426 Harriet Lehmann, 



day shortly after division had taken place in the truncus arteriosus. 

 The rudiments found in this region in earlier stages and in a 

 succeeding one, practically correspond in position to parts of the 

 complete fifth arch, and as the embryos approach the twenty-first 

 day these elements become more clearly defined. A few minor 

 variations might be noted, but in transitory structures of this nature 

 a considerable range of variation is naturally to be expected, so that 

 any variable conditions exhibited by the fifth arch (and other arches) 

 are not to be regarded as atypical or abnormal, but as the result 

 of the transitory nature of the structures involved. This arch is 

 not identical with the fifth arch of Zimmermann for the rabbit or 

 the human embryo, nor does it agree fully with the figures of the 

 fifth arch by Tandler for the rat, or for the human embryo. The 

 rudiments of a fifth arch noted in my series of rabbit embryos are 

 also of different arrangement. 



I observed such structures in several embryos not reconstructed 

 as well as in those described. The constant presence of rudiments 

 between the fourth and last aortic arches, and also the presence of 

 an additional branchial pouch, justify the conclusion that the pul- 

 monary arch is the sixth in mammals as well as in other verte- 

 brates. 



The sixth arch probably forms early in the twentieth day, since 

 it was not complete in one of the j'oungest pig embryos observed. 

 It is the last arch to form with the exception of the rudimentary 

 fifth. As in the rabbit series, the ventral end appears to develop 

 first, and the dorsal end until after the disappearance of the fifth 

 arch, retains an irregular appearance, passing through a series of 

 changes, probably related to the formation and degeneration of the 

 dorsal portion of the fifth arch. 



Elements of the pulmonary arteries appear, in one case, before 

 the sixth arch is complete, and at any rate a short pulmonary 

 branch is present upon either side very soon after the arch is 

 formed. In the rabbits observed the pulmonary arteries appeared 

 to arise slightly later. These arteries leave the sixth arches, at 

 first, about midway in their courses ; although from a side view they 

 appear to retain this position for some time, they are approaching 

 each other, because the ventral ends of the sixth arches are drawn 

 nearer together. In the embryo shown in Fig. 26, Plate 25, the 

 pulmonary arteries arise only a short distance from the union of 

 the sixth arches, and their roots lie close together, while further 



