BEHAVIOR OF ANURA TOWARD COLLOIDAL DYES 59 



on the basis that they have become temporarily inactive toward 

 the dye. We also know that these same cells would again ac- 

 tively react toward the dye in the following Spring. We there- 

 fore appear to possess evidence that certain typical cells of the 

 body may normally undergo temporary and alternating phases 

 of activity and inactivity, as regards their reactions toward col- 

 loidal acid dyes. 



It has also been observed by the writer that the reactions of 

 lymphatic endothelium toward colloidal acid dyes are much more 

 pronounced during early larval stages, than in young frogs with 

 rudimentary tails and in which both pairs of limbs are free and 

 fully formed. When a young frog of this character has remained 

 in a 1:1500 solution of dye No. 161 for seventy hours and after 

 this for twenty-four hours in tap water before killing and fixation, 

 the endothelium of the large superficial lymph sinuses of the head 

 and posterior regions does not store the dye as actively as in young 

 larvae within the time limits, at least, of this experiment. The 

 endothelium of the subvertebral lymph sinuses and some of its 

 tributaries, however, is intensely phagocytic; also the endothe- 

 lium of certain other smaller lymphatics found here and there 

 throughout the body. It is evident that large areas of lymphatic 

 endothelium which were actively phagocytic during early larval 

 stages have ceased to be so at a more advanced age. 



These observations on hibernating animals and young frogs 

 suggest the possibility that the above-mentioned variability, as 

 regards the reactions of typical tissues toward acid dyes, may also 

 be explained on the grounds of the temporary and alternating 

 states of activity and inactivity which such free and fixed cells 

 may undergo. 



In relation to recognized regions of the pharynx where dye is 

 absorbed by the epithelium, the distribution of the mononuclear 

 leucocytes in the tissues of the above-mentioned young frogs is 

 most significant. These regions, as stated on a preceding page 

 (p. 13), are thickened patches of epithelium which occupy a posi- 

 tion in both the frog and toad in the roof and floor of the pharynx. 

 Great numbers of mononuclear leucocytes loaded with dye gran- 

 ules are found in the connective tissue contiguous to these two 



