ECHINODERMS 85 



Horstadius interpreted these results to mean that the egg contains two 

 gradients, an animal one with its high point at the animal pole, and a 

 vegetative one running in the opposite direction. Regions which are high 

 on the animal gradient tend to form animal organs, and similarly for the 

 vegetative. Moreover, Horstadius proceeded to show that the two 

 gradients interact with one another. In a very beautiful series of experi- 

 ments, he combined the various layers in abnormal combinations. He 

 found, for example, that normal plutei were formed fairly frequently 

 when an-i was combined with four micromeres, and that as the number 

 of micromeres was reduced, so more and more 'animal' larvae appeared. 

 An-2, on the odier hand, required only two micromeres to produce a 

 normal larva, and became vegetative in character if more were added. 

 Veg-i was usually swung over too much to the vegetative side by the 

 addition of even one micromere; probably about a half would be enough 

 to counteract its slight preponderance of animal tendencies (Cf. Fig. 5.2). 



Both in the experiments in wliich the various zones were isolated, and 

 in those in which micromeres were grafted, there was considerable varia- 

 tion in the resulting embryos. It appears that one is dealing not only with 

 the interaction of animal and vegetative potentiaHties of various strengths, 

 but that there is also another important factor, namely a tendency for the 

 normal equihbrium between these two conditions to be restored, even 

 when the two parts originally grafted together were not in perfect 

 balance (see Lehmaim 1945, p. 64). We will fmd that a similar tendency 

 to restore a normal equihbrium condition is a very frequent influence in 

 most developmental events. It is an aspect o( individuation (p. 12). 



There are several other types of experiments in which individuation 

 is strikingly exhibited in the echinoderm embryo. The vegetative region, 

 and particularly the micromeres, have a very strong ability to influence 

 their surroundings in such a way that the latter fit into the building-up 

 of a complete or partially complete embryo. For instance, a meridional 

 half may be combined with an animal half in the i6-cell stage (Fig. 5.3). 

 From this a normal pluteus will develop, the part of the animal material 

 lying next the micromeres being, as it were, absorbed into the develop- 

 mental system of the latter and converted into gut. A rather similar result 

 is obtained if a group of micromeres is inserted among the animal cells 



B, the addition of vcg-i restricts the size of the apical tuft, but again no gut 

 develops; C, with veg-2 a relatively normal embryo is formed; D, with 

 veg-i plus the micromeres, the embryo forms a gut which is abnormally 

 large; E, the addition of the four micromeres to the animal half gives a near- 

 ly normal larva, although their mass is much smaller than the veg-2 material 

 added in C. (After Horstadius 1939.) 



