It is not to be wondered at that some discrepancy should exist between the observations of these two eminent anatomists. The researches of which we are speaking are sufficiently delicate and difficult; and even much more recent authors are far from agreeing perfectly, as far as regards the period, in which the development of the teeth begins. Serres, in his _Essai sur l’anatomie et la physiologie des dents_ (Paris, 1817), sustains the view that in the fetus he has observed the germs of all the teeth, both temporary and permanent, while Joseph Linderer (_Handbuch der Zahnheilkunde_, Berlin, 1842) says that, although he has followed the preparative method indicated by Serres, he could never discover in the fetus the germs of all the teeth. Perhaps, he adds, the time when the development of the teeth begins varies considerably in individuals, just as we remark differences in the time of eruption. In Chapter XVII of his book, Eustachius speaks of the process of formation of the teeth, which he studied in abortive fetuses, in stillborn children, in children a few months old, and also in kids. On dissecting a fetal jaw, there may be found on each side, as we have already seen, the incisors, the canines, and three molars, still soft and imperfect, separated from one another by very thin osseous partitions. Each of these teeth is enclosed within a follicle or little bag of a grayish white color, rather more mucous and glutinous than membranous, and in form somewhat like the pod of a vegetable, with the only difference that it shows an opening at one of the extremities, from which the tooth somewhat protrudes, as if it were germinating. The more recent and softer the tooth, the more its follicle has a mucous appearance and differs from the nature of membranes. As it does not adhere to the underlying tooth, it is easy to separate them. As to the tooth, it is at that period of its development partly osseous and partly mucous, since that part which later on projects from the gum soon becomes transformed into a white thin and concave scale, which gives the idea of one of the little cells of a honeycomb. This scale is harder and more conspicuous in the incisors, since these, at this stage, are better formed; the canines are less advanced in development, and the molars still less; and among these latter, those are less developed which are more distant from the canines. The deeper part of the tooth consists of a mucous and tenacious substance, harder, however, than the substance of the follicle, and of a whitish color with a tendency to dark red, translucent, and somewhat brilliant. Thus, says Eustachius, the teeth present themselves in a human fetus; but he who cannot obtain a human fetus may observe the same things in a kid.