PRESIDENTIAL ADDRESS. 407 



it is a comparatively simple matter to cut off either the first or second polar lobes 

 by means of a very fine scalpel, and this is what Crampton and Wilson did. 

 If the first polar lobe be cut off and the egg survives, it develops into a most 

 peculiar trochophore. The shape is no longer spherical but hemispherical, and 

 the flat surface is bordered by the prototroch ; in a word, there is no post- 

 trochal region. The apical plate, with its tuft of cilia, is entirely absent : the 

 pre-trochal region is covered instead with a uniform layer of very fine cilia. 

 No mesoderm is formed : the interior is filled up with a mass of endoderm 

 in which a cavity is obscurely or not at all developed. If, instead of cutting 

 off the first polar lobe, the second polar lobe be removed, a very similar larva 

 is produced ; as before, there is no post-trochal region produced and no meso- 

 derm is differentiated, but a distinct apical plate with its wisp of cilia pro- 

 duced. We can only conclude from these experiments that there are distinct 

 substances whose presence is necessary for the formation of the post-trochal 

 region and of the apical plate respectively, that both these stuffs are concen- 

 trated in the first polar lobe, but that only the stuff necessary for the formation 

 of the post-trochal part of the embryo is contained in the second polar lobe. 

 Before the second cleavage of the egg has taken place the material necessary 

 for the formation of the apical plate has become redistributed, and Wilson 

 has been able to track it to its new destination. For, when the segmenting 

 molluscan egg is subjected to the influence of sea-water free from lime, it 

 breaks up into its constituent cells just as does the sea-urchin egg. When these 

 cells are now replaced in ordinary sea-water they develop further, but they do 

 not, like the separated cells of the sea-urchin egg, produce miniature perfect 

 embryos, but, on the contrary, each continues its development as if it still 

 formed part of the original embryo. The eight-cell stage in a molluscan egg 

 consists of four large cells termed macromeres, and of four small cells termed 

 micromeres. Now when these micromeres are separated and left to develop 

 separately, in certain molluscan eggs at any rate, only one of the four 

 micromeres will give rise to an apical plate, and this cell must therefore contain 

 the special substance which was formerly in the first polar lobe. Therefore, in 

 view of these facts, we are led to what I consider the great epoch-making 

 discovery of experimental embryology, viz. the existence of specific oegan- 



FOEMING SUBSTANCES. 



This conclusion is bitterly resisted by Driesch. He has no difficulty in 

 showing that the conception of the developing organism as a machine composed 

 of juxtaposed parts is a perfectly untenable one. For no conceivable machine 

 could have its parts so arranged that one could cut a large portion out of it 

 anywhere at random and yet have the possibility of forming out of the 

 remainder an exactly similar machine of smaller size ; and yet this is true of the 

 blastula of the sea-urchin before the mesenchyme is formed. But if all the cells 

 of this blastula contained a similar organ-forming substance, then we can under- 

 stand how any sufficiently large portion of the blastula wall can round itself 

 off and give rise to a perfect embryo. To this Driesch replies that it is 

 impossiljle to form a clear conception of what an ' organ-forming ' substance is. 

 It is, of course, not an ordinary chemical substance : for the molecules of an 

 ordinary chemical substance have not the power of ' crystallising ' into arms and 

 legs and other organs, and it can hardly be supposed that substances exist the 

 individual molecules of which are miniature arms and legs. He therefore 

 maintains that all these substances are merely ' conditions ' which limit the 

 powers of the entelechy to whose efforts the real activity in organ-formation 

 must be ascribed. Now, this objection of Driesch raises a really fundamental 

 question, which is: In what, after all, does 'explanation' consist? I think 

 that close reflection on this subject will convince one that we think we have 

 ' explained ' a new phenomenon when we have successfully compared it with 

 some older phenomenon which we regard as familiar and well known. Thus we 

 imagine that we have ' explained ' the eruption of a volcano when we have 

 compared it, rightly or wrongly, to the explosion of an overheated steam boiler, 

 and the law of gravitation which ' explains ' the movements of the heavenly 

 bodies is merely a comparison of these movements with the movements of an 

 apple which falls from its parent tree to the earth. The explanation of 

 ^development by an entelechy is at bottom a comparison of the forces moulding 



