6 4 8 



ARTERIAL ARCHES OF THE AMNIOTA. 



and the basal part of the internal carotid. The trunk connecting 

 the third arterial arch with the system of the dorsal aorta persists 

 in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but 

 is lost in the remaining Reptiles and in Birds and Mammals (fig. 

 366 B, C, D). It disappears earliest in Mammals (fig. 365 C), 

 later in Birds (fig. 365 B), and still later in the majority of 

 Reptiles. 



The fourth arch always continues to give rise, as in the Anura, 

 to the system of the dorsal aorta. 



In all Reptiles it persists on both sides (fig. 366 A and B), 

 but with the division of the truncus arteriosus into three vessels 



ad 



FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS 

 OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after 

 Rathke.) 



The first two arches have disappeared in all three. In A and B the last three are 

 still complete, but in C the last two are alone complete. 



/. pulmonary artery springing from the fifth arch, but still connected with the 

 system of the dorsal aorta by a ductus Botalli; c. external carotid; <'. internal 

 carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m, rudiment of 

 fore-limb. 



one of these, i.e. that opening furthest to the left side of the 

 ventricle (e and d), is continuous with the right fourth arch, and 

 also with the common carotid arteries (c) ; while a second 

 springing from the right side of the ventricle is continuous with 

 the left fourth arch (Ji and f). The right and left divisions of the 

 fourth arch meet however on the dorsal side of the oesophagus to 

 give origin to the dorsal aorta (g). 



In Birds (fig. 366 C) the left fourth arch (h) loses its connec- 

 tion with the dorsal aorta, though the ventral part remains as 



