408 V, E. EMMEL 



ing of certain arterial branches of the aorta and an extensive 

 degeneration of others. 



Directing attention first to the degenerative changes it will 

 be recalled that the early embryonic aorta has three sets of 

 eighteen to twenty or more paired branches — dorsal, lateral, 

 and ventral. Of these rami, nearly all the dorsal vessels per- 

 sist throughout embryonic development, whereas practically 

 all the remaining vessels, with certain exceptions, subsequently 

 atrophy and disappear. Thus in the case of the human em- 

 bryo it has been shown that the primitive lateral branches of 

 the aorta which form an extensive system in the twenty-three 

 somite embryo, have in the 16 mm. to 19 mm. stage embryos 

 become largely atrophied. The single median arterial stems 

 which have replaced the extensive series of paired ventral ar- 

 teries of younger stages (Tandler, 03) and in a 5 mm. embryo 

 extend as a ''complete series of unpaired or median ventral 

 segmentals from the seventh cervical to the second lumbar 

 segment inclusive," in a 7 mm. embryo "have been reduced to 

 three main trunks" the coeliac, superior, and inferior mesenteric 

 arteries. (Keibel-Mall, '12, pp. 603, 611, 643, 653). 



Second, it is to be observed that the three remaining arterial 

 trunks to which the ventral segmental aortic arteries have been 

 reduced, undergo a remarkable shifting or caudal wandering 

 as first described by Mall in 1891 and subsequently confirmed 

 by Tandler ('03) and Broman ('08). In the human embryo 

 for example ''the coeliac artery thus wanders from the seventh 

 cervical to the twelfth thoracic segment, a displacement of some 

 eleven segments, and the superior mesenteric artery almost 

 equally as far (ten segments, second thoracic to third lumbar) ; 

 whereas the inferior mesenteric artery wanders through but 

 three segments (twelfth thoracic to third lumbar)" (Evans '12, 

 p. 647). 



Referring again to the human embryo it is of especial interest 

 with reference to our present purpose to note that all of these 

 vessels "usually attain the adult levels by the time the embryo 

 is 17 mm. long." Furthermore this caudal wandering of the 

 intestinal arteries is not b}' a displacement of the aorta on the 



