Arteries in Monsters of the Diceplialns Group 461 



much it -would have needed a normal blood supply to maintain it in a 

 healthy, A'igorous condition. The second difference referred to is the 

 apparent absence of an independent occipital of its own to supply the 

 dorsal neck region on the inner side of head A. It is apparently supplied 

 by an occipital arising by a common trunk with the one supplying the 

 inner side of head B. Both are greatly attenuated and insignificant. 



The subclavians of the double lamb give off their normal branches, 

 but since the right subclavian arises from the dorsal aorta, it does not 

 give origin to its branches near its beginning as does the left, but from 

 that portion of it which brings the branches in normal relation to the 

 parts supplied by them [Plate VI]. 



How are the arterial conditions in Teras XV to be explained? The 

 conditions in the head region have already been explained, so that all 

 that remains to be accounted for here are the arteries that lie in the trans- 

 verse zone between the heart and the region of the complete doubling. 

 This consists mainly of an extra aortic trimk, the aorto-cephalic. It is 

 readily seen that if in accordance with the theory set forth in this paper, 

 the doubling of Teras XV had extended much farther posteriorly the 

 heart as well as the anterior arteries would have been involved and thiis 

 there M'ould have been ultimately two separate hearts, probably contained 

 in two separate pericardial chambers. It is possible, however, to con- 

 ceive of a degree of doubling which involves part, but not all of the 

 heart, and it seems probable that this is the case in Teras XV. But in 

 order to understand such a condition and interpret it correctly one must 

 imagine this condition, not as in an adult heart, but as in the embryo. 



Since the early formation of the heart is the same in all vertebrates, 

 and since the arteries in all develop from six primitive arterial arches, 

 it is possible to suggest the changes that have taken place in all the terata 

 by diagrams representing the varying degrees of doubling and the changes 

 that have taken place in the primitive arches, for all are capable of 

 explanation by referring back to the normal vertebrate condition. The 

 usual diagrammatic representation of the normal transformation of the 

 six primitive arches is classic, and to compare with this I have reduced 

 the actual condition in the terata studied to similar diagrams repre- 

 senting the way in Avhich the primitive arches probably existed in these 

 creatures. For the sake of easy comparison I insert here a text figure 

 illustrating the normal mammalian changes in the primitive arterial 

 arches (Text figure 1). The solid black lines in all the diagrams 

 represent the permanent condition in the specimen, the dotted lines tlie 



