64 



ELIOT R. CLARK 



vessels to which the former capillary was connected. These re- 

 mains of the capillary shorten by the retraction of the endothelium 

 into the vessels with which they are connected, until they form 

 onl}^ a slight swelling on the surface. This eventually disappears, 

 and there is nothing left to mark the site of the former capillary. 

 In figure 11 may be seen the movement back into the connecting 

 vessel of a nucleus and a small pigment granule, from the capil- 

 lary which is undergoing retrogression. 



In this process of retrogression of blood-vessels as it is seen in 

 the living animal, there is nothing that even remotely suggests the 



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11 



Fig. 10 Several stages in the narrowing and retracting of a vessel in the 

 tad-pole's tail (rana palustris). NO C, no circulation. 



transformation of an endothelial cell into a mesenchjnme cell, or 

 of any other type of cell. The impression is gained that the 

 entire protoplasm is withdrawn into the parts of the system 

 which persist, and that none of it is lost. Certainly some of it 

 is withdrawn, as for example, in the case of the nucleus men- 

 tioned above. If any part fails to be withdrawn, it must be 

 that it is dissolved, for the last that can be seen is a thread so 

 minute that it is barely visible with high power lenses. 



In the tail of the frog larva parts of blood-vessels rarely become 

 completely isolated from the rest of the system, as has been de- 



