60 FRANKLIN P. REAGAN 



moreover may last only a few hours, if the mechanical conditions are 

 not favorable to their continued development into vessels. It is not 

 to be expected, therefore, that anything short of a very complete 

 series of such operated embryos, fixed at progressively longer intervals 

 of incubation after operation, can settle whether or not there is any 

 extension from the opposite side. 



Concerning the communication of vessels of the two sides, I 

 may say that such connections may sometimes be found in the 

 head-region ventral to the pharynx in completely isolated head- 

 meroplasts. Such cases (fig. 22) probably have Httle significance 

 for the experiments of Hahn, and Miller and McWliorter. 



Whatever may have been the vahdity of the objections urged 

 against these experimental results, the above review gives the 

 exact status of the endothelium-controversy at the time at which 

 the present work was begun. In general my results have been 

 made known through a previous publication (52). These former 

 results, together with many more recent ones are incorporated 

 in the present communication. 



IV. MATERIAL, METHODS AND OBSERVATIONS 



1 . Chick material and mechanical methods 



We have considered the previous experimental results on chick 

 material, and have enumerated the objections urged against 

 their finality. Doubtless the most serious objection to the 

 experimental work on chick material which had preceded my 

 own, was that of the possibility of ingrowth from the opposite 

 side. It had been suggested that nothing short of a study of 

 a large number of closely graded stages could furnish a basis 

 for conclusive evidence. It is evident, however, that such a 

 study might still remain open to the objection of those who 

 require still more thorough-going investigations. Such a situ- 

 ation must necessarily confront us unless there be some means 

 of altering the technique of operation, or increasing its severity 

 to an extent which will ehminate the necessity (though by no 

 means, the desirabiUty) of such an extensive study. It occurred 

 to the writer that a feasible method of ehminating the possi- 

 bility both of ingrowth and of further extension from the oppo- 



