TERATOGENESIS. 617 



Sometimes the rudimentary brain shows traces of structures which the 

 normal brain possesses, and is raised above the level of the defective skull like 

 a turban — acrania with exencephaly. With acrania are usually associated 

 facial clefts, defects in the eyes, etc. 



The malformation known as hemicrania is limited to a part of the skull, 

 usually the posterior part. The brain mass often protrudes through an opening 

 in the cranial vault and forms a mass on the back of the head or hanging down 

 upon the neck — hemicrania with exencephaly. 



In the various forms of cephalocele or cerebral hernia the roof of the skull is 

 more nearly complete and the protrusion of the cranial contents is limited 

 to circumscribed areas. The protruding mass may consist of brain substance 

 only — encephalocele, or of the membranes only — meningocele, or of both — 

 meningoencephalocele. Sometimes the brain ventricles are distended by the 

 accumulation of fluid — hydr encephalocele, or a sac formed by the membranes 

 may be distended by fluid — hydromeningocele. 



A condition known as hydrencephaly is sometimes met with. Fluid ac- 

 cumulates in the brain cavities after the skull is formed, causing a general 

 enlargement of both brain and skull, without hernia (congenital hydrocephaly) . 



A combination of hydrencephaly and cephalocele may also occur. The 

 cervical vertebrae adjoining the skull are cleft dorsally and the protruding mass 

 lies in the cleft — iniencephaly. 



Hydromicrencephaly means an accumulation of fluid with a rudimentary 

 brain and a correspondingly small skull. 



Porencephaly is a lower grade of hydromicrencephaly, in which fluid ac- 

 cumulates in the third and lateral ventricles and affects the adjacent frontal 

 and parietal lobes. If the individual lives with this malformation, the intellect 

 is impaired and the extremities contract and atrophy. 



Microcephaly and micrencephaly go together as abnormal smallness of the 

 skull and brain. The brain, aside from the diminutive size, may not be de- 

 formed. These conditions, in which the body is of the usual size, should not 

 be confused with those found in dwarfs in whom the body also is small 

 (nanocephaly) . 



In the region of the spinal cord there is a group of malformations consisting 

 of varying degrees of clefts in the vertebral canal. The clefts may remain open 

 —rachischisis— ox they may be covered by a sac-like prominence— spina bifida 

 (spina bifida cystica, rachischisis cystica). Both forms of cleft may occur in 

 any region of the vertebral column and may be limited, or involve the entire 

 column. 



The malformation known as rachischisis appears as a widely open groove 

 bounded laterally by rudimentary laminae of the vertebrae. The deformity may 

 include the entire vertebral column— holorachischisis , or it may be confined to 



