220 J. S. HUXLEY. 



similar in essentials. The larvae lived up to 3^ weeks. Some 

 ingested Nitzcliia when this was provided. Progressive dedif- 

 ferentiation was, I think, more pronounced in these HgCU-treated 

 larvae. 



Figures are given of some of the types seen during this pro- 

 gressive dedifferentiation. Fig. n shows a larva which has com- 

 pletely lost its arms and also its ciliated band, together with the 

 antero-lateral and transverse skeleton on one side. The remainder 

 of the skeleton permits it to retain some of its characteristic form. 

 The gut and its epithelium are contracted, and its spatial rela- 

 tions altered. The body-cavity is clear, with a few clumps, some 

 of pale cells, others of red pigment-cells. The general appearance 

 is much clearer and less full of cell clumps than in larvse in ad- 

 vanced stages of dedifferentiation still in the toxic solution. 



Fig. 12 shows a further stage of loss of form. Here the outline is 

 simply spheroidal. The broad oesophagus, contracted stomach and 

 thin intestine lie approximately in a straight line. The body-cav- 

 ity is very clear, with the exception of a few large clumps of cells. 

 The aboral clubs have been broken off from the rest of the skele- 

 ton. This breaking of the skeleton, it should be noted, was fre- 

 quently seen in forms where the aboral and trunk regions were 

 much dilated. The change of form of the aboral region exerts a 

 pressure inwards on the rods, at right angles to their axis, and 

 snaps the ends off; at first they lie in the position in which they 

 have been broken, thus making it easy to see how breakage oc- 

 curred. Once this happens, the oral ends of the rods will no 

 longer be pressed against the body wall, and the whole skeleton 

 thus ceases to function as a support. This is clearly seen in Fig. 

 12, a and b. 



The general appearance of the tissues in this larva, and indeed 

 in most of those in the recovery experiments, is perfectly healthy. 

 Even after the skeleton has disappeared and the spheroidal form 

 has been assumed, traces of the " lip " or pre-oral lobe may often 

 be seen near the mouth. This also, however, has disappeared in 

 the larvse shown in Fi^. 12. 



A very advanced stage of dedifferentiation is seen in Fig. 13 c. 

 The larva had shrunk considerably in size. This is apparently 

 due to the contraction of the ectoderm, which was cuboidal in- 



