ORIGIN OF VASCULAR TISSUES 61 



site side, would be that of isolating both sides from the outlying 

 blastoderm — or better still, to isolate completely the embryonic 

 body or portions of it from all the outlying blastoderm at a 

 time before the embryonic tissue is vascularized. It is evident 

 that if endothelium should then appear in the isolated tissue 

 following further incubation, such endothelium could not possi- 

 bly represent an extension or ingrowth of a yolk-sac vessel. 

 Under these conditions the question of growth from the oppo- 

 site side would not concern us, so far as the proof of local origin 

 is involved. 



As before noted, it was objected that the operations of Miller 

 and McWhorter may not have been done early enough, and 

 that their incisions may not have been made sufficiently close 

 to the embryonic tissue. In seeking a means of overcoming 

 these objections I happened on the following procedure: at the 

 time of operation it is possible to remove the yolk-sac blasto- 

 derm lateral to the hne of separation on one or both sides. In 

 case only a head-fragment is to be further incubated, it is possi- 

 ble to remove with the blastoderm that part of the body-axis 

 which normally should first contain blood-vessels. This tissue 

 is preserved at once and sectioned. In this manner one can 

 determine exactly the status of the extraembryonic vascular 

 tissues. By this means it is possible, with safety, to let the 

 embryos develop to a much more advanced stage before oper- 

 ation than would be permissible by the methods of any previous 

 observers. From this it is not to be inferred that in all cases 

 it is desirable to wait until the latest possible time of exclusion 

 of ingrowth before performing the operations. If the general 

 method be trustworthy, one should be able to get positive 

 results long before vascular tissue has developed dangerously 

 near the embryo's body. The great advantage of operating at 

 as late a stage as possible is, that the abnormaUty produced is 

 likely to be less, and growth and differentiation proceed farther 

 the older the embryo is at the time of operation. If allowed to 

 summarize what I believe to be my contributions to the experi- 

 mental technique of studying endothelium, I would say that so 

 far as mechanical methods are concer'ned they are these: com- 



