ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 



INTRODUCTION. 



This paper deals with the origin and early stages of the pulmonary vessels 

 of the chick, as demonstrated by stained serial sagittal and cross-sections of fixed 

 specimens and by injecting living embryos with dilute india ink. The serial sec- 

 tions begin at the stage of 20 somites, in which the first evidence of a pulmonary 

 system is seen in the proliferation of angioblasts from endothelial walls of estab- 

 lished vessels, although it is possible that a few of these cells may differentiate 

 from mesothelium. From these sections I have been able to show that this pro- 

 liferation of angioblasts gives rise to both the common pulmonary vein and the 

 left valve of the sinus venosus. The angioblasts spread over the ventral surface 

 of the gut, acquire a lumen, and form a capillary mesh from which the vessels of 

 the lung are evolved. After this plexus is patent and connected to the systemic 

 vessels, the changes leading to the formation of the earliest pulmonary system may 

 be followed by injections. By means of a modified technique for injection I have 

 been able to demonstrate earlier stages in these vessels than have heretofore been 

 shown and to trace the metamorphosis which this capillary plexus undergoes in 

 forming the rudimentary pulmonary vessels. The study ends at the stage of 85 

 hours' incubation, at which time the pulmonary system is definitely laid down in 

 its earliest complete form. 



Concerning the origin of the pulmonary vessels, not only is our present know- 

 ledge meager, but the views are conflicting and based on observations of embryos 

 of different forms, made with varying technique. Wax reconstruction of small 

 blood-vessels, while a valuable asset to the embryologist, is open to manifold errors, 

 and where possible should be checked up by injections. Confusion has arisen 

 from the efforts to prove or disprove the probable course of events in one embryonic 

 form from observations on another embryonic form. In recent studies of the 

 pulmonary vessels, guinea-pig, rabbit, cat, and chick embryos have been represented. 

 The finer details in the development of separate structures might follow quite 

 different courses in these several forms. Although it is to be remembered that 

 any attempt to draw conclusions for one on the basis of another is open to error, 

 this study is presented in the hope that demonstrating the developmental steps 

 of the vessels of the lung in the chick may by comparison prove of value in working 

 out the embryology of similar structures in other forms. 



In an investigation of this kind some obstacles are sure to be encountered, 

 even in so simple an embryo as the chick. In mammalian embryos these are 

 harder to overcome and offer a possible explanation for our present inadequate 



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