Hertwig, Karl Wilhelm Theodor Richard von

(b. Friedberg, Germany, 23 September 1850; d. Schlederlohe, Germany, 3 October 1937)

biology.

Hertwig was the son of Carl Hertwig. a merchant, and the former Elise Trapp. He was the younger and only brother of Oscar Hertwig, with whom he was educated and with whom he collaborated during his early years. The brothers began their university studies in 1868 under Ernst Haeckel at Jena, where they studied until 1871. In the autumn of 1872 Hertwig became lecturer in zoology at Jena, where his brother was made lecturer in anatomy and embryology. They became extraordinary professors in 1878 and three years later, the paths of their lives began to diverge when Hertwig went to Königsberg as professor while his brother remained at Jena. He held the same rank at Bonn in 1883 and at Munich in 1885, remaining at Munich until his retirement in 1925. He married Jula Braun in 1887; they had two sons and a daughter. Hertwig outlived his brother by many years and was active until the day before his death at the beginning of his eighty-eighth year.

Hertwig contributed to many fields of biology, both morphologically and experimentally. He was a protozoologist, an embryologist, and a cytologist. His earliest studies, begun under the influence of Haeckel, were in comparative morphology. In the late 1870’s he published, together with his brother, works on the nervous system, the sense organs, and the musculature of various coelenterates. This work led them to theoretical considerations of the phylogenetic relationships of two-layered coelenterates to higher, three-layered animals. There was then much speculation as to the origins and significance of the mesoderm, both ontogenetically and phylogenetically; the two Hertwigs, in a series of studies, formulated their “coelom theory” to account for the classification and phylogeny of metazoan animals. The coelom is still used as an important taxonomic criterion.

During the next decade the two brothers collaborated on important contributions to experimental embryology. They initiated experimental studies on the chemical environment of eggs in relation to artificial hybridization, producing multipolar mitoses by the use of chemical agents. Highly important for later studies on the relative roles of cytoplasm and nucleus, their results showed that sea urchin eggs can be shaken into fragments and that both the nucleate and the nonnucleate fragments can be fertilized and can subsequently develop. During the last decade of the nineteenth century Hertwig demonstrated that sea urchin eggs, after treatment with weak solutions of strychnine, can form mitotic figures and begin to divide; this was the beginning of the studies on what Jacques Loeb soon called artificial parthenogenesis. In the 1890’s Hertwig also made many studies on the cytology and life cycles of the Protozoa, particularly ciliates and heliozoans. He was especially interested in syngamy in the ciliates; and his studies on unicellular organisms, together with his wide knowledge of the zoology of higher animals, enabled him to formulate problems of broad general interest.

During the first decade of the twentieth century Hertwig emphasized the importance of maintaining constancy in the relative volumes of cytoplasm and of nucleus within the cell; when the cytoplasmic volume becomes excessive, according to his theory, the cell divides. He was led by his studies on protozoan life cycles to an interest in senescence, which he ascribed to a relative increase in nuclear volume. His studies on syngamy led also to an interest in sex and sex determination; he demonstrated that overripe frog eggs develop an excess of males, an early indication that genetic expression is subject to environmental influences. He observed in heliozoans that basophilic granules seemed to be given off by the nucleus; he called these chromidia and believed that at each mitosis they are discharged from the nucleus into the cytoplasm, to play an important role in development.

Hertwig was one of the most productive teachers in the history of zoology. His Textbook of Zoology, because of its broad outlook, was highly influential; but he also exerted great influence through his teaching. When he retired in 1925, 208 of his former students presented him with a testimonial; 117 of them were professors of zoology, many of them very well known.

BIBLIOGRAPHY

The principal references to obituaries of Richard Hertwig, to lists of his writings, and to evaluations of his work at the time of his sixtieth birthday are in R. Weissenberg. Oscar Hertwig 1849–1922 (Leipzig. 1959), pp. 56–57. The articles written at the time of his seventieth birthday are in Naturwissenschaften, 8 (1920), 767–782. The other references given by Weissenberg are to reference works likely to be found in most sizable reference libraries.

Jane Oppenheimer