204 T. H. MORGAN. 



two chromosomes present of very different sizes, as seen in the 

 figures (Fig. 4). 



In passing, it is worth while calling attention to the very dif- 

 ferent number of chromosomes found in these species of the same 

 genus ; species in fact, that are so similar that they can only be 

 distinguished with great difficulty. It does not seem probable 







FIG. 4. Phvlloxcra caryac-^lobiili. Polar spindle-equatorial plate of egg of 

 migrants. 



in the lig-ht of cases such as these that the absolute number of 



o 



the chromosomes can be a matter of any special significance. If 

 the chromosomes are all composed of the same identical sub- 

 stance it is difficult to account for their constancy in number and 

 sizes in each species. If the chromosomes are different in com- 

 position, as the conditions just mentioned would seem to indicate, 

 the differences can scarcely be associated with differences in the 

 structures of the body, since closely similar individuals are pro- 

 duced in one case with six chromosomes and in the other with 

 twenty-two chromosomes. This question is one of the most 

 puzzling problems in the whole range of cytology at the present 

 time, and it would be unwise to draw any conclusion from the 

 meagre facts known to us. I wish only to indicate as a possible 

 view that the chromosomes may be different chemically from each 

 other, and yet at the same time this difference may have no con- 

 nection with differences in the form of the body. 



Further History of the Chromosomes. --Starting with the stem- 

 mother the history of the chromosomes through the life cycle is 

 as follows : The number of chromosomes present in the polar 

 spindle of the eggs laid by the stem-mother is the somatic or 

 whole number. The number in the somatic cells of the embryo 

 that develops from this egg is also the whole number. The 

 number in the polar spindle of the male and the female eggs is, 



