418 RANDOLPH WEST 



in the various nuclei. That the typical nucleus of the fully 

 differentiated endothelial cell may be distinguished from that 

 of the indifferent mesenchyme cell we do not deny, but that in- 

 termediate stages between the two exist, in the case in question 

 we likewise hold to be true. And unless it be cut parallel to its 

 long axis, it is practically impossible to distinguish even the 

 fully differentiated endothelial nucleus from the mesenchymal 

 nucleus. 



As for the statement that these flattened rows of cells are al- 

 ways connected with a preexisting endothelium it must be re- 

 membered that practically every cell in the embryo is, at this 

 stage, in syncytial relation with every other cell, the blood cells 

 excepted. So in a certain sense a -protoplasmic connection be- 

 tween flattened cells and preexisting endothelium may be demon- 

 strated by passing over the protoplasm of indifferent mesenchyme 

 cells. To assume that because all endothelium in the embryo 

 is in syncytial relationship it is therefore derived from some 

 preexisting endothelium, appears unwarranted. Can it not be 

 said with equal truth that since the embryonic vascular endothe- 

 lium is in syncytial relationship with the mesenchyme it is 

 therefore derived from the mesenchyme? This being the case, 

 we know that there are in the mesenchyme certain flattened 

 cells which are not connected with any preexisting endothelium 

 otherwise than by means of the protoplasm of the mesenchymal 

 syncytium. The isolation of these flattened cells from any 

 other endothelium and the fact that all possible gradations 



Fig. 4 Chick 8.5 nun., iSeries 21, felide 1, How 3, Section 2. X oUU. 1 lioto- 

 micrograph of transverse section of the caudal end of the embryo. 



Fig. 5 Chick 11 mm., Series 20, Slide 1, Row 4, Section 4. X 600. Photo- 

 micrograph of transverse section ot the caudal end of the embryo. 



Figure 4. Figure 5. 



S, Coccygeal vein S, Coccygeal vein 



5, Haemal capillaries 5, Haemal capillaries 



6, Caudal muscle plate 6, Caudal muscle plate 



7, Differentiating blood cells 7, Differentiating blood cells 



S, Mesenchymal spaces 

 U). Lynii)hatic connected with \ein 



