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Stevens, Nettie Maria


(b. Cavendish, Vermont, 7 July 1861; d. Baltimore, Maryland, 4 May 1912)

cytology, heredity.

Nettie Maria Stevens came from solid New England stock. The first American Stevens came from Chelmsford, England, to Boston; his eldest son went to Chelmsford, Massachusetts, in 1663. The family remained in the area for five generations; Nettie’s father, Ephraim, was born on 24 March 1833. He married Julia Adams of Cavendish, Vermont, in 1854. Their first two children, both sons, died before Nettie’s birth. They had one more child, Emma Julia, in 1863. The family lived in Cavendish until after Julia’s death and Ephraim’s remarriage, whereupon they moved to Westford, Vermont. Nettie’s father reportedly was a hardworking and reasonably successful carpenter and handyman.

Stevens’ education began in the Westford public schools and continued at the Westford Academy, from which she graduated in 1880 as a college preparatory student. She and her sister both performed consistently well, achieving nearly perfect grades in all subjects. After graduating, Stevens taught high school, including zoology and physiology, in Lebanon, New Hampshire, for three terms. She then completed in two years (1881–1883), with nearly perfect grades, the four–year program at Westfield Normal School in Westfield, Massachusetts. She taught school until 1892 and again in 1895 and 1896, acquiring a reputation as an excellent teacher. In 1896 Stevens entered Stanford University, where she quickly decided to major in physiology. After the year 1897–1898, when she determined to work with Frank Mace MacFarland, she moved into histological work. She spent four summers (1897–1901) at Stanford’s Hopkins Seaside Laboratory, working with marine organisms. After receiving the A.B. degree in 1899, Stevens remained at Stanford, working in experimental physiology under Oliver Peebles Jenkins. She spent 1900 in the investigator’s room at the Hopkins Laboratory and completed her master’s thesis (her first publication). She credited MacFarland with having assigned her to pursue cytological studies, a focus she followed in her later major work on sex determination and chromosomes.

After Stanford, Stevens went to Bryn Mawr College, where she began working under Joseph Weatherland Warren on the physiology of frog contractions and the influence of chemicals on the force of contractions. She quickly moved, however, to work with Thomas Hunt Morgan, who was then studying regeneration in various organisms. Stevens became involved in that work and in 1901 published two regeneration studies. At first she worked with material that Morgan had brought back from Naples; then she traveled to Woods Hole in 1901 to pursue cell division in regeneration of Tubularia. From 8 October 1901 to 1 April 1902, at Morgan’s suggestion, she worked at the Naples Zoological Station, where she occupied the American women’s table and pursued studies of cut eggs, with a particular interest in the relations of cuts to the chromosomes.

Later in 1902 Stevens went on to Würzburg, to work in the laboratory of Theodor Boveri, where she began to study ovogenesis and spermatogenesis. This contact with Boveri at a particularly productive point in his chromosome studies likely had a major directive impact on Stevens’ work. She completed her Ph.D. from Bryn Mawr in 1903 and retained a professional tie with the school: as research fellow in biology (1902–1904), reader in experimental morphology (1904–1905), and associate in experimental morphology (1905–1912). She would have occupied a research professorship the next year had she not died from cancer.

In 1904 Morgan and Stevens began studying the behavior of chromosomes in aphids, work that she pursued with the support of a fellowship from the Carnegie Institution of Washington in the year 1904–1905. Several studies of germ cells in aphids appeared as a result. One paper of 1905 brought Stevens an award of $1,000 for the best scientific paper written by a woman. Another work, Studies in Spermatogenesis, marked her entry into the increasingly promising arena of sex-determination studies and chromosomal inheritance. In 1901 and 1902, Clarence Erwin McClung had suggested that there exists an extra, or accessory, chromosome in the male and that the presence of the accessory determines the male sex. In 1903 Morgan reported in his “Recent Theories in Regard to the Determination of Sex” the general opinion that an individual’s sex is not determined by external (or environmental) factors.

But scientists were not at all sure what internal factors prevailed, Morgan concluded, nor even at what point sex is determined. McClung’s suggestion had gained little support. Yet by the year 1904–1905 Stevens was actively pursuing the hypothesis that chromosomes determine sex, though not in the simple way that McClung envisioned. Both Stevens and Edmund Beecher Wilson sought to discover what role chromosomes do play. In a study that appeared as Carnegie Institution of Washington Publication 36, Stevens examined spermatogenesis in five insect species from four different groups. Two species had an extra or “accessory” chromosome in the male. But she felt that the common mealworm (Tenebrio molitor) proved most interesting for questions of sex determination because it exhibited a difference in size rather than in number of male and female chromosomes. Though cautious in her general conclusions, Stevens clearly felt that her work of 1905 on Tenebrio established that males have nineteen large chromosomes and one small one, and females twenty large ones, which implied a correlation of chromosomes with sex determination.

The period 1905 to 1908 brought a series of papers on germ cells, heterochromosomes, and the determination of sex in an attempt to elucidate the details of spermatogenesis for the unequal or “hetero” chromosomes in a range of additional species. Stevens explored what effect the different chromosomes might have, and she suggested that the evidence supported William Castle’s modified version of Mendelian heredity as well. After a leave of absence and a return research visit to Boveri’s laboratory in the year 1908–1909, Stevens continued to bring her cytological concern with chromosomes, especially heterochromosomes, and their behavior in synapsis to her work on regeneration and reproduction. Her death left this series of studies uncompleted.

Nettie Stevens achieved the highest respect from the leading biologists of her time. She failed to gain a full regular university position, no doubt largely because she was a woman. Yet she achieved an admirable career of research at the leading marine stations and laboratories. Her record of at least thirty-six publications written alone and four with a coauthor includes several major contributions and a group of studies that together constitute a central addition to the emergence of ideas of chromosomal heredity.

This woman scientist from Vermont deserves recognition as one of those providing critical evidence for a Mendelian and chromosomal theory of heredity. As Morgan wrote in his obituary note, “Her single-mindedness and devotion, combined with keen powers of observation; her thoughtfulness and patience, united to a well-balanced judgment, account, in part, for her remarkable accomplishment”.


I. Original Works. The most complete bibliography of Stevens’ work appears in the biography by Ogilvie and Choquette (see below). Her first publication, from her master’s thesis, was “Studies on Ciliate Infusoria”, in Proceedings of the California Academy of Sciences, Zoology, 3 (1901), 1–42. Her most important works include Studies in Spermatogenesis with Especial Reference to the “Accessory Chromosome”, Carnegie Institution of Washington Publication no. 36, pt. I (1905); “A Study of the Germ Cells of Aphis rosae and Aphis oenetherae,” in Journal of Experimental Zoology, 2 (1905), 313–333; Studies in Spermatogenesis. A Comparative Study of the Heterochromosomes in Certain Species of Coleoptera, Hemiptera and Lepidoptera, with Especial Reference to Sex Determination, Carnegie Institution of Washington Publication no. 36, pt. II (1906); Studies on the Germ Cells of Aphids, Carnegie Institution of Washington Publication no. 51 (1906); “A Study of the Germ Cells of Certain Diptera, with Reference to the Heterochromosomes and the Phenomena of Synapsis,” in Journal of Experimental Zoology, 5 (1908), 359–374; and “Further Studies on Reproduction in Sagitta,” in Journal of Morphology, 21 (1910), 279–319.

II. Secondary Literature. The only reliable biography of Stevens is Marilyn Bailey Ogilvie and Clifford J. Choquette, “Nettie Maria Stevens (1861–1912): Her Life and Contributions to Cytogenetics”, in Proceedings of the American Philosophical Society, 125 (1981), 292–311, which covers material in earlier biographical sketches and surveys the archival materials available. This replaces the incomplete and sometimes inaccurate study by Hans Ris in Notable American Women, VIII. 372–373. In addition, Stephen G. Brush, “Nettie M. Stevens and the Discovery of Sex Determination by Chromosomes”, in Isis, 69 (1978), 163–172, discusses her scientific work, as does Thomas Hunt Morgan, “The Scientific Work of Miss N. M. Stevens”, in Science, 36 (1912), 468–470.

Jane Maienschein

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Nettie Maria Stevens

Nettie Maria Stevens

Biologist Nettie Maria Stevens (1861-1912) discovered that chromosomes determine sex.

Nettie Maria Stevens was a biologist and cytogeneticist and one of the first American women to be recognized for her contributions to scientific research. "She … produced new data and new theories," wrote Marilyn Bailey Ogilvie in Women in Science, "yet beyond these accomplishments passed along her expertise to a new generation…. illustrat[ing] the importance of the women's colleges in the education of women scientists." Although Stevens started her research career when she was in her thirties, she successfully expanded the fields of embryology and cytogenetics (the branch of biology which focuses on the study of heredity), particularly in the study of histology (a branch of anatomy dealing with plant and animal tissues) and of regenerative processes in invertebrates such as hydras and flatworms. She is best known for her role in genetics—her research contributed greatly to the understanding of chromosomes and heredity. She theorized that the sex of an organism was determined by the inheritance of a specific chromosome—X or Y—and performed experiments to confirm this hypothesis.

Stevens, the third of four children and the first daughter, was born in Cavendish, Vermont, on July 7, 1861, to Ephraim Stevens, a carpenter of English descent, and Julia Adams Stevens. Historians know little about her family or her early life, except that she was educated in the public schools in Westford, Massachusetts, and displayed exceptional scholastic abilities. Upon graduation, Stevens taught Latin, English, mathematics, physiology and zoology at the high school in Lebanon, New Hampshire. As a teacher she had a great zeal for learning that she tried to impart both to her students and her colleagues. Between 1881 and 1883, Stevens attended the Normal School at Westfield, Massachusetts, consistently achieving the highest scores in her class from the time she started until she graduated. She worked as a school teacher, and then as a librarian for a number of years after she graduated; however, there are gaps in her history that are unaccounted for between this time and when she enrolled at Stanford University in 1896.

In 1896, Stevens was attracted by the reputation of Stanford University for providing innovative opportunities for individuals aspiring to pursue their own scholastic interests. At the age of thirty-five she enrolled, studying physiology under professor Oliver Peebles Jenkins. She spent summers studying at the Hopkins Seaside Laboratory, Pacific Grove, California, and pursuing her love of learning and of biology. During this time, Stevens decided to switch careers to focus on research, instead of teaching. While at Hopkins she performed research on the life cycle of Boveria, a protozoan parasite of sea cucumbers. Her findings were published in 1901 in the Proceedings of the California Academy of Sciences. After obtaining her master's degree—a highly unusual accomplishment for a woman in that era—Stevens returned to the East to study at Bryn Mawr College, Pennsylvania, as a graduate biology student in 1900. She was such an exceptional student that she was awarded a fellowship enabling her to study at the Zoological Station in Naples, Italy, and then at the Zoological Institute of the University of Würzburg, Germany. Back at Bryn Mawr, she obtained her doctorate in 1903. At this time, she was made a research fellow in biology at Bryn Mawr and then was promoted to a reader in experimental morphology in 1904. From 1903 until 1905, her research was funded by a grant from the Carnegie Institution. In 1905, she was promoted again to associate in experimental morphology, a position she held until her death in 1912.

While Stevens' early research focused on morphology and taxonomy and then later expanded to cytology, her most important research was with chromosomes and their relation to heredity. Because of the pioneering studies performed by the renowned monk Gregor Mendel (showing how pea plant genetic traits are inherited), scientists of the time knew a lot about how chromosomes acted during cell division and maturation of germ cells. However, no inherited trait had been traced from the parents' chromosomes to those of the offspring. In addition, no scientific studies had yet linked one chromosome with a specific characteristic. Stevens, and the well-known biologist Edmund Beecher Wilson, who worked independently on this type of research, were the first to demonstrate that the sex of an organism was determined by a particular chromosome; moreover, they proved that gender is inherited in accordance with Mendel's laws of genetics. Together, their research confirmed, and therefore established, a chromosomal basis for heredity. Working with the meal worm, Tenebrio molitor, Stevens determined that the male produced two kinds of sperm—one with a large X chromosome, and the other with a small Y chromosome. Unfertilized eggs, however, were all alike and had only X chromosomes. Stevens theorized that sex, in some organisms, may result from chromosomal inheritance. She suggested that eggs fertilized by sperm carrying X chromosomes produced females, and those by sperm carrying the Y chromosome resulted in males. She performed further research to prove this phenomenon, expanding her studies to other species. Although this theory was not accepted by all scientists at the time, it was profoundly important in the evolution of the field of genetics and to an understanding of determination of gender.

Stevens was a prolific author, publishing some thirty-eight papers in eleven years. For her paper, "A Study of the Germ Cells of Aphis rosae and Aphis oenotherae, " Stevens was awarded the Ellen Richards Research Prize in 1905, given to promote scientific research by women. Stevens died of breast cancer on May 4, 1912, before she could occupy the research professorship created for her by the Bryn Mawr trustees. Much later, Thomas Hunt Morgan, a 1933 Nobel Prize recipient for his work in genetics, recognized the importance of Stevens' ground-breaking experiments, as quoted by Ogilvie in the Proceedings of the American Philosophical Society, "Stevens had a share in a discovery of importance and her name will be remembered for this, when the minutiae of detailed investigations that she carried out have become incorporated in the general body of the subject."

Further Reading

Ogilvie, Marilyn Bailey, Women in Science: Antiquity through the Nineteenth Century, Massachusetts Institute of Technology, 1986.

Isis, June, 1978, pp. 163-72.

Proceedings of the American Philosophical Society, Held at Philadelphia for Promoting Useful Knowledge, Volume 125, American Philosophical Society, 1981, pp. 292-311. □

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Stevens, Nettie Maria

Stevens, Nettie Maria

American biologist 1861-1912

Nettie Maria Stevens was a prominent biologist who discovered that the sex of an organism was determined by a specific chromosome. Although her research career spanned less than a decade (1903-1912), she published forty papers and became one of the first American women to achieve recognition for her contributions to scientific research.

Stevens was born July 7, 1861 in Cavendish, Vermont. She graduated from the Westford Academy in 1880 and enrolled at the Westfield Normal School, a educator college founded to cerify teachers, in 1881. She received the highest scores on the college's entrance exams of any student in her class. Clearly an outstanding student, she earned the four-year teacher certification in just two years.

In 1883, Stevens graduated from Westfield Normal School with the highest academic scores in her class. After graduation she took a job as a librarian at a high school in Lebanon, New Hampshire. In 1896, at the age of 35, she transferred to Stanford University in California, where she earned a B.A. degree in General Biology in 1899 and an M.A. in physiology in 1900. She began her doctoral studies at Bryn Mawr which included a year of study (1901-1902) at the Zoological Station in Naples, Italy, and at the Zoological Institute of the University of Würzburg, Germany.

Stevens received a Ph.D. in Morphology from Bryn Mawr in 1903 and remained at the college as a research fellow in biology the following year. She was an associate in experimental morphology at Bryn Mawr from 1905 until her death from breast cancer on May 4, 1912, in Baltimore, Maryland at the age of 51.

Stevens' earliest field of research was the morphology and taxonomy of ciliate protozoa . One of her major papers in that field was written in 1904 with zoologist and geneticist Thomas Hunt Morgan, who won the Nobel Prize in 1933 for his work. Stevens' investigations into regeneration led her to a study of differentiation in embryos and then to a study of chromosomes . Steven's research showed that very young embryonic cells could not fully regenerate. In 1905 Stevens published a paper in which she announced her landmark finding that the chromosomes known as X and Y were responsible for the determination of the sex of an individual.

Stephanie A. Lanoue


Morgan, Thomas Hunt. "The Scientific Work of Miss N. M. Stevens." Science 36, no. 928 (1912):21-23.

Ogilvie, Marilyn Bailey, and Clifford J. Choquette. "Nettie Maria Stevens (1861-1912): Her Life and Contributions to Cytogenetics," Proceedings of the American Philosophical Society, 125, no. 4 (1981):10-12.

Shearer, Benjamin F., and Barbara S. Shearer. Notable Women in Life Sciences. Westport, CT: Greenwood Press, 1996.

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