642 J. PLAYFAIR MOMURRICH. 



the successive visceral clefts are formed, the posterior part of 

 the head-cavity becomes divided into successive sections, there 

 being one section for each arch. Thus, the whole head-cavity 

 becomes on each side divided into — (1) a preemandibular sec- 

 tion ; (2) a mandibular section; (3) a hyoid section; (4) 

 sections in each of the branchial arches," 



The question arises — How many are the praporal visceral 

 arches, and what are their relations to the cranial nerves, vis- 

 ceral clefts, and head-cavities ? 



If we consider Amphioxus as an ancestral type for the 

 vertebrates, we find that originally there were no prseoral vis- 

 ceral arches, and the Ascidians point to the same fact. Ac- 

 cordingly one must suppose that the anterior cleft, -which, 

 functioned as the mouth in the ancestral forms, gradually lost 

 that use, its function being as gradually assumed by succeeding 

 arches, since we have strong grounds for supposing that there 

 are in the Cranio t a arches in front of the mouth. As to the 

 number of these arches we have several theories — some main- 

 taining that there is only one, others deciding for two, and 

 others for several. As far as our present knowledge goes, I 

 consider that we are entitled to recognise two. 



The authorities upon this subject are by no means few; 

 from them the views of Parker and Marshall may be cited as 

 illustrating what has been done in this line. Parker, in his 

 paper on the skull of the Salmon,^ seems inclined to accept the 

 trabeculae cranii as a prseoral arch, but later rejects this theory. 

 He says : — ^'When we consider . . . . that the walls of the fore- 

 part of the cranium are formed by growth from the trabeculae, 

 just as posteriorly the walls are formed by growth from the 

 parachordals ; that nerves are similarly emitted through the 

 trabecular and occipital walls : when it is seen, in short, that 

 the trabeculae are neural in their relations, as completely as, 

 and in similar fashion to, the parachordals, it seems impossible 

 to resist the conclusion that the trabeculae and the para- 



1 W. K. Parker, " Oa the Sf.ructure aud Devclopmeni of the Skull iu the 

 Salmon," ' Phil. Trans.,' 1S73. 



