The acquired absence of particular parts is the result of mechanical influence or of disorganization. To the former kind belongs maiming by accident or design; for example, amputation, extirpation, and the like, which sometimes greatly resemble certain congenital defects. To the latter kind is to be referred the wasting of various organs; for instance, from spontaneous, primitive atrophy, from that consequent upon inflammation, from destructive suppuration or gangrene.

Preternaturally augmented number of parts is very frequent, and commonly congenital. It occurs in every variety of grade, from the duplication of individual diminutive parts to that of the body almost in its totality. These various degrees constitute a series, the gradations of which are so regular, that it would appear forced to divide malformations of this class into such as with a single head and trunk present duplicates of individual parts, and into such as at the same time possess double or triple head and trunk. Nor does such a distinction derive much support from the assumption that the former are due to an excess of plastic activity, the latter to the primordial existence and the fusion of twofold germs.

Bischoff(with Winslow, Haller, Meckel) opposes the following weighty arguments to this distinction, as also to the assumption of twofold germs and their fusion (Lemery, Breschet, Gurlt, Chaussier, and Adelon).

1. The malformations of this class, from the supernumerary finger or toe up to the development of two perfect embryos united only at one point, constitute so complete and uninterrupted a series, that it would be in the highest degree forced to assign for the one and for the other opposite causes, namely, excess of formative activity, and again fusion with deficiency of plastic power. Still no one can hold a supernumerary finger or toe to be due to the fusion of two embryos.

2. In twin malformations, none but the same organs, systems, or parts, whether internal or external, ever coalesce. It is invariably thorax with thorax, abdomen with abdomen, head with head, breech with breech.

Again, brain is ever found coherent with brain, bloodvessels with bloodvessels, intestine with intestine, etc.; never trachea with oesophagus, nerves with bloodvessels, and the like. This is a fact, the constancy of which precludes its explanation on the ground of accidental fusion from external causes.

3. Twin malformations manifest, for the most part, a change not limited to the parts immediately united, but pervading the entire organism. Such a change could hardly be brought about by accidental fusion.

4. Twin malformations always recur with great uniformity of character. Are external causes likely always to combine after the same fashion?

5. Again, twin malformations often occur out of the same mother, and the tendency to them is hereditary. There is neither proof nor probability of an abiding external cause, - for instance, in the maternal organs of generation.

6. At no period of development is a mechanical fusion of the ova and embryos probable; indeed, our present knowledge respecting the nature and earliest development of the ovum renders such fusion highly improbable. The pellucid zone or yelk-bag is in the highest degree unfitted for it; nor is it at all possible to press two ovula against each other in such wise as to occasion the fusion of their yelks, of their germinal vesicles, or of the zonae pellucidae of the latter. In the Fallopian tubes and the uterus, however contracted, the ovula undergo no such risk, even in multiparous animals, where they lie densely grouped together. After the embryos have become developed and shrouded within their amnia, their coalition ceases to be even conceivable. These membranes would have to undergo previous rupture; for the occurrence of twins within a single amnion is too rare and too inexplicable to be here taken into account. How slight is the disposition amongst various embryos to coalesce is shown by those cases of twins, in which, owing to confined space, the one is almost flattened by compression, without any coalition having taken place.

It results from the above that the aforesaid division of malformations with supernumerary parts, and the assumption of twofold germs and their coalition, are inadmissible. The task, therefore, still devolves • upon us of explaining this class of malformations. Accordingly we have to observe that:

1. The cause might consist in an anomalous formation of the ovum in its unimpregnated state. Bischoff adduces in favor of this view the occurrence of double yelks, as also the aforesaid hereditary character of twin formations and their recurrence out of the same mother. This cause would, however, apply only to more perfect twin formations, it being difficult to imagine a mere supernumerary finger or even extremity to have a similar origin.

2. Some have long conceived the formative power within the germ to be endowed with unusual energy, causing the development of a greater number of parts than belong to the species. The facts observed by Wolff, Von Bar, and Reichert, certainly relate to twin formations of a very early stage of development. Still this stage is too far advanced to be cited in direct support of an assumed augmentation of plastic power in the germ, as determining either an approach to the formation, out of the plastic materials of the yelk, of a double, or even of the partition of a single zona pellucida. On the other hand, these cases are of a period so early, and of characters so marked, as to render it almost self-evident that the cause of the malformation must have been a primordial one, or at least have dated from the earliest period. The multiplication of individual parts is possible at a later period, provided the germ remain for them unchanged.