DEVELOPEMENT OF EISHES. 607 



the liver a tubercle of cells buds out, which elongates, enlarges, 

 and then acquires a cavity : this is the beginning of the air- 

 bladder, ib. s. Many Fishes retain the tubular connection with 

 the alimentary canal, and those which ultimately lose the ' ductus 

 pncumaticus ' usually retain for a longer or shorter period that 

 evidence of the place and mode of origin of the air-bladder. 

 The posterior compartment of the air-bladder is first developed 

 in the Cyprinoids, which accounts for the connection of the 

 air-duct with that part : the whole posterior compartment disap- 

 pears with the duct in the Loach. In the Herring the primitive 

 place of its connection with the alimentary canal is retained. 



The ureter, q, developed from the intestine before the embryo 

 quits the ovum, communicates with the extremities of the trans- 

 verse parallel tubuli, f, formed by confluence of primitive cells in 

 the renal blastema. The cardinal veins traverse or groove the 

 renal organs, as they do the Wolflfian bodies in the embryos of 

 higher Vertebrates \ and this primitive relation of the vascular to 

 the renal system is not changed in Fishes by the substitution of 

 true kidneys for the primordial renal organs. In many Fishes a 

 cajcal process is developed from the fore, or ventral, surface of 

 the termination of the intestine, and extends forward, as a i 



bladder : its growth is arrested at various stajres in different I 



. . . . . 



sjiecies, and it is termed 'urinary bladder,' but it is the homologue l| 



or besinnino; of the allantois. 



The intestinal wall is completed, and the fissure behind the 

 liver closed, by the time the yolk is consumed. The vent opens, 

 in the Pike, on the fourth day after extrication : in the Perch 

 coloured i^articles added to the water were seen to traverse the 

 intestine, and escaj^e ' per anum ' on the sixth day.' Previously 

 the mucous walls of the gut are in contact, although the peri- 

 staltic movements are active. About the eighth day the j^resence 

 of bile is indicated by the colour of the gall-bladder and ducts. The 

 stomach expands, and divides the oesophagus from the intestine. 



After extrication the eye loses the choroidal fissure : the iris ac- 

 quires the silvery pigment. The ear-sacs assume a triangular form : 

 the two otolites grow unequally by additional calcareous layers. 



The primary enlargements of the encephalon are connected, 

 respectively, with the acoustic, optic, and olfactory nerves : the 

 anterior one, fig. 424, P, becomes chvided into prosencephalon and 

 rhinencei^halon ; the second, O, ra^ndly gains superior bulk in 

 connection with the large eyeballs, and its pineal and jiituitary 

 appendages appear as vascular membranous canals. The cere- 



' CCCXIX. p. 483. 



\m 



