50 



These pictures of injected vessels in the tail may be com- 

 pared with those referred to in Clark's studies of living speci- 

 mens in this location; and it may be further added that the 

 injections of other regions of frog larvae reveal the same 

 structure in terminal vessels of the lymphatic system wherever 

 they are invading tissue. 



The anterior outgrowth from the lymph heart plexus are 

 now conspicuous features of the head along the two lines 

 already indicated; the dorsal processes extended over the 

 brain and sense organs; and more ventrally, the jugular out- 

 runner, an enlarged saccular duct, prolonged well forward 

 in the outer wall of the branchial cavity (fig. 13). The tem- 

 poral division of the jugular tract is distended in this period, 

 forming a sinus in the later specimens (fig. 14, etc.). Distally 

 it bears a subocular projection with fine terminal filaments 

 ramifying in the tissues under the optic vesicle. Numerous 

 processes run out from the main duct into the surrounding 

 tissues, those at the ventral extremity reaching the lateral 

 margins of the submaxillary region in younger larvae of the 

 period (fig. 13). The greatly expanded cephalic sinuses, pecu- 

 liar to Anura, which begin their enlargement in the temporal 

 region in the early part of the third period (stage 3, fig. 13), 

 soon become prominent features of the head. 



Once started, distention of the jugular duct quickly pro- 

 duces a large cavity over the gills (stage 4, fig. 14), which 

 attains exaggerated proportions in succeeding stages (figs. 

 18, 19, 24, 30). An account of the main features of the de- 

 velopment of these sinuses is introduced here, since from their 

 first enlargement in this period they pass through a series of 

 remarkable changes in succeeding stages to reach the defini- 

 tive conditions described by Hoyer ('05) for 26-mm. larvae 

 of R. temporaria. 



The plexiform walls of the sinuses (figs. 18, 19) in these 

 early stages, and their many filiform processes extending into 



