FORMATION OF THE TAIL. 



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 considerably 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 pneumatic duct in the Physoclisti is due to a 

 post-larval atrophy. The region 

 of the stomach is reduced al- 

 most to nothing in the larva. 



The oesophagus becomes 

 solid, like that of Elasmobranchs, 

 and remains so for a consider- 

 able 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 com- 



f ' * iilr v a, 



FIG. 35. DIAGRAMMATIC VIEW OF THE 

 HEAD OF AN EMBRYO TELEOSTEAN, WITH THE 

 PRIMITIVE VASCULAR TRUNKS. (From Gegen- 

 baur.) 



a. auricle ; v. ventricle ; al>r. branchial 

 artery ; /. carotid ; ad. aorta ; s. branchial clefts ; 

 sv. sinus venosus ; dc. ductus Cuvieri ; n. nasal 

 pit. 



mences with the formation of a segmental 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 prone- 

 phros 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 hypertrophy. 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. 



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

 Kolliker, Vogt, etc. 



