160 



SUMMATION BY CHAIRMAN 



and the mechanisms involved in their formation, as well as similar information 

 about the cerebellum, are now available (Hicks and D'Amato, 1960a, b) ; Hicks 

 et al., 1959). There is no simple formula for the eye and retinal malformations, and 

 radiation on any day from the 9th fetal to the 8th postnatal day presents its own 

 problem, as Fig. 2 shows. Proliferation in the retina of the albino rat ceases about 

 8 days after birth. Like the forebrain, the eye passes through a series of stages in 

 which radiation does a variety of things. Restitution seems to be complete or 

 almost complete, after the severe destruction that 200 r causes on day 13, and 

 rosettes do not form. Rosettes are the cell balls that resemble distorted neural 

 tubes and form from residual neuroepithelial cells after destruction of surrounding 

 cells by radiation or other agents. Such distorted neuroepithelium continues to 

 proliferate brain, cord, or retina, as the case may be. On day 20, for complex 

 reasons, the retina is not very radiosensitive in the sense used here, yet on day 

 19 or 21, mostly because there are greater numbers of sensitive migratory cells 

 present, permanent changes occur. Of special interest for behavioral experiments 

 is the great severity of the malformations that characterize the retinas of rats 

 irradiated on the first few days after birth. From the morphogenetic standpoint, 

 some immature bipolar cells are already present at birth, but when radiation 

 destroys much of the adjacent layer of primitive migratory cells, these infant 

 bipolar cells are suddenly stimulated to grow and sprout fibers, forming a preco- 

 cious plc.xifonn zone. When the residual neuroepithelium begins to catch up, it 

 spawns another layer of bipolar cells and itself forms rosettes as it differentiates 

 into the rod cell layer. (Fig. 2). 



Malformations of the cerebellum are just as complex in their mechanisms as 

 those of the cortex, thalamus, spinal cord, or retina. Figure 3 shows drawings of 



EFFECT5 OF 150to200p 

 ON THE DEVELOPING RAT RETINA 



HEAD OEFGCTi 



RosETres 





REDuCfftrJ 



"I 



Fig. 2. The spectrum of eye malformations produced by 200 r 



2122 23 I 2 3fJS<i78 



PoiT |ta|NATftl. DATS 



Bipolar L^VER 



