TELEOSTEL 



(53 



(Lift* V "O. 



FIG, 35. DIAGRAMMATIC VIEW OF THE HEAD 

 OF AN EMBRYO TELEOSTEAN, WITH THE PRIMI- 

 TIVE VASCULAR TRUNKS. (From Gegenbaur.) 



a. auricle; ?'.. ventricle; abr. branchial 

 artery; c'. carotid; ad. aorta; s. branchial 

 clefts; sr. sinus veuosus; dc. ductus Cuvieri; 

 n. nasal pit. 



The air-bladder is formed as a dorsal outgrowth of the alimentary tract 

 very slightly in front of the liver. It grows in between the two limbs of 

 the mesentery, in which it ex- 

 tends itself backwards. It ap- 

 pears in the Salmon, Carp, and 

 other types to originate rather 

 on the right side of the median 

 dorsal line, but whether this fact 

 has any special significance is 

 rather doubtful. In the Salmon 

 and Trout it is formed consider- 

 ably later than the liver, but the 

 two are stated by Von Baer to 

 arise in the Carp nearly at the 

 same time. The absence of a pneu- 

 matic duct in the Physoclisti is 

 due to a post-larval atrophy. The 

 region of the stomach is reduced 

 almost to nothing in the larva. 



The oesophagus becomes solid, like that of Elasmobranchs, and remains 

 so for a considerable period after hatching. 



The liver, in the earliest stage in which I have met with it in the 

 Trout (27 days after impregnation), is a solid ventral diverticulum of the 

 intestine, which in the region of the liver is itself without a lumen. 



The excretory system commences with the formation of a segments 1 

 duct, formed by a constriction of the parietal wall of the peritoneal cavity. 

 The anterior end remains open to the body cavity, and forms a pronephros 

 (head kidney). On the inner side of and opposite this opening a glomerulus 

 is developed, and the part of the body cavity containing both the glomerulus 

 and the opening of the pronephros becomes shut off from the remainder 

 of the body cavity, and forms a completely closed Malpighian capsule. 



The mesonephros (Wolffian body) is late in developing. 



The unpaired fins arise as simple folds of the skin along the 

 dorsal and ventral edges, continuous with each other round the 

 end of the tail. The ventral fold ends anteriorly at the anus. 



The dorsal and anal fins are developed from this fold by local hyper- 

 trophy. The caudal fin 1 , however, undergoes a more complicated 

 metamorphosis. It is at first symmetrical or nearly so on the dorsal 

 and ventral sides of the hinder end of the notochord. This symmetry 

 is not long retained, but very soon the ventral part of the fin with 

 its fin rays becomes much more developed than the dorsal part, and 

 at the same time the posterior part of the notochord bends up towards 

 the dorsal side. 



In some few cases, e.g. Gadus, Salmo, owing to the simultaneous 

 appearance of a number of fin rays on the dorsal and ventral side of 

 the notochord the external symmetry of the tail is not interfered 

 with in the above processes. In most instances this is far from 

 being the case. 



1 In addition to the paper by Alex. Agassiz (No. 55) vide papers by Huxley, Kolliker, 

 Yogt, etc. 



