40 OBSERVATIONS ON THE DEVELOPMENT OF RAIA BATIS. 



becomes thicker from side to side ; the line of attachment of the fringe, which retains 

 its original position, being thus buried between two adjoining arches. 



From the fact that the temporary gills are formed before the permanent ones, and 

 from the outer surface of the arch, it is obvious that they cannot be — as commonly 

 described — prolongations of these last-mentioned breathing organs. 



The fringes do not cover the whole border of the arch, but are confined to its cen- 

 tral portion, and consist of from six to eight filaments each. 



We have made no observations on the formation of the internal gills, and cannot 

 therefore explain the connection which eventually exists between these and the fringes, 

 and which at a later period correspond exactly with the descriptions usually given. 







The existence of temporary branchial fringes, and their subsequent absorption, is one 

 of the most remarkable characteristics of Selachians, and one in which they differ from 

 all osseous fishes, unless it be the Lepidosiren * All vertebrates, as embryos, agree in 

 this, that they are in their early stages provided with " branchial fissures " and " arches," 

 or, as they have sometimes been called, " visceral arches." Gills or gill fringes, either 



as temporary or permanent 



formed in any scaly reptile, bird 



mammaLt Much confusion and misapprehension have arisen from the constant reit 

 eration of the opinion put forth in the early days of embryology, that all vertebrates a 

 one time have a branchial respiration, an error which is repeated by naturalists even a 

 the present day. Among Batrachians some genera, as Menobranchus, Siren, Axolotl 

 etc., have external fringes permanently attached to their branchial arches, which an 



known to be replaced by 



with, internal gills. They 



organs of respiration, for their lungs are too imperfect and rudimentary to have much 

 physiological importance. In frogs, toads, and salamanders, the external gills are 

 replaced by internal ones, and these in turn by lungs. Thus it will be seen that no 

 Batrachian is permanently provided with internal gills. 



Selachians and Batrachians agree in this, that their embryos have in their first 

 stages external fringes growing from the outer surface of the sill arches, and these 



* 



branchial 



^^x^«.x wiautmm liniments project irom tne single open;uit*x 



aperture on each side, and are long retained, if they be not permanent, in that remarkable osculant form be- 



twppn ilio rvcca/Mic* rmA *U« -._— lT1 • n * .-, . ^ _ 



t 



isnes. Uwen, Lects. on Comp. Anat. 

 ,) and Peters, Midler's Archives (1845) 



See also 



sards this animal, to 



the same effect. « The branchial arches do not, in the evolution, pass through the same phases as those of 

 fishes. They never become provided with fringes, and never perform the functions of respiratory organs." 

 Annales des Sciences Naturelles, Tom. XVII., (1862,) p. 127. 



