414 OTTO F. KAMPMEIER 



2. They are derived from the embryonic venous system either 

 by a direct transformation of certain of its channels or by the 

 fusion of multiple derivatives which have become detached from it. 



3. They arise by the confluence of mesenchymal spaces, which 

 in the mammalian embryo are frequently perivenous in formation. 



After a thorough and prolonged study of an extensive series of 

 pig embryos, the writer is forced by direct evidence to ally him- 

 self with the third position which holds that the thoracic duct has 

 an independent origin, that it is not a product of the veins either 

 by centrifugal growth or by the direct fusion and transformation 

 of venous derivatives, and that its intima is a gradual differentia- 

 tion from the mesenchymal reticulum. Essentially then he is in 

 harmony with and can confirm Huntington's conclusions. Indeed 

 so positive and convincing are the results and so perfect in their 

 agreement among themselves that the possibility of doubting 

 their accuracy would seem to be entirely excluded. 



In further corroboration of this view is the fact that some of the 

 individual stages of pig embryos show very clearly how the views 

 of the venous origin of the thoracic duct sprang into existence and 

 secured a strong foothold . During the formation of this duct the use 

 of the injection method can produce conditions, which, disregard- 

 ing all other details, would seem to corroborate the theory of its 

 centrifugal growth. Again, in certain stages there are structures 

 and data which would seem to furnish the necessary basis for the 

 other view, that it arises directly from a transformed vein or its 

 detached derivatives. But in both cases a comparative study of 

 a sufficient number of closely graded series will prove that these 

 appearances are due to the examination of an inadequate number 

 of embryos, to the tyranny of one method of investigation, or to 

 a faulty coordination of all the facts available. The appearance 

 of centrifugal growth is plainly given by injection, but this method 

 with all its advantages can only indicate the regions of completed 

 channels, or the direction of their growth, and at best serve as a 

 control by supplying negative or indirect evidence; it can never 

 portray the actual genesis of a channel or reveal the histogenetic 

 processes which are at work from the beginning. Furthermore, 

 the fact that in certain stages there are venous channels which 



