406 Harriet Lehmann, 



the pulmonar}^ arch. Dorsally tlie sixth arch runs for a short 

 distance in connection with another vessel of nearly equal size, but 

 which, after leaving- the common stem, runs towards the surface 

 just above the pulmonary arch, and ends blindly in the mesoderm 

 opposite the middle part of the sixth arch. These structures are 

 repeated on either side. The more superficial position of these rudi- 

 ments is due to the fact that they curve over the last branchial 

 pouch, while the sixth aortic arch runs just beneath it. A very 

 small portion of the branchial pouch appears to be partially con- 

 stricted off between the two vessels. For reasons given in the 

 comment following the description it seems reasonable to regard 

 these rudiments of vessels as representing a fifth aortic arch. 



The sixth arch in this embryo is small, but complete, and no 

 pulmonary artery is present. The subclavians, not included in the 

 reconstruction, arise from the dorsal aorta, about two-thirds mm 

 below the union of the aortic roots. Division in the truncus arteriosus 

 has not yet begun. 



The lack of connection between the supposed ventral remnant 

 of the first arch, and the second, makes it doubtful whether as 

 Eathke has described, the external carotid is formed from the ventral 

 remnant of the first and second arches, without at- least secondary 

 changes. A study of close stages in the eleventh and twelfth day 

 would no doubt determine this question. A comparison with the 

 conditions found in the pig will be made later. The position of the 

 blind ends of the remnants of the second arch, shows variation from 

 the condition described in the preceding stage. It is interesting to 

 note that the remnants of the hyoid arch in the embryo under con- 

 sideration are longer than those shown in Fig. 8, Plate 23, an 

 earlier stage. The manner of disappearance agrees with Rathke's 

 description of the same process. The vessels described above as 

 possible rudiments of a fifth arch, correspond in position with 

 relation to the fourth arch and dorsal aorta, to the structures noted 

 in this region in the preceeding stage. They to not agree fully 

 with Zimmermann's description of a fifth arch for the rabbit. Zimmer- 

 mann found a complete fifth arch in a rabbit of about the eleventh 

 day, but its course w^as from the truncus arteriosus to the aorta, 

 joining the latter near the dorsal end of the sixth arch. There is 

 little room for doubt however, that the rudiments in Fig. 10, Plate 23, 

 correspond to the vessel in Zimmermann's embryo. The ventral bud 

 is too small to be the entire means of ventral connection for a com- 



