Arteries in Monsters of the Dicephalus Group 463 



whence it very soon unites with its fellow to form a single tube, the 

 dorsal aorta. Anterior to the bifurcations of the truncus arteriosus 

 three arterial arches have developed in turn, and posteriorly two, making 

 in all the six primitive arterial arches. Briefly the normal changes 

 that take place in these arches in mammals are : — 



(a) The first and second arches disappear. 



(&) The dorsal trunk between the third and fourth arches disappears. 



(c) The third arch together with the anterior portion of the dorsal 

 aorta thus cut oif forms the internal carotid. 



The ventral trunk anterior to the origin of the third arch forms the 

 external carotid, and the intervening ventral portion between the third 

 and fourth arches becomes the common carotid. 



(d) The left moiety of the fourth arch together with the posterior 

 portion of the left dorsal aorta becomes the permanent mammalian aortic 

 arch. 



(e) The fifth arch, which from the beginning is rudimentary, dis- 

 appears. 



(/) A portion of the sixth arch becomes the pulmonary artery. On 

 the right, the remainder of the arch disappears, but the corresponding 

 portion on the left side remains in connection with the permanent aortic 

 arch during foetal life as the ductus Botalli. 



(g) When the aortic bulb divides longitudinally, the sixth arch comes 

 to communicate with the right ventricle, and the remaining vessels with 

 the left ventricle. 



In the diagram representing the condition in Teras XII at this early 

 embryological period (text figure 2), it will be seen that the heart and 

 arches six, five and four are normal, and that as the divergence anteriorly 

 begins to be felt, the anterior arches show corresponding changes, and in 

 proportion as the amount of duplication is slight, so also the modifica- 

 tions of the arterial arches are slight, the only difference from normal 

 being an increase in the ventral trunk of the third arch, which anteriorly 

 gives rise to the median compound external carotid. 



In the double lamb (Teras XV) it is obvious that there has been from 

 the beginning a greater degree of doubling of these elementary blood 

 vessels. There is present the beginning of two hearts and two sets of 

 arterial arches (Text figure 3). Already the aortic bulb has divided, 

 and the semi-lunar valve has developed between the ventricle and the new 

 bulbus arteriosus. Posterior to the bulbus the heart shows no duplicat- 

 ing tendencies. The communicating branch which passes from one aortic 



