J 



2IO THEORETICAL BIOLOGY 



the rule of function, and, as soon as the function begins, 

 merely furnishes it with the framework it lacks. 



MULTICELLULAR ANIMALS 



This interlocking of the two rules, which is characteristic 

 of unicellular animals, ceases altogether when we come to 

 multicellulars, in so far as these consist of permanent organs. 



The life of such animals falls into two distinct parts. In 

 the first, the organs are formed ; in the second, they are used. 

 In the first, the rule of genesis controls ; in the second, the 

 rule of function. Accordingly it is possible in multicellular 

 animals to observe the rule of genesis more clearly at a 

 given time, since it proceeds quite independently of inter- 

 ference from function. 



The first activity of the fertilised egg consists in dividing 

 equally into two halves. In the course of this process, there 

 first appears, as at every later cell-division, a remarkable 

 apparatus consisting of protoplasmic threads, which meet at 

 two nodal points. These points seem, on the one hand, to be 

 firmly anchored in the cytoplasmic network, and, on the other, 

 to be connected with the nucleus. The protoplasmic threads 

 running to the nucleus must be contractile, for, when the 

 chromosomes split longitudinally, they draw them apart as 

 far as the nodal points. In this way there arise two new 

 nuclei. The division of the protoplasm then follows. 



The first divisions of the egg transform the germ into a 

 number of cell-spherules, which are identical, for, if we 

 separate them from one another, they have the power of pro- 

 ducing two (or even more) independent animals, although of 

 half the normal size. 



Driesch succeeded in showing in sea-urchin larvae obtained 

 in this way that the half-size depended on each larva having 



