THE EMBRYOLOGY OF THE BIRD's LUNG 27 



the stage at which the first entobroiichus {Ent. I ) is given off and 

 the internal changes are to go on rapidly, accordingly, we may ex- 

 pect, a good development of blood vessels. The artery enters 

 the lung substance and passes dorsally as well as nearly parallel 

 to the lung tube. Loops of blood vessels pass from the dorsal 

 side around the lung tube and unite with the vein below. These 

 capillary loops are more abundant near the bud of the first ento- 

 bronchus and the expanded portion of the lung tube. 



At five days, twenty hours, the pulmonary artery is seen (fig. 

 60) running nearly parallel to the course of the extra-pulmonary 

 bronchus and entering the lung dorsal to the bronchus. The 

 pulmonary vein runs through the ventral region below the cen- 

 tral lung tube. 



As shown in figure 60 A, a sketch from the ventral aspect, and 

 figure 60 B, from a lateral view, the artery branches and divides 

 into capillary networks around the entobronchi, and, more cau- 

 dally, around the embryonic vestibulum from which the ectobron- 

 chi are soon to arise. The capillary network of the dorsal side, 

 having surrounded the lung tube and its outgrowths, passes ven- 

 trally, and comes into communication with the capillary net- 

 work of the pulmonary vein. 



In the course of a few hours the branching of vessels within 

 the lungs has obviously increased. Figure 61 A is the sketch of 

 the left lung and 61 B of the right lung of an embryo incubated 

 five days, twenty-two hours. In the left lung are represented 

 the chief branches of the pulmonary vein and in the right lung 

 both veins, arteries and capillaries are sketched, but the capil- 

 lary network has been simplified. The pulmonary vein is split 

 into two great divisions that form a fork over the bronchus (fig. 

 61 B, fig. 21). One arises from venules situated deep within 

 the lung substance, dorsal to the bronchus, and is designated the 

 internal branch, the other is the external branch. In examining 

 figures 60 and 61 it is to be understood that the capillary network 

 is more complex than shown in the sketches, at the same time, 

 the network was sketched with the aid of a camera, and embraces 

 those minute vessels that carried the injection and which could 

 be readily made out. 



