Association with rams had a stimulating influence on the onset of oestrus after anoestrus in the spring. Smell and sound of the rams was sufficient to provide the stimulating influence. Direct contact with rams certainly was not necessary. Ewes which had been held well separated from rams from the beginning of July were permitted varying degrees of association with rams for 11–12 hr daily for 2 weeks in November. Some could smell, hear, see, and have actual contact with the rams; others could smell, hear, and see them only; others could smell and hear them only. These ewes, along with others which had remained well separated from rams during the same period, were then run together with raddled vasectomized rams continuously. Almost all ewes experienced oestrus during the following 26 days. However, most ewes which had previously had some association with rams experienced it first within 17 days whereas most of those which had been previously kept separate from rams did so between the 18th and the 26th day. The results in all groups which were associated with rams were closely similar irrespective of the nature of the association.
Observations were made on 110 Merino rams aged between 1 and 64 weeks. They comprised four groups, the animals of which experienced different rates of growth. The weight of the testes a t any one age varied widely. It was closely related to body weight. This relation was similar in all groups. As body weight increased from 23 to 27 kg, the relative increase in the weight of the testes was much greater than that a t higher or lower body weight. The general histological characteristics of the testes were closely related to their weight, and, like it, to body weight. The tubules were present as solid or but slightly vacuolated cords in all animals which weighed less than 21 kg. Spermatozoa were not seen in the lumen of the tubules in any animal which weighed less than 27 kg, and they were present in all but three of those which weighed more than 28 kg. The relative cross-sectional area of the seminiferous tubules increased directly with the relative increase in the weight of the testes until the testes weighed at least 300 g. The weight of the epididymides was more closely related to the weight of the testes than to age or body weight. Its relation to the weight of the testes varied in the different groups. Irrespective of the age a t which the particular body weight was attained, little separation of the prepuce from the penis had occurred in animals which were lighter than 18 kg, and separation was more or less complete in most animals which were heavier than 27 kg.
Two series of observations were carried out on the effect of the presence of the ram on the incidence of oestrus in ewes. In each series a group of Merino ewes was run continuously with vasectomized rams (the continuous group) and another group (the changing group), the ewes of which had been held separate from rams, was joined with the rams for a period of one month only, fresh ewes having been introduced each month. Each series of observations occupied a period of 12-15 months. In all except the spring months most of the ewes of both the continuous group and the changing group exhibited oestrus. During the spring months of one year, 1942, most of the ewes of the changing group exhibited oestrus but many of those in the continuous group failed to do so. During the spring months of the other year, 1943, most of the ewes of the changing group failed to exhibit oestrus as well as most of those of the continuous group, but whereas the proportion which exhibited oestrus did not fall below 30 per cent. in the changing group, it fell as low as 5 per cent. in the continuous group. The differences between the continuous group and the changing group in the proportion of ewes which exhibited oestrus during September and October 1942, and October 1943, are significant at the 1 per cent. level of probability, and that during August 1942 is significant at the 5 per cent. level. These significant differences in the behaviour of the ewes must be attributed to the difference in their previous association with rams.
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