3. One species of twin formations can at the present time only be satisfactorily explained by the assumption of an ovum in ovo, - one ovum being primitively enclosed within another. We refer to twin formations from invagination or implantation, so-termed conceptionlike germination (Meckel) - diplogenese par penetration. One foetus incloses, at some part, another imperfect foetus - a foetus in foetu: or else one foetus is at some one point, commonly at the skull or palate, united with another foetus, through the medium of a more or less perfect umbilical cord. Meckel regarded the foetus in foetu as a product of conception, and sought to maintain this view by an appeal to analogy; adducing, for example, the formation of hair and teeth independently of copulation, sexless multiplication and propagation, regeneration. At the present day monosexual conception is hardly - multiplication by cotyledons or offshoots, in nowise - admissible. - Certain of the observations in point relate to cases of malformation in the early embryo, in the third and seventh month, for example, in which a conceptionlike product is simply impossible. The occurrence of ovum in ovo, in the instance of birds, at least, is proved; the intussusception of one ovum into another during development is, on the other hand, not conceivable.

4. Finally, an augmented number of parts depends not unfrequently upon arrest of development, and the anatomical excess is reduced to one of no real physiological import; for example, the true diverticulum of the intestine as the remnant of the omphalo-mesenteric duct, double frontal bones, and the like.

Malformations, with supernumerary parts, are divisible into several orders, which, with their principal species, are as follows:

1st Order

Malformations with individual parts supernumerary - head and trunk being single.

Dignathus

Malformation with supernumerary lower jaw. Caudatus. Human foetus with tail-like process at the os sacrum. Polydactylies. Malformation with supernumerary fingers. Notomeles. Monstrosity having supernumerary limbs at the back. Pygomeles, Having supernumerary limbs at the os sacrum. G-astromeles. With supernumerary limbs at the normal extremities. To which are to be added:

1. Supernumerary skull bones.

2. " vertebrae.

3. " ribs.

4. " muscles.

5. " teeth.

6. Double tongue (always superimposed).

7. Double oesophagus.

8. True diverticulum of intestine.

9. Double caecum and vermiform process.

10. Double pancreatic duct.

11. Double hepatic duct.

12. Manifold spleen.

13. Double heart.

14. Multiplicity of kidneys, probably due toarrest of development.

15.Double or triple ureters.

16. Double bladder.

17. Triple testicle (?).

18. Double penis and clitoris (?).

19. Double uterus (U. duplex, bicornis, bilocularis); to be regarded altogether as arrest of development.

20. Testicles and ovaries, seminal ducts, seminal vesicles, Fallopian tubes, uterus, etc, in the same individual.

21. Supernumerary mammae.

2d Order

Twin monstrosities, with double head and trunk.

(A.) Double Formation Of The Upper Portions Of The Body

Heteroprosopus

Two countenances; the one perfect, the other imperfect.

Dicranus

Double skull; countenance either single, or double and conjoined; lower jaw single.

Monocranus

Single skull; countenance partially double; brain double, but unequally so; three or four eyes.

Diprosopus

Double countenance; the faces and heads are completely separate, or the separation affects the faces to the zygomatic arches only; lower jaw invariably double.

Dicephalus

Two entirely separate heads, with two (seldom three) upper, and two (seldom three) lower extremities.

Thoraco-Gastrodidymus

Two heads and necks, thorax and abdomen united into one; four upper and two or three lower extremities. (The Sardinian twin sisters).

Gastrodidymus

Twins united at the lower part of the belly; the four inferior extremities branch off from the sides in pairs, at right angles.

Pygodidymus (Gurlt), Pygopages (G. St. Hilaire)

Two completely distinct bodies, conjoined at their ossa sacra or coccygis. [The well-known Hungarian sisters, Helena and Judith, born in the year 1701, who survived their 22d year].

(B.) Double Formation Of The Nether Parts Of The Body

Dipygus Or Monocephalus (Gurlt), Thoradelphus (G. St. Hilaire)

Head, neck, and thorax single; abdomina and posterior parts separate; two or four upper, always four lower, extremities.

Heterodidymus (Gurlt), Heteradelphus (G. St. Hilaire)

So-called parasite formation. A large, regularly formed body, bearing, at the chest or belly, another, more or less incomplete.

Dihypogastricus

So-called Janus formation. Double body, more or less coalescent above; separate from the umbilicus downwards. Here, either two heads are united with the two countenances (one of which is commonly defective), presenting in opposite directions; or else there is but a single (perhaps defective) countenance, with a double coalescent head. The trunk is double, united down to the umbilicus, and has four upper and four lower extremities.

Symphysocephalus (Barkow), Cephalopages (G. St. Hilaire)

Twin monstrosity united at the head; the twins may be perfect, or of the one nothing may exist except the head.

(C.) Double Formation, Both Above And Below

Diprosopus Dicedoeus (Barkow), Tetrascelus (Gurlt)

Two heads, united at the sides; thorax and abdomen coalescent; two or four upper extremities; urinary and generative organs, as also the inferior extremities, double.

Hemipages (G. St. Hil.)

The heads superficially coherent at the sides; lower jaw in common; neck, thorax, and abdomen as far down as the umbilicus, coalescent; pelves separate; four upper and four lower extremities.

Thoracodidymus (Gurlt)

Two distinct bodies united at the thorax.

Xyphopages

Two perfectly distinct bodies, united only in the vicinity of the ensiform process. (The well-known Siamese twin brothers).

3d Order

Twin malformations through implantation.

Cryptodidymus (Gurlt); so-called foetus in foetu. The greater, perfect foetus bears at some point beneath the skin, or within its natural cavities, a second, smaller, and imperfect foetus.

Omphalo-Cranodidymus

The umbilical cord, together with the rudiment of the one foetus, rooted within the skull of the other.

Epignaihus

An imperfect foetus rooted, with its bloodvessels, at the palate of a more perfect foetus.

4th Order

Triple Monstrosities

Their existence is confirmed by modern researches.

Supernumerary parts may be normal, both in form and structure; in both respects, however, they are frequently in various degrees defective.

The frequency of duplicate forms varies in different portions of the body; for example, a multiplication of viscera, or of organs of sense, is far more rare than of extremities.

It will be seen from the above, that in twin monstrosities the connection between two individuals is either a mere superficial one, occurring through the medium of skin and of bone, or else one involving, at the point of union, the blending of cavities of the body, and the union, in various degrees, of the same organs in the two individuals.

Acquired preternatural increase of number consists, in man and in the higher animals, in a multiplication of the elementary constituent parts of a tissue, - of the essential or the secondary structural elements which enter into the composition of an organ. It is, therefore, the manifestation of increase of mass or density in an organ - never of the development of new, complex ones. Still, the arrest or alienation of tissues developed for the repair of injuries, or of destructive morbid processes, does sometimes determine the formation of supernumerary apparatuses foreign to the standard of the organism; for example, anomalous excretory ducts; accessory articulations.