102 EDWARD PHELPS ALLIS, JR. 



147) to have primarily extended the full length of this arch ; and 

 if the mouth were developed from a pair of gill slits, as Dohrn 

 maintained, there would seem no reason why an anterior efferent 

 artery should not also have extended the full length of the arch. 



In the hyal arch certain of the lacunae that, in embryos, 

 represent the undeveloped anterior efferent artery are said by 

 Dohrn to become connected with the posterior efferent artery 

 by a cross-commissural vessel similar to the one that connects 

 the corresponding arteries, at the middle of their lengths, in each 

 of the branchial arches, and this hyal commissure is said to be 

 prolonged anteriorly and to fall into the primitive aortic vessel 

 of the mandibular arch. A hyo-mandibular cross-commissure 

 is thus formed, connecting an efferent hyal artery with an 

 afferent mandibular one, and it becomes the basal portion of the 

 secondary and definitive afferent artery of the spiracular gill. 

 That portion of the primitive mandibular aortic vessel which 

 lies ventral to the point where it is joined by this cross-com- 

 missure, is first called, by Dohrn, the arteria thyreoidea, but, 

 as it is said to either receive or give off, at its dorsal end, an 

 artery that is distributed to the lower jaw, Dohrn later calls 

 it the arteria thyreo-mandibularis. Still later, because this 

 artery, in embryos, still carries blood to the spiracular gill, it is 

 called the arteria thyreo-spiracularis ; but as this term implies 

 the inclusion of the arteria spiracularis of Dohrn's descriptions, 

 the term arteria thyreo-mandibularis will be here employed. 



This arteria thyreo-mandibularis of embryos is said by Dohrn 

 ('85, pp. 6-7) to lie anterior to a branch of the nervus facialis, 

 and to be separated from the efferent artery of the hyal arch by 

 a part of the corresponding (entsprechenden) hyal muscles; and, 

 as it is said to run upward behind the mandibular cartilage and 

 to have been theretofore called the arteria mandibularis (wo 

 sie bisher erwahnt ward, hiess sie A. mandibularis), there seems 

 no reason to doubt that Dohrn considered this artery of embryos 

 to be identical with the so-called arteria mandibularis of the then 

 existing descriptions of the adult fish. This arteria thyreo- 

 mandibularis is said by Dohrn to become relatively smaller and 

 less important, the older the embryo, not only in the Selachii 



