154 ADAM M. MILLER 



These finding in reptiles, and my own results in the bird, 

 warrant the conclusion that the sauropsids agree absolutely 

 genetically with the mammals in the development of the main 

 axial lymphatic lines bj^ confluence of independently formed 

 intercellular mesenchymal spaces, but that the latter are charac- 

 terized by close topographical association of these spaces with 

 temporary embryonic veins which they in large part replace, 

 while the former present no such association. In them the prog- 

 ress of thoracic duct development is not, as is the case in the mam- 

 mal, complicated by the presence of an extensive azygos venous 

 system, and axial lymphatic development, especially in the reptile, 

 occurs chiefly by confluence of mesenchymal spaces surrounding 

 the main arterial trunks. 



Chick embryo of eight days {Columbia Collection, series no. 513). 

 Reconstruction, ventral view. Figure 26. The next important 

 change in the lymphatics constituting the thoracic duct comprises 

 the further development of certain isolated spaces situated along 

 the ventro-lateral aspect of the dorsal aortic roots in the interval 

 between the cephalic end of the large ventral plexus and the 

 thoracic duct 'approach' of the jugular lymph sac. The incipient 

 stage in the formation of these spaces was illustrated in flgure 24, 

 17a. In the embryo now under consideration they have enlarged 

 and coalesced to form a continuous channel (figure 26, 17a). 

 This in turn has united with the ventral unpaired portion of the 

 thoracic duct {17) and with the thoracic duct 'approach' of the 

 lymph sac {15a). There is thus established a direct and free 

 communication between the ventral lymph plexus {17), which 

 had arisen as an independent and isolated structure, and the 

 jugular lymph sac {15). Therefore, as clearly shown in figure 26, 

 the thoracic duct 'approach' {15a), which was previously described 

 as an integral part of the jugular Ijanph sac (see fig. 23, 15a), 

 serves as the portal of entry of the thoracic duct into the lymph 

 sac (fig. 26, 15). 



The writer has in a previous article (1) shown and will here 

 again point out in a later stage that in the chick, as in reptiles 

 and mammals, a communication is established between the jugular 

 lymph sac and the great veins in this region through one or more 



