RECAPITULATION
In bringing our study of Embryology to a close, we may glance briefly at
another aspect of the subject, namely, that which emphasizes the fact
that in its development the embryo recapitulates the history of its
ancestors.
It is quite obvious that the offspring of any species of animal, if they
are to live and survive in the same kind of environment as that in which
their parents live, must resemble them somewhat closely. The only way in
which Nature can secure such a sufficiently close resemblance of
offspring to parents is by insuring that they should develop along
similar lines. So it is that we find that the whole of the life history
of an individual is more or less a recapitulation, with, of course,
variations, of that of the parents and ancestors. Each successive step
from the very beginning of the fertilisation of the ovum repeats a stage
through which previous generations have passed. If from any accident a
step in this recapitulation is omitted, the embryo is to that extent
deprived of some feature possessed by a parent or ancestor; and if this
be a sufficiently important omission, it is impossible for such an
embryo to survive. That is one way in which an embryo may differ from
its parents. That is a retrogressive change. On the other hand, such an
embryo, in addition to recapitulating the stages through which its
parents passed in development, may have something new added, something
which appears for the first time. In other words a progressive variation
may appear.
Now, since the embryo follows the same developmental track as did the
parent, passing through the successive stages of germ-cells, fertilised
ovum, embryo, f[oe]tus, infant, child, youth, and adult, it follows
that should it exhibit any additional peculiarity, unnoted in the
parent, the embryo has obviously varied progressively. That is to say,
it has pursued the same line of development together with some new
addition. On the other hand, should the offspring at any of these stages
in its career be obviously without some of the characters of previous
generations, it is as certainly due to the fact that the recapitulation
of the history of development has been, in that particular, incomplete.
In all successive cases of multicellular organisms, development by a
process of repetition of what happened in the previous generation seems
to be the rule; and it would appear that only by this means could a mass
of cells which constitute an individual grow into something sufficiently
like the parents as to be recognised for their offspring. Given the fact
that a human individual starts from a single germ-cell, it could only be
by following the same steps in development trodden by the parent that
the new individual could attain a similar growth. The object of this
similarity is, of course, to provide that the offspring may live and
survive in an environment more or less similar to that of the parents.
As Dr. Archdall Reid puts it, "the embryo starting from the same point,
must follow the same road to reach the same goal. The embryo which did
not recapitulate the history of the development of a parent would be a
monstrosity."
While, however, recapitulation in development is always more or less
clear, it does not follow that it is perfectly complete, nor perfectly
identical with the development of the parent. Indeed, on the other hand,
there is always a certain amount of variation, either progressive or
retrogressive. Progressive variation means that in addition to the
development of all the parental stages, something new has been added.
Retrogressive variation means that from the total development
experienced by the parent, something has been omitted. We are here
speaking of characters of a species, and it must not be thought that we
are referring to the characters of the embryo as if they were derived
from those of their parents. This was clearly pointed out in the earlier
portion of our study. The variations in development, to which we here
refer, take their origin in the germ-plasm which tends to repeat in each
generation similar types of development. In other words the germ-plasm
from which individuals spring is of such a nature that the embryos
arising from it show in their development a recapitulation of the
evolution of their particular species. In addition they may show
variations of either a plus or a minus character. These variations are
frequently inherited, and persist throughout succeeding generations. In
course of time, if there are many of such variations, they accumulate,
and to that extent, of course, alter the life history. That is why in
watching the development of a human embryo it is impossible to trace
accurately the early ancestral development of the race from it. It
passes through the stage of a single cell, then becomes multicellular,
and gradually assumes the form of a higher and higher type of organism.
"Manifestly the additions and subtractions have been vast. It possesses,
for instance, a placenta, an organ by which it is attached to the
mother, through which it is nourished, and which at one time is larger
than the embryo itself; but which, of course, could not have been
present in its prototypes. Nevertheless the life history unfolded by the
child is just as real, just as complete, and probably more accurate than
any written chronicle that attempts to describe the whole past of a
race." (Reid.)
"There is a history in all men's lives
Figuring the nature of the times deceased."
Here we must conclude this brief sketch of some of the principal facts
in the science of Embryology, in the hope that enough has been said to
stimulate the interest of our readers in this subject to such an extent
that they may be encouraged to pursue its study still further in one of
the many textbooks that are devoted entirely, or partly, to this matter.
We would urge in conclusion that the study is well worth while, even for
those to whom it has a non-professional interest. It should be
sufficiently obvious to any earnest thinker that the problems which are
involved in the study of embryology are precisely those which are of the
very greatest importance to humanity at large. With this subject is most
intimately bound up that of heredity itself, which has been dealt with
in another volume of this series. No true understanding of what can be
done, or what should be done, in the direction of improving the lot of
generations to come, or of making the most of the generation at present
existent, can be obtained by any who are absolutely ignorant of these
topics. It is only by their study that we realise that the human embryo,
which is to become the human individual, consists, to a very large
extent, of characters and features which are unalterably settled for it
beforehand, to which nothing can be added, and from which nothing can be
taken away. In other words, the possibilities for any individual are
those which are pre-existent in the germ-plasm from which he or she
originates. These possibilities, however, depend upon the environment in
which the embryo, infant, child, and adult is subsequently placed for
their full expansion. In many directions the inherited tendencies
transmitted by the continuity of germ-plasm are unalterably and strictly
defined. In many other directions these inherited tendencies can be so
modified, drawn out, or even partially suppressed, by suitable
surroundings of a hygienic, educative, and moral nature, that if the
process be taken in hand sufficiently early wonderful successes may
result. These results are those for which the social reformer and the
philanthropist and the serious student of sociology are earnestly
striving, but it is only by a study of the sciences of Heredity and
Embryology that accurate data can be obtained from which justifiable
conclusions may be drawn.
The great fact which embryology teaches is that the past is unfolded
stage by stage, with certain omissions and additions, so that in very
truth--
"The softest dimple in a baby's smile
Springs from the whole of past eternity,
Taxed all the sum of things to bring it there."
BIBLIOGRAPHY
The following books will be found to deal in detail with some of the
general questions in the foregoing pages:--
_The Laws of Heredity_. By G. Archdall Reid, M.B., F.R.S.E. (London.
Methuen.) This great work should be read by all who are interested in
the subject of Heredity and all the problems involved in it. It deals in
an exhaustive and interesting manner with the characteristics of living
beings, the method of development, the function of sex, the present
evolution of man, and kindred topics.
_The Greatest Life_. By Gerald Leighton, M.D., F.R.S.E. (London.
Duckworth.) This work deals with the development of character (as well
as structures) from the biological point of view. The argument is that
the _whole_ of a man, mental, moral, spiritual, as well as what is
usually termed physical, develops in accordance with biological laws.
_Text-books on Physiology_. Any of the standard books will be found to
contain accounts of the early development of the embryo and its various
tissues.
INDEX
Acquired characters, 43
After-birth, 57
Alimentary tract, 60
Allantois, 79
Amnion, 53
Amniotic space, 51
Am[oe]ba, 12
Auditory, 69
Auricles, 73
Bibliography, 90
Blastocyst, 50
Blood-vessels, 60
Body cavity, 54
Bones, 60
Brain, 61, 67
Branchial clefts, 75
Caul, 58
Cell-division, 12
Centrosome, 13
Cerebellum, 68-69
Cerebrum, 68, 69
Chorion, 79
Cochlea, 74
Conjunctiva, 71
Connective tissues, 60
Cornea, 71
Cytology, 14
Dental ridge, 75
Dental sac, 75
Ear, 74
Early development, 47
Ectoderm, 51, 59, 60
Embryology, significance, 7, 8
Embryonic area, 51, 53, 66
Enamel, 75, 76
Entoderm, 51, 59, 60
Epiblast, 54
Epidermis, 60
Eustachian canal, 74
Eye, 65, 71
Fat, 60
Fertilisation, 18, 19, 47-52
Fore-brain, 69
Germ-cells, 16, 18, 19, 24
Germ-plasm, 11, 49
Glands, 60
Hair, 60
Heart, 54, 73
Hypoblast, 54
Inborn traits, 43
Inherited traits, 43, 45
Kidneys, 60
Lens of eye, 60, 71
Liver, 60
Lungs, 60
Lymph glands, 60
Marrow, 60
Maternal blood, 80
Maturation, 48
Medulla oblongata, 68
Medullary groove, 53
Mesoderm, 52, 59, 60
Mid-brain, 68
Morula, 50
Mouth, 60
Muscles, 60
Neural canal, 53
Neural groove, 54
Nervous system, 60, 61
Nose, 60
Notochord, 54, 60, 76
Nourishment, 57, 78
Optic vesicle, 69
Origin of brain, 69
Ova, ovum, 16, 17, 48, 49
Personality, 45
Placenta, 57, 79
Primitive groove, 53
Primitive streak, 53
Recapitulation, 84-88
Reproduction, 11, 17-35
Reproductive organs, 60, 63
Segmentation nucleus, 49
Sense organs, 60
Sex, 12, 49
Smell, 70
Somatic cells, 18, 24
Sperm, 17, 47, 48
Spinal cord, 53, 67
Tactual sense, 64, 65
Teeth, 60, 61
Trachea, 60
Trophoblast, 55, 56
Thymus, 60
Thyroid, 60
Umbilical cord, 55
Uterus, 56, 57
Variations, 44, 45
Weight of embryo, 83, 88
Yolk sac, 50, 53
* * * * *
Printed by BALLANTINE, HANSON & CO
Edinburgh & London
* * * * *
The table below lists all corrections applied to the original text.
p 25: the rest are -> The rest are
p 35: Natural Selection." -> closing quotes added
p 60: alimentary tract. -> period added