22 HENRY MCELDERRY KNOWER 



These arteries arise from the dorsal aorta back of the caro- 

 tids, from which point each runs down in the anterior walls 

 of the pharynx to the sides of the ventral submaxillary region, 

 the relations and connections especially in young stages (like 

 that of fig. 1 and earlier) identifying them as the primary 

 (M.A.) arteries of the mandibular arch. 



The additions to the plexus from these arteries are drained 

 off laterally on each side through an accompanying vein which 

 runs dorsally in the pharyngeal wall back of the artery to 

 terminate in the internal jugular vein ventral to the auditory 

 vesicle just behind the eye. 



The anterior and lateral margins of the plexus in the frog- 

 are, therefore, continued into the vessels just described, and 

 do not leave blind processes projecting laterally or around 

 the corners of the mouth as shown in the reconstructions of 

 the toad. 



SECOND LYMPHATIC PERIOD 



What will be termed the second period of lymphatic develop- 

 ment occurs in frog embryos of the day following the stage 

 just described for figure 1. The larva of R. palustris has now 

 undergone a steady growth in all parts, though its primitive 

 organization is but little modified, as will become evident in 

 the following description. All larvae have hatched by the end 

 of this period (figs. 2 and 2a). The length, about 7 mm. 

 (Pollster's late 21, '37), is generally 1 mm. longer than in 

 the previous stage (fig. 1) with the tail one-half the total. 



ARRANGEMENT OF LARVAL STRUCTURES (SECOND PERIOD) 



The segments of the body and tail have expanded bringing 

 about a considerable shifting apart of the segmental arteries 

 and veins along the axis. The tail fins are broader and their 

 plexuses have spread out further peripherally. In the head, 

 the vesicles of the brain and those of the eyes and ears have 

 increased in size; and some enlargement of the pharynx is 

 noted, partly due to accumulations of connective tissue on its 

 walls. The operculum now extends further back, but still 

 leaves the gills exposed, external. 



