140 



BIOLOGICAL LECTURES. 



Another feature needs mention. The gill clefts are kept from 

 collapse by the presence of skeletal strengthening bars in the 

 tissue between them, and these are known as the visceral or 

 branchial arches. But there were not enough of these to cor- 

 respond with all the nerves, so Gegenbaur called in small carti- 

 lages found in the lips of certain sharks, which he regarded as 

 remnants of such arches. His whole results are summarized 

 by him in a table : — 



Gegenbaur did something more ; he gave the vertebrate 

 theory its final quietus, for he pointed out that in the lowest 

 fishes, where one would naturally expect, were Oken's view 

 true, that the vertebrae of the skull would be most typical, the 

 cranium was actually a solid case without the slightest trace 

 of segmentation. Further, in the higher groups, cranial " ver- 

 tebrse " can scarcely be spoken of, since the parts of which they 

 are composed are largely of dermal origin. 



Balfour was the next to introduce a new feature into the 

 discussion. As is well known, the body cavity or coelom 

 becomes divided dorsally with a series of paired pouches which 

 were formerly thought to give rise to the vertebrae, and hence 

 were called protovertebrae. They are now known to produce 



