76 CARNEGIE INSTITUTION OF WASHINGTON. 



vascular anomalies that occur in this region. Special mention should be 

 made of the fate of the right dorsal aorta, which they show is almost com- 

 pletely taken up in the formation of the proximal part of the right subclavian 

 artery, and they also show that one can not properly speak of a ventral aorta, 

 either in the human or pig embryo. 



Dr. Heuser's injections show very clearly the adaptive powers of endo- 

 thelium in the case of the first pair of aortic arches when they surrender to the 

 more caudal arches their original function of delivering blood from the heart 

 to the dorsal aortse. When this happens, they change from large trunk 

 channels to a fine capillary net with suitable feeders and drainage com- 

 munications, the same endothelium so reshaping itself as to meet only the 

 requirements of the tissues of the first gill-bars. 



Development of Pulmonary Vessels. 

 Dr. C. E. Buell has studied the origin and early stages of the pulmonary 

 vascular system in the chick, basing his observations jointly on specimens 

 prepared in serial sections and cleared specimens that had been injected with 

 india ink under an improved technique. His description and well-selected 

 illustrations carry the origin of these vessels back to stages earlier than have 

 heretofore been shown, and their metamorphosis is traced through to the time 

 the definite pulmonary vessels are laid down. One finds the same general 

 sequence of events in the angiogenesis of this region that has been referred 

 to above as occurring in other territories and with other organs. There is 

 first the laying down of an endothehal bed or net. In this case a proliferation 

 of angioblastic cells takes place from the endothelial wall of the sinus venosus 

 opposite the beginning lung-bud, and perhaps it is added to by mesenchyme 

 cells that differentiate in situ. Connecting strands of these vasoformative 

 cells acquire a lumen and plasma by cytoplasmic liquefaction resulting in a 

 capillary net along the surface of the primitive gut, an indifferent splanchnic 

 plexus whose form expresses only endothelial growth and is not yet affected 

 by the hydrodynamic necessities of supply and drainage. The second step 

 is characterized by the development within this enlarging net of primitive 

 pulmonary channels (favorable lines of blood-flow) which occurs on the com- 

 pletion of the lumen and the beginning of the circulation. There then follows 

 the period of adaptation in which the vascular channels accommodate them- 

 selves to the growing lung and the circumstances of the surrounding structures, 

 the arteries, veins, and capillaries being always adjusted in size and arrange- 

 ment to the parts, progressively stage by stage, so that at any given time they 

 are functionally adequate for that particular set of requirements, and no 



more. 



Development of the Arteries of the Forelimb. 



There are several good reasons for selecting the limb-bud as a region for 

 studying the process of angiogenesis, namely, its structural simplicity, its 

 accessibility for direct observation, and its relative isolation from the rest of the 

 body and consequent freedom from environmental complications. Dr. H. H, 

 Woollard has taken advantage of these circumstances and has completed a 

 full account of the development of the principal blood-vessels in the forelimb 

 of the pig. Starting at the time when there is first a definite capillary net, he 

 has been able to demonstrate the second or circulatory phase in the angio- 

 genesis of this region, that is, the enlargement and modification of certain 

 parts of the net to form the primary arterial and venous channels. He has 



