58 CHARLES F. W. McCLURE 



solution of pyrrhol blue has been injected. Downey (p. 443) 

 states : 



The next day the isolated segment was removed, fixed in Kelly's 



Zenker-formol mixture, imbedded and sectioned All 



of the polymorphonuclears which came in contact with the dye are 

 loaded with it, and it is concentrated in the form of the typical dye 

 granules. There is no diffuse staining of cytoplasm or nucleus, which 

 indicates that the leucocytes were living at the time of fixation. 



Downey's observations are most suggestive and indicate that 

 cells which do not ordinarily take up dye while in the circulation 

 may be made to do so in a typical manner when placed in a 

 modified environment. The fact that these cells store the dye in 

 a typical manner, certainly shows that their inability to do so 

 while in the circulation is not necessarily due to the incomplete- 

 ness of their differentiation. 



Although I believe Downey may have settled a very important 

 feature of the general problem by showing, on the basis of vital 

 staining, that such free cells cannot be classed apart from those 

 found in the tissues, we are still in doubt why some free and fixed 

 cells react toward the dye, while other contiguous cells of exactly 

 the same type often behave differently under similar environ- 

 mental conditions. While environment may finally play an 

 essential r61e in causing the free cells in the circulation to manifest 

 their activities, after entering the tissues, neither it nor an incom- 

 plete state of differentiation of the cells can wholly account for 

 the above-mentioned variability observed in larval and adult 

 animals. 



Experiments (Chapter 10) seem to show that certain typical 

 tissues of adult frogs and toads and of well-advanced larvae do 

 not react at all or do not as readily react toward the dye during 

 the Winter months, as during the Spring and Summer. It has 

 also been shown in these experiments that this is not due entirely 

 to the circumstance that these tissues may not have been suffi- 

 ciently acted upon by the dye. The circumstance that cells which 

 have previously been capable of storing the dye in the Summer 

 are unable to do so during the Winter, cannot be explained on the 

 basis of the incompleteness of their 'differentiation, but rather 



