541 



bryos is now further split off from the main venous channel by 

 fenestration into a series of apparently independent vessels which lie 

 parallel to the parent vein but which, except in a few instances, are 

 still connected with it. The veno-lymphatic ridges on the ventral sur- 

 face of the azygos veins have also increased in extent but still consist, 

 as in the 13 mm embryo, of a series of disconnected outgrowths which 

 are separated from the main venous channel by a number of fenestrae 

 (see Figs. 2, 8 and 9). 



Since the 14 mm embryo represents a veno-lymphatic stage, it is 

 evident that the transformation of the veno-lymphatics into the definite 

 thoracic and right lymphatic ducts must occur in embryos measuring 

 between 14 and 16 mm in length and that it must take place quite 

 rapidly. 



In the 15 mm embryo the veno-lymphatic anlages of the thoracic 

 and right lymphatic ducts which arise from the common jugular and 

 innominate veins are now chiefly represented by a series of apparently 

 isolated and independent spindle-shaped spaces which, for the most 

 part, are devoid of blood corpuscles and lie exactly in the line sub- 

 sequently followed by the thoracic and right lymphatic ducts. In the 

 thoracic region, however, except in a few instances, the veno-lymphatics 

 still retain their connections with the azygos veins. 



In the 15.5 mm embryo the separation of the veno-lymphatics 

 from the veins and the establishment of a series of isolated spindle- 

 shaped spaces along the entire course followed by the thoracic and 

 right lymphatic ducts is practically completed, except at the posterior 

 end of the thoracic cavity where veno-lymphatics still appear, for a 

 short distance, to be connected with the azygos veins. 



In view of the circumstance that no observable connection exists 

 between these spindle-shaped spaces in the 15.5 mm embryo and that 

 no difficulty exists in determining that both ducts are continuous 

 channels in the 16 mm embryo, I am bound to infer that these two 

 ducts are not split off from the veins as channels which are continuous 

 from the beginning, but that they are formed through a fusion of 

 multiple, independent anlages. 



There is no doubt that the veno-lymphatics from which the 

 thoracic and right lymphatic ducts are derived first make their ap- 

 pearance along the common jugular and innominate veins and that the 

 isolated spindle-shaped spaces also first appear there. It is, there- 

 fore, fair to assume that the confluence between these spaces probably 

 takes place from before backward and, such being the case, we are 

 in a position to explain the progressive manner in which, as described 



