158 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



channel. The genetic succession of mammaUan lymphatic ves- 

 sels, developing along and around atrophying antecedent embry- 

 onal veins and eventually replacing them absolutely in the topo- 

 graphical sense, has been sufficiently elaborated in the preceding 

 pages. There is no question whatever concerning the anatomical 

 fact of such replacement. My quarrel lies with the deductions 

 which the sponsors of the theory just quoted draw from the evi- 

 dence of their slides. 



If in an earlier stage they find a vein or venous radicle occupy- 

 ing a definite and clearly circumscribed topographical position in 

 relation to surrounding structures, and if they encounter in a later 

 stage a lymphatic channel in the identical position of the earlier 

 vein, they reason that this topographical coincidence proves 

 the direct transformation of the preceding vein into the succeed- 

 ing lymphatic. They^ speak of the earlier vein as an 'outgrowth' 

 from the main venous channel and describe the same as being ' de- 

 tached' or 'split off' the latter to form a Veno-lymphatic anlage,' 

 which then, by fusion with multiple equivalent detached venous 

 elements, forms finally a valid and permanent systemic lymphatic 

 channel. It appears to me that in this process of reasoning topog- 

 raphy is forced to substitute for histogenesis, and that the vital 

 intermediate genetic stages, which finally produce the result under 

 discussion, have been disregarded. I believe that this error in 

 deduction is based on perfectly correct but insufficient premises 

 and is due to the lack of sufficient material capable of demonstrat- 

 ing the intermediate histogenetic stages through which the replace- 

 ment of a venous by a lymphatic channel is accomplished. There 

 are many individual embryos of certain stages, which, interpreted 

 by themselves, tend to support strongly the hypothesis of direct 

 conversion of venous into lymphatic anlages. It requires a very 

 large number of embryos of approximately the same age in order 

 to demonstrate the true genetic processes governing the early 

 relations of the venous and lymphatic channels in the mammalian 

 embryo. Hence the futility of reasoning, from a few human em- 

 bryos, concerning primate lymphatico-venous relations, which 

 we know, from the already ascertained conditions in adult platyrrh- 

 ine and catarrhine forms, present one of the most complicated 



