CYCLOSTOMATA. 79 



The opeiiin,' of the nasal pit is in this way relatively thrown back, and at 

 the same time is caused to assume a dorsal position. This will be at once 

 understood by a comparison of tig. 43 with fig. 47. On the inner side of 

 the oral cavity a ring o£ papillae is formed (fig. 47, or.p). Dorsally these 

 papillie are continued forward as a linear streak on the under side of the 

 upper lip. A communication between the oral cavity and the branchial 

 sack is very soon established. 



The gill pouches gradually become enlarged ; but it is some time 

 before their small external openings are established. Their walls, which 

 are entirely lined by hypoblast, become raised in folds, forming the 

 branchial lamellae. The walls of the head cavities between them become 

 resolved into the contractors and dilators of the branchial sacks. The 

 extra-branchial basketwork becomes established very early (it is present 

 in the larva of 6 millimetres, about 9 days after hatching) and is shewn 

 in an older larva in tig. 47, br.s. It is nob so complicated in these young 

 larvae as in the Ammoccetes, but in Max Schultze's figure, which I have 

 reproduced, the dorsal elements of the system are omitted. On the dorsal ) 

 wall of the branchial region a ciliated ridge is formed, which may be homo- ( 

 logons with the ridge on the dorsal wall of the branchial sack of Ascidians. j 

 It has been described by Schneider in Ammocojtes. 



With reference to the remainder of the alimentary canal there is but 

 little to notice. The primitive hepatic diverticulum rapidly sprouts out 

 and forms a tubular gland. The opening into the duodenum changes from 

 a ventral to a lateral or even dorsal position. The duct leads into a gall- 

 bladder imbedded in the substance of the liver. Ventrally the liver is 

 united with the abdominal wall, but laterally passages are left by which, 

 the pericardial and body cavities continue to communicate. 



The greater part of the yolk becomes employed in the formation of the 

 intestinal wall. This part of the intestine in a nine days' larva (6-7 mm.) 

 has the form of a cylindrical tube with very thick columnar cells entirely 

 filled with yolk particles. The dorsal wall is no longer appreciably thinner 

 than the ventral. In the later stages the cells of this part of the intestine 

 become gradually less columnar as the yolk is absorbed. 



The fate of the yolk cells in the Lamprey is different from that in most 

 other Vertebrata with an equally large amount of yolk. They no doubt 

 supply nutriment for the growth of the embryo, and although in the 

 anterior part of the intestine they become to some extent enclosed in the 

 alimentary tract and break up, yet in the posterior part they become 

 wholly transformed into the regular epithelium of the intestine. 



On the ninth day a slight fold filled with mesoblastic tissue is visible 

 on the dorsal wall of the intestine. This fold appears to travel towards 

 the ventral side ; at any rate a similar but better-marked fold is visible in 

 a ventro-lateral position at a slightly later period. This fold is the com- 

 mencement of the fold which in the adult makes a half spiral, and is no 

 doubt equivalent to the spiral valve of Elasmobranchs and Ganoids. It 

 contains a prolongation of the cceliac artery, which constitutes at first the 

 vitelline artery. 



The nervous system does not undergo during the early larval period 

 changes which require a description. 



The opening of the olfactory sack becomes narrowed and ciliated 

 (fig. 47, ol). It is carried by the process already mentioned to the dorsal 



