southern ones 
when they have been brought together. This has happened in the 
case of birds (sparrow, starling, blackbird, and others) introduced into 
Australia, the goldfish in Madagascar, European ants and earthworms 
in all the southern continents. 

The early mammals were thus driven out of the northern continents 
which they first occupied. With Australia then joined to Asia, and 
South America not yet separated from North America, they were free 
to fill all the southern land masses. Then the sinking of the land cut 
off Australia, so that the true mammals were not able to follow, and 
that continent was and is the principal home of the marsupials and 
monotremes. The severance of the Americas from each other checked 
the southward migration of the higher mammals, so that primitive 
types are relatively more common in South America. Restoration of 
the land connection at Central America has, however, permitted many 
of the true mammals to reach the southern continent. The traffic was 
not all in one direction at the isthmus, since the opossums and armadillos 
reached North America from the south over this restored land connection. 

Primitiveness of Southern Faunas. — The scheme just outlined should 
have caused the faunas of the southern continents to be on the average 
more primitive than those of Eurasia and North America. For the 
mammals of Australia and South America this has already been shown 
to be true. To a less marked extent it is true also of Africa south of 
the Sahara; for there is the primitive little deerlike chevrotain, and there 
are the lemurs, the aardwolf, and the golden mole. In Madagascar is a 
host of lemurs; and if other groups of animals are to be considered, that 
island has the most primitive bird of the crane and rail group. Also 
outside the mammalia, Australia has the most primitive termites, the 
simplest insects of the butterfly-and-moth order, and some of the most 
primitive bees. The most primitive land snails are in the southern 
continents; indeed, the whole mollusk fauna of South America may be 
characterized as primitive. The three surviving genera of lungfishes are 
in the three southern continents, one genus in each. The lungfishes are 
well represented by fossils in North America and Eurasia, and the three 
living genera are plainly relicts. 

Land Connections. — The connection and separation of land masses 
postulated in the foregoing account mostly are supported by geological 


evidence; that is, they have not been invented merely to explain animal 
distribution. This is particularly true of the changes in Central America 
or the Isthmus of Panama. These changes could be safely assumed on 
geological evidence alone. 

Zoogeographers have not hesitated, however, to assume former land 
connections for which geology gives no support. Geologists have some- 
times been the authors of such connections but have based them on the 
facts of modern distribution. North America and Europe have been 
assumed to be connected through a strip of land taking in Greenland, 
arching north of the Atlantic, and joining Europe through the Scandi- 
navian Peninsula and the British Isles. An antarctic land bridge con- 
necting the tips of South America and Africa with Australia was proposed 
by the British geologist Hutton to account for the large flightless birds in 
those areas. This bridge has been adopted by many others since, but 
it seems unnecessary, for the connection of the southern continents with 
the northern land mass is adequate to account for the degree of similarity 
of the animals. A land bridge has even been thro\\'n across the middle 
of the Atlantic Ocean, from western Africa, say, to Brazil and the West 
Indies. This bridge has been employed by many students of distribution 
and is supported even now by reputable zoologists. The trend, how- 
ever, has been away from extensive land bridges. They may have 
existed, but some of them seem geologically so improbable that zoogeog- 
raphers are seeking other explanations for similarities of faunas, or are 
frankly leaving the facts unexplained rather than postulate the bridges. 

Major Realms. — From the beginnings of zoogeography, many 
attempts have been made to divide the earth into half a dozen or so 
major realms which would have significance for all kinds of animals. 
Birds were first used for such a division, then mammals. For these 
two vertebrate groups the boundaries of the realms were somewhat 
similar, and the authors of the schemes believed that other animals 
would fit into the same divisions. Much of the work of zoogeography 
has consisted of fitting groups of animals into the realms and modifying 
the boundaries when necessary. 

It has become increasingly clear, however, that different kinds of 
animals do not observe the same distributional limits, and that theo- 
retically they should not do so. Each group must be delimited by a 
different scheme. New Guinea, with respect to its earthworms, belongs 
with eastern Asia; but in its other animals it is Australian. The earth- 
worms of Ceylon, on the contrary, are of Australian types, despite the 
nearness of the island to Asia. Chile differs from the rest of South 
America in its mollusks, fresh-water fishes and earthworms, but agrees 
with other parts of the continent in its birds and mammals. It is true 
that in highly isolated areas like the Hawaiian Islands, Madagascar, and 


New Zealand, the barriers are such as to affect nearly all animal groups; 
but they have done so to very unequal degrees. 

One reason for the necessity of different distributional areas for the 
different kinds of animals is the very different history of evolution of 
each group. It makes a great difference whether, at the time of geologic 
isolation of an area, the animals in it are evolving rapidly or are rather 
stable. Madagascar, for example, is inhabited by mammals belonging 
mostly to families found nowhere else, but by amphibia, reptiles and 
insects belonging frequently even to the same genera as those of the 
African mainland. Australia is peculiar as to its mammals, but much 
like the Oriental realm (including southeastern Asia and some East 
Indian islands) in its lizards, butterflies, and earthworms. 

It seems clear now that progress in interpreting the distribution of 
animals is to be made only by working out the history of each group 


Gadow, Hans. The Wanderings of Animals. G. P. Putnam's Sons. (Chap. Ill, 

the spreading of species; Chap. V, ancient geography inferred from distribution 

and fossils.) 
Hesse, R., W. C. Allee, and K. P. Schmidt. Ecological Animal Geography. John 

Wiley & Sons, Inc. (Chap. VII.) 
Rowan, William. The Riddle of Migration. The Williams & Wilkins Company. 

(Chaps. II-IV, bird migration.) 
ScHARFF, R. F. Distribution and Origin of Life in America. Archibald Constable 

& Co., Ltd. (Chap. XII, fauna and flora of the Galapagos Islands.) 
ScHARFF, R. F. The History of the European Fauna. Charles Scribner's Sons. 

(Chap. II, general outline.) 


Many of the fundamental problems which exist in connection with 
living organisms may also be studied, and in some degree solved, with 
reference to beings, now extinct, which lived on the earth in times past. 
This biology of ancient life is termed paleontology. Paleontology may 
be defined as the science of fossil organisms. 

Fossils. — A fossil is any trace of prehistoric life. Most organisms 
have left no trace because they were soft-bodied. Organisms mth hard 
shells or skeletons had the best chance of being preserved, but even 
these were screened by a fine sieve of circumstances and most were lost. 
An animal whose bones are to be fossilized must usually be buried soon 
after death to prevent the destructive action of oxygen, water, freezing 
and thawing, and bacteria; and after it is seemingly safe the fossil is 
subject to the risk of heat and pressure which would alter it beyond 
recognition. Teeth are more likely to be preserved than bones, because 
they are highly resistant; teeth of mastodons are often saved when the 
bones of the same individuals have disintegrated. 

A fossil need not l^e any part of an organism. It may be only an 
impression, a track, or even a burrow. A dinosaur walking on clay, not 
too hard or too soft, has left its footprints to the present time. A leaf 
leaves an imprint in the silt in which it is buried, and this impression 
is a fossil. 

Similar objects buried only several thousand years ago are not 
regarded as fossils; that is a matter of definition. Fortunately not 
many objects belonging to the border line of prehistory are found, so 
that little difficulty arises from the stipulation that a fossil be prehistoric. 

How Fossils Are Preserved. — Some animals in cold regions are pre- 
served in the flesh. That happened to numerous woolly mammoths in 
Siberia (Fig. 272). They fell into crevasses in the ice, were covered 
■with snow, and at the very low temperatures were quickly frozen. Even 
the undigested food in their stomachs is recognizable in some of them. 
These bodies have been frozen for probably 20,000 years. Some frozen 
mammoths have been found in Alaska also, but only fragments of the 
flesh were preserved. Other preservatives of flesh are oil in petroleum 
lands (Poland, Galicia) and the acids of peat bogs. Human bodies have 
retained their flesh, thoroughly dried and therefore resistant to bacteria, 
in the dry southwestern parts of the United States. 




Soft parts have sometimes been preserved merely as films of carbon, 
which is the residue of the protoplasm. These films outline the body 
perfectly, around the skeleton which retains more nearly its original 

Entire insects in coniferous forests of the Oligocene epoch became 
immersed in the sticky resin on the bark of the trees, which then hardened, 
and may still show the delicate spines or the scales of the wings in butter- 
flies as clearly as in the original. 

More often only the hard parts are left — the tubes of corals (Fig. 273), 
the shells of clams, the bones of vertebrate animals. Usually these hard 

Hi iwi j^ ii WJ. ' ^ '->"T»"— 



I'lci. 272. — Muniinoth found fruzeu in .Sil)eiia in lUUl. Most of the was still on 
the body and intact. The skin is mounted in the museum of Leningrad in the posture in 
which it was found. {From Lull, "Organic Evolution," courtesy of The Macmillan Company.) 

parts must be buried before disintegration has proceeded far. They may 
rest at the bottom of a lake, and be covered by silt carried in from the 
land; they may lie on flood plains of streams and be buried under deposits 
at times of high water; they may sink in the soft mud of bogs, be buried 
in wind-blown dust, or covered with volcanic ash. Very often the 
burying material hardens into rock by the cementing action of ground 
water carrying minerals; this is particularly true of under- water deposits. 
After such hardening, the shape of the buried object is usually main- 
tained, regardless of what becomes of the material of which it is composed. 
Sometimes the entire buried shell or bone is dissolved away by ground 
water, which usually contains some carbonic acid (carbon dioxide in 
solution). The cavity thus left is a mold. If this cavity is later filled 



by minerals deposited from the ground water, the mass thus formed 
(called a cast) has the external shape of the original structure (Fig. 274). 
Both molds and casts are fossils, though they include no part of any 
living thing. 

Fig. 273. 

-Fossil chain coral, Halysites, found in Michigan. (From specimen in the 
Museum of Geology, University of Michigan.) 

Mud in which tracks were made hardened as it dried, and was resistant 
enough to keep its shape while new material was washed over it in the 
next freshet. New and old deposits hardened into rock, and the two 
slabs were readily separable at the level of the tracks. One slab bears 
molds, the other casts (Fig. 275). 


Fig. 274. — Diagram illustrating molds and casts. Horizontal shading represents sedi- 
mentary deposits, vertical shading the material subsequently filled in. a, mold of a shell 
which has been dissolved away by ground water; b, cast formed by subsequent filling in of 
the cavity of a; c, mold of a shell whose interior was filled with sediment; d, cast produced 
by filling the mold represented in c. {From Schuchert, "Historical Geology," courtesy of John 
Wiley & Sons, Inc.) 

Many bones and shells were dissolved away and replaced piecemeal. 
That is, the most soluble parts were removed first and replaced by the 
least soluble minerals which the ground water then carried. Less soluble 
portions were removed later, and replaced by minerals then prevalent. 
Different parts of the original bone are thus replaced by different minerals, 


so that even the minute anatomy is preserved. Such objects are said 
to be 'petrified (the process being called petrifaction). 

Large collections of fossils are sometimes found at ancient water 
holes, where animals congregated and died in periods of drought, or in 
asphalt pools where they were trapped and were probably attacked by 
predators which also were caught in the mire. The great collection 
of fossil bears, lions, saber-toothed tigers, horses, elephants, antelopes, 
and vultures at Rancho La Brea near Los Angeles was caught in a pit 
of tar. Caves are likewise the sites of numerous such collections. For 
the most part, however, fossil forms occur singly or in small groups, 
where they are discovered during excavations for buildings, by mine 
operations, or other accidental means. 

Fig. 275. — Natural casts of dinosaur tracks and rain imprints. {From Schuchert, "Histori- 
cal Geology," courtesy of John Wiley & Sons, Inc.) 

Paleontology Relates Two Evolutions. — Like zoogeography, paleon- 
tology treats of the interrelations of two evolutions, the evolution of the 
earth, and the evolution of living things. According to either of two 
prevalent theories of the origin of the earth, this planet was in some way 
derived from the sun, and went through a period of great heat. It is 
only the earth's history in the later cool period that concerns us in the 
study of fossils. Many of the superficial parts of the earth's crust are 
in strata of different kinds of rock. Obviously, where these strata are 
undisturbed, the lower ones were deposited first and are the oldest. In 
many places the strata have been compressed sidewise, and forced to 
rise in arches. With further lateral pressure, the arch may break, and 
the strata of one slope be shifted over the strata of the other slope. At 


the bottom of the overriding portion, an older stratum is above a younger 
one. Often this disturbance is readily recognized, but not always. 

Considerable help in recognizing disturbed strata is given by the 
fossils they contain. While the earth's crust was changing, plants and 
animals were also evolving. Animals of one period were distinctly 
different from those of another. So characteristic of a given period 
are certain kinds of animals that the fossils are known as index fossils. 
Good index fossils must be abundant and widely distributed over the 
earth, and large enough not to be overlooked. Occurrence of an index 
fossil in a stratum at one place is not, however, a complete guarantee 
that any other stratum containing such fossils was contemporaneous 
with the first. These animals had to have a certain type of environ- 
ment, and there are reasons to believe that similar environments occurred 
in different areas at different times in the earth's history. For example, 
the "red beds," made red by the oxidation of iron under certain climatic 
conditions, occur in the Conemaugh formation in Pennsylvania and 
West Virginia, and in the Wichita formation of mid-continental United 
States; but according to other evidence the Wichita is much younger 
than the Conemaugh. 

While there are other ways of correlating rock strata of different 
regions, the changes in types of animals occurring simultaneously 
with changes in the earth are among the most reliable of the means 
of identification. 

Divisions of Geological History. — Geologists use a classification of 
the earth's history which serves much the same purpose as does taxonomy 
for zoologists. The classification is known as the geological time scale. 
Major revolutions of the earth's crust caused elevation of great mountain 
systems, erosion on a grand and extremely rapid scale, and redeposit of 
the eroded material elseAvhere. As a result of these great changes, layers 
of the earth's crust having very different characteristics and containing 
very different fossils lie next to one another. These contrasts, known 
as unconformities, are used to divide geological time into five great eras. 
Within each of these eras the land of continents sank in large areas 
so that the sea invaded the land, then rose again to push the oceans back. 
On the basis of such changes, each era is divided into periods. Minor 
and local changes of the same general type are used to divide the periods 
into epochs. 

All the rocks belonging to a period constitute a system, those of an 
epoch make a series, while smaller divisions than the epochs have their 
rock formations. These terms are not generally used in this book, but 
are constantly met in geological works. 

The accompanying table gives the geological time scale as far as the 
terms are needed in an elementary study of biology. 


Geological Time Scale 




Dominant life 



Age of man 







Age of mammals and modern floras 



Age of reptiles 




Age of amphibians and lycopods 



Age of fishes 


Age of higher (shelled) invertebrates 



Age of primitive marine invertebrates 



Age of supposed unicellular life 

1 Together constituting the Carboniferous (coal age). 

Usually only a small part of this scale is represented in exposed 
strata at one place. One of the more extensive exposures of the crust 
is in the walls of the Grand Canyon in northern Arizona. The Colorado 
River at its bottom is cutting its way through granite at the rate of 
perhaps an inch in a century. Just above it in the slopes are Archeozoic 
rocks; the rim, a mile above, is Permian. Between these are mostly 
undisturbed strata in the order of the scale. 

How old the strata are in years may be computed from the trauvs- 
formation of radioactive substances. The element uranium is being 
constantly transmuted into other simpler elements, the chain ending in 
lead and helium. The rate of change is constant, and independent of 
surrounding conditions. Where uranium is present, the amount of it 
and the amount of lead are ascertained as accurately as possible. If it be 



assumed that all the lead came from uranium, and that none of the lead 
has been removed, the time required for the transformation can be 
computed. On this basis, one computation gave certain Permian strata 
an age of about 220,000,000 years, late Cambrian 400,000,000 years, 
and several pre-Cambrian formations ages ranging from 900,000,000 to 
1,800,000,000 years. The age of the earth itself has been variously 
estimated by the same method to be 3 to 6 billion years. 

Fig. 276. — Cambrian brachiopods (left) and trilobite. {From Hussey, " Historical 


Change of Animal Life. — How animals have changed during the 
millions of years since life began can be indicated here only in a general 
way. Most of the Archeozoic rocks are igneous (once molten), hence 
could not bear fossils. Unicellular organisms are supposed to have 
existed then, but there is little evidence of them. In the Proterozoic, 
which witnessed two periods of glaciation at a number of places (Utah, 
Canada), there are limy deposits undoubtedly produced by unicellular 

Fig. 277. — Ordovician graptolite (left) and snail. {From Hussey, " Historical Geology.") 

plants. Sponges, spicules, and a jellyfish which have been reported are 
still somewhat in doubt. 

Cambrian presents a great outburst of animal life of many different 
kinds. Most characteristic and most abundant were the brachiopods 
and trilobites (Fig. 276). Some shale in British Columbia contains 
marvelously preserved jeUyfishes, sea cucumbers, siliceous (glassy) 
sponges, annelid worms, and Crustacea. There were so many kinds of 
Cambrian animals that the preceding era must have included many ; but 



the long period of erosion between Proterozoic and Paleozoic destroyed 
whatever fossils there were. 

Trilobites were even more common in the Ordovician, and brachiopods 
continued abundant but mostly with shells of lime instead of horn. 
With them in this period were graptolites, snails (Fig. 277), and others. 
The first vertebrate animals, the armored ostracoderm fishes, are found 
in Ordovician but must have existed long before. The following Silurian 
preserved few fossil fishes, but they must have been present, for that 

group blossomed out extensivel}^ in the 
Devonian; these two periods are known 
as the age of fishes (see time scale). 
Among the invertebrates of these periods 
were the brachiopods (now at their peak) , 
trilobites (now on the decline), corals, 
snails, siliceous sponges, cup corals (Fig. 
278), and the scorpionlike eurypterids. 

In Mississippian time the crinoids 
(stalked echinoderms) reached their cli- 
max (some of the best-preserved ones in 
Iowa), and declined greatly in the next 
period. Clams are preserved in Pennsyl- 
vanian with their actual shells; before 
this period the shells had dissolved away 
and the fossils were only casts. The 
latter period also had many insects, some 
of them giants having a wing spread of 
over two feet, also a number of amphibia 
chiefly of the armored type. The succeed- 
ing Permian had many of these armored 
amphibia, but was chiefly distinguished 
by its great variety of reptiles, some of 
which had curious bony spines in a sail over the back (Fig. 279). 

The most characteristic invertebrate animals of the Triassic period 
were the ammonites, the most highly developed group of cephalopods 
whose evolution is described in a later section. These animals continued 
through the rest of the Mesozoic era but declined in the Cretaceous. 
Other invertebrates of the IMesozoic were crinoids, squids, and Crustacea 
(particularly crabs). The great evolution of the Mesozoic, however, was 
in the group of reptiles. On the land were the dinosaurs, in the sea the 
ichthyosaurs (looking like porpoises or sharks) and the four-paddled 
plesiosaurs, in the air the pterosaurs. Dinosaurs often had curious rows 
of dor,sal plates, as in the Jurassic Stegosaurus (Fig. 280), or shields and 
spines as in the Cretaceous Tricoratops (Fig. 281). Some of them were 

Fig. 278. — Fossil cup coral 
found in Michigan. {From speci- 
men in the Museum of Geoloyy, 
University of Michigan.) 



of huge size, as the massive 75-foot Brontosaiirus and the 10-ton Stego- 
saiirus. Other points of interest concerning the Mesozoic are that tmc 
mammals were in existence, recognizable from their teeth and jaws, and 
the first birds appeared. 

.... . ••■'"" ...'Aiiv*" -vV,i.V*T«>: 

"I ; 'I 



Fig. 279. — Permocaiboniferous reptile, Edaphosaurus cruciger; skeleton above, 
restoration below. {From specimen in Museum of Geology, University of Michigan; restora- 
tion by Prof. E. C. Case.) 

The outstanding feature of the Cenozoic is the tremendous develop- 
ment of the mammals, which rivaled that of the reptiles in the Mesozoic. 
No brief account can do them justice. The primitive members of this 
group are the marsupials, represented in North America by the opossum, 
which is little changed now from its Eocene form. Contrasted with these 
were huge forms (7 or 8 feet high) with bony protuberances on the head, 



represented in Eocene but long since extinct. This varied assemblage 
appears suddenly in the earliest Cenozoic deposits, indicating a long 
evolution before that era. The evolution of two mammals whose 
histories are most completely preserved is presented later in another 

The purpose of the brief account in this section is to show the general 
nature of the evolution of animals in relation to the evolution of the 
earth's crust. So far as it relates to the vertebrate animals the story 

Fig. 280. — Skeleton of the armored dinosaur Stegosaurus. {From Lull, "Organic Evo- 
lution," courtesy of The Macmillan Company.) 

is summarized by the diagram in Fig. 282. A similar chart for the more 
numerous kinds of invertebrates would be too confused for our purpose. 
Lines of Evolution. — Out of the wealth of fossil forms barel.y hinted 
at above it is possible to select a few groups that show especially well 
the step-by-step changes which animals have undergone. These gi'oups 
are particularly instructive because the relative ages of their members 
are not in doubt, and the differences between any two successive mem- 
bers are so small as to leave no question that they possess genetic conti- 
nuity. Such a scries of related forms is spoken of as a line of evolution. 



The lines of descent of modern elephants, horses, and cephalopods are 
especially useful for illustration. 

Evolution of Elephants. — The mastodon-elephant series shows a 
larger number of obvious changes than either of the other series named. 

Fig. 281. — Restoration of horned dinosaur Triceratops. {After Lull, from Schuchert, 
"Historical Geology," John Wiley & Sons, Inc.) 

Figure 283 will disclose the more striking steps of their evolution. The 
earliest animal recognized as belonging to the elephant series, Moeri- 
therium by name, was recovered from late Eocene and early Oligocene 

Fig. 282. — Diagram of the fossil history of the major groups of vertebrate animals, 
width of the bands indicates abundance and number of kinds. 


deposits of northern Egypt. It was slightly over 3 feet in height. The 
elephantine features are the high posterior portion of the skull (Fig. 283F') 
composed of cancellate bone, that is, bone containing open spaces; the 



elongation of the second pair of incisors in each jaw to form short tusks; 
the indication of transverse ridges on the molar teeth (F) ; and the position 
of the nasal openings some distance back of the tip of the upper jaw, 

Fig. 283. — Evolution of the head and molar teetli of the mastodons and elephants. 
The shuUs on the right are enclosed in the flesh in the form the latter is supposed to have 
had. A, A', Elephas, Pleistocene; B, Stegodon, Pliocene; C, C, Mastodon, Pleistocene; 
D, D', Triloi^hodon, Miocene; E, E', Palaeoinastodoii, Oligocene; F, F' , Moeritherium, 
Eocene. {Frorn Lull, "Organic Evolution," courtesy of The Macmillan Company.) 

indicating probably a prehensile upper lip. There were 3G teeth, and the 

neck was long enough to enable the animal to put its head to the ground. 

Palaeomastodon, which lived in Egypt and India, dates from early 

Oligocene time. It was of somewhat larger size, the posterior part of 



the .skull was distinctly higher {E') with a greater development of 
cancellate bone, and the neck was somewhat shortened. The upper 
incisors of the second pair were more elongated as tusks; the lower 
second incisors were present, but not enlarged; while all other incisors 
and the canines had disappeared. The molar teeth {E) resembled those 
of Moeritherium but were larger. The lower jaw was considerably 
elongated, and the number of permanent teeth was reduced to 26. The 
nasal openings had receded until they were just in front of the eyes, 
which is believed to indicate the existence of a short proboscis extending 
at least to the tips of the tusks. 

Trilophodon, a great migrant and consequently widespread over 
several continents in Miocene time, Avas a huge animal, nearly as large 
as modern Indian elephants. The tusks were considerably longer (-D')- 
The molar teeth were large and greatly 
reduced in number, so that only two were 
present at any one time on each side of 
each jaw. The surface of these teeth 
bore a somewhat larger number of trans- 
verse crests {D) than were present in the 
earlier forms. The lower jaw was enor- 
mously elongated, so that it projected as 
far forward as the tusks. There was a 
considerable development of cancellate 
bone in the skull, to which the supporting 
muscles of the neck were attached. The 
long lower jaw, which was not continued 
in later forms, has led paleontologists to 
conclude that Trilophodon was not in the 
direct line of descent, but that it was an 
offshoot. Dinotherium (Fig. 286, upper left), a contemporary of 
Trilophodon but with a strongly recurved lower jaw, is likewise 
regarded as a lateral branch. 

The mastodons were somewhat larger than Trilophodon, being about 
the size of the Indian elephant. The tusks {€') were much elongated 
(9 feet or more), but the lower jaw was greatly shortened and the lower 
incisor teeth were reduced or wanting. The molar teeth (Figs. 283C, 
284) were scarcely more complex than earlier forms and numbered 18 to 
20 in the permanent set. They were still crushing teeth, and the food 
must have been tender twigs and succulent plants; indeed, remains of 
such objects have been found in the region of the stomach of some of 
the fossil mastodons. 

Apparently arising from the primitive mastodons was Stegodon, 
knoAvn only from Asiatic Pliocene. Its molar teeth (Fig. 283 i?) had 

Fig. 284. — Mastodon tooth, 
showing the enormous cusps on the 
upper surface. {From a California 
specimen in the Museum of Geology, 
University of Michigan.) 



distinct transverse ridges, though not many of them, and its lower jaw 
was short. 

The extinct elephants known as mammoths belong to Pleistocene 
time, while from them or directly from Stegodon have arisen two kinds 
still living, the Indian and the African elephant. The gross features of 
the elephants are their size, short neck, long proboscis, and heavy tusks. 
The skull is very high and short (Fig. 283A'), due chiefly to the develop- 
ment of cancellate bone. As in the earlier forms, the high skull affords 
the necessary leverage for the muscles that support the weight of the 
tusks. The molar teeth are distinctly grinding teeth (Fig. 283A ; see 
also Fig. 285). Each tooth bears a number of transverse ridges, about 
10 in the African elephant and 24 or more in the Indian species. These 

A B 

Fig. 285. — Tooth of mammoth (Elephas) from the Pleistocene, showing the flat grinding 
surface and the numerous plates of enamel bound together by cement. A, side view; 
B, surface view. {From specimen discovered at Ridgeivay , Michigan, in 1912, and preserved 
in the Museum of Geology, University of Michigan.) 

ridges are worn down by the chewing of harsh food, so that the upper 
surface displays the cross sections of a number of flattened tubular 
plates of enamel enclosing dentine and bound together by cement. 
While the tusks (incisors) are of two sets, one following the other like 
milk and 'permanent teeth of other mammals, the grinders succeed one 
another in continuous fashion. As the molar teeth that appear first 
wear down they move forward in the jaw and are replaced by others 
from behind. Three permanent molars may thus successively appear 
on each side of each jaw, but the wearing and movement are slow, so 
that the interval between the appearance of the second molar and that 
of the third may be 30 years. The total number of permanent teeth, 
including the tusks, is 14. 

Correlated with the nature of the teeth of the elephants are their food 
and chewing habits. The an(;estral forms whose molars bore prominent 
elevations lived on twigs and tender herbage which they crushed in 
mastication, but the mammoths with their flattened tooth surfaces 



devoured grasses, sedges, and other harsh vegetation which they ground 
with lateral motion of the teeth upon one another. 

Fig. 286. — Restorations of heads of fossil elephantlike animals. Upper center, Moeri- 
therium; below it, Palaeomastodon; upper right, Trilophodon; upper left, Dinotherium; 
lower right, Mastodon; lower left, Elephas. {From models prepared by Ward's Natural 
Science Establishment.) 

The appearance of the heads of the series of elephantlike animals is 
imagined to have been as shown in Fig. 286. The ears are suggested 
by those of modern elephants, the proboscis by modern elephants and 
the position of the nasal openings, as 
already indicated. The general form of 
the head and tusks is, of course, accurate. 

Evolution of the Horse. — Most of the 
development of the line of descent of the 
horse took place in North America. 
Eohippus, a lower Eocene form, is the 
first member of the series recognizable as 
ancestral to the horse, though it may also 
be regarded as approximately represent- 
ing an ancestor of the tapirs and the 
rhinoceroses. It stood about 12 inches 
high and had a short head and neck (Fig. 287). The hind foot 
had three well-developed functional toes. On the outer side was a splint 
bone representing an additional toe, and on the inner side a rudiment 

Fig. 287.— Restoration of 
Eohippus. {From, model prepared 
by Ward's Natural Science Ls- 



of still another. Many living vertebrates have five digits on each hand 
or foot, and there is anatomical and embryological evidence that primitive 
vertebrates in general had five digits. These are numbered from the 
inside outward, the thumb or great toe being first, the little finger or 
little toe last. In the hind foot of Eohippus the functional toes are the 
second, third, and fourth, while the fifth is reduced to a splint bone 

ah c 

Fig. 288. — Fossil teeth of ancient horselike animals, a, tooth of Eohippus with the 
root broken; b, tooth of Mesohippus; c, tooth of Merychippus. {Photographed from 
specimens in the Zoological Laboratory of the University of Michigan.) 

and the first is rudimentary. The forefoot had four functional digits, 
the first being wholly wanting, though some old figures erroneously 
include one. In the ancestors of the horse the first digits seem to have 
disappeared first, followed by the fifth. The teeth of Eohippus were 
short of crown and relatively long of root. The upper surface bore 

several conical cusps which, howevei-, 
showed some sign of fusing to form 
transverse crests (Fig. 288). The skull 
(Fig. 289) was small, the lower jaw com- 
paratively short, and the orbit Avas placed 
well over the teeth, making the face 
relatively short. Orohippus, which lived 
in middle Eocene time, resembled Eohip- 
pus closely but lacked the splint bone of 
the forefoot (Fig. 290, left). 
Mesohippus, an Oligocene form, was about 18 inches high. It had 
only three digits on each foot (Fig. 290), but on the outer side of the 
forefoot was a splint bone representing an extra toe (the fifth). Of the 
three well-developed toes, the middle one (third) was in each foot dis- 
tinctly larger than the others. The skull (Fig. 291), except for its 
increase in size, had not changed materially. The cro\\Tis of the molar 
teeth were still low (Fig. 288) and were tuberculate, that is, provided 
with cusps on the upper surface, but the cusps were more distinctly 
united into ridges or crests. Miohippus, a little later in Oligocene, was 
somewhat larger, but otherwise much like Mesohippus (Fig. 290). 

Fig. 289.— Skull of Eohippus, 
about ^io natural size. (From 
model prepared by Ward's Natural 
Science Establishment.) 



In Merychippiis, a INIiocene animal, the feet were all three-toed (Fig. 
290), vestiges of the fifth toe being present in some specimens and wanting 
in others. The lateral toes, however, were high above the ground; the 

Fig. 290. — Fore feet of fossil horselike animals; from left to right, Orohippus, Meso- 
hippus, Miohippus, Meryehippus, Pliohippus. Of each type there are represented the 
bones and the restoration in the fiesh. (From, models prepared by Ward's Natural Science 

entire weight of the body was borne upon the middle (third) toe. The 
permanent molar teeth had moderately high crowns, and the upper sur- 
face was worn down to a flat grinding surface marked by sharp ridges of 
enamel set among dentine and cement (Fig. 288). Meryehippus was 
evidently a grazing animal, whereas its predecessors must have fed upon 
succulent herbage which was crushed, not ground. The skull was 

Fig. 291. — Skull of Mesohippus, about J^fo natural .size. (From photograph of specimen 
in Museum of Geology, University of Michigan.) 

enlarged (Fig. 292), and the lower jaw was heavier in evident relation to 
the change of the teeth. The orbit of the eye occupied a more posterior 
position relative to the teeth, making the face longer. The orbit was 
also completely closed behind by a bar of bone which in the earlier 



forms was merely a process projecting down from above. The body had 
increased to a height of 3 or 4 feet. 

Phohippus (Phocene) was not appreciably larger than the preceding 
member of the series but the two lateral toes had disappeared (Fig. 290), 
except as long splint bones. Pliohippus was thus the first one-toed 
horse. The teeth were moderately long-crowned and possessed grinding 
surfaces. The body stood about 48 inches high. 

The fossil horses of Pleistocene time were so nearly like the living 
forms as to be included with the latter in the &ame genus (Equus). The 
recent animals are 60 inches or more in height and weigh many hundreds 
of pounds. Each foot has but one toe. Two lateral toes are evidenced 

Fig. 292. — Skull of Merychippus, about ^{q natural size. {From model prepared by 

Ward's Natural Science Establishment.) 

by splint bones, and in rare cases a reversionary horse is born with exter- 
nally visible digits articulated with one of these splints on each forefoot. 
The teeth are long and columnar and grow continuously during early 
and middle life, during which time the wear at the upper surface approxi- 
mately equals the growth. The grinding surface is worn fiat, except 
that the enamel resists the abrasion more successfully than do the dentine 
and cement, so that the enamel forms sharp cutting ridges. The posi- 
tion of these ridges changes somewhat as the tooth is worn to different 
levels and the pattern of the upper surface is indicative, in a general 
way, of the age of the animal. Later in life growth of the teeth prac- 
tically ceases, and then the teeth may become quite short. The face is 
relatively longer than in the ancestral forms, since the eye is set well 
back of the teeth and the brain case has not been relatively enlai-god. 



Evolution of the Cephalopods. — ^An excellent fossil record among the 
invertebrates has been established for the tetrabranchiate (four-gilled) 
cephalopods (Mollusca), already used to illustrate the biogenetic law 
(page 255). This branch of the cephalopods is represented today by 
Nautilus, which lives in a coiled shell, externally resembling a snail shell. 
The animal lives in only a small portion of the shell near the aperture. 
The rest of the shell is divided by partitions into a number of chambers, 
from which the animal is excluded except for a small stalk that extends 
back through all of them. These partitions, or septa, represent the 
positions occupied by the animal earlier in its life. As the body grows. 

Fig. 293. — Diagrams of sutures of cephalopods, slightly more than half shown, 
orthocone; B, nautiloid; C, goniatite; D, ceratite; E, ammonite. 


it moves periodically forward into the wider part of the shell and secretes 
a partition behind itself each time it moves. 

Tetrabranchiate cephalopods have been found as fossils in Cambrian 
rocks. They became fairly abundant in early Ordovician time. At 
that time, unlike the modern Nautilus, their shells were straight cones 
(orthocones) . All later forms appear to have descended from these 

The course of evolution was as follows. The shell soon began to 
bend and in many forms became closely coiled in flat spiral form (Fig. 
210) like the shell of some snails. Owing to their resemblance to Nautilus 
these animals are called nautiloids. They were very abundant in Silurian 
time. Up to this period the septa across the shell were flat and saucer- 
like, and the sutures, the lines of junction of the septa with the wall of the 
shell, were nearly straight or only slightly curved. Later the septa 


became bent in various Avays, at least at their edges, so that the sutures 
were curved or anguhir (see Fig. 293). Forms whose sutures were of 
this curved and angular form are called goniatites, and they were al)un- 
dant in the Carboniferous period. These were to a large extent super- 
seded in Triassic time by other genera, still tightly coiled but with sutures 
thrown into a number of regular curves and sawteeth, which may be 
described as "crooked." These forms with crooked sutures are known 
as ceratites, from a very common genus keratites. And finally, in the 
forms known as ammonites, the sutures became finely crimped in a com- 
pound fashion, often producing exquisite foliaceous patterns. Fossil 
ammonites are most abundant in the Jurassic to Cretaceous strata. 

Though there were many irregidarities and overlappings in the series 
of tetrabranchiate cephalopods, the fossils show on the whole clear evi- 
dence of progress from a straight shell to one tightly coiled, and from 
nearly straight sutures to sutures that were bent, angular, crooked, and 
finely lobed. 

Prehistoric Man. — The human line of evolution is not comi)lete 
enough to offer as an example of such lines, but it has an extraordinar^y 
appeal to the modern representatives of it. Man is one of the ortler 
of primates, other members of which are the lemurs, monkeys, and 
manlike apes. There is some reason to believe that the primates evolved 
from the insectivores, the group to which moles and shrews belong. 
If a series of fossils were available to show human evolution, it should, 
assuming our surmises to be correct, lead from the insectivore t,ype, 
through forms resembling lemurs, monkeys, and apes. The later stages 
of this series would be especially useful as connecting the apes with man. 
Unfortunately, not man.y primate fossils have been foimd. The probable 
reason for the lack of fossils is that the primates have been tree-dwellers. 
Dead individuals would have dropped to the groiuid, and forested areas 
offer little chance for burial under either wind- or water-borne material. 
Fossils of man himself were not preserved in numbers until burial customs 
arose. As a result of these customs, more fossil men are known than 
fossil apes. Kinship of man and the apes must therefore be judged 
largely from homologies. Paleontology can begin to h(>lp only after 
considerable divergence has occiu-red. Nevertheless, the earliest man- 
like fossils show unmistakable leanings toward the ape structure in 
certain respects. 

One of the most primitive of the fossils appearing to connect man 
with the apes, a form usuall}^ regarded as belonging to middle Pleistocene 
tim(;, is Pilhccanthropus crectus, uncovered in some excavations in 1891 
hi Java by a Dutch army surgeon. A femur, parts of the skull, and 
several teeth were in the original find, and parts of several skulls and 
jaws and additional teeth have been added from near-by locations since. 



The craniul capacity is about 900 cc., \v}u(!li is intermediate betwecm apes 
(600 cc.) and men of today. The straight femur indicates erect posture, 
since quadrupeds have doubly curved thigh bones. The heavy brow 
ridges, rounded chin, and protruding face are all apelike. 

Also of middle Pleistocene time are a number of skulls and a few 
leg bones M^hich were found in a cavern south of Peking, China, in 1928 
and later explorations. Their massive brow ridges, low foreheads, and 
round chins are apelike, the average 1000-cc. cranial capacity inter- 
mediate, the straight femur human. Along with the remnants in this 

Fig. 294. — Restorations of prehistoric men. Left, Pithecanthropus erectus; middle, 
Homo ncanderlhalensis, modeled on the Chapelle-aux-Saints skull; right Cro-Magnon man 
modeled on type skull of the race. {From original busts by Prof. J. H. McGregor.) 

cave were crude flint implements, and charred bones of animals which 
indicated that Peking man was a hunter and knew the use of fire. 

Piltdown man, so called from Piltdown common in Sussex, south of 
London, where it was found, might on the basis of associated fossils be 
assigned a slightly earlier time than the preceding ones, but had charac- 
teristics which are indicative, in part, of a later period. The find includes 
parts of two skulls and some loose teeth. Very much like man of the 
present were the cranial capacity of 1400 cubic centimeters, which is as 
large as many European skulls now, and the poor development of brow 
ridges. Like apes were the considerable thickness of the skull bones, 
the broad low nose, and the receding chin. The skull is a mixture of 
advanced and primitive features. 

Neanderthal man, so named because the first-described specimens 
came from a cave in the Neander Valley near Dlisseldorf, Germany, 



invaded western Europe in the warm period before the last of the great 
Scandinavian glaciers. Many skulls and nearly complete skeletons have 
been found in caves in a number of countries. These men were seldom 
as tall as b}4, feet, but were powerfully built. The cranial capacity was 
1300 to 1600 cc, brow ridges were heavy, chin usually receding (though 
some had a small prominence). A restoration of Neanderthal man, with 
two other types here described, is shown in Fig. 294. The measure- 
ments of the skull are correct, but the thickness of the skin and under- 
lying connective tissue at various places, and the amount of hair, can 
only be conjectured. Well-designed flint tools (Fig. 295) were their 

main weapons, scarcely adequate 
to kill the cave bear, * mammoth, 
reindeer, and bison whose bones 
are found in the caves, so they may 
have used traps, pitfalls, and prob- 
ably stones. Neanderthal men 
were not good housekeepers, for 
debris was allowed to accumulate. 
To this untidy habit and their 
burial customs we owe our very ex- 
tensive knowledge of the anatomy 
and culture of this early human 

Toward the end of the last 
glacial epoch (late Pleistocene) 
Neanderthal man disappeared from 
Europe and was followed by the 
Cro-Magnon race. Probably it 
was a forcible displacement. The 
name Cro-Magnon comes from 
the cave in which the earliest- 
discovered skeletons of this type 
were buried. From these individuals it would be said that Cro- 
Magnon man was tall (6 feet or more), that his face was broad and flat 
(from prominent cheekbones), that his forehead was high (hence he was 
probably as intelligent as men of today), and that he was strongly built. 
But men elsewhere in southern Europe, who must presumably be assigned 
to any prevalent "type" of that time and region, were not all so tall, 
often had protruding faces, and even sloping foreheads. Thus there 
were tribes of Cro-Magnon man, just as there are tribes of American 
Indians, who are at the same time still Indians. The burials of these 
people wore evidently conducted ceremonially. Bodies were placed in 
ai'tificial positions, or were shrouded in garments of shells, or were 

295. — Neiiiiderthal flints; point 
scraper below. {From Hussey, 

" Historical Geology.") 



painted, or community tombs were walled all around with certain bones, 
as the shoulder blades and jaws of mammoths. Flint tools were brought 
to perfection, but horn, bone, and ivory were also used for that purpose 
as being more easily worked. Sewing was done with bone awl and 
needle (Fig. 296). The bow and arrow had been invented, and these 
with the spear, thrown from a short holder which remained in the hand, 
were the principal weapons. Art had a considerable development, and 
pictures of animals were cut (Fig. 297) or painted on the walls of caves. 

Fig. 296. — Cro-Magnon tools of bone; needle above, harpoon point below. 

These murals also indicate the existence of witch doctors whose bizarre 
masks are there pictured, and of dome-shaped dwellings presumably 
made of skins stretched over a framework of wood. 

Then new people began to appear from the East, from the plains of 
Persia or farther north. These newcomers migrated north of the 
Mediterranean, or south of it and across to southern Europe, or along 
the sea itself to the Atlantic Ocean and thence to the British Isles. 
They did not destroy the Cro-Magnons of southern Europe, but mixed 
with them, or by-passed and surrounded 
them. In southern France, elsewhere in 
Europe, and in the Canary Islands there 
are still people whose measurements are 
nearly identical with the Cro-Magnons 
of the first-found cave, and these are 
believed to be practically unaltered 
descendants of the Cro-Magnon race. 
With the coming of this eastern tide 
Cro-Magnon art declined, and the 
implement worker's skill deteriorated. 
But the Asiatic invaders had their 
culture, which included weaving of nets and baskets and, far more 
important, agriculture. In their Persian home they had learned to raise 
plants and animals for food — a step which made possible a tremendous 
increase in the number of people in a given area. 

Continued migration from Asia, and evolutionary developments 
within Europe itself, led to the races and cultures that have succeeded 
one another to the present time. Since the white people of North 
America are the descendants of European immigrants, the history of 

Fig. 297. — Cro-Magnon engrav- 
ing of the woolly mammoth on the 
wall of a cave in France. 


man given alcove is the history of the bulk of people of the western 
continent also. 

Man in America. — The American Indians are so plainly Mongoloids 
that they must have come from Asia; and the means of travel available 
to these people almost guarantees that they crossed the Bering Strait, 
which could have been dry. The Asiatics most like the American 
Indians are not the Chinese, but the more generalized people of central 
Asia, Tibet, or the East Indies. Migrations of these people extended to 
Patagonia on the south, and to the Atlantic seaboard, long before white 
men came to America. The Eskimos of the arctic region are more nearly 
like the Chinese and Siberians, and probably are the latest immigrants. 

Important discoveries of arrow points with fossil bison in New 
Mexico in 1927 were followed in rapid succession by other revelations 
of culture in relation to such extinct animals as horses, camels, masto- 
dons, and ground sloths. The making of pottery, an art which for some 
reason Cro-Magnon man never developed, has entered extensively into 
the later history of culture in America. The New Mexico points were 
interpi-eted as belonging to the late Ice Age, or perhaps 25,000 years 
ago, so the migration must have occurred earlier. 


BouLE, ]Marcp:llix. Fossil Man. Oliver & Boyd. (Chap. IV, Pithec'anthro})u.s; 

pp. 147-157, Heidelberg man; pp. 157-175, Piltdown man; Chap. VII, Neander- 
thal man; pp. 281-289, Cro-Magnon man.) 
HowELLS, W. Mankind So Far. Doubleday, Doran & Company, Inc. 
HussEY, R. C. Historical Geology. McGraw-Hill Book Company, Inc. 
Lull, R. S. Organic Involution. 1929 Ed. The Macmilian Company. (Chap. 

XXVI, cephalopods; Chap. XXX, dinosaurs; Cliap. XXXV, elephants; Chap. 

XXXVI, horses; Chap. XXXVII, camels. Book lacks modern viewpoint on 

factors of evolution.) 
Matthew, W. D. The p] volution of the Horse. SitpplciiHnt to American Mustniin 

Journal, January, 1903. 
OsBORN, H. F. Men of the Old Stone Age. Charles Scrilmer's Sons. (Pp. 72-84, 

Pithecanthropus; 9.5-102, Heidelberg man; 130-144, Piltdown man; 214-244, 

Neanderthal man; 289-303, Cro-Magnon man.) 


At many places in the preceding chapters tlie assumption has been 
made that the kinds of hving things on the earth haxe clianged over 
periods of time. In Chap. G possible ways of deriving multicellular 
organisms from unicellular ones were postulated, in the belief that the 
complex life of today could not always have existed. In describing the 
varied breeding habits of animals (pages 185-186), it was assumed that 
animals had evolved, but it was pointed out that the evolution of their 
habits had not closely followed their structural evolution. In the classi- 
fication of animals the basis of grouping is the supposed kinship of the 
various species, due to descent from common ancestors and ascertained 
from homology (pages 250^). The environmental relations of animals 
were shown (page 283) to involve questions of evolution, since it was shown 
that temperature could produce permanent modification of races. All 
through the discussion of geographic distribution (Chap. 21) changes in 
species were assumed to have occurred, in order to explain the position, 
size, continuity, and proximity of ranges, and the differences between 
southern and northern continents. And, finally, fossil animals (pages 
331-348) were regarded as giving positive evidence not only of evolution 
but of the direction which some evolutionary changes have taken. These 
frequent references to evolution in advance of its separate discussion 
indicate how intimately the idea of change of species is woven into the 
entire fabric of biology. It would have been impossible to discuss these 
phenomena adecjuately without relating them to evolution. Without 
repetition of the facts and discussions already presented, it is left to 
this chapter to summarize briefly with additions the reasons for believing 
such changes to have occurred, and the methods by which they may have 
been brought about. 

Evidences of Evolution. — One of the most compelling reasons for 
assuming evolution is the existence of many similarities among species 
of animals and plants. Some of these similarities have already been 
detailed in the chapter on classification. To the homologies there 
described may be added that shown by the membranous labyrinths of 
the inner ears of vertebrate animals (Fig. 298). Each has a series of 
three semicircular canals set in different planes and attached to a central 
sac; but in each group of vertebrate animals there are characteristic 




differences that make it possible to recognize the group of animals by 
the labyrinth alone. The embryos of animals also show homologies. 
Every college course in embryology is a recognition of the existence of 
types of development; for the laboratory studies, based on one or two 
animals, are used to exemplify most of the classes in a phylum. The 
homology of embryos is more spectacular when it is discovered in species 
that are not alike in the adult. This situation is more likely to arise in 

Fig. 298. — Membranous labyrinths of inner ear of various vertebrates. Each consists 
of a saccular portion from which three semiciicular canals arise. A, of a fish; B, of a frog; 
C, of a reptile; D, of a bird. (^Modified from Retzius.) 

parasitic animals, since adult parasites are frequently very degenerate. 
An excellent example is a parasite, Sacculina, found attached to the under- 
side of the abdomen of common crabs (Fig. 299). Sacculina, in the adult 
stage, is a rounded pulpy mass with practically no definite structure, 
except a host of rootlike processes which extend throughout the crab's 
body and absorb the body fluids. The embryo, however, is a three- 
cornered little animal with jointed legs which clearly marks Sacculina as 
one of the Crustacea. It is, in fact, one of the barnacles, a group in which 
adult structure is usually quite complicated (Fig. 300). 



Similarities in physiological properties are quite as abundant as are 
likenesses of structure. The enzymes of digestion are in general very 
much alike in different vertebrate animals. As a rule, protein-spJitting 
enzymes are produced and used in corresponding organs in different 
^'ertebrates. Nervous and hormone control are in most respects alike. 
Even the composition of the blood shows close similarity between animals 
whose structures are alike; the hemoglobin (page 127) has nearly the same 
crystalline chai-acters, and the serum has almost the same chemical com- 
position as shown by precipitin tests. In using this precipitin reaction 
an animal is rendered immune to, let us say, sheep blood by repeated 
injection of sheep blood into its veins. This immune blood then pro- 

FiG. 299. — Sacculina, parasitic on crabs. A, young Sacculina, shortly after hatching. 
B, young animal shown attached to its host, the crab. The projection at the anterior end 
has penetrated the chitinous ventral wall of the abdomen of the crab, only a small piece of 
the chitin being shown. C, adult Sacculina (s), consisting of a pulpy mass on the under 
side of the crab's abdomen, and a host of branching processes in the host's body. A and 
B greatly but unequally magnified, C reduced. 

duces a white precipitate when blood of a sheep or of an animal very 
similar to sheep is mixed with it, but not when blood of a very different 
sort of animal is mixed with it. The precipitate is formed only in 
response to blood of a given chemical composition, and similar composi- 
tion has been found almost solely in the blood of animals that are 
structurally similar. 

The argument from all these similarities, already advanced on page 255, 
is that only heredity — hence common ancestry — could account for them. 
But if two species of animals have come from a common source, any 
differences between them — and there always are differences — must have 
arisen 'since the time of the common ancestors. Emphasis is now to be 
put on these differences, for their origin constitutes evolution. 

The other principal evidences of evolution are derived from fossils, 


from distribution of present-day organisms over the earth, and from 
observation of the process. The rather complete series of fossil animals 
leading up to modern horses and elephants, and the series of cephalopods 
ending with extinction, as described in the preceding chapter, need no 
comment as indications of evolution. To them may be added an 
immense amount of less complete data of fossils, all of which point to 
the same conclusion, namely, that species and larger groups of animals 
and plants have changed. Geographic distribution, as repeatedly shown 
in Chap. 21, likewise requires the assumption of evolution 
to be intelligible. It should not be necessary to comment 
further upon it here. The observational evidence will 
be referred to later. 

Evolution a Change of Species. — Though evolution has 
effected a separation of groups of high rank (orders, classes, 
phyla) from one another, it has accomplished this result 
entirely by modification of species. There is no such thing 
as single wide cleavages that at once produce cA^en families 
or genera out of single common stocks. The divergence is 
everywhere a slow accumulation of small differences such 
as characterize species or varieties. When life originated, , , '"^' ,■. 

_ _ o 7 Adult free-hv- 

assuming that it did so only once, there was at first only ing barnacle of 
one species of organism. When a change occurred in a part ^^^ ^''•ti'* t^ u 
of this group, all experience indicates that the difference of its shell re- 
could have been no greater than that now existing between moved. 
species — or more probably varieties. When further changes occurred, it 
is not likely that altogether the same changes took place in both varieties, 
so that each of them gradually bi-oke up into two unlike sets of varieties 
or species. The two varieties produced by the first modification may 
thus have given rise to two species, later to two genera. By fiu'ther 
change of species, each group of species pursuing a course somewhat 
different from the other, these two genera may be supposed to have been 
transformed into families. Still further changes in species within the 
families shovdd have resulted in the degree of difference now held appi'o- 
jjriate to orders. By continued change of species, the orders may have 
diverged from one another enough to be regarded as classes and finally 
to have attained the rank of phyla. The coiu'se of evolution has been 
not to create phyla and then to proceed to split tliem up into groups of 
lower ranks, ending in species and varieties; it has ratlier gone in the 
opposite direction, beginning with species and by repeated ciianges of 
species gi-adually converting them into groups of higher rank. The pro- 
blem of evolution thus becomes that of the origin of species. 

The Nature of Species. To understand evolution it is necessary, 
therefore, to know how species arc constituted. A species may be 


thought of as a group of individuals most of which have^most of their 
inherited characteristics in common. Characteristics due to environ- 
ment and differing in individuals solel}^ because of different environ- 
mental influence are not considered. The difficult}^ in applying the 
foregoing idea lies in the word ''most," for there is much disagreement 
among taxonomists as to how much it should include. Probably no 
species that can be recognized as a species anywhere in the world has 
all of its individuals alike in all hereditary qualities. It would be possible 
to assemble groups of individuals alike in all their genes (page 224), but 
such assemblages would be much smaller than the ones now recognized 
as species. To insist that species be entirely homogeneous would simply 
multiply the number of species and would solve no problem either of 
evolution or of classification. In practice, therefore, some heterogeneity 
is admitted. As far as taxonomists agree on the grouping, all individuals 
of a species have certain qualities in common; these qualities are held to 
characterize the species. Beyond this general heritage, there are other 
characters each of which is present in some individuals, but none in all 
of them. A certain amount of variation thus exists among the indi- 
viduals of every species. Some of this variation is nearly always visible 
or otherwise capable of detection; but some of it is not seen, since it 
consists of recessive genes scattered through the population. These 
recessive genes, unless very numerous, are present more often in hetei'o- 
zygotes than in homozygotes (page 227) and do not greatly affect the 
species visibly; but they are a potential source of visible qualities in later 

Species do not as a rule cross with other species, though there are 
many exceptions. Also, species tend to occupy different ai-eas from other 
species. These are marks which help the taxonomist to recognize species 
as distinct, and their intersterility is an important agent in making them 

Origin of the Differences among Individuals. — What is*the source 
of the minority of qualities in which the individuals of a species may 
differ? Since a species is ordinarily descended from a single individual, 
it would be expected, unless the ancestor had been an extremely heterozy- 
gous organism, that its descendants would possess practically the same 
genes throughout. The existence of a number of genes which are not 
alike in all individuals indicates that some of the genes have changed in 
some individuals. Such changes of genes are the mutations already 
referred to (page 238) in the discussion of genetics. 

Mutations are not merely inventions to explain the variation within 
species; the visible changes due to them have been witnessed again and 
again in many animals and plants. Some of the first changes to be called 
mutations were observed by Hugo de Vries, one of the rediscoverers of 



Mendel's law (page 18), in the evening primrose Oenothera lamarckiana 
before the year 1890. Since that time, individuals of this species and 
others of the same genus have continued to produce offspring unlike 
themselves in some permanent way. Not a year passes without the 
production of one or more new forms. Some of them represent changes 

Fig. 301. — Mutation in Oenothera involving the length of the seed capsule. The two 
specimens at the left arc Oenothera reynoldsii mutation dehilis, a foiin which gives lise by 
mutation to the form represented by the two figures at the right, Oenothera reynoldsii 
mutation bilonga. {Photograph by Prof. H. H . Bartlett.) 

in the seed capsules (Fig. 301), others the whole habit of growth. Some 
mutations are detectible only in the adult plant, others in the young stage 
known as the rosette (Fig. 302). The alterations arising in Oenothera 
are not the simplest examples of evolutionary change, for it has been 
found that most of them are due not to simple changes of genes but to 
rearrangement of large fragments of the chromosomes and ]-egrouping of 
whole chromosomes that adhere to one another. Probably such changes 



should not be called mutations, but the name has been applied to 

]\Iodifications that are due to changes of single genes — and hence are 
true mutations — have, however, been abundantly witnessed in other 
organisms. Over a thousand alterations have occurred in pedigreed 
strains of the vinegar fly Drosophila melanogaster, and many of these 


< S C/77 > 

'! •■ -^^at^^^tiliiff- 

Fig. 302. — Mutation in Oenothera involving the rosettes, or young plants. Below 
(8 and 9), Oenothera pratincola; above (3 and 4), Oenothera pratincola mutation nummularia, 
a mutant of the preceding form. {Photograph by Prof. H. H. Bartlett.) 

are presumabl}^ changes in single genes. The first of these mutations to 
be discovered was a change from red eye to white in one fly in the labora- 
tory of Prof. T. H. Morgan in the year 1910. Since then there has been 
almost a continuous procession of mutations, affecting eyes, wings, body 
color, bristles, legs, antennae, and physiological properties (Fig. 303). 
Most of these mutants have been bred so that the mode of inheritance of 
their new characters was ascertained, and most of them turned out to be 



recessive to the wild-type characters from which they sprang. Smaller 
numbers of mutations liave been obser\^e(i to occur in other species of 
flies, and in wasps among insects; in mice, rats, rabbits, and guinea pigs 
among mammals; and in corn, barley, peas, and morning-glories among 
plants. So freely have these and other organisms mutated that the bulk 
of evolution may reasonably be assumed to follow from just such changes. 
True mutations may be supplemented by the breakage or duplication of 
chromosomes, but changes of this nature cannot be emphasized in an 
elementary discussion. 

Causes of Mutation. — What causes mutations to occur under natural 
conditions is still unknown. The genes are almost certainlj^ chemical, 
and it is likely that they are fundamentally protein. If these surmises 
are correct, mutations should be chemical modifications and of the sort 

Fig. 303. — Mutations in the vinegar flj' Drosophila melanogaster . A, normal wing; B, 
beaded ■wing; C, notch wing; D, vestigial wing; E, miniature wing; F, club wing; G, rudi- 
mentary wing; H, truncate wing; I, normal red eye; /, bar eye; K, eyeless; L, white eye. 
(C from Morgan; D and L original; the rest from Morgan, Sturtevant, Muller, and Bridges, 
courtesy of Henry Holt and Company, Inc.) 

that proteins are capable of undergoing. A century and a ciuarter ago 
Lamarck (page 17), who was the first naturalist to propound a compre- 
hensive theory of evolution, held that species changed in indirect response 
to the environment, effected thi'ough use and disuse. Lamarck knew 
nothing of the single character changes now called mutations; but, were 
his idea correct, it would mean that mutations are caused by environ- 
mental action. As Lamarck conceived the changes to occur, they con- 
stituted inheritance of acquired characters. For the individuals were 
supposed to be modified by the activity of the animals themselves which 
led to such things as overdevelopment of the muscles or stretching of the 
legs or neck. These changes were induced only in the soma or body at 
first, but he believed that the body was then capable of influencing the 
offspring in like manner. In the light of present knowledge, this influence 
of the body on the offspring would have to take the form of causing 
mutations in the germ cells while still in the body. It seems whollj^ 
unlikely that any such influence can bo exerted. The organization of 


animals appears to offer no possible mechanism whereby an altered body 
can produce in the germ cells within it any modification such that 
offspring developing from them would have the same alteration. More- 
over, though many experimental attempts to produce such changes 
have been made, no satisfactory evidence of their success has ever been 

It seems necessary, then, to exclude somatic influence from the list of 
possible causes of mutation. When mutations began to arise under 
observation in experimental cultures, it was further observed that there 
Avas no apparent difference between the environment of the one mutant 
individual and all the rest. It was long supposed, therefore, that the 
cause of mutation was an unknoAvn something within the animal, possibly 
in some way connected with its physiological processes. In recent times, 
however, it has been found that certain environmental agents are not 
stopped by the body but reach the germ cells directly. They may not 
influence the body in any detectable way yet produce modifications in the 
germ cells. The most potent of these kno\\Ti agents is X rays, and the 
most responsive organism is Drosophila. Hundreds of alterations have 
appeared in the offspring when the parents were exposed to the rays. 
Some of the modifications are visible structural changes; more of them 
have physiological effects. How much natural mutation may be due to 
such radiation is in doubt. Though there is always a certain amount of 
radiation from the earth, it appears much too feeble to account for the 
mutation that has occurred in laboratory cultures. Heat is the principal 
other agent that has been found to produce mutations, and again Droso- 
phila is the subject. While some parts of the earth have as high tempera- 
ture as was employed in these experiments, the temperate zones, where 
most of the thousand mutations of Drosophila have arisen in laboratories, 
are not among them. 

On the whole, while it must now be recognized that external agents 
may produce mutations b}^ direct action on the germ cells, the chief 
agents have not yet been discovered; and the possibility of wholly 
internal agents has not been exhausted. 

Hybridization. — Given a number of genes in which various members of 
a species are different, an important other source of variation is at hand. 
If individuals having different genes are capable of crossing, as they 
nearly always are ^dthin a single species, the genes may be combined in 
different ways. How many recombinations may be produced depends 
only on the contrasting genes. If there are only 20 spots in the chromo- 
somes at each of which, somewhere in the population, two different genes 
exist, it is possible to have over a million different kinds of individuals. 
Most species presumably have more than 20 mutant genes floating about 
in scattered members, and for each additional mutation the number of 


possible combinations is doubled. The importance of this sort of varia- 
bility in evolution can scarcely be overestimated. A species that is 
confronted by a number of environmental situations may easily be in a 
position to take advantage of several of them. Its success would thereby 
be enhanced. 

The variability that is due to combinations of genes in different ways 
is changed in its nature when genes of different pairs interact with one 
another to produce a character not like that produced by either one 
alone. An example is given on page 232 and in Fig. 201, where walnut 
comb is produced in fowls by a combination of the genes for pea and rose 
comb. The genes for brown and scarlet eye in Drosophila produce 
together a nearly white eye. Many such interactions between genes are 
known. It is indeed doubtful whether any gene fails to interact with 
those of other pairs in some way. Such interactions do not increase the 
number of different kinds of individuals which may result from recombi- 
nation of genes, but they do introduce unpredictable qualities into the 
species. This feature may likewise be highly important to a species in a 
variable environment. 

The hybridization referred to alcove h merely that occurring between 
slightly unlike individuals within the species. Whether hybridization 
occurs between two species or not depends partly on whether their 
chromosomes are similar and equally numerous. If the species has the 
same number of chromosomes and if the genes in them are in large 
measure alike, crossing is usually possible. The normal pairing of the 
chromosomes in the preparatory stages of germ cells depends on these two 
things. If the numbers of chromosomes are not equal, odd single chromo- 
somes are left over from this pairing. And if corresponding genes do not 
exist in l)oth species, the chromosomes do not unite readily. Many 
abnormalities result from these situations. The majority of species 
crosses fail to produce offspring, or the offspring are partially or wholly 
sterile. It seems unlikely, therefore, that any considerable part of evolu- 
tion is due to hybridization between species. 

The Direction of Evolution. — Evolution has taken by no means all 
of the courses that were theoretically open to it. Even if life originated 
only once, and even though the million or two species now probably in 
existence is a good round number of end products, this degree of differen- 
tiation is much less than might conceivably have occurred. The actual 
divergence of lines of descent has been considerably curtailed. What the 
other possibilities were that have not been realized, why certain species 
were produced and not others, why certain spcnnes that were produced 
survived and not others are problems to which we must now turn. Their 
solution is largely speculative but impoi'tant. 

The first element entering into the direction of evolution is the charac- 


ter of the mutations with which it starts. Some students of evohition 
have assumed that mutations are of every conceivable sort, just as a 
needle thrown on the floor may eventually, if thrown often enough, 
point in every horizontal direction. This seems an unreasonable assump- 
tion because, if genes are chemical in their nature, they should be no more 
free to enter into unlimited reactions than other substances are. Chem- 
ical substances are restricted to a certain range of reactions by the 
structure of their molecules. Furthermore, in organisms which, like 
Drosophila, have produced the greatest numbers of observed mutations, 
there is not so much variety among the mutations as a purely random 
determination of them should produce. They are too much alike, and 
some of them occur too often, to be the result of chance alone. It is 
more likely that each gene is capable of mutating in certain ways, and 
only in those ways. If this is correct, a species can evolve along any 
line which its possible mutations provide, but along no other. From 
these possible lines something has to choose. 

With respect to the combinations of genes that result from hybrid- 
ization within the species, chance probably plays an important role in 
the early stages of differentiation. When certain genes are present in a 
population in given numbers of individuals, certain combinations of 
genes are expected to occur in calculable proportions of individuals. 
Almost certainly, however, the expected proportion is never exactly 
realized. The accidental meeting and pairing of individuals will usually 
result in some small deviation from the expected result. A gene that 
ought theoretically to occur in 25 per cent of the individuals may easily 
happen to be in 28 per cent or only 22 per cent solely through chance. 
Should the deviation from expectation in the next generation happen to 
be in the same direction, the difference is accentuated. Different parts 
of a range may thus come to be inhabited by groups of individuals which, 
while still belonging to the same species, nevertheless have their genes in 
different proportions. These groups may look essentially alike, especially 
if the genes in which they differ are recessive and exist mostly in heter- 
ozygotes; but their potentialities for the future are distinctly different. 
Such differences tend to be preserved by lack of random mating. No 
individual travels the whole range of its species, so that it mates with one 
of its neighbors. When the genes become numerous enough to produce 
many homozygotes, or if they are or become dominant, the two groups of 
individuals show noticeable differences. 

It is believed that varieties of a species may arise and come to occupy 
different parts of the range, entirely as a result of random wandering and 
the accidental union and fortuitous survival of certain gene combinations. 
Possibly even a divergence great enough to mark two separate species 
may take place in this purely random manner. Beyond this degree of 



differentiation pi-ohably other factors enter. The most important of 
such factors is beheved to be natural selection. 

Charles Darwin and the Natural Selection Idea. — Though Charles 
Darwin is often popularly credited with introducing the evolution doc- 
trine, that is not correct, since, as shown in Chap. 1, the idea of evolution 
was already old in Darwin's time. His real contribution was the theory 
of natural selection. This theory made evolution seem so reasonable 
that opposition to evolution itself from intelligent people quickly fell 
away. From this fact, and from the confusion which exists between 
natural selection and evolution in Darwin's o\^^l writings, has no doubt 

come the popular misconception of 
Darwin's share in promulgating the 
evolution idea. 

The development of the natui-al 
selection concept in Darwin's mind 
is one of the fascinating romances of 
biological science. Darwin had 
come under the spell of the great 
English geologist Sir Charles Lyell 
(Fig. 304), one of whose principal 
teachings was that geological proc- 
esses of the past were essentially 
the same as those in progress now. 
Thi;; doctrine, which has been called 
iiniformitarianism, means specifi- 
cally that erosion, warping of the 
earth's crust, rise and fall of the 
land, volcanic action, etc., had been 
periods of time just as they are 
occurring now. By means of these present-day processes and no others, 
Lyell attempted to explain the development of earth features. Darwin 
was impi'essed with this method and was inclined to apply it to living 
things as well. When, therefore, from 1831 to 1830 he was privileged to 
accompany as naturalist an expedition that was traveling around the 
world on the ship Beagle, he was already in a frame of mintl to reflect 
present occurrences back into the past to see what they might explain. 

It was not until after his return from this voyage, however, that the 
idea of natural selection occurred to him. As he himself says, he got it 
from a book by Malthus, "Essay on Poi)ulation," in which it was pointed 
out that human populations tended to increase rapidly, thus leading to a 
struggle for existence. Darwin quickly saw in this situation a means of 
modifying species of other organisms; for if individuals varied, and if they 
were competing with one another, any advantage possessed by certain 

-Sir Charles Lyell, 1 797-1 S75. 

continually occurring over 




types of individuals would tend to preserve them while less favored ones 
would either suddenly or gradually disappear. If the favorable qualities 
were hereditary, as he apparently assumed they would be, the result 
would be the formation of a new species. 

For 20 years Darwin collected facts that seemed to bear on the 
possible correctness of this natural selection, but he published nothing. 
Only a few friends, including Lyell and the botanist Joseph Hooker, with 
whom he frequentl}' discussed his views, knew what conclusions he was 
reaching. Then a curious coincidence induced him to put a synopsis of 
his work into print. Alfred Russel Wallace, a young naturalist then in 
the Orient, sent to Darwin a sketch of a theory of which he desired 
Darwin's opinion. To the latter's 
surprise, this theory proved to be 
none other than the theory of 
natural selection, or survival of 
the fittest; and, as Wallace after- 
wards related, he too had first 
got the idea from reading the 
work of Malthus, "Essay on 
Population." At first Darwin 
was inclined to withhold his own 
manuscript and allow that of 
Wallace to be published. But 
since Wallace's idea was admit- 
tedly a sudden one, in favor of 
which he had collected no facts 
whatever, whereas Darwin had 
long been gathering data relati^-e 
to it, DarA\an's friends protested. 
It was finally arranged to present 
extracts from both Darwin's and 
AVallace's manuscripts simultaneously to the Linnaean Society of London, 
which was done in 1858. Darwin's theory was developed at length in 
"The Origin of Species" in 1859. The book was written in language 
intelligible to the average reader without biological training. Further- 
more, the time was ripe for such an advance. These facts, coupled with 
championship b}': T. H. Huxley (Fig. 305), who carried the evolution idea 
to the general public in lectures and popular articles, Avon a quick victory 
for the new doctrine. The history of the evolution idea in the last 60 or 
70 years has been the accumulation of new facts in support of it, the 
development of theories to account for it, the grouping of animals on the 
basis of the relationship implied in evolution, and the application of 
corollaries of evolution to other branches of biology. 

Fig. 305. — Thomas Henry Huxley, 1825- 


Operation of Natural Selection. — How natural selection is believed 
to work may be best illustrated from the standpoint of the genes. In any 
species in which a certain gene is becoming either more or less common, 
evolution is occurring. Even if the gene in question is recessive and 
even if it occurs only in heterozygotes so as never to produce a visible 
effect, if this gene is present in a gradually increasing or decreasing num- 
ber of individuals, the species is evolving. Now most genes produced by 
mutation are recessive. They cannot at first affect the visible or phys- 
iological properties of the individuals, for these organisms are hetero- 
zygous. During this early period, pure chance must be responsible for 
the fluctuation of the prevalence of the new gene, as described above 
(page 359). Many a new gene is lost before it can become established. 
Most new genes are thus lost. Evolution would be going on at a tremen- 
dous rate in some species if even a majority of mutations succeeded. Out 
of the large number that occur only a few happen to become numerous 
enough to begin to show their effects in homozygotes. Then they are 
on trial. They may confer some advantage on their possessors, such as 
longer life, more rapid growth, or greater strength. If the advantage 
is one that enables them to leave more descendants, that gene tends to 
become more prevalent. If on the contrary the new gene is harmful, in 
Guch a way that its possessor leaves fewer descendants, it is checked in 
any increase which it might otherwise enjoy. Mere harmfulness cannot 
eradicate a gene altogether if it is recessive, for it may continue to exist in 
heterozygotes beyond the reach of natural selection; but a very harmful 
gene cannot become much more abundant than the level at which hetero- 
zygotes begin to meet and mate, thus producing homozygotes. A less 
harmful gene may become slightly more abundant than this, so that some 
homozygotes appear; but it cannot replace the alternative gene which is 
superior to it. A neutral gene, one conferring neither advantage nor 
disadvantage, is at the mercy of chance. As pointed out before (page 
359), local races within a species may come into existence in different 
parts of a range by this accidental method. 

A considerable degree of variability may thus exist in any species. 
Partly it is observable, as in the distinguishable local races ; but much of i't 
is hidden in heterozygotes. At any particvilar time it is to be expected 
that a species will exhibit approximately that part of its genetic composi- 
tion which is most favorable; and ''favorable" means conducive to large 
numbers of descendants. This most advantageous group of genes would 
be expected to show, because, if they were not expressed, natural selection 
would gradually bring them to expression. If, under these circumstances, 
the environment were to change in some respect, so that certain genes 
increased and their alternates decreased in value, without much question 
the species would change toward the genetic make-up that had acquired 


enhanced usefulness. Such changes would necessarily be slow; hence the 
alteration of the environment would have to be at least semipermanent to 
accomplish any important modification of the species exposed to it. 
Even a hundred thousand generations might well prove too short for 
any important change of a species under the selective action of the 

Among groups of organisms that differ in more than the mere propor- 
tion of certain genes, selection should work more effectively. Of the 
varieties that arise by chance within a species, one or more may well fit the 
environmental situation better than others. Unless these varieties are 
kept in their separate areas by different physiological responses to features 
of the environment, which is probably not often true, the favored variety 
or varieties should gain the ascendancy. They might or might not 
crowd out their fellow varieties ; but even if they were only more abundant 
in individuals, they should have a greater share in determining any later 
evolutionary changes. In like manner, species must differ in their 
capacity to propagate, and the more capable ones should increase in 
numbers. Genera, families, orders, all higher ranks must be subject to 
this action of natural selection, but the action is always on the individuals 
that compose them. 

Adaptation. — The guidance of evolution by natural selection should 
result in a considerable degree of fitness for the environment. If individ- 
uals and species are preserved in proportion to their ability to succeed, 
their success should grow with the passage of generations. The fact that 
natural selection offers a general explanation of adaptation is one of the 
chief reasons for the rapid acceptance of Darwin's theory among biol- 
ogists. For adaptation is very widespread, and some of it is very remark- 
able. So abundant is it, and so marvelous are parts of it, that many 
naturalists have come to feel that adaptation is the outstanding feature 
of life requiring an explanation. 

It would be easy, however, to overemphasize its frequency, its degree, 
and its necessity. Most species are not so well adapted to their situ- 
ation as they conceivably could be, but they get along. Lack of satis- 
factory adaptation in certain species or larger groups seems to be proved 
by extinction. Moreover, obvious adaptation, as among taxonomic 
groups, is found most markedly in the groups of higher rank. Classes, 
orders, families are marked off from one another by such things as wings, 
gills, armor plate, webbed feet, and quills which perform definite functions 
in the lives of the individuals and often help to determine where they shall 
live. Such structures are highly adaptive. In lower ranks, however, 
this adaptiveness is much less common. Most genera of the same family 
do not make any particular use of the characters that distinguish them 
from one another, though there are exception?. Among species of the 


same genus, almost never do the distinguishing characters seem to l)e of 
2k,ny particular vakie to the individuals possessing them. This lack or 
infrequency of adaptiveness of the so-called species and genus characters 
is one of the principal reasons for adopting the view, just described, 
that varieties and species may become separated from one another by 
accidental changes in genetic composition, while natural selection does not 
exert its most powerful influence until some degree of differentiation has 
been attained. 

Pointing out the adaptations of animals has been one of the favorite 
pastimes of naturalists. Books and articles on natural history are full 
of examples, and recitation of the marvelous fitness of organisms to some 
special niche in the environment never fails to excite wonder. The 
several decades following the publication of Darwin's "Origin of Species" 
were marked by inordinate attention to the features of living things that 
enable them to cope with the environment, for to explain the develop- 
ment of any character through natural selection it was only necessary 
to find a use for it. The things most often regarded as making for suc- 
cess were ability to secure food, escape enemies, resist conditions of the 
physical environment, and attract the opposite sex. The supposed uses 
of spots and spines, colors and habits, to attain these ends were exceeded 
in marvelousness only by the ingenuity of the naturalists in devising 
them. In this period, what are probably the things of greatest impor- 
tance, the physiological qualities, were relegated to minor roles. Com- 
paratively little attention was given, for example, to resistance to disease 
and exceptional fertility. Either of these should influence the number 
of descendants more than most of the structural characters whose origins 
were sought. Plasticity, or the capacity of either an individual or a 
species to adjust itself to many types of environment, must be highly 
important but was seldom considered then. These mistakes of the early 
followers of Darwin led to a reaction against the natural selection theory 
over the end of the last century, but the doctrine has emerged again with a 
very different type of support, based on knowledge of mutations, the laws 
of heredity, and the mathematics of chance. 

It should be pointed out that adaptation is a quality of an organism 
as a whole. While in some instances one feature of an organism stands 
out as supremely important, so that other characters all yield to it in 
determining success, in most living things fitness is composed of many 
things. The success of an individual is a product of them all. An animal 
has only one life to lose or preserve. If a frog perishes in the tadpole 
stage because it has not the requisite power to withstand desiccation, it 
cannot be preserved in the adult stage by any special agility in escaping 
from enemies. Likewise, an animal gives rise to only one set of descend- 
ants. If these are few because the animal's life is short, they cannot be 


numerous because it lays eggs rapidly. It is the totality of qualities, 
some favorable, some unfavorable, that determines success, and it is on 
this total product that selection acts. 

Another point requiring emphasis is that, from the evolutionary 
standpoint, a successful species or individual is one that leaves many 
descendants. No quality is of any particular advantage to a species 
unless it entails numerous posterity. Long life may seem to be an advan- 
tage; but if it is merely a prolongation of activity after the reproductive 
period is over, the species gains nothing by it. Rapid growth is a 
good sign physiologically; but if it is expressed only in somatic tissue 
and does not result in more germ cells or more embryos or does not in 
some way enlarge families, it is useless as an element in the security of 
the species. On the whole, also, it is the far distant progeny, rather 
than the near generations, which are most important. A species so 
constituted that in its present environment it succeeds moderately but 
safely, but will in a much later environment thrive exceptionally, is 
more influential upon evolution than is a species which is exceedingly 
abundant now but dwindles in later time. These statements are, of 
course, merely definitions ; it is not possible to apply them and say which 
present species are going to be successful later. 

Isolation. — Many biologists have always believed that an important 
part of the divergence of species from one another is due to some sort of 
isolation. Attention was early called to the supposed effects of geo- 
graphic isolation, as of terrestrial organisms living on an island. The 
species in such an isolated .region are mostly different from those of the 
nearest other land areas. It is easy to see how, with different mutations 
happening to arise among island forms, and probably with a different 
sort of environment acting selectively upon them, there should be a 
gradual divergence of the two groups. Taxonomists, moreover, have 
generally held that the classification implies much more isolation than 
geographic features provide. Since species have presumably split up 
into varieties, which are free to cross with one another as far as they 
meet in the same area, and since by further divergence varieties are 
believed to advance to the rank of species, it might be supposed that 
hybridization between species would continue indefinitely. Now hybridi- 
zation should operate to remove the distinctions between species. How, 
then, have arisen the generally rather sharp lines between species? For 
there are relatively few intermediate individuals that might be regarded 
as species hybrids. 

The nature of the answer to this question is indicated by the discovery 
that most species are not fully fertile with other species, even with those 
most like them. While there are many exceptions, especially in plants, 
they are in a small minority. Some species, exen within the same 


genus, cannot be crossed; that is, they cannot or do not produce hybrid 
offspring. Other species may be crossed, but the hybrid offspring are of 
low fertihty or even completely sterile. Some species, if crossed, produce 
offspring only of the female sex, and these, since they are not partheno- 
genetic, cannot give rise to a new type. 

What causes this sterility between species or in their hybrids is only 
partially known. Difference in numbers of chromosomes is one obvious 
cause, since there can be no complete pairing and meiotic division (page 
195) of the chromosomes in a hybrid unless the chromosomes match. 
Rearrangement of the genes in the chromosomes, such as turning one 
segment of a chromosome end about, has a similar effect ; for in a hybrid 
having two chromosomes alike in genes but differing in their arrangement 
the pairing of the chromosomes is not normal. Many other chromosome 
changes may occur. When an individual has two chromosomes of the 
same sort in each pair, even if both are aberrant, it may behave normally; 
and a group of such individuals may constitute a species. But when 
they attempt to cross with individuals having chromosomes differently 
constituted, abnormalities arise. Species are just as effectively isolated 
by such chromosome changes as they would be if separated by a thousand 
miles of ocean. Indeed, it is probable that separation of species from 
one another is often rendered complete by such chromosome aberrations. 

Evolution of Domesticated Races. — One of the arguments used by 
Darwin in favor of natural selection is the fact that animals and plants 
under control by man have experienced enormous modifications. A very 
few years of selection by man have produced observable changes in 
cultivated plants, and herd records show similar though less striking 
changes in domesticated animals. The method is selection. The l:)reeder 
preserves individuals most nearly approaching his ideals in the belief 
that they will transmit the desirable qualities, and sometimes they do. 
Darwin concluded that all that was necessary to accomplish a similar 
result in wild species would be some selecting agency to replace the 
breeder. That selecting agency could not be endowed with reason or 
foresight, but highly adaptive modifications could, he believed, be pro- 
duced by selective action of the environment itself. The method, as 
conceived now, has already been outlined. 

The written histories of domestic breeds do not go back far enough 
to show the source from which any of our principal types of animals 
came. Very early records show animals already in man's control, but 
not much information about them is given. The sources of the various 
animals have been conjectured from the qualities of breeds today and 
the characteristics of wild species, but nothing is certain. The breeds of 
poultry are believed to be descended from two mid sources, the jungle 
fowl and the Malay fowl, both of the Orient. Egg-laying (juahties 


are thought to have come mostly from the former, while table birds 
have inherited more from the latter. The various breeds of pigs are 
all regarded as descendants of two wild boar species, one from Europe 
and Africa, the other from India. Dogs probably have a somewhat 
greater variety of wild ancestry, since their characteristics indicate con- 
tributions from the timber wolf of Russia, the jackal of Europe, the 
coj^ote of North America, and the dingo of Australia. Sea island cotton 
is probably derived from two wild species, upland cotton from at least 
three. Corn has an obvious relative in wild teosinte, but it is likely that 
other species of grasses are also ancestral to it. 

All this modification of breeds is evolution of a sort. That Darwin 
was justified in concluding from it that selection has been likewise the 
guiding factor in nature, some biologists have doubted. For domestic 
breeds exhibit one important quality which is uncommon in natural 
species; they are generally interfertile. The several kinds of dogs differ 
from one another structurally quite as much as wild species do; but they 
can be crossed, while wild species usually cannot be. It has often been 
argued that if selection were responsible for species formation in nature, 
these species should be as fertile with one another as are domestic varie- 
ties. This criticism overlooks one difference between the selecting agents. 
Man is vitally interested in maintaining interfertility of" his stocks, for 
his method requires that he cross them. If sterility had arisen between 
individuals, because of chromosome aberrations or for any other reason, 
those individuals would have been rejected. In nature, such individuals 
would have survived if lucky and if otherwise fit. By keeping his stocks 
fertile among themselves, and by crossing them frequently, man has 
speeded up the process of change far beyond any rate that might have 
occurred naturally. Man's goals have also been very different from 
those to which natural selection leads. But in no other important respect 
have the two processes been unlike. 

Evolution of Man. — The fossil evidence of man's origin was briefly 
outlined in the preceding chapter. Whether there has been any impor- 
tant evolution in man since he attained the capacities of Cro-Magnon 
man, for example, is uncertain. There is no historical evidence of such 
change. It is often said that man has made no progress in physical or 
mental qualities in the last 10,000 years. This statement may be true, 
but there is no way to know. It would be expected that there had been 
some evolution during that time. Man is extraordinarily heterozygous, 
and there is much hybridization between stocks. Presumably also muta- 
tions arise in man. Unless all individuals survive, and all are equally 
fertile, it is difficult to see how evolution can fail to occur. Whether 
that evolution is progress upward or not is another matter. 

Since man has guided the evolution of his flocks and herds, it would 


seem entirely possible^ that he .shouhl guides liis ()^\'n. The .science of 
eugenics aims at impr()\'ement of the race by such methods. Assuming 
that man can judge correctly which of his qualities are most desirable 
and that he can subordinate his emotions to his reason, there is no appar- 
ent obstacle to progress as far as his present genes and future mutations 
make possible. How great this progress may be it is futile to estimate, 
for no one knows what new qualities may arise through interaction of 
genes already in existence, and certainly no one can guess what genes 
will mutate or how. Predictions regarding man's future evolution are 
accordingly meaningless. 


Darwin, Charles. The Origin of Species. D. Appleton-C'ontury ("ompany, Inc. 

(Chap. XIV, recapituhition.) 
Darwin, Charles. Variation of Animals and Phmts under Domestication. D. 

Appleton-Century Company, Inc. (Introduction, a general outline of argument 

for natural selection.) 
DoBZHANSKY, T. Clcnetics and the Origin of Species. Columbia University Press. 
Ford, E. B. Mendelism and Evolution. Dial Press (Lincoln MacVeagh), Inc. 

(Chap. IV, evolution through the selection of mutations.) 
Haldane, J. B. S. The Causes of Evolution. Harper & Brothers. (Chap. V, the 

nature of adaptation.) 
Lull, R. S. Organic Evolution. 1929 Ed. The Macmillan Company. (Chaps. 

XIX-XXIV, various types of adaptations; book lacks modern viewpoint on 

factors of evolution.) 
Morgan, T. H. The Scientific Basis of Evolution. W. W. Norton <fe Company, 

Inc. (Chap. V, adaptation and natural selection; Chap. VI, nuitation.) 
Newman, H. H. Readings in Evolution, Genetics and Eugenics. University of 

Chicago Press. (Chap. XVII, criticism of natural selection.) 
Scott, W. B. The Theory of Involution. The Macmillan Company. (Chap. I\', 

evidence from paleontology.) 
Shull, a. F. Evolution. McGraw-Hill Book Company, Inc. 


Pronunciations are indicated in tlie glossary as far as possible without the aid of 

diacritical marks, but the following symbols have been necessary: 
H = the German ch; 
N = the French nasal n; 

u = the French u, pronounced by shaping the lips for sounding long oo and the 
tongue for long ee. 

Abiogenesis {ab' i o jen' e sis). The origin of living things from nonliving matter; 
same as spontaneous generation. 

Absorption. The imbiding of a liquid by osmotic or capillar}^ action. 

Acanthocephala (a kan' tho scj' a la). A group of parasitic wormlike animals some- 
times included with the Nemathelminthes. For definition see Chap. 19. 

Acetabulum {as' c tab' u lum). The socket on either side of the pelvic girdle for the 
head of the femur. 

Acetylcholine. A substance produced by nerve endings of the craniosacral sj'stem 
and serving to stimulate certain organs, to inhibit others. 

Acid. A substance which readily gives up hj'drogen ions, H+. 

Actinomorphes (ak' tin o mor' Jeez). A group of animals in Blainville's early classifi- 
cation; animals with radiating parts, such as the starfish. 

Adaptation. Fitness for the environment. In a concrete sense, an adaptive struc- 
ture, habit, or function. 

Adductor. One of the large muscles attached to the valves of a mussel shell, or the 
corresponding muscle of a glochidium ; also, one of nvmierous muscles in other 
animals which draw a structure toward the median axis. 

Adipose. Pertaining to fat. 

Adrenal. One of two or more ductless glands in close relation with the kidneys in 
most vertebrates. 

Adrenalin {ad ren' al in). A hormone produced by the adrenal medulla. 

Adsorption. The adherence of molecules of gases or dissolved substances to the 
surfaces of other bodies. 

Aeolosoma (e' o lo so' ma). A genus of worms, phylum Annelida, subclass Oligo- 

Afferent. Leading toward; said of nerve fibers which conduct impulses toward the 
central nervous system. 

Aganides {ag' a ni' deez). A genus of extinct cephalopods with bent sutures of the 
goniatite form. 

Agkistrodon piscivorus (ag Ms' tro don pis siv' o rus). A species of snake, the cotton- 
mouth moccasin. 

Alanin {al' an in). A very simple amino acid. 

Alecithal (a les' i thai). Containing httle or no yolk; said of certain eggs. 

Altricial (al Irish' al). Hatched in a weak, helpless condition; said of certain birds. 

Alveolar gland {al ve' o lev). A gland in which the lumen is inflated at certain points. 

Alveolus (al ve' o Ivs). One of the microscopic air chambers to which the bronchioles 
lead in lungs. 

Amblycorypha {atji' bli kor' i fa). A genus of katydids. 



Ambystoma {am. his' to ma). A genus of salamanders. A. maculatum, A. tigrinum, 
common species. 

Amino acid {am' i no). One of a number of organic acids containing the NHj radical 
and having certain chemical properties. These acids enter into the composition 
of all proteins and are produced by the splitting of proteins. 

Amitosis {a' mi to' sis). Cell division not involving the formation of chromosomes 
or a spindle. 

Ammonite {am' mo nite). An extinct cephalopod having a coiled shell and com- 
plicated foliaceous sutures; so called from the genus Ammonites. 

Amoeba {a me' ha). A genus of one-celled animals, a protozoon of the class Rhizop- 

Amphiaster {am' fi as' tcr). The figure produced by two asters and the connecting 
spindle in a dividing cell. 

Amphibia. A class of Vertebrata embracing the frogs, toads, salamanders, and some 
others. For definition see Chap. 19. 

Amphicoelous {am' fi see' lus). Having both ends of the centrum concave; said of 

Amphineura {arn' fi nu' ra). A class of Mollusca, the members of which are bilater- 
ally synunetrical, have a shell of eight pieces or no shell at all, and many pairs of 
gills. Chiton is an example. 

Amphioxus. A primitive fishlike animal belonging to the subphylum Cephalochorda 
of the Chordata. 

Amphiuma {am' fi u' ma). A genus of salamanders. 

Amylopsin (am' i lop' sin). A starch-digesting enzyme produced by the pancreas. 

Anabolism. The aggregate of constructive processes comprised in metabolism. 

Analogous {an al' o gus). Similar in function. 

Anaphase {an' a faze). Any stage of cell division during the passage of the chromo- 
somes from the middle to the ends of the spindle. 

Anatomy. The science which treats of the structure of animals and plants as revealed 
by dissection. It more commonly deals with the grosser features, but the finest 
details of strvicture are not excluded. 

Anaximander {an aks' i man der). A Greek physical philosopher and mathematician, 
pupil of Thales, who lived about 611-547 b.c. 

Animal pole. That part of an egg in which the protoplasm is concentrated (in eggs 
with much yolk), and which in most animals produces the nervous system, sense 
organs, etc. Other features may also characterize the animal pole. 

Anisogamete {an' i so gam' eet). One of two unlike cells which fuse in reproduction. 

Annelida {an neV i da). The phylum of animals comprising the segmented worms. 
For definition see Chap. 19. 

Anodonta. A genus of fresh-water mussels. 

Antenna {an ten' na) {pi., antennae). One of a pair of jointed appendages project- 
ing forward from the head of an insect or crustacean. 

Anthophysa (a?i' tho fi' za). A genus of colonial flagellate Protozoa whose cells are 
borne in radiating masses on a branching stalk. 

Anthothrips niger {an' Iho Ihrips ni' jer). A species of insect of the order Thysan- 
optera, commonly called thrips. 

Anthozoa {an' tho zo' a). A class of X'oelenterata, comprising the sea anemones and 
most of the corals. They have no mcdusoid form in the life cycle. 

Anus {a' nus). The posterior oix-ning of the digestive tract. 

Apoda {ap' o da). An order of Amphibia comprising the legless forms called 


Appendicular skeleton. The bones of the Hmbs and their attaching girdles in 

Arachnida (a rak' ni da). A class of Arthropoda comprising the spiders, scorpions, 

and mites. For definition see Chap. 19. 
Archaeozoic {ar' he o zo' ik). Of the earliest geological era; the oldest known rocks 

are of this era. 
Archenteron {ark en' ter on). The cavity within the endoderm of a gastrula. It 

communicates with the exterior. 
Archiannelida {ar' ki an nel' i da). A class of primitive marine worms (Annelida) 

without setae. 
Aristotle {ar' is tot' I). The most famous of the Greek naturalist philosophers, who 

lived 384-322 b.c. 
Armadillo. An armored mammal of the order Edentata, which includes also the 

sloths and anteaters. 
Arteriole. One of the smaller branches of an artery, leading to capillaries. 
Artery. A blood vessel conducting blood from the heart. 
Arthropoda {ar throp' o da). A phylum of animals, including the insects, Crustacea, 

centipedes, etc. For definition see Chap. 19. 
Articulate. To join; said of bones. 

Artiomorphes {ar' ti o mor' feez). A group of animals in Blainville's early classifica- 
tion; it comprised the animals whose bodies are bilaterally symmetrical. 
Ascaris (as' ka ris). A genvis of roundworms (Nemathelminthes) parasitic in various 

animals. A. megalocephala {meg' a lo scf a la), parasitic in the intestine of the 

Ascorbic acid. Vitamin C, the preventive of scurvy. 
Asexual. Not involving germ cells or fusion of nuclei; said of reproduction, or of an 

individual employing such a mode of reproduction. 
Assimilation. The conversion of digested foods and other raw materials into proto- 
plasmic substances. 
Association neuron. A nerve cell within the central nervous system, which helps to 

connect an afferent with an efferent neuron. 
Aster. The starlike figure composed of a centriole and the radiating lines about it; 

or the centriole may be lacking. 
Asteroidea {as' ie roi' de a). A class of Echinodermata comprising the starfishes. 

For definition see Chap. 19. 
Astral rays. The radiating lines surrounding a centriole in a dividing cell. 
Asymmetry. Absence of any kind of symmetry. 
Atoll {at' ol, or a toV). A ring- or horseshoe-shaped coral island. 
Atom. A unit of a chemical element, composed of one or more protons and electrons, 

arid usually neutrons. 
Auditory. Pertaining to hearing; applied to the nerve of hearing and the sensory part 

of the inner ear. 
Auricle. The anterior chamber of the heart in fishes, and one of the two anterior 

chambers in higher vertebrates. 
Autonomic nervous system. A system of ganglia and nerve fibers, comprising two 

mutually antagonistic groups, which center in specific parts of the central nervous 

system and regulate the involuntary responses of the heart, blood vessels, diges- 
tive tract, glands, and pupil of the eye. 
Autosome {aw' to some). Any chromosome not closely associated with sex, that is, 

not an X or Y chromosome. 
Aves {a'veez). A class of vertebrate animals comprising the birds. 


Avoiding reaction. The behavior by which Paramecium avoids obstacles of various 
kinds. It consists of stopping, moving backward, turning through an angle away 
from the oral groove, and starting forward in a new direction. 

Axial skeleton. The skull, vertebral column, ribs, sternum, and hyoid apparatus of 

Axolotl {aks' lot'I). The larval form of the tiger salamander Ambysioma tigrinum 
which reproduces while in the larval state. 

Axon {aks' one). A projection from a nerve cell which ordinarily conducts impulses 
away from the body of the cell. 

Backcross. A cross between an Fi individual and one of its parent types. 
Balanoglossus. A genus of wormlike animals doubtfully included in the phylum 

Bascanion. A genus of snakes, including the black snake or blue racer. 
Base. A substance giving rise to free hydroxyl ions, 0H~, and thereby accepting 

hydrogen ions, H"*". 
B complex. A group of related vitamins found in meats, seed coats of cereals, yeast, 

etc., including thiamin, riboflavin, niacin, and pyridoxin. 
Biconcave. Having the centrum hollow both in front and behind; said of vertebrae. 
Bidder's canal. A longitudinal tube near the median border of the kidney of certain 

Amphibia; into it the collecting tubules open. 
Bilateral symmetry. An arrangement of the parts of an object or animal body such 

that the halves on opposite sides of a certain plane (only one in number) are 

mirrored images of each other. 
Bile. The fluid secreted by the liver in vertebrates. 

Bile duct. The tube through which bile is di.scharged into the intestine. 
Binomial. Consisting of two names or terms. Applied to the system of nomencla- 
ture by which each species is given two names, one for the genus, the other for 

the species. 
Biogenetic law. The doctrine that animals in their embryonic development repeat 

the evolutionary history of the race. 
Biology. The science of life and of living things, whether plants or animals. 
Bladder. A membranous sac in which urine is stored. 

Blainville, Henri Marie Ducrotay de (5/aN veel'). French naturalist, 1777-1850. 
Blastocoele (bias' to seel). The hollow interior of a blastula. 
Blastopore. The opening through which the archenteron of an early embryo (gas- 

trula) communicates with the exterior. 
Blastostyle. In hydroids, a nontentaculate individual which produces medusae. 
Blastula {bias' tu La). An early developmental stage, consisting of a hollow ball of 

Blood platelet. One of the formed components of the blood, produced by fragmenta- 
tion of certain cells. 
Book gill. See book lung. 
Book lung. A respiratory organ composed of flat sheets joined together like pages of 

a book, found in spiders. 
Bougainvillea ramosa {boo' gin viV le a). A species of marine hydroid. 
Bowman's capsule. The expanded end of a kidney tubule, in which a glomerulus is 

Brachiopoda {brak' i op' o da). A group of marine animals of uncertain rank or 

relationship. They have a bivalve shell, the two halves of which are unequal. 

Sometimes placed in a phylum with the Bryozoa and Phoronidea. 
Bract. One of the covering (protective?) members of a siphonophore colony. 


Bradypus {brad' i pus). A genus of sloths. 

Bronchiole (brong' ki ole). One of the smaller branches of the bronchi, air tubes in 
the lungs. 

Bronchus (brotig' kus) (pi., brouchij. One of the two main branches of the trachea 
in many vertebrates. 

Brown, Robert, British botanist, 1773-1858. 

Bryozoa {bri' o zo' a). A group of marine and fresh-water animals of uncertain rank 
and relationships, mostly colonial, bearing tentacles, and commonly known 
as moss animals. Sometimes placed in a phylum with the Phoronidea and 

Buccal cavity {bitk' k'l). The most anterior division of the digestive tract of an earth- 
worm. Also the mouth cavity of other animals. 

Budding. Division of an organism into unequal parts in reproduction. 

Buflfon, Comte de {bufo^'). French naturalist, 1707-1788. 

Bufo. A genus of toads. 

Byssus. A thread attached near the adductor muscle of a glochidium ; or a bunch of 
threads attached to the foot of certain clams. 

Caecilian. One of a group of legless, wormlike Amphibia of the order Apoda. 

Caecum (see' kum). The blind pouch at the beginning of the large intestine. 

Calcarea. A class of sponges (Porifera) whose skeletons are composed of spicules 
of calcium carbonate. 

Calciferol [kal sif er ole). Vitamin D, the preventive of rickets. 

Calorie (kaV o ri). The quantity of heat required to raise the temperature of a kilo- 
gram of water 1°C.; this is a large calorie, equal to 1000 small calories. 

Cambrian (kam' bri an). Of the earliest Paleozoic time. 

Camponotus. A genus of ants. 

Canaliculus (kan' a lik' u lus). One of numerous minute channels radiating from 
each lacuna in the matrix of bone, in which slender processes of the bone cells 
are located. 

Cancellate {kan^ set late). Composed of a number of chambers separated by parti- 
tions; said of spongy bone. 

Canine (ka' nine). A tooth located laterally to the incisors. 

Capillary. One of numerous small vessels conveying blood through the tissues from 
arteries to veins or from one vein to another. 

Carapace. The hard bony covering of a turtle; also the chitinous or calcareous cover- 
ing of the cephalothorax of a crayfish or lobster. 

Carbohydrate. Any one of a class of organic substances, embracing the starches, 
sugars, cellulose, etc., which are composed of carbon, hydrogen, and oxygen, 
with the number of atoms of hydrogen and oxygen regularly in the ratio 
of 2:1. 

Carboniferous. The geological age to which the principal known coal beds belong; 
succeeding the Devonian, it is a combination of Mississippian and Pennsylvanian. 

Carchesium (kar ke' zi %im). A genus of colonial ciliated Protozoa, resembling 

Cardiac. Pertaining to or near the heart. 

Carnivore. Technically, a mammal of the order Carnivora, including the cats, dogs, 
and seals. In a popular sense, any flesh-eating animal. 

Carnivorous. Flesh-eating. 

Carotene. A yellow pigment found in carrots and many green or yellow vegetables; 
a source of vitamin A. 

Carpal. One of a number of bones in the wrist in vertebrates. 


Carpometacarpus (kar' po met a kar' pus). A compound bone in the wing of a bird, 

formed by the union of several of the metacarpals and carpals. 
Cartilage. A flexible, somewhat translucent tissue composed of cells imbedded in a 

matrix, found on the ends of bones at joints and in other situations. 
Cast. A mass of rock formed within a cavity, as the cavity of a shell or of a mold 

formerly occupied by an animal. 
Catabolism {ka tab' o liz'm). The aggregate of destructive processes comprised in 

Catalase. An enzyme which liberates oxygen from hydrogen peroxide. 
Catalyst {kat' a list). A substance which brings about a reaction but is not consumed 

in that reaction. It probably often participates in the reaction but is promptly 

Caudal. Belonging to the tail. 

Caudata. Aii order of Amphibia comprising forms with tails (salamanders, newts). 
Cell. A mass of protoplasm containing a nucleus or nuclear material. 
Cell doctrine. See cell theory. 

Cell inclusions. Nonliving objects enclosed in cells. 

Cell membrane. A thin sheet either of differentiated protoplasm, or of some sub- 
stance produced by protoplasm, surrounding a cell. 
Cell theory. The theory that all animals and plants are composed of similar units 

of structure called cells. The theory is now so well established as to be inore 

properly called the cell doctrine, and other features concerning physiology, 

development, etc., may be included in it. 
Cellulose (sel' u lose). The substance, one of the carbohydrates, of which the cell 

walls of plants are commonly composed. 
Cell wall. A nonliving structure secreted by a cell around itself. It is commonly 

composed of cellulose or chitin. 
Cement. A binding material in the composition of teeth. 
Cenozoic (se' no zo' ik). Pertaining to the most recent geological era. 
Central nervous system. The brain and spinal cord. 
Centriole. A minute body in the center of a centrosphere, and located at the end of 

the spindle of many dividing cells. 
Centrolecithal {sen' tro les' i thai). Having the yolk in a central position, surrounded 

by protoplasm at the surface; said of eggs. 
Centrosome {sen' tro some). A minute body often present in a cell, usually near the 

nucleus in a centrosphere, related in some way to the process of cell division. 

By many writers the name is used interchangeably with centriole. 
Centrosphere {sen' tro sfeer). A differentiated portion of the cytosome of a cell, 

usually near the nucleus, and typically containing a centrosome or centriole. 
Centrum. The massive portion of a vertebra ventral to the neural canal in which 

the spinal cord rests. 
Cephalochorda {sef a lo kor' da). A subphylum of Chordata, comprising the species 

of Amphioxus. For definition see Chap. 19. 
Cephalopod {sef a lo pod). One of the group Cephalopoda, to which the cuttlefishes, 

squids, and nautili belong. 
Cephalopoda {scf a lop' o da). A class of Mollusca, comprising the octopi, squids, 

cuttlefishes, and nautili, animals in which the foot is developed into a headlike 

structiu-e with eyes and a circle of arms. 
Cephalothorax {scf a lo tho' raks). A fused head and thorax, found in crayfishes and 

their allies. 
Ceratite {ser' a tite). An extinct cephalopod having a coiled shell and crooked 

sutures; named from the genus Ceratites. 


Ceratites (scr' a ti' teez). A genus of extinct cephalopods with crooked sutures; the 

common name ceratite is derived from this genus. 
Ceratium candelabrum (se ra' shi um can' de la' brum). A species of protozoon which 

forms linear colonies. 
Cerebellum. A division of the brain of vertebrates developed on the dorsal side 

anterior to the medulla. 
Cerebrum, The anterior division of the brain in vertebrates. In man it forms the 

greater part of the brain but is smaller in other vertebrates. 
Cervical. Pertaining to the neck. 
Cestoda. A class of Platyhelminthes, comprising the tapeworms. For definition see 

Chap. 19. 
Chaetogaster (ke' to gas' ter). A genus of worms, phylum Annelida, subclass Oligo- 

Chaetognatha {he tog' na tha). A group of marine animals of uncertain kinship, repre- 
sented chiefly by the arrowworm Sagitta. 
Chaetopoda {ke top' o da). A class of worms (Annelida) provided with setae, to which 

the earthworm and sandworm belong. 
Cheloniidae {kel' o ni' i dee). A family of turtles. 
Chelydidae {ke lid' i dee). A family of turtles. 
Chelydridae {ke lid' ri dee). A family of turtles. 
Chitin {ki' tin). A horny substance forming the outside skeleton of insects and many 

other animal parts. 
Chiton [ki' ton). A genus of primitive mollusks, having a shell of several pieces. 
Chloragogen cells {klo' ra go' jen). The cells of the outer layer of the intestine of the 

Chlorophyll. The green substance in chloroplasts through whose agency photosyn- 
thesis occurs. 
Chloroplast. A green plastid. 
Cholesterol {ko les' ter ol). A substance, one of the solid alcohols, found in many 

animal tissues. 
Chordata {kor da' ta). A phylum of animals including the vertebrates and a few 

others. For definition see Chap. 19. 
Chromatin {kro' ma tin). The deeply staining substance of the nucleus of a cell. 
Chromoplast. One of several kinds of colored structures or organs found in many 

plant and some animal cells. 
Chromosome. One of the rodlike or rounded bodies into which the chromatin of a 

nucleus is resolved at the time of cell division. 
Chrysemys {kris' e juis). A genus of turtles. 
Ciliate. A class of the protozoa, in which both young and adult stages are provided 

with cilia. 
Ciliophora {siV i of o ra). A subphylum of protozoa, members of which are covered 

with a pellicle, have a fixed mouth, and are usually covered with cilia; example, 

Cilium. A minute hairhke motile structure occurring on the surface of certain cells. 
Circular canal. A channel passing around a medusa near its margin. 
Circulation. The movement of the blood through a system of vessels. 
Circumpharyngeal connectives {ser' kum fa rin' je al) . Nerve cords in the earth- 
worm connecting the brain with the ventral nerve cord; so called because they 

pass around the anterior end of the pharynx. 
Citellus tridecimlineatus {si tel' lus tri des' im lin' e a' tus). A species of ground 

Class. A subdivision of a phylum ; a group of higher rank than the order. 


Clavicle. The collar bone in man. One of the bones of the ventral part of tlie 

pectoral girdle in vertebrates in general. 
Cleavage. The division or segmentation of an egg. 

Clitellum. A thickened glandular band encircling the body of an earthworm. 
Cloaca {klo a' ka). A common passageway through which the intestine, kidneys, 

and sexual organs discharge their products in some fishes, in amphibia, reptiles, 

and birds, and in a few mammals. 
Cnidoblast (?//' do blast). A cell containing a nematocyst or stinging thread in Hydra 

or other Coelenterata. 
Coagulation. Hardening; specifically, the clotting of the blood. 
Cocoon. A case in which eggs are stored and in which frequently the larvae are 

developed; also a silky covering around the pupa. 
Codosiga iko' do si' go). A genus of flagellate Protozoa having a collar around the 

Coelenterata {se Icn' ter a' ta). The phylum to which Hydra, the hydroids, jelly- 
fishes, and siphonophores belong. For definition see Chap. 19. 
Coelenteron (se len' ter on). A cavity in forms like Hydra which have only one body 

cavity. It serves the digestive and circulatory functions and is therefore also 

called the gastrovascular cavity. It has only one opening. 
Coelom (see' lome). The true body cavity, a cavity within the mesoderm on the w^alls 

of which the principal reproductive organs are located. 
Coenosarc {se' no sark). The celhilar living part of a hydroid, as distinguished from 

the j)erisarc. 
Collared epithelium. Epithelivim each of whose cells bears a collar. 
Collecting tubule. One of a number of tubes leading across the kidney of the frog 

from Bidder's canal to the ureter. 
Colloid (koV lord). A mixture in which particles invisible through a microscope but 

greater in size than molecules are held in suspension in a liquid. 
Colloidal (kol loi' dal). Contained in a liquid in aggregations submicroscopic in size 

but greater than molecules. 
Colony. A group of individuals of the same species organicall}- connected with each 

Coluber. A genus of snakes. 
Columnar epithelium. Epithelium in which the cells have one dimension distinctly 

greater than the others, that dimension being vertical to the surface covered by 

the epithelium. 
Comanchean {ko man' che an). Pertaining to Mesozoic time prior to the Cretaceous; 

formerly called lower Cretaceous. 
Common bile duct. The tube leading from the liver to the small intestine and serv- 
ing to convey bile to the small intestine. 
Compound. A substance produced by two or more elements in combination. 
Compound gland. A branching gland. 
Conemaugh iko' ne maw). A rock formation of eastern United States, belonging to 

Permocarboniferous time. 
Coniferous. Cone-bearing (as pine or cypress trees). 
Conjugation. The meeting of two cells for exchaiige of nuclear material or (by exten- 

.-^ioii of meaning) for complete fusion. 
Connective tissue. .\ tissue composed of cells and ('(MtMiii other material protluccd 

by the cells, which in its simple foiin binds organs and other tissues togeth(M'. in 

a broader sense it includes such modified tissues as cartilage, bone, tendon, and 

Contractile tissue. .\ny tissue cajjable of acti\(' contraction: as muscle. 


Contractile vacuole. A vacuole whose contents ;ir(' pon'odicallj- ejected to the outside 

of the cell in which it is contained. 
Copepod (ko' pe pod). Any one of a group of small Crustacea. 
Copulation. The act of introducing spermatozoa into the body of the female. 
Coracoid {kor' a koid). A bone of the ventral part of the pectoral girdle of vertebrate 

animals; a distinct bone in the bony fishes, amphibia, reptiles, birds, and lowest 

mammals, but fused with the scapula in t'he higher mammals. 
Cornea. The transparent bulging membrane at the front of the eye. 
Corpus luteum [pi., corpora lutea). A mass of cells invading the space in an ovary 

from which an ovum has escaped. 
Corpuscle. One of the cells of the blood. 
Cortex. The layer of gray matter which covers the cerebrum and dips into its 

folds. Also, an outer layer on various other organs, as the kidney or adrenal 

Cranial nerve. One of 10 or 12 pairs of nerves arising from the central nervous system 

within the skull. 
Craniosacral system. That part of the autonomic nervous system which centers in 

the brain and posterior portion of the spinal cord. Each organ controlled by the 

autonomic system is innervated once from it. 
Cretaceous. Pertaining to the late Mesozoic time ; so named from the chalk deposits 

characteristic of it. 
Cretinism. A developmental deficiency caused by inadequacy of the hormone 

Crinoidea {kri noi' de a). A class of Echinodermata, including the feather stars and 

sea lilies. For definition see Chap. 19. 
Crocodilini (krok' o di W ni). An order of Reptilia comprising the alligators and 

crocodiles and their allies. 
Cro-Magnon {kro man yon'). A rather highly developed race of men preceding the 

principal races of today. It dwelt, as far as known, in Western Europe. 
Crop. In the earthworm, an enlargement of the digestive tract behind the esophagus 

and in front of the gizzard. In birds, an enlargement of the esophagus for the 

temporary storage of food. 
Crustacea. A class of arthropods including the lobsters, crabs, water fleas, barnacles, 

etc. For definition see Chap. 19. 
Crystalline lens. A rounded, transparent, refractive body situated behind the pupil 

of the eye. 
Ctenophora {te nof o ra). A group of animals of uncertain rank including the comb 

jellies and sea walnuts. For definition see Chap. 19. 
Cubical epithelium. Epithelium in which the height and width of the cells are about 

Cuvier, Georges {ku vyay'). French naturalist, founder of comparative anatomy, 

Cyclostomata {si' klo sto' ma ta). A class of Vertebrata having an eellike form, a 

cartilaginous skeleton, no jaws, and no lateral fins; lampreys and hagfishes. 
Cytology. The science which deals with the structure of cells. 
Cytoplasm. The protoplasm of a cell exclusive of the nucleus. 
Cytosome. The body of a cell as distinguished from its nucleus. 

Darwin, Charles. Celebrated English naturalist, founder of the doctrine of natural 
selection, author of several works on evolution. Lived 1809-1882. 

Deciduous. Falling off at maturity or at the end of a season ; said of the leaves of trees 
which fall periodically. Applied also to trees whose leaves fall periodically. 


Deficiency disease. Any disease resulting from the lack or scarcity of some specific 

substance in the diet. 
Democritus {de mok' ri tus). Greek philosopher, known for his atomic theory, who 

lived about 460-357 b.c. 
Demospongiae {de' mo spun' ji ee). A class of Porifera (sponges). For definition see 

Chap. 19. 
Dendrite. A projection from a nerve cell which ordinarily conducts impulses toward 

the body of the cell. 
Dendritic. Treehke. 
Denticulate. Finely notched or toothed. 

Dentine. The dense bony substance composing the bulk of mammalian teeth. 
Dermatozoa {der' ma to zo' a). A group of animals (literally, the skin or touch 

animals) in Oken's early classification. It comprised the invertebrates. 
Dero. A genus of worms, phylum Annelida, subclass Oligochaeta. 
Determinate. Leading infallibly to a given end result from a given beginning; said 

of development in which each cleavage cell produces a certain structure and 

nothing else, regardless of experimental interference. 
Devonian (de vo' ni an). Of middle Paleozoic age, next following the Silurian. 
Dextrin. Any one of several related carbohydrates derived by hydrolysis from starch, 

among them being erythodextrin, achroodextrin, and maltodextrin. 
Diaphragm {di' afram). A partition; specifically, the partition between the thorax 

and abdomen of a mammal. 
Diffusion. The spreading of the molecules of one substance among those of another. 
Digestion. The conversion of food into soluble substances which may diffuse through 

Dinosaur {di' no sawr). One of an order of extinct reptiles of Mesozoic time, mostly 

of large size. 
Dinotherium {di' no the' ri um). An extinct elephantlike animal from the Miocene. 
Dioecious {di ee' shus). Having the male and female organs in separate individuals; 

said of species. 
Diogenes {di oj' e neez). Greek natural philosopher of the fifth century before Christ, 

born at ApoUonia. 
Diploblastic. Composed of two layers of cells. 
Diploid {dip' laid). Double; specifically, the double number of chromosomes found 

in the somatic cells, and in germ cells before meiosis, in bisexual animals. Cf. 

Dipnoi {dip' no i). A subclass of Pisces, fishes with an air bladder functioning as a 

lung; the lungfishes. 
Disaccharide {di sak' a ride). A carbohydrate whose molecule can be split into two 

molecules of simple sugar (monosaccharide). 
Dominant. Receiving expression when only one determining gene is present, and 

in the presence of the gene for a contrasted recessive character; said of inherited 

characters that are exhibited by heterozygotes. 
Dorsal. Pertaining to the back; hence, usually, upper. 
Dorsal aorta. A large artery formed, in fishes, by the union of vessels coming from 

the gills, and passing backward in the dorsal region. 
Dorsal root. The dorsal one of two roots by which a spinal nerve is connected with 

the spinal cord. Its fibers are sensory in function. 
Drosophila {dro sof i la). A genus of flies, of which the vinegar fly (D. melanogaster, 

met' a no gas' ter) is a common species. 
Duodenum {du' o de' num). The first of three divisions of the small intestine. 
Dutrochet, Rene Joachim Henri {dii' tro' shay'). French physiologist, 1776-1847. 


Dyad. A double body formed by the division of a tetrad into two parts. Its two 
parts may be derived from the same chromosome or from different chromosomes. 

Echinoderm (e ki' no derm). One of the Echinodermata. 

Echinodermata (e ki' no der' ma to). The phylum of animals including the starfishes, 

sea urchins, sea cucumbers, brittle stars, etc. For definition see Chap. 19. 
Echinoidea (ek' i noi' de a). A class of Echinodermata, comprising the sea urchins, 

sand dollars, and heart urchins. For definition see Chap. 19. 
Echinorhynchus (e ki' no ring' kus). An Acanthocephalan worm. 
Ecology {e koV oji). The branch of biology dealing with the relation of animals or 

plants to their environment. 
Ectoderm. The outer layer of cells of a gastrula, or the representative of this layer 

in later stages. 
Ectosarc. The outer layer of protoplasm in cells in which the outer and inner proto- 
plasm differ distinctly in structure, as in Amoeba. 
Edaphosaurus (e daf o saw' rus). An extinct lizardlike reptile bearing a spiny fin 

on its back, from Permocarboniferous rocks of North America. 
Effector. A structure specialized for some specific response; also the nerve carrying 

impulses to such a structure. 
Efferent. Leading from; said of nerve fibers which conduct impulses away from the 

central nervous system. 
Elasmobranchii (e laz' mo brang' ki i). A class of Vertebrata comprising the sharks, 

skates, rays, torpedoes, and chimaeras. For definition see Chap. 19. 
Electrolysis (e lek troV i sis). Decomposition of an ionized substance in solution by 

passing an electric current through the solution. 
Electrolyte. A substance which, because it ionizes, is in solution capable of conduct- 
ing an electric current and of being decomposed by the current. 
Electron. A unit of negative electric charge entering into the composition of atoms. 
Element. One of the approximately 90 primary forms in which matter exists. 
Elephas {eV e fas). A genus of animals including living elephants and their fossil 

relatives of Pleistocene time. 
Elodea {eV o de' a). A genus of aquatic plants. 
Embryo. An undeveloped animal while still in the egg membrane or in the maternal 

Embryology. The science which deals with the development of the embryo, or 

young stages, of animals or plants. 
Embryonic. Pertaining to an embryo. 
Empedocles {em ped' o kleez). Greek philosopher and poet, born in Sicily. Lived 

about 490-430 B.C. 
Emulsion. A mixture of two liquids or semiliquid substances, neither one soluble 

in the other, the one being in the form of separate droplets suspended in the 

Emulsoid. A mixture consisting of a liquid in which are distributed particles of a 

liquid or semisolid substance which are exceedingly minute yet larger than 

Emys (e' mis). A genus of turtles of the family Testudinidae. 
Enamel. The very hard, polished calcareous substance forming the surface layer or 

internal plates in the teeth of mammals. 
Endocrine secretion. A secretion which must leave the gland by diffusion, not 

through a duct. 
Endoderm. The inner layer of cells of a gastrula, or the representative of this layer 

in later stages. 


Endosarc. The inner mass of protoplasm in cells in which the outer and inner proto- 
plasm differ in structure. 
Endoskeleton. A skeleton within the fleshy parts, as in vertebrate animals. 
Energy. The capacity for performing work. It is kinetic when employed in pro- 

dvicing motion or heat, potential when stored in chemical combination. 
Enterokinase {en' ter o ki' nase). An enzyme produced in the small intestine and 

serving to convert trypsinogen into trypsin. 
Enteron. A digestive system open at both ends. 
Enteropneusta {en' te rop nu' sta). A subphylum of Chordata, wormlike animals, 

of which Balanoglossus and Cephalodiscus are representatives. 
Entomology. The zoology of insects. 
Enzyme {en' zime). An organic substance which brings about a chemical reaction 

but is not consumed bj- that reaction. Probably it participates in the reaction 

but is promptly restored. 
Eocene (e' o seen). Of the earliest Cenozoic and Tertiary time. 
Eohippus {e' o hip' pus). The earliest known ancestor of the horse, an extinct animal 

of Eocene time. 
Epidermis. The outer of the two principal layers of the skin. Also an outer layer of 

cells in general. 
Epistylis (rp' i sti' lis). A genus of colonial ciliated Protozoa, resembling Vorticella. 

E. flavicans {flav' i kanz), one of the species. 
Epithelial. Pertaining to an epithelium; as epithelial tissues or structures. 
Epithelium. A layer of cells at the surface of a tissue or organ, or lining a cavity. 
Epoch. One of the divisions of a period in the geological time scale. 
Equation division. A division in which chromosomes are duplicated, producing two 

equal cells ; said of one of the divisions of germ cells as contrasted with the other 

or meiotic division. 
Equatorial. In the plane of a great circle halfway between the poles; said of a cleavage 

plane of an egg. Also, in a middle position in other objects. 
Equatorial plate. The flattened group of chromosomes on the middle of the spindle 

of a dividing cell. Also, the plane which they approximately occupy. 
Equus {e' kwus). A genus of animals including the living horse and some of its fossil 

relatives of Pliocene and Pleistocene time. 
Era. One of the five major divisions of geological time. 
Erepsin (e rep' sin). A proteolytic enzyme produced in the small intestine. 
Ergosterol {er goa' ter ol). A substance, chemically a solid alcohol, obtained from 

ergot, a fungus. On irradiation with ultraviolet it possesses strong antirachitic 

Erosion. The wearing away (of rocks) through the action of water and other agencies. 
Esophagus (e sof a gr(/.s). In the earthworm, a narrow passage leading from the 

pharynx to the crop. In vertebrates, the passage between the pharj-nx and the 

Estrogen. A hormone or grouj) of liornioiics produced by the follicles of tlie human 

ovary; several other names have been applied to it. 
Euarctos (// ark' lose). A genus of bears, including the western black l)ear. 
Eudorina elegans {u' do ri' nn). A species of colonial chlorophyll-bearing organism 

whose cells are imbedded in a spherical jell^ylike mass. 
Euglena ('/ gle' na). A genus of green flagellate Protozoa. 
Eustachian tube {u sta' ki an). A passage between the pharynx and the tympanum 

or middle ear. 
Eutheria {n the' ri a). A subclass of Mammalia comprising tlic A-iviparous mammals. 
Eutrephoceras in' t re fox' er ns). .\ genius of extinct ceiilialo])ods rcsemhiing Nautilus. 


Evagination. The folding of a layer of cells outward from an enclosed cavity. 
Evolution. The gradual or sudden change of animals or plants through successive 

Evolve. To change; to undergo evolution. 

Excretion. The elimination of waste substances. Also a substance excreted. 
Exhalent. Breathing out; applied to one of the siphons of a clam or mussel. 
Exophthalmic goiter {cks' of thai' mik). A disease resulting from overactivity of the 

thyroid gland, and having as one of its symptoms the bulging of the ej-es. 
Exoskeleton. A skeleton on the outside of the body, as in the arthropods. 
Extensor. A muscle whose contraction straightens a joint. 
External respiration. The passage of oxygen from the surrounding air or water to 

the blood. 

Fi (ef unin'). An individual or generation of individuals resulting from the crossing 

of two unlike parents. An abbreviation of the words first filial. 
Fi {ef too'). An individual or generation of individuals resulting from the mating of 

two Fi individuals as parents. An abbreviation of the words second filial. 
Family. A taxonomic group of higher rank than the genus but below the order. 
Fat. A compound of glycerol and one or more fatty acids. 
Fatty acid. An organic acid entering into the composition of fats. 
Fauna. Collectively, the animals of a given region or of a given period of time. 
Femur. The single bone of the thigh in vertebrates above the fishes. 
Feral. Plscaped from domestication. Also, sometimes, wild. 
Fertilization. The union of an egg with a spermatozoon, a process requisite, in the 

higher animals, to the development of the egg. 
Fetus (/(' tus). The embryo of a mammal while still in the uterus. 
Fibril. One of the longitudinal contractile threads of a voluntary muscle cell. 
Fibrin. A substance in blood which forms much of the clot on escape from the 

Fibrinogen (fi. brin' ojen). A soluble protein carried in blood plasma, from which 

the insoluble fibrin of a clot is formed. 
Fibula (fib' u la). The outer one of two bones in the lower leg of vertebrates except 

the fishes. 
Fission. The division of an organism into two approximately equal parts ; or, simply, 

Flagellate iflaj' el late). Possessing flagella. As a noun, a flagellate protozoon. 
Flagellum [pi., flagella). A long whiplike motile projection from a cell. 
Flame cell. A cell having a hollow interior in which a bunch of vibratile cilia are 

located, forming part of a protonephridium. 
Flexor. A muscle whose contraction bends a joint. 
Fluke. Any one of several species of trematode worms. 
Follicle. A layer of cells surrounding some object, as an ovum in an ovary. 
Foot. The basal muscular part of a clam or snail, variously modified in many other 

mollusks. Also the terminal part of a leg, the base of Hydra, etc. 
Foraminal aperture (/o ram' i nal). In a sponge gemmule, the opening in the shell 

through which the young sponge escapes when it begins to develop. 
Formation. The rocks belonging to one of the minor divisions (lower than epoch) 

of geological time. 
Fossil. The remains, or other indication, of a prehistoric animal or plant. 
Fructose. A simple sugar (monosaccharide) found in fruit juices, and one of the 

products (with glucose) obtained by breaking down sucrose (cane sugar); same 

as levulose. 


Funiculus {fu nik' u lus). A muscular strand which draws the body of a br^yozoan 

into a U shape. 
Furcula (fur' ku la). The wishbone of a bird, consisting of the fused clavicles of the 

two sides. 

Galactose (ga lak' lose). A simple sugar (monosaccharide) obtainable by breaking 
down lactose, or lipids of the brain. 

Galen. Famous Greek physician and anatomist, born about a.d. 130. His writings 
were long the highest authority in medical science. 

Gall bladder. A pouch in which the bile secreted by the liver is stored. 

Gamete (gajn' eet). A germ cell, or other cell which fuses with a second cell in repro- 

Gametogenesis (ga me' to jen' e sis). The ripening of germ cells. 

Ganglion (gang' gli on) (pi., ganglia). A mass of nerve cell bodies, usually forming 
a thickening in the course of a nerve. 

Gastric. Pertaining to the stomach. 

Gastrocnemius (gas' trok ne' mi us). A large muscle in the calf of the leg in verte- 
brate animals. 

Gastropoda [gas Irop' o da). A class of Mollusca including snails and slugs, mollusks 
whose bilateral symmetry is often obscured by a coiled bodj' and shell. 

Gastrotheca. A genus of frogs. 

Gastrovascular. Serving the functions of digestion and circulation. 

Gastrovascular cavity. See coelenteron. 

Gastrula {gas' tru la). An early developmental stage, formed from a blastula, com- 
monly by the invagination of the vegetative pole of the latter. The gastrula 
consists of two layers of cells (ectoderm and endoderm) surrounding a cavity 
which communicates with the exterior. 

Gastrulation. The formation of a two-layered embryo from a blastula, by invagina- 
tion of the vegetative pole, by delamination, or otherwise. 

Gemmule. A group of cells forming a reproductive body in fresh-water sponges. 

Gene. Something in a germ cell or other cell which is responsible for a hereditary 

Generic (je ner' ic). Pertaining to a genus. 

Genetics. The science of heredity, variation, sex determination, and related phe- 

Genital. Concerned with reproduction. 

Genus (je' nus) (pL, genera, jen' e ra). A group of species having so many structural 
features alike that they must be regarded as having sprung from common ances- 
try; a group of lower rank than the family. 

Geoflfroy Saint-Hilaire, Etienne (zho frwa' sa^ ie lair'). French naturalist, 1772- 

Gephyrea (je fi' re a). A group of wormlike aninuils of doubtful rank and relation- 
ships. Thej^ have sometimes been referred to the Annelida. 

Germ cell. A cell capable of reproduction, or of sharing in reproduction, as con- 
trasted with the somatic or body cells which are sterile; or, more strictly, a repro- 
ductive cell that has undergone, or will undergo, or whose cell descendants will 
. undergo, oogenesis or s{)orniatogenesis before partici])ating in reproduction. 

Germ layers. The embryonic layers, ectoderm, endoderm, and mesoderm; so called 
because, in a sense, each one contains the "germ" of certain adult structures. 

Gill. A structure having a surface enlarged usually by branching or folding, which 
serves a respiratory fimction. 

Gill bar. The tissue between two gill clefts. 


Gill cleft. One of several openings from the pharynx to the sides of the neck or head 

of a vertebrate embryo or adult; derived from a gill pouch. Also called gill slit. 
Gill plate. The thickened patch of ectoderm in an embryo from which gills are 

Gill pouch. One of several evaginations from the sides of the anterior part of the 

digestive tract in the embryos of vertebrate animals. In some animals they 

break open to the outside, becoming gill clefts. 
Gizzard. In the earthworm, a thick-walled portion of the alimentary tract behind 

the crop. In birds, the posterior muscular division of the stomach. 
Gland. An organ whose function is the secretion of something to be used in, or ejected 

from, the body. 
Glaucomys. A genus of flying squirrels. 

Glenoid fossa. The cavity into which the head of the humerus fits. 
Glochidium (glo kid' i um). The young stage of a mussel, which becomes temporarily 

attached to fishes. 
Glomerulus {glo mer' u lus). A coil of blood capillaries at the end of each tubule in 

the kidney of a vertebrate animal. 
Glossozoa {glo' so zo' a). A group of animals (literally, tongue animals) in Oken's 

early classification. It comprised the fishes. 
Glottis. A slitlike opening in the larynx at the anterior end of the trachea in 

Glucose. Grape sugar. 
Glycerol {glis' er ole). An alcohol entering into the composition of fats and having 

the empirical formula C3H!i(OH)3. Same as glycerin. 
Glycine {glV seen). The simplest of the amino acids. 
Glycogen {gW ko jen) . Animal starch ; a common form of stored carbohydrate food 

in animal tissues. 
Gmelin, Johann {gma' lin). German botanist, 1748-1804. 
Golgi apparatus {gole' jee). A structure of various shapes, often a network, and of 

unknown function, found in many cells, usually near the nucleus. 
Gomphoceran {gam fas' er an) . Any extinct cephalopod resembling Gomphoceras, 

whose shell was short and wide. 
Gonad {go' nad). An organ in which germ cells (either oogonia or spermatogonia) 

are produced or lodged. 
Gonangium {go nan' ji um). A reproductive structure in a hydroid, consisting of a 

blastostyle, its attached medusa buds, and a gonotheca. 
Goniatite {go' ni a tile). An extinct cephalopod having a coiled shell and bent or 

angular sutures; so named from the genus Goniatites {go' ni a ti' teez). 
Gonionemus {go' ni o ne' mu-s). A genus of jellyfishes. 
Gonium. A genus of colonial flagellate organisms in which the cells are arranged in a 

flattened plate. 
Gonophore. One of the reproductive members of a siphonophore colony. Also, a 

medusa or medusalike member of a hydroid. 
Gonotheca. A transparent sheath forming the outer part of a gonangium of a 

Gopherus {go' fer us). A genus of turtles. 
Grantia. A genus of calcareous marine sponges. 

Graptemys {grap' te 7nis). A genus of turtles of the familj' Testudinidae. 
Graptolite {grap' to lite). An extinct group of colonial hydroid coelenterates of 

Cambrian and Devonian time. 
Graze. To eat grass or similar herbage. 
Gregaloid. Loosely adhering in an irregular mass. 


Grew, Nehemiah. English botanist, 1641-1712. 

Gullet. A tube leading from the posterior end of the oral groove in Paramecium to 
the interior of the cell. 

Habitat. The kind of place in which an organism lives. 

Halogen. One of a family of chemical elements including chlorine, iodine, bromine, 

and fluorine. 
Halysites {hal' i si' teez). A genus of extinct chain corals. 
Haploid. Single; referring to the reduced number of chromosomes in the mature 

germ cells of bisexual animals. Cf. diploid. 
Harvey, William. English physician and physiologist, 1578-1657. 
Hemoglobin. A reddish protein contained in the red blood cells. 
Heparin {hep' a rin). A substance extracted from liver, carbohydrate in nature with 

amino and phosphate components, used to prevent clotting of blood. 
Hepatic portal system. The portal system leading from the stomach, intestine, 

pancreas, and spleen to the liver. 
Herbivorous. Plant-eating. 
Heredity. The occurrence, in organisms, of any qualities due to the nature of the 

protoplasm of which they are made. 
Hermaphrodite. An organism possessing both male and female organs. Also 

(adjective), possessing the organs of both sexes. 
Herpetology [her pe toV oji). The zoology of reptiles and Amphibia. 
Heteromita lens. A species of flagellate protozoon. 

Heteromorphes {het' er o mor' Jeez). A group of animals in Blainville's early classifi- 
cation; animals of irregular form, mainly sponges and Protozoa. 
Heterozygote. An organism to which its two parents have contributed unlike genes 

with respect to some inherited character, and which in turn produces two kinds 

of germ cells with respect to that character. 
Heterozygous. Of the nature of a heterozygote. 
Hexactinellida {heks ak ti neV It da). A class of Porifera (sponges) whose spicules 

are composed of silica. 
Hipparion {hip pa' ri on). An extinct horselike animal of ^Miocene and Pliocene 

time in North America and Europe. 
Hippocampus {hip' po kam' pus). A genus of fishes of bizarre form resembling in 

part tlie head of a horse. 
Hippocrates {hip pok' ra teez). Greek physician. Father of Medicine, 460-359 (?) B.r. 
Hirudinea {hi' ru din' e a). A class of Annelida comprising the leeches. For defini- 
tion see ( 'hap. 19. 
Holothurioidea {ho' lo thu' ri oi' de a). A class of Echinodermata, comprising the 

sea cucumbers. For definition see Chap. 19. 
Homo. The genus of animals comprising man. 
Homolecithal ihn' mo Ics' i thai). Having little yolk, nearly evenly distributed; said 

of ("figs. 

Homologous iho inol' o giis). Originating in the same wa\' in evolution; said of organs 

or structures. 
Homology. Siniilaiit y of origin in evolution; api)lied to organs that arise in the 

same way. 
Homozygote. An organism whose two parents contributed to it similar genes for 

some inherited character, and whose germ cells are therefore all alike with respect 

to that character. 
Homozygous {ho' ino zi' gi(s). Of the Jialure of a homozygote. 
Hooke, Robert. English natural philosopher and mathematician, 1635-1703. 


Hooker, Sir Joseph Dalton. English botanist, 1817-1911. 

Hormone [hor' mone). A secreted substance which stimulates activity in an 

Humerus. The single bone of the upper arm in Amphibia and the higher vertebrates. 
Huxley, Thomas. English biologist and lecturer, 1825-1895. 

Hybrid. The offspring of two parents unlike one another in some heritable character. 
Hybridization. The process of crossing animals having unlike heritable characters, 

thereby producing animals possessing genes for the traits of both parents. 
Hydra. A small tubular fresh-water animal with tentacles and stinging organs, 

belonging to the phylum C'oelenterata. 
Hydranth. A hydralike tentacle-bearing member of a hydroid colony. 
Hydroid. A colonial coelenterate, the individuals of which resemble the hydra in 

certain respects. 
Hydrorhiza {hi' dro ri' za). That part of a hydroid colony which is attached to the 

Hydrotheca. The tough transparent sheath surrounding a hydranth of a hydroid; 

an expansion of the perisarc. 
Hydroxy! ion [hi droks' il). The radical 0H~ produced in solutions of bases. 
Hydrozoa. A class of Coelenterata, including Hydra, the hydroids, jelly fishes, and 

some corals. For definition see Chap. 19. 
Hyla. A genus of tree frogs. 
Hyoid. A bone or group of bones or cartilages located at the base of the tongue or 

in a corresponding situation. 
Hypodermis. An external layer of cells beneath a secreted cuticle, as in the earth- 
worm and in insects and Crustacea. 
Hypohippus. An extinct horselike animal of Miocene time in North America. 
Hypostome. A projection from the center of the circle of tentacles in a hydra or 

one of the hydroids. It is perforated by the mouth. 

Ileum {iV e um). The last and usually longest of three divisions of the small intestine. 
Ilium {iV i um) {pL, ilia). The dorsal bone of the pelvic girdle in Amphibia and the 

higher vertebrates. 
Incisor. One of the front cutting teeth of a mammal. 

Incubation. The warming of eggs, resulting in acceleration of their development. 
Indeterminate. Not leading necessarily to a given end result from a given beginning: 

said of development in which cleavage cells may, if disturbed, produce some 

structure other than that which they would have produced without interference. 
Ingestion. The taking in of food. 
Inhalent. Breathing in; applied to one of the siphons of clams and mussels, to 

certain pores of sponges, and to other passages. 
Innominate bone. The single bone formed by the fusion of three bones of the pelvic 

girdle in man. This name is not usually applied in the case of other vertebrates, 

though fusion of the bones of the girdle commonly occurs. 
Insecta. A class of Arthropoda having one pair of antennae, three pairs of legs, and 

tracheae for respiration; the insects. 
Insectivore. Technically, a mammal of the order Insectivora, including the moles, 

shrews, and hedgehogs. In a popular sense, any insect-eating animal. 
Insertion. The place of attachment of the distal end of a muscle or its tendon. 
Insulin (m' su lin). An endocrine secretion produced by the islands of Langerhans 

in the pancreas; its function is control of sugar metabolism. 
Internal respiration. The transfer of oxygen from the blood to the surrounding cells; 

true respiration. 


Interphase. The stage in the cycle of a cell in which it is not dividing: the so-called 
"resting" stage. 

Interstitial cells. The cells of a testis which lie between the seminiferous tubules. 

Intracellular. Within a cell. 

Invagination. The folding of a layer of cells inward into a cavity. 

Ion. An atom or group of atoms bearing an electric charge. 

Ischium {is' ki um) {pi., ischia). The posterior of two ventrally located bones of the 
pelvic girdle of vertebrate animals above "the fishes. 

Islands of Langerhans. Groups of isolated cells in the pancreas, which produce 

Isogamete {i' so gam' eet). One of two gametes of equal size which fuse in reproduc- 

Isogamy {i sog' a mi). Fusion of like gametes in reproduction. 

Isolation. In evolution, the inability of species to cross with one another. 

Jejunum. The second of three divisions of the small intestine. 

Jensen, Zacharias. Dutch inventor of the microscope about 1591. 

Jugular vein. A large vein returning blood from the head. 

Jurassic. Of middle Mesozoic age; named from rocks in the Jura mountains. 

Karyokinesis {ka' ri o ki ne' sis). Same as mitosis. 

Kidney. The chief organ for the excretion of nitrogenous wastes in most vertebrates. 

Also an excretory organ in certain other animals. 
Kinostemidae {ki' no stcr' ni dee). A family of turtles. 

Labial palp. One of two pairs of flattened organs beside the mouth of mussels. 

Labyrinth. The inner ear of vertebrates. 

Lacteal (lak' tc al). One of the minute vessels leading from the intestine to the 

• lymph ducts. 
Lactose. Milk sugar, a disaccharide found in the milk of mammals. 
Lacuna. A space in the matrix of bone which contains in life a bone cell. 
Lamarck, de, Jean Baptiste, etc. Celebrated French naturalist and proponent of 

evolution, 1744-1829. 
Lamella. A layer. 

Lamprey. An eellike animal of the class Cyclostomata. 
Lampsilis {lamp' si lis). A genus of fresh-water mussels. 
Large intestine. The enlarged portion of the digestive tract following the small 

Larva. A free-living developmental stage of an animal in which certain adult 

organs are still lacking or in which organs are present that are lacking in the 

Lateral fold. One of two ridges of skin along the back of certain species of frogs, 

extending lengthwise at either side. 
Lecithin {Ics' i thin). One of a number of lipoid substances common in egg yolk, 

nerve tissue, and other kinds of cells. 
Leeuwenhoek, van, Anton {lay' ven hook). Dutch naturalist and microscopist, 1632- 

Leiolopisma {li' o lo piz' ma). A genus of skinks (lizards). 
Lemming. A rodent of the family Muridae, to which the rats, mice, and muskrats 

Lepas anatifera {le' pas an' a tif cr a). A species of barnacle (subclass Cirripedia 

of the Crustacea). The goose barnacle. 


Leptinotarsa {lep' tin o tar' sa). A genus of leaf-eating beetles to which the common 

potato beetle belongs. 
Leptodactylus (lep' to dak' ti lus). A genus of frogs. 
Lernaeopoda {ler' ne op' o da). A copepod (Crustacea) parasitic on the gills of certain 

Leucocyte. A white blood cell. 
Linear. Arranged in a line or row. 

Lingula. A genus of brachiopods, a group of uncertain relationships. 
Linkage. The occurrence of the genes for two or more hereditary characters in the 

same pair of chromosomes. 
Linnaeus, Carolus {lin ne' us). Swedish botanist and naturalist, author of the 

binomial system of nomenclature and an artificial classification of animals and 

plants, 1707-1778. 
Lipid {Up' id). Any organic compound of the class including true fats, and the 

compounds of fats with other substances such as phosphates and sugars. 
Liver. A gland which secretes bile and other substances. 

Loxoceras {loks os' er as). A genus of extinct cephalopods of the orthocone type. 
Lumbar. Pertaining to the loins, the region of the back posterior to the ribs. 
Lumbricus terrestris {lum bri' kus). A species of earthworm. 
Lung. A respiratory organ in the vertebrates. 
Lycopod (W ko pod). A club moss. 
Lyell, Sir Charles. British geologist, 1797-1875. 

Lymph. A clear fluid containing colorless cells found in lymph vessels. It is essen- 
tially blood without its red cells and with much less of the proteins. 
Lymphatic system. A system of vessels conveying lymph in vertebrates. 
Lymph capillaries. The smaller vessels of the lymphatic system. 
Lymph node. A connective tissue enlargement in a lymph vessel, which removes solid 

materials from the lymph and produces one kind of white blood cell. 

Macronucleus. The large nucleus in a cell or organism having two nuclei of unequal 

Macrosiphum sanborni (niak' ro si' fum). A species of insect, one of the plant lice, 

living on chrysanthemum plants. 
Malpighi, Marcello {mahl pee' gee). Italian anatomist, founder of microscopic 

anatomy, 1628-1694. 
Malthus, Thomas Robert. English political economist, author (1803) of Essay on. 

Population, who hved 1766-1834. 
Maltose. Malt sugar. 
Mammal. One of the Mammalia. 
Mammalia. A class of vertebrates having hairy bodies, producing young within the 

body of the mother, and nourishing the young after birth with milk secreted by 

the mother. 
Mammalogy. The zoology of mammals. 
Mammoth. An elephantlike animal of prehistoric times. 

Mantle. A sheet of tissue, typically quite thin, which secretes the shell in mollusks. 
Mantle fibers. Fibers lying about the periphery of the spindle of a dividing cell 

and extending from the centrioles to the chromosomes. 
Manubrium {ma nu' bri xim). A projection from the center of the subumbrella of a 

medusa, corresponding to the hypostome of a hydranth, and bearing the mouth 

at its end. 
Marginal bone. One of a ring of bones around the margin of the carapace of a 



Marsupial. A mammal having a pouch in which the young are carried (for example, 

the opossum and the kangaroo). As an adjective, possessing a pouch; as the 

marsupial frog. 
Mastigophora {mas' ti gof o ra). A class of protozoa, characterized by flagella. 
Mastodon {mas' to don). An extinct genus of elephantlike animals of Pliocene and 

Pleistocene time. 
Maternal. Pertaining to or derived from the mother. 
Matrix. The noncellular material in which the cells of bone and cartilage are 

Matter. That of which any physical object is composed; anything which can occupy 

Maturation. A process which germ cells undergo before they become functional, 

consisting of one or two cell divisions; if of two divisions, the chromosomes remain 

imduplicated in one of them. 
Medulla oblongata. The enlargement of the anterior end of the spinal cord in verte- 
brates, commonly regarded as the posterior division of the brain. 
Medusa (pZ., medusae). A jellyfish; the free-swimming member of many hydroid 

Megapode {meg' a pode). A bird of the family Megapodiidae, the mound birds and 

jungle fowls. 
Meiosis {mi o' sis). Separation of maternal from paternal chromosomes in oogenesis 

or spermatogenesis. Also, according to some, the two divisions of oogenesis or 

Mendel, Gregor. Austrian monk and plant breeder, founder of modern movement 

in genetics, and author of Mendel's law of heredity. Lived 1822-1884. 
Mendel's law. The law that genes for inherited characters separate from one 

another and recombine in various ways in the germ cells. 
Meridional {me rid' i o nal). Passing through the animal and vegetative poles; said 

of certain cleavage planes of an egg. 
Merychippus imer' i kip' pus). An extinct horselike animal of Miocene time. 
Mesenchyme {mes' en kime). A tissue composed of cells of irregular shape, loosely 

joined in a network with extensive meshes. 
Mesentery {mes' en ter i). A double sheet of tissue, continuous with the peritoneum, 

which supports an organ (such as the intestine) from the body wall. 
Mesoderm. A layer of cells between the ectoderm and endoderm. 
Mesohippus. An extinct animal of Oligocene time, ancestral to the horse. 
Mesozoa. A group of degenerate animals of uncertain rank and relationship, onco 

regarded as intermediate between protozoa and metazoa; hence the name. 
Mesozoic. Pertaining to the geological era between the Paleozoic and Cenozoic, 

or the age of reptiles. 
Metabolism {me tab' o liz'tn). The sum total of the chemical processes going on in 

Metacarpal. One of the bones forming the body of tli(^ liand or forefoot in vertolnates. 
Metagenesis. The occurrence of two or more forms of individual in the same sj)ecies, 

one of which reproduces sexually and one or more asexually. 
Metamere. See somite. 
Metamerism {me tam' er iz'ni). The condition of being divided into a number of 

similar metameres or somites. 
Metamorphosis {met' a mor' fo sis). The transformation of a larva into an adult. 
Metaphase. That stage of cell division in which the chromosomes are in the equa- 
torial plate. The chromosomes are typically (hii)li('ate(l in this stage. 
Metatarsal. One of the bones forming the body of tlu' iliindi loot of vertebrates. 



Metatheria. A subclass of mammals including the marsupials or pouched mammals. 
Metazoon. An animal composed of many cells. Although the term contrasts an 

animal with the protozoa, it is not a name of any taxonomic group of animals. 
Miastor. A genus of fhes; the larvae are often paedogenetic. 
Microgromia socialis. A species of protozoon which forms a gregaloid colony in a 

gelatinous supporting substance. 
Micronucleus. The smaller nucleus in a cell or organism having two nuclei of unequal 

Micropyle. A small hole in the shell of an egg through which the spermatozoon enters 

in fertilization. 
Microstomum {mi Aros' to mum). A genus of rhabdocoele flatworms. 
Microtus. A genus of field mice. 

Miocene. Belonging to middle Tertiary time; succeeding the Oligocene. 
Miohippus {mi' o hip' pus). An extinct horselike animal from the Oligocene. 
Mirbel, Charles Francois {mccr hcV). French botanist, 1776-1854. 
Mississippian. The hfth period of the Paleozoic era, following the Devonian and 

preceding the Pennsylvanian. 
Mitochondria {mi' to kon' dri a). Objects of unknown function and of various shapes 

(threadlike, rod-shaped, or granular) found in the cytosome of many cells. 
Mitosis. Cell division involving the formation of chromosomes, spindle fibers, etc. 

Also called karyokinesis. 
Moeritherium {me' ri the' ri um). An extinct animal from the Eocene of Egypt, 

probably an early ancestor of the elephants. 
Molar. One of the grinding teeth of a mammal, back of the incisors and canines. 
Mold. A cavity in a rock representing the form of an animal or plant or other 

object whose remains formerly occupied the cavity. 
Molecule. Usually a group of atoms behaving as a imit of the substance which they 

compose. It is the smallest particle which possesses the chemical nature of the 

Mollusca. The phylum of animals including the clams, snails, cuttlefishes, etc. 

For definition see Chap. 19. 
Mollusk. One of the ^Nlollusca. 
Monoecious {mo nee' shus). Having the organs of both sexes in the same individual 

which is thus a hermaphrodite; said of species. 
Monosaccharide {mon' o .sak' a ride). A simple sugar; one which cannot be broken 

down into simpler sugars. 
Monotreme. One of the Monotremata (Prototheria) ; an egg-laying mammal having 

a cloaca. 
Morgan, T. H. Leading American geneticist, 1866-1945. 

Morphology. The branch of biolog,y which deals with the structure of living things. 
Motor. Pertaining to movement; applied to a neuron which conveys impulses result- 
ing in muscular movement, glandular action, and the like. 
Motor root. The ventral one of two roots by which a spinal nerve is connected with 

the spinal cord. So called because its fibers have a motor function. 
Motor unit. The group of muscle cells innervated by a single nerve fiber. 
Muellerian duct {mul le' ri an). A tube formed in the embryo of most vertebrate 

animals, becoming the oviduct in the female and degenerating (with few excep- 
tions) in the male. 
Muscle. An aggregation of contractile cells. 
Mustelus mustelus {mus te' lus). A species of shark. 
Mutation. A heritable modification arising in an organism. 
Myelin. A fatty substance forming a sheath around many nerve fibers. 


Myofibril (mi' o ji' hril). One of the contractile threads in a voluntary muscle cell. 
Myosin {mi' o sin). A common protein in muscle. 

Myotome. One of the segments into which certain muscles are divided. 
Myriapoda (meer' i ap' o da). A class of Arthropoda having tracheae, one pair of 

antennae, and many unspecialized legs; centipedes and millipedes. 
Mysis. A genus of Crustacea having all appendages two-branched; also a larval 

stage of other Crustacea in which all appendages are two-branched. 
Myxedema (miks' e de' ma). A disease whose symptoms are puffy tissues, reduced 

metabolism, and mental depression, caused by deficient thyroid action. 

Nacre. The pearly substance secreted by mollusks upon their shell or other objects. 

Nais. A genus of fresh-water worms, phylum Annelida, subclass Oligochaeta. 

Nasal pit. The ectodermal depression in an embryo which forms much of the nostril. 

Natrix. A genus of snakes. N. rhombifera, N. sipedon, two of the species. 

Natural history. A descriptive account of things in nature, particularly animals and 
plants, though the term is sometimes used to include minerals, rocks, climate, 

Natural selection. The survival of the fittest individuals or the fittest species in a 
variable population. 

Nauplius {naiv' pli ris). The earliest larval stage of shrimps, barnacles, and some 
other Crustacea. 

Nautiloid. One of the extinct cephalopods resembling, Nautilus. 

Nautilus. An animal belonging to the Cephalopoda, living in a coiled shell divided 
into chambers. 

Neanderthal man (no ahn' der tahl). A primitive man whose remains have been 
found in various places in Europe. 

Necator. The genus of roundworms to which the hookworm belongs. 

Nectocalyx {nek' to ka' liks) {pi., nectocalyces, nek' to ka' li seez). One of the swim- 
ming members of a siphonophore colony. 

Necturus. A genus of salamanders; the mud puppy. 

Nemathelminthes {nem' a thel min' theez). The phylum of roundworms and their 
allies. For definition see Chap. 19. 

Nematocyst {nem' a to sisi). One of the stinging bodies of Hydra and other coelen- 

Nematode {nem' a tode). Any roundworm of the class Nematoda, phylum Nemathel- 

Nematomorpha {nem' a to mar' fa). A group of wormlike animals of uncertain 
affinities. They have usually been doubtfully included in the Nemathelminthes. 
For definition see Chap. 19. 

Nemertinea {nem' er tin' e a). A group of wormlike animnls of uncertain relation- 
ships. They are regarded by some as a class of Platyhelminthes. For definition 
see Chap. 19. 

Nephridiopore {ne frid' i o pore). The external opening of a nephridium. 

Nephridium {ne frid' i um). An excretory organ of certain invertebrate animals 
(worms, mollusks, etc.), approximately corresponding in function to the kidney 
of vertebrates. It is commonly a coiled tube, as in the earthworm. 

Nephrostome (ne/' ro stome). The opening at the inner end of a nephridium as in 
the earthworm. Also an opening (originally like that in the earthworm) con- 
necting the coelom with the blood vessels of the kidney in certain Amphibia. 

Nereis, A genus of marine worms, phylum Annelida. 

Nerve. A bundle of axons or dendrites of nerve cells or of both axons and dendrites. 


Nervous tissue. Tissue capable of transmitting impulses; as the tissues of the brain, 

spinal cord, and nervos. 
Net knot. A thickened portion of the chromatin of a cell nucleus. 
Neural arch. That part of a vertebra above the centrum and neural canal. 
Neural canal. The opening in a vertebra through which the spinal cord extends. 
Neural crest. One of a number of groups of cells at the sides of the brain and spinal 

cord of an embryo, from which ganglia and nerves are developed. 
Neural fold. One of the ridges of ectoderm forming the earliest development of the 

nervous system. 
Neural groove. An elongated depression between the neural folds of an embryo. 
Neural spine. A projection rising from the middle of the neural arch of a vertebra. 
Neural tube. The tube formed beneath the ectoderm by the union of the neural 

folds along their crests. 
Neurilemma. The thin cellular covering of a nerve fiber. 
Neuromuscular. Combining the functions of contraction and the transmission of 

Neuron {nu' rone). A nerve cell. 
Neutron. A particle, like a proton but without electric charge, entering into the 

composition of the nuclei of most atoms. 
Niacin (nz' a sin). The antipellagra vitamin, part of the B complex. 
Nicotinic acid {nik' o tin' ik). Same as niacin. 

Nomenclature {no' men kla' lure). A system of naming; terminology. 
Nostril. One of the external openings of the nasal chamber. 
Notochord (no' to kord). A cylindrical rod of cells beneath the nervous system of an 

embryo (adult of some animals). It is the forerunner of the spinal column of the 

vertebrate animals. 
Notophthalmus {no' tof thai' mus). A genus of salamanders. 
Nuchal plate {nu' kal). In turtles, the median plate of the carapace at the anterior 

Nuclear membrane. A thin film of protoplasm surrounding the nucleus of a cell. 
Nuclear sap. The liquid forming the bulk of the nucleus of a cell. 
Nucleolus {nu kle' o lus). A small, usually rounded body found in the nuclei of many 

cells, which is of different chemical composition from the rest of the nucleus. 

Its function is uncertain. 
Nucleus. A highly refractive, deeply staining body of specialized protoplasm found 

within nearly all cells. 
Nudibranch {nu' di brank). One of a group of marine moUusks. 

Obelia. A genus of hydroids, or colonial hydralike animals of the phylum Coelen- 

Octopus. A genus of devilfishes (moUusks) having eight arms. 

Oenothera (e' no the' ra). A genus of plants to which the evening primroses belong. 
Oken, Lorenz. German naturalist and transcendentalist philosopher, 1779-1851. 
Olfactory. Pertaining to the sense of smell. 

Oligocene. Of early Tertiary time, between Eocene and Miocene. 
Oligochaeta {ol' i go ke' to). A subclass of Chaetopoda (Annelida), including chiefly 

terrestrial and fresh-water worms with relatively few setae which do not rest on 

fleshy outgrowths but project directly from the body wall. The earthworm is an 

Onychophora {on' i kof o ra). A class of primitive Arthropoda having tracheae and 

one pair of antennae. Peripatus is an example. 


Oocyte (o' o site). A female germ cell subsequent to the initiation of oogenesis and 
prior to the second division. An oocyte is designated primary during the growth 
period and prior to the first division; secondary after the first division and before 
the second. 

Oogenesis (o' o jen' e sis). The series of changes imdergone by female germ cells 
in preparation for reproduction. 

Oogonium (o' o go' ni urn). One of the early germ cells of a female animal, prior to 
the beginning of oogenesis. 

Operculum (o per' ku lum). A fold of skin covering the gills and gill clefts in some 
amphibian larvae; also a similar covering of the gills in fishes. 

Ophiuroidea {o' fi u roi' de a) . A class of Echinodermata, comprising the brittle 
stars. For definition see Chap. 19. 

Ophthalmozoa {of thai' mo zo' a). A group of animals (literally, eye animals) in 
Oken's early classification. The term was synonymous with Thricozoa and 
comprised the mammals. 

Opisthocoelous (o pis tho see' lus). Having the centrum concave behind and convex 
in front; said of vertebrae. 

Optic nerve. The nerve of sight. 

Oral groove. The spiral depression on one side of Paramecium, leading to the gullet. 

Order. A group of animals forming a subdivision of a class, and being composed 
of one or more families. 

Ordovician. Of early Paleozoic time, succeeding the Cambrian. 

Organ. A group of cells or tissues performing some specific function. 

Organism. A living being, whether plant or animal. 

Organismal theory. The theory that parts of an organism owe their nature to the 
nature of the whole. 

Organizer. A substance which controls some feature of embryonic development. 

Origin. The place of attachment of the proximal end of a muscle. 

Ornithology. The zoology of birds. 

Orohippus. One of the earliest known ancestors of the horse, an animal of Eocene 
time in North America. 

Orthoceras (or thos' er as). A genus of extinct cephalopods of the orthocone type. 

Orthocone. One of the early cephalopods that lived in a straight shell. 

Osculum. An opening through which water leaves the passages of a sponge. 

Osmosis. The diffusion of a substance through a membrane in response to unequal 
distribution of that substance on opposite sides of the membrane. 

Osmotic pressure. Objectively defined, the pressure that will just prevent diffusion 
of a solvent into a solution when the two are separated by a semipermeable mem- 
brane. Also, the pressure due to the greater kinetic energy of the molecules of a 
solvent on one side of a semipermeable membrane than on the other, due to the 
presence of a solute on the side exhibiting the lesser kinetic energy of the solvent. 

Otozoa. .\ group of animals (literally ear animals) in Oken's early classification. 
It comprised the birds. 

Ovary. The organ in which the immature germ cells of a female animal are 

Oviduct. A tube through which the eggs of a female animal leave the body. 

Oviparity. The condition of being oviparous. 

Oviparous (o vip' a rus). Egg-laying. 

Oviposition. The laying of eggs. 

Ovisac. A chamber for the storage of eggs. b(>ing in some cases a lateral pouch of 
the oviduct, as in the earthworm. 

Ovoviviparity {o' vo viv' i par' i ti). The condition of being ovoviviparous. 


Ovoviviparous (o' vo vi vip' a rus). Producing young from eggs that are retained in 
the oviduct during their development, but without attachment to the oviduct, 
and wholly from nutrition stored in the egg. 

Ovum. An egg; a relatively large passive cell which, in preparation for reproduction, 
has undergone one or two unequal divisions. 

Oxidation. The chemical process of combining with oxygen. 

Paedogenesis (pr' do jen' c sis). Sexual maturity in an animal otherwise immature; 
the capability possessed by some species of reproducing wliile in the larval con- 

Palaeomastodon (pa' le o mas' to don). A genus of extinct animals belonging to the 
clej)hant ancestry, found in the Oligocene of Egypt and India. 

Paleontology. The science wliich treats of prehistoric life on the earth, now repre- 
sented by fossils. 

Paleozoic (pa' le o zo' ik). Pertaining to the geological era prior to the Mesozoic, 
when amphibia, fishes, and the higher shell-bearing invertebrates were the domi- 
nant forms. 

Pancreas. A gland which secretes a fluid containing several digestive enzymes and 
discharges into the intestine. 

Pandorina. A genus of colonial flagellate organisms in which the cells are held in 
a spheroidal jellylike mass. P. monim (trio' rum) is one of the species. 

Paramecium. A genus of ciliated protozoa. 

Parasite. An animal which lives in or on another species of animal (its host), at the 
expense of the latter. 

Parasitism. The condition of being a parasite. 

Parathyroid. One of a pair (or two pairs) of small ductless glands closely associated 
with the thyroid. 

Parietal bone. One of a pair of bones on the posterior upper part of the skull of 
vertebrate animals. 

Parthenogenesis (par' the no jen' e sis). The development of an egg without fertiU- 

Parthenogonidia (par' the no go nid' i a). The asexually reproducing cells of Volvox. 

Paternal. Pertaining to or derived from the father. 

Pectoral girdle. A group of connected bones serving to attach the bones of the fore- 
limbs of vertebrate animals to the rest of the skeleton. 

Peking man. An early hvmian type somewhat resembling the Piltdown and Neander- 
thal types, found in China. 

Pelecypoda (pel' e sip' o da). A class of Mollusca having bivalve shells and a bilobed 
mantle; the clams and mussels. 

Pellagra (pel la' gra). A condition of malnutrition accompanied by eruption of the 

Pellicle. A thin skin or film on the surface of a cell. 

Pelvic girdle. A group of bones serving to join the bones of the hind limbs of verte- 
brate animals to the rest of the skeleton. 

Penis. The copulatory organ in the male of many animals. 

Pennsylvanian. The sixth period of the Paleozoic era, following the Mississippian 
and preceding the Permian. 

Pentadactyl (pen' ta dak' til). Having five fingers or toes. 

Pepsin. An enzyme of the stomach of vertebrate animals, whose function is digestion 
of many kinds of protein. ' 

Pepsinogen (pep sin' n jen). An inactive substance from which the enzyme pepsin is 


Period. One of the divisions of an era in the geological time scale. 

Periodic. Occurring at rather regular intervals; said of migration which depends 

on the seasons or on the age of the migrating animals. 
Peripheral nervous system. In general, the nerves, collectively ; the nervous system 

aside from the brain and spinal cord or other central cord. 
Perisarc. The tough sheath surrovmding the stalk and branches of a hydroid. 
Peritoneum {per' i to ne' urn). A sheet of cells covering the viscera and lining the 

body cavity in many animals. 
Permeable. Permitting the passage of both liquids and dissolved substances. 
Permian. Belonging to the close of the Paleozoic era. 
Petrifaction. The piecemeal substitution of mineral matter for the body substance 

of dead animals or plants. 
Phalanx {fa' lanks) {pL, phalanges, fa Ian' jeez). Any one of the bones of the fingers 

or toes in vertebrate animals. 
Pharynx {far' inks). In an earthworm, the thick-walled portion of the digestive 

tract just posterior to the buccal pouch and in front of the esophagus. In verte- 
brates, the portion of the digestive tract at the back of the mouth, into which the 

gill clefts open. 
Phoronidea {fo' ro nid' e a). A small group of marine animals, of which Phoronis is 

the onl}'^ genus, of uncertain relationship to other animals. Sometimes placed in 

a phylum with the Bryozoa and Brachiopoda. 
Photosynthesis. The construction of glucose from carbon dioxide and water by 

the energy of sunlight in the presence of chlorophyll. 
Phylum. One of a dozen or more major groups into which the animal kingdom is 

divided; in general, the largest group of which it can be said that the members 

are related. 
Physalia. A very complex colonial coelenterate, one of the siphonophores. 
Physiology. The branch of biology which deals with the functions of animals and 

plants, and the processes going on in them. 
Piltdown. A locality in Sussex, England, near wliich primitive human fossils have 

been found. 
Pineal body {jnn' e al). A structure on the dorsal side of the brain in vertebrate 

animals. Because of its similarity, in development, to the embryonic stages of an 

eye, it is often called the pineal eye and is believed by many to be a vestigial 

sense organ. 
Pisces {pis' seez). A class of vertebrate animals including the fishes. For definition 

see Chap. 19. 
Pithecanthropus {pilh' e kan thro' pus). An extinct apelike and manlike animal 

believed to be closely related to the early ancestry of man. 
Pituitary {pi tu' i ta ri). A glandular organ beneath the brain composed in part of 

nervous tissue. 
Placenta. A vascular tissue dovetailing into the wall of the uteiiis on one side and 

(connected with the umbilical cord on the other, thus forming an intimate nutritive 

connection betweeen the embryo and the mother in viviparous animals. 
Planaria. A genus of flatworms, phyhmi Platyhelminthes. 
Planula {plan' u la). A ciliated larva consisting of a solid elliposidal mass of cells, 

dov(^loped from the fertilized egg of a medusa or similar organism. 
Plasma. The liquid part of the blood. 
Plasmodroma. A subphylum of protozoa devoid of cilia. 
Plastid. One of several kinds of protoplasmic bodies in cells, like the green bodies 

in {)lant cells, which are centers of chemical activity. 
Plastron. The flat plate of bones on the ventral side of a turtle. 


Platelet, See blood platelet. 

Plato. A Greek pliilosopher, pupil of Socrates and teacher of Aristotle. Lived 

about 427-347 b.c. 
Platyhelminthes {plat' i hel min' theez). The phylum of flatworms. For definition 

see Chap. 19. 
Pleistocene {plise' to seen). Belonging to the epoch following Pliocene in the Tertiary. 
Pleodorina (pie' o do ri' no). A minute spherical organism composed of cells of two 

sizes embedded in a jellylike substance. P. californica {kaU i jor' ni ka), with 

numerous small cells; P. illinoisensis {iV li noi zen' sis), with four small cells. 
Plethodon {pleth' o don). A genus of salamanders. 
Pliny {plin' i). Roman naturahst (a.d. 23-79) and author of works on natural 

Pliocene. Pertaining to the epoch of Tertiary time following Miocene. 
Pliohippus. An extinct animal of Pliocene time, closely resemljling the horse. 
Plumatella. A group of fresh-water bryozoa. 
Pneumatophore {nu' ma to fore'). A capsule enclosing gas, serving to float a siphono- 

phore colony. 
Podophrya (po dof ri a). A protozoon belonging to the class Suctoria. 
Polar body. A small nonfunctional cell, one of the two cells produced Ijy each divi- 
sion in oogenesis. 
Polarity. The condition of exhibiting or possessing different properties in different 

parts; the condition of a cell in wliich the protoplasm is unlike in different parts 

of the cell. 
Pole. A differentiated part or extremity, as of an egg, or of the spindle of a dividing 

Poloc3rte. The small cell produced at either of the divisions of oocytes in oogenesis. 

Same as polar body. 
Polychaeta (poU i ke' la). A subclass of Chaetopoda (Annehda) including those 

marine j^^orms having numerous setae borne on fleshy outgrowths at the sides of 

the somites. Nereis, the sandworm, is an example. 
Polymorphic. Having a variety of forms. 

Polymorphism. The existence of two or more kinds of individuals within a species. 
Polyneuritis. A disease due to vitamin Bi (thiamin) deficiency. 
Polyorchis (poZ' i or' kis). A genus of jelly fishes. 
Polyp. One of the feeding individuals of a hydroid or coral colony or simple related 

Polysaccharide {poV i sak' a ride). A carbohydrate whose molecule can be split into 

many molecules of simple sugar (monosaccharide). 
Porcellio. A genus of sowbugs (Isopoda, Crustacea). 

Porifera {po rif er a). The phylum of animals comprising the sponges. For defini- 
tion see Chap. 19. 
Portal system. A blood vessel or group of vessels beginning and ending in capillaries. 
Postcava. A large vein leading to the heart from behind or below. 
Poterioceras {po te' ri os' er as). A genus of extinct cephalopods of the gomphoceran 

F*recipitin {pre sip' i tin). A substance which produces a precipitate when two blood 

sera are mixed. 
Precocial. Able to run about as soon as hatched; said of certain birds. 
Precoracoid. A ventrally situated bone or cartilage of the pectoral girdle in Amphibia 

and some reptiles. 
Primary. For application to spermatocytes, see spermatocyte. For application to 

oocytes, see oocyte. 


Primate. A mammal of the order including man and the apelike animals. 

Priority, law of. The rule that the name first given a species along with a description 

is the one that shall be accepted when different names have been applied to the 

same species. 
Proboscis {pro bos' sis). The trunk of an elephant, consisting of the elongated nose 

and upper lip. Also a fleshy projection of other sorts. 
Procoelous {piv see' lus). Having the anterior end of the centrum concave, the 

posterior end convex; said of vertebrae. 
Procyon (pro' si on). The genus of Carnivora to which the raccoon belongs. 
Proglottis (pi., proglottides, pro glot' ti deez). One of the individuals in a chain of a 

Prophase. Any early stage of mitotic cell division, prior to the equatorial plate. 
Prosecretin (pro' se kre' tin). A substance in the walls of the small intestine from 

which secretin is produced. 
Prostomium. A rounded projection overhanging the mouth of an earthworm. 
Protein. One of many organic substances, compounds of amino acids, which therefore 

contain carbon, hydrogen, nitrogen, and oxygen and often other elements. The 

molecules are large and very complex. Lean meat and egg albumen contain 

quantities of proteins. 
Proterospongia haeckeli {pro' ter o spun' ji a hek' el i). A species of protozoon which 

forms gregaloid colonies. 
Proterozoic. Belonging to the era preceding the Paleozoic. 
Proteus. A genus of salamanders. 
Prothrombase. A substance from which an enzyme of clotting (of blood) is 

Proton, A particle bearing a positive electric charge entering into the composition 

of the nuclei of atoms. 
Protonephridium. A primitive excretory organ consisting of flame cells and con- 
necting tubes. « 
Protoplasm. The living matter of which animals and plants are essentially composed. 
Prototheria. A subclass of Mammalia, including the egg-laying mammals such as 

the duckbill Ornithorhynchus and the spiny anteater Ecliidna. 
Protozoa. One-celled animals. The phylum comprising the one-celled animals, 

including colonial forms in which the cells of the colony are, at least potentially, 

all alike. 
Protozoology. The zoology of the protozoa. 

Pseudemys (su' de mis). A genus of turtles of the family Testudinidae. 
Pseudopodium (su' do po' di U7n) (pi., pseudopodia). A blunt Hngerlike projection 

thrust out by Amoeba and other rhizopods. 
Ptarmigan (tar' mi gan). Any one of several species of birds related to the grouse 

and partridges. 
Ptyalin (ti' a lin). The starch-digesting enzyme of the saliva. 
Pubis (pL, pubes, pu' hc.ez). The anterior one of two ventrally ])la('(Hl bones in the 

pelvic girdle of vertebrate animals abovi; th(> fishes. 
Pulmonary circulation. The circiUation of the blood through the lungs, as distin- 
guished I'roiii that, through the body in general (syst(>niic). 
Pulsating vacuole. Same as contractile vacuole. 

Pupa. A quiescent stage in lh(> development of an insect, just before the adult con- 
dition is reached. 
Purkinje, Jan Evangelista (poor keen' ya). Bolicniian physiologist in the University 

of Prague, 1 787-1 Xf)9. 
Pus. A collection of white cells at a wound or i)lace of infection. 


Pylorus (pi W rus). The opening from the stomach to the intestine. 
Pyridoxin. "N^itamin Be, the antidermatitis vitamin. 

Quadrate. One of the bones of the skull; in birds and reptiles and bony fishes, the 
bone from wliich the lower jaw is suspended. 

Race. A group of individuals having certain characteristics in common because of 

common ancestry. 
Radial canal. One of four tubes extending from the middle to the margin of a medusa. 
Radial symmetry. An arrangement of the parts of an object or organism such that 

it is capable of being divided into halves that are mirrored images of one another, 

by two or more planes all of which pass through a common longitudianl axis. 
Radiating canal. One of a series of collecting channels surrounding the pulsating 

^■acuoles of Paramecium and similar protozoa. 
Radical. A group of atoms behaving as a unit in reactions. 
Radio-ulna. The fused radius and ulna of frogs and toads. 
Radius. The bone of the lower arm located on the thumb side in Ampliibia and 

the higher vertebrates. 
Rana. A genus of frogs. R. cantabrigensis, the wood frog; R. catesbeiana, bidl 

frog; R. clamitans, green frog; R. palustris, pickerel frog; R. pipiens, leopard frog. 
Range. The area occupied by a species or larger taxonomic group of animals or 

Ray, John. English naturaHst, 1627-1705. 
Reaction. Any response of an animal to a stimulus; also any chemical change taking 

place in a substance, particularly a change involving some other substance as well. 
Recapitulation theory. See biogenetic law. 
Receptor. An organ which is especially sensitive to certain stimuh and serves to 

initiate impulses in nerve fibers. 
Recessive. Not being produced when the gene for a contrasted dominant character is 

also present; said of inherited characters that are not developed in heterozygotes. 
Reciprocal. Involving the same types of individuals, but with the sexes reversed; 

said of two crosses, in one of wliich the female possesses the same characteristic 

as does the male in the other cross. 
Rectum. The terminal portion of the large intestine in the higher vertebrates. In 

vertebrates with a cloaca, the term is sometimes applied to the part of the large 

intestine anterior to the cloaca. 
Reduction. Cell division in which chromosomes are not duplicated but merely 

separated from one another after having previously come together in pairs, as 

occurs in one of the two divisions in the ripening of most germ cells. 
Reflex. Same as reflex action. 
Reflex action. An action performed as a result of an impulse which passes over a 

reflex arc. 
Reflex arc. A group of two or more neurons, one of them afferent, another efferent, 

so connected as to be able to transmit impulses resulting in reflex actions. 
Regeneration. The production of lost parts by organisms. 
Relict. A li\ing remnant of an otherwise extinct group of organisms. 
Renal corpuscle. One of numerous bodies in the kidneys of vertebrate animals, each 

composed of the expanded end of a kidney tubule (Bowman's capsule) and an 

enclosed knot of blood capillaries (glomerulus). 
Rennin. An enzyme i)roduced by the gastric glands and having the property of 

coagulating milk. 
Reproduction. Tlie formation of new individuals among organisms. 


Reptilia. A class of vertebrate animals including the snakes, lizards, crocodiles, 

turtles, and some others. For definition see Chap. 19. 
Respiration. The gaseous metabolism of protoplasm, including elimination of carbon 

dioxide, usually absorption of oxygen, and, according to some physiologists, the 

chemical reactions which consume oxygen or produce carbon dioxide. 
Retina {ret' i no). The sensitive inner layer of the eye of vertebrates and some other 

Retractile. Capable of being withdrawn. 
Rhabdocoele {rab' do seel). A flatworm (Platyhelminthes) of the order Rhabdo- 

Rhinozoa (ri' no zo' a). A group of animals (literally, nose animals) in Oken's early 

classification. It comprised the reptiles. 
Rhizopoda (n zop' o da). A class of Protozoa having a form that is changeable 

through the production of pseudopodia; example. Amoeba. 
Rhynchocephalia (ring' ko se fa' U a) . An order of Reptiha, comprising only one 

living form, Sphenodon, of the New Zealand region. 
Riboflavin (ri' bo fla' vin). Vitamin B2, the preventive of scaliness of skin, tendency 

to cataract, etc. 
Rodent. A gnawing mammal, a member of the order Rodentia (rats, mice, squirrels, 

Rodentia. The order of mammals including the rodents (rats, mice, squirrels, etc.). 
Rotifera (ro tif er a). A group of animals (the rotifers) usually regarded as a separate 

phylum, but of uncertain position in the animal kingdom. For definition see 

Chap. 19. 

Sacculina (sak' ku W na). A degenerate crustacean, related to the barnacles, para- 
sitic on crabs. 

Sacral. Pertaining to the sacrum, the region between the hips. 

Sacrum. A group of vertebrae, more or less fused, in the region between the hips. 

Sagitta (so jit' ta). A marine animal of small size, sometimes called the arrowworm, 
but not a true worm at all. Its relationship to other animals is obscure. 

Salientia (sa' li en' shi a). An order of Amphibia including the tailless forms (frogs, 

Saliva. The fluid secreted by the salivary glands about the mouth. 

Salivary. Pertaining to saliva, the fluid secreted into the mouth in mammals. 

Salt. A compound, other than an acid or base, which in solution produces ions. 

Sarcolemma. The membrane surrotmding a striated muscle cell. 

Sarcoplasm. The protoplasm of a striated muscle cell, as distinguished from the 
enclosed myofibrils. 

Sargasso sea. A great eddy in an ocean, enclosing masses of seaweeds; with capital- 
ized initials the name may be limited to the eddy of the North Atlantic Ocean. 

Sargassum. A genus of seaweeds. 

Scaphiopus (ska fi' pus). A genus of spadefoot toads. 

Scaphites (.skaf i' ieez). A genus of extinct cepliMlopods of the ammonitic form. 

Scaphopoda (skaf op' da). A class of MoUusca in which the shell and mantle are 
tubular, as in Dentalium. 

Scapula. The shoulder blade; a bone of the pectoral girdle, located on or near the 
dorsal side of the body. 

Schleiden, Matthias (shli' den). German botanist, 1804-1881. 

Schultze, Max (shooW sa). German biologist and anatomist, 1825-1874. 

Schwann, Theodor (shvahn). German physiologist and anatomist, 1810-1882. 


Sciuridae (si u' ri dee). The family of rodents including the flying squirrels, squirrels, 

marmots, and chipmunks. 
Sciurinae (si' u ri' nee). The subfamily of Sciuridae comprising the marmots, 

squirrels, and cliipmunks. 
Sciuromorpha (si' u ro mor' fa). The suborder of rodents comprising the squirrellike 

Sciurus (si u' rus). The genus including the arboreal squirrels. 
Scolex. The enlarged attaching organ from which are budded off the proglottides 

of a tapeworm chain. 
Scyphozoa (si' fo zo' a). A class of Coelenterata, jellyfishes of large size which have 

no hydroid form in the Ufa cycle. 
Secondary. For application to spermatocytes, see spermatocyte. For application 

to oocytes, see oocyte. 
Secretin (se kre' tin). A substance produced in the small intestine and serving to 

stimulate secretion by the pancreas and liver. 
Secretion. The act of producing from the blood or other fluids or substances in the 

protoplasm some new material to be used in metabolism or otherwise. Also the 

new substance thus formed. 
Segmentation. Same as cleavage. 
Self-fertilize. To fertilize the eggs of an individual by spermatozoa of the same 

Semicircular canal. One of several curved tubes forming part of the inner division 

of the ear in vertebrates. 
Seminal receptacle. An organ in a female animal for the reception and storage of 

spermatozoa from the male. 
Seminal vesicle. One of several bodies closely connected with the testes in the earth- 
worm, in which a large part of the development of the spermatozoa takes place. 

Also, an enlargement in the vas deferens or similar duct in which spermatozoa 

may be stored in various animals. 
Semipermeable membrane. A membrane which allows some substances to pass 

through it, but retards or excludes others. 
Sensory. Pertaining to sensation; applied to a neuron which transmits an impulse 

resulting in sensation, or by extension to any other receiving neuron whether 

concerned with sensation or not. 
Septum. A partition. 

Series. The rocks, collectively, which belong to a geological epoch. 
Serum. The yellowish fluid which escapes from a blood clot; it is approximately 

the plasma without any fibrinogen. 
Sessile. Attached directly, as distinguished from stalked. Sometimes, also, 

attached, as distinguished from free-Uving. 
Seta (pi. setae, se' tee). A spine; specifically, one of the spines projecting from the 

somites of an earthworm and used for locomotion. 
Sex-linked. Associated with sex; said of hereditary characters the genes for which 

are in the X chromosomes associated with sex. 
Sexual. Involving the production of true germ cells, or the fusion of nuclei; said 

of reproduction, or of an individual employing such a mode of reproduction. 
Shoal. A shallow place in a body of water; also a sandbank or bar which makes the 

water shallow. 
Silurian. Of middle Paleozoic time, between Ordovician and Devonian. 
Sinus node. A mass of rather undifferentiated tissue in the right auricle of the heart 

which receives stimuU and initiates the heart beat. 


Siphon. A passageway for currents of water; as the clefts l)etweeii the lialvcs of 

the mantle of mussels where the edges do not meet, or the tulie on tlie Aeiitnil 

side of a scjuid or cuttlefish. 
Siphonophora {si' fo nof o ra). An order of Hydrozoa (C'oelenterata), the members 

of wliich form highly polymorphic colonies. Example, Physalia, the Portuguese 

Siphonops {si' fo nops). A genus of caecilians (Apoda, Amphibia). 
Siren. A genus of salamanders. 
Skeleton. A framework of hard parts serving for support, protection, or movement, 

or a combination of these functions, in animals. 
Slime tube. A sheath of mucous material secreted on the surface of an earthworm 

at the time of mating. 
Small intestine. That part of the intestine of vertebrates immediately following 

the stomach, as distinguished from the large intestine. 
Smooth muscle. Muscle composed of nonstriated, uninucleate, spindle-shaped cells. 

It is common in the intestine, bladder, and glands of vertebrates. 
Socrates {sok' ra teez). Greek philosopher who lived about 470-399 b.c. 
Solanum (so la' num). A genus of plants including the common potato, nightshade, 

and many others. 
Solution. A liquid containing another substance in the form of particles not greater 

than molecules in size. 
Soma. The body, as contrasted with the germ cells. 
Somatic. Pertaining to the body; when applied to cells, referring to the sterile bodj- 

cells in contrast to the germ cells which are reproductive. 
Somite. One of the segments into which the body of a worm or arthropod or othei- 

segmented animal is divided. 
Species {pL, species). A group of animals or plants so nearly alike that, in general. 

they might have sprung from the same parents. (The term is rather arbitrarily 

used, however.) 
Specific. Pertaining to a species. 

Sperm. One of the male germ cells in an animal or plant ; also called sperm cell. 
Spermary. See testis. 
Spermatheca. See seminal receptacle. 
Spermatid. One of the two cells formed by the second division in spermatogenesis. 

By transformation in shape the spermatids become mature spermatozoa. 
Spermatocyte {sper' ma to site'). A male germ cell between the beginning of sperma- 
togenesis and the second division in that process. A spermatocyte is called 

primary during the growth period and prior to the first division; secondary aft(>r 

the first division but ])rior to the second. 
Spermatogenesis {sper' iim to jen' e sife). The ripening of male germ cells. 
Spermatogonium {sper' nia to go' ni um)^ {pi, spermatogonia!. One of the early 

germ cclis of a male animal, prior to the Ix'giiuiing of spermatog(>nesis. 
Spermatophore {sper' ma to fore'). A mass of spermatozoa, sometimes resting upon 

a stalk or being otherwise attached, as in some salamanders. 
Spermatozoon {sper' ma to zo' on) {pL, spermatozoa). The male germ cell in animals. 
Sphenodon (sfen' o don). A genus of reptiles of tlie order Rhynchocephalia. Only 

on(; living species is known. 
Spheroid. Of nearly s])lierical shape. 
Spicule. A body of various shapes commonly of calcareous or siliceous material, 

forming part of the skeleton of a sponge. 
Spinal cord. That part of the central nervous system of vertebrate animals lying 

behind the brain and largely enclosed in a chaiuicl in the vertebrae. 


Spinal nerve. One of the paired nerves arising by two roots from the spinal cord. 
Spindle. A group of structures resembling threads, in the form of a spindle, formed 

in the cytoplasm of a cell during mitosis. 
Spiracle. In frog tadpoles, an opening through which water passes out of the gill 

(•hanil)er on one side. In insects, one of a inunber of openings on the sides of 

the body tlu'ough wliich air is introduced to or ejected from the tracheae. 
Spireme {spi' reem). The coiled or tangled thread formed by the chromatin of a cell 

prior to division. 
Spirostomum {spi ros' to nmm). A genu^ of ciliated protozoa. 
Splint. .\ bone at either side of the foot of the horse and some of its relatives, being 

the remnant of a lost toe. 
Spongilla. A genus of fresh-water sponges. 

Spongin. The horny material of the skeleton of the bath sponges. 
Spontaneous generation. Same as abiogenesis. 
Sporadic. Occurring at irregular intervals, often without apparent reason; said of 

migration of animals. 
Spore. One of a great variety of reproductive cells usually having protective cover- 
ings. Often the term is limited to asexual reproductive cells. The word is often 

compounded with qualifying prefixes or preceded by quahfying adjectives. 
Sporozoa. A class of protozoa, parasites usually without locomotor organs or mouth. 
Sporulation. The formation of spores; sometimes applied to multiple division of the 

nucleus followed by fragmentation of the cj'tosome, which occurs in the spore 

formation of certain species. 
Squamata (skwa ma' to). An order of reptiles to wliich the snakes, lizards, and 

cliameleons belong. 
Squamosal. A bone of the posterolateral region of the skull of vertebrates. In the 

mammals it suspends the lower jaw, but not in the other vertebrates. 
Squamous epithelium {skwa' ■rnus). Epithelium whose cells are low and flat. 
Statoblast (skit' o blast). A gemmulelike bodj' by means of wlrich many Bryozoa 

reproduce asexually. 
Steapsin {ste ap' sin). The fat-splitting enzyme of the pancreatic fluid. 
Stegodon (steg' o don). A genus of extinct animals, related to the elephants, from 

the Pliocene of southern Asia. 
Stegosaurus {steg' o saw' rus). A genus of dinosaurs bearing rows of plates set verti- 
cally on the back, belonging to Jurassic and Comanchean time. 
Stejneger, Leonhard {sti' ne ger). A living American herpetologist. 
Stentor. A genus of ciliated protozoa. 
Sternum. The breastbone; present in most vertebrates except fishes and some 

Stimulus. A change in the environment or some internal condition wliich produces a 

reaction in an organism. 
Stomach. An enlargement in the anterior part of the digestive tract of many animals; 

certain phases of the digestion of food occur there. 
Storeria. A genus of snakes. S. occipitomaculata {ok sip' i to mak' u la' ta); S. 

dekayi {de kay' i). 
Stratified. Arranged in strata or layers; said of epithelia, geological deposits, etc. 
Stratum (pL, strata). A laj^er; specifically, a layer of sedimentary rock. 
Stratum corneum. The thin outermost layer of cells in the skin of certain animals 

(as the frog). 
Striated muscle. Muscle composed of cylindrical, cross-banded, multinucleate cells 

(except in the heart). Skeletal muscles in vertebrates are of this kind. 
Striation. A stripe; as the crosslines of voluntary muscle cells. 


Stylonychia {sti' lo nik' i a). A genus of ciliated protozoa. 

Subepithelial cells. In Hydra, rounded cells lodged among the epithelial cells, often 

near the base of the latter. 
Sucker. An attaching organ beneath the head of a frog tadpole ; a similar organ on 

the scolex of a tapeworm colony; also the attacliing organ of leeches. 
Sucrose. Common table sugar, a disaccharide derived from cane or beets. 
Suctoria. A class of ciliated protozoa which bear no cilia when adult, but have tybe- 

like tentacles. 
Surface phenomena. A group of physical and chemical phenomena characteristic 

of surfaces (of cells, particles, fine pores, etc.) 
Sustentative {sus ten' ta tiv). Supporting; applied to connective tissue and other 

supporting tissues. 
Suture. The line of junction between a septum of a cephalopod shell and the outer 

wall of the shell. Also the immovable joint between two flattened bones, as 

those of the skull. 
Swammerdam, Jan {swahm' me?- dahm). Dutch naturalist, anatomist, and ento- 
mologist, 1637-1680. 
Sweat gland. One of the excretory organs of the skin. 
Sylvius, Jacques Dubois. French anatomist, 1478-1555. 
Symbiosis {sim' hi o' sis). The association of two species of animals for their mutual 

Symbiotic. Of the nature of symbiosis. 
Symmetry. The state of being symmetrical, or capable of being divided by a hne or 

plane into two parts which are mirrored images of each other. 
Sympathin. A substance produced by nerve endings of the thoracolumbar sj^stem 

and serving to inhibit certain organs, stimulate others. 
Synapse (sin aps'). The point of contact of two neurons. 
Synapsis (sin ap' sis). The pairing of maternal with paternal chromosomes earlj- in 

the maturation of the germ cells. 
Synapta. A genus of sea cucumbers. 
Syncytium (sin sish' i urn). An undivided mass of protoplasm containing several or 

many nuclei. 
Synonym (sin' o nim). A taxonomic name which is rejected because it is a duplicate. 
Synura. A genus of colonial flagellate protozoa. 

System. A collection of organs concerned with the same general function, as diges- 
tion. Also, the rocks, collectively, which belong to a geological period. 
Systematic botany. See taxonomy. 
Systematic zoology. See taxonomy. 
Systemic circulation. The circulation of the blood through the body in general, as 

distinguished from that through the lungs or limgs and skin (pulmonary or 

pulmocutaneous) . 

Tadpole. The larva of a frog, or certain other animals. 

Tail. A slender posterior appendage. In a spermatozoon, the whiplike propelling 

organ behind the head and mid-piece. 
Tamiasciurus (ta' mi a .sr n' rus). The subgenus of the genus Sciurus including the 

red squirrels. Sciurus (Tamiasciurus) hudsonicus loquax {hud son' i kus lo'- 

kwaks), the southern Hudsonian red squirrel. 
Tarsal. One of a number of bones in the ankle of most vertebrate animals. 
Tarsometatarsus (tar' so met' a tar' sus). A compound bone in the leg of a l)ini, 

formed of several of the metatarsals and tarsals. 
Taxonomy. The science of the classification of animals or plants. 


Teleostomi {te' le os' to mi). A subclass of Pisces comprising the true fishes. They 

have a skeleton partly or wholly of bone and respire by means of gills. 
Telolecithal {teV o les' i thai). Containing much yolk, crowded toward the vegetative 

pole; said of eggs. 
Telophase {tel' o faze). The final phase of mitotic cell division, in which the nuclei 

are reconstructed. 
Tentacle. One of a number of armlike projections from hydroids, Bryozoa, Nautilus, 

and other animals. Also one of certain elongated individuals of a siphonophore 

Termite. One of an order of insects called "white ants," but not really ants. 
Terrapene iter' a pee' nee). A genus of turtles of the family Testudinidae. 
Terrigenous {ter rij' e nus). Derived from the land; as applied to lake bottoms, 

composed of material washed in from the land, as distinguished from material of 

organic origin. 
Tertiary {ter' shi a' ri). The single period of Cenozoic time. 
Test. A hard outer covering, capsule, or shell; as of a sea urchin. 
Testis. The organ in which the male germ cells are lodged and developed. 
Testosterone {les ios' ter one). A hormone produced by the interstitial cells of the 

testis; it controls development of secondary sexual characters and sex behavior. 
Testudinata {tes tu' di na' ta). An order of reptiles, comprising the turtles. 
Testudinidae {les' tu din' i dee). A family of turtles. 
Tetrad. A quadruple body formed, during the growth period in the ripening of 

germ cells, from the union of two chromosomes which at the same time become 

Thales {Iha' leez). Greek philosopher and astronomer who lived about 640-546 B.C. 
Thamnophis {Iham' no fis) . A genus of garter snakes. T. butleri {but'leri); T. 

proximus {proks' i runs); T. sackeni {sak' en i); T. sauritus {saw ri' tus). 
Theophrastus {the' o fras' tus) . Greek philosopher, founder of botany, who lived 

about 372-287 b.c. 
Thermocline. A layer of water in a lake in which the temperature falls at least 1°C. 

for each additional meter of depth. 
Thiamin {thi' a min). Vitamin Bi, the preventive of polyneuritis or beriberi. 
Thoracic. Pertaining to the thorax or chest. 
Thoracolumbar system. That part of the autonomic nervous system which centers 

in the middle portion of the spinal cord. Each organ controlled by the auto- 
nomic system is innervated once from it. 
Thorax. A middle portion of the body of many animals, between head and abdomen. 
Thricozoa {Ihrik' o zo' a). A class of animals (hair animals) in Oken's early classifi- 
cation. It comprised the mammals which Oken also called Ophthalmozoa. 
Thrombase. An enzyme which brings about the conversion of fibrinogen into fibrin 

in the clotting of the blood. 
Thromboplastin. A substance which converts prothrombase into thrombase in the 

clotting of the blood ; it is found in blood platelets and many cells. 
Thymus. A ductless gland located near the gill clefts, or in the neck, or in the anterior 

part of the thorax in vaiious vertebrates. 
Thyroid. A ductless gland located in the ventral part of the pharynx. 
Thyroxin {thi roks' in). The hormone of the thyroid gland. 
Tibia. The inner one of the two bones in the lower leg of vertebrates, except the 

Tibiofibula. The fused tibia and fibula of some Amphibia. 
Tibiotarsus. A compound bone in the leg of a bird, formed of the tibia and certain 

of the tarsal bones. 


Tissue. A group of cells of similar structure forming a contiiuious mass or 

a-tocopherol (aV Ja to kof er ole). Vitamin E, the antisterility vitamin of rats. 
Tonsil. A glandular organ at the side of the throat. 
Trachea {tra' ke a). The tube conveying air to and from the lungs in vertebrates. 

Also an air tube in insects and some other invertebrates. 
Tracheal gills. Threadlike or leaflike projections in which tracheae have their l)egii^- 

ning in certain aquatic insect larvae. 
Trachelocerca {tra' ke lo ser' ka). A genus of ciliated protozoa. 
Transverse process. One of a pair of projections at the sides of a vertebra in most 

vertebrate animals. 
Trematoda {trem' a to' da). A class of Platyhelminthes, parasitic flatworms with 

suckers and without cilia. 
Triassic. Of the earliest Mesozoic time. 
Triceratops {tri ser' a tops)^ A genus of three-horned dinosaurs of late Cretaceous 

time in western North America. 
Trichinella (trik' i neV la). A genus of parasitic roundworms, the cause of the disease 

Triclad. Having the digestive tract divided into three branches; said of an order of 

Trilobite (tri' lo bite). A primitive crustacean of Paleozoic time, having the body 

partially divided by longitudinal grooves into three lobes. 
Trilophodon {tri lof o don). An extinct genus of animals from the Miocene of several 

continents; related to the elephants. 
Trionychidae (tri' o nik' i dee). A family of turtles. 

Triploblastic (trip' lo bias' tik). Composed of three fundamental layers of cells. 
Triturus. A genus of salamanders. 
Trochophore. A form of free-swimming larva characteristic of many worms, mol- 

lusks, and rotifers. 
Trypsin. A protein-splitting enzyme produced by the pancreas. 
Trypsinogen (trip sin' o jen). The inactive substance from which the enzyme trypsin 

is produced. 
Tube feet. Tubular protusions from the arms of echinoderms, which serve as organs 

of locomotion. 
Tubercula pubertatis (tu ber' ku la pii' ber ta' lis). Two thick glandular ridges on the 

clitellum of an earthworm near the ventral surface. 
Tuberculate. Bearing cusps or conical prominences; said of teeth. 
Tubular gland. A gland whose lumen is of about uniform bore throughout. 
Tunicata (tu' ni ka' ta). A subphylum of Chordata, including the sea squirts, s(>a 

pork, salpas, etc. For definition see Chap. 19. 
Turbellaria (tur' bcl la' ri a). A class of Platyhelminthes, ciliated flatworms leading 

a free existence. 
Type. In systematic zoology, an individual or group which is formally held to Im> 

typical of the species or larger group to which it belongs; as, the type specimen of 

a species, the type species of a genus, or the type geims of a family. 
Typhlosole (tif lo sole). A ridge resulting from the infolding of the dorsal intestinal 

wall of the earthworm. 

Ulna. The bone of the little-finger side of the forearm in Anipliibia and tlu^ higher 

Umbilical cord. A ropelike cord in which blood vessels pass bet\v«!en an embryo and 

the placenta in viviparous mammals. 


Unconformity. A sharp contrast, often a lack of parallelism, between adjoining rock 
strata, cansod by a long period of erosion. 

Uniformitarianism. The doctrine that geological processes of the past were similar 
to those of the present time. 

Unisexual. Involving but one sex, the female; applied to parthenogenetic reproduc- 

Unit character. A hereditary trait that behaves as a unit in transmission, being 
capable of inheritance independently of other luiit characters. 

Universal symmetry. An arrangement of the parts of an object or organism such 
that it is capalsle of being divided into symmetrical halves by an infinite number 
of planes passing in any direction through a central point. 

Urea (u re' a). A substance, C0(NH-))2, produced by the decomposition of proteins 
and some other substance in organisms. 

Ureter (it re' ter). A tube conducting urine awaj' from the kidney. 

Urethra {u re' thra). The duct by wliich urine is discharged from the bladder. 

Urinary bladder. A bag in which urine is stored. 

Urine. The liquid waste excreted bj^ kidneys. 

Uriniferous tubule. One of the many coiled tubes making up the bulk of the kidney 
in vertebrates. 

Urinogenital system. A group of organs concerned with both excretion and reproduc- 
tion in vertebrates. 

Uterus («' te rus). A modified portion of the oviduct in which the eggs undergo at 
least part of their development. Strictly the term uterus is apphcable only in 
animals in which the developing embryo becomes attached to the wall of the 

Vacuole. A region within a cell occupied by a liquid other than protoplasm, usually 
water with various substances in solution. 

vVagina. The passage leading from the uterus to the exterior in many animals. 

Valence. A measure of the mmiber of other elements or radicals with which a given 
element or radical may combine; it is determined by the number of electrons in 
the outer layer. 

Variety. In taxonomy, a division of a species; a group of individuals within a species 
tliat differ in some minor respect from the rest of the species. 

Vascular tissue. Blood or lymph, or the more liquid parts of blood-producing organs. 

Vas deferens (vas' def er enz) (pi., vasa deferentia, vas' a def er en' shi a). A duct 
conveying spermatozoa from the testis to the exterior. 

Vas efferens {vas'' ef fer enz) {pi., vasa efferentia, vas' a ef fer en' shi a). One of a 
number of minute tubes leading away from a testis, serving to convey the sper- 
matozoa. They lead into a larger tube called in many cases the vas deferens. 

Vaucheria {vaw ke' ri a). A multinucleate fresh-water alga. 

Vegetative. Concerned with nutrition. When applied to an egg, meaning that side 
near wliich the yolk is accumulated (vegetative pole). 

Vein. A vessel conveying toward the heart blood which has already traversed capil- 
laries since leaving the heart. 

Ventral. Literally, pertaining to the belly; hence, usually, lower. 

Ventricle. The posterior chamber of the heart in fishes, amphibia, and some reptiles, 
and one of the two posterior chambers in higher vertebrates. Its function is the 
propulsion of the blood through the main arteries and connecting vessels. 

Vermiform appendix. A narrow blind pouch forming a prolongation of the caecum. 

Vertebrata. A subphylum of the phylum Chordata, comprising the backboned 
animals. For definition see Chap. 19. 


Vertebrate, adj. Possessing a backbone, n. An animal having a backbone. 

Vesalius {ve sa' li us). Belgian anatomist and court physician, 1514-1564. 

Villus {pi., villi). One of the fingerlike projections from the inner surface of the small 

Virchow, Rudolf {veer' no). German physiologist and pathologist, 1821-1902. 
Visceral. Pertaining to the viscera, or organs contained in some large cavity of the 

body ; applied in the vertebrates chiefly to the organs of the abdomen, in clams to 

the digestive organs and glands above the foot. 
Viscosity. The resistance offered by a substance to the relative movement of its 

Visual purple. A light-sensitive pigment in the retina. 
Vitamin. One of several substances common in leafy vegetables, animal fats, and 

elsewhere, which are necessary for specific aspects of metabohsm in animals. 
Viviparity {viv' i par' i ti). The condition of being viviparous. 
Viviparous {vi vip' a rus). Producing young from eggs that are retained in the uterus 

of the mother, with the aid of nutrition derived from the mother through a 

placenta and umbilical cord. 
Volvox. A small spherical organism composed of flagellated green cells embedded in 

jelly, in a single layer around a liquid interior. Sometimes regarded as an animal, 

though more properly included among plants. 
Vorticella. A cihated protozoon attached to a contractile stalk. 

Wallace, Alfred Russel. EngUsh naturalist, 1823-1913. 

X body. An object in the cytosome of some of the early cleavage cells of Sagitta, 

which marks the germ cells. 
X chromosome. A chromosome closely associated with the determination of sex. In 

many animals the female has two of them, the male only one. 
Xenophanes (ze nof a neez). Greek philosopher who lived about 570-480 b.c. 
Xerophthalmia {ze' rof thai' mi a). A dry, lusterless condition of the eyeball. 

Y chromosome. A chromosome possessed only by the males of many species. It 
behaves in spermatogenesis much as if it were homologous with the X 

Yolk plug. The remnant of the vegetative cells last to be drawn into the interior of a 
gastrula in certain embryos. 

Zoogeography. The branch of zoology treating of the geographical distribution of 

Zooid. One of the members of a hydroid or siphonophore colony. Often, in a 

restricted sense, a particular kind of individual, as a hydranth. 
Zoology. The science of animals. 
Zygapophysis {zi' ga pof i sis). One of four short projections, two in front and two 

behind, extending from the upper portion of a vertebra. Those of the posterior 

pair articulate with the anterior pair of the vertebra next behind. 
Zygote, A cell or individual produced by the fusion of two cells or their nuclei in 

the process of sexual reproduction. 


The following list of 16-mm. motion pictures and 35-mm. filmstrips 
can be used to supplement some of the material in this book. These 
visual aids can be obtained from the producer or distributor shown with 
each title. (The addresses of these producers or distributors are listed 
at the end of the bibliography.) In many cases these visual aids can 
also be procured from your local film library or local film distributor. 

The running time (min) of the film and whether it is silent (si) or 
sound (sd), filmstrip (FS) or color (C) are listed with each title. All 
those not listed as color are black and white. 

Each film has been listed only once, usually in the first chapter to 
which it is applicable. However, in many cases it can be used advan- 
tageously in several of the other chapters. 


Eyes of Science (Bausch & Lomb 45min si). — Shows Galileo with his 
early telescope; Leeuwenhoek and his simple microscope; and today's 
lenses and tubes that have given scientists the "super eye." 

Marvels of the Microscope (Gut lOmin sd). — Microscopic studies of 
water plants and minute forms of animal hfe. 

Unseen Worlds (Ganz lOmin sd). — Explains the intricacies of the 
newly developed electron microscope. 


Chemical Reactions (Brandon 20min sd). — Explains the composition 
of an atom; relationship between nucleus and electrons; chemical reac- 

Electrons (EBF 11 min sd). — Shows phenomena associated with con- 
duction of electricity in liquids, gases, and vacuums. 

Molecular Theory of Matter (EBF 11 min sd). — Molecular hypothesis 
illustrated by animation; behavior of molecules in various conditions; 
Brownian movement. 


Protoplasm — the Beginning of Life (Bray ISmin si). — Protoplasm 
shown in characteristic motion in one-celled and many-celled hosts. 

Living Cell (EBF 15min si). — Shows single-celled organisms and many- 
celled organisms under the microscope. 



Green Plant (EBF ISmin si). — ^Shows that living things are dependent 
for food upon the green plant; the latter's processes of foodmaking and 
growth are diagramed. 

Nitrogen Cycle (EBF 15min si). — Shows how nitrogen compounds 
serve as a key to the transfer of energy in nature and how animals depend 
upon plants for food. 


Body Framework (EBF 15min si). — Function of skeleton; structure, 
chemical composition, growth and repair of bones; main types of joints. 

Muscles (EBF 15min si). — The structure and use of muscles are 


Digestion (EBF 15min si).— Covers complete digestive tract. 

Digestion of Foods (EBF lOmin sd). — A summary of the digestive 
process; show^s relation of circulatory and nervous systems to the diges- 
tive process. 

Alimentary Tract (EBF llmin sd). — Treats in detail motihty phe- 
nomena of the gastrointestinal tract by means of actual photography. 


Breathing (EBF 15min si). — Explains action of diaphragm, breathing, 
and lung structure and function. 

Mechanisms of Breathing (EBF lOmin sd). — The breathing mecha- 
nism in operation. 


Blood (EBF 12min si). — Illustrates the separation of plasma from 
]>l()()d cells, protein and salts from plasma, etc. 

Circulation (EBF ISmin si). — Traces circulatory system; ('omi)ares 
liuman heart with that of the frog. 

Control of Small Blood Vessels (Lutz 2()min si). — Illustrates both the 
sti-uctural and physiological features of the arterioles, precapillaries and 

Heart and Circulation (l^^^BF lOmin sd). — Detailed explanation of the 
mechanics of the pulmonary and systemic systems. 


Work of the Kidney (l*]IU<' liinin sd). ^Detailed exposition of the 
kidneys and their functions. 



Reactions in Plants and Animals (Harvard llmin sd). — Characterizes 
the concepts of stimiikis and reaction and presents a study of different 
types of reactions in plants and animals. 

Nervous System (Brandon 150min si). — Study of development of 
nei'vous system with special sections on development of early nervous 
systems; reflex actions; spinal cord; the brain; conditioned reflexes and 

Nervous System (EBF lOmin sd). — Shows structure of the nervous 
system; nerve impulse. 


How Animal Life Begins (NYU llmin sd). — The fundamentals of 
reproduction in the rabbit; cell growth involved in animal reproduction 
is used to illustrate human reproductive processes. 

In the Beginning (USD A 17min sd). — Prologue to life, shows ovula- 
tion, fertilization, and early development of mammalian egg. 

Reproduction among Mammals (EBF lOmin sd). — Presents the story 
of embryology, using the domestic pig. 

Reproduction in Plants and Lower Animals (B&H 15min si). — Shows 
the process of fertilization, conjugation, and mitosis as well as reproduc- 
tion by budding. 


Development of Bird Embryo (EBF 15min si). — Shows development 
from early stages to hatching. 

Frog (EBF lOmin sd). — Portrays life cycle of the frog and develop- 
ment of the embryo. 

Salamanders and Their Young (Rvitgers 15min si). — Shows the habi- 
tat and l^reeding of salamanders. 

Snapping Turtle (EBF llmin sd). — Presents complete hfe story of 
this reptile. 

Insects : Their Growth and Structure (USDA FS). — Shows types and 
kinds of insects; external anatomy; internal anatomy; and life cycles. 

Heredity (EBF lOmin sd). — ]\Iendelian laws of inheritance presented. 


Animal Life (Harvard lOmin sd). — A review of the main types of ani- 
mals: protozoans, sponges, coelenterates, echinoderms, worms, molluscs, 
crustaceans, insects, and vertebrates. 


Parade of Invertebrates I-IV (Rutgers lOmin si C). — In four reels; 
shows numerous types of invertebrates. 

Microscopic Animal Life (EBF 15min si). — Shows four single-celled 
animals and one multicellular animal. 

Marine Communities (Rutgers 15min si C). — Shows how many types 
of undersea life associate in communities. 

One-celled Animals (EBF 15min si). — An excellent study of the 


Lost World (EBF 15min si). — Exhibition in motion of extinct pre- 
historic animals recreated in full-scale, hfelike models. 

History of Horse in North America (Cal 20min sd). — Demonstrates 
advancement of the horse in fifty million years. 

Evolution (Gut 30min sd). — Presentation of theories of the origin and 
development of the earth and its living inhabitants; prehistoric animals. 

Monkey into Man (NYU 20min sd). — A study of monkey Ufe showing 
family and social life and variation in brain power among them; compares 
most intelligent of apes and man with a brief review of man's develop- 
ment from the primitive stages to modern evolution. 

Fingers and Thumbs (NYU 20 min sd). — Traces the development of 
man's hands; evolution from earhest form of life to the ape is clearly 
depicted as well as the actual development of the hand in the ape family. 


Bausch & Lomb Optical Co., 635 St. Paul St., Rochester, N.Y. 

B&H — Bell & Howell Company, 1801 Larchmont Ave., Chicago. 

Brandon Films, Incorporated, 1600 Broadway, New York 19 

Bray Pictures Corporation, 729 Seventh Ave., New York 

Cal — University of California, Extension Division, Department of Visual 

Instruction, 301 California Hall, Berkeley, Calif. 
EBF — Encyclopaedia Britannica Films, 1841 Broadway, New York 17 
Ganz, Wilham J. Company, 40 E. 49th St., New York 
Gut— Gutlohn, Walter O. Inc., 25 W. 45th St., New York 19 
Harvard Film Service, School of Education, Lawrence Hall No. 4, Cam- 
bridge 38, Mass. 
Lutz, Brenton R., 088 Boylston St., Boston, Mass. 
NYU — New York University Film Library, Washington Square, New 

Rutgers University, Box 78, New lirunswick, N.J. 

USDA — U. S. Department of Agriculture, Motion Pictui-c Division, 
Washington, D.C. 


Boldface numbers refer to pages bearing illustrations of items indexed. 

Abiogenesis, 159 
Absorption, 106 

of food, 104 
Acanthocephala, 275 
Accidental dispersal, 320 
AcetabuliuTi, 92, 93 
Acetylcholine, 148 
Acids, 35, 36 
Actinomorphes, 246 

Adaptation, as quality of whole organism, 

resulting from natural selection, 363- 

taxonomic ranks showing, 363, 364 
Adipose tissue, 82 
Adrenal glands, 153, 154, 155 
Adrenalin, 155 
Adsorption, 45 
Afferent neuron, 144, 145 
"Age and Area" hypothesis, 310 
Age of earth, 331 

in geological periods, 331 
Alecithal egg, 199 

cleavage of, 201 
AU-or-none rule, 53 
Altricial birds, 190 
Alveolar gland, 84, 85 
Alveoli, 114, 115 
Amino acids, 41 
Amitosis, 62 
Ammonite, 332 

sutures of, 343 
Amoeba, 24, 51, 260 

food vacuole of, 101 
Amphibia, 277, 279 

evolution of, 335 
Amphicoelous vertebrae, 92 
Amphineura, 271, 272 
Amphioxus, 276 
Amylopsin, 105 
Anabolism, 46 

Analogy, 251, 252 

Anaphase, 56, 58 

Anatomy, comparative, 13-14 

Anaximander, 2 

Animal communities, 290-291 

relation of, to vegetation, 305, 306 
Animal pole, 199 
Anisogamete, 161 
Annehda, 269-270 
Anthophysa, spheroid and dendritic 

colony of, 68 
Anthozoa, 263, 264 
Anus, of earthworm, 101, 102 

in embryo, 208, 210, 212 

in vertebrate, 103 
Apathetic animals (Lamarck), 246 
Appendicular skeleton, 90, 92, 93 
Arachnida, 273, 274, 275 
Archenteron, 73, 204, 206 
Archeozoic, 330 
Archiannelida, 270 
Aristotle, 3, 4, 5, 6 
Arms, homology of, 253 
Arteries, 122, 123, 129 
Arterioles, 129 
Arthropoda, 272-275 
Artiomorphes, 246 
Ascaris, chromosomes in, 59 

cleavage of, 203 
Ascorbic acid, 110 
Asexual reproduction, 159, 169-174, 176 

of Paramecium, 169 
Assimilation, 46. 49 
Association neuron, 145, 146 
Associations, 290-291 

in vegetated areas, 305, 306 
Asteroidea, 268 
Asymmetry, 78, 79 
Atolls, 303 
Atom, 31-33 

"Atomic" theory of the universe, 3 
Auricle, 123, 124 
Auriculoventricular bundles, 128 




Auriculoventricular node, 128 

Autonomic n(>rvou« system, 143, 146, 147 

Autosomal luikage, 236, 237 

Autosome, 236 

Aves, 279 

Axial skeleton, 90, 91 

Axon, 143, 144 


Backcross, 227 

two-pair, 230, 231 
Balanoglossus, 276 
Barnacle, 300 

homology of, with Sacculina, 350, 361 
Bases, 35, 3(i 
B complex, 111 

Beagh\ Darwin's voyage on the, 360 
Beetle, 272 
Beriberi, 111 
Bidder's canal, 136, 178 
Bilateral symmetry, 77, 78 
Bile, 103, 105 
Bile duct, 103 

iji embryo, 211, 212 
Biogenetic law. 74, 255, 256, 257 
Biology, defined, 1 

history of, 1-20 

scope of, 20-