442 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tion, and apical regions are correspondingly decreased. In the case of 

 o«i, normal development with four micromeres seems rather difficult to 

 account for on the basis of substance gradients. An^ represents the high- 

 est concentration of animal substance; and, since it is less than a fourth 

 of the whole polar pattern, there can be no great difference in concentra- 

 tion in it. The micromeres supposedly represent the highest concentra- 

 tion of vegetal substance. To produce an effective vegetal gradient this 

 substance must diffuse from the micromeres. To balance the animal gra- 

 dient it must approach the concentration of normal development; but in 

 that case we should expect no great difference in concentration of either 

 substance, since only the two ends of the polar axis are represented. 

 Evidently there must be mutual ''suppression" between the two gradients 

 as the vegetal substance diffuses from the micromeres, if anything like 

 normal development is to result, or else the accumulation at the two 

 poles of the two substances must be postulated; and these assumptions 

 raise further questions and difficulties. Similar difficulties arise with re- 

 gard to aw2 and vegi,; each represents only a fraction of the gradients, and 

 the micromeres another fraction. How do whole gradients arise from 

 these? The concept of dynamic gradients which can be altered in length, 

 height, or slope and obUterated or reversed by quantitative metabolic 

 differences seems to present fewer difficulties and is in Hne with observa- 

 tions on normal sea-urchin development and on reconstitution in many 

 animals. If a substance diffuses from the micromeres, may it not be simply 

 an activating substance rather than one producing some specific differ- 

 entiation? 



Effects of the micromeres with veg^ appear anomalous, in terms of con- 

 centration gradients. The ring, veg2, is supposedly entirely entodermal, 

 but there is reconstitution of ectoderm from its more apical region and, 

 with micromeres present, exogastrulation. But, according to Horstadius' 

 experimental records seven of thirteen larvae from isolated vcg^ without 

 micromeres developed exogastrulae ; and of seventy-three with one or 

 more micromeres, thirty-seven showed different degrees of exogastrula- 

 tion, and thirty-six died early with little development. In normal develop- 

 ment veg^ does not form ectoderm at all; but when isolated, with or with- 

 out micromeres, ectoderm is reconstituted. According to the substance 

 hypothesis, it contains only a low concentration of animal substance but 

 a high concentration of vegetal substance. How can the animal substance 

 become effective in the presence of the high concentration of the other? 

 The reconstitution of ectoderm in veg2 is apparently quite similar to re- 



