DEVELOPMENT OF THE THORACIC DUCT 147 



to further compare in this respect the reconstruction of series 

 214 (14 mm. fig. 190) with that of series 143 (15.5 mm. fig. 192). 

 In the former embryo the secondary para-azygos channel of the 

 right side (34), developed by confluence of components of the 

 right ventro-medial azygos plexus, only persists in its caudal 

 portion, and has in the rest of its original extent been detached and 

 surrounded by the extraintimal lymphatic channel. On the other 

 hand, in the 15.5 mm. series 143 (fig. 192) this normal process of 

 detachment and replacement by extraintimal lymphatic anlages 

 has been delayed. Fig. 192 shows the para-azygos ventro-medial 

 channel (84} as an extensive tributary trunk of the right azygos 

 vein, cephalad of the level C. Only a few areas of the extrain- 

 timal lymphatic development occur, as yet, along its course. Fur- 

 ther development of these and their confluence will produce the 

 condition already seen in the earlier embryo, series 214, which is 

 typical for the average embryo of the 14 mm. stage. Thus 

 unusual and atypical delay of lymphatic development occurs in 

 some embryos of this period in the region above specified, and pro- 

 duces a definite and uniform aberrant picture. The correct 

 interpretation of the same is given by the conditions obtaining in 

 the average embryo of this period. I lay some stress on the facts 

 just described, because isolated observation of an instance of this 

 character might readily lead to the erroneous assumption that the 

 detached venous bag (34) of figs. 263 and 264 became directly 

 transformed into the lymphatic channel of the thoracic duct occupy- 

 ing in the later stages the identical topographical position and 

 maintaining the same relations to surrounding structures. This 

 error can only be avoided by determining, in numerous embryos 

 of this period, the typical and average condition of lymphatic 

 development. 



Thus in the average 16 mm. cat embryo the originally separate 

 and independent extraintimal anlages of the two thoracic ducts 

 have become confluent, and form from this stage onward usually 

 a continuous and uninterrupted lymphatic channel, which is con- 

 nected with the general venous system through the jugular lymph 

 sac, by union with the thoracic duct approach of the latter. 



From this stage forward the subsequent growth and readjust- 



