1. The cytological study of 4–8 hr. old zygotes of seven species of Plasmodium, i.e. P. gallinaceum, P. berghei, P. cynomolgi, P. gonderi, P. inui, P. knowlesi and P. vivax, revealed a uninucleate condition at the resting phase.
2. Ookinetes of these species at 12–24 hr were uninucleate and of the usual dimorphic form.
3. In the present studies post-zygotic meiosis was demonstrated in the early oocysts of the above seven species of Plasmodium. The times of investigation for the beginning and completion of meiosis in the early oocysts of these species of Plasmodium were as follows:
4. Different forms of spireme at prophases were observed in the early oocysts of the above species of Plasmodium. This fact indicated that each species has its own characteristic form of spireme; for instance, ‘bow-shaped’ in P. gallinaceum, beaded, filamentous net-work in P. cynomolgi.
5. The diploid and haploid number of chromosomes were demonstrated step by step, during the first reduction division in the early oocysts of seven species of Plasmodium.
6. The succeeding nuclear divisions following the post-zygotic meiosis were traced in early oocysts of seven species of Plasmodium. The divisions took place mitotically, giving rise to tetra- to multinucleate oocysts. The haploid group of chromosomes in each species was quite distinct during the different mitotic phases found in these oocysts.
7. The results obtained from the cytological study of different meiotic and mitotic phases found in the early oocysts of seven species of Plasmodium show that the number of chromosomes is a specific character, for instance, P. gallinaceum has a complex of two haploid chromosomes, P. cynomolgi, four; P. gonderi, three; P. inui, four; P. knowlesi, P. berghei and P. vivax, two.