297 
In regard to the aortic arches in the rabbit, the observations 
show lack of uniformity as in the case of pig embryos. Only one 
observer (ZIMMERMANN) has reported a complete arch between the 
systemic and pulmonic. In his specimen this consisted of a slender 
vessel extending from the truncus arteriosus to the aortic root and 
ending very close to the pulmonic arch. Miss LEHMANN found irreg- 
ular vascular elements in the rabbit between the systemic and pul- 
monic arches, but in none of her specimens was a complete arch 
present. She suggests the probable reason for this in saying: “No 
complete arch was found, but I was not in possession of a closely 
graded series of embryos in the eleventh and twelfth days, in which 
to follow the history of this vessel.” 
Lewis emphasizes also in the rabbit the great irregularity of the 
vascular elements between the fourth and pulmonic arches, which vary 
according to his observations in different individuals and on opposite 
sides of the body in the same embryo. The observations of Lewis 
and Miss LEHMANN are in accord, except that Lewis has shown wider 
variability in the vascular elements in question. In connection with 
the above review of the conditions exhibited in rabbit embryos, it is 
interesting to add the observations of ZIMMERMANN on the phases of 
disappearance of the fifth arch, which, it will be remembered, he found 
complete in one specimen. In the process of breaking down, according 
to ZIMMERMANN, the arch forms an anastomosis with the fourth arch 
and thereafter separates from the truncus arteriosus, then, the portion 
of the arch between the anastomosis and the aorta becomes obliterated, 
leaving for a very short time, only a small spur from the aortic root. 
After the disappearance of the latter, the final remnant of the arch 
consists of a sprout from the caudad surface of the fourth arch with 
a blind diverticulum extending ventrally towards the truncus. 
In embryos of the rat TANDLER observed a spur from the caudad 
surface of the fourth arch and, later, a broad connection between the 
fourth and the pulmonic. The latter he interprets as a modified fifth 
arch, and this conclusion is rendered more probable by the observ- 
ation of Miss LEHMANN showing a connecting vessel between the fourth 
and the pulmonic arches in the pig (cf. Fig. 10). 
In a sheep embryo TANDLER found on both sides of the pharynx 
a vessel extending from the distal end of the pulmonic arch into the 
territory between the systemic and pulmonic arches but was unable to 
determine its ventral connection. 
We thus have in mammalian embryos an interesting gradation in 
the vascular elements between the fourth and pulmonic arches: a) a 
