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, fœtus, 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]
Amœba,[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]

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