JACQUES LOEB 545 



possible that the 'eggs must be punctured at a certain stage, e.g. at 

 the period after the first polar body is given off and before the second 

 polar body is extruded in order to develop or in order to develop 

 normally, but this can only be decided by further experiments. 



In these experiments the eggs were not covered with blood since it 

 was found in previous experiments that this did not improve the 

 yield of parthenogenetic tadpoles. 



Many, sometimes the majority, of the parthenogenetic tadpoles 

 did not metamorphose even within a year although they grew nor- 

 mally. It was suspected that this was due to an imperfect develop- 

 ment of the thyroid gland or to lack of iodine in the food. In one 

 tadpole, over a year old, the attempt was made to bring about meta- 

 morphosis by the feeding of thyroid gland from cattle. Three feed- 

 ings were sufficient to cause metamorphosis within 2 weeks. 



