82 Florence R. Babin. 



subject has been necessarily confusing to those who have not followed 

 the subject carefully. However, now that the origin and method of 

 growth of the lymphatic system has been cleared up, so that the funda- 

 mental morphology is understood, the controversy has had this great 

 value, that it has brought up for analysis and discussion every con- 

 ceivable method of growth. Lymphatics do not arise as dilated per- 

 ipheral tissue spaces after the manner of the ccelom as the earlier 

 embryologists thought ; they do not grow by the addition of hollow con- 

 nective tissue cells, as Schwann and Virchow thought; they do not rise 

 as perineural spaces, nor by fenestration of a vein, nor by extra-intimal 

 clefts, nor by the progressive addition of connective tissue spaces, nor 

 by the addition of detached blood vessels, but they bud from the veins 

 and grow by the sprouting of their endothelial wall. 



IX. CONCLUSIONS. 



The most important result of this study on the morphology of the 

 lymphatic system is the emphasis it throws on the importance of 

 endothelium as a tissue. The angioblast is one of the early tissues 

 to be differentiated; it is not an inert lining for vessels, but an actively 

 growing functioning tissue. In its place of growth it is a syncytium 

 of actively amoeboid protoplasm. Mollier (99) has shown that in the 

 spleen it may be reticular; Mall (86 and 87) has shown that it may 

 give rise to reticulum. Undoubtedly the further development of our 

 knowledge of endothelium depends on the development of the new 

 experimental anatomy. 



The lymphatic endothelium buds off from the veins. It is always 

 a little different in appearance from the endothelium of the veins, and 

 the lymphatic capillary is different in size and form from the blood 

 capillaries. The growing lymphatic tip has the remarkable character- 

 istic that it avoids the blood capillaries, while it is attracted by other 

 lymphatic capillaries. 



Endothelium is the essential tissue of the lymphatic system. In the 

 lower vertebrates lymph hearts are formed by the addition of striated 

 muscle to primary lymph sacs. In the higher forms lymph glands 

 are formed by the development of lymphocytes around the ducts. 

 This takes place not only in the wall of the primary lymph sacs, but 

 along plexuses of ducts, so that there are primary and secondary 

 lymph glands. 



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