2O LIBBIE H. HYMAN. 



end and special sense organs ; for during this time there are two 

 centers of high susceptibility, the anterior end of the embryo and 

 the growing region of the primitive streak. Application of 

 depressing agents at the proper times when such processes are 

 occurring would tend to prevent the occurrence of unions, and the 

 formation of folds, bends, turns, etc., and would induce ab- 

 normalities in particular structures, such as the sense organs and 

 limb buds. It is not, however, necessary in the case of structures 

 requiring a very high rate of activity for their normal develop- 

 ment that the inhibiting agents be applied at the time they are 

 developing (contrary to the opinion of Stockard, '21); for any 

 general lowering of the metabolism of an embryo at an early 

 stage hinders the future development of such structures. Thus it 

 has been shown that the usual typical abnormalities appear 

 when the egg is subjected to depressing agents before fertilization 

 or when the sperm are injured (for literature on this matter see 

 Hyman, '21). Probably structures which develop at a low 

 metabolic level cannot be inhibited or altered unless depressing 

 agents are applied at the time of their greatest developmental 

 activity. 



As noted long ago by Dareste, abnormalities can be induced 

 in the chick by a variety of methods and such abnormalities 

 bear no specific relation to the agent employed. I shall here 

 review the different agents which have been used and the ab- 

 normalities obtained by their action, considering only single 

 monsters. 



Dareste ('836, '91, p. 103 ff.) found that shaking eggs induces 

 abnormalities of development. Eggs incubated immediately 

 after a railroad journey are apt to develop abnormally but yield 

 normal embryos if several days elapse between the journey and 

 incubation. Eggs shaken violently on a machine yield a high 

 percentage of abnormalities whether incubated immediately 

 or some time after being shaken. In about half of the eggs 

 so treated blastoderms without embryos were found. When 

 embryos were present these were generally abnormal, exhibiting 

 such conditions as doubled hearts, defective eyes, reduced size, 

 defective brains, spina bifida, and absence of the posterior end. 



According to Dareste and others (Dareste, '91, pp. 161, 168), 



