102 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



embryos into the more definite channel of the broncho-mediastinal 

 trunk with larger caliber. 



Figs. 154 to 158 show the region of the broncho-mediastinal 

 channel in four successive sections in a 20 mm. cat embryo 

 (series 241, slide xix, sections 19, 20, 21 and 22, x 225). Fig. 

 154 again gives a topographical view of the entire field, while 

 the same is cut down to the ventral portion in the remaining plates. 

 In all the multi-meshed lymphatic plexus of the preceding stage 

 is replaced by the more definite and larger channel of the broncho- 

 mediastinal trunk proper (57), in the classical situation between 

 the pulmonary arteries {10) ventrally and the trachea {9), vagi 

 {22) and permanent ventral mediastinal veins {18) dorsally. 



This stage marks the practical attainment of adult conditions 

 for the lymphatic channel of the broncho-mediastinal trunk, and 

 from this period on its further development is merely a question 

 of continued growth and adaptation to the environment in es- 

 tablishing secondary connections with the tributary channels, 

 developed independently, the preazygos segment of the thoracic 

 duct, and (usually) the ventral prolongation of the subclavian 

 approach of the jugular lymph sac. 



As will be seen from the foregoing analysis of the development 

 of the broncho-mediastinal lymphatic trunk in embryos of the 

 cat, the principles of mammalian systemic lymphatic ontogenesis 

 defined in Part I of this memoir are consistently sustained by 

 the actual evidence afTorded. The development of the perive- 

 nous extraintimal anlages which form the basis for the subse- 

 quent evolution of the systemic lymphatic channels, is a uniform, 

 and remarkably consistent process. 



The existence of these spaces in this and other regions of the 

 embryo has been so often denied by recent contributors to the 

 subject, or, if admitted, explained in every possible way except 

 through the correct interpretation, that I publish in this paper 

 a series of micro-photographs of a considerable number of sec- 

 tions taken from embryos of all the important stages. Of course 

 these observations could be indefinitely multiplied. The larger 

 the embryological collection, and the better the individual prep- 

 arations are fixed and stained, the more conclusive will be the 



