22 WILLIAM A. LOCY AND OLOF LARSELL 



4. THE DEVELOPiMENT OF THE PULMOXARY ARTERY 



In describing the external appearance of the embryonic lung 

 the pulmonary artery and pulmonar}^ vein were noted as a part 

 of the surface view. We shall now give a more detailed account 

 of the method of origin of the puhnonary blood vessels and of 

 the embryonic changes that they undergo. 



The pulmonary artery is formed by the union of two parts, one 

 of which, the proximal end, sprouts from the sixth aortic arch, 

 and the other, the distal end, begins in the lung wall and grows 

 towards the sprout from the aortic arch. 



The vascularization of the walls of the lung precedes the forma- 

 tion of the pulmonary artery and the distal extremity is the first 

 formed. Examination of sections of the fifty- two hour stage 

 shows the presence of rounded vascular spaces in the mesenchjmia 

 of the lung primordium. When first formed these vascular 

 spaces are of small extent and can seldom be traced through 

 more than two sections, but the examination of numerous speci- 

 mens of this age shows that they are fairly constant in appearance 

 and as to their position in the median and dorsal portions of the 

 lung parenchyma. These represent the rudimentary condition 

 of the vascular area of the lungs. 



As development proceeds, the vascular spaces assume greater 

 definiteness, and by extension come together, and fuse forming 

 an incipient network of capillary-like canals. Between the sev- 

 enty-fifth and the eighty-second hour, in particular, these spaces 

 show with increasing definiteness, and by the eighty-second hour 

 of development the longer vascular spaces can be traced through 

 twelve or more sections. These are intermingled with other 

 spaces of less extent, all occupying the area of the lung in which, 

 at a little later period, the pulmonary artery is formed. In the 

 early formed vascular spaces it is not obvious which are des- 

 tined to give rise to the artery and which to the pulmonary vein. 

 This account agrees in essential particulars with Evans' obser- 

 vations of the development of blood vessels from capillary spaces. 



These vascular changes in the mesenchyma of the lung wall 

 begin before the formation of the sixth aortic arch. In many of 



