GANOIDEI. 119 



The fin-rays become formed as in Teleostei, and parts of 

 the continuous embryonic fin gradually undergo atrophy. The 

 dorsal limb of the embryonic tail, as has been shewn by Wilder, 

 is absorbed in precisely the same manner as in Teleostei, leaving 

 the ventral lobe to form the whole of the permanent tail-fin. 



BIBLIOGRAPHY. 



(92) Al. Agassiz. " The development of Lepidosteus." Proc. Amer. Acad. of 

 Arts and Sciences, Vol. xm. 1878. 



General observations on the Embryology of the Ganoids. 



The very heterogeneous character of the Ganoid group is clearly shewn 

 both in its embryology and its anatomy. The two known types of formation 

 of the central nervous system are exemplified in the two species which have 

 been studied, and these two species, though in accord in having a holoblastic 

 segmentation, yet differ in other important features of development, such as 

 the position of the yolk etc. Both types exhibit Teleostean affinities in the 

 character of the pronephros ; but as might have been anticipated Lepidosteus 

 presents in the origin of the nervous system, the relations of the hypoblast, 

 and other characters, closer approximations to the Teleostei than does 

 Acipenser. There are no very prominent Amphibian characters in the 

 development of either type, other than a general similarity in the segmenta- 

 tion and formation of the layers. In the young of Polypterus an interesting 

 amphibian and dipnoid character is found in the presence of a pair of true 

 external gills covered by epiblast. These gills are attached at the hinder 

 end of the operculum, and receive their blood from the hyoid arterial arch 1 . In 

 the peculiar suctorial disc of Lepidosteus, and in the more or less similar struc- 

 ture in the Sturgeon, these fishes retain, I believe, a very primitive vertebrate 

 organ, which has disappeared in the adult state of almost all the Vertebrata ; 

 but it is probable that further investigations will shew that the Teleostei, and 

 especially the Siluroids, are not without traces of a similar structure. 



1 Vide Steindachner, Polypterus Lapradei, &c., and Hyrtl, " Ueber d. Blutgefasse, 

 &c." Sitz. IViencr Akad., Vol. LX. 



