100 CARNEGIE INSTITUTION OF WASHINGTON. 



the nuclei and rapidly enlarge, so that in from one to two hours there 

 is complete destruction of the cytoplasm and nuclei of the central 

 part of the angioblastic cords, resulting in the formation of a clear 

 plasma, the periphery of the mass being preserved as an endothelial 

 boundary. 



The liquefaction of the cytoplasm occurs both in the loops of the 

 angioblastic plexuses and in masses of angioblasts that are still isolated, 

 in the latter case resulting in small vesicles that subsequently join the 

 main plexus. These observations were made for the most part in the 

 area pellucida of the yolk sac, but the same phenomena were also 

 seen within the body of the embryo. The dorsal aorta in the trunk 

 region and a portion of it within the head were seen to differentiate 

 in situ in the above manner. The angioblasts are not all converted 

 into endothelium and blood-plasma; a portion of them take part in 

 the formation of red blood-corpuscles. During the lumen formation 

 in the angioblastic strands small clumps of the original angioblasts 

 become partially separated by liquefaction of the cytoplasm around 

 them. Such masses soon show the presence of hemoglobin and con- 

 stitute blood-islands that eventually float away in the blood-plasma 

 and break apart as free, red blood-cells. Blood-islands can also be 

 seen to be derived secondarily from the endothelium. This work has 

 been continued by Professor Sabin during the past year, and a further 

 account will be embodied in our next report. 



The development of the lymphatic vessels of the heart has been 

 studied in embryo pigs by means of injection methods, by Mr. James 

 R. Cash. He finds that the lymphatics arise from two sources — the 

 right lymphatic duct and the tracheal plexus, both of which send 

 vessels to the lungs and act as pathways of drainage in the adult heart. 

 A pericardial plexus is formed, which covers the entire heart and 

 reaches its greatest density when the embryo is about 60 mm. long, 

 decreasing with subsequent growth. From the injections it appears 

 that when this plexus is approximately complete its vessels invade 

 the heart-muscle and by anastomosis form the myocardial plexus. 

 This becomes less dense as the deeper points of the myocardium are 

 reached. At the 60 mm. stage no lymphatic vessels could be seen to 

 extend into the myocardium, although short lymphatic buds were 

 observed along the larger vessels of the pericardial plexus. Injections 

 at this period failed to fill any of the pericardial lymphatics, and it is 

 therefore probable that this plexus is almost completely formed before 

 the invasion of the myocardium. Like all other lymphatics, those of 

 the heart develop along the path of the blood-vessels. The author 

 finds, however, that the pattern of the lymphatics does not correspond 

 to that of the blood-vessels, and also that the lymphatics are not so 

 numerous as has been supposed. 



