REJUVENESCENCE IN EMBRYO AND LARVA 413 



In both starfish and sea-urchin the susceptibility increases very 

 greatly, and more in the starfish than in the sea-urchin, up to the 

 early gastrula stage and then begins to decrease sHghtly as the 

 larval structure begins to develop. At this stage the cells have 

 lost the differentiation of the egg, the chemically active protoplasm 

 has undergone great increase at the expense of the inactive substance 

 and has attained the maximum, and from this stage on the develop- 

 ing organism begins to grow old. 



TABLE VIII 



Starfish {Asterias forbesii) 



KCNo.oi mol. 



Stage of Average Survival Time 



Development ia Hours and Minutes 



Unfertilized egg undergoing maturation 1 1 • 30 



30 minutes after fertilization 1 1 . 30 



2-8 cells 10. 30 



64-1 28 cells 5.30 



Blastulae before movement 1.15 



Early gastrulae 1.35 



Advanced gastrulae i . 20 



Young bipinnaria larva 3 . 00 



TABLE IX 



Sea-Urchin (Arbacia punctulata) 

 KCN 0.00s mol- 



Stage of Average Survival Time 



Development in Hours and Minutes 



Unfertilized egg 8.15 



20 minutes after fertilization 6. 45 



4-8 cells 5.45 



Late cleavage 3-3° 



Early gastrulae 2.15 



Advanced gastrulae 3 ■ 00 



Prepluteus 3.30 



In this coimection it is of great interest to note that in the starfish 

 and sea-urchin and various other species the late blastula and early 

 gastrula stages appear to be critical stages in development under 

 many experimental conditions, e.g., in experimental partheno- 

 genesis, in many hybrids and under the action of various external 

 agents. Development may proceed with little or no disturbance 



