EMBRYONIC DEVELOPMENT 399 



the chick, Strangeways and Fell have shown that fragments of normal 

 embryonic tissues may be cultivated in vitro in a medium made from the 

 undiluted extract of the tissue of radiated chick embryos. However, 

 the tissue of radiated embryos was incapable of growth even in normal 

 plasma. Such results as these indicate that death of the tissues of a 

 radiated embryo is not due to the formation of stable toxic substances in 

 body fluids. 



Butler has come to similar conclusions with regard to amphibian 

 development. It has been demonstrated that with proper dosage of 

 X-rays one may prevent the development and also the regeneration of 

 limbs in Amblystoma larvae (Butler, 27, 29). In this case the question 

 arises as to whether the prevention of limb development and of limb regen- 

 eration is due to a direct action of the radiation on the cells which go into 

 the formation of the limbs, or whether toxic substances are formed in 

 the body which act upon the cells and prevent their differentiation. 

 Recent experiments by Butler have shown that if one limb bud be 

 shielded with lead and the remainder of the body be radiated, the 

 shielded limb on the radiated larva will develop normally. Likewise, 

 radiation of the posterior half of an Amblystoma larva, while the anterior 

 half of the body is shielded, results in the growth of the anterior limbs and 

 the complete suppression of the posterior limbs. It seems clear, there- 

 fore, from these experiments, as well as from the experiments of Strange- 

 ways and Fell, that the suppression of tissue growth and the initiation of 

 tissue degeneration result from a direct effect of radiation on the tissues 

 themselves and not from toxic substances contained within body fluids. 



Gaskell (38) in his study of radiated chicks reported particularly 

 on the mitotic changes which occurred within the tissues. He found 

 that radiation brought about either a partial or a complete diminution of 

 mitotic activity. Whether or not the embryo recovered, according to 

 Gaskell, depended on the extent to which mitotic activity was diminished. 

 This effect of X-rays on mitotic activity in chick embryos has also been 

 studied by Regaud, Lacassagne, and Jovin (80) and by Butler (28). 



The wall of the developing neural tube provides particularly favorable 

 material for a study of the effects of radiation on cells in different phases 

 of mitotic activity. Two types of cells are found in the wall of the neural 

 tube of young chick embryos; one type, the germinal cells, with nuclei 

 conspicuously active in mitosis, is restricted to the innermost layer of the 

 neural wall; the other type of cell, with nuclei in the resting state and none 

 in division, forms the main mass of the neural wall and is known as 

 the mantle layer. Butler has shown that a study of the neural tube 

 after radiation with a single nonlethal dose of X-rays reveals a series of 

 cellular changes as follows : an initial period of mitotic depression during 

 which mitotic figures in the germinal cells disappear entirely; a period of 

 pseudo-recovery during which mitotic activity reappears; a period of 



