174 



AVERS. 



[Vol. VI . 



and posterior vertical and external horizontal semicircular canals. 

 These rudiments are readily visible from the exterior." 



His two figures are taken from embryos rendered transparent 

 and sketched through the tissues from the side. 



Mitrophanow (1890, 196), who has worked upon the lateral line 

 system of Elasmobranch fishes, has recently announced that 

 the auditory saucers and the sense organs of the lateral line 

 system in the auditory region of the head have a common origin 

 from a thickened band of epithelium, which begins in the spinal 

 suture and takes in on both sides of the body the regions sup- 

 plied by the VII + VIII and the IX + X nerves from where it 

 descends to the gill region. First of all, the ear saucer sepa- 

 rates from the sensory band which 

 is thus cut in two parts, an anterior 

 belonging to the VII, and a posterior 

 lying in the territory of the IX. 

 This separation is indicative of an 

 earlier condition of independence, 

 which has been obliterated by the 

 shortening and modifying in other 

 ways of the ontogenetic processes. 

 This investigator observed epithe- 

 lial ridges similar to those de- 

 scribed by Kupffer in Petromyzon 

 in the territory of the V, but he 

 concluded that they were not re- 

 lated to the canal organs of the 

 adult. 



The early history of the develop- 

 ment of the Elasmobranch auditory 

 capsule does not differ materially 

 from that of other forms except in 

 the rate of growth. In this respect 

 it is a much more favorable object 

 for the study of the development of 

 the internal ear than any other ver- 

 tebrate with which I am acquainted. 

 Cut 17 shows the auditory area of 

 the skin after it has assumed the saucer shape, this condition 

 of the organ following on the preliminary thickening of the 



Cut jy. — The head of an embryo 

 Shark (^Acanthias vulgaris), from 

 nature, magnified 20 times. The 

 figure shows tlie saucer-shaped de- 

 pression containing the insinking 

 sense organ which is to be con- 

 verted into the auditory sense organs 

 of the Shark. 



a.v Auditory saucer (vesicle). 



ep Epiphysis. 



f Fore-brain. 



g Gill region. 



/. ! Upper and lower hind-brain region. 



ni Mid-brain. 



