204 



MARINE BIOLOGICAL LABORATORY. 



the study of the development of the internal ear than 

 any vertebrate with which I am acquainted. Fig. 8 shows 

 the auditory involution after it has assumed the saucer- 

 shape already alluded to. This condition is the result 

 of a concomitant thickening and sinking of the ecto- 

 derm, which over the area of the saucer represents one 

 of the canal sense-organs of the lateral line system, 

 and the whole process from the beginning on is but a 



Fig. 8. — The head of an embryo Shark 

 {Acanihias vulgaris), ixovci nature, magni- 

 fied 20 times. The figure shows the saucer- 

 shaped depression containing the insinking 

 sense-organ which is to be converted into 

 the auditory sense-organs of the Shark. 



a.v Auditory saucer (vesicle). 

 ep Epiphysis. 

 / Fore-brain. 

 g Gill region. 



^, I Upper and lower hind-brain region. 



m Mid-brain. ^ 



repetition of the formation of the canal organ and its 

 canal, as seen in Amia. As the saucer-shaped thick- 

 ening sinks below the surface, the opening on the 

 surface grows smaller, the bottom of the saucer in- 

 creases in size, and the resulting structure is an 

 auditory vesicle distinctly flask-shaped (Fig. 9, a. v). 

 The neck of the flask grows longer, and finally appears 

 bent backwards and inwards, owing to the increase in 

 size of the head in this region, which causes a transla- 



