Foetus, the name given to the mammalian embryo from the time when the changes which take place in the ovum after fertilisation have resulted in the formation of the rudiment of the offspring until birth. The human festus at the end of the first month of pregnancy is about one-third of an inch in length, the rudimentary eyes and ears are just distinguishable, and four bud-like processes represent the future limbs. By the end of the third month the embryo exceeds three inches in length; the head is separated from the trunk by the formation of a distinct neck; ossification has commenced in most of the bones, and the placenta or after-birth, through which the fcetus is now to derive its nourishment, is already formed. By the end of the fifth month the foetus weighs nine or ten ounces; the various features of the face have considerably approximated towards their ultimate shape, and fcetal movements can now be felt by the mother. By the end of the seventh month the foetus has attained a length of some fifteen inches, and weighs about forty ounces. After this period the foetus becomes viable - i.e. if born, it may possibly be reared. The fcetus at term averages about twenty inches in length, and weighs about seven pounds; the fontanelles, a large anterior and smaller posterior membranous interspace between the bones of the skull, are widely open; ossification is still very incomplete, too, in the bones of the limbs and trunk; the series of changes in the circulatory apparatus which adapt the child for the change from the foetal circulation to that of the distinct individual, are now well-nigh complete, though the closure of the foramen ovale, which connects the two ventricles of the heart, and the complete degeneration of the fcetal structures known as the ductus arteriosus and ductus venosus, are not effected until a few days after birth. The peculiarities of the foetal circulation cannot be discussed in detail here; suffice it to say that the pulmonary circulation is, of course, not yet established, and that aeration of the blood is effected in the placenta, while the structure of the fcetal heart is adapted to the special conditions for securing a flow of blood to the various parts which obtain.