210 THE BEGINNINGS OF LIFE. 



Dr. Thomson says, c The origin of the embryo-cell is still 

 involved in obscurity ,' and when he adds, 'Most 

 ovologists are disposed to connect it in some <way or other 

 with the previously existing germinal vesicle, or some 

 part of its contents, and more especially the nucleus,' 

 we can only recognize in this statement an evidence of 

 the enormous amount of influence which the old doctrine 

 concerning the potentiality of the nucleus once exercised 

 over the minds of physiologists. As Dr. Thomson frankly 

 admits, there is no direct evidence that can be produced 

 in favour of such an hypothesis : it would probably 

 never have been advanced had it not been for the 

 old doctrines concerning the marvellous powers of 

 the nucleus, which we have now gradually learned to 

 discard. The fact that segmentation does actually com- 

 mence in certain ova where no nucleus or embryo-cell 

 is present just as the protoplasmic contents of a spore- 

 case, or of an encysted Protomyxa, may break up into 

 separate living units in spite of the absence of a nu- 

 cleus should go far to convince us that such a body is 

 not in the least necessary, in order that the phenomena 

 of segmentation and development may be initiated. 

 Although, therefore, it may be present in many cases, 

 and may seem to take the initiative by its early divi- 

 sion, we must not on this account suppose that any 

 influence or power emanating from the embryo cell is 

 the cause of the segmentation of the yolk-mass : we 

 should rather regard both sets of phenomena as merely 

 associated changes, each alike being referrible to the 



