ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 21 



extension of a capillary net from the ventral aorta. Unknown to Bremer, Fedorow, 

 in a Russian publication, antedated the former's work by a similar description 

 of the origin of the pulmonary artery in the embryo of the guinea-pig. Bremer 

 (19126), in a fourth paper, generously acknowledged the priority of Fedorow's work. 

 Huntington, basing his observations on reconstructions from the cat embryo, 

 holds that the artery is formed by the "organization of a distinct arterial channel 

 in the ventral portion of the post-branchial plexus." Thus far his observations 

 coincide with my own on the chick. Concerning the origin of the original plexus, 

 he states that it is derived from the dorsal aorta and links up secondarily with the 

 ventral aorta: 



' ' The so-called outgrowth from the pulmonary sixth arch serves merely as the point 

 of junction, at which after coalescence with the pulmonary plexus, the blood is carried 

 from the ventral segment of the sixth arch into this prepared channel of the pulmonary 

 artery. The outgrowth would be more correctly defined as the pulmonary arterial 

 tap or approach of the sixth arch." 



Huntington's description of the origin of the splanchnic plexus in the cat is 

 quite different from the condition met with in embryos of the guinea-pig, rabbit, 

 and chick. It may be possible that the cat is individual in this respect. Fedorow, 

 using guinea-pig embryos, described an extension of capillaries from the ventral 

 aorta. A similar observation is made by Bremer in rabbit embryos. My chick 

 embryos show an extension of angioblasts from the ventral aorta. However, this 

 is but a part of the whole process and there are other factors which contribute to 

 the formation of the splanchnic plexus. In considering this we must realize that 

 the splanchnic plexus consists of more than merely that portion giving rise to the 

 pulmonary arteries; it lies caudal to the fourth aortic arch and includes the devel- 

 oping hepatic system as well. In the chick the different parts of the plexus are 

 derived from different structures. The cephalic (pre-pulmonic or post-branchial) 

 portion of the plexus is formed from angioblasts derived from the endothelium 

 of the dorsal aorta, ventral aorta, and sinus venosus. The post-caval portion is 

 largely from the dorsal aorta and partially from the sinus venosus. The cardinal 

 veins may also contribute to both parts of the plexus, although I have not seen any 

 direct proliferation from them. They are joined to the plexus at a very early stage, 

 namely, at 35 somites. It is also possible that certain of the angioblasts may dif- 

 ferentiate from mesenchyme and contribute to this formation. 



In order to understand the origin of the pulmonary artery, it is necessary to 

 consider that portion of the splanchnic plexus lying between the fourth aortic arch 

 and the sinus venosus at the level of the lung-bud. The pulmonary artery, and the 

 pulmonary arch (sixth) as well, are persisting channels in this capillary bed. 



As to the origin of the capillary plexus, it is derived from angioblasts that 

 proliferate from endothelium of established vessels. From the dorsal aorta angio- 

 blasts spread out ventrally over the surface of the gut. From the ventral aorta 

 they extend caudally under the surface of the gut. From the sinus venosus, as a 

 part of the common pulmonary vein, the angioblasts spread laterally, caudally, and 

 cranially, so that the ventral surface of the primitive gut is covered with a network 



