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.
(ii) That in other defects accessory structures are sometimes produced in the neighbourhood, as if Nature, being baulked in her efforts of development at one spot, expends her energies in some less useful addition elsewhere. The accessory auricles and auricular appendages seen in macrostoma are cited as illustrations of this; and any precanine external to the cleft is maintained to be of a similar character. Whilst fully admitting the plausibility of such teaching, I cannot see that it explains such osseous development as occurs in Albrecht’s Kiel skulls, or such a regular appearance of an incisor external to the cleft as that indicated by the figures quoted above. The less frequent existence of an accessory tooth in the os incisivum seems much more readily explicable on such a ground.
To summarize the principal points as to the development of ordinary harelip:
1. That the intermaxilla is derived from the union of four ossific portions, two on either side of the median line, and that these are to be known respectively as the endo- and meso-gnathion, whilst the superior maxilla is termed the exognathion.
2. That these ossific portions are developed from the internal and external nasal projections of the fronto-nasal process respectively, and that ordinarily the central and lateral incisors are developed one from each segment. Occasionally an accessory incisor is developed between the other two from the endognathion.
3. That the external nasal process does not enter into the formation of the upper lip, but terminates superficially in the depression immediately below the ala nasi (see [p. 57]).
4. That simple harelip, where the cleft is limited to the soft parts, is due to the non-union of the superficial portions entering into the formation of the lip.
5. That in alveolar harelip the cleft passes between the endo- and meso-gnathion.