286 SCOTT. ' [Vol. I. 



the " sense-bud," and, as the latter is somewhat constricted at 

 the top, the opening of the pit is smaller than its interior. The 

 organs in other parts of the head and in the trunk region I have 

 not detected till more advanced stages ; but their mode of for- 

 mation would seem to be the same. The distribution of these 

 organs in the Ammocoete and the adult is well figured by Lan- 

 gerhans, and the remarkably high position taken by the lateral 

 nerve is followed by the organs of the lateral line, which are 

 separate " sense-buds," and not a continuous line of epithe- 

 lium. 



It may, no doubt, be eventually proved by some more fortu- 

 nate observer that these sense-organs arise in the lamprey, just 

 as they do in the Selachians, from the epiblastic thickenings 

 which form the ganglia of the cranial nerves. But, assuming 

 the correctness of the account here given, which is to be re- 

 garded as the primar}^ mode of formation, that seen in Pctromy- 

 zo7t, or that shown by the Selachians? The evidence, it seems 

 to me, points very strongly to the view that the Selachians 

 exhibit the primitive method, especially when we remember the 

 remarkable retardation which affects the development of nearly 

 all the sense-organs of Pctromyzon. For, aside from the fact 

 that the process is much more clearly and completely known 

 in the case of the Selachians, the fusion of the cranial nerve- 

 rudiments with the epiblast (whether that fusion be itself a 

 primary or a secondary process) is unintelligible except in 

 connection with the development of the sense-organs ; nor can 

 we suppose that the latter have become differentiated from the 

 skin otherwise than in connection with the nerves. Especially 

 is it difficult to understand how a branch of a cerebral nerve, the 

 lateral, came to extend itself throughout the entire length of 

 the trunk, unless that extension were conditioned by an accom- 

 panying extension of the sensory organs which primarily 

 belonged to the head alone. So far, I think, Beard's position 

 is altogether reasonable and probable.- I cannot follow him, 

 however, in regarding these organs as especially branchial ; 

 they appear rather as segmental organs of the head, and their 

 occurrence in front of the mouth by no means implies the 

 former existence of pre-oral gill-clefts, as, indeed, Beard admits, 

 though much of his reasoning tacitly implies the existence of 

 pre-oral clefts. 



