476 A. W. BELLAMY 
tion and recovery. 2) In general it was found that any type of 
abnormality could be produced by, a) regulating the concentra- 
tion or intensity of action of a given agent; b) regulating the 
length of exposure of the egg to the agent—in connection with a 
and c; c) varying the stage in development at which the egg or 
embryo is exposed to the experimental conditions. 3) It was 
further emphasized that, fundamentally, the frog egg and em- 
bryo reacted to the agent or conditions used not specifically, 
but quantitatively. In other words, the end result is primarily 
a function of the intensity of action of the agent or condition 
used, the length of exposure, and the physiological condition of 
the eggs at the time of exposure to the experimental conditions. 
The previous report dealt primarily with experimentally pro- 
duced modifications seen in the earlier stages of development—up 
to and including gastrulation. This report is concerned for the 
most part with the modifications seen in the later stages of de- 
velopment—gastrulation to the time of hatching or a little later. 
I am under obligation to Prof. C. M. Child, in whose laboratory 
the work was begun in 1916, for aid in the way of suggestion and 
criticism. . 
To facilitate the presentation of the experimental data and to 
give a brief sketch of the results of the experiments, the charac- 
teristic features of the different types of modified development are 
described below, together with a statement of the general experi- 
mental conditions under which the different types of modifica- 
tions may be expected to appear. 
TYPES OF MODIFIED DEVELOPMENT 
In spite of the almost endless variety of abnormal forms that 
appear in experiments of this nature, the types of abnormalities 
obtained fall readily into four general categories, according as 
development has been inhibited or accelerated, or according as 
the egg or embryo acclimates to the experimental conditions, or, 
on removal to water, recovers. Furthermore, however different 
in appearance embryos of these different types may be, there is 
at least one characteristic feature common to all of them. The 
response of the egg or embryo is differential, whether the condi- 
