1859 PAPER ON FISH-DEVELOPMENT 221 



investigated led to the paper at the Linnean already 

 mentioned. 



The paper on fish-development was mainly based 

 upon dissections of the young of the stickleback. 

 Fishes had been divided into two classes according as 

 their tails are developed evenly on either side of the 

 line of the spine, which was supposed to continue 

 straight through the centre of the tail (homocercal), 

 or lopsided, with one tail fin larger than the other 

 (heterocercal). This investigation showed that the 

 apparently even development was only an extreme 

 case of lopsidedness, the continuation of the "chorda," 

 which gives rise to the spine, being at he top of the 

 upper fin, and both fins being developed on the same 

 side of it. Lopsidedness as such, therefore, was not 

 to be regarded as an embryological character in 

 ancient fishes ; what might be regarded as such was 

 the absence of a bony sheath to the end of the 

 "chorda" found in the more developed fishes. 

 Further traces of this bony structure were shown to 

 exist, among other piscine resemblances, in the Am- 

 phibia. Finally the embryological facts now observed 

 in the development of the bones of the skull were of 

 great importance, "as they enable us to understand, 

 on the one hand, the different modifications of the 

 palato-suspensorial apparatus in fishes, and on the 

 other hand the relations of the components of this 

 apparatus to the corresponding parts in other Verte- 

 brata," fishes, reptiles, and mammals presenting a well- 

 marked series of gradations in respect to this point. 



