TIIK CHROMOSOMES OF Till-: CHICK SOMA. 445 



DISCUSSION. 



The morphological data on the behavior of the chromosomes 

 in the developing embryo, although admittedly scant, has so 

 far given no clue to the manner in which the chromosomes may 

 contribute their potentialities to the growing organism. Before 

 a study of somatic chromosomes had been made it seemed 

 reasonable to expect to find the various highly differentiated cells 

 of the body with chromosome numbers, morphology or behavior 

 at variance both with those found in other tissues and with the 

 specific number and general characteristics found in the gonads. 

 This has been found not to be the case in at least three forms, 

 the pig (i), the evening primrose (2) and the chick (5). There 

 was some fragmentation of the somatic chromosomes in the 

 first two forms but it was shown that none of the chromatin was 

 lost. Furthermore this fragmentation was neither specific for 

 any particular tissue nor constant in amount. In general the 

 chromosome situation in the soma seems to be entirely similar to 

 that found in the unreduced gonad cells. This is a matter 

 difficult to understand in the present state of our information. 

 Of all the characteristics that must be controlled or borne by or 

 at least associated with the chromosomes (as shown by the studies 

 on Drosophila) few have any chance for expression in the majority 

 of the somatic cells and tissues and must therefore be inhibited 

 in one way or another. The cells of the lower forms of animal 

 life largely retain the germ-like power of reproducing the portion 

 lost or even an entire organism following injury. This power is 

 perhaps even more marked in plants. As differentiation becomes 

 more extreme in the animal kingdom the ability to regenerate a 

 part or a whole animal from cells already specialized becomes less 

 and less until in the highest types of animals somatic cells are 

 usually able to produce only somatic cells like themselves. 

 Yet the chromosomes in these highly specialized cells have, in 

 the examples studied, been found to be entirely similar to those 

 in the germ cells containing the possibilities for a complex animal 

 or plant. This may suggest that the chromosomes are function- 

 less in the differentiated Soma. Or it may be that having 

 contributed their share in the production of the specialized tissue 

 are thereafter inactive as far as are concerned the general somatic 



