LYMPHATICS IN FROG LARVAE 33 



Many attempts fail to supply intermediate steps between 

 the two extremes. 



On the one hand, there are specimens like that in figure 1, 

 with a single vessel from a small heart plexus distributed to 

 a limited dorsal area over the cervical cord; or others are like 

 figure 3, with a larger plexus and two or three processes 

 limited in distribution dorsally (Dsl.Cn.). These represent 

 the beginning stage, while on the other hand many injections 

 of the same period exhibit an extended and well-developed 

 dorsal system like those illustrated in figures 5 to 12. (See 

 plate 5 for figs. 8 to 12.) 



Figure 8 illustrates the earliest injection of the dorsal lym- 

 phatics of the series. It is selected to represent the primary 

 dorsal lymph plexus soon after its establishment. The early 

 stage of the larva is evidenced by the small size and simple 

 forms of hind-brain, ear vesicles and stomach fold. It is a 

 specimen from the early second period. On the right side, 

 the lymph heart plexus is seen against the lateral muscles, 

 sending two vessels dorsally. These ramify over the cord, 

 and combine with two similar vessels from the left heart 

 plexus to form a simple dorsal lymph plexus from which a 

 median trunk grows back over the cord. 



As the dorsal lymphatics spread posteriorly, and connec- 

 tions with the lateral system multiply, these become arranged 

 in a series on either side, and divert the flow of lymph from 

 the tail outward through the bilateral system thus formed, 

 illustrated in figures 9 and 10. 



The demonstration of the posterior extension of the main 

 dorsal duct to one-half the length of the tail as part of the 

 system in the early stage, pictured in figure 6, is an important 

 feature not hitherto seen. It is difficult to inject at this time, 

 on account of the considerable resistance from its fine caliber 

 and close-meshed plexiform character. Hence, the extent and 

 character of the duct must be carefully verified in a number 

 of injections of the same period (figs. 2, 7, 8, 10). The inter- 

 ruption of continuity in figure 6 was necessary to demonstrate 

 the entire duct in the tail in correlation with the development 

 of other lymphatics of this specimen. In figure 6, the caudal 



