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.It could form, for example, if adiploid sperm (having two complete sets of chromosomes) fusedwith a haploid ovum (having one complete set of chromosomes),producing an embryo having three sets of chromosomes.McClintock s training and studies as assistant in Sharp scytology class had offered her a thorough exposure to the his-tory and techniques of staining a method for accentuating cel-lular structures for microscopic studies.Randolph and Sharp hadlearned a new staining technique developed by cytologist JohnBelling descriptively known as the squash technique.Belling sfind called for fixing cells on a slide, staining them with a carminestain, and then applying the glass cover with the thumb to flattenthem carefully so the inner structure could be seen.Together,Randolph and McClintock used the smear technique to analyzethe chromosomes of the pollen parent cells from the triploid plant.They also developed a hypothesis for how this unusual plant cameto be, and they published their results in the American Naturalistin February 1926.Their teamwork ended there, as Lee Kass discovered, becauseMcClintock felt she had done most of the work and should havereceived first listing as coauthor.As a result, a distancing developedbetween her and the Plant Breeding Department, where Randolph sside of the story probably received a friendlier hearing, if onlybecause he was working closely with Emerson. Science as a Way of Finding Out 37The two had also disagreed about a practice McClintock appar-ently had of jumping in to solve a problem without an invitationIn addition, Randolph s progress on the problem of identifying themaize chromosomes was blocked because he could only determinelarge from small maize chromosomes and had not yet found a meth-od for seeing distinguishing characteristics between chromosomes ofdifferent sizes.There was also controversy about the base number ofchromosomes for maize because different strains had different basenumbers and researchers had reported different numbers for variousstrains of maize.McClintock, who always loved a good puzzle, decided to try herhand.Later she would remark, Well, I discovered a way in which hecould do it, and I had it done within two or three days the wholething done, clear, sharp, and nice. But as science historian Lee B.Kass pointed out in correspondence with the authors, the project took a long time [it was] not so easy. In fact, Kass points out,while tradition has it that McClintock solved the problem while stilla graduate student, she did not succeed in solving several importantaspects until 1928 29, after she had completed her Ph.D.in 1927and after Randolph published a paper in 1928 establishing the basenumber of chromosomes as 10.Meanwhile, upon receiving her doc-torate, to put food on the table, she accepted an appointment as aninstructor at Cornell.By 1929, her thesis was published in Genetics, a key journalin the field.She also found time, between teaching assignments,to succeed in finding two new angles on the stubborn problemof chromosome morphology.First, as she said, she looked in adifferent place.Randolph had been studying maize chromosomesduring what appeared to be the best opportunity the metaphase,when they appeared to get larger and fatter and, therefore, easierto see.McClintock also made successful use of the metaphasestage to find identifying characteristics on the chromosomes.Asan instructor, McClintock figured out a way to use the metaphaseof mitotic haploid chromosomes in the first division of the devel-oping pollen grain the gametophyte (n) stage to identify the10 maize chromosomes.She especially noticed that she could tellhow each chromosome differed in length in these mitotic haploid 38 BARBARA MCCLINTOCKchromosomes: She saw that a narrowing, the centromere, occurredin different locations on different chromosomes, creating armsof different lengths.One even had a sort of satellite, or moon,attached to the main strand by a slender thread, or stalk.In fact,it so happened that all 10 chromosomes had clearly recognizablemorphological (structural) markers, serving to orient the travelerlike a street sign or, more aptly put, like a landmark.She publishedthese findings as a short but major article in the prestigious jour-nal Science, announcing the haploid number (10) of chromosomesfound in maize as well as a diagram, known as an ideogram, of all10 chromosomes.This one feat alone showed McClintock s early accomplish-ment, but she had in mind doing much more.Her success withidentifying the maize mitotic haploid chromosomes, publishedin 1929, was just beginning, and in the next three years, she con-tinued to produce a series of papers that, in the words of geneti-cist Marcus Rhoades, clearly established her as the foremostinvestigator in cytogenetics. Thanks to McClintock s knowledge,experience, and unusual talents, with a little practice she soonbecame expert at Belling s stain technique.By making a few wiz-ardly adjustments to improve the clarity of results for maize, shesucceeded in gathering a highly viewable sample of every chromo-some at each stage of cell division and replication in both mitoticand meiotic chromosomes.By 1929 30, she also had the idea of looking at the process ofcell division during the pachytene stage of meiosis.During this stage,in certain strains of corn, the chromosomes are more slender andstretched out and less condensed.McClintock found that the chro-mosomes were much easier to study before they became so dense.She also could see a narrowing at the area where the two chroma-tids attached themselves to the stretched-out spindle fibers.Whileobserving chromosomes in the pachytene stage, in corn plants pro-vided by Dr.C.R.Burnham, she also observed a knobby structure onsome chromosomes.The absence or presence and location of theseknobs offered another way to tell the chromosomes apart.(For moreinformation, see Kass and Bonneuil s paper published in 2004 andKass s 2003 publication on this subject.) Science as a Way of Finding Out 39McClintock was therefore able to identify the different maizechromosomes in 1929, and by 1933, in pachytene stages, the chro-mosomes were numbered 1 through 10, from longest to shortest, bycontrasting their shapes, measuring their lengths, and comparingtheir patterns.This important episode in her early career alreadyshowed her uncanny ability to combine flawless laboratory tech-nique with visual acuity, sure-footed interpretation of what she saw,and highly cerebral and imaginative integration of all the pieces to acomplex puzzle. 3Scientist at Work(1927 1931)arbara McClintock began her research career knowing exactlyBwhat she wanted to do: explore cytogenetics or, in other words,work with chromosomes and their genetic content and expressions.For fruit flies, researchers had already discovered that specific linkedgenes were always carried on the same chromosome
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