OBSERVATIONS ON THE DEVELOPMENT OF U\\\ BATIs. 41 



fringes have the same structure in both .♦ r J lie Selachians still further give with 

 frogs, toads, and salamanders, in the fact that the outer fringes are absorbed, and are 



placed by 



They differ from them, however, in tin f< 



Selachians retain their internal gills p. rinanently through life, while, if such <\ist 

 at all in the Batrachians just mentioned, it is only during the larval stage, and th y 

 are soon replaced by lungs. Selachians may therefore be said to pass through stages 

 analogous to the first and second stages of Anourous Batrachians and ialainanders. 



The other changes which the fissures pass through before the skate acquires its 

 permanent form are as follows. The seventh fissure is closed up at a vei \ earh period, 

 about the time that the dorsals are beginning to be formed. "While the first arch bends 



d is drawn forward as already described in connection with the formation of the jaw-, 

 at the same time becomes broader, so as to widen the distance betA\< en the mouth and 

 e first fissure, or the second, after the first is partially closed. The inner part of the 



first closes up, while the 



open (fig. 5, a), is somewhat enlarged, and 



retains its relative position to the eye. It is very soon widely separated from the other 

 fissures by the rapid growth of the intervening parts, and still further by the extension 

 of the pectoral fins forward between this remnant of the first fissure and those behind 

 it, the former being thus thrown to the upper, and the latter to the under surface. The 

 unclosed portion of the first branchial fissure is thus converted into the spiracle. 



The transformation thus described is of very great interest when compared with the 

 changes which occur in the corresponding fissures of the air-breathing vertebrates, and 

 enables us to establish an unexpected homology. Reichcrt,f in his most important 

 investigations of the development of the gill arches (" visceral Bogen ") of the pig, has 

 shown that in this animal the first fissure is gradually separated from the others by the 

 widening of the second arch, and for a time, even after all the others are closed up, 

 forms a direct opening from the side of the neck into the pharynx. Afterwards it is 



* Cornalia states that these respiratory fringes are not found in the u cotylophorus " sharks, as in < ntrina, 

 for in such the foetus forms a direct communication with the oviduct of the parent, and ihe fringes are there- 

 fore unnecessary. This statement may be questioned with propriety, on the ground of analog}. un)<-.- il were 

 based upon observations made upon very young embryos. This, however, does not app< ar to have been the 

 case, for the figure of the foetus referred to by him as evidence (see Carus, Entwickelung der Thiere im 

 Allgemeine, Tab. VI. fig. 9) shows that the specimen was quite advanced, and has reached a period when the 

 fringes might have been absorbed. It is quite probable that, as the young of such sharks have the advantage 

 of a vitelline placenta, the fringes would disappear at a very early stage. See Cornalia, Sulle Branchie 

 Transitorie dei Feti Plagiostomi, (Nizza, 1856,) p. 22. 



t Meckel's Archives, (1837,) p. 120. 



VOL. IX. 6 



