98 LFAUBOSTEUS. 



after about the end of the third week, and by this time the operculum 

 has grown considerably, and the gills have become well developed. 



The most remarkable changes in the later periods are those of the 

 mouth. 



The upper and lower jaws become gradually prolonged, till they 

 eventually form a snout ; while at the end of the upper jaw is placed 

 the suctorial disc, which is now considerably i^educed in size (fig. 68, sd). 

 The "fleshy globular termination of the upper jaw of the adult 

 Lepidosteus is the remnant of this embryonic sucking disc." (Agassiz, 

 No. 92.) 



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

 tinuous embryonic fin gradu- 

 ally undergo atrophy. The 

 dorsal limb of the embryonic 

 tail, as has been shewn by 

 Wilder, is absorbed in pre- 

 "■ cisely the same manner as in 



Fig. 68. Head of an advanced larva op Tplpmtpi Ipavino- thp vpntral 

 Lepidosteus. (After Parker.) leleostei leaving tne ventral 



7 • r, ^t +1,^ ^ifonf^,.-, r^u. ow vn lobe to lorm the whole 01 the 



oh openings 01 the olractory pit; sd. re- 



mains of the larval suctorial disc. permanent taii-hn. 



Bibliography. 



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

 Arts and Sciences, Vol. xiii. 1878. 



General observations on the Emhryology 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 difier 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 pre.'sents 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 segmentation and formation of the layers. In the young of 

 Polypleius an interesting amphibian and dipnoid character is found in the 

 pr-esence 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^ In the peculiar suctorial disc of Lepi- 

 dosteus, and in the more or less similar sti'ucture 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. 



^ Vide Steindachner, Polypterus Laprodei, &c., and HjTtl, " Ueber d. Blutgefasse, 

 &c." Sitz. Iflener Akad., Vol. lx. 



