DEVELOPMENT OF VESSELS WITHOUT HEART 189 



The vessels of the area opaca and the area pellucida 



Soon after the operation a difference in the arrangement of the 

 capillaries of the area pellucida and of the area opaca begins to 

 appear, which becomes quite marked about the sixtieth hour of 

 inculcation (twenty-seven hours after the operation). In the 

 former the spaces between the vessels are seen to be growing 

 larger and the capillaries less numerous than in the latter (fig. 

 8). A trace of this is noticeable in figures 3 and 4. In the older 

 specimens this condition is more pronounced. Figure 9 shows an 

 embryo of four and one-half days in which the difference in the 

 appearance of the capillaries is becoming decidedly marked and 

 the two areas are separated by a rather definite border around the 

 peripherv of the area pellucida. In still older embryos the cap- 

 illaries within the area i:)ellucida become smaller in diameter 

 and more scattered, and in embryos of seven or eight days (fig. 

 10) are broken up and appear as little streaks and puddles of 

 blood, which are, in reality, isolated endothelial lined vesicles 

 and spaces. A somewhat similar, though less intense, change 

 occurs in the area opaca. Here the process is slower, and con- 

 sists chiefly of the narrowing of many of the capillaries, some of 

 which become solid and separate in the middle. Figure 11 

 shows a small portion of the area opaca in an eight day chick 

 (seven days after operation), in which there are a great number 

 of solid cords, some of which have broken in twain and begun to 

 retract. The beginning of the process of narrowing and retrac- 

 tion of the capillaries in the area opaca is apparent by the fifth 

 or sixth day, and is well demonstrated by the increasing diffi- 

 culty of injection, which has been mentioned above. 



The sinus terminalis 



The sinus terminalis, or vena terminalis, as it is termed by 

 Popoff, will now be considered. This relatively large embryonic 

 vessel is present before the heart is formed (Lillie, '08, pp. 87- 

 88). Popoff has described it as beginning to 'degenerate' (break- 

 up) in chicks of forty somites, which would be somewhere around 

 the eighty-fifth or nintieth hour of incubation, the breaking-up 



