96 GENERAL DEVELOPMENT. 



preference the darkest situations. It soon straightens out, and, 

 with the disappearance of the yolk, the tail becomes narrower 

 than the head. A large caudal fin becomes developed. 



When the larva is about twenty days old, it bears in most 

 anatomical features a close resemblance to an Ammoccetes ; 

 though the histological differences between my oldest larva 

 (29 days) and even very young Ammocoetes are considerable. 



The mouth undergoes important changes. The upper lip becomes much 

 more prominent, forming of itself the anterior end of the body (fig. 47, /). 

 The opening 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 fig. 43 with fig. 47. On the inner side of the 

 oral cavity a ring of papillae is formed (fig. 47, or.p}. Dorsally these papillae 

 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 

 fig. 47, br.s. It is not 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 homologous with 

 the ridge on the dorsal wall of the branchial sack of Ascidians. It has been 

 described by Schneider in Ammoccetes. 



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 (67 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 



