EVOLUTION OF ORGANS IN THE CHORDATA. 283 



muscles came into relation with the eye is that the light 

 reached the latter, when the medullary tube began to close, 

 through the ectodermal pit of a prseoral gill-cleft. This ecto- 

 dermal branchial pit is now the lens of the eye, whose peculiar 

 mode of formation is thus explained. The vascular part of the 

 same gill arch is retained in the choroid gland of Teleosteans, 

 which receives its blood supply from the pseudobranchial vein, 

 and the arteria centralis retinae, which is the efferent artery of 

 the lens branchia. This hypothesis explains the vessels of the 

 campanula Halleri, of the pecten of Reptiles and Birds, the em- 

 bryonic lens vessels of Mammals, as remnants of the blood- 

 vessels of the branchia represented by the lens. Leaving the 

 eye, Dohrn next goes on to support his view that almost the 

 whole of the head except the brain represents visceral or ven- 

 tral structures, just as the tail contains only dorsal structures, 

 and asserts his belief that attempts to estimate the number of 

 myotomes in the head are all in vain. In his opinion the 

 cerebral nerves have lost those branches which innervated 

 myotomes and their derivatives, and have, in consequence of 

 the extraordinary enlargement and complication of the ventral 

 region, increased to a corresponding degree their visceral 

 branches, at the same time having undergone great alterations 

 in distribution on account of the changes of relative position 

 among the gill arches. Thus, the attempts of Van Wighe and 

 others to diagnose dorsal branches of the cranial nerves are 

 founded in mistaken views. A ramus dorsalis of a spinal nerve 

 never innervates a mucous tube, any more than the ramus 

 dorsalis, so called, of a cranial nerve innervates myotomes and 

 muscles of a dorsal fin. Again, Dohrn points out how necessary 

 it is to understand more accurately the anatomy and develop- 

 ment of the vertebrate organs before constructing complete 

 and simple schemes which reduce the head to a number of 

 myotomes as formerly to a certain number of vertebrae. A 

 great anatomist once said that if he wished to read romances 

 he knew better specimens than histories of creation where- 

 with to amuse himself, k propos of which Dohrn points out 

 that if phylogenies are to be compared with romances it is as 



VOL. XXVII, PART 2. NEW SER. U 



