460 MARIE A. HINRICHS. 



control series was kept in the dark. There were further controls 

 as follows; (a) in the light (in diffuse daylight throughout the 

 experiment), and (6) in the dark, of each of the following: 

 normal eggs fertilized by normal sperm (stained and unstained), 

 unfertilized eggs (stained and unstained). This gave a check on 

 accidental fertilization during preparation of the material, also 

 on any possible injurious effect of the dyes. The norm for the 

 series was furnished by the embryos resulting from the ferti- 

 lization of normal eggs by normal sperm. 



The results obtained with visible radiation following sensi- 

 tization, and with ultraviolet radiation were essentially similar, 

 except that the length of the effective periods of exposure to 

 ultraviolet radiation was always much shorter; for example, 

 differential inhibition without recovery could be obtained by 

 exposing sperm (1/3,600 per cent.) for 10-15 seconds. 



Results. Since exposure of either sex component (before 

 fertilization) to radiation from any one of the various light 

 sources produces differential modification of development, and 

 since the degree of modification is in general proportional to the 

 dosage of radiation, it seems best to consider the results obtained 

 all together, and to describe first the types obtained, and later 

 the conditions under which one type or the other predominated. 



The relative effectiveness of the various light sources may be 

 determined by noting the duration of exposure required in each 

 case to produce a given proportion of differentially inhibited 

 forms. 



A. Differential Inhibition. Long-continued or very strong 

 radiation of eggs (fertilized or unfertilized), and of sperm before 

 using in the fertilization of normal eggs, produces embryos which 

 show marked evidences of permanent differential inhibition. 

 They are characterized by having the development of the oral 

 lobe, aboral arms, and the region between them, more or less 

 impaired or even entirely suppressed (Figs. 3-24). ! Inhibition 

 of development of the median anterior region results in the 

 production of plutei in which the aboral arms are closer together 

 than normal (Fig. 4), closely parallel (Figs. 5-7), or even partially 



1 Figures were drawn with the aid of a Bausch and Lomb euscope, and represent 

 an approximate magnification of 48 diameters, following reduction. 



