98 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



opment are seen on the opposite side (4~5) and (not labelled) in 

 the center of the field, but the structures on the left side are, 

 as in the earlier stages (cf. series 189) better developed and 

 larger. 



The succeeding figures, 120, 121 and 122 (sections 7, 10 and 

 11) show the same structures, which can be distinctly followed 

 for some distance caudad of the last section figured. In fig. 122 the 

 original connection of the large upper venous kernel of the left side 

 (4) with one of the components of the peritracheal venous plexus 

 (18) is still suggested by the arrangement of the adjacent cells, 

 although the degenerating vein has been cut off from all actual 

 communication with the permanently valid channels of the 

 mediastinal venous plexus. 



The 14 mm. stage is so important, and the results gained by 

 the examination of a large number of embryos so conclusive in 

 respect to the true ontogenesis of the systemic lymphatic channels, 

 that I have selected a well marked example and publish here 

 eighteen consecutive, or nearly consecutive, sections from the 

 same individual. I do this in order to show the extent to which 

 extraintimal perivenous lymphatic development has proceeded 

 in this stage, in respect to the length of the resulting segments of 

 the future lymphatic channel, still separated from each other, 

 and to accentuate, by a large number of successive illustrations 

 from the same embryo, the histogenesis of the lymphatic anlages 

 in their relation to the decadent embryonic venules which they 

 eventually entirely replace. The conception involved by the 

 theory of extraintimal systemic lymphatic development has 

 proved apparently, to judge from published comments, difficult 

 to understand in some laboratories from the histo-mechanical 

 standpoint. The following eighteen reproductions of sections 

 of a 14 mm. embryo (series 214, slide xiii) surely explain, with- 

 out detailed description, the principles underlying lymphatic 

 ontogenesis in the mammalian embryo. I very gladly welcome 

 the opportunity, which this series of Wistar publications offers, 

 for presenting them. 



The plates comprised in this group are shown in order cephalo- 

 caudad in figures 123 to 141 inclusive. In all of them the ex- 



