128 EDWARD PHELPS ALLIS jr.. 



vessel iiiiuiediately joins its fellow of the oj)posite side to form a 

 short median trunk which joins the anterior end of the truncus ar- 

 teriosus. The heart still connects with the truncus arteriosus be- 

 tween the first and second afferent branchial arteries. The efferent 

 artery of the first branchial arch extends downward and forward con- 

 siderably beyond the point at which the afferent artery of the arch 

 arises from the truncus arteriosus, but it in no place closely ap- 

 proaches any part of the prespiracular artery. There is, however, at 

 this age, a vessel that can be traced through a limited number of 

 sections and that lies postero-mesial and parallel to the anterior end 

 of the efferent artery. In certain series of sections no connection of 

 this vessel or space with the other vessels of the region could be 

 traced. In others it was certainly directly connected with the efferent 

 artery of the arch , while in still others it seemed to be connected 

 with the afferent artery, or with both the afferent and efferent arteries. 

 The most frequent and positive connection was with the anterior end 

 of the efferent artery, this artery thus turning mesially and then 

 backward into the postero-mesial portion of its arch. The posterior 

 end of this postero-mesial vessel lies not far from the ventral arm 

 of the ventral bend of the prespiracular artery, and in one series of 

 sections it there turned downward a short distance toward that artery. It 

 is thus unquestionably the beginning of the connection found between 

 these two arteries in 12 mm and older specimens, the primary con- 

 nection of the prespiracular artery with the postspiracular artery then 

 aborting. 



There are thus, in young embryos of Amia, two unbroken currents of 

 blood sent upward, in the region of the ceratohyal and hyomandibular, 

 from a ventral to a dorsal longitudinal vessel, the vessel that conducts 

 one of the currents passing anterior to the rudimentary spiracular 

 cleft and the other one posterior to it. Near their dorsal ends these 

 two vessels are connected by a transverse cross bar that has a nearly 

 longitudinal position. In 9 mm embryos a third current is established, 

 lying posterior to the second one, in the gill cover; but, if it can be 

 considered as an independent current, it certainly passes through a 

 capillary network, thus probably never being a direct and unbroken 

 current as both the others are in early stages. In 12 mm embryos 

 the anterior one of these two or three currents has become arterial 

 by reason of a secondary connection established between the vessel that 

 transmits it and the ventral end of the efferent artery of the first 



