410 V. E. EMMEL 



Fourth, there are certain important cytological conditions 

 to be considered in the degenerating arteries themselves. Many 

 of these vessels are found compactly filled with basophilic stain- 

 ing cells (cf. figures 1 da, 7 and 11). Such conditions are found 

 near the aortic origin of the artery and may extend for short 

 distances into the ramus, occasionally continuing to a point where 

 the degenerating vessel is lost in the mesenchyma. Eryth- 

 rocytes are strikingly deficient in such regions and may in- 

 deed be entirely lacking throughout the vessel, a condition evi- 

 dently indicative of the reduction if not complete cessation of 

 the circulation through these retrograding arteries. The com- 

 ponent cells of these intra-arterial cell masses piay take a some- 

 what lighter stain but they otherwise appear cytologically iden- 

 tical with those of the aortic clusters. They are phagocyti- 

 cally active (fig. 11, in) and undergo cell multipUcation (fig. 

 7, d). In regions of the artery not thus occluded the endothelial 

 cells are frequently rounded up or swollen and project into the 

 lumen of the vessel. The transitional stages to be found be- 

 tween the still intact endothelial cells and the intra-arterial 

 masses seem to leave no doubt but that the latter have arisen 

 in situ from the lining endothelium of the retrograding vessel. 

 Finally, there remains the crucial fact of an intimate relation- 

 ship between these intra-arterial masses and the aortic clusters. 

 This is illustrated in figure 6 in which the intra-arterial cell 

 mass (iam) when followed toward the aorta is found to terminate 

 in an aortic cluster situated within the lumen of the aorta. It 

 may be observed that the component elements merge into each 

 other with no evident line of demarcation between them. In- 

 deed the cytological conditions and morphological relations are 

 such as to justify regarding the phenomenon as essentially 

 comparable to a partial evisceration of the contents of the de- 

 generating artery into the lumen of the aorta. Such a relation 

 of aortic clusters and nitra-arterial masses is of frequent occur- 

 rence. It is also iiuceworthy that in many cases where such 

 a relationship is apparently lacking the aortic cluster is, how- 

 ever, situated in a well marked depression or concavity in the 

 aortic wall, and that some of these depressions are in relation 



