During this period, the dominant research frame centered on the male-factor in fertility, treating sperm quality and counts as central determinants of reproductive outcomes. Researchers employed systematic semen analyses and large-scale comparisons between known fertile and infertile populations, establishing early diagnostic thresholds and epidemiological approaches. Attention also broadened to the milieu of fertilization, with emphasis on seminal plasma effects and the emergence of immunological and genetic considerations that reshape clinical evaluation of male fertility.