No. I.] THE VERTEBRATE EAR. 155 



been partly used up in the formation of the ear. A glance at 

 Figs. 15, 16, PI. I, and Fig. 4, PI. IX, will show how beautifully 

 and completely this connection is kept through the vicissitudes 

 of the ontogenetic changes. 



When the lateral line organs have made their appearance, the 

 auditory grooves communicate with their line at points equi- 

 distant from the median line on either side of the head. The 

 preservation of the anatomical continuity of the internal ear 

 with the surface organs of the lateral line system is sufficient 

 evidence to establish the thesis on my title page were no other 

 evidence forthcoming. This structural continuity is broken 

 later on in most Elasmobranch species, though it persists in a 

 few forms throughout adult life. In the young Torpedo (PL I, 

 Fig. 16) the relation of the end of the auditory groove to the 

 aural sense organ canal is still plainly visible. The groove ends 

 between two sense organs and marks the place from which the 

 original sense organ of the ear migrated. 



Development of the Organs of the Lateral Line in Ichthyophis 

 according to the Brothers Sarasin. 



The first traces of the lateral line organs appear very early 

 in development. They are first indicated by a thickening of the 

 epidermis in more or less circular patches, which thickening is 

 due to a lengthening of the epithelial cells of both layers, but 

 most noticeably of the outer layer. The apex of the organ 

 sinks inward, while at the same time the cells of the lower 

 layer grow longer in a narrow circle which surrounds the central 

 depression, thereby making its walls higher and the pit deeper. 

 One or more of the cells of the outer layer which lie in the 

 centre of the depression grow larger and rapidly assume the 

 form of the adult sense cells. They soon occupy the entire 

 thickness of the epithelial layer from the bottom of the pit to 

 the basement membrane. The Sarasin brothers believed that 

 all other sensory cells of an organ were produced by the divis- 

 ion of a single cell, the " Sinnesurzelle." 



The epidermal cells of the second layer, which lie immedi- 

 ately underneath the sense cells, are shoved to one side and 

 begin to lengthen, sending a prolongation upwards around and 

 among the sense cells ; when fully developed, they form the 

 long staves or supporting cells of the completed organ. 



