PROTOZOA 295 



cluster of homogeneous cells, but a harmonious whole, 

 the very different parts of which always come to lie at 

 the right spot ? We must admit the impotence of science 

 in face of these embryonic mysteries. We know nothing 

 of the forces at work here ; but we can say with some 

 confidence that no supernatural power controls the 

 development of an animal, so that each separate process 

 shall bring it a step nearer the complete construction. 

 The co-operation of the parts has clearly been brought 

 to its present height by natural selection, and the forces 

 at work in embryonic development are certainly not 

 different from those that we know in organic nature as 

 attraction and repulsion, strain and release. 



We saw in the sixth chapter that there are in the 

 ovum minute particles that represent the rudiments of 

 the future organs. We may now add that it has been 

 claimed that these particles are the chromatin that is 

 found in the nucleus of every cell, and so of the ovum, 

 in the shape of a number of minute corpuscles. 1 We 

 know that this chromatin also divides in the cleavage 

 of the cell, and this is done by so fine a piece of 

 mechanism that one daughter-nucleus receives just as 

 many halves of the particles as the other. If this 

 substance ^presents the rudiments of the future organs, 

 as we have good reason to believe, the ontogeny of an 

 animal would run somewhat as follows. In the first, 



1 We know from the following experience that the nucleus is the 

 chief constituent of the celL It has been proved experimentally that 

 in protozoa that were cut into pieces, only those parts remained alive 

 and grew into fresh animals which contained a fragment of the nucleus. 

 Pieces without nucleus died ; even pieces of an amoeba without nucleus 

 could not creep along, but floated about aimlessly in the water. 



