THE VITAL IMPETUS 



135 



But since either of the blastomeres may develop into 

 an entire embryo, the mechanism must also be present 

 in each of them, and 

 since in the i6-cell 

 stage each blastomere 

 may develop an entire 

 embryo, it must be 

 present in each -of the : 

 sixteen blastomeres. 

 A three - dimensional 

 mechanism is there- 

 fore capable of division 

 down to certain limits. 

 Suppose now that 



we allow the sea-urchin egg to develop normally up 

 to the blastula stage. In this stage it is a hollow 

 sphere, the wall of which is a single layer of cells. 

 It is similar all round, that is, we cannot distinguish 

 between top and bottom, right and left, anterior and 

 posterior regions ; but since it develops into a larva 

 in which all these distinctions become apparent very 

 soon, it must possess the three-dimensional mechanism, 

 since the ^activity of the developmental process is 

 going to produce different structures in each direction. 



Now the blastula, by very careful 

 manipulation can be divided, cut 

 into parts with a sharp knife. Since 

 it is similar all round the direction 

 of the cut is purely a matter of 

 chance. It can be cut through along 

 the planes 12, 34, 56, 78, for in- 

 stance ; really there are an infinite 

 number of planes along which the 

 blastula can be cut into two separate parts, and 

 the direction of the plane is not a matter of choice, 



FIG. 1 6. 



