REPRODUCTION AND HEREDITY 313 



though containing the same ' primary constituents,' will yet 

 assume such widely divergent appearances, forms at first a for- 

 midable obstacle. But we must consider that though the 

 nucleus is the real carrier of the heritable qualities the structure 

 of the ovum nevertheless regulates the course of segmentation 

 and the shape of the embryonic cells. 



Let us take a simple case. The ovum of the sea-urchin 

 possesses only a small quantity of food-yolk which is almost 

 uniformly distributed throughout the entire plasm. As a result 

 the egg-body meets a division in all parts with approximately 

 equal resistance, and the young segmentation-cells are therefore 

 of approximately equal size. In the frog-egg the vegetal pole 

 we know to be strongly loaded with the solid food-yolk which 

 hinders segmentation, and we see, therefore, that in the animal 

 pole which is poor in yolk the process of division takes place 

 far more easily and rapidly. Thus numerous small plasmic cells 

 are formed here, while at the vegetal end originate large cells 

 full of yolk. In spite of the equivalence of the nuclei, which at 

 this stage we are still able to demonstrate experimentally, the 

 fate of the individual embryonic cells is widely divergent, simply 

 owing to the unequal distribution of the yolk in the ovum. It 

 seems further clear that with progressing division young cells are 

 continually changing their relation to one another and to the grow- 

 ing embryo ; that the individual elementary organisms obtain a 

 continually decreasing share in the entire life-work, in proportion 

 as their multiplication proceeds ; that, further, in space as well 

 as in time, they meet unequal conditions ; and that, therefore, the 

 external factors exercise their influences upon the different cells 

 in a different manner. It is true that we are not in a position 

 exactly to define the stimuli which determine the shape of 

 cells and their special task in the complete organism, and cause 

 only a well-defined part of their inherited ' rudiments ' to develop, 

 but we may at least regard it as probable that in the conditions 

 mentioned are given the essential causes for the future deter- 

 mination of the form. The assumption of a differentiating effect 

 of external factors is far more in accord with our observations of 

 the far-reaching influence of environment upon appearance and 

 structure of the developed organisms, and our mind finds in this 

 conception greater satisfaction ; for the mysterious process of 



