SECT. 3] AND ORGANISATION 553 



is founded on another must be rejected — yet to think that heredity- 

 will build organic beings without mechanical means is a piece of 

 unscientific mysticism." 



Such opposition to mechanical explanations in embryology has 

 of course long been dead, and there are many authors who have put 

 forward such theories to account for the phenomena of differentiation 

 in the embryo. But these theories are for the most part quite physical, 

 depending on physical properties such as elasticity and torsion, so 

 that they cannot be considered in detail in this book. It is also 

 very unfortunate that, owing to the exceedingly small size of the 

 parts undergoing such changes, practically no direct work has been 

 done, and investigators have focused their energies on the prepara- 

 tion of models, made of a variety of materials, such as rubber and 

 plasticine, which can be made, like the yeast pills of His, to exhibit 

 the phenomena of gastrulation and the like. Roux in his valuable 

 review of the technique of " Entwicklungsmechanik " has described 

 such rubber models, and an even greater collection of them is to be 

 found in the paper of Rhumbler. Rhumbler gives in full the litera- 

 ture on this subject (e.g. Morgan; Schaper & Cohen; and Spek), and 

 has many illustrations of invagination models, etc. (see especially his 

 Section F onwards) . These investigations are interesting indeed, but 

 they do not contribute a great deal to our knowledge of what happens 

 in the early stages of embryonic development, although they cer- 

 tainly set forth a number of ways in which it might conceivably 

 happen. Robertson's theory of cell-division, again, in which lecithin 

 was supposed to be broken down at the two nuclear regions of the 

 dividing cell to provide phosphoric acid for new nuclein, and the 

 resulting free choline to diffuse away, reaching its greatest concentra- 

 tion in the equatorial plane at right angles to the line joining the 

 two nuclei, was never shown to hold in actual fact. Like so many 

 of the "Nachahmung" models, it rested upon an unwarranted 

 simplification of the material under discussion. Nevertheless, it was 

 an ingenious suggestion, and, in spite of McClendon's criticisms, it 

 may still be found to contain a modicum of truth, but the reason 

 why it and others like it will not be taken up in detail here is because 

 they are not sufficiently close to the facts. The purpose of this book 

 is to give all the facts that are known about the physico-chemical 

 aspects of embryonic development, and not the theories, which, 

 indeed, would demand a much larger treatise. 



