14 .MARIE A. HIXRICHS. 



EXPLANATION OF PLATE I. 



FIGS. 1-3. Young plutei showing successive stages of early disintegration by 

 photolysis. Lateral view. Fig. 2. Aboral view. Fig. 3. 



FIGS. 4-6. Normal three-day plutei. Fig. 4, Unstained, developed in dark. 

 Fig. 5, Stained, developed in dark. Fig. 6, Lateral view of Fig. 5. 



FIGS. 7-22. Differential inhibition. Figs. 7-8, lateral view. Aboral arms and 

 oral lobe slightly reduced. Figs. 9-11. Approximation of arms indicating inhi- 

 bition of development of the median anterior region. Figs. 12-13, Complete 

 reduction of median anterior region. Single median arm. (Figs. 7-13 Two- 

 day larvae. For normals see Fig. 48.) Fig. 14. Three-day larva. Union of two 

 arms anteriorly, due to the reduction of the median anterior region. Figs. 15-16. 

 More complete inhibition of development. Figs. 17-21. Exogastrulae, indicating 

 a difference in susceptibility to growth-inhibiting influences. Endoderm cells 

 have continued to grow at the expense of ectoderm cells. Fig. 21. The widely 

 diverging aboral arms indicate a secondary differential recovery of the median 

 anterior region. 



FIGS. 23-27. Differential recovery. The anterior median region has recovered 

 from a temporary inhibition and has regained and finally surpassed its normal 

 growth rate. Oral lobe is over-developed. (Fig. 26. The angle of divergence 

 between the aboral arms is much greater than normal.) 



FIGS. 28-30. Differences in degree of action of visible radiation. (A) Exposed 

 without previous staining, Fig. 28. (B) After staining, Figs. 29, 30. The in- 

 hibitory effect is greater in B. Both A and B were exposed for the same period 

 of time. 



