4 DIRECT GROWTH OF VEINS BY SPROUTING. 



Having determined that the primary point about the vascular system is 

 that it starts by the differentiation of a new cell, which increases by division and 

 by sprouting, it is of first importance to determine whether the differentiation of 

 new angioblasts is limited in time or whether it continues throughout life, either 

 generally or in certain specific places. This question was tested by restudying the 

 regeneration of blood-vessels, after intestinal anastomoses made in adult dogs, 

 in conjunction with Dr. Halsted and Dr. Holman (1920), who performed the 

 operations. No evidence could be found of a differentiation of new angioblasts; 

 rather, the vascular system was restored by an active division of preexisting endo- 

 thelium of small arteries, veins, and capillaries, involving a return of this endothe- 

 lium to its embryonic angioblastic condition. Thus in these studies the new vessels, 

 when first formed, were connected with the old, but showed a lumen as irregular 

 as the lumen of embryonic vessels during their transformation from solid angio- 

 blastic masses. It thus seems likely that we must look for a phase in embryonic 

 or fetal development when the differentiation of new angioblasts ceases, all sub- 

 sequent new growth of vessels being accounted for by the division of preexisting 

 endothelium. Thus the complete story of the development of the vascular system 

 must take into consideration how far each vessel arises by the differentiation of 

 angioblasts and how far by division and sprouting, and when, for each organ or 

 area of the body, the differentiation of new vasoformative cells ceases. 



As far as we have gone in this study, it has been found that throughout the first 

 7 days of incubation in the chick there is a differentiation of new angioblasts to 

 be made out in the area pellucida. This differentiation of new angioblasts is 

 extremely extensive during the whole of the second day; from the third day on it 

 becomes relatively greatly diminished ; but almost any blastoderm up to the seventh 

 day of incubation, which is as far as the process has yet been followed, will show one 

 or two small vesicles unconnected with the main plexus. It is an interesting 

 point that the solid masses of angioblasts are much rarer than the vesicles, only one 

 or two masses of angioblasts having been found in about 80 specimens, while most 

 of the specimens show one or two vesicles. The reason for this is that the lique- 

 faction takes place in a short time, only one or two hours being required to trans- 

 form a solid mass into a hollow vesicle, while it takes a long time for the vesicles 

 to join the main plexus. One only rarely sees the process during the time of 

 observation of a single specimen, representing on an average 5 hours. This dif- 

 ference in duration in the two processes explains why the isolated vesicles are so 

 much more common in sections than the solid masses of angioblasts. 



Concerning the primary vessels of the embryo, it was first noted that a large 

 part of the dorsal aorta of the chick could be seen in the living blastoderm to 

 differentiate in situ from angioblasts. In this volume is a study of the origin of 

 the pulmonary vessels in the chick, by Buell. He has demonstrated that the 

 period in which the vessels begin, i. e., on the second day of incubation, is a stage 

 in which the vessels are represented by a mass of solid angioblasts. These angio- 

 blasts first appear as a solid mass of cells connected with the wall of the sinus 

 venosus and are readily distinguishable by their structure from undifferentiated 



