1. Adler, H. C. (1957). Thesis. Royal Veterinary and Agricultural College, Copenhagen, Denmark.
2. Akpokodje, J. U. (1984). Paper presented at the 4th Annual Conference, Nigerian Society for the Study of Reproduction, NAPRI, ABU, Zaria, Nigeria.
3. Barlett, D. E., Hasson, E. V. &Teetar, K. G. (1947).American Veterinary Medical Association Journal,110, 114–120.
4. Clark, B. L. &Dufty, J. H. (1978).Australian Veterinary Journal,54, 530–531.
5. Finlay, R. C., Ruckerbauer, G. M. &Stovell, L. P. (1985)Canadian Journal of Comparative Medicine,49, 231–232.