542 



ON INCREASE IN COMPLEXITY 



[PT. Ill 



would mistake for organisms. The 'something going on' which we 

 call a thunderstorm, for example, is very complicated. An organism 

 from some points of view is comparatively simple, otherwise biology 

 would not have got as far as it has, and this simplicity appears to 

 be the outcome of its organisation. What happens in development 

 is not merely an increase in complexity nor an increase in spatial 

 structure, but a gradual rise in the level of organisation of the 

 developing organism". 



It has often been said that the interesting thing about any magni- 

 tude is not so much its absolute as its relative size, and not so much 

 its relative size as the rate at which its relative size is changing. The 

 first question to ask therefore in 

 the discussion of the physico- 

 chemical aspect of differentia- 

 tion is whether the rate of 

 differentiation is the same as 

 the rate of growth at each 

 embryonic stage. That the two 

 processes are independent in 

 the sense that one can be in- 

 duced without the other has 

 long been known. Panum, for 

 instance, as long ago as i860 

 observed chick embryos with 

 well-developed blastoderms but no primitive streak, but he did not 

 associate this with any definite causative factor. Then Broca found he 

 could get this growth without differentiation by keeping fertile eggs 

 for a month or more at room temperature before incubating them, 

 and Dareste observed that very high as well as very low temperatures 

 maintained during the first 24 hours of development would bring 

 about the same effect. Rabaud confirmed this : for further information 

 see the review of Tur. Finally, Edwards found that cell-division with- 

 out embryonic organisation always occurred in eggs incubated from 

 20 to 27°. Continued growth may take place also in the primitive 

 streak stage as well as in the simple blastoderm when there is not a 

 sufficient degree of temperature to permit of normal differentiation. 

 Fig. 89, taken from Edwards' paper, shows the effects he obtained. 

 The controlled action of temperature can also bring about nuclear 

 division without corresponding cell-division in Echinus^ according to 



" 12° 23° 

 TemperaJture 



Fig. 89. 



