No. 2.] VERTEBRATE CEPHALOGENES/S. 237 



organ has retained in its inclosed state the tendencies of growth 

 possessed by the surface organs, atid in the vertebrates above the 

 IchtJiyopsida offers the only remnant of the perfected primitive 

 canal system of sense organs. 



The position of the ear capsule does not jnark a divide between 

 two morphologically differejit portions of the brain, nor has this 

 capsule played any part in tJie formation of the brain contours. 



No explanation has ever been offered of the origin of those 

 peculiar structures, the semicircular canals, of which Foster 

 says, " But the peculiar features of the semicircular canals sug- 

 gest almost irresistibly that they are special agents " in the 

 equilibration of the body. 



The solution here offered makes clear why these canals are 

 semicircjilar, and why they, as canals, have such special and 

 morphologically significant relations to the sense organs in the 

 ampullae, and it further helps us materially in forming a judg- 

 ment as to the function of these organs which have so long 

 proved a fruitful source of speculation, experiment, and differ- 

 ence of opinion. 



Our knowledge of the development of the sense organs of 

 the lateral line, and of the canal system, so intimately associated 

 with them, we owe to several writers, but E. P. Allis has given 

 us the most detailed and accurate account of these organs as 

 they occur in Amia. Briefly stated, the steps of the develop- 

 mental process are as follows : i. The layers of the ectoderm 

 thicken over certain areas and sink below the general level of 

 the surface of the body. These thickened bodies lie thus in 

 the bottom of relatively extensive surface depressions of the 

 general body surface. The ultimate sense organs are meanwhile 

 forming, and as they do so each sinks below the bottom of its 

 depression and lies in a pit, the lips of which soon grow upwards 

 and inwards towards each other. By their coalescence they 

 form an arch over the organ, which is soon converted into a 

 canal, by the rapid growth of the edges of the arch away from 

 the organ, and their fusion along the median line of the depres- 

 sion over which they arch as they grow. 



Canals originating thus, continue to grow until they meet 

 some other canal opening, or until the exhaustion of the impulse. 

 In the former case the two terminal pores of the canal fuse into 

 one ; in the latter the primitive canal possesses a strictly terminal 



