14 THE BIOLOGICAL PROBLEM OF TO-DAY 



that the nucleus is the bearer of the hereditary 

 material, from the evolutionistic interpretation 

 given it by Weismann. 



A paper on ' The Blastopore and Spina Bifida/ 

 and an occasional lecture on ' Old and New Theories 

 of Development/ gave me the opportunity of deal- 

 ing with Roux's mosaic theory, although that not 

 only shows learning, but apparently is the out- 

 come of experiment. I advocated in its place the 

 theory that ' the embryological development of an 

 organism is no mosaic work. The parts of an 

 organism develop in relation to each other, the 

 development of a part depending upon the develop- 

 ment of the whole.' The labours of Roux, as well 

 as the valuable researches of Driesch, induced 

 me to carry out a series of experiments with the 

 object of getting a surer basis for my epigenetic 

 conception of development. The results of these 

 were published recently under the title, On the 

 Value of the First Cleavage-cells in the Formation 

 of the Organs of Embryos. 



In the latter treatise I confined myself advisedly 

 to the exposition and interpretation of the results 

 of my investigations, having in view a subsequent 

 discussion of the more theoretical bearings of my 

 results. It is this that sees the light in the present 

 book. 



As for many years I have occupied myself with 

 the problem of development, pursuing observation 

 and framing theory, there is due to nryself and to 

 others an exposition of the position I have assumed 

 in many of my treatises, but in a more connected 



