282 Herbert M. Evans. 



main blood-vessels present in successive stages. They tell us little 

 of the beginnings or of the method of development of any of these 

 vessels. 



Significant advances in anatomy have owed much to improve- 

 ments in the method of investigation. Thus von Lenhossek has 

 declared it probable that a complete nerve cell, with all its proc- 

 esses had never been seen before the introduction of Golgi's method 

 of staining. 



In almost all instances, the study of the development of the 

 vascular system has been done merely by the usual methods of histolog- 

 ical investigation, and, as a consequence, has suffered keenly the 

 limitations this method entails. The shrinkage which dehydra- 

 tion always produces in tissues prepared for serial sectioning is 

 unavoidable. In perfect technique, this shrinkage is never great and 

 is so uniform that great distortion cannot result; but it is present, 

 nevertheless, and on no system or tissues of the embryo does it 

 impress itself, even though slightly, as it does on the embryonic 

 vessels. These delicate endothelial tubes easily collapse and the 

 finest of them almost invariably do so. Thus the observer is for- 

 tunate to trace clearly the chief vessels and their tributaries. If 

 new vessels, in the embryo as in the adult, arise by simply capillary 

 sprouts and plexuses, these must imperfectly be seen, and so in the 

 development of most of the vessels we have doubtless missed these 

 earliest and important stages. 



But more serious errors than this have arisen, for the employ- 

 ment of tissue prepared in the usual way has often led the observer 

 to consider the larger uncollapsed vessels as constituting the entire 

 system present, and thus led him to the conception of the outgrowth 

 of vascular trunks as such. This, however, is never the case. 



To recognize the origin and the method of growth of the early 

 blood-vessels, we must have a complete picture of the vascular tree, 

 of all its capillaries and even of their endothelial sprouts. Can 

 any method accomplish this much? 



Fortunately the experience of this laboratory during the last ten 

 years has shovm the existence of such a method. I refer to the 

 method of injection. 



