It should be remembered that cleft palate is a factor which predisposes to death by infectious diseases whose local manifestations are in the mouth and throat; hence the majority of the degenerates in whom cleft palate occurs are liable to die before the completion of their fifth year. Hare-lip (Fig. [32]) is an exceedingly hereditary disorder, as Murray, Demarquay, Trélat, Hutchinson, Féré, Marsh, Lucas, Ireland, and others have shown. It occurs with great frequency in almost all forms of degeneracy. Kiernan, of Chicago, has found that 5 per cent. of the hereditary lunatics in that city and in New York City have hare-lip.
FIG. 32.
How far degeneracy may turn back the page of evolution is excellently illustrated in the cyclopian descendants of degenerate families elsewhere cited from Kiernan, of Chicago. While there is a seeming conflict as to the primitive eye type of vertebrates between morphologists represented by Howard Ayres[203] (who claims that the eyes were derived from the median eye of the ascidian lancelet) and Semper[204] (who is of the opinion that the existing vertebrate eyes represent the paired eyes of a hypothetical annelid precursor); still both opinions are fully reconcilable through the results of study of the ascidian and lancelet eye collated with cyclopian and triophthalmic (three-eyed) degeneracies in man, the human eye and the third eye of reptiles like the hatteria of New Zealand. The eye of the ascidian tadpole agrees fundamentally with the type of eye peculiar to the vertebrates in that the retina is derived from the wall of the brain. On this account it is called a myolonic eye. In the typical invertebrate eye, on the contrary, the retinal cells are differentiated from the external ectoderm.
The ascidian eye, however, differs, as Osborn[205] remarks, essentially from the paired eyes of the craniate (skulled) vertebrates in that the lens as well as the retina is derived from the wall of the brain. The lens of the lateral eye of the vertebrates is derived by an invagination of the ectoderm, which meets and fits in the retinal cup at the end of the optic vesicle. The ascidian eye, however, agrees in respect to the origin of its lens with the parietal or pineal eye of the lizard, in which the lens is likewise derived from cells which form part of the wall of the cerebral outgrowth which gives rise to the pineal body.
The pineal body is another of those remarkable rudimentary structures whose constant presence in all groups of vertebrates forms such an eminently characteristic median outgrowth from the dorsal wall of the brain (thalamencephalon), the distal extremity of which dilates into a vesicle and becomes separated from the proximal portion. The distal vesicle becomes entirely constricted off from the primary epiphysial (pineal) outgrowth of the brain, and the parietal nerve does not represent the primitive connection of the pineal eye with the roof of the brain, but arises quite independently of the proximal portion of the epiphysis.
The remote ancestors of the vertebrates possessed a median, unpaired, myolonic eye, which was subsequently replaced in function by the evolution of the paired eyes. The cyclopic condition occurs very frequently among human monstrosities, much more frequently than among animals, Hannover claims,[206] but this is clearly due to the fact that human monstrosities are much more frequently recorded. Of the 120 cases I have been able to collect from literature, 56 presented other evidences of degeneracy than cyclopic conditions, and 60 had neuropathic or other taint in the ancestry. Dareste[207] has shown that the production of a single eye, the changes in the structure of the mouth, the atrophy and abnormal situation of the olfactory apparatus and of the vesicle of the hemispheres, all result from an arrest of development. The determining influence must be exerted very early in the life history of the embryo.
