80 HENKY MCELDERKY KNOWER 



The third vein is identified by the attached anterior lymph 

 heart, and receives posterior and anterior tributary veins 

 from the dorsal plexuses, to form divisions of the anterior 

 vertebral vein, though they do not yet appear as definite 

 branches. 



It has been explained that the two first intersegmental veins 

 form parts of the great superficial cutaneous venous system, 

 for which they serve as outlets. 



The lymphatics of this stage, illustrated in figures 13 to 17, 

 may be readily located in relation to the blood vessels, with 

 the lymph heart in figure 38 as a guide. The heart and lateral 

 lymph plexus lie immediately superficial to the primary seg- 

 mental veins, while the peripheral venous loops arch over in 

 the outer body wall. These larvae, still between 7 mm. and 

 8 mm. in length, are selected from injections of both lym- 

 phatics and blood vessels in the third period, which is identi- 

 fied by the S-shaped digestive tract, distended cavities, etc. 

 (See pp. 45, 46, and Pollister's late 22 ( '37) for descriptions of 

 the characters of this period.) The larvae of this period corre- 

 spond to those of the toad in which the anterior segmental 

 veins are described as becoming detached in a connected 

 group, and transformed into lymphatics. The facts just re- 

 viewed for frogs evidently permit of no such interruption in 

 the history of the anterior veins in these Anura. 



The gradual differentiation of the peripheral blood vessels, 

 with fixation of veins characteristic of late stages, is illus- 

 trated in figures 39 and 40. The earlier figure (fig. 39) is 

 from a larva of R. palustris with 'globular body-form,' and 

 first loop (Int.Flx.) of the intestinal spiral, corresponding to 

 the specimens illustrated in figures 23 to 27 for lymphatics. 

 As was explained for the lymphatic plexuses of this period, 

 the plexuses of blood vessels are also in a state of transition, 

 with larger veins emerging from the network to become the 

 definitive trunks which appear in late stages. 



The neural plexus of the hind-brain, with the small vessels 

 which run out to connect with the lateral first segmental veins, 

 are particularly well shown in figure 39; and the distinct, 



