FIELDS AND GRADIENTS IN NORMAL ONTOGENY 323 



an exogastrula, without apical organ, cilia, or mouth. The situation 

 here is complicated by the fact that the gradient-field (vegetative- 

 animal) concerned with inducing gastrulation interacts with the 

 previously established animal-vegetative gradient-system of the 

 egg. As a result, not only are organising capacities graded with 

 distance from the vegetative pole, but so are the capacities for being 

 organised. 



It has been found^ that the production of a properly proportioned 

 pluteus larva is dependent in the first place on the presence of some 

 of the vegetative pole material. This material acts as an organiser, 

 and is normally to be found in the micromeres : micromeres grafted 

 into abnormal situations will induce gastrulation and the formation 

 of a secondary set of main organs where they are grafted ; they will 

 organise the neighbouring tissues so as to make them conform to 

 the normal morphology of a larva, and to the new polarity set up 

 by the graft. But the organising capacities are not restricted to the 

 micromeres, for, if they are removed, it is found that the next most 

 vegetative region, disc veg. 2, is capable of forming a pluteus with 

 proportions approximating to those of the normal. It is well known 

 that lithium salts produce exogastrulation in Echinoderm larvae, 

 i.e. a reinforcement of the vegetative potencies." It is therefore in- 

 teresting to find that lithium salts induce gastrulation and the 

 awakening of organiser properties in isolated animal halves^ (see 



also p. 337). 



In the second place, the production of a perfect pluteus larva is 

 dependent on a balance between animal and vegetative material, 

 and it has been possible to study this balance quantitatively. In 

 order to obtain a well-proportioned pluteus it is necessary to add 

 one micromere to disc veg. i, two micromeres to disc an. 2, and four 

 micromeres to disc a?i. i. Excess of material from the animal pole 

 leads to imperfect gastrulation and abnormal enlargement of the 

 apical organ, excess of material from the vegetative pole leads to 

 exogastrulation and reduction in the extent of the ciliated area. 



Similarly, it is possible to observe gradual approximation to 

 normal proportions when macromeres are added to a complete 

 animal hemisphere. Isolated, the animal hemisphere gives a 



^ Horstadius, 193 1. ^ Herbst, 1895. 



^ von Ubisch, 1929. 



21-2 



