60 NERVOUS SYSTEM OF VERTEBRATES. 



in front of the auditory pit, one in front of the other, which give 

 rise to the supraorbital and infraorbital lines of sense organs and 

 whose ganglia are closely related to the trigeminus and profundus 

 ganglia. It is evident that in the early history of vertebrates the 

 acustico-lateral system occupied three segments and that pits 

 were formed to contain the sense organs. It is probable that 

 at first the portions of the system in the three segments were 

 independent of one another. 



In typical fishes the main rows of sense organs are enclosed in 

 canals similar to the lateral line canal. In addition to these chief 

 rows there are often auxiliary rows or groups of pit organs and 

 in many cases the typical canals are incompletely formed, the 

 organs remaining as pit organs. In cyclostomes and amphibia 

 canals are not formed. In selachians there appear along the course 

 of the main rows of organs depressions which sink in as pits and 

 then become deep, narrow tubes and eventually extend beneath 

 the surface to end in one of several masses of jelly-like tissue 

 lodged in concavities of the skull. The deep ends of the tubes 

 expand and develop sense organs which are related in structure 

 to the lateral line organs. These are the ampullae of Lorenzini. 

 The mode of development of the vesicles of Savi and of the 

 nerve sacs of ganoids, both of which belong to the acustico- 

 lateral system of organs, is not known. 



The complete hiatus in the neural crest in front of the anlage 

 for the VII nerve is connected in some way with the development 

 of the acustico-lateral system opposite this point. Probably 

 this system actually uses up the material of this part of the neural 

 crest. Forward from this, overlying the region of the cerebellum 

 and midbrain, is a large segment of the neural crest which gives 

 rise to the trigeminus group of nerves. It is necessary to say group 

 of nerves, because the adult trigeminus is formed from two nerves 

 which arise independently in vertebrate embryos. 



From the crest covering neuromeres v and vi in selachians grow 

 down two ganglionic anlages which remain connected peripherally 

 so that they form an U-shaped loop, having two connections 

 with the brain. The caudal of these two anlages forms the gang- 

 lion of the N. trigeminus proper and gives rise to the sensory por- 



