202 W. B. CHAPMAN 



and are thus very slowly traversed, atrophy, or disappear alto- 

 gether. The progressive breaking-iip of the capillaries which, in 

 this experiment, is general throughout the area pellucida, and 

 gradually extends to the area opaca, is clearly this same process 

 on a much larger scale. The little lakelets of blood that are left 

 being due to the inability of the vessels to entirely discharge 

 their contents. 



VIII. SUMMARY 



In summarizing the chief factors that have appeared in this 

 investigation, the following are the more important: 



1. In chick embryos, in which the heart has been removed 

 before the establishment of the circulation, the embryo and area 

 vasculosa remains alive for seven or eight days after the opera- 

 tion. A limited amount of growth takes place in the embryo 

 proper, while the area vasculosa spreads out over the yolk until 

 it may reach a diameter of approximately 45 millimeters. 



2. The development of the blood vessels in the area vasculosa 

 is not entirely inhibited, but the process proceeds in a normal 

 manner for a short period beyond the time at which the circula- 

 tion usually commences. During this time there are formed in 

 the extra-embryonic region vessels identical with the normal an- 

 terior vitelline veins, which fuse anterior to the embryo as in 

 normal chicks, while the sinus terminalis passes through a cycle 

 of development and regresson which markedly imitates the nor- 

 mal. On the other hand, other vessels which normally differen- 

 tiate early, the omphalo-mesenteric arteries, as well as the om- 

 phalo-mesenteric veins, and the posteror vitelline vein, are not 

 formed. 



3. With the expansion of the vascular area, there is a continued 

 formation of new capillaries; the property of sprout formation 

 apparently being retained until death. After the third day, 

 however, this is confined to the marginal portions of the area. 

 Near the embryo, in the area pellucida, new formation has ceased 

 at three days, and regressive changes commence; connecting cap- 

 illaries are retracted, leaving isolated or nearly isolated endo- 

 thelial blisters distended with fluid or blood cells. This regres- 

 sive process advances gradually into vessels of the area opaca. 



