The development of the mammary glands follows the same course during all first pregnancies, regardless of the time of the breeding season. When a second or third pregnancy occurs at the beginning of the breeding season, the development of the glands during the pregnancy resembles the development during the first pregnancy once the embryos have reached the primary amniotic cavity stage. In all other pregnancies the development is complicated by a concurrent lactation and later involution. However, the structure of the glands during lactation is essentially the same, regardless of which pregnancy it follows, and of whether the lactation overlaps with a subsequent pregnancy, or occurs during anoestrus. Therefore it seems that the presence of developing embryos does not influence the course of lactation in any way. Owing to this similarity, the structure of the glands is the same in both the second and third pregnancies until the developing embryos are approximately 5 mm long, if the pregnancy occurs during the middle or at the end of the breeding season. It appears that the young are weaned when the developing embryos reach this size, i.e. 5 mm long. However, the changes in the glands during involution are affected not only by the time of the breeding season, but also by the pregnancy of the individual animal. When involution occurs during anoestrus it proceeds until a final state, only slightly more developed than the virgin condition, is reached. When involution occurs concurrent with the next pregnancy it differs according to the pregnancy of the individual animal. After a third pregnancy, which commenced either at the beginning or during the middle of the breeding season, the animal usually dies of old age before involution is established, but after a first or second pregnancy commencing at these times, involution takes place during the subsequent pregnancy. In these animals the extent to which the involution proceeds is governed by the size attained by the developing embryos at that particular time. When the involution occurs during a second pregnancy, i.e. after the first period of lactation, the most involuted condition is found when the embryos are 32-6 mm long. When it occurs during a third pregnancy, i.e. after the second period of lactation, involution is completed earlier, when the embryos are only 13-7 mm long, and this involution is more pronounced than that occurring during the second pregnancy. It is difficult to explain why this first period of involution should be more gradual, and take so much longer than the second. Moreover, in the latter there is a definite point whereafter involution is striking and rapid; this does not occur during the first period of involution. The only feasible explanation I can offer is that possibly, during the first period of pregnancy, lactation, and then involution, the regular cycle has not yet been established in the mammary glands. After this stage they may be fully mature, and the cycle stabilized.
Longitudinal Study of Shiga Toxin-Producing Escherichia coli Shedding in Sheep Feces: Persistence of Specific Clones in Sheep Flocks
