6i2 ON INCREASE IN COMPLEXITY [pt. iii 



of development by genetics and experimental embryology and 

 especially by chemistry may lead to the discovery of the physiological 

 action of the genes, but for the present we may confess ignorance." 

 A great step forward in this direction was taken by Goldschmidtt 

 in his important book on physiological genetics. He assumes that 

 genes are primarily of the nature of enzymes or substances which 

 can excite the action of enzymes. Working on the caterpillars of 

 Lymantria dispar he found that by crossing the European and Japanese 

 races, intersexes could be obtained, and he was able to identify their 

 various grades between the two poles of maleness and femaleness 

 with the time-process of development, i.e. with critical points earlier 

 or later in ontogeny. His conclusions were, firstly that the velocity 

 of the sex-determining (and all morphogenetic) reactions was pro- 

 portional to the quantity of the genes present, secondly that all the 

 morphogenetic reactions go on side by side, the most rapid one 

 controlling development and, thirdly, that the morphogenetic re- 

 actions involve determination-hormones brought into being by gene 

 action. In other words, the genes evoke the organisers, which evoke 

 the morphogenesis. It is obvious that the co-operation of the chemist 

 and the geneticist in the investigation of embryonic development will 

 be very fruitful in the future. With the work of Onslow on the 

 chemistry of coat-colour and of Brink and his collaborators on the 

 waxy gene in maize and its chemical effects, such co-operation may 

 already be said to have begun. Brink & Abegg, in an important 

 passage, point out that, though at present there is little likelihood 

 of valuable results emerging from further chemicafstudy of nuclear 

 material, i.e. the genes themselves, there is every chance of success 

 in the investigation of the chemical field of action of the genes. The 

 manner in which these units function in ontogeny plainly offers 

 a prodigious field for the future work of chemical embryologists. 

 Interesting reviews of this subject are those of F. R. Lillie and of 

 T. H. Morgan. 



This section may fitly be concluded with the words of Sir W. B. 

 Hardy: "Let us consider the egg as a physical system. Its poten- 

 tialities are prodigious and one's first impulse is to expect that such 

 vast potentialities would find expression in complexity of structure. 

 But what do we find? The substance is clouded with particles, but 

 these can be centrifuged away leaving it optically structureless but 

 still capable of development. . . . On the surface of the egg there is 



