Embryonic history of the aortic arches in mammals. 391 



vertebrates. The diagrams already given show the state of knowledge 

 when he took up the work. In 1843, as based chiefly upon studies of 

 embryos of the pig, sheep and cow, he describes for mammals five pairs 

 of aortic arches. Four of these lie in the four branchial pouches, and 

 the fifth behind the last gill-slit. The main part of the first disappears 

 before the fifth is established. A persistent remnant of the first indicates 

 that portion which united it dorsally with the second, and is continued 

 forward to the brain by the anterior prolongations of the dorsal aorta. 

 A little later, corresponding changes take place in the second arch, and 

 the anterior portion of the dorsal aorta appears as a branch of the 

 third arch. Degeneration in the second arch begins in its middle portion. 

 A slender branch runs forward from the ventral part of the third arch 

 which may be regarded as a remnant of the first and second arches, 

 rather than an outgrowth from the third, since it was noted in embryos 

 in which the second arch was still present. This vessel forms the Carotis 

 facialis. Somewhat later, the portion of the third arch uniting it 

 dorsally with the fourth, disappears and the remainder of the arch is 

 continuous with the anterior portion of the dorsal aorta. The short ventral 

 portion of the third arch between the fourth and the Carotis facialis forms 

 the common carotid stem, and its larger branch, mentioned above is the 

 Carotis cerebralis. The left fourth arch persists as the systemic, and is 

 continued caudad by the left aortic root and the dorsal aorta. The left 

 vertebral artery is given off slightly above the place where the fourth 

 and fifth arches join, and bends near its origin, passing forward. The 

 left subclavian arises from it. The right fourth arch persists, considerably 

 diminished in diameter, and is continued by the right aortic root to a 

 point about mid-way between the dorsal end of the fifth arch and the 

 union of the aortic roots. Here it gives off the right subclavian. The 

 remainder of the aortic root disappears. The right vertebral artery is 

 given off from a point in the right fourth arch corresponding to the place 

 of origin of the left A. vertebralis. Of the fifth pair of arches, the right 

 disappears, completely, and Rathke looked in vain in mammals for a 

 vessel passing from this arch or any other upon the right side, to the 

 lungs. Near the origin of the fifth arch, a branch is given off which 

 passes backward to the lungs, soon after they appear. This forms the 

 posterior portion of the pulmonary stem, and the part of the fifth arch 

 between this branch and the heart forms its upper portion. The remainder 

 of the left fifth arch forms the ductus arteriosus Botalli. 



In reference to Raïhke's conception of the manner of formation of 

 the pulmonary artery in mammals, it may be stated that he probably did 

 not have material favorable for observing those early stages which show a 

 symmetrical development of a pulmonary branch from either fifth arch, and 

 the changes which bring about the conditions described by him. Beemer's 

 (1902) investigations (to be noted below) have shown that Rathke's 

 diagrams do not represent the actual origin of the pulmonary artery. 



Rathke's paper of 1857 treats of the aortic arches in Saurians and 

 also contains his well known schemes illustrating the transformation of the 

 aortic arches in Amniota. His diagrams have been widely copied in text- 



