178 W. B. CHAPMAN 



The question now arises, whether this further development of 

 the vascular system, which takes place after the circulation is 

 established is dependent upon the mechanical factors concerned 

 with the circulation or not. According to Thoma, it is, while 

 the observations of Roux and of Patterson indicate that the 

 sinus terminalis will develop in the absence of a heart-beat, 

 and that possibly the same may be true of other yolk-sac 

 vessels. The observations of Loeb, Knower, and Stockard 

 indicate the development of some main vessels when no circu- 

 lation is present, but do not give a complete picture. It there- 

 fore still remains to be determined, precisely, how much vascu- 

 lar development takes place independently of the circulation. 



III. METHOD OF INVESTIGATION 



The chick was selected for this study on account of its rapid 

 development, because it is possible to secure material for experi- 

 mentation during most of the year, and because of the ease 

 of injecting and studying the extra-embryonic vessels, which lie 

 in a single plane, and which early develop a very characteristic 

 pattern. After some experimentation, I decided upon the thirty- 

 third hour of incubation as the proper time for operation, and 

 afterwards, endeavored to arrange my operations so that they 

 would come as nearly the hour mentioned as possible. At this 

 time, the chick has about twelve somites. Miss Sabin has ob- 

 served that the heart begins beating at the ten somite stage, but 

 that the circulation does not start until after fifteen or sixteen 

 somites have formed, which would mean about the thirty-eighth 

 hour of incubation.' Therefore, any time between the thirtieth 

 and thirty-eighth hour of incubation would have sufficed for 

 accurate results in this experiment. 



The technic employed in this work was suggested by Professor 

 Clark and modified to suit the experiment. It is as follows: 



The egg is taken from the incubator and placed in a warm- 

 box. The shell is sterilized at the point to be opened by wiping 

 with a small cloth saturated with alcohol. As soon as the alcohol 

 has evaporated, a small hole is scraped through the shell with the 

 point of a scalpel using care not to puncture the shell membrane. 



