526 INVEETEBEATA CHAP. 



When the gastrulae of Spliaerechinus are cut up, even if the 

 archenteron has already differentiated itself into oesophagus, stomach, 

 and rectum, if the fragment contains a part of the gut this will 

 differentiate itself again into oesophagus, stomach, and rectum on a 

 smaller scale. 



This result Driesch holds to be incompatible with any physical 

 or chemical explanation, but to render necessary the hypothesis of an 

 indwelling non-material entity the entelechy, which directs the 

 material at its disposal to purposeful ends. Driesch will not speak 

 of organ-forming substances, but of conditions, and he claims that 

 when the cytoplasm of the egg has been regulated to a certain extent 

 by the entelechy, in a certain direction, it becomes " stitfer " and 

 more incapable of " regulation " in a different direction. 



But there is the most indubitable evidence that, whether we can 

 form a mental picture of them or not, such things as onmn-formini> 



- 1 - O O O 



substances do exist ; for instance, when the developing tail of a 

 lizard, after an injury, can be made to give rise to two tails merely 

 by indenting the growing rudiment, what other explanation can lie 

 given ? Can the indwelling entelechy be forced to alter its whole 

 method of action by such an external influence ? Even Driesch's 

 pons asinorum can be got over, as will be shown below. 



When the blastulae of sea-urchins, instead of the segmentary 

 eggs, are exposed to heat, the gut develops as an external evagination 

 instead of an invagination. This external appendage can be cut off, 

 and in this way an anenterous larva is formed. Such a larva can 

 develop the first four pluteus arms and the stomodaeuni. In this 

 way it can be proved that the formation of the stomodaeum is in- 

 dependent of the gut. 



Herbst (1893, 1895, 1896) took up the task of testing the action 

 of different chemicals on the course of the development. He made 

 solutions of different salts in distilled water, of approximately the same 

 strength as is constituted by the total salt in sea-water. Small 

 quantities of these solutions were then added to the sea- water in which 

 the eggs to be experimented with were placed. He found that the 

 results obtained depended chiefly on the basic radicle in the salt, and 

 that they were inversely proportional to the molecular weight of the 

 salt used ; or, in other words, directly proportional to the number 

 of molecules. Most of the bases tried produced a larva devoid of 

 calcareous centres. Such larvae never produced larval arms. 



Herbst's most startling result was obtained by the use of the salts 

 of lithium. To obtain these results the eggs must remain in the 

 mixture of sea-water and lithium solution from the time they are 

 fertilized till the blastula stage is reached. If removed sooner the 

 lithium salt produces the same result as the other salts used, and 

 eggs submitted to the action of the solution after sixteen cells have 

 been formed do not yield the typical result now to be described. 



If eggs are left in the lithium mixture from the time they are 

 fertilized until the blastula stage is reached, then they develop into 



