Agenesia is an incomplete and imperfect development of an organ or part and aplasia is absence of an organ or part.
Acrania is absence of most or all of the bones of the cranium.
Amelia is absence of one or more limbs.
Anencephalia is absence of the brain.
Hypocephalia is incomplete development of the brain.
Hemicrania is absence of half of the head.
Exencephalia is defective skull with brain exposed or extruded. If the protruding brain contains a ventricle which is filled with excessive amount of fluid, the malformation is a hydrencephalocele.
Arhinencephalia is absence or rudimentary development of the olfactory lobe with corresponding lack of development of the external olfactory organs.
Agnathia is absence of the lower jaw.
Anophthalmia is absence of one or both eyes.
Abrachia is absence of the forelimbs.
Abrachiocephalia is absence of forelimbs and head.
Adactylia is absence of digits.
. Excess of Development
1. Congenital hypertrophy
Hemi hypertrophy (partial)
2. Increase in the number of a part
Polyotia (ears)
Polyodontia(teeth)
Polymelia (limbs)
Polydactylia(digits)
Polymastia (mammary gland)
Polythelia(teats)
Twins Entirely Separate
Although separate, these twins are in a single chorion. One twin as a rule is well developed; the other is malformed (acardius). In the malformed foetus there is arrested development of the heart, lungs, and trunk. Such monsters may lack a head (acephalus), limbs and other recognizable features (amorphous), or the trunk (acormus).
Twins United
These twins are more or less completely united and are of symmetrical development.
TWINS UNITED
A. Anterior Twinning: The anterior part of the individual is double, the posterior single.
Pygopagus – united in the pelvic region with the bodies side by side.
Ischiopagus – united in the pelvic region with the bodies at an obtuse (not pointed) angle.
Dicephalus – two separate heads; doubling may also affect the neck, thorax and trunk.
Diproosopus – doubling in the cephalic region without complete separation of heads; only the face doubled.
B. Posterior Twinning: The posterior part is double, the anterior single.
Craniopagus – brains usually separated; bodies as a rule at an acute angle.
Cephalothoracopagus – union of head and thorax.
Dipygus – doubling of posterior extremities and posterior part of body.
C. Twinning Almost Complete: Duplication of the whole trunk or the anterior or posterior extremities with parallel, ventral arrangement of the foetuses. The pair is joined in the region of the thorax, and also often in the abdominal region.
Thoracopagus – united only by the thorax.
Prosopothoracopagus – besides the union the thorax the abdomen, the head and neck are united.
Rachipagus – thorax and lumbar portion of the spinal column united.