146 THE CHROMOSOMES 
effect caused by isolation, for it had been shown by 
Driesch and others that when the first two cells of a 
sea-urchin egg that has been normally fertilized are 
separated, each forms a perfect embryo. Such cells, 
although containing only half the cytoplasm, contain 
Fic. 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, a; or three cells get a full set, b; or 
two cells, c¢; or one cell, d; 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 
