DEDIFFERENTIATION IN ECHINUS LARVAE. 221 



stead of flattened. No trace of skeleton was present. (The ani- 

 mal was examined both in vivo and as a stained preparation.) 

 Large, brownish aggregations were seen in the interior, together 

 with an apparently closed and solid pale vesicle, perhaps the 

 stomach. Curious irregularities of the ectoderm were observed 

 at one pole. These were also seen in several other specimens (cf. 

 Fig. 12 &). 



The general resemblance of this specimen to much-dedifferenti- 

 ated individuals of the Ascidian Clavellina is striking (Driesch, 

 '06; Schultz, '07; and my own unpublished observations). There 

 are numerous points of difference, as one would expect in such 

 widely different organisms, but the following essential similarities 

 exist: (i) the assumption of the spheroidal form; (2) the ag- 

 gregation of free cells to form dark masses; (3) the regression of 

 epithelial cells to the cuboidal condition; (4) the conversion of 

 internal structures into closed vesicles; (5) the congested condi- 

 tion of the body spaces, consequent upon contraction. 



5. ON THE GENERAL EFFECT OF MERCURY IN DILUTE SOLUTIONS. 



In order to get some more accurate idea of the processes occur- 

 ring in a weak solution of a mercury salt, some experiments were 

 carried out on the gill of Mytilns. I have to thank Mr. J. Gray, 

 of King's College, Cambridge, for some suggestions. 



A preliminary test with various strengths of HgCl 2 solution 

 showed that in very weak solutions marked disintegration of the 

 tissue took place before ciliary action was stopped. 



The point to be tested was whether the effect of Hg + ions 

 was proportional to the strength of solution used, or was a pro- 

 gressive effect, proportional to the total amount of mercury in 

 the solution. 



(A} Five ringer bowls were prepared, and 4 pieces of gill placed 

 in each. One contained 50 c.c. of sea-water as control ; the others 

 50 c.c. of /375,ooo HgCL. The solution in Nos. i and 2 was 

 not changed. In No. 3, it was changed after 2, 4, 6, 8 hours and 

 again after 24 hours. In No. 4, it was changed every Y^ hour for 

 8 hours, and again after 24 hours. 



After 3 hours, there was slightly more disintegration in No. 

 4 than elsewhere. After 6 hours, all the pieces in No. 4 had their 



