=Extra-Uterine Pregnancy.= The tube is a bad place for the ovum to grow and develop, because the tube cannot stretch to such an extent as the uterus can, nor can it furnish the embryo such good nourishment as the uterus can. Occasionally, however, it happens that the impregnated ovum remains in the tube and develops there; we then have a case of what we call _extra-uterine_ (outside-of-the-uterus) or _tubal_ pregnancy. Extra-uterine pregnancy is also called _ectopic_ pregnancy, or ectopic gestation. Unless diagnosed early and operated upon, the woman may be in great danger, for after a few weeks or months the tube generally ruptures. From the moment the spermatozooen has entered the ovum, a process of _division_ or _segmentation_ commences. The ovum, which consists of one cell, divides into two, the two into four, the four into eight, the eight into sixteen, these into thirty-two, these into sixty-four, 128, 256, 512, 1,024, until they can no longer be counted. This mulberry mass of cells arranges itself into two layers, with a cavity in between. And from these layers of cells there develop gradually all organs and tissues, until a fully formed and perfect child is the result. If two ova are impregnated at the same time by two spermatozoa, the result is twins.[5] I might mention here that the moment the ovum is impregnated, i.e., joined by a spermatozooen, it is called technically a zygote; it is also called embryo, and this name is applied to it until the age of five or six weeks. Some use the term embryo up to two or three months. After that, until it is born, it is called fetus. A study of the development of the embryo and the formation of the various organs from one single cell, the ovum, vitalized or fecundated by another single cell, the spermatozooen, is the most wonderful and most fascinating of all studies. But that belongs to the domain of Embryology, which is a separate science.