If the first vaccination be found imperfect in character, that is, if it has not properly 'taken,' the operation should be repeated at the earliest opportunity. It has been recommended, in all cases, to perform a second vaccination not later than the sixth or eighth year. If small-pox be prevailing, it is proper to vaccinate all who have not been vaccinated within three or four years. In any event, re-vaccination at or after the period of puberty is of extreme importance. It will give additional security even to those whose original vaccination was perfect. In some cases, the susceptibility to small-pox is not wholly exhausted by one vaccination. Inasmuch as it is desirable for every one to escape this disease, even in its most modified form, re-vaccination should always be performed, as it affords a very sure and trustworthy means of such escape. After successful re-vaccination, small-pox, even in its mildest shape, is rarely met with. In girls especially, in whom the changes which occur at puberty are most marked, re-vaccination should be performed about the age of fourteen. GROWTH AND DEVELOPMENT. During infancy the body grows with great rapidity. About the end of the third year one-half of the adult height of the body is attained. After this period growth is more gradual; for in order to reach the remaining half, about eighteen years more are required. At twenty years of age the height is somewhat more than three and a half times that at birth, and the weight about twenty times. Development does not go on at an equal rate in all parts of the body. The lower limbs, small at birth, increase proportionally more rapidly, while the head, relatively large at birth, developes more slowly. The muscular system is gradually strengthened. At the end of the third month the infant is able, if in good health, readily to support its head; at the fourth month it can be held upright; at the ninth month it crawls about the floor; by the end of the year it is able with assistance to step; and between one and two years, at different times, according to its vigor and activity, it acquires the power of standing and walking alone. The periods of greatest and least growth of the child are, on the one hand, spring and summer; on the other, autumn and winter. It has long been known that animals grow more rapidly in the spring than at any other season of the year. This has been attributed to the abundance of herbage they are then able to obtain. It has been ascertained by actual measurement, that children grow chiefly in the spring.