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-urinary 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 lymphatico-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 history of lym- 

 phatic and venous organization in all amniote types heretofore 

 examined is so strikingly consistent and so uniformly constant 

 that my personal conviction of the truth of the above statements 

 is very firmly rooted. 



