Cytological observations were systematically employed for detecting cellular morphogenesis and chromosomal behavior of apogamous development of new sporophytes from individually cultured male gametophytes of Pyropia haitanensis. The results showed that the apical parts of premature male gametophytes initially underwent obvious apogamy, which primarily appeared in the functionally specialized spermatangial mother cells (SSMCs) that would not develop into highly differentiated spermatangia but change their developmental fate and become brownish red in color. These brownish red spermatangial mother cells (SMCs) were released accompanying the spermatia dissociated from mature spermatangia and then rarely formed circular carpospore-like cells (CLCs) and carpospore-like germlings (CLGs) that eventually developed into normal apogamous sporophytic conchocelis. Chromosome counts indicated that spermatangia and spermatia were haploid (n = 5), whereas SSMCs, CLCs, and CLGs were diploid (2n = 10). Furthermore, filamentous conchocelis and conchosporangial branch cells of apogamous sporophytes were diploid and no haploid, aneuploid, and polyploid were observed, suggesting that spontaneous chromosome doubling occurred in the process of apogamy and initiated rarely SSMCs rather than released spermatia. Moreover, pit connections were observed in the whole development of apogamous sporophytes like in heterozygous sporophytes. Mitotic chromosomal behaviors appeared during the first two divisions of conchospores that normally grew into haploid tetrads and developed into unisexual male gametophytes with the characteristics of apogamy in the aging cultures.