454 Philippine Journal of Science i»w 



formation was done by von Baer(i) in 1827. The last-named 

 author claimed that five pairs of aortic arches were present in the 

 embryos of all vertebrates that developed out of water, but that 

 these were never all present at any one time. Thirty years later 

 Rathke (7) drew up schemes to represent their transformation in 

 Amniota, based upon the assumption that there were only five 

 pairs of these arches represented in the embryonic stage of the 

 higher vertebrates. For a long time the diagrams of Rathke 

 were accepted as standards to represent the history of the de- 

 velopment of these arches. 

 Lehmann(5) says: 



The number of arches recognized by Rathke however was thrown into 

 question by van Bemmelen in 1886, who first pointed out the presence 

 in reptiles and the chick, of a rudimentary arch between the fourth and 

 the supposed fifth. Subsequent observations, notably those of Zimmermann 

 (1889), and Tandler (1902), have led to the recognition of six aortic 

 arches in mammals. The discovery of a rudimentary fifth arch in 

 mammals makes the number of the aortic arches for Amniota the same 

 as in Dipnoi and Amphibia, and establishes an identity, as regards the 

 place of origin of the pulmonary artery in all lung-breathing vertebrates. 

 The diagrams of Boas have accordingly replaced those of Rathke. 



Evans, (3) writing in Keibel and Mall's Human Embryology 

 on abnormal aortic branchings, says in part: 



The variations in the great vessels arising from the aortic arch have been 

 known for a long time and could be explained satisfactorily on an embryo- 

 logical basis ever since the work of Rathke. (1843). 



To explain my cases of abnormality of origin, it would seem 

 advisable to review the development and transformation of the 

 aortic arches and segmental arteries occurring in normal mam- 

 malian embryos (see figs. 1 and 2) . In human embryos 4 to 5 

 millimeters in length five aortic arches are recognized to develop 

 in the following order: First, second, third, fourth, and sixth. 

 The fifth aortic arch is transitory, appearing in embryos 7 mil- 

 limeters long; it soon degenerates. 



Both the descending or dorsal aortse, between the first and 

 third arches, together with the third aortic arches, give origin 

 to the internal carotid arteries; while the two ventral aortae, 

 between the third and fourth arches, form the common carotids ; 

 those between the first and third arches become the external 

 carotids after the disappearance of the first and second aortic 

 arches. The portion of the dorsal aorta between the third and 

 fourth arches disappears. 



The fourth arch, including the short ventral stem between 

 the fourth and sixth arches, becomes the permanent aortic arch 



