STEVENS'S STUDIES ON SEX DETERMINATION:
The uncertainty over sex determination is borne out by Stevens's early research. Together with Morgan, she did experiments on aphids to test the claim that temperature alters sex ratios in populations. These experiments failed to confirm the externalists' hypothesis. In her early cellular studies of aphids (which she did by herself), Stevens also failed to detect McClungs accessory chromosomes. Thus, at the end of 1904, she concluded that although it seemed likely that sex was somehow determined by the eggs and sperm, exactly how it was determined remained unclear.
During the next two years, Stevens completed a comparative, cellular study of several species of insects, drawn from a diverse group of beetles (Coleoptera), butterflies and moths (Lepidoptera), and true bugs (Hemiptera). This was painstaking work. First she dissected the tiny gonads from the insects. These were fixed in a preservative solution, embedded in paraffin blocks, and sliced into very thin sections. The tissue sections were then mounted on microscope slides and stained with one of several dyes. Careful observations of the tissue sections revealed gametes in various stages of development. If a cell had been just beginning to divide and the tissue had been cut at just the right angle, all of the chromosomes were clearly visible and could be accurately counted.
In the common mealworm, Tenebrio molitor, Stevens found that the body cells of females contained 20 large chromosomes, while body cells in males contained 19 large chromosomes and 1 small chromosome (the Y chromosome). She also found that although mealworm eggs always contained 10 large chromosomes, there were two types of sperm: 50 percent contained 10 large chromosomes, 50 percent contained 9 large chromosomes plus the 1 small Y chromosome. This pattern was found in the other species that she studied except that, in a few species, the small (Y) chromosome was completely absent. In these cases, 50 percent of the sperm contained a set of chromosomes identical to that found in eggs, while in the other 50 percent the number of chromosomes was one fewer than that found in eggs. Stevens's results could be generalized as follows: for any given species, all eggs are the same, but there are two distinct types of sperm.
In her conclusion, Stevens pointed out that a male is produced whenever an egg is fertilized by a sperm carrying the small Y chromosome (or lacking this chromosome); a female is produced whenever an egg is fertilized by a sperm carrying the large X chromosome. Having stated this conclusion, however, she refused to claim that the X and Y chromosomes could accurately be described as sex chromosomes.” These chromosomes seemed to play some hereditary role in sex determination, but how they did so was still unclear. In other words, Stevens had found an important correlation between chromosomes and sex determination, but she couldn't prove causation. She cautiously concluded that further evidence was needed before one could speak confidently of sex chromosomes.”
With the benefit of hindsight, Stevens's conclusion seems curiously conservative, but other prominent biologists were even more hesitant about drawing general conclusions from a relatively small sample of data. Wilson made similar observations of chromosomes and published his results at about the same time Stevens did. Like Stevens, he refused to endorse the idea of sex chromosomes. Although he admitted that chromosomes provided the best working hypothesis" for explaining sex determination, he remained open to the possibility that they were simply indicators, rather than determiners of sex. Morgan was even more reluctant to accept the idea of sex chromosomes. Some of Stevens's contemporaries, particularly embryologists, never acknowledged that chromosomes might play a role in sex determination.
Nettie gained notoriety after her death in 1912, from Thomas Hunt Morgan. He stated:
Modern cytological work involves an intricacy of detail, the significance of which can be appreciated by the specialist alone; but Miss Stevens had a shre 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.
The uncertainty over sex determination is borne out by Stevens's early research. Together with Morgan, she did experiments on aphids to test the claim that temperature alters sex ratios in populations. These experiments failed to confirm the externalists' hypothesis. In her early cellular studies of aphids (which she did by herself), Stevens also failed to detect McClungs accessory chromosomes. Thus, at the end of 1904, she concluded that although it seemed likely that sex was somehow determined by the eggs and sperm, exactly how it was determined remained unclear.
During the next two years, Stevens completed a comparative, cellular study of several species of insects, drawn from a diverse group of beetles (Coleoptera), butterflies and moths (Lepidoptera), and true bugs (Hemiptera). This was painstaking work. First she dissected the tiny gonads from the insects. These were fixed in a preservative solution, embedded in paraffin blocks, and sliced into very thin sections. The tissue sections were then mounted on microscope slides and stained with one of several dyes. Careful observations of the tissue sections revealed gametes in various stages of development. If a cell had been just beginning to divide and the tissue had been cut at just the right angle, all of the chromosomes were clearly visible and could be accurately counted.
In the common mealworm, Tenebrio molitor, Stevens found that the body cells of females contained 20 large chromosomes, while body cells in males contained 19 large chromosomes and 1 small chromosome (the Y chromosome). She also found that although mealworm eggs always contained 10 large chromosomes, there were two types of sperm: 50 percent contained 10 large chromosomes, 50 percent contained 9 large chromosomes plus the 1 small Y chromosome. This pattern was found in the other species that she studied except that, in a few species, the small (Y) chromosome was completely absent. In these cases, 50 percent of the sperm contained a set of chromosomes identical to that found in eggs, while in the other 50 percent the number of chromosomes was one fewer than that found in eggs. Stevens's results could be generalized as follows: for any given species, all eggs are the same, but there are two distinct types of sperm.
In her conclusion, Stevens pointed out that a male is produced whenever an egg is fertilized by a sperm carrying the small Y chromosome (or lacking this chromosome); a female is produced whenever an egg is fertilized by a sperm carrying the large X chromosome. Having stated this conclusion, however, she refused to claim that the X and Y chromosomes could accurately be described as sex chromosomes.” These chromosomes seemed to play some hereditary role in sex determination, but how they did so was still unclear. In other words, Stevens had found an important correlation between chromosomes and sex determination, but she couldn't prove causation. She cautiously concluded that further evidence was needed before one could speak confidently of sex chromosomes.”
With the benefit of hindsight, Stevens's conclusion seems curiously conservative, but other prominent biologists were even more hesitant about drawing general conclusions from a relatively small sample of data. Wilson made similar observations of chromosomes and published his results at about the same time Stevens did. Like Stevens, he refused to endorse the idea of sex chromosomes. Although he admitted that chromosomes provided the best working hypothesis" for explaining sex determination, he remained open to the possibility that they were simply indicators, rather than determiners of sex. Morgan was even more reluctant to accept the idea of sex chromosomes. Some of Stevens's contemporaries, particularly embryologists, never acknowledged that chromosomes might play a role in sex determination.
Nettie gained notoriety after her death in 1912, from Thomas Hunt Morgan. He stated:
Modern cytological work involves an intricacy of detail, the significance of which can be appreciated by the specialist alone; but Miss Stevens had a shre 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.
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