THE GONADS OF THE FOWL 91 



The large endothelial cells thus produced may become more 

 and more isolated from the endothelium and finally migrate 

 toward the surrounding mesenchyme or fall into the lumen of the 

 vessel. It seems likely that their migration into the former may 

 be easier in the case of the veins and small capillaries than in the 

 arteries. The presence of concentrically disposed mesenchyme 

 cells around the latter prevents, or at least hampers, the migra- 

 tion of the young cells toward the mesenchyme. It is true that 

 this mechanical obstacle may be surmounted to some extent on 

 account of the plasticity and amoeboid movements of the cells 

 produced by the endothelium, through which these elements may 

 work their way among the fibroblasts and become eventually free. 



While there can be little doubt about the formation of large 

 cells in the endothelial lining of the blood-vessels, the ultimate 

 fate of the elements thus produced could not be established in 

 the slides. The occurrence of granulopoietic agglomerations in 

 the vicinity of the vessels might at first lead one to suppose that 

 the endothelium takes an active part in the granulopoietic pro- 

 cess. But, at least in the case of the gonads, the formation of 

 granulopoietic foci around the vessels is rather due to the pres- 

 ence of undifferentiated mesenchyme cells in the perivascular tis- 

 sue than to the existence of an active proliferation in the walls of 

 the vessels. Cases in which large endothelial cells are being pro- 

 duced are so restricted in late stages of development that it is 

 impossible to find transitional stages in this process. The close 

 proximity of the hemocytoblasts to the endothelium does not 

 afford conclusive evidence of their formation in the latter, since 

 such cells could have arisen in the mesenchyme enveloping the 

 blood-vessel. Therefore, it is largely a matter of interpretation 

 whether or not the endothelium contributes to the formation of 

 the clusters of granular leucocytes The occurrence of special 

 phagocytic cells resembling hemocytoblasts in the clusters sug- 

 gests that the former might have originated in the endothelium. 

 This, however, cannot be accepted without further study. 



The formation of granulocytoblasts takes place by the appear- 

 ance of scattered minute granules around the sphere of the hemo- 

 cytoblasts, the granules growing meanwhile new granules arise 

 in the cytoplasm. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 28, NO. 1 



