22 ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 



of angioblasts. This sheet of angioblasts later forms a network of capillaries con- 

 necting the dorsal and ventral aorta? to the sinus venosus through the common pul- 

 monary vein. There are also connections to anterior and posterior cardinal veins. 



This capillary plexus, meeting the fate of all embryonic capillary meshes, 

 is changed into individual vessels, certain ones of which increase in size and take 

 over the function of the smaller capillaries, leading to the atrophy and loss of the 

 latter. This process is followed in the splanchnic plexus. I have already shown 

 how the tributaries of the pulmonary vein are evolved in this manner. In a similar 

 way the arteries are formed. In figure 8, along the junction of the lateral and 

 ventral surface of the gut on each side, is a capillary vessel which arises from the 

 ventral aorta, extends caudalward, following a diagonal course to the laterodorsal 

 surface of the lung rudiment, where it connects with other capillary vessels, the 

 forerunners of the corresponding branches of the common pulmonary vein. It 

 is possible to inject the vessel at 60 hours' incubation (35 somites). 



It is interesting to note that the lumen of the artery can be injected before 

 the pulmonary arch is patent, showing that the artery antedates the arch. -This 

 does not agree with the observations of Huntington in cat embryos, in which he 

 states the arches are formed before the arteries. 



PULMONARY ARCHES. 



The pulmonary arches (sixth) arise in a manner slightly different from that 

 of the other aortic arches. The difference is largely chronological. The fact 

 that the arches are formed in conjunction with the splanchnic plexus and hence 

 may be regarded as a part of that capillary net does not cover the whole process, 

 as there are certain differences in origin that must be considered. The arches are 

 formed later than the pulmonary artery and vein and other capillaries in the 

 splanchnic plexus. It is possible to inject these vessels before a lumen is established 

 in the arches, although the dorsal and ventral primordia can be seen. Figure 7 

 shows such a stage. 



The pulmonary arch on each side arises from two sources. The first or dorsal 

 rudiment often has a double origin, part from the dorsal aorta and part from the 

 fourth aortic arch at the angle formed by the union of these two vessels. This is 

 the most constant relation, although some injections show it coming almost entirely 

 from the fourth arch near its junction with the dorsal aorta. It curves ventrally 

 around the last pharyngeal pouch and is connected with a similar process extending 

 dorsally from the ventral aorta. The lumina of the dorsal aorta and fourth arch 

 penetrate the dorsal angioblastic cord from above, often separately for a short dis- 

 tance, then uniting and extending ventrally. In a similar manner the lumen of the 

 ventral aorta extends dorsally into the ventral angioblastic cord. The two lumina 

 meet behind in the fourth pharyngeal pouch, completing the pulmonary arch. 

 This occurs in chicks of 35 somites. 



It is possible to inject both the dorsal and ventral primordia before the arch 

 is complete (fig. 7). In embryos a few hours older it is possible to inject the whole 

 arch, the large, pouch-like lumina of the two rudiments being connected by a 



