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SCIENCE 



[N. S. Vol. XLIV. No. 1127 



using this method, I find, just as did His, 

 that blood-vessels begin by a differentia- 

 tion of cells. It is difficult to be sure of the 

 first cells in the living chick which become 

 angioblasts, but by the time the first cleft 

 appears which indicates the position of the 

 two upper myotomes there is an extensive 

 plexus of bands of cells in the area vascu- 

 losa. In watching these bands of cells in 

 the living specimen, I thought for some 

 time that they could be differentiated by a 

 slightly greater refractility than the rest 

 of the tissue; but this did not prove to be 

 an adequate criterion, for when the syn- 

 cytium of mesenchyme forms in the later 

 stages it makes a network of the tissue 

 which is just as refractile. Moreover, in 

 the study of the early vessels in the living 

 blastoderm it is extremely difficult to tell 

 which is the vessel and which the inter- 

 space. However, I found that the bands of 

 endothelium or the definite vessels which 

 form from them would suddenly change 

 their appearance over wide areas, becom- 

 ing intensely refractile and very granular 

 and opaque. In this stage, which is so 

 striking that it can be seen under low pow- 

 ers of the microscope, the vessels lose all ap- 

 pearance of being hollow ; and I soon found 

 that this was because every cell was passing 

 into the phase of cell-division. This was 

 proved by the rows of spindles in stained 

 specimens. 



The extent of cell-division in these chick 

 embryos is most interesting. At times wide 

 areas of the endoderm cell divide and be- 

 come so opaque as to entirely obscure the 

 cells beneath, and one has to wait until the 

 endoderm becomes clear again. The dif- 

 ference in the reaction of the bands of 

 endothelium and the syncytium of mesen- 

 chyme to cell division is a guide in the 

 study of the early differentiation of blood- 

 vessels. When the bands of endothelial 

 cells divide the cells remain together: the 



outline of each cell becomes distinct, but 

 they do not separate. In the case of the 

 division of the cells of a syncytium of 

 mesenchyme, however, many of the proc- 

 esses are withdrawn and the cell-body 

 rounds up, so that it stands out as if it 

 were an isolated cell, as has been described 

 by Margaret Reed Lewis in tissue-cultures. 

 Thus in areas in which it becomes very diffi- 

 cult to trace the ultimate strands of endo- 

 thelium it may be necessary to wait for the 

 phase of cell-division in one or the other 

 tissue in order to make the distinction. In 

 watching the vessels of the area vasculosa, 

 one gets the suggestion that there may be 

 a rhythm in cell-division. For example, if 

 the area pellucida around the posterior end 

 of the embryo be considered as divided into 

 an inner and an outer zone, either all the 

 vessels of the inner zone or all those of the 

 outer zone may be found in cell division at 

 the same time. 



The vessels of the original plexus increase 

 in size by cell division and new vessels are 

 constantly formed within the plexus by 

 numerous sprouts that grow out to connect 

 its meshes. Beside this growth within the 

 plexus there is an active differentiation of 

 new endothelial cells, which can be watched 

 in the living chick. In the early stages, up 

 to five or six somites, there is no syncytium 

 of mesenchyme and the wandering cells 

 are scanty in number. Individual spindle- 

 cells are thus clearly seen. They divide 

 and at once show the essential character- 

 istic of endothelium — that is, the tendency 

 to form bands. Either an individual cell, 

 or bands of two or three cells, send out 

 tiny processes toward the older bands of 

 endothelium, which at once respond by 

 sending out tiny processes to meet the new 

 ones. Thus endothelium consists of cells 

 which differentiate as spindle-cells from 

 the mesenchyme, and show at once two 

 characteristics, first a tendency to remain 



