The accessory tooth in the os incisivum met with in some cases of alveolar harelip is maintained by Albrecht to be a reappearance of this lost middle incisor; and his explanation of such an occurrence seems very feasible, viz. that the existence of the alveolar cleft prevents the naso-palatine artery from anastomosing with the posterior palatine, and thus the vascular supply to the os incisivum is greater than it should be under normal circumstances; hence, there being a superabundance of nutrient material, nature uses such in the restoration of a structural unit ordinarily suppressed. The same fact (viz. the absence of the usual anastomosis) may explain why the mesognathion is (even when demonstrably present) always small and the lateral incisor not infrequently stunted or absent, and so answers the objection to this theory which has been raised on the ground that in cases of alveolar harelip an incisor external to the cleft is not invariably present. Hence the dental formula in cases of alveolar harelip will vary as follows: In tetraprodontous jaws— CI₂=I₁I₁=I₂C, or C=I₁I₁=C; in hexaprodontous jaws— CI₂=IₐI₁I₁Iₐ=I₂C, or C=IₐI₁I₁Iₐ=C, where Iₐ represents the accessory incisor, the other letters as on p. 51. The former of each of these pairs of formulæ represent the mesognathion and lateral incisor as present, the latter as absent.[59] Notwithstanding the mass of positive evidence which is steadily accumulating in favour of Albrecht’s theory, there are still some careful observers who will not admit its truth. Some attack it on the ground that the intermaxilla does not consist of four portions (Kölliker, &c.); with such we have already dealt. Others object to it on the ground that any teeth existing external to the cleft are merely accessory teeth, or due to a bifidity of the canine. The arguments with which they support their opinion are derived from the following considerations: (i) That in normal development accessory teeth do certainly occur, as in the case mentioned above (p. 53), whilst in a few instances of abnormality the same condition has been noted (_e. g._ a few cases recorded by Kölliker;[60] see also Fig. 19, showing a facial cleft in which several accessory teeth are present along the inner margin). And the explanation given of such facts is that the involution of mucous membrane from which the teeth are developed is continuous along the alveolar ridge, and not localised to the definite spots from which the teeth subsequently erupt. This, however, is merely an opinion still _sub judice_, and not absolutely proven.