14 VASCULOGENESIS IN THE CAT 



be attended with sensations, even emotions, on the part of the 

 observer. Yet it cannot thereby be proved that the vessels end 

 blindly; and we ought not to assume, because of the delicacy of 

 embryonic structures that their physical properties are different 

 in kind, or obey other general laws of stress and strain than larger 

 tubes which are more accessible to examination. 



But while the results of injection demonstrate the permeabil- 

 ity of the vessel wall, as do in life the passage of cells and fluids, 

 they do not afford us the opportunity of ascertaining the structure 

 of the wall or the nature of its orifices. They simply give us the 

 concept of permeability, which it is not permissible, without fur- 

 ther grounds, to translate into porosity. Every fact of extrava- 

 sation can be admitted and the integrity of a completely closed 

 vessel wall still be maintained if we introduce the concept of a 

 structureless (invisible) membrane, the resistance of which to 

 pressure is practically nil. 



Helly 16 has shown this with admirable clearness in his interest- 

 ing work upon the vessels of the spleen. He confirmed previous 

 observations of the passage of injections through the vessel walls, 

 whether solutions, suspensions, colloidal masses, or foreign blood 

 corpuscles were used. He further washed out the red blood cor- 

 puscles from the pulp spaces by saline irrigation, and caused their 

 numbers in the pulp to increase by retrograde injection (obstruc- 

 tion of the splenic vein) and finally in sections of 5 /z observed 

 both erythrocytes and leucocytes in passage through the walls of 

 the sinuses. Yet he considered the wall complete. 



Was zunachst den Widerstand anlangt, welchen die Gefasswand dem 

 Durchtritte fester und fliissiger Bestandteile entgegenzusetzen vermag, 

 so ist ersichtlich, dass derselbe bei den venosen Capillaren nur sehr gering, 

 an gewissen Stellen der Wand tiberhaupt fast gleich null ist; gleicht sie 

 doch, von der Flache betrachtet, sehr einem Gitter, dessen Llicken viel- 

 fach gross genug sind, um ein rotes Blutkorperchen ohne jede merk- 

 liche Formveranderung durchtreten zulassen. Dem zwischen beiden 

 Bestandteilen des Gitters, den inneren, parallel zur Langsachse des 

 Gefasses angeordneten, stabformigen Endothelzellen und den ausseren, 

 quer um dasselbe verlaufenden Kreisfasern, befindliche unmessbar 

 dtinnen strukturlose Hautchen kann wohl kein irgend nennenswerter 

 Einfluss im Sinne einer Behinderung der Diapedese zugeschrieben wer- 

 deri und dies umso weniger, als das gedachte Hautchen sehr hinfallig ist 



16 1903, Arch. f. mikr. Anat., Bd. 61, p. 245. 



