Female students were no longer exceptional by the late 1890s. Between 1878, the year in which Aletta Jacobs became the first woman to take her final medical examinations at the University of Groningen, and 1905, when Ritzema Bos finally left Amsterdam for Wageningen, the proportion of female university students increased from 2 % in the academic year 1895 to over 18 % in 1904.

The percentage of female biology students was traditionally much higher (25 % in 1898; 63 % in 1913). A variety of explanations have been suggested for this. One is that biologists were regarded as peace-loving people who communed with nature in a way that gave them ‘profound joy, pleasure and wisdom’ – something that evidently appealed to girls more than boys. Another is that biology was ‘a good area in which… to find comfort for unfulfilled desires; one did not have to be a brilliant biologist to derive pleasure from it’, as an admirer of Westerdijk's once wrote to her. Then there were some professors who whispered that ‘the ladies come here not to study biology, but to catch a husband. You never see them after their bachelor's exams. Once they get engaged, they take to embroidering cushions.’

Johanna Westerdijk (known familiarly as ‘Hans’), who was born on 4 January 1883 and grew up in Amsterdam, gravitated to biology from her sheer love of nature – along with music, dancing and literature – which she greatly preferred to typical girls’ pastimes like embroidery and playing with dolls. As a young girl, and the eldest child of a general practitioner in Amsterdam, she drew inspiration – like hundreds of her fellow townspeople – from the immensely popular nature books written by the Amsterdam teachers Eli Heimans and Jacobus Thijsse. But it was not until she attended the lectures of Hugo de Vries that she became enthralled with the scientific aspects of biology.

‘Strangely enough, however, [De Vries] would not allow the three new firstyear students who arrived in 1900 to do practical laboratory work,’ she wrote. ‘Out of sheer rebelliousness I applied to the Willie Commelin Scholten Phytopathology Laboratory, where Dr C.J.J. van Hall, a man of my acquaintance and a former assistant of De Vries’s, took me under his wing and instructed me on the anatomy of plants.