Final answer:
The haploid cells in spermatogenesis that develop into functional spermatozoa are called spermatids, which undergo spermiogenesis to become mature sperm.
Step-by-step explanation:
During spermatogenesis, the haploid cells that require further differentiation in order to become functional spermatozoa are called spermatids. After the process of mitosis and the two rounds of meiosis, the primary spermatocytes divide to form secondary spermatocytes, which then undergo meiosis II to produce spermatids. These spermatids are initially round with a central nucleus and a significant amount of cytoplasm.
Then a transformation process known as spermiogenesis occurs, where the early spermatids undergo morphological changes, reducing the cytoplasm and shaping themselves into mature sperm with a flagellum, a compact head, and a middle piece rich in mitochondria. This detailed structuring is vital for the functionality of the sperm as it allows for motility and the efficient delivery of genetic material to the ovum.