DEVELOPMENT OF THE MAMMALIAN SPLEEN 291 



cells which are derivatives of the tissue bordering them. In 

 two- to six-day larvae, the lacunae are bounded by cells that 

 begin to assume the flattened and elongated structure of a definite 

 endothelium. 



Mollier is of the opinion that a capillary system is present from 

 the very beginning. His interpretation of its development is 

 embodied in the following quotation: 



Ich bin der Ansicht dass es sich hier um ein allgemeines gestaltendes 

 Prinzip handelt, dass liberall die erste Entwicklung von Gefassen auf 

 diesen Weg aus mesenchymatosen Material (Reticulum) erfolgt. Wie 

 ein grosseres Gefass durch die starkere Betonung und Entwicklung 

 bestimmter Bahnen eines capillaren Netzes ausgestattet wird so meine 

 ich, ist der gleiche Vorgang schon ontogenetisch friiher tatig um durch 

 Betonung und Entwicklung bestimmter Reihen von Maschenraumen 

 im Mesenchym-reticulum die ersten Capillarnetze zu schaffen. 



He does not deny the possibiUty of blood-vessels growing by 

 sprouting, but he interprets the sprouting process as a production 

 of mesenchyme cells which at the same time are converted into 

 'Gefasszellen.' 



Sabin, working with pig embryos, comes to conclusions very 

 similar to those reported by Mollier. By an injection of the 

 splenic artery it was found that in a pig 3 cm. long the entire 

 splenic circulation consists of a capillary network which extends 

 throughout the organ. This condition is maintained until the 

 fetus is 7.5 cm. in length. ''Thus the spleen confirms the general 

 principle that the primitive circulation of any organ is in the form 

 of a capillary network out of which the arteries and veins are 

 formed." The spleen is characterized by persistence of the primi- 

 tive capillary network until the embryo is 10 cm. long when the 

 type of circulation characteristic of the adult spleen is established. 



Sabin's results are quite contrary to the observations of Dan- 

 chakoff on normal spleen development in the chick. The latter 

 worker found that the earliest vascular elements of the spleen are 

 exclusively venous. Slits later appear in the mesenchyme which 

 connect with the early vessels. These slits or sinuses connect 

 together and form a network, thus giving the early spleen a 

 spongy structure. At a much later stage (twelve to thirteen 



