This study was conducted in order to observe the changes in cellular association of seminiferous tubules from 4 to 22 weeks of age and to obtain the cycle and relative duration of seminiferous epithelia from 24 weeks of age in male ducks. Fifety-five male ducks were used in the experiment and divided into 11 groups, consisting of 5 male ducks each, with 2 weeks intervals from 4 to 24 weeks of age. The body and tests weight showed most rapid increase during 4 to 6 weeks and 18 to 22 weeks of age, respectively. The seminiferous tubules were abruptly enlarged in diameter of tubules during 18 to 22 weeks of age. Gonocytes were seen from 4 to 6 weeks of age, however they were not observed as from 8 weeks of age. Both type Ap spermatogonia and type Ad spermatogonia occurred from 8 to 12 weeks of age, while spermatocytes and spermatids were beginning to appear at 16 weeks and 18 weeks of age, respectively. Spermatozoa were first observed at 20 weeks of age. Full spermatogenic activity was completed at the age of 20 weeks. Average paired weight of the testes in male ducks was 78g at 24 weeks of age and its ratio spermatogenic activity was completed at the age of 20 weeks. Average diameter of seminiferous epithelium at 22 weeks of age was 232 micro m, and average numbers of Sertoli cell, spermatogonia, spermatocyte, spermatids and spermatozoa in the cross section of seminiferous epithelium were 15.30, 59.08, 41.78, 71.11 and 165.30, respectively. Spermatogonia and spermatids were classified into 2 and 4 types, respectively. The cycle of the seminiferous epithelium could be divided into 5 stages at 24 weeks of age. The relative frequencies of stages from I to V were 13.5 %, 25.0 %, 22.3 %, 20.6 % and 18.7 %, respectively. Thus, establishment of spermatogenesis in male ducks were beginning to appear at 20 weeks of age

The present study was carried out to investigate the effects of nitrate (KNO3) on the thyroid glands in rabbits which were administrated KNO3 of 1g/kg/day for 6 weeks. Growth rate, and serum levels for T4, T3 and TSH were observed every week. The histological changes and the weights of the thyroid glands were observed in 6 weeks. The mean growth rates of experiments were decreased significantly in the 1st week, but were increased a little from the 5th to 6th week compared with those of controls. The serum levels for T4 of the experimental group manifested significantly decreased values than those of control through the experimental term consistently and the serum levels for T3 were greatly decreased in the 3rd and 4th weeks. TSH contents of the serum were not changed through the experimental term. The mean weight of the thyroid gland in the experiment was decreased significantly after 6 weeks compared with that of cotrol. In the experimental group, the color of colloid in the thyroid follicles revealed deeper eosinophilic and the heights of the follicular epithelial cells were taller than those of controls. The colloid in the thyroid follicles revealed depletion. As summarized above, the observations suggest that nitrate can be an antithyroid substance in rabbits and it leads the thyroid glands to hypofunctional state