608 TEXT-BOOK OF EMBRYOLOGY. 



system (anencephaly) are produced. In these experiments the malformations 

 follow retarded closure or lack of closure of the neural tube. 



DEFECTS IN THE REGION OF THE FACE AND NECK, AND THEIR ORIGIN. 



Associated with malformations of the brain there is a group of defects which 

 involve the eyes and nose, and to which the term cydocephaly has been applied. 

 The cerebral hemispheres are derivatives of the fore-brain. Sometimes they 

 fail to develop properly and are represented by a single mass occupying a 

 considerable portion of the cranial cavity. The eyes primarily represent 

 lateral, symmetrical outgrowths from the fore-brain vesicle. If the cerebral 

 hemispheres fail to develop, the development of the eyes is profoundly influ- 

 enced. Instead of being widely separated there may be any degree of mal- 

 formation from a mere narrowing of the distance between them to a complete 

 fusion into a single organ within a single medial orbit synophthalmia or cyclo- 

 pia. Within this orbit the eye may possess double or partially blended cor- 

 neae, pupils, lenses, and optic nerves, or it may have single structures. 



Since the fronto-nasal process, which plays an important part in the forma- 

 tion of the nose, depends for its normal shape upon the development of the fore- 

 brain region, various degrees of malformation of the nose almost invariably 

 accompany cyclopia. In a typical cyclops the nose is reduced to a fleshy mass 

 protruding from the frontal region. 



It is not unusual to find clefts of the upper lip (hare lip) and of the palate 

 (cleft palate) associated with cyclopia; for the normal union of the fronto- 

 nasal and maxillary processes depends upon the development of the fore-brain 

 region. The branchial arches likewise may be affected with resulting mal- 

 formations of the mouth and external ear. The two ears may be united across 

 the ventro-medial line synotus or cydotus, and the mouth slit may be absent 

 cydostomus. 



The eye may also be the seat of local defects. It may remain abnormally 

 small micr ophthalmia, or incompletely developed, or may be entirely lacking 

 anophthalmia. The eyelids may enclose an abnormally narrow fissure 

 ankyloblepharon, or the fissure may be wanting cryptophthalmia, or the lids 

 may be adherent to the eyeball symblepharon. Sometimes there is a cutaneous 

 fold which partly fills the inner canthus like the nictitating membrane in lower 

 forms epicanthus. 



Malformations of the face are not uncommon, all such congenital defects 

 being due to incomplete fusion of the processes which form the jaws and 

 greater part of the face (see page 148). In extreme cases there is an early 

 and complete arrest of development of all the parts which normally form the 

 face aprosopus. Arrested development of the first pair of branchial arches 



