458 WILLIAM A. LOCY AND OLOF LARSELL 



part of the pharynx. The lung pouches are smooth and do not 

 as yet exhibit surface irregularities. 



Injected specimens of this stage frequently show blood vessels 

 running along the ventral surface of each lung and uniting in the 

 median plane at a point where the lung pouches join the pharynx 

 (fig. 6). From this place of union a vessel leads into the left 

 atrium of the heart. Another vein, coming from the front, 

 passes along the ventral surface of the laryngo-tracheal groove 

 and joins the stem vessel that leads into the left atrium. The 

 blood vessels on the lungs are the beginnings of pulmonary veins 

 and they are commonly injected before the pulmonary artery is 

 established. Sections show however that vascular spaces for the 

 formation of the distal extremity of the ]Dulmonary artery are 

 already present in the lung walls. 



In the closing hours of the fourth day the trachea becomes 

 differentiated from the posterior portion of the laryngo-tracheal 

 groove. It may be definitely distinguished in an embryo with 

 39 somites (estimated as in the 94-hour stage), and, by the 100th 

 hour, it is well defined. This is not readily evident in surface 

 views but in optical section (fig. 25, 4 days, 4 hours) the connec- 

 tion of the trachea with the pharynx and with the bronchi is 

 well exhibited. At the distal end of the lung tube is an enlarge- 

 ment that foreshadows the abdominal air-sac. At its proximal 

 end, on each side, a short portion of the bronchus lies between 

 the anterior limits of the lung and is the first appearance of the 

 extra-pulmonary bronchus. These extra-pulmonary bronchi join 

 the trachea which is of larger calibre than the bronchi. The 

 oesophagus makes a rather abrupt dorsal bend away from the 

 trachea, and then, with a more gentle curvature continues cau- 

 dally and, bending downward, passes between the lungs. 



During the fifth day the lung grows larger and begins to show 

 surface irregularities. Figure 8 shows the appearance of the left 

 lung territory as exposed by dissection in a specimen of the 4^- 

 day stage. The lung pouch of this specimen has grown dorsally 

 so as to extend across the path of the aorta. Its distal extremity 

 exhibits a protuberance which is the beginning of a lobe in which 

 lies the expanded end of the mesobronchus. At about this 



