64 CHARLES R. STOCKARD 



this individual the heart is a solid string and the blood had never 

 circulated. The liver presents a dense appearance, no blood 

 vessels are seen and blood corpuscles are entirely absent. The 

 general differentiation and condition of the tissues are, how- 

 ever, fairly normal and not at all degenerate. The intestinal 

 epithelium is typical in structure. Above the intestine the well 

 differentiated dorsal aorta is shown with connective tissue fibers 

 abundantly present in its wall and a definite endothelial lining. 

 The lumen of this aorta, however, has never contained any 

 type of blood cells and the only solid particles within it are a 

 slight coagulum near the center of the vessel. Above the dor- 

 sal aorta are the two Wolffian ducts and between them under 

 the notochord are a few mesenchymal cells which represent 

 more posteriorly the remains of the intermediate cell mass. 

 Almost all of the erythrocytes in this mass have completely de- 

 generated or have been destroyed by mesenchymal cells. 



The embryos without a circulation thus furnish a definite 

 means of establishing the actual haematopoetic value of any 

 organ. They demonstrate that unless the blood current reaches 

 the organ and thereby introduces embryonic blood cells into it 

 the organ iself is incapable of giving rise to blood cells. 



This experiment also demonstrates with equal force the inabil- 

 ity of vascular endothelium to form blood cells in the fish. I can 

 see no reason if vascular endothelium possesses a blood forming 

 power why the aorta and other interior vessels of these embryos 

 are invariably empty and never contain any type of blood cell. 

 It cannot reasonably be claimed that this inability is due to the 

 abnormal condition of the embryo having taken away the power 

 of the endothelium to form blood cells, since it is so absolutely 

 demonstrated that real blood forming material in other portions 

 of the embryo possesses its perfectly normal capacity to produce 

 blood and does produce it in a very abundant fashion. 



These embryos furnish no evidence to indicate that there is 

 any connection or association between the mesenchymal cells 

 which are to form the connective tissue and those destined to 

 form blood cells. There is no instance of a tendency for con- 

 nective tissue cells to change into blood cells or of blood cells 

 to give rise to any type of connective tissue cells. 



