SOME CURRENT THEORIES 455 



lecithin, specifically, but by the disappearance of the yolk as a 

 nutritive supply. If the organism obtains nutrition from without, 

 the formation of both nuclear substance and cytoplasm may go on 

 for a long time, but sooner or later the gradual ''dilution" of the 

 protoplasm begins to make itself felt. It may be that the synthesis 

 of the nuclein of the nucleus is, as Loeb has suggested, an auto- 

 catalytic reaction, but the important point is that any attempt to 

 interpret the period of early embryonic development as a whole in 

 terms of autocatalysis fails to take account of features of great 

 biological importance. 



As regards Robertson's further evidence, his experiments on 

 the retardation of development by means of lecithin must be pre- 

 sented in rhuch more complete form before they can be regarded 

 as convincing. To establish as a fact a change so important as the 

 reversal of embryonic development requires extended and careful 

 experimentation. There is no evidence, from Robertson's descrip- 

 tion, of anything more than a toxic effect of the lecithin preparation, 

 and for the present we can only regard his conclusion as based on 

 very inadequate evidence. 



While the autocatalytic theory of growth is interesting and 

 doubtless of value as regards certain aspects of growth, it is at best 

 only a partial theory and can never be applied to the growth process 

 as a whole. The great periodic changes in growth during senescence 

 and rejuvenescence not only do not follow the laws of autocatalytic 

 reactions, but are determined by a complex of factors of which some 

 are only indirectly connected with chemical reactions of any kind. 

 From the laws of simple chemical reactions alone we can never 

 hope for anything more than partial and inadequate interpretations 

 of the complex biological processes, such as growth and reduction, 

 differentiation and dedifferentiation, senescence and rejuvenescence. 



REFERENCES 



Bernstein, J. 



1898. "Zur Theorie dcs Wachstums und der Bcfruchtung." .In/;. /. 

 Entwickclungsmecli ., \'1I. 



Blackman, F. F. 



1909. "The Manifestations of the Principles of Chemical Mechanics in 

 the Living Plant," Report of the jStli Meeting of the Brit. Assoc. 



for the Adv. of Sci. 



