DEVELOPMENT OF VESSELS WITHOUT HEART 193 



process was often not complete. Figure 14 is a camera lucida 

 drawing of a section of the sinus terminalis as it appeared in a 

 chick during the ninth day of incubation. The vessel had dis- 

 appeared over half of its length, and, on the side drawn, appeared 

 so thin as to resemble a wide band of endothelium, sending out 

 sprouts from its outer margin and gradually shading off into 

 normal capillaries towards the inside. 



A condition somewhat similar to the ones described above was 

 found in one of the eggs that I had been incubating as a normal. 

 In this egg, (incubated six days) for some reason, the embryo 

 had not developed normally and appeared as a small shapeless 

 mass of protoplasm lying in the center of the area pellucida. The 

 area vasculosa, however, was covered with a rich plexus of capil- 

 laries which were clearly alive and growing. There was no sign 

 of a border vein around this plexus, and the capillaries were 

 sending out numerous sprouts around their outer margin. 



Fig. 13 A camera lucida drawing of a section of the area opaca in the region 

 of the sinus terminalis which has broken up into capillaries. At this point, the 

 capillaries have apparently grown. Operated chick of 3 days' incubation. 



Fig. 14 Camera lucida drawing of the sinus terminalis in a chick during the 

 ninth day of incubation (8 days after operation). Enl. 47 X. 



Fig. 15 Camera lucida drawing of a section of the outer border of the area 

 opaca where the capillaries are highly vegetative and are sending out nimierous 

 sprouts. Operated chick of 105 hours incubation. Enl. 47 X. 



Fig. 16 Camera lucida drawing of the outer edge of the area opaca of a nor- 

 mal chick in which the sinus terminalis has broken up into capillaries. Drawn 

 for comparison with figure 11. Incubated 107 hours. Enl. 44 X. 



