Chromosomes of Rattus norvegicus in spermatogonial mitosis and 1st and 2nd meiosis are illustrated by a series of photomicrographs. The following observations highlight this demonstration: 1. The Y chromosome remains positively heteropycnotic throughout the three division cycles while the X becomes positively heteropycnotic at the onset of the 1st meiosis and remains heteropycnotic until the end of the 2nd meiosis. 2. In the mitotic and the two meiotic prophases, regions of the autosomes adjacent to the kinetochore are marked by heterochromatin. At diplotene, one chiasma, as a rule, is seen to lie near the heterochromatin. 3. The X and the Y during 1st meiotic prophase are embedded in the sex vesicle. The arrangement of the X and the Y at diplotene suggests the partially homologous nature of the X and the Y. The chiasma formation between the X and the Y, however, appears to be limited to the extremely short second arms which may be euchromatic. The 1st meiotic division is reductional to the X and the Y.