146 



THE CHROMOSOMES 



effect caused by isolation, for it had been shown by 

 Driesch and others that when the first tw^o cells of a 

 sea-urchin egg that has been normally fertihzed are 

 separated, each forms a perfect embryo. Such cells, 

 although containing only half the cytoplasm, contain 



Fig. 38. — Diagram to show five combinations of chromosomes result- 

 ing from the first division of dispermic eggs, in which either each cell gets 

 one complete set of chromosomes, o; or three cells get a full set, 6; or 

 two cells, c; or one cell, (/; or none of the four cells, e, get a full set. 

 (After Boveri.) 



a full set of chromosomes. The difference, therefore, 

 between these cells and isolated cells from dispermic 

 eggs would seem to be due mainly to their different 

 chromosomal contents. 



Further evidence in favor of the chromosomal hy- 

 pothesis is found in certain cases of hybrids between 



