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EMBRYOLOGY
THE BEGINNINGS OF LIFE
BY GERALD LEIGHTON, M.D., F.R.S.E.
AUTHOR OF "THE GREATEST LIFE," "BRITISH SERPENTS"
"HUXLEY: HIS LIFE AND WORK," ETC.
LONDON: T. C. & E. C. JACK
67 LONG ACRE, W.C., AND EDINBURGH
NEW YORK: DODGE PUBLISHING CO.
* * * * *
CONTENTS
CHAP. PAGE
I. THE CELL AND THE INDIVIDUAL 7
II. PROBLEMS OF REPRODUCTION 15
III. PROBLEMS OF REPRODUCTION (_continued_) 23
IV. THE MAKING OF A MAN 36
V. FERTILISATION AND EARLY DEVELOPMENT 47
VI. EARLY DEVELOPMENT 53
VII. THE BEGINNINGS OF THINGS 59
VIII. THE BEGINNINGS OF THINGS (_continued_) 62
IX. THE BEGINNINGS OF THINGS (_continued_) 66
X. THE BEGINNINGS OF THINGS (_continued_) 73
XI. HOW THE EMBRYO IS NOURISHED 78
XII. RECAPITULATION 84
BIBLIOGRAPHY 90
INDEX 91
* * * * *
EMBRYOLOGY
CHAPTER I
THE CELL AND THE INDIVIDUAL
What is Embryology, and what is its significance or interest to the
ordinary educated man and woman? The answer to the question is the
justification for the appearance of the following pages, and one may
regard it as a somewhat striking fact, that in the production of a
series of works of which this volume is one, those responsible for the
subjects should have deemed it advisable to include Embryology.
Embryology may be defined as that part of the science of Biology which
deals with the formation of a new individual or embryo. The definition
itself ought to be sufficient to explain the significance of the subject
for every one, because one can hardly conceive of any more profoundly
important knowledge than that which tells of the mode of origin, manner
of growth, and ultimate birth of an entirely new being. In the absence
of such accurate knowledge it is quite obvious that all one's ideas
concerning the manner in which the new individual is to be treated must
have a more or less haphazard, or at least empirical, basis. In fact
only when the science of Embryology, or the development of the
individual, becomes a part of the ordinary everyday mental equipment of
those who are responsible for bringing into the world new individuals,
and subsequently protecting and handling them, will it be reasonable to
expect that these new individuals are dealt with in the best possible
manner. In a word it is evident that education, using that term in the
very widest possible sense, can never be anything more than a blind
groping in the dark until those into whose hands it is entrusted realise
and know at least the most important fundamental facts concerning
development. It is lack of this kind of knowledge which has been
responsible for so much of the mistaken systems of the past in dealing
with the young, and it is the spread of this knowledge which alone is
the hope of better things in the future. Wherever knowledge is absent
superstition is rife, and in no sphere of life is this more painfully
obvious than in connection with the subject which we are about to study.
It would have been entirely impossible for many of the stupid and even
cruel methods of mental and physical treatment which have been meted out
to the young children in the past to have been tolerated for a moment
had this knowledge been available and sufficiently widespread.
Possessing it, a flood of light is thrown upon the fascinating and
otherwise obscure problems of heredity; and thus it lays open the pages
of the past for those who care to read them. Possessing it also it
throws upon the mental screen pictures of possibilities in the future
for all those who have eyes to see. So the study of Embryology links up
the past with the present and joins the present with the future. Is it
not, therefore, obvious that the study of such a subject means dealing
with problems the importance of which it is impossible to exaggerate;
problems which the parent, the teacher, the social reformer, the
politician, and the philanthropist will grapple with in vain unless they
call in science to their aid? Such is the meaning and significance of
the subject of our study.
In the widest sense of the word Embryology, therefore, deals with all
manner of living things, be they plant or animal. But since our purpose
here is to state, as far as possible in the space at our disposal, the
facts which are of particular importance in relation to the human
subject, we shall only glance at the rest of living creatures. A brief
look at them, however, is quite necessary in order to appreciate what
follows. Let us be quite clear of what we are in search. We want to know
as far as possible what it is that goes to the making of a man. What is
the origin of the new individual? Where does the embryo come from? What
elements are concerned in its formation? Where do these elements come
from? How are they subsequently built up into the type of the species to
which they belong? From what source do they gain their nourishment? What
influences of a degenerative nature are likely to affect them? These are
the questions which it is the business of the Embryologist to answer,
and these are the questions the answers to which afford the explanation
of man in the making. Surely they merely require to be stated that their
significance may be appreciated.
We may now glance very briefly at the simplest facts which bear upon the
subject, and which must precede our detailed study. The necessity for
reproduction and development is involved in the universal fact of
death. In all except the very simplest forms of life--those consisting
of one simple mass of protoplasm--the individual sooner or later
perishes, and if it were not that there were some methods by means of
which the individuals could give rise to new individuals obviously the
species would come to an end. No matter to what great age an individual
animal may live, and there are some such as the tortoises which do live
for centuries, sooner or later death overtakes them, and in all,
investigation of their structure shows that nature has made provision
for the carrying on of the race by means of new individuals.
Every living creature, be that creature simple or complicated, animal or
vegetable, man or a jellyfish, starts life as one single cell. The very
simplest living individuals never consist of anything else but one
single cell, and it is in these primitive forms of life alone that what
we call death can not be said to occur. Such a simple cell, after living
for a certain period, simply divides itself into two halves, each of
which gradually assumes the size and shape of what we may term the
parent cell. The first individual has simply become two separate
individuals. These two in their turn after another period of independent
existence, again each divide, thus giving rise to four, and so on. Now
here, although the original parent cell no longer exists _as a cell_,
the actual material of which it was composed still exists in the cells
which came into existence as the result of this division. The original
cell, therefore, may be literally said to have been deathless, or
immortal, though not everlasting. This is a profound thought, and one
which must be grasped at the very commencement of our study of
development, because it is one to which we shall have to recur again and
again when we come to study the cells which give rise to human beings,
in whom, too, there is a deathless continuity of cell protoplasm, or
germ-plasm as it is then called. It is upon this fact that the whole
science of Embryology depends.
The important idea to be learned from observing this process of
reproduction in the single-celled animal is this: that there is nothing
here which we may term the body of an animal as opposed to any of its
parts. The one cell is both body and organs, and everything else; in
itself it has the capacity of performing all the functions necessary for
life, including that of reproduction for the perpetuation of the
species. No part of the cell is set on one side for any special purpose
such as happens in the bodies of higher animals. There are no special
elements which go to the producing of the next generation, none of the
cells which in a mammal, for example, we call "germ-cells." The whole
individual is one cell. In fact one might almost say that there is no
individual, but only race, or if we regard the cell as an individual
then it is all germ-plasm. That is the important fact to be learned in
the reproduction of single cells.
There are some single cells, such as those of the yeast, which reproduce
in a slightly different manner, namely, by budding off a portion of
themselves and finally becoming separate, and this might be regarded as
a slightly higher stage, in so far as the original cell from which the
bud came may be still identified; but in reality the process differs
very little from that first described.
Then we may note that very low in the scale of living things there is a
process of reproduction known as conjugation, in which, although the
cells of the species appear to be all alike, yet, nevertheless, two of
them join together for purposes of reproduction. In other words we have
here a process of cell-union before we have the cell-division which
follows. It is important to note at this stage that the creatures which
we have mentioned, and even some more highly organised, such as an
am[oe]ba, which has a nucleus, go through these simple or complicated
reproductive processes in the total absence of anything which could
suggest a distinction of sex. In these cases the individuals are
obviously all of one sex, and, therefore, the distinction of sexes into
male and female is evidently something which has been added later in the
scheme of evolution, not for the purpose of reproduction itself, but for
something which is to be added to that.
Then in the slightly higher animals and plants we come to those in which
many cells go to the making of the individual, the multicellular
individuals, and amongst these we very soon see the origin of what is
termed specialisation of function. That is to say, in these higher
creatures which consist of many numbers of cells arranged so as to form
one individual, certain cells are set apart for one purpose and others
for another. Some may be for digestion, some for purposes of movement,
and others for reproduction. Here we have a new phenomenon, namely, the
setting aside of certain cells in a multicellular individual which from
the very beginning are capable of one function alone, namely,
reproducing the species. The higher one goes in the scale of life the
more striking and obvious this fact becomes, and as we shall see when
we come to the vertebrate kingdom, this setting aside of the cells which
are to produce the individuals of the next generation is the key to the
solution of the most difficult of our problems.
In these highest forms of life, however, the cell itself is becoming a
much more complicated thing than that lowly form which we first noted as
dividing into two to form two new individuals. Indeed, the cells in the
highest animals and plants are immensely complicated in their structures
and functions, and especially in connection with the changes which take
place in the nucleus of such cells. Not only the nucleus but another
small object within the cell which is neither part of the nucleus nor
part of the cell protoplasm, also is very important, and this structure
is termed the "centrosome." In fact this little body apparently begins
the whole process of cell-division by itself dividing into two parts.
Then the nucleus follows suit, and ultimately the whole cell divides.
The nucleus itself is a complicated structure, as is especially seen
during the processes of division, in which it breaks itself up into a
number of thread-like portions, and the number of these is always the
same in any given species, a fact which is of great importance in
reproduction. Why do we mention these apparently dry details? Because in
these minute and complicated nuclear movements the whole problems which
are at the bottom of development and heredity lie. The problems of life
itself can only be solved by the study of what takes place in these
minute portions of cells. It is here that the new formation of an
individual begins, and although it is no part of our purpose here to
detail all the complicated processes of nuclear division, it is
essential, in order to grasp the meaning of our subject, that we should
realize that in the changes within the cell life with its variations
begins.
The study of these wonderful cell processes, a work which demands the
most patient investigation and high technical skill, has reached such a
stage that it is a science of its own, and is called the science of
"Cytology," or the science of cells, which has been made possible only
in comparatively recent years by the invention of microscopes having
great powers of magnification, and by the application of elaborate
methods of staining to the cells themselves.
We can say no more about these processes here, but the foregoing
paragraphs may perhaps be sufficient to show us how important it is to
grasp these simple facts of cell life in their bearing upon development
itself.