Chapter VIII

, where also his services as a histologist are recorded.

Huxley took a great step toward unifying the idea of germ-layers throughout the animal kingdom, when he maintained, in 1849, that the two cell-layers in animals like the hydra and oceanic hydrozoa correspond to the ectoderm and endoderm of higher animals.

Kowalevsky (Fig. 68) made interesting discoveries of a general bearing. In 1866 he showed the practical identity, in the early stages of development, between one of the lowest vertebrates (amphioxus) and a tunicate. The latter up to that time had been considered an invertebrate, and the effect of Kowalevsky's observations was to break down the sharply limited line supposed to exist between the invertebrates and the vertebrates. This was of great influence in subsequent work. Kowalevsky also founded the generalization that all animals in development pass through a gastrula stage--a doctrine associated, since 1874, with the name of Haeckel under the title of the gastræa theory.

Beginning of the Doctrine of Germinal Continuity.--The conception that there is unbroken continuity of germinal substance between all living organisms, and that the egg and the sperm are endowed with an inherited organization of great complexity, has become the basis for all current theories of heredity and development. So much is involved in this conception that, in the present decade, it has been designated (Whitman) "the central fact of modern biology." The first clear expression of it is found in Virchow's _Cellular Pathology_, published in 1858. It was not, however, until the period of Balfour, and through the work of Fol, Van Beneden (chromosomes, 1883), Boveri, Hertwig, and others, that the great importance of this conception began to be appreciated, and came to be woven into the fundamental ideas of development.

[Illustration: Fig. 68.--A. Kowalevsky, 1840-1901.]

Influence of the Doctrine of Organic Evolution.--This doctrine, although founded in its modern sense by Lamarck in the early part of the nineteenth century, lay dormant until Darwin, in 1859, brought a new feature into its discussion by emphasizing the factor of natural selection. The general acceptance of the doctrine, which followed after fierce opposition, had, of course, a profound influence on embryology. The latter science is so intimately concerned with the genealogy of animals and plants, that the newly accepted doctrine, as affording an explanation of this genealogy, was the thing most needed.

The development of organisms was now seen in the light of ancestral history, rudimentary organs began to have meaning as hereditary survivals, and the whole process of development assumed a different aspect. This doctrine supplied a new impulse to the interpretation of nature at large, and of the embryological record in particular. The meaning of the embryological record was so greatly emphasized in the period of Balfour that it will be commented upon under the next division of our subject.

The period between Von Baer and Balfour proved to be one of great importance on account of the general advances in knowledge of all organic nature. Observations were moving toward a better and more consistent conception of the structure of animals and plants. A new comparative anatomy, more profound and richer in meaning than Cuvier's, was arising. The edifice on the foundation of Von Baer's work was now emerging into recognizable outlines.

The Period of Balfour, with an Indication of Present Tendencies

Balfour's Masterly Work.--The workers of this period inherited all the accumulations of previous efforts, and the time was ripe for a new step. Observations on the development of different animals, vertebrates and invertebrates, had accumulated in great number, but they were scattered through technical periodicals, transactions of learned societies, monographs, etc., and there was no compact science of embryology with definite outlines. Balfour reviewed all this mass of information, digested it, and molded it into an organized whole. The results were published in the form of two volumes with the title of _Comparative Embryology_. This book of "almost priceless value" was given to the world in 1880-1881. It was a colossal undertaking, but Balfour was a phenomenal worker. Before his untimely death at the age of thirty-one, he had been able to complete this work and to produce, besides, a large number of technical researches. The period of Balfour is taken arbitrarily in this volume as beginning about 1874, when he published, with Michael Foster, _The Elements of Embryology_.

[Illustration: Fig. 69.--Francis M. Balfour, 1851-1882.]

His University Career.--Balfour (Fig. 69) was born in 1851. During his days of preparation for the university he was a good student, but did not exhibit in any marked way the powers for which later he became distinguished. At Cambridge, his distinguished teacher, the late Sir Michael Foster, recognized his great talents, and encouraged him to begin work in embryology. His labors in this field once begun, he threw himself into it with great intensity. He rose rapidly to a professorship in Cambridge, and so great was his enthusiasm and earnestness as a lecturer that in seven years "voluntary attendance on his classes advanced from ten to ninety." He was also a stimulator of research, and at the time of his death there were twenty students engaged in his laboratory on problems of development.

He was distinguished for personal attractiveness, and those who met him were impressed with his great sincerity, as well as his personal charm. He was welcomed as an addition to the select group of distinguished scientific men of England, and a great career was predicted for him. Huxley, when he felt the call, at a great personal sacrifice, to lay aside the more rigorous pursuits of scientific research, and to devote himself to molding science into the lives of the people, said of Balfour: "He is the only man who can carry out my work."

His Tragic Fate.--But that was not destined to be. The story of his tragic end need be only referred to. After completing the prodigious labor on the _Comparative Embryology_ he went to Switzerland for recuperation, and met his death, with that of his guide, by slipping from an Alpine height into a chasm. His death occurred in July, 1882.

The memorial edition of his works fills four quarto volumes, but the "Comparative Embryology" is Balfour's monument, and will give him enduring fame. It is not only a digest of the work of others, but contains also general considerations of a far-seeing quality. He saw developmental processes in the light of the hypothesis of organic evolution. His speculations were sufficiently reserved, and nearly always luminous. It is significant of the character of this work to say that the speculations contained in the papers of the rank and file of embryological workers for more than two decades, and often fondly believed to be novel, were for the most part anticipated by Balfour, and were also better expressed, with better qualifications.

The reading of ancestral history in the stages of development is such a characteristic feature of the embryological work of Balfour's period that some observations concerning it will now be in place.

Interpretation of the Embryological Record.--Perhaps the most impressive feature of animal development is the series of similar changes through which all pass in the embryo. The higher animals, especially, exhibit all stages of organization from the unicellular fertilized ovum to the fully formed animal so far removed from it. The intermediate changes constitute a long record, the possibility of interpreting which has been a stimulus to its careful examination.

Meckel, in 1821, and later Von Baer, indicated the close similarity between embryonic stages of widely different animals; Von Baer, indeed, confessed that he was unable to distinguish positively between a reptile, a bird, and a mammalian embryo in certain early stages of growth.

In addition to this similarity, which is a constant feature of the embryological record, there is another one that may be equally significant; _viz._, in the course of embryonic history, sets of rudimentary organs arise and disappear. Rudimentary teeth make their appearance in the embryo of the whalebone whale, but they are transitory and soon disappear without having been of service to the animal. In the embryos of all higher vertebrates, as is well known, gill-clefts and gill-arches with an appropriate circulation, make their appearance, but disappear long before birth. These indications, and similar ones, must have some meaning.

Now whatever qualities an animal exhibits after birth are attributed to heredity. May it not be that all the intermediate stages are also inheritances, and, therefore, represent phases in ancestral history? If they be, indeed, clues to ancestral conditions, may we not, by patching together our observations, be able to interpret the record, just as the history of ancient peoples has been made out from fragments in the shape of coins, vases, implements, hieroglyphics, inscriptions, etc.?

The Recapitulation Theory.--The results of reflection in this direction led to the foundation of the _recapitulation theory_, according to which animals are supposed, in their individual development, to recapitulate to a considerable degree phases of their ancestral history. This is one of the widest generalizations of embryology. It was suggested in the writings of Von Baer and Louis Agassiz, but received its first clear and complete expression in 1863, in the writings of Fritz Müller.

Although the course of events in development is a record, it is, at best, only a fragmentary and imperfect one. Many stages have been dropped out, others are unduly prolonged or abbreviated, or appear out of chronological order, and, besides this, some of the structures have arisen from adaptation of a particular organism to its conditions of development, and are, therefore, not ancestral at all, but, as it were, recent additions to the text. The interpretation becomes a difficult task, which requires much balance of judgment and profound analysis.

The recapitulation theory was a dominant note in all Balfour's speculations, and in that of his contemporary and fellow-student Marshall. It has received its most sweeping application in the works of Ernst Haeckel.

Widely spread throughout recent literature is to be noted a reaction against the too wide and unreserved application of this doctrine. This is naturally to be expected, since it is the common tendency in all fields of scholarship to demand a more critical estimate of results, and to undergo a reaction from the earlier crude and sweeping conclusions.

[Illustration: Fig. 70.--Oskar Hertwig in 1890.]

Nearly all problems in anatomy and structural zoölogy are approached from the embryological side, and, as a consequence, the work of the great army of anatomists and zoölogists has been in a measure embryological. Many of them have produced beautiful and important researches, but the work is too extended to admit of review in this connection.

Oskar Hertwig, of Berlin (Fig. 70), is one of the representative embryologists of Europe, while, in this country, lights of the first magnitude are Brooks, Minot, Whitman, E.B. Wilson, and others.

Although no attempt is made to review the researches of the recent period, we cannot pass entirely without mention the discovery of chromosomes, and of their reduction in the ripening of the egg and in the formation of sperm. This has thrown a flood of light on the phenomena of fertilization, and has led to the recognition of chromosomes as probably the bearers of heredity. The nature of fertilization, investigated by Fol, O. Hertwig, and others, formed the starting-point for a series of brilliant discoveries.

The embryological investigations of the late Wilhelm His (Fig. 71) are also deserving of especial notice. His luminous researches on the development of the nervous system, the origin of nerve fibers, and his analysis of the development of the human embryo are all very important.

Recent Tendencies. Experimental Embryology.--Soon after the publication of Balfour's great work on "Comparative Embryology," a new tendency in research began to appear which led onward to the establishment of experimental embryology. All previous work in this field had been concerned with the structure, or architecture, of organisms, but now the physiological side began to receive attention. Whitman has stated with great aptness the interdependence of these two lines of work, as follows: "Morphology raises the question, How came the organic mechanism into existence? Has it had a history, reaching its present stage of perfection through a long series of gradations, the first term of which was a relatively simple stage? The embryological history is traced out, and the palæontological records are searched, until the evidence from both sources establishes the fact that the organ or organism under study is but the summation of modifications and elaborations of a relatively simple primordial. This point settled, physiology is called upon to complete the story. Have the functions remained the same through the series? or have they undergone a series of modifications, differentiations, and improvements more or less parallel with the morphological series?"

[Illustration: Fig. 71.--Wilhelm His, 1831-1904. At Sixty-four Years.]

Since physiology is an experimental science, all questions of this nature must be investigated with the help of experiments. Organisms undergoing development have been subjected to changed conditions, and their responses to various forms of stimuli have been noted. In the rise of experimental embryology we have one of the most promising of the recent departures from the older aspects of the subject. The results already attained in this attractive and suggestive field make too long a story to justify its telling in this volume. Roux, Herbst, Loeb, Morgan, E.B. Wilson, and many others have contributed to the growth of this new division of embryology. Good reasons have been adduced for believing that qualitative changes take place in the protoplasm as development proceeds. And a curb has been put upon that "great fault of embryology, the tendency to explain any and every operation of development as merely the result of inheritance." It has been demonstrated that surrounding conditions have much to do with individual development, and that the course of events may depend largely upon stimuli coming from without, and not exclusively on an inherited tendency.

Cell-Lineage.--Investigations on the structural side have reached a high grade of perfection in studies on cell-lineage. The theoretical conclusions in the germ-layer theory are based upon the assumption of identity in origin of the different layers. But the lack of agreement among observers, especially in reference to the origin of the mesoderm, made it necessary to study more closely the early developmental stages before the establishment of the germ-layers. It is a great triumph of exact observation that, although continually changing, the consecutive history of the individual cells has been followed from the beginning of segmentation to the time when the germ-layers are established. Some of the beautifully illustrated memoirs in this field are highly artistic.

Blochman (1882) was a pioneer in observations of this kind, and, following him, a number of American investigators have pursued studies on cell-lineage with great success. The researches of Whitman, Wilson, Conklin, Kofoid, Lillie, Mead, and Castle have given us the history of the origin of the germ-layers, cell by cell, in a variety of animal forms. These studies have shown that there is a lack of uniformity in the origin of at least the middle layer, and therefore there can be no strict homology of its derivatives. This makes it apparent that the earlier generalizations of the germ-layer theory were too sweeping, and, as a result, the theory is retained in a much modified form.

Theoretical Discussions.--Certain theoretical discussions, based on embryological studies, have been rife in recent years. And it is to be recognized without question that discussions regarding heredity, regeneration, the nature of the developmental process, the question of inherited organization within the egg, of germinal continuity, etc., have done much to advance the subject of embryology.

Embryology is one of the three great departments of biology which, taken in combination, supply us with a knowledge of living forms along lines of structure, function, and development. The embryological method of study is of increasing importance to comparative anatomy and physiology. Formerly it was entirely structural, but it is now becoming also experimental, and it will therefore be of more service to physiology. While it has a strictly technical side, the science of embryology must always remain of interest to intelligent people as embracing one of the most wonderful processes in nature--the development of a complex organism from the single-celled condition, with a panoramic representation of all the intermediate stages.

FOOTNOTES:

[Footnote 3: As Whitman has pointed out, Aristotle taught epigenesis as clearly as Harvey, and is, therefore, to be regarded as the founder of that conception.]

[Footnote 4: The discovery is also attributed to Hamm, a medical student, and to Hartsoeker, who claimed priority in the discovery.]

[Footnote 5: _De Formatione Intestinorum, Nova Commentar, Ac. Sci. Petrop._, St. Petersburg, XII., 1768; XIII., 1769.]

[Footnote 6: Besides biographical sketches by Stieda, Waldeyer, and others, we have a very entertaining autobiography of Von Baer, published in 1864, for private circulation, but afterward (1866) reprinted and placed on sale.]

[Footnote 7: It is of more than passing interest to remember that Pander and Von Baer were associated as friends and fellow-students, under Döllinger at Würzburg. It was partly through the influence of Von Baer that Pander came to study with Döllinger, and took up investigations on development. His ample private means made it possible for him to bear the expenses connected with the investigation, and to secure the services of a fine artist for making the illustrations. The result was a magnificently illustrated treatise. His unillustrated thesis in Latin (1817) is more commonly known, but the illustrated treatise in German is rarer. Von Baer did not take up his researches seriously until Pander's were published. It is significant of their continued harmonious relations that Von Baer's work is dedicated "An meinen Jugendfreund, Dr. Christian Pander."]

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