230 J- S. HUXLEY. 



alive, but, it should be emphasized, with apparently quite healthy 

 tissues. 



The fact that such larvae ingested diatoms in a normal fashion 

 is noteworthy. The possibility is thus raised of producing, by 

 environmental changes, permanent larval forms. (Cf. Perkins, 

 '02.) Such permanent larval forms of small size, but hereditarily 

 determined, are of course known in many species, especially in 

 the male sex. 



The action of mercury is apparently to precipitate and put out 

 of action a definite quantity of the living molecules in the cell. 



POSTSCRIPT. 



Owing to delay in the arrival of German periodicals after the 

 war, I did not see the important paper by Runnstrom ('17) until I 

 had not only completed the work here recorded, but also written the 

 paper. Runnstrom has demonstrated that young Echinoid plutei 

 treated with ZnSo 4 in very w<eak solutions undergo dedifferentia- 

 tion-changes very similar to those observed by me. He also finds 

 that dedifferentiation occurs spontaneously in a certain proportion 

 of larvae in every culture, especially from over-ripe eggs. The 

 specimens noted by me on p. 213 above are doubtless of this cate- 

 gory. Finally, he describes very similar dedifferentiation as the 

 result of starvation, from which recovery is possible. Recovery 

 is also possible in the ZnSo 4 larvae when replaced in sea-water. 



I will not summarize his results further, except to say that they 

 show conclusively that dedifferentiation may be produced by 

 many agencies, and may be reversible. 



There are one or two points of general theoretical interest 

 which may be noted. While remarking on the resemblance of 

 the changes seen at metamorphosis to those produced by starvation, 

 Runnstrom says that an essential difference between the two is 

 the rapidity of the metamorphic changes. On the other hand, 

 reduction by means of toxic agencies is more rapid, and it should 

 be remembered that at metamorphosis the existence of rapidly 

 growing imaginal organs will drain the rest of the organism 

 very quickly. The rapidity of dedifferentiation in the zooid of 

 Perophora is very great once it has started. The observations 

 of Professor MacBride (p. 226) also corroborate strongly the idea 



