LYMPHATICS IN FROG LARVAE 53 



special relations of the early duct first injected and described 

 for figure 20, this view of its origin seems the most reasonable. 

 The final direct transformation of the primary duct into a 

 collecting duct of the typical system of sinuses of the sub- 

 maxillary region lends further support to the view of its 

 nature as a persisting local end-product of invasion from the 

 general lymph system of the body. 



Most of the early injections of the submaxillary lymphatics 

 are in larvae slightly older than that represented in figure 20 

 (and in fig. 13) and lymph sinuses are seen in place of the 

 simple branched duct. This second type of injection is illus- 

 trated in figure 21, from two contrasting specimens, the 

 sinuses on the right, R, being well filled, while those on the 

 left, L, are only lightly outlined. (The larval organs and the 

 lymph sinuses on the right are drawn from the same speci- 

 men as that shown in figure 14, w r hile figure 21, L, is inserted 

 from a different specimen.) 



Comparisons of these injections of the first sinuses, with 

 the simple form of the system shown in figure 20, demon- 

 strate essentially the same pattern with like distribution of 

 unit parts in both types, the early sinuses being grouped 

 around the central vessel which extends across the region 

 from the temporal sinus in place of the primary duct, as if 

 derived directly from this duct and its branches. This indeed 

 appears to be a fact; the dilated central sinuses (fig. 21) 

 arising from the main body of the primary duct by continued 

 expansion of its w r all begun in the earlier stage (fig. 20) ; 

 while the circum-oral lymphatic and the triangular sac repre- 

 senting the first phase of the mandibular sinus in front of 

 the axial vessel (central sinus) (fig. 21) arise from anterior 

 projections of the early duct toward the mouth. 



The relations of the circum-oral sinus and its origin from 

 connections with the temporal sinus are definitely exposed on 

 its first appearance and in injections of all stages (figs. 20, 

 14, 19, 21, 22). It is distinctly a centrifugal outgrowth. On 

 the other hand, the early stages of the expansion of the man- 

 dibular sinus in its differentiation from the anterior median 

 process of the primary duct are not so easily followed, on 



