5o8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tion exogastrulation may result. According to this suggestion, the effect 

 of Hthium in these cases is merely lowering of the levels of the primary 

 gradient and decreasing dominance of the apical region, so that the basal 

 activation is possible within the smaller scale of organization. The de- 

 crease in scale of these apical halves is quite similar to the decrease in 

 scale by inhibiting conditions in the reconstitution of Tuhularia and 

 Corymorpha pieces. Pieces giving rise to apical partial forms in well- 

 aerated water may develop as complete individuals after certain degrees 

 of inhibition. At present the evidence for a specific effect of lithium on a 

 vegetal substance or gradient does not appear convincing. Apical halves 

 of sea-urchin embryos occasionally reconstitute entoderm and mesen- 

 chyme and gastrulate in normal environment, according to von Ubisch 

 (1936a, h). He suggests that this takes place only under the most favor- 

 able conditions, but in the hght of other experimental evidence it appears 

 probable that such development results from slightly inhibiting conditions 

 producing lower levels of metabolism and consequently smaller scale of 

 organization. 



Reconstitution of isolated basal halves is essentially similar to reconsti- 

 tution of a piece of hydroid stem or planarian body after removal of apical 

 or anterior regions, but Runnstrom and Horstadius have interpreted it in 

 terms of the two opposed concentration gradients.^ Basal halves include 

 the most basal part of the presumptive ectoderm {veg^, Fig. 145), the 

 whole presumptive entoderm {veg^, Fig. 145), and the primary mesen- 

 chyme. According to Horstadius, they give rise to a variety of forms 

 ranging from normal plutei, through various modifications of pluteus form 

 and more or less ovoid larvae with gut proportionally too large and with 

 abnormal, often excessive skeleton, to exogastrulae. The teratological 

 forms resemble forms resulting from differential inhibition by various 

 agents. In the forms which approach or attain normal proportions there 

 is evidently considerable reconstitution of ectoderm. In apical halves the 

 animal gradient is supposed to gain the upper hand, according to Hor- 

 stadius; one might expect, then, that in the basal halves the vegetal gra- 

 dient would gain the upper hand, but it certainly does not in the forms 

 that reconstitute ectoderm. These cases also seem to be similar to post- 

 embryonic reconstitutions in the lower invertebrates. After isolation from 

 the higher gradient-levels of the apical half more or less activation of the 

 more apical levels of the basal half apparently increases scale of the ecto- 

 derm and raises gradient-levels so that apical regions can develop. This 



s Runnstrom, 1928a; Horstadius, 1928a, h, 1935. 



