66 WHAT IS LIFE 



then happens ? The egg goes on its way segmenting 

 as if nothing had happened, there is no re-arrange- 

 ment of the nuclei, a most significant fact, and 

 finally a perfectly normal larva of frog or sea- 

 urchin is the result. That is a frog or a sea-urchin 

 has been developed by means of a series of events 

 which one may safely say had never occurred 

 before, a tolerably clear proof that there is within 

 A'/ ) the egg a power which is able to steer it even 

 Tf J through seas before unsailed by any egg. 

 Nereis \ A further development of this is to be studied 

 in experiments made by Wilson on the eggs of 

 nereis, an annelid. It is a little difficult to de- 

 scribe this experiment without becoming highly 

 technical, so that those who are familiar with the 

 real facts of the case must deal leniently with the 

 writer if he endeavours to make the circumstances 

 clear by simplifying things to the utmost extent. 

 In the normal development of nereis, then, it is 

 known that certain cells will develop into certain 

 parts of the later organism and others into different 

 and distinct parts. 



Now if the egg be allowed to develop under 

 pressure as in the experiments above detailed, the 

 lines of division are vertical in all cases, so that the 

 segmented egg finally forms a plate, a flat plate, of 

 eight cells. Now if these cells, thus formed into a 

 plate, are released from the pressure under which 



