LARVAL DEVELOPMENT IN THE SEA URCHIN 109 



such gradients, when they do arise, shall differ from that pro- 

 duced in the normal egg by the same factors. The demonstra- 

 tion of the existence of metabolic gradients in the unfertilized 

 egg by this method is then not complete. The evidence from 

 death gradients is also unsatisfactory on this point, because, 

 with the low metabolic rate in the unfertilized egg, differences 

 in rate are at least slight (Child, 16 c). According to Boveri 

 ('01 a, '01 b) the apico-basal axis of embryo and larva coincides 

 with the axis of the growing oocyte in Strongylocentrotus livi- 

 dus, but Garbowski ('05) has shown that such coincidence does 

 not occur in all cases, and, according to Wilson and Mathews 

 ('95), the embryonic axis in Toxopneustes may form any angle 

 with the axis determined by the position of the polar bodies 

 and of the nucleus after maturation, while in Asterias these two 

 axes coincide. 



These various observations afford no adequate basis for gen- 

 eral conclusions. If they are all correct we must conclude that 

 in echinoderms the apico-basal axis of the embryo may coincide 

 with that of the growing oocyte, or may be determined de novo 

 by factors acting on later stages. If the original axis of the 

 growing oocyte is a metabolic gradient, it is evidently not very 

 strongly marked or permanently fixed in the protoplasm, and 

 other factors acting differentially on the egg (Child, '15 b, Chaps. 

 II, V.) may determine a new effective gradient, as is appar- 

 ently the case in Asterias (Wilson and Mathews, '95; Child, 

 '15 a), and as Garbowski's observations indicate in some cases in 

 Strongylocentrotus (Garbowski, '05). If any metabolic gradient 

 is present in the unfertilized egg of Arbacia it is apparently 

 slight (Child, '16 c), and it seems probable that the differential 

 conditions associated with fertilization, or perhaps, in some cases, 

 the differential action of other external factors might determine 

 a new effective gradient. In general, the observations, as far as 

 they go, suggest that the apico-basal embryonic axis coincides 

 with the axis of the growing oocyte, except where the differen- 

 tial action of other factors is sufficient to determine a new effec- 

 tive gradient, and this is very probably true for many other 

 animal eggs. 



