Tin Origin and Development of the Lymphatic System. 



Bremer (15), offers a consistent as well as a constructive theory 

 of the vascular system which goes far to modify the force of the 

 following words of Koelliker quoted by Riickert and Mollier (128, 

 p. L019) : " Ueber die erste Bildung der Gefasse und des Blutes 

 herrschen wohl ebenso viele Ansichten, als Forscher sich ueber 

 diesen Gegenstand ausgesprochen haben, was auf jeden Fall be- 

 weist, dass derselbe zu den schwierigsten gehort." The theory 

 thus outlined is that the fundamental morphology of the vascular 

 system is based on the specificity of the endothelium, or in the 

 words of His (47, p. 325), on the fact " Zu den am friihesten sich 

 sondernden Gewebsanlagen gehort der Gefasskeiin oder Angioblast. 

 Seine Sonderung erfolgt sehr scharf, und sein Wachsthum geht 

 nach durchaus eigenthtimlichen Gesetzen vor sich." It includes 

 the discovery of Mall (83-87) that endothelium may produce reticu- 

 lum, a process which he has shown takes place both in the liver 

 and in the heart; but maintains that there is an early differentia- 

 tion of two tissues, namely, endothelium and mesenchyme, so that 

 the angioblasts once formed give rise to all the vascular endothelium 

 of the body. 



The opposing theories in the varying forms of the origin of blood 

 vessels from tissue spaces, their growth by the addition of tissue 

 -I Hires or by the addition of connective tissue cells, and the differ- 

 entiation of endothelium from mesenchyme over extensive areas 

 within the body wall find their most recent evidence in the works 

 of Riickert (127), Riickert and Mollier (128), Halm (40), Bon- 

 net (14) and Maximow, 1909 (90, p. 511). These views, however, 

 must be traced through the earlier works of Gotte (37) and Reich- 



ert (121-123). The evidence for the continued origin of bl 1 



vessels from the mesenchyme is for the most part from the inter- 

 pretation of appearances in sections. Hahn's work, however, in 

 connection with the difficult point of the origin of the heart and 

 aorta is experimental. He removed the vascular membranes on 

 one side of early chick embryos and obtained a heart anlage and 

 aorta on both sides. He thought that he could entirely exclude a 

 growth from one side to the other as well as remnants of the mem- 

 branes on the same side and concluded that both heart and aorta 

 arise in situ from the mesenchyme and not from the endoderm of 

 the yolk sac not from an ingrowth from the extraembryonal mem- 

 branes. 



