National Geographic (via Greg Laden) reports on a fisherman caught a pregnant Dusky Shark (Carcharhinus obscurus) off the coast of Mexico. When he opened his catch he was surprised to find, along with 9 normal foetuses, an albino cyclops.
Cyclopia is an extreme form of Holoprosencephaly, a chromosomal defect that occurs in many animals, including humans.
Holoprosencephaly is characterized by the failure of the prosencephalon (the forebrain of the embryo) to develop. During normal development, the forebrain is formed and the face begins to develop in the fifth and sixth weeks of pregnancy. Holoprosencephaly is caused by a failure of the embryo’s forebrain to divide to form bilateralcerebral hemispheres (the left and right halves of the brain), causing defects in the development of the face and in brain structure and function.
There are three classifications of holoprosencephaly. Alobar holoprosencephaly, the most serious form in which the brain fails to separate, is usually associated with severe facial anomalies. Semilobar holoprosencephaly, in which the brain’s hemispheres have a slight tendency to separate, is an intermediate form of the disease. Lobar holoprosencephaly, in which there is considerable evidence of separate brain hemispheres, is the least severe form. In some cases of lobar holoprosencephaly, the patient’s brain may be nearly normal….
Ethmocephaly is the least common facial anomaly. It consists of a proboscis separating narrow-set eyes with an absent nose and microphthalmia (abnormal smallness of one or both eyes). Cebocephaly, another facial anomaly, is characterized by a small, flattened nose with a single nostril situated below incomplete or underdeveloped closely set eyes.
Although rarely seen in wild animals, due to the high mortality rate, it is not unknown in domestic animals.
In humans, most of these children show some form of trisomy 13. In its more severe instances, infants rarely live beyond 12 months, and many foetuses die in the womb. The least severe cases result in the well-known condition of a cleft lip (although trisomy 13 is not the only cause).
There are many images of animals and children with cyclopia on the web, but I’ll let you find them yourself.