Schakff — On Dohni's Theories on the Origin of Vertebrates. 19 



along with, the artery, shed their blood into the former. The 

 artery then loses all connexion with the anterior vein, which alone 

 represents the aortic arch. The posterior vein also becomes 

 independent, and joins the aortic arch of the next gill-arch, so 

 that ultimately each aortic arch receives its blood from two very 

 unequally large veins of two different gill-arches. 



On that part of the visceral arch which is nearest the intestine 

 (the proximal part) the mesoderm cells condense to form the 

 rudimentary cartilaginous arch. From the distal parts of the 



III 



Figure m., longitudinal section through the head of an embryonic Petromyzon (after Dohrn) 



ep. = epiphysis. 



na. = nasal invagination. 



hy. = hypophysial invagination. 



ect. = ectoderm. 



ul. = upper lip. 



mo. = mouth, or stomodacum. 



11. = lower lip. 



ent.= entoderm. 



mesoblastic somite in the interior of each arch, the musculi inter- 

 branchiales are developed; the dorsal parts of the same form the 

 constrictor superficialis. The musculi interarcuales arise from the 

 dorsal proximal parts of the mesoblastic somite. The muscle which 

 quite at the beginning was constricted off from the proximal portion 

 of the mesoblastic somite by the venous commissures and the 

 cartilaginous arch is the adductor arcus visceralis. (See fig. v.) 



The two middle portions of the cartilaginous arch are formed 

 from the original cartilaginous piece. They are articulated to 



C2 



