PRESIDENTIAL ADDRESS. 405 



maintained that the embryos which he obtained were real half -embryos and 

 not merely distorted whole ones. Hertwig's position seemed to be upheld by 

 the remarkable experiments of Driesch on the eggs of the sea-urchin. Many of 

 these experiments have become so well known that they have, so to speak, 

 escaped from zoological literature into popular literature, and have even 

 become incorporated in current philosophy. It will therefore be necessary to 

 examine Driesch's work critically, although limits of space forbid us dealing 

 with his experiments in detail, and a very brief description of the more 

 important must suffice. 



The first and in many ways the most striking of Driesch's experiments was 

 that of separating the first two blastomeres of the sea-urchin's egg from one 

 another by. violent shaking. When this was done he found that each of the 

 separated blastomeres developed into a perfect larva of reduced size. Driesch 

 hailed this as a, final proof that the doctrine of ' Specific organ-forming areas ' 

 of His which had been endorsed by Roux was fundamentally false. This con- 

 elusion he was able to back up by further experiments, especially after his 

 methods had been improved by the discovery made by his friend and co-worker 

 Herbst that when sea-urchin eggs were allowed to develop in artificial sea-water 

 from which lime had been excluded the blastomeres separated from one another 

 spontaneously. Driesch showed that one of the first four blastomeres would 

 develop into a perfect larva, and that in some few cases one of the first eight 

 blastomeres would do likewise. 



Driesch asserted that the fate of a cell was a function of its position in the 

 embryo, not of its inborn specific quality. He showed that when eggs were 

 allowed to develop under pressure the first eight cells, instead of forming two 

 tiers of four cells each, were spread out in one plane. If the membrane of 

 the egg had been burst these cells did not return to their positions when the 

 pressure was removed, but at the next cleavage formed a double-layered plate 

 of sixteen cells, eight in each layer ; and yet this structure would in favourable 

 circumstances develop into a perfectly normal embryo. Now it follows from 

 this that cells which under normal circumstances would have formed the lower 

 pole of the larva must form the sides. To similar conclusions Hertwig was 

 led when he examined the development of frogs' eggs submitted to pressure, 

 either by being sucked into narrow glass tubes or by being pressed between 

 glass plates. He maintained that the dividing planes separating the blasto- 

 meres were formed along the lines of pressure, or, in other words, that growth 

 took place at right angles to the pressure : that the nuclei of the developing 

 egg could he juggled about like a handful of marbles without altering the result. 



Driesch then showed that if the blastula into which the sea-urchin egg 

 develops be cut into pieces, these pieces if not too small will close up and form 

 miniature blastulas which will develop by the invagination of their lower 

 poles into gastrulre and further into the well-known pluteus larvfe. Previously 

 to the occurrence of invagination, cells are budded from the lower pole into the 

 cavity of the blastula; these are termed mesenchyme. If the blastula be cut 

 in pieces after this has occurred, these pieces may still heal up and form 

 miniature blastulse ; but only blastuloB derived from the lower pole of the 

 original blastula will become converted into gastrulre and form guts — those 

 derived from the upper pole remain gastrulse until they die. Another instance 

 of the same thing was observed in the case of the gastrulse of the star-fish. 

 These are sausage-shaped — not hemispherical, like the gastrulaa of the sea- 

 urchin — and hence comparatively easy to cut across. The gut reaches from the 

 posterior pole only about half-way up. When the gastrula is bisected the 

 stump, including the fragment of the gut attached to the blastopore, will 

 regenerate the missing parts and form a smaller gastrula which will develop into 

 a perfect larva. But if, before bisection has been performed, the apex of the 

 gut has grown out into the thin-walled vesicle from which the coelom is 

 developed and this is removed by the operation, then, although the stump will 

 heal up and a miniature gastrula will be formed, this gastrula will never form a 

 coelom. 



Driesch talks of positive and negative determination of the potencies 

 of portions of the gi'owing embryo. To take an instance ; when the mesenchyme 

 has been formed in the blastula of the sea-urchin, the lower portion of the 



