ARREST OF DEVELOPMENT. 39 



ment, giving rise to median fissures, or the cranio-facial axis fails 

 and undergoes atrophy or absorption, and the lateral arches unite 

 prematurely, so that fusion of the lateral organs results. In 

 the first case there is usually some shortening of the cranio- 

 facial axis, at least in extreme cases. In the second form the 

 axis becomes narrowed, or is absent in parts of the skull, and the 

 corresponding nervous and facial structures fail to be developed. 

 The presence of lateral parts corresponding to the several por- 

 tions of the axial region affords a strong indication that the axial 

 parts are not absent from the commencement of development, 

 but that they fail to be developed and become atrophied at an 

 early stage. The most common form originates in the absence 

 or failure of the parts developed from the investing mass in front 

 of the notochord, the olfactory lobes and nasal capsules are then 

 deficient. When the investing mass fails more posteriorly the 

 eyes and mesencephalon are found wanting. When the invest- 

 ing mass fails in front of the ear-capsules, so that the basi- 

 occipital ends in a point between them, the mandible is absent ; 

 and in extreme cases, when the structures normally developed 

 in the investing mass in front of the ear-capsules all fail, the 

 tympanic cavities are fused in front. It frequently happens that 

 the mesencephalic or epencephalic region are deficient, whilst 

 the anterior and posterior portions of the cranio-facial axis are 

 developed. The changes which produce these abnormal condi- 

 tions probably commence in the investing mass, and not in the 

 subsequently formed bones, since failure as frequently occurs 

 between the bask>ccipital and basisphenoid as in either of these 

 bones. In this case the anterior part of the basioccipital and 

 the posterior part of the basisphenoid are equally arrested. The 

 nerve-centres appear to be always arrested at points correspond- 

 ing to the deficient portions of the axis, so that it appears im- 

 probable that the malformation originates in deficiency of the 

 facial arches. Rarely one lateral half only of a portion of the 

 cranium is arrested, the other being normally developed. 



IN FISHES. 



140. The anterior portion of an eel with complete deficiency of 

 the face in front of the eyes. The mandible extends be- 



