238 J. S. HUXLEY. 



PLATE II. 



FIG. 6. Outline of Q-day control pluteus when transferred to the solution 

 n/ 1, 000,000 HgCl 2 . The skeleton is omitted. Anterior view. 



FIG. 10. Differential disintegration of gastrula in n/ 1,000,000 HgCl^. The 

 nature of the spherical non-disintegrated mass was not ascertained. 



FIGS. 11-15. Stages of progressive dedifferentiation of plutei after re- 

 placement in sea-water from toxic solutions. 



FIG. ii. Form without trace of ciliated band. Gut epithelium cuboidal 

 (occasionally contracting). Trunk ectoderm thin. 2 days in KCN M/IOO.OOO, 

 17 days in sea-water, m = mouth. 



FIG. 12. Completely spheroidal forms. 



FIG. i2a. With aboral ends of spicules broken. Gut straight, with cub- 

 oidal epithelium. No trace of ciliated band, oral lobe, or typical form. 

 Some branched mesenchyme cells, m = mouth. 2 days in HgCL n/i, 000,000, 

 10 days in sea-water. 



FIG. izb. Another specimen from the same vessel, 3 days later. Faintly 

 motile. Gut reduced to a closed vesicle with cells aggregated round it. 

 A curious projection with sharp lobes in oral region. 



FIG. 13. Extreme reduction. 



FIG. 130. Non-motile, with remains of aboral ends of spicules, a dense 

 mass of cells surrounding a gut-vesicle, and very thin ectoderm, i day in 

 KCN n/ 25,000, 13 days in sea- water. 



FIG. i3&. Still further reduction. Remains of only one spicule. Dense 

 interior, with no visible trace of internal organs. 4 days in KCN 71/25,000, 

 10 days in sea-water (with Nitzchia). 



FIG. i3C. Faintly motile. No trace of spicules. Oral projection as 

 in i2b. Several brown aggregations of cells, and a vesicle presumably de- 

 rived from the gut. 2 days in HgCL n/ 1,000,000, 30 days in sea-water (with 

 Nitzchia) . 



FIG. 14. A larva not quite so much dedifferentiated as 120. 



FIG. 15. A much-reduced larva, showing protrusion of the left spicule 

 at both ends. Where it protrudes aborally, a ring of thickened ectoderm 

 surrounds it. The other spicule is absent ; presumably it has simply fallen 

 out. The oesophagus has partially disintegrated. 3 days in HgCl 2 , n/i,ooo,ooo, 

 2 days in sea-water. 



FIG. 16. A larva treated like that shown in Fig. 7. 



