416 Harriet Lehmann, 



and upon its posterior side three ingrowths of the surrounding tissue 

 cross its lumen, and just beneath these lie two small ventrally 

 directed spurs from the dorsal aorta. A third spur leaves the upper 

 side of the sixth arch, near its dorsal end. Between these spurs, 

 and the one described above as leaving the ventral part of the 

 fourth arch, lie a chain of small sinuses situated about midway 

 between the fourth and sixth arches. In this specimen two distinct 

 entodermal pouches are present between the fourth and sixth 

 arches. 



The pulmonary aortic arch in this stage has manifestly increased 

 in size, so that its diameter nearly equals that of the third arch. 

 The rudiments of the pulmonary arteries are developed symmetrically 

 as to position, one from the posterior side of either sixth arch, about 

 midway in its course. The pulmonary artery of the left side is 

 however, a more slender vessel than that of the right. 



Division in the truncus arteriosus has not yet taken place. A 

 reconstruction of the vertebral artery is shown in Fig. 20, Plate 24, 

 but no attempt has been made to show with exactness its irregular 

 outline. The subclavians arise from the aortic trunks about Vi ^^ 

 behind the union of the aortic roots. 



A remnant of the first arch is still present in this stage, in 

 connection with the dorsal aorta. A slender but somewhat larger 

 dorsal remnant of the second arch persists, but I am unable to say 

 just what changes have brought about the greater separation of 

 the posterior ends of the vessels which have previously been regarded 

 as ventral remnants of the second arch. A study of close stages 

 would, I think, show intermediate changes. 



The rudimentary structures described in connection with the 

 fourth and sixth arches, by comparison with later stages, may be 

 fairly considered as representing a stage in the formation of the 

 fifth arch, since the rudiments occupy positions with relation to the 

 fourth and sixth arches, clearly in correspondence with the same 

 parts of a complete fifth arch in a later stage, and with similar 

 incomplete vessels in other embryos. The presence of two distinct 

 branchia] pouches between the fourth and sixth arches also adds 

 evidence in favor of considering these structures as parts of one 

 incomplete fifth arch. These rudiments appear a short time before 

 division takes place in the truncus arteriosus. Tandler finds a 

 rudiment of a fifth arch in the rat in a stage almost identical in 

 development to the one under consideration. The origin of the 



