4 M \KIJ-: A. HIXRICHS. 



(Child, '16, '20, '23), and since these regions are also the first to be 

 modified by sublethal doses of such agents (Child, 'i6b, '24), it 

 seemed likely that those regions of the developing embryo which 

 showed the greatest degree of developmental modification (see 

 also Child, 'i6&), as for example, the oral lobe and aboral arms, 

 would also show the earliest evidences of disintegration in lethal 

 doses of visible light following sensitization (Figs. 1-3). 



Modification of development in these regions is brought about 

 by an exposure made much earlier than at the pluteus stage. 

 A short exposure made immediately after fertilization will produce 

 abnormal inhibited development of these active regions. Two 

 possibilities suggest themselves; either a general metabolic 

 lowering of developmental rate at the time of exposure, produces 

 an effect at first latent, and not appearing until later, at a highly 

 susceptible stage of development ; or, there is an early difference 

 in the relative susceptibility of regions or "Anlagen" present in 

 the fertilized egg. The latter seems more probable. (See also 

 Hinrichs, '25, now in press.) 



As a result of inhibition of development by means of radiation, 

 forms are produced in which the anterior region of the developing 

 embryo is relatively less developed than other regions. The 

 oral lobe, which represents the apical region, and the aboral 

 arms which together with the oral lobe, forms the anterior region 

 of the pluteus, show the greatest effect (Figs. 7-11). The angle 

 between the arms is greatly reduced. Often there is but a single 

 median arm (Figs. 12, 13) or one partially fused anteriorly 

 (Fig. 14). 



If the inhibitory effect has been even more marked, the aboral 

 arms and oral lobe may be undeveloped, and there remains an 

 ovoid mass which may have short parallel skeletal arms embedded 

 in the tissue, one on either side of the archenteron, which latter 

 has but one opening to the exterior (Fig. 15). A final stage of 

 inhibition shows an apolar spherical mass with non-direct i\ < 

 swimming (Fig. 16). Exogastrulae (Figs. 17-22) are also com- 

 monly found in cultures in which inhibition has been produced. 

 (See also Herbst, '95, and MacArthur, '24.) 



B. Differential Recovery. As examples of differential recovery 

 obtained by short early exposures in which the harmful effect is 



