STRUCTUEE AND DEVELOPMENTAL RATE 257 



ment from such a state, parts that should progress at a dispropor- 

 tionately fast rate are unable to attain such supremacy and all 

 portions of the embryo start at about the same rate. The usual 

 developmental balance and inequalities in rate among the parts 

 are lost and thus the typical form of the individual which actually 

 depends upon these inequalities in rate of growth becomes modified. 



3. The types of deformities following a stop in development as 

 well as those occasionally resulting from a slowing of the rate 

 are similar to the defects produced by all experimental methods. 

 Practically any deformity recorded in the literature other than 

 those resulting from germinal variations or mutations may be 

 induced by lowering the temperature and thus modifying the 

 developmental rate. 



4. By an interruption of development during late cleavage 

 stages a considerable percentage of twins and double individuals 

 may be produced. When the eggs of the sea-minnow, Fundulus 

 heteroclitus, are subjected to temperatures of 5° or 6°C. during 

 cleavage stages, development is almost stopped. On returning 

 such eggs to a summer temperature, after several days' sojourn 

 in the refrigerator, there will follow a high mortality, but many 

 specimens will resume development producing a significant per- 

 centage of twins and a number of variously deformed conditions 

 along with a good proportion of normally formed young fish. 



Arresting or stopping development of the same eggs during 

 the same developmental stages by diminishing the available sup- 

 ply of oxygen will be followed by closely similar results. 



The eggs of the trout are naturally much more inclined to 

 develop into double individuals than are those of Fundulus. 

 When the oxygen supply during early development is not abun- 

 dant, a great many twin and double trout specimens are frequently 

 found to occur. 



All of these double conditions result from arrests during very 

 early stages of development, invariably before the process of blas- 

 topore formation has in any way begun. No duplicities or 

 twins have been found to occur among the great numbers of fish 

 eggs which have been arrested during postgastrular stages of 

 development. 



