STRUCTURE AND DEVELOPMENTAL RATE 173 



nature. Here again it would seem to be strongly indicated that a 

 connection of primary importance existed between the retardation 

 of development and the origin of the double specimens. 



A number of similar experiments with low temperature arrests 

 could be reviewed, but they would differ little in their general 

 results from those above. We may, therefore, pass to an analy- 

 sis of another type of experiment before undertaking a general 

 consideration of the significance of the results. 



b. Arresting development by reducing the oxygen supply and the 



occurrence of double individuals and twins in the 



trout and Fundulus 



Cellular proliferation which is so important an element in 

 development is a great energy-consuming process. No doubt the 

 interruptions in cell proliferation which were described in the 

 preceding section as due to low temperatures are actually caused 

 by a lower rate of oxidation which takes place at such tempera- 

 tures. In nature development is not only interrupted at times 

 by indirectly lowering the rate of oxidation through temperature 

 changes, but also by directly reducing the oxidation rate through 

 a lack of free oxygen. In the present section we may review 

 some of the consequences of lowering developmental rate by 

 directly reducing the available oxygen supply. 



The methods employed have been extremely crude, just such 

 methods as nature might frequently use. With such methods the 

 results are, of course, more variable than might be obtained from 

 highly refined manipulations, yet the variations themselves are 

 quite instructive. Experiments with Fundulus eggs may first 

 be considered. 



1 . Results with Fundidus. The eggs of Fundulus are demersal 

 and are supplied with long thread-like processes which normally 

 serve to entangle them on the blades of sea-grass or other objects 

 among which they are deposited by the female. This arrange- 

 ment serves to keep the eggs near the surface, and to insure 

 contact with a better oxygen supply than might be obtained 

 should they lie in the sand or silt of the bottom. When these 

 eggs are developed in the laboratory they are kept in small glass 



