156 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



lymphatic vessels, genetically a derivative from the former, but 

 in no sense responsible for the development of the latter. It is 

 perhaps possible to illustrate my meaning by a comparison drawn 

 from an entirely foreign field. The pronephric, and the subse- 

 quent Wolffian duct, in approaching the cloaca, is met by a re- 

 sponsive outgrowth of the latter and eventually gains its perma- 

 nent opening into the genito-urinary cavity by union with the 

 same. Nobody would hold this cloacal horn responsible for the 

 Wolffian duct development by a process of 'outgrowth' of 'bud- 

 ding.' The embryonal veins respond in exactly the same way to 

 the approach of the independently developed systemic lymphat- 

 ics. This response in the typical and prevalent mammal takes 

 the form of a rudimentary and foreshortened lymphatico-venous 

 heart, and continues to function as such throughout the life of the 

 individual, as the jugular lymph sac. There is here certainly a 

 distinct genetic principle involved. We, to a certain extent, dis- 

 regard the cloacal participation in the final establishment of the 

 Wolffian duct connection with the genito-urinar}- sinus, because 

 the ontogeny of the duct in itself presents such marked and strik- 

 ing stages. In exactly the same general way I believe that we are 

 prone to misinterpret the vertebrate Ij^mphatico-venous junctions, 

 unless we recognize lymphatico-venous hearts and their remnants 

 in their true morphological significance, as links uniting struc- 

 tures genetically distinct and of different origin. In the matter 

 here under discussion this does not mean a double genesis for the lym- 

 phatic vessels, part derived from the veins, part by independent 

 mesenchymal confluence of intercellular spaces. This would be 

 no more in accordance with the actual facts than a genetic de- 

 scription ascribing a portion of the Wolffian duct to cloacal 'out- 

 growth,' while another portion is credited to independent develop- 

 ment. I would refrain from laying stress on these facts were 

 they based on guess-work, but the entire genetic histoiy of lym- 

 phatic and venous organization in all amniote types heretofore 

 examined is so strikingly consistent and so uniforml}- constant 

 that my personal conviction of the truth of the above statements 

 is very firmly rooted. 



