Hertwig, Wilhelm August Oscar

(b. Friedberg, Hessen, Germany, 21 April 1849; d. Berlin, Germany, 25 October 1922)

zoology.

Hertwig was the elder son of Carl Hertwig and Elise Trapp. The family interest in science was keen, his father having been trained in chemistry under Liebig at Giessen. After the birth of Hertwig’s brother Richard, the family moved to Mühlhausen in Thuringen. There Oscar and Richard were educated together, and on the advice of the Gymnasium head, Wilhelm Osterwald, who had taught Ernst Haeckel in Merseberg, they went to Jena and came under Haeckel’s influence. In his brief but charming recollections of Haeckel, published in 1919, Oscar left no doubt as to the great debt he owed the Jena zoologist, on whose advice he forsook chemistry for medicine.

Hertwig spent from 1868 to 1888 at Jena with the exception of two short periods at Bonn. He became assistant professor of anatomy at Jena in 1878 and professor three years later. From 1888 until 1921 he occupied the first chair of cytology and embryology and directed the new Anatomical-Biological Institute. He became a member of the Leopoldina Academy in Jena and the Prussian Academy of Sciences in Berlin. Hertwig married Marie, the daughter of the teacher Wilhelm Gesenius, in 1884. They had two children, Gunther (1888) and Paula (1889).

Hertwig’s prize essay (Jena, 1871) and doctoral thesis (Bonn, 1872) were devoted to traditional developmental studies, but after reading Leopold Auerbach’s Organologischen Studien (Breslau, 1874) he became deeply interested in the nature of the fertilization process. The views most widely held at that time were either that the spermatozoa make contact with the egg, thus stimulating its development by the transmission of a subtle mechanical vibration akin to the supposed action of a ferment (the contact theory advocated by G. W. Bischoff), or that they penetrate the egg and their chemical constituents become commingled with the egg yolk. Consequently, when Auerbach found two nuclei in the fertilized egg he assumed they had originated from the mixture of the chemical constituents of sperm and egg, and not from any previous nuclei.

Auerbaeh’s work left Hertwig curious and dissatisfied. As an anatomist conscious of the role of organized structures in the cell, he expected to discover some structural continuity between the nuclear contents of the egg before and after fertilization. He was aware of the distinctive staining reactions of the nucleus and of the hereditary role that Haeckel had assigned it. He also knew that botanists had accepted Nathanael Pringsheim’s observation of the penetration of the antherozoid into the oogonium in Oedogonium. At this juncture in Hertwig’s career, Haeckel was about to take Richard Hertwig on a research trip to the Mediterranean. Hertwig rashly resigned his new assistantship at Bonn to join them, and it was on this trip that he found the sea urchin (Toxopneustes lividus) which so admirably suited his research purposes.

Because of its small size, finely divided yolk, and absence of any noticeable membrane, the sea urchin was remarkably transparent. Using fresh material and material fixed in acetic or osmic acid, Hertwig followed the fate of the egg and sperm nuclei with ammoniacal carmine solution. To his delight he was able to detect the presence of a remnant of the egg nucleus before and during entry of the spermatozoon and the fusion of both nuclei five to ten minutes later. Although he was mistaken in describing this nuclear structure in the egg as the nucleolus, he was able to state categorically that he had observed no breaking up of that nucleus, and hence could maintain the morphological continuity between it and the cleavage nuclei of the developing embryo. More important for the understanding of heredity, however, was his observation that only one spermatozoon is required to fertilize one egg. The entry of further spermatozoa is prevented by the formation of a vitelline membrane, which spreads around the surface of the egg starting from the cone of attraction at the point of entry of the spermatozoon.

In the winter of 1875 Hertwig wrote this work up for the Habilitalionsschrifr, which he defended the following November, when he was called upon to evaluate the statement: “The egg cell passes through no monera stage in its development.” This theme was surely suggested by Haeckel, according to whom the egg passed through a nuclear-free or “monera” stage, in harmony with the principle of recapitulation. Hermann Fol and Edouard van Beneden also published papers that year on the subject of fertilization, but neither man, in interpreting his findings, freed himself completely from the chemical view in which nuclear continuity was denied. Hertwig, on the other hand, clung to this conception even after it had been superseded by the more correct theory of chromosome continuity. It was not until T. H. Morgan’s work became well known in Germany that Hertwig fully accepted the modern version of this theory, which included the exchange of genes between chromosomes by crossing-over.

Hertwig returned to the subject of the egg in his famous paper, “Das Probleme der Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung” (1885). Here Pflüger’s evidence against the existence of a structural differentiation in the egg was rejected, and the profound influence of Naegeli’s idioplasm theory on Hertwig is apparent. He called the Munich botanist’s postulate of the equivalence of male and female germinal substances Naegeli’s “Vererbungsaxiom,” which for Hertwig became the equivalence of egg and sperm nuclei; the nuclein or chromatin (he early considered them identical) was then both the fertilizing substance and the idioplasm.

This acceptance of the genetic primacy of the nucleus contrasted with the more cautious and critical attitudes of Eduard Strasburger, Wilhelm Waldeyer, and Max Verworn. Yet Hertwig’s writings were more restrained than those of August Weismann, whose uninhibited speculations Hertwig termed “weismannische Naturphilosophie.” He opposed Weismann’s doctrine of differentiation by selective loss of idioplasm and instead asserted the correct doctrine of the genetic equivalence of all body cells. Weismann’s doctrine of ancestral germ plasms or “ids” was objectionable to Hertwig; it involved unnecessary speculation. Hertwig thought it was enough to consider the problem of increase of nuclear mass—or as Naegeli had conceived it, increase of idioplasm—which repeated fertilizations would cause. The essential function of polar bodies was therefore to remove half the nuclear substance, but not to make the resulting eggs qualitatively different. If for every character more than one determinant exists, then an equal division of the entire egg nucleus will in no way alter the hereditary constitution of the several eggs produced. These polar bodies were to be looked upon as relics of what in earlier evolutionary history were functional eggs. In 1890 Hertwig showed that spermatogenesis is equivalent to oogenesis and involves the formation from one sperm mother cell of a tetrad of spermatozoa, but unlike the egg “tetrad” all four products of the sperm tetrad are functional.

For many years Hertwig was an opponent of Weismann’s doctrine that reduction division leads to qualitative differences between gametes and of Carl Rabl’s and Theodor Boveri’s insistence on the individuality of the chromosomes. Nevertheless, he was a lifelong exponent of the study of heredity through cytology and in 1909, at the age of sixty, with his son and daughter, he began to study the biological effects of the irradiation of eggs, spermatozoa, and embyros with X rays.

Hertwig wrote a series of papers on the germ layer theory with his brother Richard. They began by questioning the so-called specificity of the germ layers in a paper (1878) that applied this theory to the medusae. Four years earlier Haeckel had published his gastraea theory, according to which all organisms from sponges upward pass through a stage in which the embryo consists of two layers, the inner formed by invagination of the outer layer. These two layers had been recognized in medusae by T. H. Huxley in 1849. The Hertwigs rightly questioned the evidence for the existence of a third germ layer, the mesoderm, and assigned such tissues either to ectoderm or endoderm.

In their Studien zur Blattertheorie, parts 2 and 4 (1881, 1882), the Hertwigs tried to eliminate the confusion over the origin of mesoderm and the relation of the body cavity of vertebrates to the archenteron of the gastrula. They thought to find a universal distinction between structures arising from cells detached from existing germ layers, which they termed mesenchyme, and those arising by invagination of the endoderm, the true mesoderm. The body cavity in higher organisms, they asserted, is secondary in origin, being formed entirely by such invagination and not partly by cavity formation within solid tissue. The cells of the invaginated layer thus surround this cavity or “coelom.”

In the search for phylogenetic relationships and homologies between diploblastic and triploblastic animals, the coelom theory was useful at a time when the prevailing fashion in embryology was phyletic and descriptive. It failed in that it modified one rigid scheme—the gastraea theory—by introducing another and thus did not stimulate experimental embryology.

Hertwig traveled little, rarely attended international meetings, and was known chiefly through his extensive writings. His most popular textbooks were Lehrbudi der Entwicklungsgeschichte des Menschen und der Wirbeltiere (1886–1880) and Die Zelle and die Gewebe (1893–1898), now known as the Allgemeine Biologie, after the title of the second edition (1906).

Hertwig did not succumb to the beguiling neatness and finality of Haeckel’s biogenetic law. While Haeckel stimulated him, Naegeli profoundly influenced him. The Lamarckian, anti-Darwinian, and morphological-chemical features of Naegeli’s work were all echoed by Hertwig. It was Hertwig who wrote in approval of Paul Kammerer’s work, who vehemently attacked the concept of natural selection in Das Werden der Organismen (1916), and who upheld the chemical study of the cell while belittling the biological significance of Emil Fischer’s work. Hertwig believed that proteins undergo a series of postmortem changes before they can be characterized by the techniques of the organic chemists. To learn about the proteins of the living cell the chemist must adopt more comprehensive methods and aims. He must become a biologist and above all a morphologist.

Hertwig’s exposure to chemistry had been slight. His early success in routing the chemical theory of fertilization and in reestablishing the morphological conception in terms of nuclear continuity no doubt set him in the direction of histological rather than histochemical studies of the cell. He paid lip service to chemistry but never appreciated the distinction between nucleic acid and nucleoprotein; consequently, he was content to equate chromatin with nuclein and to call it a protein.

In his retrospective essays, including his magnificent “Dokumente zur Geschichte der Zeugungslehre” (1918), Hertwig emphasized the conceptual advance made in 1875 when the process of fertilization and the transfer of hereditary material became firmly associated in the nuclear theory of sexual reproduction. The demonstration of nuclear continuity and conjugation in his Habilitationsschrift was decisive in establishing this theory and was surely his greatest contribution to science.

BIBLIOGRAPHY

I. Original Works. A bibliography with 121 entries is included in Weissenberg’s biography (see below). Hertwig wrote seven books: Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbeltiere, 2 vols. (Jena, 1886–1888; 10th ed., 1915); French ed. (Paris, 1891); English ed. (London, 1892); Italian ed. (Milan, 1894); Die Zelle und die Gewebe, 2 vols. (Jena, 1893–1898); 2nd ed. entitled Allgemeine Biologie (Jena, 1906; 7th ed., 1923); Zeit-und Streitfragen der Biologie (Jena, 1894); English ed. (London, 1896); Die Elemente der Entwicklungslehre des Menschen und der Wirbeltiere. Anleitung und Repetitorium für Studierende und Aerize (Jena, 1900; 6th ed., 1920); Das Werden der Organismen. Eine Widerlegung von Darwin’s Zufallsiheorie (Jena, 1916; 3rd ed., 1922); Zur Abvehr des ethischen, des sozialen, des politischen Darwinismus (Jena, 1918): und Der Staat als Organismus. Gedanken zur Entwicklung der Menschheit (Jena, 1922). Hertwig edited and contributed to Handbuch der vergleichenden und experimentellen Entwicklungslehre der Wirbeltiere, 3 vols. (Jena, 1901–1906).

His important papers on fertilization are “Beiträge zur Kenntniss der Bildung, Befruchtung und Theilung des thierischen Eies,” in Morphologisehes Jahrbueh: I, 1 (1876), 347–434; II. 3 (1877), 1–86; “Weitere Beiträge...,” II, 3 , 271–279. His embryological work, with special reference to the germ layer theory, the coelom theory, and fertilization, are in Zeitschrift für Naturwissenschaften (Jena): “Ueber das Nervensystem und die Sinnesorgane der Medusen,” 11 (1877), 355–374: “Ueber die Entwicklung des mittleren Keimblattes der Wirbelthiere,” 15 (1882)” 286–340; 16 (1883). 247–328; “Wclehen Kinfluss übt die Schwerkraft auf die Theilung der Zellen?,” 18 (1885). 175–205; and “Das Problem der Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung,” 18 (1885), 276–318.

Among the papers he published in the Archiv für mikroskopische Anatomic und Entwicklungsmechanik are the following: “Vergleich der Ei- und Samenbildung bei Nematoden. Eine Grundlage für celluläte Strcitfragen,” 36 (1890), 1–138; “Weitere Versuche über den Einfluss der Zentrifugalkraft auf die Entwicklung thierischer Eier,” 63 (1904), 643–657; “Dokumente zur Geschichte der Zeugungslehre. Eine historische Studie als Abschluss eigener Forschung,” 90 , sec. 2 (1918), 1–168. His last three papers appeared in the Deutsche medizinische Wochenschrift: “Zur Erinnerung an Ernst Haeckel,” 45 (1919), 1031; “Der jetzige Stand der Lehre von den Chromosomen,” 48 (1922), 9–10; and “Die Erblichkeitslehre, ihre Geschichte und Bedeutung für die Gegenwart,” 48 (1922), 1239–1240.

His researches in the use of radiation and chemical agents in the study of heredity and embryology were published mostly in the Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin, Math.-nat. Klasse. These include “Die Radiumstrahlung in ihrer Wirkung auf die Entwicklung tierische Eier” (1910), 221–233; “Neue Untersuchungen über die Wirkung der Radium-strahlung auf die Entwicklung tierischer Eier” (1910), 751–771; “Mesotherium versuche an thierischen Keimzellen, ein experimenteller Beweis für die Idioplasmanatur der Kernsubstanzen” (1911). 844–873; “Veränderung der idioplasmatischen Beschaffenheit der Samenfäden durch physikalische und chemische Eingriffe” (1912), 554–571; “Keimesschädigung durch chemische Eingriffe” (1913). 564–582: “Die Verwendung radioaktive Substanzen zur Zerstörung lebender Gewebe” (1914), 894–904. Hertwig’s history of the Anatomical-Biological Institute in Berlin is in Max Lenz, Geschichle der Königlich Friedrich-Wilhelms-Universität zu Berlin, III (Halle, 1910), 141–154.

II. Secondary Literature. The best account of Hertwig’s life and work is Richard Weissenberg, Oskar Hertwig 1849–1922. Leben und Werk eines deutschen Biologen, no. 7 in the series Lebensdarstellungen deutscher Naturforscher (Leipzig, 1959); the excellent bibliography by Rudolph Zaunick for this work gives a generous list of obituary notices, of which the best is by Franz Keibel in Anatomischer Anzeiger, 56 (1923), 372–383. In addition, Keibel’s valuable discussion of Haeckel’s views on the theory of recapitulation is available in “Haeckels biogenetisches Grundgesetz und das ontogenetische Kausalgesetz von Oskar Hertwig,” in Deutsche medizinische Wochenschrift, 37 (1911). 170–172. An overly critical view of the Hertwig brothers” contribution to the germ layer theory is in J. M. Oppenheimer, Essays in the History of Embryology und Biology (Cambridge, Mass., 1967).

For an appreciation of the Hertwigs’ fruitful suggestions in experimentation, see Fritz Balzer, Theodor Boveri: Life and Work of a Great Biologist, D. Rudnick, trans. (Berkeley–Los Angeles, 1967). The most authoritative treatment of Hertwig’s standpoint on cytological questions is Frederick Churchill, “Hertwig, Weismann, and the Meaning of Reduction Division Circa 1890,” in Isis, 61 (1970), 429–457.

Robert Olby