24 WILLIAM A. LOCY AND OLOF LARSELL 



passes on the ventral surface of each lung; these two unite into 

 a trunk vessel situated in the median plane, and this, in turn, 

 passes to the left atrium of the heart. Anteriorly is situated 

 another vein that runs along the median line of the laryngo-tra- 

 cheal groove and also unites with the trunk vessel that leads into 

 the left atrium. In this specimen the pulmonary artery was 

 not seen. 



There is considerable individual variation in the time of forma- 

 tion of the blood vessels, so that the precise time and the degree 

 of development is not identical in corresponding specimens. 

 Some of this obser^-ed variation may be owing to imperfect in- 

 jection. Nevertheless, the method of formation of the pulmon- 

 ary artery is sufficiently definite to leave little room for doubt. 

 By the beginning of the fifth day there is a stem vessel in the 

 lung wall and a short spur from the ventral end of the sixth aor- 

 tic arch. These are directed towards each other but they are 

 separated by a very obvious interval, they constitute the proxi- 

 mal and distal ends of the future pulmonary artery. 



Sections of the 96-hour stage (fig. 58) show on each side a 

 short spur from the ventral part of the sixth arch. About twelve 

 hours later (4| days) we find in surface views (fig. 57) an almost 

 completed pulmonary artery. There is, however, satisfactory 

 evidence in the injected specimens thi :h.e spur from the sixth 

 arch is not the only growing point in ii.e formation of the artery. 

 On the contrary, it meets a forward growing vessel from the 

 lung, which has been formed through the medium of the vascu- 

 lar spaces already described, and which precede its appearance. 

 Fourteen injected specimens of the middle of the fifth day (4| 

 days) were dissected. All showed the complete outline of the 

 pulmonary artery, but in every specimen it was noticeable that 

 the pulmonary artery was not of the same calibre throughout 

 its course. Both ends were well developed, but about midway 

 between the two ends the diameter was reduced, so that it pre- 

 sented the appearance of a slender tread. This is well shown in 

 figure 57 which represents the dissection of an injected specimen 

 of four and one-half days development. The distal division of 

 the artery, from the sixth arch, is shorter than the proximal di- 



