84 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



from the aortic sac and then becomes connected midway with a channel to the 

 dorsal aorta. The free end of this vessel on either side constitutes the primitive 

 pulmonary artery; the proximal part, together with the connection with the 

 dorsal aorta, forms the pulmonary arch. Buell's injections of the chick show that 

 the primitive pulmonary artery arises in this form not from the ventral-arch sprout 

 but from the aortic sac, and that it is secondarily carried up upon the arch by the 

 rapid development of the ventral end of the arch. Huntington describes in the 

 cat the development of an isolated channel from the pulmonary plexus which is 

 later tapped by a short outgrowth from the pulmonary arch to form each primitive 

 artery. 



The observations to be given on the development of the human primitive 

 pulmonary arteries were made for the early stages from the study of cross-sections 

 alone and in larger embryos by the preparation of models. 



Embryos in which the vessels are well distended show that the earliest pul- 

 monary plexus is already present at the time of establishment of the fourth arch. 

 At this time the endodermic lung-bud is connected with the esophagus for most 

 of its length. A net of large capillaries and of angioblastic cords extends backward 

 from the aortic sac under the caudal pharyngeal-pouch complex and for a short 

 distance up its posterior surface along the course of the later pulmonary arch. 

 From its caudal extremity this plexus also sends a less developed net a little dis- 

 tance along the under surface and side of the laryngeal rudiment and common 

 tracheo-esophageal mass. At this time large capillaries can be seen extending 

 down as a plexus from the aorta into the esophagus. Later, there is a continuous 

 tracheo-esophageal net of uniform character due to the meeting of the two earlier 

 territories. 



The earliest primitive pulmonary artery that could be recognized with cer- 

 tainty by a study of cross-sections was in an embryo with well-distended vessels 

 soon after the completion of the fourth aortic arch. It could perhaps be demon- 

 strated still earlier by total injections. At this time the lung-bud was of consider- 

 able length and the primary lobes well elongated. A slightly later stage is shown 

 on plate 2, figures 33 and 34. 



A search was made for an arterial rudiment in the tracheo-esophageal groove 

 independent of the pulmonary arterial outgrowth from the aortic sac, such as 

 Huntington believes to exist in the cat. The result was entirely negative, though 

 6 embryos of the proper stage in excellent state of preservation and with moderate 

 vascular distention were examined. The region showed no vessel of greater than 

 capillary caliber until the extension from the sac-vessel had reached into it. Some 

 rather larger endothelial tubes were found on the dorsal surface of the lung-bud 

 bifurcation, even before the fourth aortic arch was complete, but when followed in 

 then development they proved to be the rudiments of the vein shown in figures 

 33 and 34. Buell has seen this earlier in his chick injections and terms it the 

 "cephalic pulmonary tributary." He finds it to be a transitory vessel. 



The succeeding history of the primitive arteries is connected with the trans- 

 formations of the pulmonary arch. The study of our series led to the same con- 



