These consist in diminution or augmentation of the normal number of organic parts. It is not rare for both to be found united in one individual, one part presenting a deficiency, another, in virtue of the law of compensation, an excess of formation. Thus, monstrosities, in which otherwise deficiency predominates, will exhibit a superfluous finger or toe; double twin malformations, on the contrary, deficiency in various parts.

Deficiency or absence of individual parts, or diminished number of plural organs, are frequent, - for example, the absence of entire extremities, of individual fingers and toes; amongst the viscera, of one of the kidneys. It is either congenital or acquired. In the former case it includes malformations with deficiency, in a stricter sense. There is scarcely any part that has not been found wanting, without detriment to the entirety of the rest of the body. In this respect, however, certain relations or sympathy may not be overlooked, by virtue of which the absence of one part is paired with that of another part. Thus, in acephali the heart is almost always absent, very commonly, too, the entire thoracic viscera, together with the liver, the spleen, the pancreas, - an example of compliance with rule perfectly inexplicable; for neither can development from an identical germ, nor functional dependence of the organs here be argued. Occasionally so many parts are absent at once, that nothing is born save a head, a single extremity, or a shapeless mass. The absence of parts is very often, though by no means invariably, due to arrest of development.

The principal kinds of malformation that pertain hither are the following (GeoffVoy St. Hilaire, Gurlt, Bischoff):

1. Amorphics, Anideus

A shapeless mass consisting of cutis, areolar tissue, fat, and a few bones; is never found but in association with a perfect twin. It probably results from an early destruction of the germ.

2. Acephalus

The head alone may be wanting, or with it more or less of the trunk, so that, in fact, nothing may be present save a pelvis with the inferior extremities, or with one of these only. It is for the most part associated with a twin. Even though the trunk be present, the heart is usually absent; the respiratory organs probably always; liver, spleen, and pancreas commonly; stomach and intestinal canal are generally very defective; the uropoietic and generative organs are mostly present, though incomplete. It is in some instances perhaps de-ducible from injury to or destruction of the germ, or from the disturbance occasioned by a twin.

3. Pseudacephalus, Paracephalus

Rudiment of head, with the rest of the body either entire or defective. These are sometimes twin cases, and are for the most part the consequence of hydrocephalus.

4. Aprosopus

The face, and especially the eyes, nose, and mouth, wanting. The skull is diminutive, and exhibits the ears coalescing, either in front or above. The brain is always very defective. The pharynx terminates, superiorly, in a caecal sac. Here again, probably, destruction - splitting of the medullary tube, as also of the dorsal plates, at their anterior part, takes place at an early period. Hence the non-development of the anterior brain-cell with eyes and nose, and in like manner the superior arches of the cranial vertebrae, of the parietal and of the frontal vertebrae, and the consequent inclining towards each other of the temporal bones for the closure of the vertebrae. Hence, probably, also, the non-development of the anterior visceral arches, involving absence of the inferior maxilla and of the facial bones, with anterior junction of the external ears, which emanate from the second and third visceral arches.

5. Microcephalics

Diminutive, incomplete head; similar to the foregoing, except that the inferior jaw is present, proving the development of the first visceral arch.

6. Anophthalmus

Absence of both eyes or of one eye only. Eyelids and lachrymal organs are present, although often defective - coalescent. Perhaps for the most part dependent upon dropsical destruction of the eye-vesicles, rudiments of the optic nerves being commonly discoverable.

7. Absence Of The Eyelids

Absence Of The Eyelids; an arrest of formation, these organs being of later development.

8. Absence Of Iris

Absence Of Iris; in like manner, an arrest of formation.

9. Anotus

Absence of the external ear, - deficient development of the external portion of the first visceral groove.

10. Brachyrhynchus

Shortness of nasal prominence, owing to deficiency of intermedial jawbones.

11. Acormus

One rudimental head with a regular twin, or with a triple birth. Most probably due to mechanical hindrance to development, and to destruction occasioned by the other foetus or foetuses.

12. Oligospondylus

Absence of vertebrae or of semi-vertebrae, is owing either to an anomaly of original germinal development, or else to the coalescing of two or more vertebrae or semi-vertebrae.

13. Anaedceus

Absence of the entire generative apparatus, or of the external organs of generation only; very rare as an independent malformation, where the individual is otherwise normally formed. It is an arrest of development, the said parts not being evolved out of the germ.

14. Peromelus And Micromelus

The limbs are wanting or maimed.

It is frequently an arrest of development; it may, however, result from mechanical influence, - from the severing of members through strangulation.

15. Phocomclus

Monstrosity with phocal extremities, the hands issuing directly from the shoulders, the feet from the pelvis, whilst the intervening parts are either wanting or merely rudimental. It is an arrest of development often dependent upon hydrocephalus or spina bifida.

16. Parosomus

Various deformities caused by the absence of individual parts.

17. Absence Of Various Individual Organs Of The Thorax Or Abdomen

Absence Of Various Individual Organs Of The Thorax Or Abdomen, of a lung, the liver, the spleen, the stomach or its blind sac, a portion of intestine, etc. It is either an arrest of development or the result of morbid destruction.

A particular kind of diminution of number consists in the symphysis or fusion of kindred formations; fusion of two fingers, toes, ribs, vertebrae, of the inferior extremities, of the kidneys, obliteration of the uterus. A deficiency of various grades is included under cleft-formations.