DEDIFFERENTIATION IN ECHINUS LARV^. 215 



The controls had arms about 25 per cent, longer than Fig. I, c. 

 It will be seen that in all cases the arms have diminished in 

 length. They have also become more dense in appearance, being 

 in the more advanced examples quite opaque. The diminution is 

 rarely less than that seen in Fig. i, c, and ranges from about 20 

 per cent, to about 80 per cent. The skeleton does not protrude 

 at the tips of the arms, and the missing parts have apparently 

 been resorbed. Another marked feature in the large majority of 

 the specimens is the dilatation of the trunk and aboral regions 

 (Fig. i, a-c), which leads to a bulgy form, usually with a wide 

 separation of the aboral ends of the spicules. 



This may be explained as an effect of differential susceptibility, 

 the more susceptible oral region contracting before the aboral, 

 which then becomes distended with fluid. In some specimens 

 (Fig. i, c) the trunk ectoderm was seen to be definitely thinner 

 than normal, implying distention. Other specimens (Fig. i, d and 

 e) did not show these phenomena at all markedly; in every case, 

 such non-distended individuals exhibited extreme arm-resorption, 

 and were also well below the average size. These were pre- 

 sumably individuals of general low susceptibility, in which all 

 regions had suffered, the aboral ectoderm also being contracted, 

 and the total volume of body-fluid being in some manner dimin- 

 ished. In point of fact, they were usually seen (Fig. i, d) to 

 have abnormally thick aboral and trunk ectoderm. 



The number of pigment-granules appeared in general to have 

 increased. 



The appearances after 48 hours were very similar, with an 

 increase in the amount of arm-resorption, and of aboral contrac- 

 tion. 



After 72 hours, none had more than quite vestigial arms (Fig. 

 2, a), while the majority were almost or quite armless (Fig. 2, c). 

 Very few now showed the separation of the aboral ends of the 

 spicules, and not so many the swelling of the trunk region (Fig. 

 2, c}. Many, on the contrary, were very narrow, especially at 

 the aboral pole, and showed a condensation of the mesenchyme 

 cells to form clumps (Fig. 2, a). In Fig. 2, b, the aboral region 

 of the same specimen is shown under a higher power. The large 

 clumps of mesenchyme cells are extremely like the clumps of 



