324 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



blastula, three-quarters of the surface of which is covered by the 

 ciHa of the apical organ. Addition of half a macromere gives a 

 larva in which the apical organ is reduced almost to normal pro- 

 portions ; a ciliated band and a stomodaeum are formed, but no gut. 

 Addition of a whole macromere gives a little pluteus in which the 

 gut is, however, too small. Addition of two macromeres gives a 

 perfect pluteus. The addition of four micromeres produces roughly 

 the same effect as that of one macromere. Thus, in proportion to 

 total bulk, the organising capacity of the micromeres is far higher 

 than that of the macromeres, since their size is only about one- 

 thirtieth of that of the macromeres (fig. 150). 



If in place of a whole animal half, an isolated disc an. i had been 

 used, the addition of four macromeres would have resulted in the 

 formation of a perfect pluteus. This again shows that the morpho- 

 genetic effects of the organiser material are dependent on the level 

 (within the main gradient) of the tissues which they are organising. 

 A further proof of this is given by the following fact. An isolated 

 veg. I disc will invaginate a little gut ; but the addition of an animal 

 hemisphere to veg. i prevents the latter from gastrulating at all. 



If corresponding amounts are removed from both ends of the 

 gradient, the remaining tissue is still able to form a pluteus. Thus 

 discs an. 2, veg. i and veg. 2, together, are able to form a properly 

 proportioned larva. But the zones which have been removed, an. i 

 and the micromeres, are together also able to give rise to a proper 

 pluteus. It is therefore possible to obtain two perfect larvae after 

 section at right angles to the egg-axis, provided only that the 

 balance between animal and vegetative potencies is preserved. 



The importance of these facts needs no emphasising. They show 

 that the morphogenetic properties of the organiser in the Echino- 

 derm larva are located at one end of a gradient ; that these capacities 

 are not localised in any given tissue, but diminish gradually with 

 increasing distance from the vegetative pole, along the gradient, and 

 that the degree of organisation produced is quantitatively depend- 

 ent, first upon the difference of level (along the main gradient) 

 between organising material and material to be organised, and 

 secondly upon the relative amounts of the two kinds of material. 



