To these belong:

(A.) Cyclopia

In this malformation we find in the forehead a single eye, or the two eyes blended into one. It is met with under every gradation of the fusion of both eyes. The nose is either wanting, or defective, being frequently represented by an imperforate proboscis-like appendix, which overhangs the one eye or the two united. The mouth is sometimes normal, sometimes misshapen - nay, the entire infra-frontal countenance may be wanting. The ethmoid, nasal, lachrymal, turbinated bones, the vomer, the superior maxillary and palatine bones, the pterygoid processes, are often all or severally absent; the anterior lobes of the brain invariably so. One explanation of this deformity is based upon Huschke's hypothesis, of both eyes being developed out of a single primitive rudiment, subsequently divided in twain by the interposition of the nasal and facial parts. An arrest in the development of these parts might then, indeed, suffice to occasion the mischief. Bischoff, however, firmly maintains that the two eyes originate at once, distinct and separate, from the anterior primitive brain-cell, and he derives the cyclopian deformity from an arrest in the development of this cell causing the too close approximation and eventual fusion of the rudiments of the two eyes. As this defective development of the brain-cell frequently causes a defective development of the anterior portion of the plastic material for the chorda dorsalis, and often for the anterior process of the first visceral arch, it would thus occasion the absence, before alluded to, of the aforesaid facial bones.

(C.) Monotia, Agnathus, Otocephalus

The two ears approach each other more or less below the skull, and finally coalesce. The inferior maxilla is wanting. The superior maxillary, the zygomatic, the palatine bones, along with the pterygoid processes, are either, in like manner, wanting, or else inadequately developed. The mouth is absent or very diminutive. The skull is normal, but the face small, and in brutes projects after the fashion of a proboscis. Bischoff considers this deformity referable to an arrest of development of the first visceral arch, intercepting or impairing the growth of all the said bones, and thus promoting the mutual approximation of the two ears beneath the skull. Were the internal organs of hearing implicated, the source would needs reside in a defective development of the third primitive brain-cell.

(D.) Monopodia, M. Syrenomeles - Siren-Malformation

The two lower extremities, more or less perfectly developed as to their individual parts, are blended into a single one. The pelvis, the sexual and urinary organs, are wanting or imperfect; the intestinal canal is defective beyond the caecum, and the anus invariably absent. The extremities, moreover, have revolved upon their axes, the direction of the patella and of the poples of the knee being reversed. It is founded in a faulty development of the lower end of the trunk and of its organs, the rudiments of which approximate too closely towards each other, and ultimately coalesce.

Syndactylus, Aschysto-Dactylus

Here the fingers or toes are imperfectly separated. It is an arrest of development, the rudiment of hand and foot, even when distinctly cognizable, not manifesting at first any division of fingers and toes.

(E.) Fusion Of Kidneys, Testicles, And Ovaries

This, according to Bischoff, is not due to arrest of development, - even these organs not originating from a single rudiment, - but rather to a defective development of the intermediate formations occasioning fusion of the rudiments.