Arteries in Monsters of the Dicephalus Group 455 



and since the present study is designed especially to delineate a series 

 of consecutive stages of a given cosmobiotic series, the arterial system 

 will illustrate the theory emphasized in this paper that such a series 

 may be conceived as a gradual separation of component halves beginning 

 with normal individuals and ending with duplicate twins. Of such a 

 series, which my specimens represent, the monster in question, Teras 

 XII, comes first and represents an initial stage in the splitting of the 

 organism, a process which here involves only the anterior part of the 

 head. Therefore there is present in this creature the forepart of two 

 separate heads, that is, two snouts and both sides of two faces, but owing 

 to the lack of division of the remainder of the head the inner sides of the 

 two faces are close together and all intervening tissues are greatly 

 cramped. Nevertheless, such division as is present is responsible for 

 the physiological necessity of a double blood supply to the divided region, 

 and nature has responded to the call as best it could. The division has 

 not yet extended far enough posteriorly to demand two separate external 

 carotids to the inner sides of the two components, but one median vessel 

 has been able to meet the emergency by giving off double the normal 

 number of branches, one moiety of which supplies component A, the 

 other component B. Thus the median external carotid is made up of 

 elements of the inner external carotid of A and also of B (A's left and 

 B's right), and for this reason I have called the main median artery 

 "compound." 



ISTormally a common carotid, external carotid, and internal maxillary 

 form a single compound curve. If such vessels should develop in a space 

 too crowded to permit them to assume their natural curve, they would 

 become elongated into a straight tube. This is the condition in the 

 median region of Teras XII ; the median compound external carotid and 

 internal maxillary have been forced to develop as a straight tube owing 

 to the cramped condition in this region. 



An examination of Plate V will demonstrate that in the neck region 

 and on the outer sides of the componental head the amount of space and 

 other conditions are normal, and here the arteries are distributed in nor- 

 mal manner. This is also true in the median region wherever the topo- 

 graphical changes leave room for normal behavior, as for example, the 

 distribution of the branches of the median compound external carotid 

 which supply the inner lateral and dorsal sides of the two components. 

 Only a single hypothetical branch, or capillary connection between the 

 median compound external carotid and the outer external carotid of 



