412 V. E. EMMEL 



IV. DISCUSSION CONCERNING ENDOTHELIAL TISSUE AS 



CONTRIBUTING TO THE CELLULAR ELEMENTS 



OF THE BLOOD 



On the basis of the present results it appears evident that the 

 cell clusters in the embryonic aorta furnish an instance of the 

 vascular endothelium contributing cellular elements to the 

 circulating blood. Since this is not entire agreement with 

 one of the postulates of the angioblast theory, namely that all 

 the blood cells of the organism are direct descendants from 

 the early embryonic blood islands, it becomes of interest to 

 note the conditions under which this endothelial activity is 

 taking place. The close association of the aortic clusters with 

 degenerating vessels directs attention to the occurrence in these 

 retrogressive vessels of stimulative factors to which the en- 

 dothelium reacts in the manner under consideration. As stated 

 b}" Thoma ('93) and elaborated by Mall ('06) in the case of the 

 embryonic liver, a vessel in which there is a reduction of the 

 circulation below normal tends to shrink and disappear. The 

 occurrence of such a retardation of circulation in the atrophy 

 of the aortic rami is indicated by the marked absence of red 

 blood corpuscles (p. 410). With the circulation practically at a 

 standstill it is not improbable that diminished oxidation and 

 inhibition of gaseous interchange are in part at least produc- 

 tive of mildly abnormal chemical and toxic conditions conducive 

 to the phagocytic activities, endothelial proliferation and con- 

 sequent formation of intra-arterial cell masses and aortic clusters. 



In support of this conclusion attention may be called to the 

 emphasis being more recently attached to such abnormal intra- 

 vascular conditions as stimulative to endothelial activity. 

 Thus Mallory ('00 and '14, pp. 165-166, 183) advances grounds 

 for the conclusion that certain dilute and weak toxines stimulate 

 endothelial proliferation and phagocytes and maintains that 

 in this manner arise the macrophags encountered in many dis- 

 eases. Batchelor ('14) and Scott ('14) record marked prolifera- 

 tive endothelial changes and phagocytic activity in hepatic 

 vessels occluded l)y artificially produced emboli and wounds. 



