LYMPHATICS IN FROG LARVAE 23 



The most striking change at this time is the appearance of 

 the stomach (fig. 2a) as a conspicuous fold to the left of the 

 liver, now grown larger. The yolk mass is otherwise still a 

 simple tube, unchanged except in some distention anteriorly 

 and the beginning of the formation of a hind-gut, though there 

 is yet no bend here. There is now only a single vitelline vein 

 on the ventral surface to the right (fig. 2a). 



Changes in the digestive tube furnish the most conspicuous 

 guide for ready identification of the early developmental 

 stages. Accordingly it will be convenient to refer to the series 

 furnished by Liu and Li ('30) for the changes in the shape, 

 development of subdivisions, and shifting of parts in the 

 digestive system of R. nig. However, my series begins earlier 

 than the first stage of the Liu-Li series, before the appearance 

 of the stomach; hence my second stage, figure 2 (Pollister's 

 late 21, '37), will correspond to their figure 1. 



Stage 2 of this paper will then be understood to include 

 embryos exhibiting a more or less marked stomach fold, be- 

 ginning in the younger specimens of the period which covers 

 about a day. The rest of the gut is still unmodified p<>-t- 

 riorly, much the same as in stage 1. 



The submaxillary vessels 



The submaxillary blood vessels shown in figures 2a and '_!l> 

 for stage 2 of R. palustris have retained the characters already 

 described for the earlier stage, though the network now lies 

 in two layers, a superficial and a deep sheet, and has spread, 

 especially around the posterior angles. The external carotid 

 arteries and jugular veins are well shown and are connected 

 as before. A new transverse vein has been formed in the 

 plexus on either side to drain the lateral extensions medially 

 into the exiernal jugular (figs. 2a and 2b). The arteries and 

 veins from the mandibular arch described for the earlier >iauv 

 as joining the plexus at the anterior angles, and laterally, are 

 also seen in these figures, but appear best in side view, drawn 

 in figure 2. They are connected as in the previous stage 

 (fig. 1) and leave no blind processes to extend forward around 

 the mouth. 



