ARREST OF DEVELOPMENT. 43 



imperfect condition of the middle ear and tympanic 

 bones. The right Eustachian tube is marked by a black 

 bristle ; it is a mere groove open below. 



159. A vertical section of the head of a Human foetus in which 

 the palate is entirely deficient. A groove above the 

 mouth, marked by a bristle, represents the nasal cavity. 

 The cribriform plate of the ethmoid is very small, its 

 vertical plate is absent. The palato-pterygoid and max- 

 illary process (suborbital arch) are absent. The eye is 

 abnormally small. The external ear is represented by a 

 small lobule; there is no meatus. The cavity of the skull 

 occupied by the middle cerebral lobe is much enlarged ; 

 the remainder of the cranial cavity is correspondingly 



contracted. 







13. Prosencephalic arrest. 



Syn. Cyclops, Gurlt. 



Ehinocephalus, Cyclocephalus, Ethmocephalus, Geoff. 



W. Vrolik* first ascribed the origin of the following forms to 

 arrest of development of the anterior portion of the investing 

 cartilage of the notochord ; and there can be little doubt that 

 this is the most satisfactory explanation yet arrived at. The 

 trabecular arch and the structures arising from it are always 

 absent or defective. The nasal septum is either entirely absent, 

 or is represented by a defective vomer. The whole ethmoid bone 

 is usually wanting. There are traces of inferior spongy bones 

 in the frontal proboscis, a process arising from {fie union of the 

 naso-frontal processes. There is sometimes a single nostril, 

 sometimes a pair beneath the eyes, between the united sub- 

 ocular arches, when a defective nasal cavity exists between the 

 maxillae. More frequently the proboscis is perforated by a 

 nostril, and contains a cavity extending back to the skull, in 

 which a pair of cartilaginous spongy bones are situated. In 

 such cases the maxillary bones are very massive and are fused in 

 the median line. The orbits are united into a single cavity, and 

 the eyes are more or less perfectly fused, sometimes forming a 

 single globe. The brain exhibits unmistakable signs of arrest. 



* Over den Aard en Oorsprong der Cyclopie : Amsterd. 1834. 

 Miillor's Arch. 1836. 



