THE SKELETON OF THE VEKTEBRATE HEAD. 101 



which the cephalic haemal arches are peculiar. The hsemal 

 arches of the head are in 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 Kemak) becomes from the first 

 united to the corresponding portion of the cephalic primordial 

 vertebral system (Kopfurwirbelplatte) ; and the former, instead 

 of dividing into two layers one for the wall of the alimentary 

 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 clefts. 



The hsemal portion of the head, therefore, is distinguished 

 from the corresponding portion of the trunk, in presenting 

 metasomatomic clefts, in having no serous cavity, and in hav- 

 ing the haemal axis external to the hsemal arches of its sclero- 

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

 explain these various peculiarities of the head in the haema- 

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

 which bear upon the cephalic exclusion of the haemal axis. 

 The anterior portion of the primordial alimentary tube, from 

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

 vitellary margin, consists essentially of two parts ; a cephalic 

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

 the " visceral laminae," from the so-called first pair of laminae 

 backwards, and becomes developed into the pharynx ; and a 

 cervico-thoraco-abdominal portion, bounded laterally by the 

 anterior portion of the primordial vertebral system of the 

 trunk and the corresponding portions of the primary ventral 

 wall. The primordial haemal axis (heart 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 

 afterwards developed the oesophagus, stomach, duodenum, liver, 



