THE EMBRYOLOGY OF THE 



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vision from the lung. The slender thread-like portions shows the 

 region of junction between the two ends. It would appear, there- 

 fore, that the first rudiment of the pulmonary artery begins in 

 the mesenchyma of the lung walls in the form of vascular spaces, 

 which by extension and union form a network of connecting pas- 

 sages, and from these arise the distal end of the pulmonary ar- 

 tery. The proximal division arises slightly later, springing from 



Fig. 57 Dissection of the lung territory of an embryo incubated four and 

 one-half days. Shows the pulmonary artery not yet completed in its middle 

 course. Drawn bv G. H. A. Rech. 



the sixth aortic arch, and then growing to meet the distal divi- 

 sion from the lungs. When their union is effected the pulmonary 

 artery is established. Figures 12 and 13, part I, show the sur- 

 face appearance at five and one-half days after union of the two 

 parts of the pulmonary artery. 



Marshall ('92) was, we believe, the first to maintain that the 

 "pulmonary arteries appear in the walls of the lung about the 



