80 FRANKLIN P. REAGAN 



aortae are sometimes abnormally small, and sometimes abnor- 

 mally large. That they are lined with true endothelium can 

 hardly be doubted. Their position is sufficiently constant to 

 assure us of their significance. 



g. Endothelium independent of mesothelium. The work of 

 Bremer (4) has revived our interest in the coelom theory of 

 the origin of endothelium. Bremer found that coelomic meso- 

 thelium furnishes a source of endothelial tissue in the body-stalk 

 of the early human embryo; he found that the body-stalk 

 vascularized before yolk-sac vessels appear. Here was clearly 

 a case in which the blood-vessels did not represent an ingrowth 

 from the yolk. Bremer made the suggestion that all (udothe- 

 lium might arise from mesothelium. He accounted for the 

 presence of sauropsidan vitelline vessels prior to coelom forma- 

 tion on the yolk by postulating their origin from a "premeso- 

 thelial stage of mesoderm." I have previously pointed out (52) 

 that Riickert has shown in case of isolated blood-islands, that 

 cell groups proliferated from this early vascular tissue cleave to 

 form slit-like cavities which later unite with other similarly 

 formed cavities to contribute to the formation of the coelom. 



In my previous work (52) I attempted to produce self-vascu- 

 larizing meroplasts from regions devoid of coelomic tissue, but 

 was unable to do so. Since that time it has been found possible 

 to operate sufficiently close to the median line in the posterior 

 region to exclude coelomic mesothelium as such, and yet obtain 

 endothelial cavities of local origin undoubtedly representing 

 aortic endothelium. Such a condition is shown in figure 31. 

 In the experiment which yielded this condition, the incision 

 E-F (fig. 1) was made in the posterior region of the primitive 

 streak. The longitudinal incision was just lateral to the meso- 

 blastic somite where the mesoderm was uncleft. Certainly the 

 aorta here could not have come from an epithelial sort of meso- 

 derm; mesothelial origin seems to be excluded. It might be 

 argued that the somite contains cells that are potentially meso- 

 thelium, in analogy with the conditions in lower vertebrates. 

 This may very well be admitted. In fact it is possible, in an 

 earlier stage at least, that any of the mesenchyme cells were 



