TENNENT, DAVID HILT

(b. Janesville, Wisconsin, 28 May 1873; d. Bryn Mawr, Pennsylvania, 14 January 1941)

biology.

Tennent was one of four children born to Thomas Tennent, a contractor, and his second wife, Mary Hilt. Thomas also had two children by his first wife. Young David thus grew up in a large family, and lived a rather rigorous and austere life as a child. He became interested in science early and hoped to study medicine, but he was prevented from doing so by the accidental death of his father in 1893. He first became a licensed pharmacist, then in 1895 entered Olivet College in Michigan, where he received the B.S. in 1900. In 1904 he received the Ph.D. from the Johns Hopkins University, where he had studied under W. K. Brooks.

While a graduate student, Tennent spent one year as a substitute instructor at Randolph-Macon College. In 1904 he became instructor in biology at Bryn Mawr College, where he taught for thirty-four years. After his retirement he was a research professor there until his death. In 1909 Tennent married Esther Maddux; their only child, David Maddux, born in 1914, became a research biochemist.

The major part of Tennent’s work was in marine biology, and consisted principally of experiments on marine eggs. While a graduate student, he spent several summers at the U.S. Fisheries laboratories in Beaufort, North Carolina; later he spent a number of summers in the Dry Tortugas at the biological laboratory of the Carnegie Institution. He worked also at a number of other marine laboratories in the United States and abroad, and spent two sabbatical leaves in Japan.

Tennent concentrated most of his efforts on the study of echinoderm development. His particular interest was in the role of the nucleus, which he studied both as an embryologist and as a cytologist. His most important investigations dealt with the study of echinoderm hybridization.

Tennent performed cross fertilizations between species, genera, orders, and classes within the Echinodermata. He found that after exposing the eggs of some species (such as Lytechinus) to monovalent cations, they could be more easily fertilized by foreign sperm than by sperm of their own species. It was known that chromosomes are often eliminated from the mitotic spindles in hybrids. Tennent, a skilled cytologist, developed an intimate knowledge of the configurations of the chromosome sets and thereby established that it was the paternal chromosomes that were eliminated, presumably as a result of incompatibility between maternal cytoplasm and paternal chromatin. He also showed that, depending on the number of paternal chromosomes eliminated, the embryo develops more or less, sometimes solely, under the influence of the maternal chromosomes. Tennent distinguished for the first time between the autosomes and the sex chromosomes of a number of echinoderms; and by hybridizing two forms that differ in the shape of their sex chromosomes, he established for the first time that the male is the digametic sex in sea urchins.

Like Boveri and Driesch before him, Tennent wished to determine the time at which the influence of the paternal chromosomes first manifests itself in development. Boveri had performed only preliminary experiments, and Driesch only crude ones, to elucidate this point. By crossing species that differed clearly in several features, Tennent confirmed the conclusions of Boveri and Driesch that maternal factors control the earliest phases of development, and that the paternal factors are expressed later in development. Another important contribution to the study of echinoderm hybrids was his demonstration that the direction of development toward the character of the maternal or the paternal species could be altered by changing the hydrogen ion concentration of the seawater in which the hybrids were maintained.

Although he was never a complete innovator in his investigations, the skill, caution, objectivity, and patience with which Tennent worked enabled him to establish on a firm foundation cytological and developmental truths that had merely been intimated by some of his more original predecessors.

BIBLIOGRAPHY

The most detailed biography of Tennent, with a complete bibliography of his articles, is M. S. Gardiner, “Biographical Memoir of David Hilt Tennent 1873–1941,”, in Biographical Memoirs. National Academy of Sciences, 26 (1951), 99–119.

Jane Oppenheimer