It was formerly believed that the human being existed in miniature in the first cell and that its development during pregnancy was entirely a matter of increase in size. But the microscope has disproved this, and we now know that embryonic development comprises both growth and evolution. Much of the information accepted to-day is, of course, speculative, having been deduced from observations made upon the reproductive processes of lower mammals, since the youngest human ovum which has been discovered and examined was probably two weeks old. But the evidence points quite convincingly to the belief that the early stages of development consist of proliferation of and alterations in the kinds of cells, their arrangement into groups, and a differentiation of the functional activity of these groups of cells before the mass assumes human form and develops organs. As to terminology, some authorities call this mass the embryo during this stage of grouping and differentiation, which corresponds to the first six weeks of pregnancy, and the fetus from then until the time of delivery. By others it is designated the ovum during the first two weeks of pregnancy, the embryo from the third to the fifth week, after which it is known as the fetus. From the nurse’s standpoint these distinctions are of no consequence, for the mass may safely be called a fetus from the time that the expectant mother looks to the nurse for guidance and care. It is scarcely warrantable to take the time and space which would be necessary to trace in detail through its various stages the intricate development of the human body, with its attached membranes. But the whole question is so important and so interesting that we shall at least have a word of description as to its size and characteristics at successive periods. Although the exact length of time required for the maturation of the fetus is not known, it is estimated that two hundred and eighty days, or ten lunar months, elapse between the beginning of the last menstrual period and the beginning of labor. And in spite of the difference in size among the mothers, it is found that the products of conception develop and grow at a fairly uniform rate of speed. A new human being is the ultimate result of conception, but the chorion, amnion, placenta and umbilical cord must also be created to serve as aids in building and protecting the developing child during its uterine life. The part played by these accessory structures is so vital, in spite of being temporary, that it will be well for us to look into their origin and functions before considering the fetus itself which they serve.