90 



MARY ELMORE SAUER AND DONALD DUNCAN 







Fig. 12. Electron microscope picture of a tangential section of the neural tube. 

 X 15.000. Line indicates 1 micron. 



not represent e.ssential DNA but an excess formed in a period of derant^ed 

 metabolism or the reversal of a synthesizinc; cell to its presynthetic stage. A 

 related idea is an accumulation in the cytoplasm of nuclear material which 

 difTuses freely but is normally present in only small amount. RNA increases 

 in the cytoplasm followin" irradiation (Mitchell, 1942). Consequently, more 

 DNA from the nucleus is transforming into RNA of the cytoplasm. Some of 

 the cytoplasmic bodies might result from a derangement in this process. 



This has been a fascinating study, but it has raised more questions than 

 it has answered. What is the potentiality of early neural tube cells for resoip- 



FiGs. 9-11 are on page 89. 



Fig. 9. Feulgcn stain of the brain of a 4J/j-day chick embryo iriadiated with 500 r 

 3 hours previously. The lumen is at the top of the figure. Newcomer fixative. X 750. 



Fig. 10. Feulgen stain of the brain of a 3- to 3'/; -day chick embryo which had 

 been irradiated with 500 r 3J/2 hours pre\iously. The lumen is at the top of the figure. 

 Most of the bodies contain both Feulgen-positive and negati\e regions. Carnoy fixative. 

 X 1450. 



Fig. 11. Neural tube of a 2J/2-day chick embryo irradiated 8 hours previously with 

 500 r. The lumen is at the left. Newcomer fi.xative; May-Griinwald and Giemsa stain. 

 X 1400. 



