ECHINODERMS 87 



complete itself. Thus vertebrate induction may involve both individua- 

 tion and something else, which has been named 'evocation' ; in the early- 

 stages of the echinoderm, we seem to have only the former. 



The hypothesis of two gradients fmds very strong support in the work 

 described above. But innumerable questions immediately suggest them- 

 selves. What are these things gradients of? Are they localised in any par- 

 ticular part or structure of the egg ? And how do they produce their 



Figure 5.4 



Grafts of the micromeres (black) at different levels along the an-veg axis. 

 They migrate into the blastocoel cavity as primary mesoderm, and induce 

 the formation of an archenteron from the material near them. This induced 

 gut (indicated in black in the right-hand column) is larger the nearer the 

 graft is to the vegetative pole. (After Horstadius.) 



effects ? None of these questions can be answered with any assurance, but 

 knowledge is increasing rapidly about them. As regards the first, 

 we know that the gradients can be influenced by chemical substances (Lin- 

 dahl 1942, Gustafson 1950). The vegetative tendencies are strengthened by 

 the action of lithium salts. If complete eggs are treated with lithium, they 

 form too large a gut, producing exogastrulae similar to those formed 

 when veg-2 is isolated. Similarly, isolated animal halves can be brought 

 to develop into normal larvae by lithium. The animal tendencies are 

 increased by early treatments with thiocyanide. This, and other similar 



