THE SKELETON OF THE VERTEBRATE HEAD. KH 



wliich the cephalic liremal arches are peculiar. The lui-nial 

 arches of the head are iii immediate contact with the ali- 

 mentary tube ; they are lined by the mucous membrane, 

 which is also in contact with their centrums. There is, in 

 fact, no extension of the peritoneo-pleuropericardiac space into 

 the head. The cephalic portion of the primary abdominal 

 wall (Kopfseitenplatte of Remak) becomes from the first 

 united to the corresponding portion of the cephalic primordial 

 vertebral system (Kopfurwirbelplatte) ; and the fonner, instead 

 of dividini::^ into twolayere — one for the wall of the alimentar}' 

 tube, and another for the wall of the visceral cavity, with a 

 serous space between them as in the trunk — becomes, in con- 

 junction with the latter, perforated by the branchial cleft.s. 



The luTmal portion of the head, therefore, is distinguished 

 from the corresponding portion of the trunk, in presenting 

 metasomatomic clefts, in having no serous ca^'ity, and in hav- 

 ing the hajmal axis external to the hremal arches of its sclero- 

 tomes. We are not yet in possession of sufficient data to 

 explain these various peculiarities of the head in the ha?ma- 

 pod. I must direct attention, however, to the followiug facts, 

 which bear upon the cephalic exclusion of the haemal axis. 

 The anterior portion of the primordial alimentary tube, from 

 the cid-de-sac in which it terminates in front, back to its 

 vitellary margin, consists essentially of two parts ; a cephalic 

 portion, terminated by the cid-de-sac, is bounded laterally by 

 the "visceral lamina^" from the so-called first pair of lamina3 

 backwards, and becomes developed into the pharynx ; and a 

 cervico-thoraco-abdomiual portion, bounded laterally by the 

 anterior portion of the primordial vertebral system of the 

 trunk and the coiTesponding portions of the primary ventral 

 wall. The primordial haemal axis (licart and branchial artery) 

 is formed within the pericardiac space, on the inferior aspect 

 of the posterior or trunk portion of the tube from which are 

 aftenvards developed the cesophagus, stomach, duodenum, liver, 



