The type B spermatogonia were proportionately rounded cells with distinct spherical nuclei. The chromatin substance was observed as distinct clumps which usually adhere to the nuclear membrane. The nucleoli were located at the centre of the nucleus, but deviations were also noted. The cytoplasm was lightly eosinophilic (Figure 7). Prior to transformation of gonocytes into spermatogonia, an intermediate prespermatogonial stage occurred which had been ascertained as “quiescent” by Guraya [25]. The transformation of the gonocytes into spermatogonia was considered as initiation of spermatogenesis [26]. The first appearance of spermatogonia in the seminiferous tubules was an important factor as it denotes the onset of spermatogenesis. In the present study, the transformation of gonocytes to spermatogonia was observed in the seminiferous epithelium at 4 months of age (group-III) indicating the initiation of spermatogenesis at this age in Assam goat. However, Baishya et al. [27] recorded the initiation of spermatogenesis at 69 days of age in the Assam goats, which was much earlier as compared to 4 months of age (group-III) observed in this study. As the study was made at 2 and 4 months of age, the possibility of initiation of spermatogenesis after 2 months and prior to attaining 4 months of age could not be ascertained in the present work. The age of initiation of spermatogenesis was reported to be variable in different animals, which was recorded as 70 days in the Dwarf Nigerian lamb [21], 150 days in rams [6], 120 days in Brown Swiss crossbred bulls [28], and 350 days in buffalo [18].

The meiotic phases of the primary spermatocytes were recognized in the kids from 4 months of age (group-III) onwards. The leptotene primary spermatocytes were uniformly rounded cells with spherical nuclei. The chromatin substance was organized to form dense filamentous network which filled the nucleus completely. The cytoplasm was lightly stained. The nuclei were distinct spheroid shaped (Figure 8).

The zygotene primary spermatocytes were marked by their deeply stained nuclei, which were usually notched, but occasionally rounded in shape. The chromatin substance appeared coarse. The nuclei were ill defined, and nuclear envelope was indistinct. The cytoplasm of the cells was pale stained (Figure 8).

The pachytene primary spermatocytes were characterized by large spherical cells, and their nuclei were uniformly spheroid. The chromatin material appeared in coarse filamentous arrangement leaving irregular interstices in the nucleoplasm. The nuclear envelope was indistinct, and nucleoli were small and faintly stained. The cytoplasmic rim was comparatively broad and faintly stained (Figure 8).

The diplotene primary spermatocytes were characterized by their largest size of the spermatogenic series. They were large, rounded cells with spherical nuclei. The chromatin material was more loosely arranged. They were stained lightly than the pachytene nuclei. The nuclear membrane was indistinct, and the nucleoli were rarely perceptible. The cytoplasmic rim surrounding the nucleus was weakly eosinophilic (Figure 9).

The secondary spermatocytes were smaller in size than the primary spermatocytes but larger than the round spermatids. They were rounded in shape, and their nuclei were spherical with centrally placed small dot-like clumps of chromatin interconnected with fine filamentous network. The nuclear membrane was distinct with indistinct nucleoli. The cytoplasm was scant and eosinophilic. These cells were seen in the goats from 6 months of age (group-IV) onwards.

The round spermatids (Figure 9) were the smallest cells of the spermatogenic series. They possessed spherical nuclei and had thin peripheral cytoplasmic rim. The chromatin substance appeared granular with 2 to 3 larger irregular aggregates which stained more intensely than the surrounding nuclear material. The nuclear membrane was thin and prominent.

The elongated spermatids (Figure 7) were characterized by elongated nuclei with condensed chromatin. They stained more deeply and either remained attached to the Sertoli cell cytoplasm or migrated centripetally towards the lumina of the seminiferous tubules.

The spermatozoa (Figure 8) were usually found attached to the Sertoli cells at their ad luminal border and in the lumen of the seminiferous tubules after their release from the Sertoli cells. The head was elongated—oval in shape. The present study confirmed the first occurrence of spermatozoa attached to the ad luminal border of the Sertoli cells or in the lumina of the seminiferous tubules at 6 months of age (group-IV), indicating the onset of puberty at this age in male Assam goats. The first occurrence of spermatozoa had been reported at 18 weeks of age in the Dwarf Nigerian ram [21], 165 days in native rams of Iran [29], and 20 weeks in Korean native goat [12]. The initiation of puberty as evidenced by presence of sperm cells in the ejaculated semen was found to be at 29 weeks in the Beetal X Assam local male kids [30].

The histological characteristics of all the spermatogenic cells recorded in the present study in the Assam goats at various ages were in agreement with the earlier findings reported in the Dwarf Nigerian ram [21], domestic animals [31], goat [32], and native rams of Iran [29].