28 THE GROWTH OF THE BKAIX. 



tion ; evolution in the sense that within the substance 

 of the egg there was present, in an extremely reduced 

 form to be sure, the perfect and complete animal, to 

 which the egg was destined to give rise ; and it was 

 held that the changes accompanying development con- 

 sisted in an enlargement of this miniature. In this 

 gross form the theory was soon discarded, but it is still 

 current in a modern guise. The animal is no longer con- 

 sidered to be pre-existent as a formed miniature, but it 

 is assumed by many that condensed in this small bit 

 of living matter are particles, molecules, if you please, 

 which in some way have marked out for them a fixed 

 course of development. In a sense, therefore, the future 

 animal is still looked upon as pre-existent in the egg, 

 since we know of no device by which any widely 

 different species of animal can be made to grow from 

 that particular cell. Yet despite the fact that the 

 lines are so narrowly fixed, along which the eggs of 

 different species develop, there are only slight cyto- 

 logical differences to be observed among those from 

 different animals ; and we have by no means reached 

 the point where, by the examination of an egg-cell alone, 

 any detailed prediction can be made concerning the 

 form into which it might develop. The theory just 

 mentioned probably expresses many of the facts of 

 development, but there are other facts, such as the 

 production of several embryos from a single egg or 

 complete embryos from fractions of the egg, which show 

 it to be at least incomplete. The significance of these 

 non-conforming observations will be best understood 

 from a few examples. 



It has been shown repeatedly that if from a cell the 

 nucleus be removed, the cytoplasm ceases to grow, and 

 shortly dies. When thus isolated, the nucleus also 

 dies. Recently Wilson has studied the development of 



