THE SKELETON OF THE VERTEBRATE HEAD. 195 



the fish is the homologue of the corresponding arch in the 

 amphibian ; of the posterior horns of the hyoid, and their 

 associated elements in the bird ; and of the posterior horns 

 and body of the hyoid in the mammal. 



It has not yet been determined upon what developmental 

 change the suspension of the hyoidean arch of the fish to its 

 mandibular arch depends. It is probably of the same nature 

 as that which occurs in the tadpole, with this difference, that 

 the upper portion of the hyoidean arch disappears in the fish, 

 without developing a stapedial ossicle ; while its lower portion 

 remains permanently connected to the mandibular arch, in- 

 stead of regaining an attachment to the cranium. 



The hyoidean and branchial arches of the fish are provided, 

 as has been already stated, with a well-developed double series 

 of actinapophyses, for the support of the branchiostegal 

 membrane and the branchial laminae. These actinapophyses 

 in the fish are foreshadowed in the tadpole by the tubercular 

 margins of its branchial styles. 



The question may now be put if we are brought by 

 reference to the development of the parts to allocate to the 

 three post-stomal sclerotomes, haemal arches, consisting re- 

 spectively of the sclerous parts developed in the three anterior 

 post-stomal visceral laminae, to what sclerotomes are we to 

 refer the potential or actual haemal arches in the remaining 

 visceral laminae ? For reasons already stated, they cannot be 

 disposed of by referring them to a splanchno-skeleton, because 

 in that case the hyoidean arch or arches, and apparently the 

 mandibular arch also, must be referred to the same categoiy. 

 Neither can they be referred to any of the cervical, or trunk 

 sclerotomes ; because it would appear that the visceral walls 

 of the head are alone perforated by clefts. We are not yet 

 prepared to answer the question. It involves, as it appears to 

 me, the investigation of a residual quantity, the solution of 

 which will require some information in reference to certain 



