Amongst other structures which arise from the important layer of ectoderm are the teeth. Of these there are during life two sets, a temporary and a permanent. The temporary teeth, though they do not make their appearance till after the birth of the embryo, still are partly developed during embryonic life, lying embedded in the tissues until the familiar process known as "cutting the teeth" takes place. This is, of course, merely the time of their external appearance. The first stage in the development, however, is a thickening of the epithelium of the gums in a direction which is to correspond with the line where the teeth will eventually pierce. This thickening is called the "dental ridge." This grows downwards into the underlying tissue in flask-shaped growths. From the neck of each of these flasks there is a small projection which indicates where the permanent teeth will ultimately be. This first stage is termed that of the _enamel germ_. This becomes surrounded by cells which ultimately form a _dental sac_. Next, tissue from below grows into the flask, and the further growth of this gives rise to the _enamel organ_. Finally, enamel itself and dentine are developed, and the embryonic tooth remains covered under the gums until it cuts them. So far we have considered merely the mode of development of the most important organs of the body, but we have said nothing of the most important supporting structure, namely, the skeleton. The earliest appearance of anything in the shape of a skeleton is the structure known as the notochord, a structure of immense importance and interest in the embryology of all vertebrate animals, in which it is a temporary thing only. The first appearance of this notochord in lower animals is the earliest indication of the vertebrate type, because it is found that in the higher vertebrates it is the forerunner of the bony spinal column and the skull. It appears first as a groove underneath the medullary groove, of which we have already spoken, and its two lips unite to form a cavity, as did those of the medullary groove. In this case, however, the groove becomes a solid rod, then termed the notochord, and it lies immediately under the medullary groove itself, which, as we have seen, gives rise to the central nervous system. In the course of development, masses of cells come to arrange themselves on each side of the notochord, which they eventually include, and at the same time they grow upwards and around the spinal cord which is thus enclosed. Later on these surrounding portions become cartilage, and, still later, bone; the notochord meanwhile gradually disappearing where the bony spinal column appears. This primitive vertebrate structure therefore, of the notochord, has the all-important function of coming to enclose, and thus protect, the spinal cord and nervous system.