434 CHARLES RUSSELL BARDEEN 



per cent of the control specimens were carried to this stage. 

 Late in the season fertilized eggs obtained under natural condi- 

 tions frequently show some abnormalities of development and 

 at this period a certain percentage of the control specimens show 

 abnormalities even during the earlier stages of development. 

 Note is made wherever this occurred. 



To carry tadpoles through the later stages of development and 

 metamorphosis special precautions are necessary. A large 

 amount of well aeriated water and an abundance of food supply 

 is necessary and even then a considerable number of apparently 

 normal tadpoles usually fail to complete a normal metamorphosis. 



In the tables where the percentage of 'normal specimens' is 

 given it is to be understood that by this term is meant the per- 

 centage of tadpoles developing into large well developed tadpoles 

 a few of which were isolated and followed through metamorphosis. 

 When the percentage of isolated specimens which metamor- 

 phosed was approximately equivalent to the percentage of a 

 similar set of control specimens undergoing metamorphosis the 

 whole group from which the experiment specimens were isolated 

 was considered 'normal.' With greater facilities at hand it 

 might have been possible to determine the percentage of the whole 

 group capable of undergoing metamorphosis as compared with 

 the control in each experiment. It was, however, possible to 

 do this merely in the few experiments in which it has been noted. 



The effects of exposure may be roughly subdivided into the 

 following subdivisions, although it is difficult to draw sharp 

 lines of division between them. 



1. Development stopped during cleavage. Cleavage more 

 or less abnormal figs. 1 and 6. 



2. • Gastrulation abortive or abnormal. May stop early fig. 7, 

 or lead to : 



a. spina bifida specimens (figs. 2 and 3,) and specimens 

 with large anus (figs. 8, 9 and 10). 



b. hemi-embryos (figs. 4 and 5). 



3. Gastrulation complete though- more or less abnormal 

 No distinct larval differentiation (fig. 11). 



