The hindbrain proceeds to develop normally, allowing for the child to be carried to term, but the diencephalon and ocular lobe remain small and underdeveloped. Ectodermal mesenchyme
Mesenchyme is formed in the developing embryo and will eventually become cartilage and bone. When ectodermal mesenchyme fails to migrate into the head region of the embryo, the skull will not be able to form. What exactly causes the failure of mesenchymal migration is unknown. Diagnosis
Acrania can be diagnosed early in pregnancy through an ultrasound. This abnormality appears during the beginning or end of the fourth week of the fetuss development. An absence of the skull is needed in order to make a diagnosis. A presence of brain tissue will confirm the diagnosis of acrania and differentiate it from other developmental problems such as anencephaly. Prognosis
Anencephaly is a fatal condition. Infants with anencephaly are stillborn in about 75 percent of cases. Newborns who survive die within several hours, days, or weeks. This disease is rare, occurring in 1 in 20,000 live births. In order to better manage an acrania diagnosis, early detection is of extreme importance. Families may choose either to terminate the pregnancy, or to carry the fetus to term, depending on the laws where they live. Acrania may cause a fetus to spontaneously abort before reaching term. References
External links

Acalvaria (Acrania) at NIHs Office of Rare Diseases
Structural closure in term babies occurs within 3 to 7 days. After it closes, the remnant is known as ligamentum venosum. If the ductus venosus fails to occlude after birth, it remains patent (open), and the individual is said to have a patent ductus venosus and thus an intrahepatic portosystemic shunt (PSS). This condition is hereditary in some dog breeds (e.g. Irish Wolfhound). The ductus venosus shows a delayed closure in preterm infants, with no significant correlation to the closure of the ductus arteriosus or the condition of the infant. Possibly, increased levels of dilating prostaglandins leads to a delayed occlusion of the vessel. == References ==