Complex sentence: In 1674, he and a fellow student, assisted by Van Leeuwenhoek, were the first to observe semen, a situation that would later lead to a priority dispute between Hartsoeker and Leeuwenhoek over the discovery of spermatozoids.
Split into simpler sentences: In 1674, he and a fellow student, assisted by Van Leeuwenhoek, were the first to observe semen. This situation later led to a priority dispute between Hartsoeker and Leeuwenhoek over the discovery of spermatozoids.