PRESIDENTIAL ADDRESS. 409 



distinction between ' condition ' and ' effective cause ' is rather a metaphysical 

 one, we may proceed to show that the supposititious indwelling entelechy can be 

 entirely baulked and misled in its aim by slightly different arrangements of the 

 organ-forming substances. The eggs of the frog contain two different cyto- 

 plasmic substances easily distinguishable by the naked eye; one of these is of a 

 dark colour, and the other of a light colour. When the experiment was per- 

 formed of fixing a frog's egg upside down to a slide so that it could not rotate, 

 and allowing it to develop in this position, it was found that the nervous system 

 of the tadjJole was still produced on the side of the egg which was uppermost. 

 This can be understood when it is realised that the dark substance is of a lesser 

 specific gravity than the white substance, and that the substances re-arrange 

 themselves under the influence of gravity. If, however, frogs' eggs are fixed 

 to one slide and compressed by having another slide clamped on the top of them, 

 and are allowed to divide into two in this position, and if the slides be then 

 turned upside down and the development allowed to continue, a double monster 

 is produced'! — that is, a tadpole sometimes with two heads and sometimes with 

 two tails. Now, Driesch defines his entelechy as a ' rudimentary psychoid 

 which knows and wills what it wants to produce ' ; but we may safely affirm that 

 no intelligent psychoid ever desired to produce a result like this, and in this 

 case nothing has been either added to or subtracted from the egg. But if we 

 try to give an explanation in terms of organ-forming substance, we succeed 

 much better. We may assert with confidence that the formation of a normal 

 eynhryo ix the consequence of the arrangement of tJie dark and light substances 

 in a certain spatial relation to one another. When the egg is inverted this fixed 

 relation is maintained owing to the influence of gravity, since, as we have seen, 

 the two substances have different specific weights ; but when the egg has been 

 divided into two and is then inverted, then the division plane between the two 

 cells causes a readjustment of the positions of the two substances within each 

 cell as if each cell were a whole egg, and thus arises the tendency for each 

 cell to develop into a Avhole embryo. If the same experiment be tried with a 

 newt's egg — in which, however, the various organ-forming substances are not 

 distinguishable by the naked eye — the result is to produce, not a double monster, 

 but two completely separate embryos. Now, if we analyse closely wherein lies 

 the difference, in the distribution of these substances in the two-cell stage of a 

 normal egg and of an egg which has been compressed and inverted during the 

 first cleavage, we find that it can only consist of a slight re-entrant angle in the 

 outline of the black substance as it crosses the division plane separating the two 

 cells. In the nomial egg the black substance forms an evenly curved cap in the 

 two-cell stage ; in the compressed egg this cap is bent inwards in the middle. 

 Yet this slight difference is supposed to be sufficient to deceive the entelechy 

 and baulk it of the fulfilment of its purpose. In the newt's egg, where the 

 materials are apparently more mobile, the re-entrant angle is more acute, and 

 here the duplicity becomes so great as to produce two completely separate 

 embryos. That the difference in outline is in reality the factor which causes 

 the doubling is proved by a large number of additional experiments. Thus 

 Herlitzka, experimenting, not with the segmenting egg but with the blastula of 

 the newt, was able to show that, by constricting it with a fine hair so as to 

 indent the anterior outline, he was able to produce a two-headed embryo ; Loeb, 

 by placing the blastulse of the sea-urchin Arhaeia before they had escaped from 

 the egg membrane in water of diminished salinity, was able to cause them to 

 ewell so as to tear rents in the membrane and to produce extrusions of the 

 blastular wall. These rounded extrusions begin to develop like separate 

 embryos, forming their own guts. 



We thus come to the conclusion that for the present we may dismiss the 

 conception of the entelechy from our minds as a working hypothesis and adopt 

 instead the conception of organ-forming substances, and we may now proceed 

 to inquire what further can be learnt about these extraordinary materials. In 

 some cases it can be shown that what determines the fate of a particular region 

 of the embryo is, not the presence or absence of a certain substance, but its 

 presence in greater amount than in neighbouring regions. The classic example 

 of this kind of thing is the egg of Ascaris, the Nematode worm as worked out 

 by Boveri. We are, most of us, aware that the development of this egg used 



