406 TRANSACTIONS OF SECTION D. 



blastular wall is posit.ively determined by having conferred on it the power of 

 producing a gut, whilst the upper portion of the blastular wall is negatively 

 determined in being deprived of the power of producing a gut : whereas, as we 

 have seen, previous to the formation of mesenchyme either half could produce a 

 gut. 



Driesch then began experiments on a totally different kind of eggs, viz. 

 those of the ctenophore Beroii. These eggs are much larger than those of the 

 sea-urchin and have an abundant supply of yolk. The first step in develop- 

 ment is the division of the egg by longitudinal furrows into a wreath of eight 

 cells. Now it is a comparatively easy matter to separate one or two of these 

 from the rest; and the remainder will then develop into an imperfect ctenophore 

 with seven or six, instead of the customary eight, ciliated ribs. It is therefore 

 evident that the material for one particular rib must be localised in one 

 particular blastomere. Driesch even succeeded in proving that this speciali- 

 sation existed before the egg had divided into cells at all, for he cut pieces of 

 protoplasm from unfertilised eggs, and those that survived developed into 

 ctenophores with an imperfect number of ribs. 



Driesch and Hertwig, on the one hand, and Roux, on the other, drew 

 opposite conclusions from the results of their experiments. 



Roux regarded — as His did before hun — each element of the embryo as 

 imbued with its own specific organ-forming capacity, which he attributed to a 

 substance termed by him its ' Idioplassox.' The power of regenerating lost 

 parts could not be attributed to the Idioplasson ; so, in order to account for it, 

 a new substance, the ' Reserve-idioplasson,' was invented, which came into 

 play only when by experiment or accident one part was separated from the rest. 

 Driesch, as we all know, boldly asserted the existence of an ' Entelechy ' or 

 arranging spirit which out of the material at its disposal constructed the 

 organism which it knew and willed. Thus the inability of the upper half of a 

 blastula, once the mesenchyme was formed, to produce a perfect larva was 

 explained by Driesch on the assumption that as development proceeded the 

 protoplasm became relatively more stiffened or stereotyped and less easy of 

 manipulation by. the entelechy, and the fact that the egg of a ctenophore would 

 not endure the removal of a blastomere without giving rise to an imperfect 

 organism was attributed to an early or precocious ' stiffening ' of the proto- 

 plasm. Roux would have attributed many of Driesch's results to the action 

 of his Reserve-idioplasson, to which Driesch retorted that by a parity of 

 reasoning all development might be construed as regeneration : ' everything is 

 wanting at the beginning except a single cell, which regenerates all the rest.' 



Hertwig did not go so far as Driesch in calling up spirits from the void ; 

 but his explanation must be characterised as vague and misty : he speaks, as 

 we have seen, of the fate of a oell being a function of its position, and of the 

 development of organs being a result of the reciprocal action of different cells 

 on each other. But it is obvious that if differentiation is to spring from this 

 an initial difference must exist; for the reciprocal action of similar cells on 

 one another would give everywhere a similar result. 



The next step in advance came from the study of molluscan eggs first by 

 Crampton and then by E. B. Wilson, who confirmed and extended Crampton's 

 results. In the eggs of certain Mollusca the first cleavage of the egg seems 

 to divide it, not into two, but into three cells. The third ' cell ' is, however, 

 devoid of a nucleus, and, before the next cleavage, it melts into one of the two 

 remaining cells. This transitory cell is known as the ' first polar lobe.' At 

 the next cleavage five cells are apparently produced, but again one of these 

 is a transitory ' second polar lobe ' which melts into one of the remaining four 

 before the cleavage following. After this cleavage a third polar lobe is 

 extended and reabsorbed in the same way. The egg of a mollusc normally gives 

 rise to a characteristic larva termed a trochopore : this, as all know, is a more 

 or less spherical structure girdled by a belt of powerful cilia known as the 

 prototroch, and having at the apex of one hemisphere a thickened plate — the 

 Apical Plate bearing a tuft of long cilia. This hemisphere is known as the pre- 

 TROCHAL hemisphere ; at the end of the other hemisphere, termed the post- 

 TROCHAL, is situated the embryonic mouth or blastopore. This opening leads into 

 a sac-like gut, at the sides of which are situated two masses of mesoderm. Now 



