During the fourth week of human development the neuropore in a normally developing fetus closes. When this process is either interrupted or never initiated,<unk>  occurs. The desmocranium becomes a membranous coverage instead of forming the epidermis of the scalp. Whether from being blocked by amniotic bands or by just not initiating, the migration of mesenchyme under the ectoderm does not occur. Because this migration does not occur, the skull, and all involved muscles, are never formed. Without the presence of the neurocranium, the brain fails to separate into two separate lobes. 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.
Acrania
Macrodontia 
Macrotia
Microcoria
Spotted lunulae
Amastia
Macrostomia
Polycephaly